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MECHANISTIC TARGET OF RAPAMYCIN; MTOR

MECHANISTIC TARGET OF RAPAMYCIN; MTOR

Alternative titles; symbolsMAMMALIAN TARGET OF RAPAMYCINFKBP12-RAPAMYCIN COMPLEX-ASSOCIATED PROTEIN 1; FRAP1FK506-BINDING PROTEIN 12-RAPAMYCIN COMPLEX-ASSOCIATED...

Alternative titles; symbols

  • MAMMALIAN TARGET OF RAPAMYCIN
  • FKBP12-RAPAMYCIN COMPLEX-ASSOCIATED PROTEIN 1; FRAP1
  • FK506-BINDING PROTEIN 12-RAPAMYCIN COMPLEX-ASSOCIATED PROTEIN 1
  • FRAP
  • FRAP2
  • RAFT1

Other entities represented in this entry:

  • MTOR COMPLEX, INCLUDED; MTORC, INCLUDED
  • MECHANISTIC TARGET OF RAPAMYCIN COMPLEX 1, INCLUDED; MTORC1, INCLUDED
  • MECHANISTIC TARGET OF RAPAMYCIN COMPLEX 2, INCLUDED; MTORC2, INCLUDED

HGNC Approved Gene Symbol: MTOR

Cytogenetic location: 1p36.22 Genomic coordinates (GRCh38): 1:11,106,534-11,273,496 (from NCBI)

▼ Description
MTOR is a highly conserved protein kinase that is found in 2 structurally and functionally distinct protein complexes: TOR complex-1 (TORC1) and TORC2. TORC1 is a key regulator of cell growth and proliferation and mRNA translation, whereas TORC2 promotes actin cytoskeletal rearrangement, cell survival, and cell cycle progression (summary by Jacinto et al., 2004 and Thoreen et al., 2012).

▼ Cloning and Expression
To identify the target for the FKBP12-rapamycin complex in human, Brown et al. (1994) used a FKBP12/glutathione-S-transferase fusion protein and glutathione affinity chromatography to purify a 220-kD bovine brain protein which bound the FKBP12-rapamycin complex. They designed oligonucleotide probes based on the bovine protein sequence and screened a human Jurkat T-cell cDNA library. Their complete human cDNA for FRAP encoded a predicted 2,549-amino acid protein with a calculated molecular mass of approximately 300 kD. Brown et al. (1994) showed by Northern blot analysis that the 7.6-kb gene transcript was present in a variety of human tissues. They noted that, while the precise functions of FRAP and its yeast homologs TOR1/TOR2 are unknown, the C-terminal regions of these proteins share amino acid homology (approximately 21% identity on average) with several phosphatidylinositol kinases; see 171834.

In a review, Hay and Sonenberg (2004) described the domain structure of MTOR. The N-terminal half of the protein contains 20 tandem HEAT repeats, which are implicated in protein-protein interactions. Each HEAT repeat consists of 2 alpha helices of about 40 amino acids. The C-terminal half contains a large FRAP-ATM (607585)-TRRAP (603015) (FAT) domain, followed by the FKB12- and rapamycin-binding domain, a serine/threonine kinase catalytic domain, a negative regulatory domain, and a C-terminal FAT (FATC) domain necessary for MTOR activity.

▼ Gene Function
FKBP12-rapamycin associated protein (FRAP) is one of a family of proteins involved in cell cycle progression, DNA recombination, and DNA damage detection. In rat, it is a 245-kD protein (symbolized RAFT1) with significant homology to the Saccharomyces cerevisiae protein TOR1 and has been shown to associate with the immunophilin FKBP12 (186945) in a rapamycin-dependent fashion (Sabatini et al., 1994). Brown et al. (1994) noted that the FKBP12-rapamycin complex was known to inhibit progression through the G1 cell cycle stage by interfering with mitogenic signaling pathways involved in G1 progression in several cell types, as well as in yeast. The authors stated that the binding of FRAP to FKBP12-rapamycin correlated with the ability of these ligands to inhibit cell cycle progression.

Rapamycin is an efficacious anticancer agent against solid tumors. In a hypoxic environment, the increase in mass of solid tumors is dependent on the recruitment of mitogens and nutrients. When nutrient concentrations change, particularly those of essential amino acids, the mammalian target of rapamycin (mTOR/FRAP) functions in regulatory pathways that control ribosome biogenesis and cell growth. In bacteria, ribosome biogenesis is independently regulated by amino acids and ATP. Dennis et al. (2001) demonstrated that the human mTOR pathway is influenced by the intracellular concentration of ATP, independent of the abundance of amino acids, and that mTOR/FRAP itself is an ATP sensor.

Castedo et al. (2001) delineated the apoptotic pathway resulting from human immunodeficiency virus (HIV)-1 envelope glycoprotein (Env)-induced syncytia formation in vitro and in vivo. Immunohistochemical analysis demonstrated the presence of phosphorylated ser15 of p53 (191170) as well as the preapoptotic marker tissue transglutaminase (TGM2; 190196) in syncytium in the apical light zone (T-cell area) of lymph nodes, as well as in peripheral blood mononuclear cells, from HIV-1-positive but not HIV-1-negative donors. The presence of these markers correlated with viral load (HIV-1 RNA levels). Quantitative immunoblot analysis showed that phosphorylation of ser15 of p53 in response to HIV-1 Env is mediated by FRAP and not by other phosphatidylinositol kinase-related kinases, and it is accompanied by downregulation of protein phosphatase 2A (see 176915). The phosphorylation is significantly inhibited by rapamycin. Immunofluorescence microscopy indicated that FRAP is enriched in syncytial nuclei and that the nuclear accumulation precedes the phosphorylation of ser15 of p53. Castedo et al. (2001) concluded that HIV-1 Env-induced syncytium formation leads to apoptosis via a pathway that involves phosphorylation of ser15 of p53 by FRAP, followed by activation of BAX (600040), mitochondrial membrane permeabilization, release of cytochrome C, and caspase activation.

Fang et al. (2001) identified phosphatidic acid as a critical component of mTOR signaling. In their study, mitogenic stimulation of mammalian cells led to a phospholipase D-dependent accumulation of cellular phosphatidic acid, which was required for activation of mTOR downstream effectors. Phosphatidic acid directly interacted with the domain in mTOR that is targeted by rapamycin, and this interaction was positively correlated with mTOR's ability to activate downstream effectors. The involvement of phosphatidic acid in mTOR signaling reveals an important function of this lipid in signal transduction and protein synthesis, as well as a direct link between mTOR and mitogens. Fang et al. (2001) concluded that their study suggested a potential mechanism for the in vivo actions of the immunosuppressant rapamycin.

Kim et al. (2002) and Hara et al. (2002) reported that MTOR binds with RAPTOR (607130), an evolutionarily conserved protein with at least 2 roles in the MTOR pathway. Kim et al. (2002) showed that RAPTOR has a positive role in nutrient-stimulated signaling to the downstream effector S6K1 (608938), maintenance of cell size, and MTOR protein expression. The association of RAPTOR with MTOR also negatively regulates MTOR kinase activity. Conditions that repress the pathway, such as nutrient deprivation and mitochondrial uncoupling, stabilize the MTOR-RAPTOR association and inhibit MTOR kinase activity. Kim et al. (2002) proposed that RAPTOR is a component of the MTOR pathway that, through its association with MTOR, regulates cell size in response to nutrient levels.

In mammals, MTOR cooperates with PI3K (see 171834)-dependent effectors in a biochemical signaling pathway to regulate the size of proliferating cells. Fingar et al. (2002) presented evidence that rat S6k1 alpha-II, Eif4e (133440), and Eif4ebp1 (602223) mediate Mtor-dependent cell size control.

Hara et al. (2002) showed that the binding of RAPTOR to MTOR is necessary for the MTOR-catalyzed phosphorylation of 4EBP1 in vitro and that it strongly enhances the MTOR kinase activity toward p70-alpha (S6K1). Rapamycin or amino acid withdrawal increased, whereas insulin strongly inhibited, the recovery of 4EBP1 and RAPTOR on 7-methyl-GTP sepharose. Partial inhibition of RAPTOR expression by RNA interference reduced MTOR-catalyzed 4EBP1 phosphorylation in vitro. RNA interference of C. elegans Raptor yielded an array of phenotypes that closely resembled those produced by inactivation of CE-Tor. Thus, the authors concluded that RAPTOR is an essential scaffold for the MTOR-catalyzed phosphorylation of 4EBP1 and mediates TOR action in vivo.

Vellai et al. (2003) demonstrated that TOR deficiency in C. elegans more than doubles its natural life span. The absence of Let363/TOR activity caused developmental arrest at the L3 larval stage. At 25.5 degrees C, the mean life span of Let363 mutants was 25 days compared with a life span of 10 days in wildtype worms.

By immunoprecipitation analysis, Kim et al. (2003) identified GBL (612190) as an additional subunit of the MTOR signaling complex in human embryonic kidney cells. GBL bound the kinase domain of MTOR and stabilized the interaction of raptor with MTOR. Loss-of-function experiments using small interfering RNA showed that, like MTOR and raptor, GBL participated in nutrient- and growth factor-mediated signaling to S6K1 and in control of cell size. Binding of GBL to MTOR strongly stimulated MTOR kinase activity toward S6K1 and 4EBP1, and this effect was reversed by stable interaction of raptor with MTOR. Nutrients and rapamycin regulated the association of MTOR with raptor only in complexes that also contained GBL. Kim et al. (2003) proposed that GBL and raptor function together to modulate MTOR kinase activity.

Huntington disease (HD; 143100) is an inherited neurodegenerative disorder caused by a polyglutamine tract expansion in which expanded polyglutamine proteins accumulate abnormally in intracellular aggregates. Ravikumar et al. (2004) showed that mammalian target of rapamycin (mTOR) is sequestered in polyglutamine aggregates in cell models, transgenic mice, and human brains. Sequestration of mTOR impairs its kinase activity and induces autophagy, a key clearance pathway for mutant huntingtin (613004) fragments. This protects against polyglutamine toxicity, as the specific mTOR inhibitor rapamycin attenuates huntingtin accumulation and cell death in cell models of HD, and inhibition of autophagy has converse effects. Furthermore, rapamycin protects against neurodegeneration in a fly model of HD, and the rapamycin analog CCI-779 improved performance on 4 different behavioral tasks and decreased aggregate formation in a mouse model of HD. The data provided proof of principle for the potential of inducing autophagy to treat HD.

