The human MJD gene: genomic structure and functional characterization of the promoter region

Spinocerebellar Ataxia Type 3 Pathophysiology-Implications for Translational Research and Clinical Studies

The spinocerebellar ataxias (SCA) comprise a group of inherited neurodegenerative diseases. Machado-Joseph Disease (MJD) or spinocerebellar ataxia 3 (SCA3) is the most common autosomal dominant form, caused by the expansion of CAG repeats within the ataxin-3 (ATXN3) gene. This mutation results in the expression of an abnormal protein containing long polyglutamine (polyQ) stretches that confers a toxic gain of function and leads to misfolding and aggregation of ATXN3 in neurons. As a result of the neurodegenerative process, SCA3 patients are severely disabled and die prematurely. Several screening approaches, e.g., druggable genome-wide and drug library screenings have been performed, focussing on the reduction in stably overexpressed ATXN3(polyQ) protein and improvement in the resultant toxicity. Transgenic overexpression models of toxic ATXN3, however, missed potential modulators of endogenous ATXN3 regulation. In another approach to identify modifiers of endogenous ATXN3 expression using a CRISPR/Cas9-modified SK-N-SH wild-type cell line with a GFP-T2A-luciferase (LUC) cassette under the control of the endogenous ATXN3 promotor, four statins were identified as potential activators of expression. We here provide an overview of the high throughput screening approaches yet performed to find compounds or genomic modifiers of ATXN3(polyQ) toxicity in different SCA3 model organisms and cell lines to ameliorate and halt SCA3 progression in patients. Furthermore, the putative role of cholesterol in neurodegenerative diseases (NDDs) in general and SCA3 in particular is discussed.

High throughput compound screening in neuronal cells identifies statins as activators of ataxin 3 expression

The spinocerebellar ataxias (SCA) comprise a group of inherited neurodegenerative diseases. SCA3 is the most common form, caused by the expansion of CAG repeats within the ataxin 3 (ATXN3) gene. The mutation results in the expression of an abnormal protein, containing long polyglutamine (polyQ) stretches. The polyQ stretch confers a toxic gain of function and leads to misfolding and aggregation of ATXN3 in neurons. Thus, modulators of ATXN3 expression could potentially ameliorate the pathology in SCA3 patients. Therefore, we generated a CRISPR/Cas9 modified ATXN3-Exon4-Luciferase (ATXN3-LUC) genomic fusion- and control cell lines to perform a reporter cell line-based high-throughput screen comprising 2640 bioactive compounds, including the FDA approved drugs. We found no unequivocal inhibitors of, but identified statins as activators of the LUC signal in the ATXN3-LUC screening cell line. We further confirmed that Simvastatin treatment of wild type SK-N-SH cells increases ATXN3 mRNA and protein levels which likely results from direct binding of the activated sterol regulatory element binding protein 1 (SREBP1) to the ATXN3 promotor. Finally, we observed an increase of normal and expanded ATXN3 protein levels in a patient-derived cell line upon Simvastatin treatment, underscoring the potential medical relevance of our findings.

Aripiprazole Offsets Mutant ATXN3-Induced Motor Dysfunction by Targeting Dopamine D2 and Serotonin 1A and 2A Receptors in C. elegans

