RNA Interference-Based Therapy for Spinocerebellar Ataxia Type 7 Retinal Degeneration

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant neurodegenerative disease characterized by loss of motor coordination and retinal degeneration with no current therapies in the clinic. The causative mutation is an expanded CAG repeat in the ataxin-7 gene whose mutant protein product causes cerebellar and brainstem degeneration and retinal cone-rod dystrophy. Here, we reduced the expression of both mutant and wildtype ataxin-7 in the SCA7 mouse retina by RNA interference and evaluated retinal function 23 weeks post injection. We observed a preservation of normal retinal function and no adverse toxicity with ≥50% reduction of mutant and wildtype ataxin-7 alleles. These studies address an important safety concern regarding non-allele specific silencing of ataxin-7 for SCA7 retinal therapy.     

Progress in pathogenesis studies of spinocerebellar ataxia type 1.

Spinocerebellar ataxia type 1 (SCA1) is a dominantly inherited disorder characterized by progressive loss of coordination, motor impairment and the degeneration of cerebellar Purkinje cells, spinocerebellar tracts and brainstem nuclei. Many dominantly inherited neurodegenerative diseases share the mutational basis of SCA1: the expansion of a translated CAG repeat coding for glutamine. Mice lacking ataxin-1 display learning deficits and altered hippocampal synaptic plasticity but none of the abnormalities seen in human SCA1; mice expressing ataxin-1 with an expanded CAG tract (82 glutamine residues), however, develop Purkinje cell pathology and ataxia. These results suggest that mutant ataxin-1 gains a novel function that leads to neuronal degeneration. This novel function might involve aberrant interaction(s) with cell-specific protein(s), which in turn might explain the selective neuronal pathology. Mutant ataxin-1 interacts preferentially with a leucine-rich acidic nuclear protein that is abundantly expressed in cerebellar Purkinje cells and other brain regions affected in SCA1. Immunolocalization studies in affected neurons of patients and SCA1 transgenic mice showed that mutant ataxin-1 localizes to a single, ubiquitin-positive nuclear inclusion (NI) that alters the distribution of the proteasome and certain chaperones. Further analysis of NIs in transfected HeLa cells established that the proteasome and chaperone proteins co-localize with ataxin-1 aggregates. Moreover, overexpression of the chaperone HDJ-2/HSDJ in HeLa cells decreased ataxin-1 aggregation, suggesting that protein misfolding might underlie NI formation. To assess the importance of the nuclear localization of ataxin-1 and its role in SCA1 pathogenesis, two lines of transgenic mice were generated. In the first line, the nuclear localization signal was mutated so that full-length mutant ataxin-1 would remain in the cytoplasm; mice from this line did not develop any ataxia or pathology. This suggests that mutant ataxin-1 is pathogenic only in the nucleus. To assess the role of the aggregates, transgenic mice were generated with mutant ataxin-1 without the self-association domain (SAD) essential for aggregate formation. These mice developed ataxia and Purkinje cell abnormalities similar to those seen in SCA1 transgenic mice carrying full-length mutant ataxin-1, but lacked NIs. The nuclear milieu is thus a critical factor in SCA1 pathogenesis, but large NIs are not needed to initiate pathogenesis. They might instead be downstream of the primary pathogenic steps. Given the accumulated evidence, we propose the following model for SCA1 pathogenesis: expansion of the polyglutamine tract alters the conformation of ataxin-1, causing it to misfold. This in turn leads to aberrant protein interactions. Cell specificity is determined by the cell-specific proteins interacting with ataxin-1. Submicroscopic protein aggregation might occur because of protein misfolding, and those aggregates become detectable as NIs as the disease advances. Proteasome redistribution to the NI might contribute to disease progression by disturbing proteolysis and subsequent vital cellular functions.     

Polyglutamine-expanded ataxin-7 activates mitochondrial apoptotic pathway of cerebellar neurons by upregulating Bax and downregulating Bcl-x(L)

