全周 (9AM - 6PM)

我们和你在一起

Extra info thumb
VAMP-ASSOCIATED PROTEIN B AND C; VAPBC

VAMP-ASSOCIATED PROTEIN B AND C; VAPBC

Alternative titles; symbolsVESICLE-ASSOCIATED MEMBRANE PROTEIN-ASSOCIATED PROTEIN BVAMP-ASSOCIATED PROTEIN BDVAP33A, DROSOPHILA, HOMOLOG OFOther entities represe...

Alternative titles; symbols

  • VESICLE-ASSOCIATED MEMBRANE PROTEIN-ASSOCIATED PROTEIN B
  • VAMP-ASSOCIATED PROTEIN B
  • DVAP33A, DROSOPHILA, HOMOLOG OF

Other entities represented in this entry:

  • VAMP-ASSOCIATED PROTEIN C, INCLUDED

HGNC Approved Gene Symbol: VAPB

Cytogenetic location: 20q13.32 Genomic coordinates (GRCh38): 20:58,389,210-58,451,100 (from NCBI)

▼ Description
The VAPB gene encodes a protein that is a member of the vesicle-associated membrane protein (VAMP)-associated protein (VAP) family. VAPB plays a role in the unfolded protein response (UPR), a process that suppresses the accumulation of unfolded proteins in the endoplasmic reticulum (ER) (Kanekura et al., 2006).

▼ Cloning and Expression
By searching an EST database for human homologs of the Aplysia 33-kD VAMP-associated protein (Vap33), Nishimura et al. (1999) identified cDNAs encoding VAPA (605703), VAPB, and VAPC. Sequence analysis predicted that the 243-amino acid VAPB protein, which is 60% homologous to VAPA, contains a conserved N-terminal domain, an alpha-helical coiled-coil domain, and a C-terminal transmembrane domain. The 99-amino acid VAPC protein is a splice variant of VAPB that retains the N-terminal 70 residues but lacks the coiled-coil and transmembrane domains. Northern blot analysis detected a major 2.5-kb VAPB transcript and 1.1- and 8.7-kb minor VAPB transcripts in all tissues tested. Expression of VAPB was less intense than that of VAPA or the 1.9-kb VAPC transcript. SDS-PAGE analysis demonstrated that the transmembrane domain of recombinant VAPA interacted with VAPA and VAPB fusion proteins.

De Vos et al. (2012) found that a portion of VAPB localized to mitochondria-associated membranes, a specialized ER domain in close apposition with mitochondria.

Using immunohistochemical analysis, Larroquette et al. (2015) found that Vapb was expressed in motor neurons of mouse spinal cord, but not in glial cells or sensory neurons. Little to no Vapb was detected in cerebellum, cerebrum, or hippocampus. In motor neurons, Vapb localized to cell bodies and dendrites and colocalized with ER markers.

▼ Gene Structure
Chen et al. (2010) noted that the VAPB gene contains 6 exons.

▼ Mapping
The finding by Nishimura et al. (2004) of a missense mutation in the VAPB gene as the cause of a form of amyotrophic lateral sclerosis (ALS8; 608627), which had been mapped to 20q13.3, demonstrated this as the localization of the VAPB gene.

▼ Gene Function
In COS-7 cells and mouse NSC34 cells, Kanekura et al. (2006) demonstrated that wildtype human VABP localized to the endoplasmic reticulum and that its overexpression promoted the unfolded protein response. In contrast, mutant VAPB (P56S; 605704.0001) was insoluble, shifted to non-ER subcellular locations, and did not induce UPR. Although the studies indicated that mutant VAPB was a loss of function mutation, cotransfection experiments showed that mutant VAPB inhibited the ability of wildtype VAPB to mediate UPR, consistent with a dominant-negative effect.

Using immunohistochemistry and Western blot analysis, Teuling et al. (2007) found that VABP was widely expressed in both neuronal and nonneuronal human and mouse cell lines. The protein localized to the endoplasmic reticulum. In the CNS, the highest levels of VABP expression were found in mouse and human spinal cord motor neurons.

Using yeast 2-hybrid and coimmunoprecipitation analyses, De Vos et al. (2012) found that endogenous human VAPB interacted with the outer mitochondrial membrane protein PTPIP51 (FAM82A2; 611873). The cytoplasmic N-terminal domain of VAPB specifically interacted with the cytoplasmic C-terminal domain of PTPIP51. Knockdown of either VAPB or PTPIP51 in HEK293 cells delayed mitochondrial calcium uptake following IP3R (ITPR1; 147265)-induced calcium release from ER stores, resulting in increased peak cytosolic calcium concentration. De Vos et al. (2012) concluded that PTPIP51 directs VAPB localization to mitochondria-associated membranes and that both VAPB and PTPIP51 are involved in calcium exchange between the ER and mitochondria.