The atypical antipsychotic aripiprazole is a Food and Drug Administration-approved drug for the treatment of psychotic, mood, and other psychiatric disorders. Previous drug discovery efforts pinpointed aripiprazole as an effective suppressor of Machado–Joseph disease (MJD) pathogenesis, as its administration resulted in a reduced abundance and aggregation of mutant Ataxin-3 (ATXN3) proteins. Dopamine partial agonism and functional selectivity have been proposed as the main pharmacological mechanism of action of aripiprazole in the treatment of psychosis; however, this mechanism remains to be determined in the context of MJD. Here, we focus on confirming the efficacy of aripiprazole to reduce motor dysfunction in vivo, using a Caenorhabditis elegans (C. elegans) model of MJD, and on unveiling the drug targets required for its positive action against mutant ATXN3 pathogenesis. We employed pharmacogenetics and pharmacological approaches to identify which dopamine and serotonin receptors are critical for aripiprazole-mediated improvements in motor function. We demonstrated that dopamine D₂-like and serotonin 5-HT_1A and 5-HT_2A receptors play important roles in this process. Our findings strengthen the relevance of dopaminergic and serotoninergic signaling modulation against mutant ATXN3-mediated pathogenesis. The identification of aripiprazole’s cellular targets, relevant for MJD and perhaps other neurodegenerative diseases, may pave the way for prospective drug discovery and development campaigns aiming to improve the features of this prototypical compound and reduce side effects not negligible in the case of aripiprazole.

Epigenetic profiles in polyglutamine disorders

The dominant polyglutamine (polyQ) disorders are a group of progressive and incurable neurodegenerative disorders, which are caused by unstable expanded CAG trinucleotide repeats in the coding regions of their respective causative genes. The most prevalent polyQ disorders worldwide are Huntington’s disease and spinocerebellar ataxia type 3. Epigenetic mechanisms, such as DNA methylation, histone modifications and chromatin remodeling and noncoding RNA regulation, regulate gene expression or genome function. Epigenetic dysregulation has been suggested to play a pivotal role in the pathogenesis of polyQ disorders. Here, we summarize the current knowledge of epigenetic changes present in several representative polyQ disorders and discuss the potentiality of miRNAs as therapeutic targets for the clinic therapy of these disorders.

Sequence analysis of 5' regulatory regions of the Machado-Joseph disease gene (ATXN3)

Machado-Joseph disease (MJD) is a late-onset autosomal dominant neurodegenerative disorder, which is caused by a coding (CAG)(n) expansion in the ATXN3 gene (14q32.1). The number of CAG repeats in the expanded alleles accounts only for 50 to 75 % of onset variance, the remaining variation being dependent on other factors. Differential allelic expression of ATXN3 could contribute to the explanation of different ages at onset in patients displaying similar CAG repeat sizes. Variation in 5' regulatory regions of the ATXN3 gene may have the potential to influence expression levels and, ultimately, modulate the MJD phenotype. The main goal of this work was to analyze the extent of sequence variation upstream of the ATXN3 start codon. A fragment containing the core promoter and the 5' untranslated region (UTR) was sequenced and analyzed in 186 patients and 59 controls (490 chromosomes). In the core promoter, no polymorphisms were observed. In the 5' UTR, only one SNP (rs3814834) was found, but no improvements on the explanation of onset variance were observed, when adding its allelic state in a linear model. Accordingly, in silico analysis predicted that this SNP lays in a nonconserved position for CMYB binding. Therefore, no functional effect could be predicted for this variant.

Machado-Joseph disease/spinocerebellar ataxia type 3

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), may be the most common dominantly inherited ataxia in the world. Here I will review historical, clinical, neuropathological, genetic, and pathogenic features of MJD, and finish with a brief discussion of present, and possible future, treatment for this currently incurable disorder. Like many other dominantly inherited ataxias, MJD/SCA3 shows remarkable clinical heterogeneity, reflecting the underlying genetic defect: an unstable CAG trinucleotide repeat that varies in size among affected persons. This pathogenic repeat in MJD/SCA3 encodes an expanded tract of the amino acid glutamine in the disease protein, which is known as ataxin-3. MJD/SCA3 is one of nine identified polyglutamine neurodegenerative diseases which share features of pathogenesis centered on protein misfolding and accumulation. The specific properties of MJD/SCA3 and its disease protein are discussed in light of what is known about the entire class of polyglutamine diseases.

