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 steadystate 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.Keywords: mutant huntingtin; striatum; transcription initiation sites; quantitative PCR.Huntington's disease (HD) is a progressive neurodegenerative disorder, characterized by a decline in motor function and cognition, as well as the development of psychiatric symptoms [1]. HD develops when an individual inherits one copy of the HD gene with an extended polyglutamineencoding CAG repeat [2]. The number of CAG repeats is inversely correlated with the age of onset of the disorder [3]. The extended polyglutamine tract in mutant huntingtin confers an abnormal function that ultimately causes neurodegeneration of a subpopulation of cells in the basal ganglia. In addition, a reduction in the level of normal huntingtin may also be detrimental to the survival and function of neurons [4,5].One of the earliest known changes in human HD patients is the loss of cannabinoid receptors [6]. Immunohistochemistry and radio-ligand binding assays of postmortem human brains at different ages and stages of HD have demonstrated that CB1 receptors decrease on nerve terminals in the globus pallidus [7] and substantia nigra [6,8] prior to cell loss. Similarly, CB1 mRNA levels decline in the striatum of transgenic HD mice [8,9]. The mechanism by which mutant huntingtin causes changes in CB1 mRNA levels has not yet been determined and it is ...

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.

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