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant neurodegenerative disorder caused by polyglutamine-expanded ataxin-7. In the present investigation, we expressed disease-causing mutant ataxin-7-Q75 in the primary neuronal culture of cerebellum with the aid of recombinant adenoviruses. Subsequently, this in vitro cellular model of SCA7 was used to study the molecular mechanism by which mutant ataxin-7-Q75 induces neuronal death. TUNEL staining studies indicated that polyglutamine-expanded ataxin-7-Q75 caused apoptotic cell death of cultured cerebellar neurons. Mutant ataxin-7-Q75 induced the formation of active caspase-3 and caspase-9 without activating caspase-8. Polyglutamine-expanded ataxin-7-Q75 promoted the release of apoptogenic cytochrome-c and Smac from mitochondria, which was preceded by the downregulation of Bcl-x(L) protein and upregulation of Bax protein expression in cultured cerebellar neurons. Further real-time TaqMan RT-PCR assays showed that mutant ataxin-7-Q75 upregulated Bax mRNA level and downregulated Bcl-x(L) mRNA expression in the primary neuronal culture of cerebellum. The present study provides the evidence that polyglutamine-expanded ataxin-7-Q75 activates mitochondria-mediated apoptotic cascade and induces neuronal death by upregulating Bax expression and downregulating Bcl-x(L) expression of cerebellar neurons.     

A computerized database-scan to identify c-MYC targets

Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disease caused by the expansion of a polyglutamine tract in the protein ataxin-7, a protein of unknown function. In order to analyze the expression pattern of wild type ataxin-7 in detail, the murine SCA7 gene homolog was cloned and the expression pattern in mice analyzed. The SCA7 mouse and human gene exhibit a high degree of identity at both DNA (88.2%) and protein (88.7%) level. The CAG repeat region, known to be polymorphic in man, is conserved in mouse but contained only five repeats in all mouse strains analyzed. The arrestin homology domain and the nuclear localization signal found in human ataxin-7 is also conserved in the murine homolog. Expression of ataxin-7 was detected during mouse embryonic development and in all adult mouse tissues examined by northern and western blots. In brain, immunohistological staining revealed an ataxin-7 expression pattern similar to that in human, with ataxin-7 expression in cerebellum, several brainstem nuclei, cerebral cortex and hippocampus. Our data show high conservation of ataxin-7 both structurally and at the level of expression, suggesting a conserved role for the protein in mice and humans.     

A Drosophila homolog of the polyglutamine disease gene SCA2 is a dosage-sensitive regulator of actin filament formation

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disorder caused by the expansion of a CAG repeat encoding a polyglutamine tract in ataxin-2, the SCA2 gene product. The normal cellular function of ataxin-2 and the mechanism by which polyglutamine expansion of ataxin-2 causes neurodegeneration remain unknown. In this study we have used genetic and molecular approaches to investigate the function of a Drosophila homolog of the SCA2 gene (Datx2). Like human ataxin-2, Datx2 is found throughout development in a variety of tissue types and localizes to the cytoplasm. Mutations that reduce Datx2 activity or transgenic overexpression of Datx2 result in female sterility, aberrant sensory bristle morphology, loss or degeneration of tissues, and lethality. These phenotypes appear to result from actin filament formation defects occurring downstream of actin synthesis. Further studies demonstrate that Datx2 does not assemble with actin filaments, suggesting that the role of Datx2 in actin filament formation is indirect. These results indicate that Datx2 is a dosage-sensitive regulator of actin filament formation. Given that loss of cytoskeleton-dependent dendritic structure defines an early event in SCA2 pathogenesis, our findings suggest the possibility that dysregulation of actin cytoskeletal structure resulting from altered ataxin-2 activity is responsible for neurodegeneration in SCA2.     

Histone deacetylase-3 interacts with ataxin-7 and is altered in a spinocerebellar ataxia type 7 mouse model

Spinocerebellar ataxia type 7 (SCA7) is caused by a toxic polyglutamine (polyQ) expansion in the N-terminus of the protein ataxin-7. Ataxin-7 has a known function in the histone acetylase complex, Spt/Ada/Gcn5 acetylase (STAGA) chromatin-remodeling complex. We hypothesized that some histone deacetylase (HDAC) family members would impact the posttranslational modification of normal and expanded ataxin-7 and possibly modulate ataxin-7 function or neurotoxicity associated with the polyQ expansion. Interestingly, when we coexpressed each HDAC family member in the presence of ataxin-7 we found that HDAC3 increased the posttranslational modification of normal and expanded ataxin-7. Specifically, HDAC3 stabilized ataxin-7 and increased modification of the protein. Further, HDAC3 physically interacts with ataxin-7. The physical interaction of HDAC3 with normal and polyQ-expanded ataxin-7 affects the toxicity in a polyQ-dependent manner. We detect robust HDAC3 expression in neurons and glia in the cerebellum and an increase in the levels of HDAC3 in SCA7 mice. Consistent with this we found altered lysine acetylation levels and deacetylase activity in the brains of SCA7 transgenic mice. This study implicates HDAC3 and ataxin-7 interaction as a target for therapeutic intervention in SCA7, adding to a growing list of neurodegenerative diseases that may be treated by HDAC inhibitors.     

