| dc.description | Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a mutation in the gene that encodes huntingtin. Although mutant huntingtin is widely expressed throughout the brain and body, the most pronounced neuropathology of HD is of the eventual loss of GABAergic medium spiny neurons in the caudate and putamen. Prior to cell death, intranuclear mutant huntingtin decreases mRNA levels of a small percentage of genes expressed in the caudate and putamen of HD patients and in the striatum of several different models of transgenic HD mice. The mechanism of this mutant huntingtin-induced decrease in selected mRNAs has not yet been defined. It has been hypothesized that mutant huntingtin interacts with transcription factors, sequesters these proteins from active transcriptional complexes thereby leading to the observed decrease in steady-state mRNA levels. The goal of this research was to study the effect of the N-terminal fragment of mutant huntingtin on the expression of the dopamine- and cAMP-regulated phosphoprotein (DAR-PP-32) and preproenkephalin (ppENK) genes to define the mechanism by which mutant huntingtin down-regulates steady-state mRNA levels of specific genes. The N-terminal fragment of mutant huntingtin lowered steady-state levels of DAR-PP-32 and ppENK mRNA in the brains of R6 transgenic HD mice to ∼ 40-50% of the levels observed in young R6 and wild-type mice. Steady-state levels of DARPP-32 mRNA were not affected in the kidneys of these mice even though the N-terminal fragment of mutant huntingtin was also expressed in the kidney. The N-terminal fragment of mutant huntingtin altered transcription from all start sites of the proximal DARPP-32 and the ppENK promoters. The activity of DARPP-32 and ppENK promoter deletion constructs was lower in the presence of the N-terminal fragment of mutant huntingtin in immortalized striatal cell lines but no difference in transcription factor binding to these promoters were detected. Transient transfection experiments demonstrated that short-term expression of the N-terminal fragment of mutant huntingtin exerted cell- and promoter-specific transcriptional repression of the DARPP-32, ppENK and cytomegalovirus (CMV) promoters. The effects of the N-terminal fragment of mutant huntingtin on transcription were also observed in vitro in the presence of purified N-terminal fragment of mutant huntingtin and nuclear proteins isolated from R6 transgenic HD mice. The presence of the amino terminus of huntingtin protein with an expanded polyglutamine repeat in combination with factors present in the wild-type forebrain nuclear extracts decreased the stability of the proteins that interacted with the CMV promoter. Dysregulation of transcription is an early step in HD pathogenesis that likely causes a cascade of changes throughout the brain, neuronal dysfunction and eventually death of susceptible neurons. The N-terminal fragment of mutant huntingtin in concert with tissue- and promoter-specific neuronal factors induce gene-specific transcriptional dysregulation. The N-terminal fragment of mutant huntingtin decreased transcription, not by sequestering proteins away from the promoter, but by directly interacting with the proteins that form the preinitiation complex, altering the stability of the preinitiation complex, which reduced the rate of transcription. | en_US |
