Spinocerebellar ataxia type 12 (SCA12) is a progressive, autosomal dominant, neurodegenerative disorder caused by an expansion of a CAG/CTG trinucleotide repeat on chromosome 5q32; both the disease phenotype and the causative mutation were initially described by our group (Holmes et al, 1999). While the disease is one of most common forms of SCA in India, and scattered SCA12 pedigrees have been detected around the world, perhaps the most intriguing aspect of SCA12 is that the repeat falls in a putative promoter of PPP2R2B, a gene encoding regulatory subunits of the trimeric enzyme phosphatase 2A (PP2A). Functional PP2A consists of a structural unit, one of two catalytic units, and one of ~30 regulatory subunits, with the N-terminal region of the regulatory subunits serving to target the holoenzyme to specific intracellular sites. Dysregulation of PP2A has been directly linked to tau hyperphosphorylation in Alzheimer's disease, and to multiple other neurodegenerative diseases. We hypothesize, based on preliminary data from cell overexpression models, that the SCA12 repeat expansion leads to increased expression of PPP2R2B isoform B1, and that this overexpression leads to dysregulation of PP2A activity and neurotoxicity. However, it has not been possible to confirm these observations, as human SCA12 brain material is not available and PPP2R2B Is not expressed in leukocytes or lymphoblasts. To test our hypothesis, we will use fibroblasts from skin biopsies of patients with SCA12 to generate induced pluripotent stem cells (iPSCs)(Aim 1). We will then determine the effect of the mutation on PPP2R2B expression and other cellular properties in the fibroblasts and in the IPSCs differentiated into forebrain neurons (Aim 2). The potential public health benefits of this project are three fold: 1) a better understanding of how repetitive DNA can influence gene expression, 2) a better understanding of SCA12 pathogenesis, with the potential of detecting targets for therapeutic agents, and 3) new insight into the role of PP2A in the pathogenesis of neurodegenerative disease.
Spinocerebellar ataxia type 12 (SCA12) is a disabling, hereditary, neurodegenerative disorder caused by a repeat expansion mutation in the gene PPP2R2B. We propose to generate and characterize induced pluripotent stem cells from skin samples of patients with SCA12; we will use the cells to better understand the pathogenesis of SCA12, and potentially to shed light on more common neurodegenerative disorders, including Alzheimer's disease.