Scott et al. (2004) found that signaling through Tor and its upstream regulators, Pi3k and Rheb (601293), was necessary and sufficient to suppress starvation-induced autophagy in the Drosophila fat body. In contrast, a downstream Tor effector, S6k, promoted rather than suppressed autophagy, suggesting S6K downregulation may limit autophagy during extended starvation.

Hay and Sonenberg (2004) reviewed the roles of MTOR in protein synthesis, cell growth and proliferation, synaptic plasticity, and cancer.

Brugarolas et al. (2004) showed that downregulation of Mtor by hypoxia in mice required de novo transcription and expression of Redd1 (607729) and an intact Tsc1 (605284)/Tsc2 (191092) complex.

Beuvink et al. (2005) showed that the drug RAD001 (everolimus), a rapamycin derivative, dramatically enhanced cisplatin-induced apoptosis in wildtype p53 but not mutant p53 tumor cells. The use of isogenic tumor cell lines expressing either wildtype MTOR cDNA or an MTOR mutant unable to bind RAD001 demonstrated that the effects of RAD001 resulted from inhibition of MTOR function. Beuvink et al. (2005) showed that RAD001 sensitized cells to cisplatin by inhibiting p53-induced p21 (116899) expression. This effect was attributed to a small but significant inhibition of p21 translation, combined with the short half-life of p21.

Kwon et al. (2003) found that inhibition of Mtor decreased the seizure frequency and death rate in mice with conditional Pten (601728) deficiency, prevented the increase in Pten-deficient neuronal soma size in young mice, and reversed neuronal soma enlargement in adult mice. Mtor inhibition did not decrease the size of wildtype adult neurons. Kwon et al. (2003) concluded that MTOR is required for neuronal hypertrophy downstream of PTEN deficiency, but it is not required for maintenance of normal neuronal soma size. They proposed that MTOR inhibitors may be useful therapeutic agents for the treatment of brain diseases resulting from PTEN deficiency, such as Lhermitte-Duclos disease (see 158350) or glioblastoma multiforme (137800).

Akt/PKB (164730) activation requires the phosphorylation of ser473. Sarbassov et al. (2005) showed that in Drosophila and in human cells TOR and its associated protein rictor are necessary for ser473 phosphorylation, and that a reduction in rictor or mTOR expression inhibited an AKT/PKB effector. The rictor-mTOR complex directly phosphorylated Akt/PKB on ser473 in vitro and facilitated thr308 phosphorylation by PDK1 (605213).

Holz et al. (2005) showed that MTOR and S6K1 maneuvered on and off the EIF3 (see 602039) translation initiation complex in HEK293 cells in a signal-dependent, choreographed fashion. When inactive, S6K1 associated with the EIF3 complex, while the S6K1 activator MTOR, in association with RAPTOR, did not. Hormone- or mitogen-mediated cell stimulation promoted MTOR/RAPTOR binding to the EIF3 complex and phosphorylation of S6K1. Phosphorylation resulted in S6K1 dissociation and activation, followed by phosphorylation of S6K1 targets, including EIF4B (603928), which, upon phosphorylation, was recruited into the EIF3 complex. Holz et al. (2005) concluded that the EIF3 preinitiation complex acts as a scaffold to coordinate responses to stimuli that promote efficient protein synthesis.

Cota et al. (2006) demonstrated that mTOR signaling plays a role in the brain mechanisms that respond to nutrient availability, regulating energy balance. In the rat, mTOR signaling is controlled by energy status in specific regions of the hypothalamus and colocalizes with neuropeptide Y (162640) and proopiomelanocortin (POMC; 176830) neurons in the arcuate nucleus. Central administration of leucine increases hypothalamic mTOR signaling and decreases food intake and body weight. The hormone leptin (164160) increases hypothalamic mTOR activity, and the inhibition of mTOR signaling blunts leptin's anorectic effect. Thus, Cota et al. (2006) concluded that mTOR is a cellular fuel sensor whose hypothalamic activity is directly tied to the regulation of energy intake.

Laviano et al. (2006) questioned the clinical validity of the experiments performed by Cota et al. (2006) given that in human conditions such as hepatic encephalopathy and cancer, and in malnourished uremic patients undergoing hemodialysis, supplementation with 7 grams per day of leucine, which comprises 50% of a branched-chain amino acid mix, improves appetite and muscle protein synthesis. Cota et al. (2006) responded that their experiments were done in healthy rats of normal weight to investigate the physiologic role of hypothalamic mTOR in the regulation of food intake.

Bernardi et al. (2006) identified PML (102578) as a critical inhibitor of neoangiogenesis (the formation of new blood vessels) in vivo, in both ischemic and neoplastic conditions, through the control of protein translation. Bernardi et al. (2006) demonstrated that in hypoxic conditions PML acts as a negative regulator of the synthesis rate of hypoxia-inducible factor 1-alpha (HIF1A; 603348) by repressing mTOR. PML physically interacts with mTOR and negatively regulates its association with the small GTPase RHEB (601293) by favoring mTOR nuclear accumulation. Notably, PML-null cells and tumors displayed higher sensitivity both in vitro and in vivo to growth inhibition by rapamycin, and lack of PML inversely correlated with phosphorylation of ribosomal protein S6 (180460) and tumor angiogenesis in mouse and human tumors. Thus, Bernardi et al. (2006) concluded that their findings identified PML as a novel suppressor of mTOR and neoangiogenesis.

Li et al. (2006) demonstrated that Tor1 is dynamically distributed in the cytoplasm and nucleus in yeast. Tor1 nuclear localization is nutrient-dependent and rapamycin-sensitive: starvation or treatment with rapamycin causes Tor1 to exit from the nucleus. Tor1 nuclear localization is critical for 35S rRNA synthesis, but not for the expression of amino acid transporters and ribosomal protein genes. Li et al. (2006) further showed that Tor1 is associated with 35S ribosomal DNA (rDNA) promoter chromatin in a rapamycin- and starvation-sensitive manner; this association is necessary for 35S rRNA synthesis and cell growth. Li et al. (2006) concluded that the spatial regulation of Tor1 complex 1 (TORC1; see later) might be involved in differential control of its target genes.

Raab-Graham et al. (2006) found that the mTOR inhibitor rapamycin increased the Kv1.1 (KCNA1; 176260) voltage-gated potassium channel protein in hippocampal neurons and promoted Kv1.1 surface expression on dendrites without altering its axonal expression. Moreover, endogenous Kv1.1 mRNA was detected in dendrites. Using Kv1.1 fused to the photoconvertible fluorescence protein Kaede as a reporter for local synthesis, Raab-Graham et al. (2006) observed Kv1.1 synthesis in dendrites upon inhibition of mTOR or the N-methyl-D-aspartate (NMDA) glutamate receptor (see 138251). Thus, Raab-Graham et al. (2006) concluded that synaptic excitation may cause local suppression of dendritic Kv1 channels by reducing their local synthesis.

Hoyer-Hansen et al. (2007) showed that Ca(2+)-induced autophagy in mammalian cells utilized a signaling pathway that included CAMKK2, AMPK (PRKAA2; 600497), and mTOR. Ca(2+)-induced autophagy was inhibited by BCL2 (151430) but only when BCL2 was localized to the endoplasmic reticulum.

The activity of mTOR is regulated by RHEB, a Ras-like small GTPase, in response to growth factor stimulation and nutrient availability. Bai et al. (2007) showed that RHEB regulates mTOR through FKBP38 (604840), a member of the FK506-binding protein (FKBP) family that is structurally related to FKBP12 (186945). FKBP38 binds to mTOR and inhibits its activity in a manner similar to that of the FKBP12-rapamycin complex. RHEB interacts directly with FKBP38 and prevents its association with mTOR in a GTP-dependent manner. Bai et al. (2007) concluded that their findings suggested that FKBP38 is an endogenous inhibitor of mTOR, whose inhibitory activity is antagonized by RHEB in response to growth factor stimulation and nutrient availability.

Cunningham et al. (2007) showed that mTOR is necessary for the maintenance of mitochondrial oxidative function. In skeletal muscle tissues and cells, the mTOR inhibitor rapamycin decreased the gene expression of the mitochondrial transcriptional regulators PGC1-alpha (604517), estrogen-related receptor alpha (ESRRA; 601998), and nuclear respiratory factors, resulting in a decrease in mitochondrial gene expression and oxygen consumption. Using computational genomics, Cunningham et al. (2007) identified the transcription factor yin-yang 1 (YY1; 600013) as a common target of mTOR and PGC1-alpha. Knockdown of YY1 caused a significant decrease in mitochondrial gene expression and in respiration, and YY1 was required for rapamycin-dependent repression of those genes. Moreover, inhibition of mTOR resulted in a failure of YY1 to interact with and be coactivated by PGC1-alpha. Cunningham et al. (2007) concluded that they identified a mechanism by which a nutrient sensor (mTOR) balances energy metabolism by means of the transcriptional control of mitochondrial oxidative function.

Mao et al. (2008) demonstrated that mTOR is targeted for ubiquitination and consequent degradation by binding to the tumor suppressor protein FBXW7 (606278). Human breast cancer cell lines and primary tumors showed a reciprocal relation between loss of FBXW7 and deletion or mutation of PTEN (601728), which also activates mTOR. Tumor cell lines harboring deletions or mutations in FBXW7 are particularly sensitive to rapamycin treatment, suggesting to Mao et al. (2008) that loss of FBXW7 may be a biomarker for human cancers susceptible to treatment with inhibitors of the mTOR pathway.

To test for the role of intrinsic impediments to axon regrowth, Park et al. (2008) analyzed cell growth control genes using a virus-assisted in vivo conditional knockout approach. Deletion of PTEN, a negative regulator of the mTOR pathway, in adult retinal ganglion cells promoted robust axon regeneration after optic nerve injury. In wildtype adult mice, the mTOR activity was suppressed and new protein synthesis was impaired in axotomized retinal ganglion cells, which may have contributed to the regeneration failure. Reactivating this pathway by conditional knockout of the TSC1 gene (605284), another negative regulator of the mTOR pathway, also led to axon regeneration.

