At least eight hereditary neurodegenerative disorders, including Huntington's disease, are caused by an expanded glutamine tract. We previously established a model system for studying polyglutamine (polyQ) neurotoxicity in C. elegans (PNAS 96, 179-184, 1999). The N-terminus of the mutant human huntingtin protein, which contains an expanded glutamine tract, is expressed in the well-characterized ASH neurons. Expression of the expanded huntingtin polyglutamine tract leads to age-dependent degeneration of the ASH neurons. To identify genes that modulate polyQ toxicity, we are performing an RNA interference (RNAi) screen in the sensitized
rrf-3 (
pk1426 ) strain using a library provided by the Marc Vidal laboratory. We first validated our system by using a previously identified genetic enhancer of polyQ toxicity,
pqe-1 (PNAS 99, 17131-6, 2002). The vast majority of ASH neurons expressing expanded huntingtin fragments in
pqe-1(
rt13) are dead in young animals. In young
rrf-3 mutant animals expressing huntingtin fragments, feeding dsRNA of
pqe-1 causes 50% of ASH neurons to degenerate compared to 0% in the feeding control. In aged
rrf-3 mutant animals expressing expanded huntingtin fragments, 80% of ASH neurons are affected compared to 50% of ASH neurons in the control experiment. We are focusing on RNAi of those genes whose decrease in function results in sterility or reduced viability, since these candidates may have been missed in the original genetic screen for polyQ enhancers (PNAS 99, 17131-6, 2002). Candidate genes that enhance neurodegeneration to a similar level as feeding dsRNA of
pqe-1 (80% ASH neurodegeneration) are being retested. Several interesting candidate genes have emerged. Reducing the expression of the molecular chaperone HSP40 (F39B2.10) enhances ASH neurodegeneration. Protein misfolding has been previously implicated in polyQ neurotoxicity in several systems, validating our experimental approach. RNAi of the B-chain of a sodium potassium transporting ATPase (C17E4.9) also enhances polyQ neurotoxicity. In a complementary approach, we designed an assay system to test the effect of pharmacological agents on polyQ neurotoxicity in C. elegans (collaboration with Hemant Varma, Stockwell lab, Whitehead Institute). Since our RNAi screen revealed C17E4.9 as an enhancer of polyQ toxicity, we are initially focusing the pharmacological tests on drugs that specifically inhibit the activity of the sodium potassium transporting ATPase. Identification and characterization of genes isolated from the RNAi screen will provide insight into pathogenic mechanisms underlying polyQ-induced neurodegeneration.