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ATAXIN 7; ATXN7

ATAXIN 7; ATXN7

HGNC Approved Gene Symbol: ATXN7Cytogenetic location: 3p14.1 Genomic coordinates (GRCh38): 3:63,863,143-64,003,461 (from NCBI)▼ DescriptionATXN7 is a transcr...

HGNC Approved Gene Symbol: ATXN7

Cytogenetic location: 3p14.1 Genomic coordinates (GRCh38): 3:63,863,143-64,003,461 (from NCBI)

▼ Description

ATXN7 is a transcription factor that appears to be critically important for chromatin remodeling at the level of histone acetylation and deubiquitination. It is a core component of 2 different transcription coactivator complexes: the SPT3 (SUPT3H; 602947)/TAF9 (600822)/GCN5 (KAT2A; 602301) acetyltransferase (STAGA) complex, which has histone acetyltransferase activity, and the USP22 (612116) deubiquitination complex (summary by Sopher et al., 2011).

▼ Cloning and Expression

By positional cloning, David et al. (1997) identified the causative gene for spinocerebellar ataxia-7 (164500). The ATXN7 gene has a 2,727-bp open reading frame predicting an 892-amino acid polypeptide, designated ataxin-7, with a nuclear localization signal and a polyglutamine (polyQ) tract. In the consensus cDNA sequence, a 9-residue polyglutamine tract is encoded by CAG repeats at codons 30 to 39. Mutated disease alleles have expanded repeats (see MOLECULAR GENETICS).

Einum et al. (2003) identified an ATXN7 splice variant, termed ataxin-7b, which contains an additional 67 basepairs between exons 12 and 13, and predicts a 945-amino acid protein with a molecular mass of 101 kD. Northern blot analysis revealed a 4.5-kb transcript expressed predominantly throughout the CNS. Immunohistochemistry revealed intense cytoplasmic staining in cerebellar Purkinje cells, inferior olivary neurons, and retinal photoreceptor cells. Einum et al. (2003) suggested that expanded polyglutamine repeats in ataxin-7b also contribute to neurodegeneration and that expression of multiple polyglutamine-containing proteins may result in selective patterns of neurodegeneration.

Sopher et al. (2011) identified human ATXN7 variants that initiate from an alternative promoter (P2A) and transcription start site in the 3-prime end of intron 2. These variants differ from previously reported ATXN7 transcripts in their 5-prime UTR, but the translational start site is unchanged.

Strom et al. (2002) cloned the mouse Sca7 cDNA from a brain cDNA library. The deduced 867-amino acid protein shows 88.7% identity with the human protein. The mouse CAG region contained only 5 repeats in 3 different strains examined. Northern blot analysis detected expression of Sca7 in all adult mouse tissues, with highest expression in heart, brain, liver, and kidney.

▼ Gene Structure

Michalik et al. (1999) reported that the ATXN7 gene contains 13 exons that range in size from 69 to 979 bp. The ATG initiation codon at position 554 of the cDNA occurs in exon 3 at position 12, and the coding region extends to the first 5 codons of exon 13. The CAG repeat is located in exon 3, starting at codon 30. The introns vary in size from 233 bp to about 40 kb, resulting in an overall size for the ATXN7 gene of about 140 kb. Sequence analysis of intron 7 (491 bp) revealed a polymorphic GT/AC repeat, a useful intragenic marker for ATXN7 in segregation studies.

Sopher et al. (2011) identified an alternative promoter (P2A) in ATXN7 that initiates transcription at an alternate start site in the 3-prime end of intron 2. Variants initiated from P2A contain a single first exon comprising the last 400 bp of intron 2 and exon 3, or instead have a shorter exon (2A) spliced to exon 3. The translational start site is unchanged. Binding sites for the transcriptional regulator CTCF (604167) flank exon 3. Mice have a single Atxn7 promoter corresponding to human P2A. Sopher et al. (2011) also identified identified SCAANT1 (ATXN7AS1; 614481), an ATXN7 antisense transcript on the opposite strand. SCAANT1 transcription begins in ATXN7 intron 4 and covers exon 4, exon 3, including the translational start site and the CAG repeat region, and the P2A promoter of ATXN7.

▼ Mapping

Gross (2012) mapped the ATXN7 gene to chromosome 3p14.1 based on an alignment of the ATXN7 sequence (GenBank AF032105) with the genomic sequence (GRCh37).

▼ Gene Function

Kaytor et al. (1999) examined the subcellular localization of ataxin-7 in transfected COS-1 cells, using ATXN7 cDNA clones with different CAG repeat tract lengths. In addition to a diffuse distribution throughout the nucleus, ataxin-7 associated with the nuclear matrix and the nucleolus. The location of the putative ATXN7 nuclear localization sequence (NLS) was confirmed by fusing an ataxin-7 fragment with chicken muscle pyruvate kinase, a normally cytoplasmic protein. Mutation of this NLS prevented protein from entering the nucleus. Thus, expanded ataxin-7 may carry out its pathogenic effects in the nucleus by altering a matrix-associated nuclear structure and/or by disrupting nucleolar function.