Genomewide copy number analyses of human cancers identified a frequent 5p13 amplification in several solid tumor types, including lung (56%), ovarian (38%), breast (32%), prostate (37%), and melanoma (32%). Using integrative analysis of a genomic profile of the region, Scott et al. (2009) identified a Golgi protein, GOLPH3 (612207), as a candidate targeted for amplification. Gain- and loss-of-function studies in vitro and in vivo validated GOLPH3 as a potent oncogene. Physically, GOLPH3 localizes to the trans-Golgi network and interacts with components of the retromer complex, which in yeast has been linked to TOR signaling. Mechanistically, GOLPH3 regulates cell size, enhances growth factor-induced mTOR signaling in human cancer cells, and alters the response of an mTOR inhibitor in vivo. Thus, Scott et al. (2009) concluded that genomic and genetic, biologic, functional, and biochemical data in yeast and humans established GOLPH3 as a novel oncogene that is commonly targeted for amplification in human cancer, and is capable of modulating the response to rapamycin, a cancer drug in clinical use.

Mutations in the TSC1 (605284) and TSC2 (191092) genes cause tuberous sclerosis (191100 and 613254, respectively); the protein products of these genes form a complex in the TOR pathway that integrates environmental signals to regulate cell growth, proliferation, and survival. DiBella et al. (2009) showed that morpholino knockdown of zebrafish Tsc1a led to a ciliary phenotype including kidney cyst formation and left-right asymmetry defects. Tsc1a localized to the Golgi, but morpholinos against it, nonetheless, acted synthetically with ciliary genes in producing kidney cysts. Consistent with a role of the cilium in the same pathway as Tsc genes, the TOR pathway was found to be aberrantly activated in ciliary mutants, resembling the effect of Tsc1a knockdown, and kidney cyst formation in ciliary mutants was blocked by rapamycin. DiBella et al. (2009) suggested a signaling network between the cilium and the TOR pathway wherein ciliary signals can feed into the TOR pathway and where Tsc1a may regulate the length of the cilium itself.

Araki et al. (2009) demonstrated that mTOR is a major regulator of memory CD8 T-cell differentiation and that the immunosuppressive drug rapamycin has immunostimulatory effects on the generation of memory CD8 T cells. Treatment of mice with rapamycin following acute lymphocytic choriomeningitis virus infection enhanced not only the quantity but also the quality of virus-specific CD8 T cells. Similar effects were seen after immunization of mice with a vaccine based on nonreplicating virus-like particles. In addition, rapamycin treatment also enhanced memory T-cell responses in nonhuman primates following vaccination with modified vaccinia virus Ankara. Rapamycin was effective during both the expansion and contraction phases of the T cell response; during the expansion phase it increased the number of memory precursors, and during the contraction phase (effector to memory transition) it accelerated the memory T cell differentiation program. Experiments using RNA interference to inhibit expression of mTOR, raptor (607130) or FKBP12 (186945) in antigen-specific CD8 T cells showed that mTOR acts intrinsically through the mTORC1 (mTOR complex 1; see later) pathway to regulate memory T-cell differentiation. Araki et al. (2009) concluded that their studies identified a molecular pathway to regulate memory T-cell differentiation and provided a strategy for improving the functional qualities of vaccine- or infection-induced memory T cells.

Sestrins (see 606103) are conserved proteins that accumulate in cells exposed to stress, potentiate adenosine monophosphate-activated protein kinase (AMPK; 602739), and inhibit activation of TOR (mTOR). Lee et al. (2010) showed that the abundance of Drosophila sestrin is increased upon chronic TOR activation through accumulation of reactive oxygen species that cause activation of c-Jun N-terminal kinase (see 601158) and transcription factor Forkhead box O (Foxo; see 136533). Loss of Drosophila Sesn resulted in age-associated pathologies including triglyceride accumulation, mitochondrial dysfunction, muscle degeneration, and cardiac malfunction, which were prevented by pharmacologic activation of AMPK or inhibition of TOR. Hence, Lee et al. (2010) concluded that Drosophila Sesn appears to be a negative feedback regulator of TOR that integrates metabolic and stress inputs and prevents pathologies caused by chronic TOR activation that may result from diminished autophagic clearance of damaged mitochondria, protein aggregates, or lipids.

Using naive CD8 T (OT-I) cells from Rag2 (179616) -/- mice, Rao et al. (2010) showed that IL12 (161560) enhanced and sustained antigen and B7.1 (CD80; 112203) costimulatory molecule-induced mTor kinase activity via Pi3k and Stat4 (600558) pathways. Blocking mTor activity with rapamycin reversed IL12-induced effector functions through loss of Tbet (TBX21; 604895) expression. Rapamycin treatment of IL12-conditioned OT-I cells also induced Eomes (604615) expression and memory T cell precursors with greater antitumor efficacy. Rao et al. (2010) concluded that mTOR is the central regulator of transcriptional programs determining effector and/or memory cell fates of CD8+ T cells.

Yu et al. (2010) showed that mTOR signaling in rat kidney cells is inhibited during initiation of autophagy, but reactivated by prolonged starvation. Reactivation of mTOR is autophagy-dependent and requires the degradation of autolysosomal products. Increased mTOR activity attenuates autophagy and generates protolysosomal tubules and vesicles that extrude from autolysosomes and ultimately mature into functional lysosomes, thereby restoring the full complement of lysosomes in the cell--a process Yu et al. (2010) identified in multiple animal species. Thus, Yu et al. (2010) concluded that an evolutionarily conserved cycle in autophagy governs nutrient sensing and lysosome homeostasis during starvation.

Ketamine results in a rapid antidepressant response after administration in treatment-resistant depressed patients. Li et al. (2010) observed that ketamine rapidly activated the mTOR pathway, leading to increased synaptic signaling proteins and increased number and function of new spine synapses in the prefrontal cortex of rats. Moreover, blockade of mTOR signaling completely blocked ketamine induction of synaptogenesis and behavioral responses in models of depression. Li et al. (2010) concluded that these effects of ketamine are opposite to the synaptic deficits that result from exposure to stress and could contribute to the fast antidepressant actions of ketamine. Furthermore, Li et al. (2010) demonstrated that another compound, which selectively acts on NR2B (138252), had similar effects to ketamine, suggesting that this effect is mediated through NMDA receptors.

Sathaliyawala et al. (2010) found that the Mtor inhibitor rapamycin impaired mouse Flt3l (FLT3LG; 600007)-driven dendritic cell (DC) development in vitro, with plasmacytoid DCs and classical DCs most profoundly affected. Depletion of the Pi3k-Mtor negative regulator Pten facilitated Flt3l-driven DC development in culture. Targeting Pten in DCs in vivo caused expansion of Cd8-positive and Cd103 (ITGAE; 604682)-positive classical DCs, which could be reversed by rapamycin. Increased Cd8-positive classical DC numbers caused by Pten deletion correlated with increased susceptibility to Listeria infection. Sathaliyawala et al. (2010) concluded that PI3K-MTOR signaling downstream of FLT3L controls DC development, and that restriction by PTEN ensures optimal DC numbers and subset composition.

Protein synthesis and autophagic degradation are regulated in an opposite manner by mTOR, whereas under certain conditions it would be beneficial if they occurred in unison to handle rapid protein turnover. Narita et al. (2011) observed a distinct cellular compartment at the trans side of the Golgi apparatus, the TOR-autophagy spatial coupling compartment (TASCC), where (auto)lysosomes and mTOR accumulated during Ras-induced senescence. mTOR recruitment to the TASCC was amino acid- and Rag guanosine triphosphatase (e.g., 612194)-dependent, and disruption of mTOR localization to the TASCC suppressed interleukin-6/8 (147620/146930) synthesis. TASCC formation was observed during macrophage differentiation and in glomerular podocytes; both displayed increased protein secretion. Narita et al. (2011) concluded that the spatial coupling of cells' catabolic and anabolic machinery could augment their respective functions and facilitate the mass synthesis of secretory proteins.

Using ribosome profiling, Hsieh et al. (2012) uncovered specialized translation of the prostate cancer genome by oncogenic mTOR signaling, revealing a remarkably specific repertoire of genes involved in cell proliferation, metabolism, and invasion. Hsieh et al. (2012) extended these findings by functionally characterizing a class of translationally controlled proinvasion mRNAs that direct prostate cancer invasion and metastasis downstream of oncogenic mTOR signaling. Hsieh et al. (2012) developed a clinically relevant ATP site inhibitor of mTOR, called INK128, which reprograms this gene expression signature with therapeutic benefit for prostate cancer metastasis.

Terenzio et al. (2018) found that mTOR was activated and then upregulated in injured axons, owing to local translation of mTOR mRNA. This mRNA was transported into axons by the cell size-regulating RNA-binding protein nucleolin (NCL; 164035). Furthermore, mTOR controlled local translation in injured axons. This included regulation of its own translation and that of retrograde injury-signaling molecules such as importin beta-1 (602738) and STAT3 (102582). Deletion of the mTOR 3-prime UTR in mice reduced mTOR in axons and decreased local translation after nerve injury. Both pharmacologic inhibition of mTOR in axons and deletion of the mTOR 3-prime UTR decreased proprioceptive neuronal survival after nerve injury. Terenzio et al. (2018) concluded that mRNA localization enables spatiotemporal control of mTOR pathways regulating local translation and long-range intracellular signaling.

Cipponi et al. (2020) described stress-induced mutagenesis in multiple in vitro and in vivo models of human cancers under nongenotoxic drug selection, paradoxically enhancing adaptation at a competing intrinsic fitness cost. A genomewide approach identified MTOR as a stress-sensing rheostat mediating stress-induced mutagenesis across multiple cancer types and conditions. These observations were consistent with a 2-phase model for drug resistance, in which an initially rapid expansion of genetic diversity is counterbalanced by an intrinsic fitness penalty, subsequently normalizing to complete adaptation under the new conditions. Cipponi et al. (2020) concluded that their model suggested synthetic lethal strategies to minimize resistance to anticancer therapy.