▼ Molecular Genetics
By the study of candidate genes within the region of 20q13.3 that showed linkage to an atypical form of ALS called ALS8 (608627), Nishimura et al. (2004) identified a novel mutation in the VAPB gene (P56S; 605704.0001). Subsequently, they identified the same mutation in patients from 6 additional kindreds but with different clinical courses, such as ALS8, late-onset spinal muscular atrophy (182980), and a typical severe ALS with rapid progression. Although it was not possible to link all of these families, haplotype analysis suggested a founder effect. Members of the vesicle-associated proteins are intracellular membrane proteins that can associate with microtubules and have been shown to have a function in membrane transport. The data suggested that clinically variable motor neuron diseases may be caused by dysfunction in intracellular membrane trafficking.

Kirby et al. (2007) did not identify mutations in the VAPB gene in 301 cases of ALS from the United Kingdom, including 23 familial and 278 sporadic cases.

Landers et al. (2008) identified a P56S mutation in the VAPB gene in 1 of 80 families with ALS. The family with the mutation was of Brazilian origin. No other clearly pathogenic mutations were identified. In 1 other family, they identified a 3-bp in-frame deletion (478delCTT), resulting in loss of ser160, that was also found in 0.45% of controls. In vitro expression studies of del-ser160 VAPB showed wildtype cytoplasmic localization. Landers et al. (2008) concluded that VAPB mutations are not a common cause of ALS and that the 3-bp deletion they identified in 1 family was not causative for the disorder.

In 1 of 107 non-Brazilian probands with ALS, Chen et al. (2010) identified a heterozygous mutation in the VAPB gene (T46I; 605704.0002). In vitro functional expression studies in COS-7 and neuronal cells showed that the T46I mutation formed intracellular protein aggregates and ubiquitin aggregates, ultimately resulting in cell death.

▼ Animal Model
Chai et al. (2008) demonstrated that the Drosophila Dvap33a gene is the structural and functional homolog of the human VAPB gene. Hypomorphic and null Dvap33a alleles expressed in neurons caused a severe decrease in bouton number and an increase in bouton size. Conversely, overexpression of Dvap33a in neurons induced a highly significant increase in the number of boutons with a concomitant decrease in their size. Electrophysiologic and electron microscopic studies showed that these structural alterations were associated with compensatory changes in the physiology and ultrastructure of synapses, which maintained evoked responses within normal boundaries. These compensatory changes were determined by changes in expression of glutamate receptor subunits. Targeted expression of human VAPB in Drosophila neurons with hypomorphic or null Dvap33a alleles rescued the morphologic and electrophysiologic mutant phenotype. Transgenic expression of mutant Dvap33a in Drosophila recapitulated major hallmarks of human neuronal diseases, including locomotion defects and neuronal death with aggregate formation. The findings implicated a role for human VAPB in synaptic homeostasis.

In transgenic mice, Aliaga et al. (2013) found that expression of human VAPB with the P56S mutation caused various motor behavioral abnormalities, including progressive hyperactivity. Accumulation of mutant VAPB triggered ER stress, leading to increased proapoptotic Chop (DDIT3; 126337) expression in both corticospinal and spinal motor neurons. Mutant transgenic mice experienced significant loss of corticospinal motor neurons, but no obvious degeneration of spinal motor neurons.

Moustaqim-Barrette et al. (2014) found that cortical neurons of vap-null Drosophila showed ER stress with accumulation of ubiquitinated proteins, concomitant with development of a progressive flight defect. Loss of vap also caused redistribution of oxysterol-binding protein (OSBP; 167040) from ER to Golgi. Flies expressing vap with an ALS8 mutation (P56S) showed a less severe phenotype than vap-null flies. Expression of human OSBPL8 (606736), which lacks the vap-binding sequence found in Drosophila osbp, attenuated ER stress and accumulation of ubiquitinated proteins and partly rescued the flight defect in vap-null flies and flies with the ALS8 mutation.

Larroquette et al. (2015) replaced the wildtype Vapb gene in mice with P56S mutant Vapb. Heterozygous and homozygous P56S Vapb knockin mice were obtained at the expected mendelian ratio, developed normally, and had life spans similar to wildtype mice. However, heterozygous and homozygous P56S Vapb knockin mice showed dose- and age-dependent defects in motor tasks compared with wildtype mice. P56S Vapb knockin mice also showed age- and dose-dependent cellular pathologic defects in motor neurons, with loss of P56S Vapb in the ER and accumulation of the mutant protein in cytoplasmic inclusions, where it localized with ubiquitinated proteins. P56S Vapb knockin motor neurons showed chronic mild atrophy, induction of ER stress, and autophagic response prior to onset of motor defect. Soleus muscle of P56S Vapb knockin mice showed mild partial denervation and morphologic changes at synaptic boutons at neuromuscular junctions.