Lebre et al. (2001) used a 2-hybrid approach to screen a human retina cDNA library for ataxin-7-binding proteins and isolated R85, a splice variant of Cbl-associated protein (CAP; 605264). R85 and CAP are generated by alternative splicing of the SH3P12 gene, which was localized on chromosome 10q23-q24. The interaction between ataxin-7 and the SH3P12 gene products was confirmed by pull-down and coimmunoprecipitation. SH3P12 gene products are expressed in Purkinje cells in the cerebellum. Ataxin-7 colocalizes with full-length R85 in cotransfected COS-7 cells and with one of the SH3P12 gene products in neuronal intranuclear inclusions in brain from a SCA7 patient. The authors proposed that this interaction may be part of a physiologic pathway related to the function or turnover of ataxin-7.

To analyze the effects of overexpression of mutant ataxin-7 protein, Zander et al. (2001) established ATXN7 cell culture models in kidney and neuroblastoma cell lines. The cells readily formed anti-ataxin-7-positive fibrillar inclusions and small, nuclear electron-dense structures. Comparable to the inclusions in human SCA7 brain tissue, there were consistent signs of ongoing abnormal protein folding, including the recruitment of heat-shock proteins and proteasome subunits. Occasionally, sequestered transcription factors were found. Activated caspase-3 (600636) was recruited into the inclusions in both the cell models and human SCA7 brain, and its expression was upregulated in cortical neurons, suggesting that it may play a role in the disease process. On the ultrastructural level, there were signs of autophagy and nuclear indentations, indicative of a major stress response in cells expressing mutant ataxin-7.

Matilla et al. (2001) used a 2-hybrid assay to show that ataxin-7 interacts with the ATPase subunit S4 of the 19S regulatory complex of the 26S proteasome (602706). The ataxin-7/S4 association was modulated by the length of the polyglutamine tract, whereby S4 showed a stronger association with the wildtype allele of ataxin-7. Endogenous ataxin-7 localized to discrete nuclear foci that also contained additional components of the proteasomal complex. Immunohistochemical analyses suggested alterations either of the distribution or the levels of S4 immunoreactivity in neurons that degenerate in SCA7 brains. Immunoblot analyses demonstrated reduced levels of S4 in SCA7 cerebella without evident alterations in the levels of other proteasome subunits. The authors suggested a role for S4 and ubiquitin-mediated proteasomal proteolysis in the molecular pathogenesis of SCA7.

By combining profile-based sequence analysis with genomewide functional data in model organisms, Scheel et al. (2003) found that ATXN7 is orthologous to the yeast open reading frame Ygl066c. Evidence suggested that Ygl066c is a component of the SAGA histone acetyltransferase complex. Scheel et al. (2003) suggested the finding had implications for disease pathogenesis by providing a direct connection between SCA7 and histone acetylation.

A putative ataxin-7 yeast ortholog, SGF73 (Ygl066c), is a component of the SAGA (Spt/Ada/Gcn5 acetylase) multisubunit complex, a coactivator required for transcription of a subset of RNA polymerase II (see 180660)-dependent genes (Gavin et al., 2002; Sanders et al., 2002). Helmlinger et al. (2004) showed that ataxin-7 is an integral component of the mammalian SAGA-like complexes TFTC (TATA-binding protein-free TAF-containing complex) and STAGA (SPT3 (602947)/TAF9 (600822)/GCN5 (602301) acetyltransferase complex). Immunopurified ataxin-7 complex retained histone acetyltransferase (see 603053) activity, characteristic for TFTC-like complexes. The authors identified a 71-amino acid domain in ataxin-7 that is required for interaction with TFTC/STAGA subunits and is conserved highly through evolution, allowing the identification of a ATXN7 gene family. This domain contains a conserved cys3-his motif that binds zinc, forming a new zinc-binding domain. Polyglutamine expansion in ataxin-7 did not affect its incorporation into TFTC/STAGA complexes purified from SCA7 patient cells. The authors concluded that ataxin-7 is the human ortholog of the yeast SAGA SGF73 subunit and is a bona fide subunit of the human TFTC-like transcriptional complexes.