MTOR Complexes 1 and 2

Jacinto et al. (2004) identified 2 distinct mammalian TOR complexes: TORC1, which contains TOR, LST8 (612190), and RAPTOR (607130), and TORC2, which contains TOR, LST8, and RICTOR (609022), which they called AVO3. Like yeast TORC2, mammalian TORC2 was rapamycin-insensitive and functioned upstream of Rho GTPases to regulate the actin cytoskeleton. TORC2 did not regulate S6K (see 608938) activity. Knockdown of TORC2, but not TORC1, prevented paxillin (602505) phosphorylation, actin polymerization, and cell spreading.

Sarbassov et al. (2004) identified a RICTOR (609022)-containing MTOR complex that contains GBL (LST8) but not RAPTOR. The RICTOR-MTOR complex did not regulate the MTOR effector S6K1 and was not bound by FKBP12 (186945)-rapamycin. Rapamycin treatment of human embryonic kidney cells eliminated the binding of MTOR to RAPTOR, but did not affect the interaction of MTOR with RICTOR. Knockdown of RICTOR caused accumulation of thick actin fibers throughout much of the cytoplasm in HeLa cells, loss of actin at the cell cortex, altered distribution of cytoskeletal proteins, and reduced protein kinase C (PKC)-alpha (see 176960) activity. Sarbassov et al. (2004) concluded that the RICTOR-MTOR complex modulates the phosphorylation of PKC-alpha and the actin cytoskeleton, similar to TOR signaling in yeast.

The multiprotein mTORC1 protein kinase complex is the central component of a pathway that promotes growth in response to insulin, energy levels, and amino acids and is deregulated in common cancers. Sancak et al. (2008) found that the Rag proteins, a family of 4 related small guanosine triphosphatases (GTPases) (RAGA, 612194; RAGB, 300725; RAGC, 608267; and RAGD, 608268), interact with mTORC1 in an amino acid-sensitive manner and are necessary for the activation of the mTORC1 pathway by amino acids. A Rag mutant that was constitutively bound to guanosine triphosphate interacted strongly with mTORC1, and its expression within cells made the mTORC1 pathway resistant to amino acid deprivation. Conversely, expression of a guanosine diphosphate-bound Rag mutant prevented stimulation of mTORC1 by amino acids. Sancak et al. (2008) concluded that the Rag proteins do not directly stimulate the kinase activity of mTORC1, but, like amino acids, promote the intracellular localization of mTOR to a compartment that also contains its activator RHEB (601293).

Dowling et al. (2010) inhibited the mTORC1 pathway in cells lacking the eukaryotic translation initiation factor 4E binding proteins EIF4EBP1 (602223), EIF4EBP2 (602224), and EIF4EBP3 (603483) and analyzed the effects on cell size, cell proliferation, and cell cycle progression. Although the EIF4EBPs had no effect on cell size, they inhibited cell proliferation by selectively inhibiting the translation of mRNAs that encode proliferation-promoting proteins and proteins involved in cell cycle progression. Thus, Dowling et al. (2010) concluded that control of cell size and cell cycle progression appear to be independent in mammalian cells, whereas in lower eukaryotes, EIF4E binding proteins influence both cell growth and proliferation.

Rosner et al. (2009) reported that the mTORC1-mediated consequences on cell cycle and cell size were separable and did not involve effects on mTORC2 activity. However, mTORC2 itself was a potent regulator of mammalian cell size and cell cycle via a mechanism involving the Akt (see 164730)/TSC2 (191092)/Rheb (601293) cascade.

Heublein et al. (2010) stated that path, a Drosophila amino acid transporter, functions in nutrient-dependent growth via MTORC1. They showed that the human orthologs of path, PAT1 (SLC36A1; 606561) and PAT4 (SLC36A4; 613760), had similar growth regulatory functions when expressed in flies. Knockdown of PAT1 or PAT4 in human MCF-7 breast cancer cells or HEK293 cells via small interfering RNA inhibited cell proliferation without affecting cell survival, similar to the effect of MTOR knockdown. Knockdown of PAT1, PAT4, or MTOR reduced phosphorylation of the MTORC1 targets S6K1, S6, and 4EBP1, but had a much smaller effect on signaling through PI3K and AKT and had no effect on MTORC2. Knockdown of PAT1, PAT4, or MTOR in serum- and nutrient-starved cells reduced amino acid-dependent MTORC1 signaling following refeeding. Conversely, overexpression of PAT1 in starved cells enhanced the sensitivity of the MTORC1 response to amino acids during refeeding. Heublein et al. (2010) hypothesized that PAT1 and PAT4 participate in amino acid sensing and contribute to the MTORC1 response to amino acids.

Sengupta et al. (2010) showed that mTORC1 controls ketogenesis in mice in response to fasting. The authors found that liver-specific loss of TSC1 (605284), an mTORC1 inhibitor, led to a fasting-resistant increase in liver size, and to a pronounced defect in ketone body production and ketogenic gene expression on fasting. The loss of raptor (607130), an essential mTORC1 component, had the opposite effect. In addition, Sengupta et al. (2010) found that the inhibition of mTORC1 is required for the fasting-induced activation of PPAR-alpha (170998) and that suppression of NCoR1 (600849), a corepressor of PPAR-alpha, reactivates ketogenesis in cells and livers with hyperactive mTORC1 signaling. Like livers with activated mTORC1, livers from aged mice have a defect in ketogenesis, which correlates with an increase in mTORC1 signaling. Moreover, Sengupta et al. (2010) showed that suppressive effects of mTORC1 activation and aging on PPAR-alpha activity and ketone production are not additive, and that mTORC1 inhibition is sufficient to prevent the aging-induced defect in ketogenesis. Thus, Sengupta et al. (2010) concluded that their findings revealed that mTORC1 is a key regulator of PPAR-alpha function and hepatic ketogenesis and suggested a role for mTORC1 activity in promoting the aging of the liver.

Hsu et al. (2011) defined the mTOR-regulated phosphoproteome by quantitative mass spectrometry and characterized the primary sequence motif specificity of mTOR using positional scanning peptide libraries. Hsu et al. (2011) found that the phosphorylation response to insulin is largely mTOR-dependent and that mTOR exhibits a unique preference for proline, hydrophobic, and aromatic residues at the +1 position. The adaptor protein growth factor receptor-bound protein-10 (GRB10; 601523) was identified as an mTORC1 substrate that mediates the inhibition of phosphoinositide 3-kinase (PI3K; see 171834) typical of cells lacking tuberous sclerosis complex-2 (TSC2; 191092), a tumor suppressor and negative regulator of mTORC1.

Yu et al. (2011) used large-scale quantitative phosphoproteomics experiments to define the signaling networks downstream of mTORC1 and mTORC2. Characterization of an mTORC1 substrate, Grb10, showed that mTORC1-mediated phosphorylation stabilized Grb10, leading to feedback inhibition of the PI3K and extracellular signal-regulated/mitogen-activated protein kinase (ERK/MAPK; see 176872) pathways. Grb10 expression is frequently downregulated in various cancers, and loss of Grb10 and loss of the well-established tumor suppressor phosphatase PTEN (601728) appear to be mutually exclusive events, suggesting that Grb10 might be a tumor suppressor regulated by mTORC1.

Amino acids activate the Rag GTPases, which promote the translocation of mTORC1 to the lysosomal surface, the site of mTORC1 activation. Zoncu et al. (2011) found that the vacuolar hydrogen proton-adenosine triphosphatase ATPase (v-ATPase; see 607027) is necessary for amino acids to activate mTORC1. The v-ATPase engages in extensive amino acid-sensitive interactions with the Ragulator, a scaffolding complex that anchors the Rag GTPases to the lysosome. In a cell-free system, ATP hydrolysis by the v-ATPase was necessary for amino acids to regulate the v-ATPase-Ragulator interaction and promote mTORC1 translocation. The results obtained in vitro and in human cells suggested that amino acid signaling begins within the lysosomal lumen. Zoncu et al. (2011) concluded that their results identified the v-ATPase as a component of the mTOR pathway and delineated a lysosome-associated machinery for amino acid sensing.

Yilmaz et al. (2012) found that Paneth cells, a key constituent of the mammalian intestinal stem cell (ISC) niche, augment stem cell function in response to calorie restriction. Calorie restriction acts by reducing mTORC1 signaling in Paneth cells, and the ISC-enhancing effects of calorie restriction can be mimicked by rapamycin. Calorie intake regulates mTORC1 in Paneth cells, but not ISCs, and forced activation of mTORC1 in Paneth cells during calorie restriction abolishes the ISC-augmenting effects of the niche. Finally, increased expression of bone stromal antigen-1 (BST1; 600387), an ectoenzyme that produces the paracrine factor cyclic ADP ribose, in Paneth cells mediates the effects of calorie restriction and rapamycin on ISC function. Yilmaz et al. (2012) concluded that their findings established that mTORC1 non-cell-autonomously regulates stem cell self-renewal, and highlighted a significant role of the mammalian intestinal niche in coupling stem cell function to organismal physiology.

Thoreen et al. (2012) used high-resolution transcriptome-scale ribosome profiling to monitor translation in mouse cells acutely treated with the mTOR inhibitor Torin-1, which, unlike rapamycin, fully inhibits mTORC1. Their data revealed a surprisingly simple model of the mRNA features and mechanisms that confer mTORC1-dependent translation control. The subset of mRNAs that are specifically regulated by mTORC1 consists almost entirely of transcripts with established 5-prime terminal oligopyrimidine (TOP) motifs, or, like Hsp90ab1 (140572) and Ybx1 (154030), with previously unrecognized TOP or related TOP-like motifs that were identified. Thoreen et al. (2012) found no evidence to support proposals that mTORC1 preferentially regulates mRNAs with increased 5-prime untranslated region length or complexity. mTORC1 phosphorylates a myriad of translational regulators, but how it controls TOP mRNA translation was unknown. Remarkably, loss of just the E4-BP family of translational repressors, arguably the best characterized mTORC1 substrates, is sufficient to render TOP and TOP-like mRNA translation resistant to Torin-1. The 4E-BPs inhibit translation initiation by interfering with the interaction between the cap-binding protein eIF4E (133440) and eIF4G1 (600495). Loss of this interaction diminishes the capacity of eIF4E to bind TOP and TOP-like mRNAs much more than other mRNAs, explaining why mTOR inhibition selectively suppresses their translation.

