Thesis: Ph. D. in Neuroscience, Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, 2016.Cataloged from PDF version of thesis.Includes bibliographical references (pages 247-264).The polyglutamine diseases are the most common form of inherited neurodegenerative disorders. Each of the polyglutamine diseases stems from the same underlying cause: a CAG expansion mutation in the coding region of a gene. This gives rise to a protein with an expanded glutamine repeat stretch. Despite the fact that all polyglutamine diseases are caused by the same type of mutation, the CAG expansion in different genes gives rise to different diseases, with differentially vulnerable neuronal populations and distinct pathologies. One of the most well-known polyglutamine disorders is Huntington's disease (HD), which results from a CAG repeat expansion in the huntingtin (Htt) gene. HD is characterized by psychiatric symptoms, cognitive decline, and movement disturbances, especially chorea. Interestingly, some presumed HD patients exhibited HD-like symptoms and characteristic striatal degeneration, but did not harbor a mutation in Htt. This led to the discovery of the Huntington's disease-like (HDL) disorders. One such disorder is Huntington's disease-like 2 (HDL2). Recent studies identified a specific polyglutamine protein hypothesized to contribute to HDL2 pathology. Given the similarities between HD and HDL2 patients, I used Drosophila to model these two genetically distinct disorders to compare polyglutamine-induced toxicity. This work represents the first time HDL2 has been modeled in Drosophila, and the first characterization of HDL2 polyglutamine protein pathology. My investigation highlights many distinctions between expanded Htt and HDL2 polyglutamine proteins. Importantly, my research demonstrates that nuclear localization of the polyglutamine protein is critical to disease pathogenesis in HDL2, but not HD. I also present the results from an in vivo RNAi screen to search for novel suppressors of toxicity in our HD and HDL2 models. Analyzing top RNAi suppressors from both models indicates different pathogenic pathways are at play in these two polyglutamine diseases, but some mechanisms may be shared. We conclude that while HD and HDL2 have similar clinical profiles, distinct pathogenic mechanisms contribute to the two neurodegenerative disorders.by Megan Krench.Ph. D. in Neuroscience