Spinocerebellar ataxia type 7 (SCA7) is an inherited neurological disorder characterized by cerebellar and retinal degeneration. SCA7 is caused by CAG/polyglutamine (polyQ) repeat expansions in the ataxin-7 gene. We set out to determine the molecular basis of SCA7 neurodegeneration by focusing on polyQ-ataxin-7 transcriptional and epigenetic dysregulation, as ataxin-7 is a core component of the transcription co-activator complex STAGA, which possesses histone acetyltransferase and histone deubiquitinase activity. By pursuing unbiased transcriptome analysis, we discovered that SCA7 involves impaired regulation of Ca++ homeostasis. Transcription factor binding site analysis of SCA7 down-regulated genes revealed putative binding sites for peroxisome proliferator-activated receptors, which are known pathway targets of Sirtuin-1 (Sirt1), a NAD+- dependent deacetylase implicated in lifespan extension and neuroprotection, whose function was impaired in SCA7. Based upon these results, we crossed Sirt1 over-expressing mice with PrP-fxSCA7 92Q BAC mice, and with SCA7 266Q knock-in mice. We observed amelioration of cerebellar and retinal degeneration, and rescue of Purkinje cell synaptic dysfunction in Sirt1-SCA7 bigenic mice. We documented abnormalities in bioenergetics and organelle quality control in SCA7 mice and neuronal progenitor cells (NPCs) from patient stem cells. Using mass spectrometry, we detected significant reductions in NAD+ in SCA7 brain, accompanied by activation of poly-ADP ribose polymerase 1 (PARP1), which led us to assay for DNA damage, and we noted increased gH2AX levels in neurons from SCA7 mice. As USP22 deubiquitinase function promotes DNA repair, and the ataxin-7 homologue ataxin-7-like 3 (Atxn7-L3) promotes deubiquitinase module function, we examined Atxn7-L3, and found that Atxn7-L3 interacts with PARP1, and Atxn7-L3 over-expression can rescue impaired histone H2B deubiquitination. These results, together with unbiased ChIP-Seq analysis of H3K9/14 acetylation in SCA7 mice, support a model in which epigenetic dysregulation yields increases DNA damage, PARP1 hyperactivation, and excess NAD+ utilization to promote SCA7 disease pathogenesis. As genetic defects in DNA repair cause inherited cerebellar ataxias, and recent studies implicate DNA damage and PARP1 activation, excessive NAD+ utilization may commonly occur in ataxias and retinal disease, offering an opportunity for rationale therapy development. In this project, we will define the bioenergetics defects, mitochondrial dysfunction, and Ca++ flux phenotypes in SCA7 to determine the basis of Sirt1 dysfunction and contribution of NAD+ depletion to SCA7 pathogenesis. We will assess DNA damage and PARP activation in SCA7, and consider the role of Atxn7-L3 and USP22 in epigenetic alterations linked to DNA damage in SCA7. Finally, we will test if modulation of Atxn7-L3, USP22, or PARP1 can rescue DNA damage and molecular pathology in SCA7 cell models, NPC patient cells, and mice, and will determine if treatments aimed at modulating these pathogenic processes will be effective in related cerebellar ataxias and retinal degenerations.
We have sought the molecular basis of spinocerebellar ataxia type 7 (SCA7) neurodegeneration by focusing on polyglutamine-expanded ataxin-7 transcription and epigenetic dysregulation, and after discovering that SCA7 involves impaired regulation of Ca++ homeostasis, we performed unbiased transcription factor binding site analysis, which indicated that Sirtuin-1 dysfunction promotes SCA7 pathogenesis, a hypothesis that we validated by documenting a remarkable rescue of SCA7 disease phenotypes in two different SCA7 mouse models crossed with Sirt1 over-expressing mice. Abnormalities of bioenergetics and organelle quality control in SCA7 mice and neuronal progenitor cells from patient stem cells then led us to consider a role for NAD+ depletion driven by poly-ADP ribose polymerase 1 (PARP1) hyperactivation in response to increased DNA damage and impaired DNA repair, which may result from SCA7 epigenetic dysregulation. We now wish to continue this line of investigation, and propose a series of experiments intended to determine if epigenetic dysregulation, mitochondrial dysfunction, DNA damage, PARP1 hyperactivation, or excess NAD+ utilization promote SCA7 disease pathogenesis, and we plan to evaluate genetic and pharmacological modulation of targets lying on these pathways as possible therapies for SCA7, and will also test the hypothesis that shared pathogenic processes, occurring in SCA7 and related, inherited cerebellar or retinal degenerations, offer an opportunity to develop treatments with potential for broad application.
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