Efeyan et al. (2013) generated knock-in mice that express a constitutively active form of RagA (612194), RagA(GTP), from its endogenous promoter. RagA(GTP/GTP) homozygous mice developed normally but failed to survive postnatal day 1. When delivered by cesarean section, fasted RagA(GTP/GTP) neonates die almost twice as rapidly as wildtype littermates. Within an hour of birth wildtype neonates strongly inhibit mTORC1, which coincides with profound hypoglycemia and a decrease in plasma amino acid concentrations. In contrast, mTORC1 inhibition does not occur in RagA(GTP/GTP) neonates, despite identical reductions in blood nutrient amounts. With prolonged fasting, wildtype neonates recover their plasma glucose concentrations, but RagA(GTP/GTP) mice remain hypoglycemic until death, despite using glycogen at a faster rate. The glucose homeostasis defect correlates with the inability of fasted RagA(GTP/GTP) neonates to trigger autophagy and produce amino acids for de novo glucose production. Because profound hypoglycemia does not inhibit mTORC1 in RagA(GTP/GTP) neonates, Efeyan et al. (2013) considered the possibility that the Rag pathway signals glucose as well as amino acid sufficiency to mTORC1. Indeed, mTORC1 is resistant to glucose deprivation in RagA(GTP/GTP) fibroblasts, and glucose, like amino acids, controls its recruitment to the lysosomal surface, the site of mTORC1 activation. Thus, the Rag GTPases signal glucose and amino acid concentrations to mTORC1, and have an unexpectedly key role in neonates in autophagy induction and thus nutrient homeostasis and viability.

Robitaille et al. (2013) used quantitative phosphoproteomics to identify substrates or downstream effectors of the 2 mTOR complexes. mTOR controlled the phosphorylation of 335 proteins, including CAD (carbamoyl phosphate synthetase-2/aspartate transcarbamoylase/ dihydroorotase; 114010). The trifunctional CAD protein catalyzes the first 3 steps in de novo pyrimidine synthesis. mTORC1 indirectly phosphorylated CAD-S1859 through S6 kinase (S6K; see RPSKB1, 608938). CAD-S1859 phosphorylation promoted CAD oligomerization and thereby stimulated de novo synthesis of pyrimidines and progression through S phase of the cell cycle in mammalian cells. Ben-Sahra et al. (2013) independently showed that activation of mTORC1 led to the acute stimulation of metabolic flux through the de novo pyrimidine synthesis pathway. mTORC1 signaling posttranslationally regulated this metabolic pathway via its downstream target S6K1, which directly phosphorylates S1859 on CAD. Growth signaling through mTORC1 thus stimulates the production of new nucleotides to accommodate an increase in RNA and DNA synthesis needed for ribosome biogenesis and anabolic growth.

Zeng et al. (2013) demonstrated that mTORC1 signaling is a pivotal positive determinant of regulatory T cell (Treg) function in mice. Tregs have elevated steady-state mTORC1 activity compared to naive T cells. Signals through the T cell antigen receptor (TCR; see 186880) and interleukin-2 (IL2; 147680) provide major inputs for mTORC1 activation, which in turn programs the suppressive function of Tregs. Disruption of mTORC1 through Treg-specific deletion of the essential component raptor (607130) leads to a profound loss of Treg-suppressive activity in vivo and the development of a fatal early-onset inflammatory disorder. Mechanistically, raptor/mTORC1 signaling in Tregs promotes cholesterol and lipid metabolism, with the mevalonate pathway particularly important for coordinating Treg proliferation and upregulation of the suppressive molecules CTLA4 (123890) and ICOS (604558) to establish Treg functional competency. By contrast, mTORC1 does not directly affect the expression of Foxp3 (300292) or anti- and proinflammatory cytokines in Treg cells, suggesting a nonconventional mechanism for Treg functional regulation. Finally, Zeng et al. (2013) provided evidence that mTORC1 maintains Treg function partly through inhibiting the mTORC2 pathway. Zeng et al. (2013) concluded that their results showed that mTORC1 acts as a fundamental rheostat in Tregs to link immunologic signals from TCR and IL2 to lipogenic pathways and functional fitness, and highlighted a central role of metabolic programming of Treg suppressive activity in immune homeostasis and tolerance.

Loss of MTM1 (300415), a phosphatase that can dephosphorylate PtdIns(3)P, causes X-linked myotubular myopathy (310400) in humans and in the Mtm1 -/- mouse model. Fetalvero et al. (2013) found that mTORC1 activity was inhibited in Mtm1 -/- mouse skeletal muscle, concomitant with increased content of PtdIns(3)P, ubiquitinated proteins, and lipidated proteins normally degraded via autophagy. Mtm1 -/- muscle also showed accumulation of defective mitochondria with decreased COX enzyme activity. No change in mTORC1, mitochondria, or content of nondegraded proteins was observed in liver, heart, or brain of Mtm1 -/- mice. Overnight fasting activated mTORC1-dependent inhibition of autophagy in wildtype, but not Mtm1 -/-, skeletal muscle. Inhibition of hyperactivated mTORC1 normalized autophagy and rescued muscle mass in Mtm1 -/- mice. Fetalvero et al. (2013) concluded that MTM1 is involved in the regulation of mTORC1 and autophagy specifically in skeletal muscle.

Thedieck et al. (2013) showed that astrin (SPAG5; 615562) functioned as a negative regulator of MTORC1 following exposure of HeLa cells to cell stresses, such as arsenite, hydrogen peroxide, or excessive heat. Astrin localized to centrosomes in unstressed cells, but localized to stress granules following induction of stress granules by cell stress. Astrin competed with MTOR in binding RAPTOR and sequestered RAPTOR to stress granules, inhibiting the MTORC1 apoptotic response to stress. Knockdown of astrin via small interfering RNA resulted in MTORC1 assembly and activation in both stressed and unstressed cells. Thedieck et al. (2013) concluded that astrin-mediated inhibition of apoptosis may be beneficial in preventing healthy cells from undergoing apoptosis upon transient stresses or metabolic challenge.

Bar-Peled et al. (2013) identified the octameric GATOR (GTPase-activating protein (GAP) activity toward RAGs) complex as a critical regulator of the pathway that signals amino acid sufficiency to mTORC1. GATOR is composed of 2 subcomplexes, GATOR1 and GATOR2. Inhibition of the GATOR1 subunits DEPDC5 (614191), NPRL2 (607072), and NPRL3 (600928) makes mTORC1 signaling resistant to amino acid deprivation. In contrast, inhibition of the GATOR2 subunits MIOS (615359), WDR24, WDR59 (617418), SEH1L (609263), and SEC13 (600152) suppresses mTORC1 signaling, and epistasis analysis shows that GATOR2 negatively regulates DEPDC5. GATOR1 has GAP activity for RAGA (612194) and RAGB (300725), and its components are mutated in human cancer. In cancer cells with inactivating mutations in GATOR1, mTORC1 is hyperactive and insensitive to amino acid starvation, and such cells are hypersensitive to rapamycin, an mTORC1 inhibitor. Thus, Bar-Peled et al. (2013) concluded that they had identified a key negative regulator of the RAG GTPases and revealed that, like other mTORC1 regulators, RAG function can be deregulated in cancer.

Rodgers et al. (2014) showed that the stem cell quiescent state is composed of 2 distinct functional phases, G0 and an 'alert' phase that they termed G-Alert. Stem cells actively and reversibly transition between these phases in response to injury-induced systemic signals. Using genetic mouse models specific to muscle stem cells, Rodgers et al. (2014) showed that mTORC1 activity is necessary and sufficient for the transition of satellite cells from G0 into G-Alert and that signaling through the hepatocyte growth factor (HGF; 142409) receptor c-Met (see 164860) is also necessary. Rodgers et al. (2014) also identified G0-to-G-Alert transitions in several populations of quiescent stem cells. Quiescent stem cells that transition into G-Alert possess enhanced tissue regenerative function. Rodgers et al. (2014) proposed that the transition of quiescent stem cells into G-Alert functions as an 'alerting' mechanism, an adaptive response that positions stem cells to respond rapidly under conditions of injury and stress, thus priming them for cell cycle entry.

Zhang et al. (2014) showed that as well as increasing protein synthesis, mTORC1 activation in mouse and human cells also promotes an increased capacity for protein degradation. Cells with activated mTORC1 exhibited elevated levels of intact and active proteasomes through a global increase in the expression of genes encoding proteasome subunits. The increase in proteasome gene expression, cellular proteasome content, and rates of protein turnover downstream of mTORC1 were all dependent on induction of the transcription factor NRF1 (NFE2L1; 163260). Genetic activation of mTORC1 through loss of the tuberous sclerosis complex tumor suppressors TSC1 (605284) or TSC2 (191092), or physiologic activation of mTORC1 in response to growth factors or feeding, resulted in increased NRF1 expression in cells and tissues. Zhang et al. (2014) found that this NRF1-dependent elevation in proteasome levels serves to increase the intracellular pool of amino acids, which thereby influences rates of new protein synthesis. The authors therefore concluded that mTORC1 signaling increases the efficiency of proteasome-mediated protein degradation for both quality control and as a mechanism to supply substrate for sustained protein synthesis.

Reduced expression of SMN (SMN1; 600354) causes spinal muscular atrophy (SMA; see 253300). Kye et al. (2014) found that expression of microRNA-183 (MIR183; 611608), but not its primary transcript, was increased in Smn-knockdown rat primary neurons, concomitant with impaired axonal growth, impaired local translation of Mtor in neurites, and reduced Mtor pathway-dependent neurite protein synthesis. Mir183 was also elevated in SMA model mice and in SMA patient-derived fibroblasts. Codepletion of Mir183 and Smn in rat neurons rescued the axonal phenotype and increased Mtor expression in neurites. Kye et al. (2014) identified an Mir183-binding site in the 3-prime UTR of the Mtor transcript, and Mir183 bound directly to this site and inhibited Mtor translation. Inhibition of Mir183 in vivo partly alleviated the disease phenotype in SMA model mice. Kye et al. (2014) concluded that axonal MIR183 and the MTOR pathway contribute to SMA pathology.

