PROJECT SUMMARY
Spinocerebellar ataxia type 12 (SCA12) is an autosomal dominant neurodegenerative disease,
discovered and characterized by our group, notable clinically for tremor, gait abnormalities, and neuropsychiatric
syndromes, and neuropathologically for atrophy in cerebral cortex and cerebellum. SCA12 is caused by a
CAG/CTG expansion mutation in PPP2R2B, a gene encoding brain-specific regulatory units of protein
phosphatase 2A (PP2A). Normal alleles carry 4 to 31 triplets, whereas disease alleles carry 43 to 78 triplets.
SCA12 is the second most common ataxia in North India, with cases of SCA12 detected in the United States,
Italy, and China. There is no treatment. Our preliminary data indicates that PPP2R2B contains at least 16 exons
with multiple alternative splice variants, resulting in at least 8 protein isoforms (termed B¿1-B¿8) with different N
terminal domains, among which Bß1 and Bß2 are by far the most abundant and best characterized. The N-terminal domains contribute to the localization of the PP2A holoenzyme, and hence their relative abundance
may be critical in establishing patterns of protein phosphorylation. The repeat is located in the 5’ region of exon
7 in a predicted promoter region, but is also contained within the transcripts of alternative splice variants. Based
on extensive preliminary data, we hypothesize that the CAG repeat expansion in SCA12 alters PPP2R2B
expression and splicing, leading to changes in PP2A targeting that contribute to SCA12 pathogenesis by
alteration of phosphoproteome. Secondarily, we hypothesize that missplicing induced by repeat expansion
generates a transcript encoding a toxic polyserine tract that contributes to SCA12 pathogenesis. Our aims are
each designed to explore a specific aspect of SCA12 pathogenesis, and simultaneously to explore more general
principles of neurodegeneration related to repeat expansion and the function of PP2A, using genetically
engineered mice and human iPSCs. In Aim 1, we will determine the effect of the SCA12 mutation on PPP2R2B
expression. In Aim 2, we will determine the effect of abnormal PPP2R2B expression on PP2A substrate targeting
and the phosphoproteome. In Aim 3, we will determine how changes in PP2A substrate targets and aberrantly
expressed polyserine contribute to neurotoxicity. If our hypothesis is correct, SCA12 pathogenesis derives from
repeat-expansion-induced changes in PPP2R2B expression that 1) alters PP2A substrates targets and hence
the phosphoproteome, and 2) generates toxic polyserine tracts. Establishing these pathways will establish
multiple potential targets for therapeutic intervention in SCA12. At the same time, our experiments will more
generally establish the potential effect of repeat expansion on promoter and splicing function, and the role of
PP2A dysregulation in neurodegeneration, of particular interest given evidence of PP2A dysregulation in other
neurodegenerative disorders, such as Alzheimer’s disease.