Huntington's Disease (HD) is one of 8 dominant human neurodegenerative disorders caused by expansion of an unstable CAG-trinucleotide repeat encoding polyglutamine (polyQ). How expanded polyQ stretches cause disease is unknown. We generated a C. elegans model for polyQ-mediated cellular dysfunction by expressing fragments of huntingtin, the protein product of the HD locus, containing polyQ stretches of 2, 23 (control) or 95 and 150 (toxic) residues in the ASH sensory neuron. Expression of Htn-Q150, but not Htn-Q2, Htn-Q23 or Htn-Q95 causes an ASH dye-filling defect, but no ASH cell death, in 8-day-old animals. Despite the absence of cell death this dye-filling defect is dependent on
ced-3 function, suggesting that Htn-Q150 activates the apoptotic cell death pathway. Htn-Q150 coexpression with subthreshold levels of a second toxic (but non-apoptotic!) transgene, an OSM-10::GFP fusion protein, causes enhanced dye-filling defects and cell death in 3- and 8-day-old animals. This cell death depends on
ced-3 function, supporting the notion that Htn-Q150 activates the apoptotic cell death pathway. Htn-Q150 and OSM-10::GFP coexpression (but neither alone) leads to a strong defect in nose touch response (Faber et al., 1999 PNAS 96, 179-184). Our current efforts include 1) Targeting Htn-Q150 expression to other cell types to create phenotypes more amenable to high throughput screens. 2) Screening (10,000 animals to date) for recessive enhancers of the Htn-Q150 mediated ASH dye-filling defect in 3d animals (unenhanced
rt13 ) and one integrated array ( rtIs12 ) that maintain their enhancing potential in 4x backcrossed (4x) animals. Another putative enhancer has just been isolated.
rt13 enhancement is array dependent; 79% (n = 160) and 1% (n = 150) of
rt13 (4x) ASH neurons are dye-filling defective at 3d in the presence or absence of an Htn-Q150 array, respectively. Importantly, two new Htn-Q150 arrays generated in
rt13 (4x) animals by microinjection reproduce the ASH dye-filling defect of 73% (n = 162). Further analysis of
rt13 is underway.