Liang et al. (2014) generated a mosaic Tsc1-knockout mouse model in which mutant mice developed renal mesenchymal lesions that recapitulated perivascular epithelioid cell tumors (PEComas) found in patients with TSC. The authors found that YAP (YAP1; 606608) was upregulated by MTOR in mouse and human PEComas. Genetic or pharmacologic inhibition of Yap blunted abnormal proliferation and induced apoptosis of mouse Tsc1/Tsc2-deficient cells in culture and in mosaic Tsc1-knockout mice. Yap accumulated in cells lacking Tsc1/Tsc2 due to impaired degradation of Yap by autophagy in an Mtor-dependent manner. Liang et al. (2014) proposed that YAP is a potential therapeutic target for TSC and other disease with dysregulated MTOR activity.

Rebsamen et al. (2015) and Wang et al. (2015) independently identified SLC38A9 (616203) as an amino acid sensor for lysosomal mTORC1. Rebsamen et al. (2015) found that SLC38A9 transported radiolabeled glutamine, arginine, and asparagine, but not leucine or histidine, when reconstituted into proteoliposomes.

Using knockdown HEK293 cells and knockout mouse embryonic fibroblasts, Jewell et al. (2015) found that functional RAGA and RAGB were required for activation of lysosomal mTORC1 by leucine. In contrast, ARF1 (103180) was required for activation of lysosomal mTORC1 by glutamine.

By mass spectrometric analysis of proteins that immunoprecipitated with Ragulator subunits from HEK293T cells, Schweitzer et al. (2015) identified the lysosomal membrane protein C17ORF59 (BORCS6; 616599). Epitope-tagged C17ORF59 immunoprecipitated all Ragulator subunits, but not RAG GTPases. Schweitzer et al. (2015) found that C17ORF59 disrupted RAG-Ragulator complexes in vitro and in HEK293 cells, caused mislocalization of RAG GTPases away from lysosomes, and inhibited mTORC1 recruitment to lysosomes and activation in response to amino acid stimulation. Overexpression of C17ORF59 did not alter Ragulator localization at lysosomes. Schweitzer et al. (2015) concluded that C17ORF59 is a negative regulator of mTORC1 activation.

Ben-Sahra et al. (2016) found that mTORC1, which stimulates anabolic processes underlying cell growth, increases metabolic flux through the de novo purine synthesis pathway in various mouse and human cells, thereby influencing the nucleotide pool available for nucleic acid synthesis. mTORC1 had transcriptional effects on multiple enzymes contributing to purine synthesis, with expression of the mitochondrial tetrahydrofolate cycle enzyme MTHFD2 (604887) being closely associated with mTORC1 signaling in both normal and cancer cells. MTHFD2 expression and purine synthesis were stimulated by activating transcription factor-4 (ATF4; 604064), which was activated by mTORC1 independent of its canonical induction downstream of EIF2A (609234) phosphorylation. Thus, mTORC1 stimulates the mitochondrial tetrahydrofolate cycle, which contributes 1-carbon units to enhance production of purine nucleotides in response to growth signals.

By database and coimmunoprecipitation analyses, Chantranupong et al. (2016) found that CASTOR1 (GATSL3; 617034) and CASTOR2 (GATSL2; 617033) interacted with GATOR2 subunits. CASTOR1 and CASTOR2 robustly interacted with themselves and with each other to form homo- and heterooligomers. Arginine bound CASTOR1, but not CASTOR2, and CASTOR1-CASTOR2 dimers bound roughly half as much arginine as CASTOR1 homodimers. Arginine disrupted interaction between the GATOR2 complex and CASTOR1 homodimers or CASTOR1-CASTOR2 heterodimers, but it had no effect on interaction between GATOR2 and CASTOR2 homodimers. Consequently, CASTOR1 homodimers inhibited mTORC1 activity in an arginine-dependent manner, whereas CASTOR2 homodimers inhibited mTORC1 activity in an arginine-independent manner. Depletion of CASTOR1 via Cas9 made the mTORC1 pathway in human cell lines insensitive to deprivation of arginine, but not to deprivation of leucine or all amino acids. Overexpression of CASTOR1 reduced the sensitivity of the arginine-induced mTORC1 pathway, but it did not alter its maximal activity, whereas overexpression of CASTOR2 reduced maximal arginine-induced mTORC1 activity. Chantranupong et al. (2016) concluded that CASTOR1 associates with GATOR2 in an arginine-dependent manner to inhibit mTORC1 signaling in the absence of arginine, and that CASTOR2 constitutively associates with GATOR2 to dampen mTORC1 signaling.

Castellano et al. (2017) identified cholesterol, an essential building block for cellular growth, as a nutrient input that drives mTORC1 recruitment and activation at the lysosomal surface. The lysosomal transmembrane protein SLC38A9 (616203) is required for mTORC1 activation by cholesterol through conserved cholesterol-responsive motifs. Moreover, SLC38A9 enables mTORC1 activation by cholesterol independently from its arginine-sensing function. Conversely, the Niemann-Pick C1 protein (NPC1; 607623), which regulates cholesterol export from the lysosome, binds to SLC38A9 and inhibits mTORC1 signaling through its sterol transport function. Castellano et al. (2017) concluded that, thus, lysosomal cholesterol drives mTORC1 activation and growth signaling through the SLC38A9-NPC1 complex.

Using HEK293 cells, Gu et al. (2017) found that SAMTOR (BMT2; 617855) bound the GATOR1-KICSTOR (see 617420) supercomplex, and that SAMTOR-GATOR1-KICSTOR inhibited MTORC1 signaling at lysosomes. Binding of S-adenosylmethionine (SAM) to SAMTOR interfered with binding of SAMTOR to GATOR1-KICSTOR and permitted MTORC1 signaling. Methionine starvation reduced SAM levels, promoting association of SAMTOR with GATOR1-KICSTOR and inhibition of MTORC1 lysosomal signaling. The authors concluded that SAMTOR senses methionine availability via SAM binding and thereby links methionine availability with MTORC1 signaling.

Di Malta et al. (2017) found that MiT/TFE transcription factors (including MITF, 156845; TFEB, 600744; and TFE3, 314310) control mTORC1 lysosomal recruitment and activity by directly regulating the expression of RagD (608268). In mice, this mechanism mediated adaptation to food availability after starvation and physical exercise and played an important role in cancer growth. Upregulation of MiT/TFE genes in cells and tissues from patients and murine models of renal cell carcinoma, pancreatic ductal adenocarcinoma, and melanoma triggered RagD-mediated mTORC1 induction, resulting in cell hyperproliferation and cancer growth. Thus, Di Malta et al. (2017) concluded that this transcriptional regulatory mechanism enables cellular adaptation to nutrient availability and supports the energy-demanding metabolism of cancer cells.

▼ Biochemical Features
Crystal Structure

Yang et al. (2013) reported cocrystal structures of a complex of truncated mTOR and mammalian lethal with SEC13 protein-8 (mLST8; 612190) with an ATP transition state mimic and with ATP-site inhibitors. The structures revealed an intrinsically active kinase conformation, with catalytic residues and a catalytic mechanism remarkably similar to canonical protein kinases. The active site is highly recessed owing to the FKBP12 (186945)-rapamycin-binding (FRB) domain and an inhibitory helix protruding from the catalytic cleft. mTOR-activating mutations map to the structural framework that holds these elements in place, indicating that the kinase is controlled by restricted access. In vitro biochemistry showed that the FRB domain acts as a gatekeeper, with its rapamycin-binding site interacting with substrates to grant them access to the restricted active site. Rapamycin-FKBP12 inhibits the kinase by directly blocking substrate recruitment and by further restricting active-site access. Yang et al. (2013) concluded that the structures also revealed active-site residues and conformational changes that underlie inhibitor potency and specificity.

Cryoelectron Microscopy

Aylett et al. (2016) resolved the architecture of the human mTORC1 complex, containing mTOR with subunits Raptor (607130) and mLST8, bound to FK506-binding protein (FKBP; 186945)-rapamycin, by combining cryoelectron microscopy at 5.9-angstrom resolution with crystallographic studies of Chaetomium thermophilum Raptor at 4.3-angstrom resolution. The structure explained how FKBP-rapamycin and architectural elements of mTORC1 limit access to the recessed active site. Consistent with a role in substrate recognition and delivery, the conserved amino-terminal domain of Raptor is juxtaposed to the kinase active site.

Prouteau et al. (2017) reported that in the budding yeast, glucose withdrawal, which leads to an acute loss of TORC1 kinase activity, triggers a similarly rapid Rag GTPase-dependent redistribution of TORC1 from being semiuniform around the vacuolar membrane to a single, vacuole-associated cylindrical structure visible by super-resolution optical microscopy. Three-dimensional reconstructions of cryoelectron micrograph images of these purified cylinders demonstrated that TORC1 oligomerizes into a higher-level hollow helical assembly, which Prouteau et al. (2017) named a TOROID (TORC1 organized in inhibited domain). Fitting of the mammalian TORC1 structure into the helical map revealed that oligomerization leads to steric occlusion of the active site. Guided by the implications from their reconstruction, Prouteau et al. (2017) presented a TOR1 allele that prevented both TOROID formation and TORC1 inactivation in response to glucose withdrawal, demonstrating that oligomerization is necessary for TORC1 inactivation. Prouteau et al. (2017) concluded that their results revealed a novel mechanism by which Rag GTPases regulate TORC1 activity and suggested that the reversible assembly and/or disassembly of higher-level structures may be an underappreciated mechanism for the regulation of protein kinases.

▼ Pathogenesis
Pancreatic Neuroendocrine Tumors

Jiao et al. (2011) explored the genetic basis of pancreatic neuroendocrine tumors (PanNETs) by determining the exomic sequence of 10 nonfamilial PanNETs and then screened the most commonly mutated genes in 58 additional PanNETs. Jiao et al. (2011) found mutations in genes in the mTOR pathway in 14% of the tumors, a finding that could potentially be used to stratify patients for treatments with mTOR inhibitors. The most frequently mutated genes specify proteins implicated in chromatin remodeling: 44% of the tumors had somatic inactivating mutations in MEN1 (613733), and 43% had mutations in genes encoding either of the 2 subunits of a transcription/chromatin remodeling complex consisting of DAXX (603186) and ATRX (300032). Clinically, mutations in the MEN1 and DAXX/ATRX genes were associated with better prognosis.