p80 coilin,a coiled body-specific protein,interacts with ataxin-1, the SCA1 gene product

Spinocerebellar ataxia type 1 (SCA1) is an autosomal-dominant neurodegenerative disorder characterized by ataxia and progressive motor deterioration. SCA1 is associated with an elongated polyglutamine tract in ataxin-1, the SCA1 gene product. Using the yeast two-hybrid system and co-immunoprecipitation experiments, we have found that p80 coilin, coiled body-specific protein, binds to ataxin-1. In further experiments with deletion mutants, we found that the C-terminal regions of ataxin-1 and p80 coilin were essential for this interaction. In HeLa cells that have been co-transfected with ataxin-1 and p80 coilin, the p80 coilin protein co-localizes with ataxin-1 aggregates in the nucleoplasm. However, immunohistochemical analysis and immunofluorescence assays showed that mutant ataxin-1 aggregates do not redistribute p80 coilin's dot-like structures in the Purkinje cells of SCA1 transgenic mice. This feature of the interaction between ataxin-1 and p80 coilin suggests that p80 coilin might be implicated in altering the function of ataxin-1.     

脊髓小脑共济失调第7型的临床特征及基因突变研究

对一脊髓小脑性共济失调(Spinocerebellar ataxia, SCA)家系的患者进行临床特征及相关基因突变研究。对该家系进行详细的病史采集, 并对患者行视力、眼底血管造影、眼底拍照、视觉诱发电位、视网膜电图以及头颅MRI等辅助检查; 采用聚合酶链反应分别扩增SCA1、SCA2、SCA3、SCA6、SCA7、SCA17及DRPLA基因的CAG重复序列, 用8%变性聚丙烯酰胺凝胶电泳及直接测序进行突变分析。结果2名患者主要表现为小脑性共济失调、视力下降、眼底视网膜色素变性、小脑和脑干萎缩; 并存在SCA7基因的突变, 而未发现SCA1、SCA2、SCA3、SCA6、SCA17及DRPLA基因突变。说明该家系为SCA7突变家系, SCA7基因中CAG三核苷酸重复拷贝数的异常扩增是其致病原因。     

Ataxin-10, the spinocerebellar ataxia type 10 neurodegenerative disorder protein, is essential for survival of cerebellar neurons

Spinocerebellar ataxia (SCA) type 10, an autosomal dominant disease characterized by cerebellar ataxia, is caused by a novel pentanucleotide (ATTCT) repeat expansion in the SCA10 gene. Although clinical features of the disease are well characterized, nothing is known so far about the affected SCA10 gene product, ataxin-10 (Atx-10). We have cloned the rat SCA10 gene and expressed the corresponding protein in HEK293 cells. Atx-10 has an apparent molecular mass of approximately 55 kDa and belongs to the family of armadillo repeat proteins. In solution, it tends to form homotrimeric complexes, which associate via a tip-to-tip contact with the concave sides of the molecules facing each other. Atx-10 immunostaining of mouse and human brain sections revealed a predominantly cytoplasmic and perinuclear localization with a clear restriction to olivocerebellar regions. Knock down of SCA10 in primary neuronal cells by small interfering RNAs resulted in an increased apoptosis of cerebellar neurons, arguing for a loss-of-function phenotype in SCA10 patients.     

Structural and functional analysis of ataxin-2 and ataxin-3.

Spinocerebellar ataxia types 2 (SCA2) and 3 (SCA3) are autosomal-dominantly inherited, neurodegenerative diseases caused by CAG repeat expansions in the coding regions of the genes encoding ataxin-2 and ataxin-3, respectively. To provide a rationale for further functional experiments, we explored the protein architectures of ataxin-2 and ataxin-3. Using structure-based multiple sequence alignments of homologous proteins, we investigated domains, sequence motifs, and interaction partners. Our analyses focused on presumably functional amino acids and the construction of tertiary structure models of the RNA-binding Lsm domain of ataxin-2 and the deubiquitinating Josephin domain of ataxin-3. We also speculate about distant evolutionary relationships of ubiquitin-binding UIM, GAT, UBA and CUE domains and helical ANTH and UBX domain extensions.