In an SCA7 transgenic mouse model, La Spada et al. (2001) found that the cone-rod dystrophy involved altered photoreceptor gene expression due to interference with CRX (602225), a homeodomain transcription factor containing a glutamine-rich region. By coimmunoprecipitation analysis of CRX and ATXN7 truncation and point mutants, Chen et al. (2004) determined that the ATXN7-interacting domain of CRX localized to its glutamine-rich region and the CRX-interacting domain of ATXN7 localized to its glutamine tract. Nuclear localization of ataxin-7 was required to repress Crx transactivation, and the likely nuclear localization signals were mapped to the C-terminal region of ataxin-7. Using chromatin immunoprecipitation, the authors showed that both Crx and ataxin-7 occupied the promoter and enhancer regions of Crx-regulated retinal genes in vivo. Chen et al. (2004) suggested that one mechanism of SCA7 disease pathogenesis may be transcription dysregulation, and that CRX transcription interference may be a predominant factor in SCA7 cone-rod dystrophy retinal degeneration.

Regulation of ATXN7 by ATXN7AS1

By quantitative RT-PCR of human human tissues, Sopher et al. (2011) observed inverse expression of ATXN7 and its antisense transcript, SCAANT1 (ATXN7AS1; 614481). ATXN7 expression was high in cortex, cerebellum, striatum, and liver and weak in lung and kidney, whereas SCAANT1 expression was weak in brain tissues and liver and high in lung and kidney. Qualitative RT-PCR revealed abnormally elevated ATXN7 expression and reduced SCAANT1 expression in SCA7 patient fibroblasts and peripheral blood samples. Sopher et al. (2011) generated transgenic mice expressing an approximately 13.5-kb human ATXN7 minigene construct containing SCAANT1, the alternative promoter P2A, the ATXN7 translational start site in exon 3, a pathogenic CAG expansion in exon 3, and either wildtype or mutant CTCF (604167)-binding sites flanking exon 3. Transgenic mice with mutant CTCF-binding sites, but not those with wildtype CTCF-binding sites, showed reduced CTCF binding, elevated ATXN7 expression, and reduced SCAANT1 expression and developed features of SCA7. Knockdown of CTCF in human retinoblastoma cells significantly reduced expression of SCAANT1 and increased ATXN7 mRNA expressed from the P2A promoter, but not from the canonical upstream promoter. Chromatin immunoprecipitation analysis revealed repressive chromatin modifications in the P2A promoter with SCAANT1 expression. Sopher et al. (2011) concluded that SCAANT1 is under positive transcriptional control by CTCF and negatively regulates ATXN7 expression.

Yang et al. (2015) showed that polyQ-expanded ATX7 sequestered USP22 into inclusions when overexpressed in HEK293T cells. Sequestration of USP22 by polyQ-expanded ATX7 depended on interaction between the zinc finger domain of ATX7 and the N-terminal domain of USP22 and resulted in insoluble aggregates of USP22. PolyQ expansion in ATX7 reduced the catalytic activity of USP22 and impaired the deubiquitinating function of USP22.

▼ Molecular Genetics

In patients with SCA7 (164500), David et al. (1997) identified a highly unstable CAG repeat expansion of the ataxin-7 gene (607640.0001). On mutated alleles, CAG repeat size was highly variable, ranging from 38 to 130 repeats, whereas on normal alleles it ranged from 7 to 17 repeats. Gonadal instability in SCA7 was greater than that observed in any of the known neurodegenerative diseases caused by translated CAG repeat expansions, and the instability was particularly striking on paternal transmission (for a detailed discussion, see 'Genetic Anticipation' in 164500).

▼ Animal Model

By using constructs with tissue-specific promoters, Yvert et al. (2000) generated transgenic mice which expressed mutant human ataxin-7 in either Purkinje cells or retinal rod photoreceptors. Mice overexpressing full-length mutant ataxin-7(Q90) either in Purkinje cells or in rod photoreceptors had deficiencies in motor coordination and vision, respectively. In both models, an N-terminal fragment of mutant ataxin-7 accumulated within ubiquitinated nuclear inclusions that recruited a distinct set of chaperone/proteasome subunits. A severe degeneration was caused by overexpression of ataxin-7(Q90) in rods, whereas a similar overexpression of normal ataxin-7(Q10) had no obvious effect. The degenerative process was not limited to photoreceptors, and secondary alterations were seen in postsynaptic neurons. The authors suggested that proteolytic cleavage of mutant ataxin-7 and transneuronal responses are implicated in the pathogenesis of SCA7.

To study the mechanism of polyglutamine neurotoxicity in SCA7, La Spada et al. (2001) generated a transgenic mouse model of SCA7 that expressed ataxin-7 with 92 glutamines in the CNS and retina. They observed a cone-rod dystrophy type of retinal degeneration. Using yeast 2-hybrid studies, La Spada et al. (2001) demonstrated that ataxin-7 interacts with CRX, a nuclear transcription factor predominantly expressed in retinal photoreceptor cells. Mutations in the CRX gene cause cone-rod dystrophy-2 (120970) in humans. Coimmunoprecipitation experiments colocalized ataxin-7 with CRX in nuclear aggregates. Using a rhodopsin promoter-reporter construct, La Spada et al. (2001) observed that polyglutamine-expanded ataxin-7 suppressed CRX transactivation. With electrophoretic mobility shift assays and RT-PCR analysis, they observed a reduction in CRX binding activity and reductions in CRX-regulated genes in SCA7 transgenic retinas. The data suggested that the SCA7 transgenic mice faithfully recapitulated the process of retinal degeneration observed in human SCA7 patients. The authors hypothesized that ataxin-7-mediated transcription interference of photoreceptor-specific genes may account for the retinal degeneration in SCA7, and thus may provide an explanation for how cell-type specificity is achieved in this polyglutamine repeat disorder.

