iPS Cells for Investigation of HDL2 and HD Pathogenesis

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Huntington's disease (HD) is an autosomal dominant neurodegenerative disease, characterized by abnormalities of movement, cognition and emotion, with relentless progression until death ~20 years after disease onset. HD is caused by expanded CAG repeats in exon 1 of the Huntingtin (HTT) gene. Substantial advances have been made into understanding the neurobiology of HD. Nonetheless, as of yet no treatment exists that stops or substantially slows disease progression, and it has proven difficult to prioritize among the >100 proposed pathogenic mechanisms to focus on those most likely to lead to therapeutic advances. HDL2, discovered and genetically defined by the Margolis group, is a rare, autosomal dominant neurodegenerative disorder, clinically and neuropathologically very similar from HD. Like HD, the neuropathology of HDL2 is characterized by cortical and striatal neurodegeneration and the presence of neuronal protein aggregates. HDL2 is caused by a CTG/CAG expansion on chromosome 16q24. Normal alleles contain 6-28 triplets, while pathogenic repeats range from 40-59 triplets, again remarkably similar to HD. In the CTG orientation, the repeat falls in the gene junctophilin-3 (JPH3). We have hypothesized that the HDL2 mutation leads to neurodegeneration via a combination of loss of JPH3 expression, toxicity of the sense strand transcript containing an expanded CUG repeat, and expression of polyglutamine from a cryptic gene on the antisense strand. The relative contribution and interactions of these mechanisms remains unknown, and modeling HDL2 has proven challenging. We now propose (Aim 1) to generate and characterize induced pluripotent cells from fibroblasts of individuals with HDL2. The pluripotency of the cells will be systematically investigated. In aim 2, we will then differentiate the iPS cells into neurons, including a subpopulation with a striatal phenotype. Cells will be characterized with neuronal and striatal-specific markers to determine the differentiation pattern, and we will determine the extent to which these cells recapitulate findings observed in HDL2 brain and model systems. We will determine the survival, electrophysiological profiles, vulnerability to glutamate toxicity, and vulnerability to BDNF withdrawal of these cells compared to controls. The public health implications of developing HDL2 iPS cells as a tool for studying HDL2 are several fold: improved understanding of HDL2 itself, new insights into fundamental pathogenic processes relevant to other repeat expansion diseases, and the opportunity to find pathogenic points of convergence between HD and HDL2 that will lead to a focus on therapeutic targets of most promise for both diseases.