Ataxin-2 (ATXN2) is a ubiquitously expressed RNA-binding protein (RBP) conserved across eukaryotic species from yeast to human. Trinucleotide (CAG) repeat expansion mutations of the poly-glutamine (polyQ) tract of the ATXN2 gene are associated with several neurodegenerative diseases including spinocerebellar ataxia 2 (SCA2) and amyotrophic lateral sclerosis (ALS), a fatal adult-onset disorder characterized by the progressive death of motor neurons of the brain and spinal cord. In vivo models recapitulate this link to motor neuron degeneration and ALS, as ataxin-2 is reported to be a toxic modifier of TDP-43, a protein most commonly associated with both the familial and sporadic forms of ALS. Studies have supported this toxic gain-of-function model, as reduction of ataxin-2 levels seems to serve a protective role against neuron degeneration while simultaneously prolonging survival in Atxn2-null mice in the presence of TDP-43 overexpression. At the same time, endogenous neural- specific functions of ataxin-2 are still poorly understood, as are its direct role in mediating ALS pathogenesis. This project aims to uncover the transcriptome-wide regulatory role of ataxin-2 in the central nervous system in order to mechanistically characterize the interaction between ATXN2 mutations and motor neuron death. This proposal seeks to use molecular and genomic techniques, including eCLIP-seq, RNA-seq and ribosome profiling, to map the physical and functional ?interactome? of ataxin-2 in mouse brain and spinal cord, as well as in human iPSC-differentiated motor neurons. To address the connection to motor neuron disease, this proposal also aims to generate an isogenic iPSC model using CRISPR/Cas9 technologies by knocking-in ALS-associated polyQ expansion sequences into the ATXN2 genomic locus. These cell lines will then be differentiated into mature motor neurons in order to study neural-specific transcriptional and translational perturbations introduced through repeat expansions. This study will directly characterize the role of ataxin-2 in neuron-specific RNA metabolism, as well as identify specific target genes disrupted with polyQ mutations. If successful this research will define the underlying molecular mechanisms linking ataxin-2 to ALS risk, identify novel disease-relevant pathways that potentially serve as targetable avenues for therapy, and provide a broadly applicable disease modeling strategy to investigate the direct genetic influence of other repeat expansion mutations in future work.
Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease in the adult population that manifests through progressive muscle atrophy, loss of voluntary motor activity, and eventual death in those afflicted. Although poly-glutamine repeat expansions in the ATXN2 gene significantly overlap with ALS risk, the direct molecular contribution of ataxin-2 in causing motor neuron degeneration remains largely unknown. This study aims to characterize the regulatory roles of ataxin-2 in mediating neural-specific RNA metabolism and neuronal physiology, and will strive to use relevant disease modeling strategies to uncover causative pathways and potential therapeutic targets in ataxin-2-associated ALS pathogenesis.