▼ ALLELIC VARIANTS ( 2 Selected Examples):

.0001 AMYOTROPHIC LATERAL SCLEROSIS 8
SPINAL MUSCULAR ATROPHY, LATE-ONSET, FINKEL TYPE, INCLUDED
AMYOTROPHIC LATERAL SCLEROSIS, TYPICAL, INCLUDED
VAPB, PRO56SER
In a large white Brazilian family with atypical ALS (ALS8; 608627), Nishimura et al. (2004) found a heterozygous 166C-T transition in exon 2 of the VAPB gene, leading to a pro56-to-ser (P56S) mutation. Subsequently the authors demonstrated the same mutation in patients from 6 additional kindreds in which the clinical course varied, including some with late-onset spinal muscular atrophy (Finkel type; 182980) and some with typical severe ALS with rapid progression (see 105400). Although it was not possible to link all of these families, haplotype analysis suggested founder effect. In vitro functional expression studies in rat hippocampal neurons and HEK293 cells showed that the P56S mutation disrupted the normal subcellular distribution of the VAPB protein and caused intracellular aggregates. Unlike the wildtype protein, the mutant P56S protein did not colocalize with either the Golgi apparatus or the endoplasmic reticulum (ER).

Nishimura et al. (2005) analyzed 7 polymorphic markers around the VAPB gene in an index case from each of the Brazilian families with P56S mutation previously reported by Nishimura et al. (2004) and in 9 Brazilian Portuguese controls. They found evidence for a common founder for all families regardless of ancestry, with a founding event 23 generations ago, consistent with the Portuguese colonization of Brazil.

Teuling et al. (2007) found that the P56S mutant protein formed cytosolic aggregates in all cell types examined, including mouse and human nonneuronal cells. These aggregates did not colocalize with markers for the ER. Further studies showed that the mutant protein acted in a dominant-negative manner by recruiting wildtype VAPB to the aggregates and disrupting normal protein and cellular function.

Landers et al. (2008) identified the P56S mutation in affected members of a Brazilian family with ALS. The mean age at onset was between 45 and 55 years with survival varying from 5 to 18 years. The mutation was not identified in 79 additional ALS families.

Millecamps et al. (2010) identified the P56S mutation in 1 (0.6%) of 162 French probands with familial ALS. The patient was of Japanese descent, representing the first non-Brazilian reported to carry this mutation. Three other family members had motor neuron disease, suggesting autosomal dominant inheritance. The patient had long disease duration with onset in the legs during the sixth decade. Millecamps et al. (2010) suggested that the finding of the P56S mutation in a Japanese patient may reflect the Portuguese trading connection with the Far East and Brazil in the mid-16th century.

De Vos et al. (2012) found that VAPB with the P56S mutation showed significantly higher affinity than wildtype for the outer mitochondrial membrane protein PTPIP51 (FAM82A2; 611873). Increased binding with PTPIP51 resulted in accumulation of VAPB at mitochondria-associated membranes in the ER and elevated calcium uptake by mitochondria following release of calcium from ER stores. Expression of human VAPB with the P56S mutation also disturbed calcium handling in cultured rat cortical neurons following depolarization.

Using cultured embryonic rat cortical neurons, Morotz et al. (2012) found that expression of human VAPB with the P56S mutation (VAPB-P56S) significantly slowed anterograde axonal transport of mitochondria. Studies in rat neurons and HEK293 cells showed that expression of VAPB-P56S increased resting intracellular Ca(2+) concentration and disrupted the interaction between tubulin (see 191130) and the mitochondrial membrane Rho GTPase MIRO1 (RHOT1; 613888). Expression of VAPB-P56S had no effect on the amount of TRAK1 (608112) or kinesin-1 (see 602809) associated with MIRO1.

.0002 AMYOTROPHIC LATERAL SCLEROSIS 8
VAPB, THR46ILE
In a non-Brazilian patient with ALS8 (608627), Chen et al. (2010) identified a heterozygous 137C-T transition in the VAPB gene, resulting in a thr46-to-ile (T46I) substitution in a highly conserved residue important for the interaction with lipid-binding proteins. The mutation was found in 1 of 107 probands with familial ALS and was not found in 257 controls. The 73-year-old male patient presented with wasting of the small muscles of the hands. He also had fasciculations of the leg, and later developed speech and swallowing difficulties. The diagnosis was confirmed by nerve conduction studies. The patient had a brother with ALS who died within 4 months of diagnosis from pneumonia, but DNA was not available for testing. In vitro functional expression studies in COS-7 cells and neuronal showed that the T46I mutation formed intracellular protein aggregates and ubiquitin aggregates, ultimately resulting in cell death. The mutant protein was unable to activate the unfolded protein response pathway, as measured by lack of activation of IRE1 (ERN1; 604033), and the effect was dominant-negative. Expression of the equivalent T48I mutation in Drosophila resulted in aggregate formation in neurons and nerve fibers, cell degeneration, fragmentation of the endoplasmic reticulum, and upregulation of chaperone proteins. Muscle was also adversely affected. Chen et al. (2010) also postulated that disturbances in lipid metabolism may play a role in the pathogenesis of ALS.

Tags: 20q13.32