▼ Mapping
Moore et al. (1996) assigned the FRAP gene to chromosome 1p36 by fluorescence in situ hybridization (FISH). Lench et al. (1997) mapped the FRAP gene to 1p36.2 by FISH following radiation-hybrid mapping to that general region. Chromosome 1p36.2 is the region most consistently deleted in neuroblastomas. Given the role of PIK-related kinase proteins in DNA repair, recombination, and cell cycle checkpoints, the authors suggested that the possible role of FRAP in solid tumors with deletions at 1p36 should be investigated. Onyango et al. (1998) established the order of genes in the 1p36 region, telomere to centromere, as CDC2L1 (176873)--PTPRZ2 (604008)--ENO1 (172430)--PGD (172200)--XBX1--FRAP2 (FRAP1)--CD30 (153243).

▼ Molecular Genetics
Smith-Kingsmore Syndrome

In a girl with megalencephaly, intractable seizures, and facial dysmorphism (Smith-Kingsmore syndrome, SKS; 616638), Smith et al. (2013) identified a de novo heterozygous missense mutation in the MTOR gene (C1483F; 601231.0001). The mutation was found by exome sequencing and confirmed by Sanger sequencing. Smith et al. (2013) noted that Lee et al. (2012) had identified a somatic missense mutation at the same MTOR residue (C1483Y) in brain cells derived from a patient (HME-1563) with hemimegalencephaly and seizures and had postulated a gain-of-function effect.

In 3 Aboriginal Australian sibs with macrocephaly, intellectual disability, facial dysmorphism, and small thoraces, Baynam et al. (2015) identified a de novo heterozygous missense mutation in the MTOR gene (E1799K; 601231.0002). Peripheral blood cells derived from 1 of the patients showed increased mTOR activity when stimulated, and the increased response was inhibited by coincubation with rapamycin. The findings were consistent with a gain-of-function effect at the cellular level. Because the 3 sibs had different fathers and the mutation was not detected in the mother's peripheral blood, Baynam et al. (2015) suggested that the mother was gonadal mosaic for the mutation.

Mroske et al. (2015) identified a de novo heterozygous E1799K mutation in 2 brothers with SKS. The mutation, which was found by exome sequencing and confirmed by Sanger sequencing, was not found in the peripheral blood of either parent, suggesting it orginated as a consequence of gonadal mosaicism. Based on molecular modeling, Mroske et al. (2015) noted that the E1799K mutation occurs in the FAT domain, which clamps onto the kinase domain and negatively regulates MTOR activity. Disruption of this residue may destabilize the protein and shift it to a more active state, which could result in increased protein synthesis during brain development or possibly trigger an inflammatory reaction within the brain.

In 8 patients, including a mother and daughter and a pair of monozygotic twins sisters, with variants of SKS, Moller et al. (2016) identified de novo heterozygous germline missense mutations in the MTOR gene (see, e.g., 601231.0009 and 601231.0010). The mutations were found by exome sequencing of a custom gene panel and confirmed by Sanger sequencing. Functional studies of the variant and studies of patient cells were not performed.

In 2 sibs, born of unrelated German parents, with SKS, Moosa et al. (2017) identified a de novo heterozygous E1799K mutation in the MTOR gene. The mutation, which was found by exome sequencing and confirmed by Sanger sequencing, was not detected in the parents' peripheral blood. Additional sequencing indicated that the mutation was located on the paternal chromosome, which Moosa et al. (2017) suggested was consistent with paternal gonadal mosaicism. Functional studies of the variant and studies of patient cells were not performed, but the authors suggested that its recurrence indicates a mutational hotspot within the gene.

Focal Cortical Dysplasia, Type II, Somatic

In brain tissue resected from 12 children with seizures due to focal cortical dysplasia type II (FCORD2; 607341), Lim et al. (2015) identified 9 different de novo somatic missense mutations in the MTOR gene (see, e.g., 601231.0003 and 601231.0004). The mutations in the first 4 patients were found by found by whole-exome sequencing and verified by several methods; subsequent mutations were found in an additional 73 patients with FCD type II who underwent sequencing of the MTOR gene. The mutations were not found in the patients' blood samples. The allelic frequencies of the mutations ranged from about 1 to 12%. Overall, MTOR mutations were found in 15.6% of 77 patients with FCD type II who were studied. Transfection of 3 of the mutations into HEK293 cells showed that they resulted in constitutively increased MTOR kinase activity compared to controls. Inhibition of mTOR with rapamycin suppressed cytomegalic neurons and seizures in mutant mice.

In brain tissue of 6 unrelated patients with FCD type II, Nakashima et al. (2015) identified 4 different de novo somatic missense mutations in the MTOR gene (see, e.g., 601231.0005 and 601231.0006). The mutations in the first 2 patients were found by whole-exome sequencing; subsequent mutations were found by direct screening of the MTOR gene in additional patients. Overall, mutations were found in 6 (46%) of 13 individuals with FDCT2b. Mutant allele frequencies in brain tissue were very low (range about 1 to 9%). Transfection of the mutations into HEK293 cells showed that all resulted in constitutive activation of mTOR, with increased phosphorylation of 4EBP, the direct target of mTOR kinase. Lesion-specific brain tissue from affected individuals showed increased phosphorylation of S6K (RPS6KB1; 608938) compared to controls, consistent with a gain of function of the mTOR pathway. Nakashima et al. (2015) concluded that somatic MTOR mutation caused hyperactivation of the mTOR-signaling pathway, which is involved in growth, migration, and maturation of neurons and glial cells. Aberrant activation of this pathway can result in the formation of dysmorphic neurons and balloon cells, particularly during brain development.

In resected brain tissue from 6 (37%) of 16 patients with FCD type II, Moller et al. (2016) identified somatic missense mutations in the MTOR gene (see, e.g., 601231.0005 and 601231.0008). The mutations were found by targeted sequencing of the MTOR gene and other genes in the MTOR pathway. The mutant allele frequency was low, less than 7% in patient brain tissue. Hyperactivation of the mTOR pathway as shown by S6 intense phosphorylation was observed in dysmorphic neurons of patients with FCD IIa and FDC IIb, in contrast to apparently normal adjacent neurons. The findings were consistent with hyperactivation of the mTOR pathway.

▼ Animal Model
Murakami et al. (2004) generated mTor-knockout mice by disrupting the kinase domain of mouse mTor and found that mTor +/- mice were normal and fertile, but that mTor -/- mice exhibited embryonic lethality shortly after implantation. Although homozygous blastocysts appeared normal, their inner cell mass and trophoblast did not proliferate in vitro. Mutation analysis showed that the 6 C-terminal amino acids of mTOR are essential for kinase activity and are necessary for normal cell size and proliferation in embryonic stem cells. Murakami et al. (2004) concluded that mTOR controls both cell size and proliferation in early mouse embryos and embryonic stem cells. Independently, Gangloff et al. (2004) observed defects in mTor -/- mice but not in mTor +/- mice confirming the findings of Murakami et al. (2004).

By generating mice with a deletion of mTor specifically in T lymphocytes, Delgoffe et al. (2009) demonstrated that mTor activation is not necessary for normal activation and IL2 (147680) secretion but is necessary for Th1 (see IFNG, 147570), Th2 (see IL4, 147780), and Th17 (see IL17, 603149) differentiation. Under fully activating conditions, mTor-null T cells differentiated into Foxp3 (300292)-positive regulatory T cells. This differentiation was associated with hyperactive Smad3 (603109) activation in the absence of exogenous Tgfb (190180). T cells lacking the Torc1 protein complex did not divert to a regulatory pathway, thus implicating both Torc1 and Torc2 in preventing the generation of regulatory T cells. Delgoffe et al. (2009) suggested that MTOR kinase signaling regulates the commitment to effector or regulatory T cell lineages.

Harrison et al. (2009) found that rapamycin feeding increased both median and maximum life span in genetically heterogeneous mice, even when started late in life. The effect was observed in both male and female mice and occurred in the absence of diet restriction or reduction in body weight. Rapamycin did not change the distribution of presumptive causes of death.

N-acylethanolamines (NAEs) are lipid-derived signaling molecules, which include the mammalian endocannabinoid arachidonoyl ethanolamide. Given its involvement in regulating nutrient intake and energy balance, the endocannabinoid system is an excellent candidate for a metabolic signal that coordinates the organismal response to dietary restriction and maintains homeostasis when nutrients are limited. Lucanic et al. (2011) identified NAEs in C. elegans and showed that NAE abundance is reduced under dietary restriction and that NAE deficiency is sufficient to extend life span through a dietary restriction mechanism requiring PHA4, a homolog of FOXA1 (602294). Conversely, dietary supplementation with the nematode NAE eicosapentaenoyl ethanolamide not only inhibited dietary restriction-induced life span extension in wildtype worms, but also suppressed life span extension in a TOR pathway mutant. Lucanic et al. (2011) concluded that their study demonstrated a role for NAE signaling in aging and indicated that NAEs represent a signal that coordinates nutrient status with metabolic changes that ultimately determine life span.

Calorie restriction, which increases life span and insulin sensitivity, is proposed to function by inhibition of mTORC1, yet paradoxically, chronic administration of rapamycin substantially impairs glucose tolerance and insulin action. Lamming et al. (2012) demonstrated that rapamycin disrupted a second mTOR complex, mTORC2, in vivo and that mTORC2 was required for the insulin-mediated suppression of hepatic gluconeogenesis. Further, decreased mTORC1 signaling was sufficient to extend life span independently from changes in glucose homeostasis, as female mice heterozygous for both mTOR and mLST8 (612190) exhibited decreased mTORC1 activity and extended life span but had normal glucose tolerance and insulin sensitivity. Lamming et al. (2012) concluded that mTORC2 disruption is an important mediator of the effects of rapamycin in vivo.

Cina et al. (2012) targeted Mtor disruption to mouse podocytes. Mutant mice developed proteinuria at 3 weeks and end-stage renal failure by 5 weeks after birth. Podocytes from mutant mice exhibited accumulation of autophagosomes, autophagolysosomal vesicles, and damaged mitochondria. Similarly, human podocytes treated with rapamycin accumulated autophagosomes and autophagolysosomes.