Toward understanding Machado-Joseph disease

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is the most common inherited spinocerebellar ataxia and one of many polyglutamine neurodegenerative diseases. In MJD, a CAG repeat expansion encodes an abnormally long polyglutamine (polyQ) tract in the disease protein, ATXN3. Here we review MJD, focusing primarily on the function and dysfunction of ATXN3 and on advances toward potential therapies. ATXN3 is a deubiquitinating enzyme (DUB) whose highly specialized properties suggest that it participates in ubiquitin-dependent proteostasis. By virtue of its interactions with VCP, various ubiquitin ligases and other ubiquitin-linked proteins, ATXN3 may help regulate the stability or activity of many proteins in diverse cellular pathways implicated in proteotoxic stress response, aging, and cell differentiation. Expansion of the polyQ tract in ATXN3 is thought to promote an altered conformation in the protein, leading to changes in interactions with native partners and to the formation of insoluble aggregates. The development of a wide range of cellular and animal models of MJD has been crucial to the emerging understanding of ATXN3 dysfunction upon polyQ expansion. Despite many advances, however, the principal molecular mechanisms by which mutant ATXN3 elicits neurotoxicity remain elusive. In a chronic degenerative disease like MJD, it is conceivable that mutant ATXN3 triggers multiple, interconnected pathogenic cascades that precipitate cellular dysfunction and eventual cell death. A better understanding of these complex molecular mechanisms will be important as scientists and clinicians begin to focus on developing effective therapies for this incurable, fatal disorder.

Compromised mitochondrial complex II in models of Machado-Joseph disease

Machado-Joseph disease (MJD), also known as Spinocerebellar Ataxia type 3, is an inherited dominant autosomal neurodegenerative disorder. An expansion of Cytosine-Adenine-Guanine (CAG) repeats in the ATXN3 gene is translated as an expanded polyglutamine domain in the disease protein, ataxin-3. Selective neurodegeneration in MJD is evident in several subcortical brain regions including the cerebellum. Mitochondrial dysfunction has been proposed as a mechanism of neurodegeneration in polyglutamine disorders. In this study, we used different cell models and transgenic mice to assess the importance of mitochondria on cytotoxicity observed in MJD. Transiently transfected HEK cell lines with expanded (Q84) ataxin-3 exhibited a higher susceptibility to 3-nitropropionic acid (3-NP), an irreversible inhibitor of mitochondrial complex II. Increased susceptibility to 3-NP was also detected in stably transfected PC6-3 cells that inducibly express expanded (Q108) ataxin-3 in a tetracycline-regulated manner. Moreover, cerebellar granule cells from MJD transgenic mice were more sensitive to 3-NP inhibition than wild-type cerebellar neurons. PC6-3 (Q108) cells differentiated into a neuronal-like phenotype with nerve growth factor (NGF) exhibited a significant decrease in mitochondrial complex II activity. Mitochondria from MJD transgenic mouse model and lymphoblast cell lines derived from MJD patients also showed a trend toward reduced complex II activity. Our results suggest that mitochondrial complex II activity is moderately compromised in MJD, which may designate a common feature in polyglutamine toxicity.

Different methylation of the TNF-alpha promoter in cortex and substantia nigra: Implications for selective neuronal vulnerability

Increasing evidence has linked inflammatory processes to neurodegenerative disorders, including Alzheimer's and Parkinson's disease (PD). Tumor necrosis factor alpha (TNF-alpha) is a key inflammatory cytokine and several studies linked increased TNF-alpha to dopaminergic cell death in PD. The TNF-alpha promoter sequence contains several CpG dinucleotides located within or next to transcription factor binding sites. To test the hypothesis whether the methylation state of the TNF-alpha promoter contributes to increased expression of TNF-alpha in PD we compared DNA from different brain regions (substantia nigra pars compacta (SNpc) and cortex) of PD patients and neurologically healthy, age and sex matched controls by bisulfite sequencing of the TNF-alpha promoter region. The TNF-alpha promoter DNA from SNpc was significantly less methylated in comparison to DNA from cortex; however both in PD patients and controls. Although there was a tendency for hypomethylation in PD, our analysis of the 10 CpGs in the TNF-alpha core promoter region (-258 to -35 relative to the TSS) revealed no particular pattern in PD patients compared to control and identified no particular hypomethylated position in cortex or SNpc DNA. Electrophoretic mobility shift and luciferase reporter assays showed that methylation of specific solitary CpG in the TNF-alpha promoter resulted in reduced binding of the transcription factors AP-2 and Sp1, respectively, and suppressed TNF-alpha promoter activity. The brain region specific methylation state of solitary CpG in the TNF-alpha promoter thus determines transcription factor binding efficacy and TNF-alpha expression. A lesser degree of methylation of the TNF-alpha promoter in SNpc cells could underlie the increased susceptibility of dopaminergic neurons to TNF-alpha mediated inflammatory reactions.