Helmlinger et al. (2002) showed that R6 transgenic mice express mutant huntingtin (HTT; 613004) in the retina, leading to severe vision deficiencies and retinal dystrophy. Comparable early and progressive retinal degeneration and dysfunction have been described in R7E mice (Yvert et al., 2000). These abnormalities are reminiscent of other retinal degeneration phenotypes (in particular rd7/rd7 mice) in which photoreceptor cell loss occurs. Helmlinger et al. (2002) suggested that the NRL (162080) pathway and photoreceptor cell fate may be altered in the retina of R6 and R7E mice.

Libby et al. (2003) generated lines of transgenic mice carrying either a ATXN7 cDNA construct or a 13.5 kb ATXN7 genomic fragment with 92 CAG repeats. Whereas the cDNA transgenic mice showed little intergenerational repeat instability, the genomic fragment transgenic mice displayed marked intergenerational instability with an obvious expansion bias. Selective deletion of the 3-prime genomic region significantly stabilized intergenerational transmission of the ATXN7 92 CAG repeat. The authors suggested that cis-information present on the genomic fragment may drive the instability process. Small-pool and standard PCR analyses of tissues from genomic fragment mice revealed large repeat expansions in their brains and livers, but no such changes were found in any tissues from cDNA transgenic mice that underwent neurodegeneration. As large somatic repeat expansions were absent from the brains of ATXN7 cDNA mice, Libby et al. (2003) proposed that neurodegeneration may occur without marked somatic mosaicism, at least in these mice.

Bowman et al. (2005) assessed the ubiquitin-proteasome system (UPS) using transgenic mice with 266 CAG repeats and a ubiquitin (191339) reporter gene. Reporter levels were low during the initial phase of disease, suggesting that neuronal dysfunction occurs in the presence of a functional UPS. Late in disease, there was a significant increase in reporter levels specific to the most vulnerable neurons, resulting from increase in ubiquitin reporter mRNA. No evidence for general UPS impairment or reduction of proteasome activity was seen. The differential increase of ubiquitin reporter among individual neurons directly correlated with the downregulation of a marker of selective pathology and neuronal dysfunction in SCA7. There was an inverse correlation between the neuropathology revealed by the reporter and ataxin-7 nuclear inclusions in the vulnerable neurons. Bowman et al. (2005) proposed a protective role for polyglutamine nuclear inclusions against neuronal dysfunction and excluded significant impairment of the UPS in polyglutamine neuropathology.

Janer et al. (2010) identified ATXN7 as target for sumoylation in vitro and in vivo. Sumoylation did not influence the subcellular localization of ATXN7 nor its interaction with components of the TFTC/STAGA complex. Expansion of the polyglutamine stretch did not impair the sumoylation of ATXN7. SUMO1 (601912) and SUMO2 (603042) colocalized with ATXN7 in a subset of neuronal intranuclear inclusions in the brain of SCA7 patients and Atxn7 knockin mice. In a COS-7 cellular model of SCA7, there were 2 populations of extranuclear inclusions: homogeneous and nonhomogeneous. Nonhomogeneous inclusions showed significantly reduced colocalization with SUMO1 and SUMO2, but were highly enriched in Hsp70 (HSPA1A; 140550), 19S proteasome, and ubiquitin. These were characterized by increased staining with the apoptotic marker caspase-3 (CASP3; 600636) and by disruption of PML nuclear bodies (see 102578). Preventing the sumoylation of expanded ATXN7 by mutating the SUMO site increased both the amount of SDS-insoluble aggregates and of CASP3-positive nonhomogeneous inclusions, which are toxic to the cells. Janer et al. (2010) concluded that sumoylation influences the multistep aggregation process of ATXN7, and they implicated a role for ATXN7 sumoylation in SCA7 pathogenesis.

▼ ALLELIC VARIANTS ( 1 Selected Example):

.0001 SPINOCEREBELLAR ATAXIA 7
ATXN7, (CAG)n REPEAT EXPANSION

In 18 patients from 5 families with spinocerebellar ataxia-7 (SCA7; 164500), David et al. (1997) reported a highly unstable CAG repeat expansion of the ataxin-7 gene. Gonadal instability was pronounced and was associated with paternal transmission.

Tags: 3p14.1