Lim et al. (2015) found that transfection of the L2427P MTOR mutation (601231.0003) into embryonic developing mouse cortex resulted in neuronal migration defects, cytomegalic neurons, and seizures associated with aberrantly increased mTOR kinase activity. Treatment with rapamycin rescued the cytomegalic neurons and seizure activity.

▼ History
Crino (2008) noted that rapamycin was discovered in the 1970s as a macrolide antibiotic and antifungal in a soil sample from Rapa Nui, also known as Easter Island.

▼ ALLELIC VARIANTS ( 10 Selected Examples):

.0001 SMITH-KINGSMORE SYNDROME
MTOR, CYS1483PHE
In a girl with megalencephaly, intractable seizures, and facial dysmorphism, and umbilical hernia (SKS; 616638), Smith et al. (2013) identified a de novo heterozygous c.4448G-T transversion in the MTOR gene, resulting in a cys1483-to-phe (C1483F) substitution at a highly conserved residue. The mutation was found by exome sequencing and confirmed by Sanger sequencing. The patient died of pneumonia at age 19 months.

.0002 SMITH-KINGSMORE SYNDROME
MTOR, GLU1799LYS
In 3 Aboriginal Australian sibs, aged 2, 3, and 7 years, with intellectual disability, macrocephaly, seizures, facial dysmorphism, and small thoraces (SKS; 616638), Baynam et al. (2015) identified a de novo c.5395G-A transition in exon 39 of the MTOR gene, resulting in a glu1799-to-lys (E1799K) substitution at a conserved residue. Peripheral blood cells derived from 1 of the patients showed increased mTOR activity when stimulated, and the increased response was inhibited by coincubation with rapamycin. The findings were consistent with a gain-of-function effect at the cellular level. Because the 3 sibs had different fathers and the mutation was not detected in the mother's peripheral blood, Baynam et al. (2015) suggested that the mother was gonadal mosaic for the mutation.

Mroske et al. (2015) identified a de novo heterozygous E1799K mutation in 2 brothers with SKS. The mutation, which was found by exome sequencing and confirmed by Sanger sequencing, was not found in the peripheral blood of either parent, suggesting it orginated as a consequence of gonadal mosaicism. The mutation was not present in the dbSNP, 1000 Genomes Project, or ExAC databases. Based on molecular modeling, Mroske et al. (2015) noted that the E1799K mutation occurs in the FAT domain, which clamps onto the kinase domain and negatively regulates MTOR activity. Disruption of this residue may detabilize the protein and shift it to a more active state.

Mirzaa et al. (2016) reported 4 children, including identical twins, with a mosaic MTOR mutation E1799K present at an alternative allele ratio of about 50% in saliva or white blood cells; no neurologic tissue was available for quantitation. All 4 children had symmetric megalencephaly and intellectual disability, but limited or no polymicrogyria. The twins had autism.

In 2 sibs, born of unrelated German parents, with SKS, Moosa et al. (2017) identified a de novo heterozygous E1799K mutation in the MTOR gene. The mutation, which was found by exome sequencing and confirmed by Sanger sequencing, was not detected in the parents' peripheral blood. Additional sequencing indicated that the mutation was located on the paternal chromosome, which Moosa et al. (2017) suggested was consistent with paternal gonadal mosaicism. Functional studies of the variant and studies of patient cells were not performed.

.0003 FOCAL CORTICAL DYSPLASIA, TYPE II, SOMATIC
MTOR, LEU2427PRO
In brain tissue resected from 2 unrelated children with seizures due to focal cortical dysplasia type II (FCORD2; 607341), Lim et al. (2015) identified a de novo somatic c.7280T-C transition in the MTOR gene, resulting in a leu2427-to-pro (L2427P) substitution at a highly conserved residue in the kinase domain. The mutation, which was found by whole-exome sequencing and verified by several methods, was not found in the patients' blood samples or in the 1000 Genomes Project database (May 2, 2013). Transfection of the mutation into HEK293 cells showed that it resulted in constitutively increased kinase activity compared to controls. The allelic frequencies in the affected brains ranged from about 7 to 12%, and cytomegalic neurons from the brains of these patients showed enrichment of the mutant allele. In addition, the abnormal neurons had increased S6K phosphorylation (RPS6KB1; 608938) compared to controls, consistent with a gain of function of the mTOR pathway. Transfection of the L2427P mutation into embryonic developing mouse cortex resulted in neuronal migration defects, cytomegalic neurons, and seizures associated with aberrantly increased mTOR kinase activity. Treatment with rapamycin rescued the cytomegalic neurons and seizure activity.

.0004 FOCAL CORTICAL DYSPLASIA, TYPE II, SOMATIC
MTOR, LEU2427GLN
In brain tissue resected from a 10-year-old girl with seizures due to focal cortical dysplasia type II (FCORD2; 607341), Lim et al. (2015) identified a de novo somatic c.7280T-A transversion in the MTOR gene, resulting in a leu2427-to-gln (L2427Q) substitution at a highly conserved residue in the kinase domain. The mutation, which was found by whole-exome sequencing and verified by several methods, was not found in the patient's blood samples or in the 1000 Genomes Project database (May 2, 2013). Transfection of the mutation into HEK293 cells showed that it resulted in constitutively increased kinase activity compared to controls. The allelic frequencies in the affected brain ranged from about 3 to 5%.

.0005 FOCAL CORTICAL DYSPLASIA, TYPE II, SOMATIC
MTOR, SER2215TYR
In 2 unrelated patients with seizures due to focal cortical dysplasia type II (FCORD2; 607341), Nakashima et al. (2015) identified a de novo somatic c.6644C-A transversion in the MTOR gene, resulting in a ser2215-to-tyr (S2215Y) substitution at a highly conserved residue in the kinase domain. Transfection of the mutation into HEK293 cells showed that it caused constitutive activation of the mTOR pathway.

Moller et al. (2016) identified a somatic S2215Y mutation in the MTOR gene in brain tissue resected from 2 unrelated French boys with FCD type II. The mutations, which were found by deep sequencing of a targeted gene panel, were not found in patient blood. The S2215Y variant was not found in the ExAC database. The mutant allele frequency was low: about 1% in 1 patient and 3.6% in the other. Functional studies of the variant were not performed, although patient dysmorphic neurons showed intense S6 phosphorylation, consistent with hyperactivation of the mTOR pathway.

Mirzaa et al. (2016) reported an additional patient with focal cortical dysplasia and seizures in whom the S2215Y mutation was present at an alternative allele fraction of 0.035 in brain tissue but absent in saliva.

.0006 FOCAL CORTICAL DYSPLASIA, TYPE II, SOMATIC
MTOR, LEU1460PRO
In resected brain tissue from 2 unrelated patients with seizures due to focal cortical dysplasia type II (FCORD2; 607341), Nakashima et al. (2015) identified a de novo somatic c.4379T-C transition in the MTOR gene, resulting in a leu1460-to-pro (L1460P) substitution at a highly conserved residue in the FAT domain. Transfection of the mutation into HEK293 cells showed that it caused constitutive activation of the mTOR pathway.

.0007 FOCAL CORTICAL DYSPLASIA, TYPE II, SOMATIC
MTOR, TRP1456GLY
In resected brain tissue from an infant with seizures due to focal cortical dysplasia type II (FCORD2; 607341), Leventer et al. (2015) identified a de novo somatic c.4487T-G transversion in the MTOR gene, resulting in a trp1456-to-gly (W1456G) substitution at a highly conserved residue in the FAT domain. The mutation was found by whole-exome sequencing and confirmed by Sanger sequencing. The frequency of the mutant allele in patient brain samples was 8.3%; the mutation was not found in lymphocyte-derived DNA. Functional studies of the variant were not performed, but patient brain samples showed phospho-S6 ribosomal immunostaining in cytomegalic neurons consistent with constitutive mTOR pathway activation. Mutations in nearby residues had been reported to cause increased MTOR activity.

.0008 FOCAL CORTICAL DYSPLASIA, TYPE II, SOMATIC
MTOR, SER2215PHE
In resected brain tissue from 3 unrelated French patients with seizures due to focal cortical dysplasia type II (FCORD2; 607341), Moller et al. (2016) identified a somatic c.6644C-T transition (c.6644C-T, NM_004958.3) in the MTOR gene, resulting in a ser2215-to-phe (S2215F) substitution in the kinase domain. The mutations, which were found by deep sequencing of a targeted gene panel, were not found in patient blood. The S2215F variant was not found in the ExAC database. The mutant allele frequency was low, ranging from about 1 to 7%. Functional studies of the variant were not performed, although patient dysmorphic neurons showed intense S6 phosphorylation, consistent with hyperactivation of the mTOR pathway.

Mirzaa et al. (2016) reported 2 additional patients with focal cortical dysplasia and seizures in whom the S2215F mutation was absent in saliva but present in brain, at alternative allele fractions of 0.055 in one and 0.012-0.086 in the other.

.0009 SMITH-KINGSMORE SYNDROME
MTOR, PHE1888CYS
In a pair of 23-year-old monozygotic twin sisters with Smith-Kingsmore syndrome (SKS; 616638), Moller et al. (2016) identified a de novo heterozygous germline c.5663A-C transversion (c.5663A-C, NM_004958.3) in the MTOR gene, resulting in a phe1888-to-cys (F1888C) substitution at a conserved residue in the FAT domain. The mutation, which was found by exome sequencing of a custom gene panel and confirmed by Sanger sequencing, was not found in the ExAC database. Functional studies of the variant and studies of patient cells were not performed.

.0010 SMITH-KINGSMORE SYNDROME
MTOR, MET2327ILE
In a 2.5-year-old girl with Smith-Kingsmore syndrome (SKS; 616638), Moller et al. (2016) identified a de novo heterozygous germline c.6981G-A transition (c.6981G-A, NM_004958.3) in the MTOR gene, resulting in a met2327-to-ile (M2327I) substitution at a conserved residue in the kinase domain. The mutation, which was found by exome sequencing of a custom gene panel and confirmed by Sanger sequencing, was not found in the ExAC database. Functional studies of the variant and studies of patient cells were not performed.

Tags: 1p36.22