Ataxin-3 represses transcription via chromatin binding, interaction with histone deacetylase 3, and histone deacetylation

Ataxin-3 (AT3), the disease protein in spinocerebellar ataxia type 3 (SCA3), has been associated with the ubiquitin-proteasome system and transcriptional regulation. Here we report that normal AT3 binds to target DNA sequences in specific chromatin regions of the matrix metalloproteinase-2 (MMP-2) gene promoter and represses transcription by recruitment of the histone deacetylase 3 (HDAC3), the nuclear receptor corepressor (NCoR), and deacetylation of histones bound to the promoter. Both normal and expanded AT3 physiologically interacted with HDAC3 and NCoR in a SCA3 cell model and human pons tissue; however, normal AT3-containing protein complexes showed increased histone deacetylase activity, whereas expanded AT3-containing complexes had reduced deacetylase activity. Consistently, histone analyses revealed an increased acetylation of total histone H3 in expanded AT3-expressing cells and human SCA3 pons. Expanded AT3 lost the repressor function and displayed altered DNA/chromatin binding that was not associated with recruitment of HDAC3, NCoR, and deacetylation of the promoter, allowing aberrant MMP-2 transcription via the transcription factor GATA-2. For transcriptional repression normal AT3 cooperates with HDAC3 and requires its intact ubiquitin-interacting motifs (UIMs), whereas aberrant transcriptional activation by expanded AT3 is independent of the UIMs but requires the catalytic cysteine of the ubiquitin protease domain. These findings demonstrate that normal AT3 binds target promoter regions and represses transcription of a GATA-2-dependent target gene via formation of histone-deacetylating repressor complexes requiring its UIM-associated function. Expanded AT3 aberrantly activates transcription via its catalytic site and loses the ability to form deacetylating repressor complexes on target chromatin regions.

Gene structure and expression of the mouse APOBEC-1 complementation factor: multiple transcriptional initiation sites and a spliced variant with a premature stop translation codon

Editing of apolipoprotein (apo) B mRNA is mediated by an enzyme-complex that consists of the catalytic cytidine deaminase APOBEC-1 and the mRNA binding protein APOBEC-1 complementation factor or APOBEC-1 stimulating protein (ACF/ASP). Here we describe the detailed characterization of the structure, expression and splicing pattern of the mouse ACF/ASP gene. ACF/ASP mRNA is mainly expressed in mouse liver, small intestine and kidney. The deduced protein sequences of ACF/ASP from mouse and man share an identity of 93%. The mouse ACF/ASP gene consists of 12 exons and gives rise predominantly to full-length transcripts. To a minor extent (<10%) ACF/ASP mRNA with unspliced exon 8 is generated in liver, kidney and small intestine that encodes a truncated protein with a predicted molecular weight of 43 kDa. The promoter of the mouse ACF/ASP gene lacks a canonical TATA-box, but contains a cluster of Sp1 binding sites and uses multiple transcriptional initiation sites. Transfection studies demonstrated a preference of this promoter for cell lines derived from the gastrointestinal tract and proved the location of the promoter core region. The high sequence identity between man and mouse-much higher as observed for APOBEC-1-indicates a strong evolutionary constraint on the structure-function relationship of ACF/ASP, most probably due to a central role in editing and processing of apo B mRNA.