Mutant huntingtin increases nuclear corepressor function and enhances ligand-dependent nuclear hormone receptor activation.

There is increasing evidence that transcriptional dysregulation is important in Huntington's disease pathogenesis. The transcriptional protein, nuclear corepressor (NCoR), acts to repress transcription of nuclear hormone receptors, such as the thyroid hormone receptor (TR) and retinoic acid receptor, in the absence of their appropriate ligand. NCoR has been shown to bind to the mutated huntingtin protein in a yeast two-hybrid screen. This aberrant interaction may have profound effects on both the function of the NCoR protein and on its control of nuclear hormone receptor-mediated transcription. To test this hypothesis, reporter gene assays were performed in inducible PC12 cell lines expressing exon 1 of the human huntingtin protein (Htt) with either a 25 or 103 polyglutamine (Q) repeat. Expression of mutant 103Q protein appears to enhance the ability of NCoR to repress TR-mediated transcription in the absence of ligand. Western analyses indicated that the expression of the mutant 103Q Htt protein did not change the endogenous NCoR levels in the HD103Q PC12 cells when compared to uninduced cells. Interestingly, using GST pull-down assays we found that a mutant Htt exon 1 construct with 53 polyglutamine (HD53Q) did not bind to NCoR in a polyglutamine-dependent fashion. These findings suggest that an aberrant NCoR-Htt interaction does not exist in vitro. Expression of the mutant 103Q protein was also found to enhance ligand-dependent activation of TR and retinoic acid receptor. In vitro binding data shows that TR binds to HD53Q in the presence of ligand. Taken together these data suggest that Htt may function as a transcriptional coactivator of nuclear hormone receptors.