Brain-specific factors in combination with mutant huntingtin induce gene-specific transcriptional dysregulation.

Mutant huntingtin lowered steady-state levels of DARPP-32 mRNA in the brain but not kidney of R6 transgenic HD mice by repressing transcription from one of two promoters. The activity of DARPP-32 promoter deletion constructs were lower in the presence of mutant huntingtin in immortalized striatal cell lines but no difference in transcription factor binding to the promoter was detected. The activity of CMV, TK and HPRT promoters was also affected by mutant huntingtin in these cell lines. Transient transfection experiments demonstrated that short-term expression of mutant huntingtin exerted a cell- and promoter-specific transcriptional repression. In in vitro experiments, transcription of the CMV promoter was reduced in the presence of striatal proteins and mutant huntingtin. It is likely that select combinations of trans-acting factors, co-activators and components of the Pol II holoenzyme acting in concert provide the basis for both the gene- and tissue-specific effects of mutant huntingtin.

Structure, expression and regulation of the cannabinoid receptor gene (CB1) in Huntington's disease transgenic mice.

Loss of cannabinoid receptors (CB1) occurs prior to neurodegeneration in Huntington's disease (HD). The levels and distribution of CB1 RNA were equivalent in 3-week-old mice regardless of genotype demonstrating that the specific factors and appropriate chromatin structure that lead to the transcription of CB1 were present in the striatum of young R6/2 and R6/1 transgenic HD mice. The expression of the mutant HD transgene led progressively to decreased steady-state levels of CB1 mRNA in neurons of the lateral striatum, which was dependent on the size of the CAG repeat and relative expression of the gene encoding mutant huntingtin (HD). Although it is known that the coding region of CB1 is contained within a single exon in mice, rats and humans, the 5'-untranslated region of the mouse gene remained to be defined. CB1 mRNA is encoded by two exons separated by an 18.4-kb intron. Transcription of CB1 occurred at multiple sites within a GC-rich promoter region upstream of exon 1 encoding the 5'-UTR of CB1. There was no difference in the selection of specific transcription initiation sites associated with higher levels of CB1 expression in the striatum compared to the cortex or between the striata of wild-type and HD transgenic mice. The progressive decline in CB1 mRNA levels in R6 compared to wild-type mice was due to decreased transcription, which is consistent with the hypothesis that mutant huntingtin exerts its effects by altering transcription factor activity. The cell-specific conditions that allow for increased transcription of CB1 in the lateral striatum compared to other forebrain regions from all transcription start sites were affected by the expression of mutant huntingtin in a time-dependent manner.

Mutant huntingtin affects the rate of transcription of striatum-specific isoforms of phosphodiesterase 10A.

Huntington's disease (HD) is caused by the inheritance of a copy of the gene encoding mutant huntingtin with an expanded CAG repeat. Phosphodiesterase 10A (PDE10A) mRNA decreases in transgenic HD mice expressing exon 1 of the human huntingtin gene (HD). The mouse PDE10A mRNA is expressed through alternative splicing and polyadenylation in a tissue-specific manner and that transcription of striatal PDE10A mRNA is driven by two promoters. PDE10A2 is the predominant isoform of the gene is expressed in the striatum. Using in situ hybridization and quantitative RT-PCR, we determined that decreased steady-state levels of PDE10A2 mRNA were caused by an altered transcription initiation rate rather than by post-transcriptional mRNA instability in HD mice. Transcription from three initiation sites located within a 50-bp region in the PDE10A2-specific promoter was differentially affected by the presence of the mutant huntingtin transgene. The mouse and human PDE10A2 promoters are highly conserved with respect to the relative position of cis-regulatory elements. Several transcription factors that have been shown to interact with mutant huntingtin, including Sp1, neuron restrictive silencing factor, TATA-binding protein and cAMP-response element binding protein, are unlikely to be involved in mutant huntingtin-induced PDE10A2 transcriptional dysregulation.