The experiments proposed here are to address a fundamental question in HD;whether the sequence context of the expanded polyglutamine (polyQ) in Huntingtin (Htt) contributes significantly to the mechanisms of toxicity. In other words, to what extent is the toxicity of the polyQ expansion in mutant Htt influenced by the presence of different flanking sequences in the various fragments or full-length Htt? To address this, we will generate a new collection of transgenic C. elegans expressing various N-terminal proteolytic cleavage fragments and full-length Htt, each containing short (Q15), medium (Q40), and long (Q128) polyQ expansions. This will allow for comparison, in a single animal model system, of the biological and biochemical phenotypes of different Htt protein fragments expressed in neurons. Because our laboratory has previously established the C. elegans behavioral, cellular and genetic responses to polyQ alone, these studies should reveal to what extent cellular toxicity is influenced by contributions of the flanking Htt sequences. As toxicity undoubtedly represents the integration of multiple molecular defects, we will perform genome-wide RNAi screens to identify genetic modifiers. If the same modifiers are identified for transgenic lines expressing different length Htt fragments, we will conclude that the molecular basis for Htt toxicity is not influenced by varying sequence context. Alternatively, if distinct classes of modifiers are revealed by these genetic screens, we will conclude that the Htt flanking sequences have a strong influence on the cellular machinery. The results of this study are likely to have important implications for the analysis and interpretation of many current studies on Htt expression in other transgenic model systems. RELEVANCE TO PUBLIC HEALTH: The research proposed here is relevant to public health in that it directly examines the genetic/molecular mechanisms of polyglutamine diseases, a subset of neurodegenerative diseases of aging. By understanding the molecular underpinings of disease, we will be better poised to develop therapeutic strategies to combat disease symptoms in affected patients.