The HTT CAG-Expansion Mutation Determines Age at Death but Not Disease Duration in Huntington Disease.

Huntington disease (HD) is caused by an expanded HTT CAG repeat that leads in a length-dependent, completely dominant manner to onset of a characteristic movement disorder. HD also displays early mortality, so we tested whether the expanded CAG repeat exerts a dominant influence on age at death and on the duration of clinical disease. We found that, as with clinical onset, HD age at death is determined by expanded CAG-repeat length and has no contribution from the normal CAG allele. Surprisingly, disease duration is independent of the mutation's length. It is also unaffected by a strong genetic modifier of HD motor onset. These findings suggest two parsimonious alternatives. (1) HD pathogenesis is driven by mutant huntingtin, but before or near motor onset, sufficient CAG-driven damage occurs to permit CAG-independent processes and then lead to eventual death. In this scenario, some pathological changes and their clinical correlates could still worsen in a CAG-driven manner after disease onset, but these CAG-related progressive changes do not themselves determine duration. Alternatively, (2) HD pathogenesis is driven by mutant huntingtin acting in a CAG-dependent manner with different time courses in multiple cell types, and the cellular targets that lead to motor onset and death are different and independent. In this scenario, processes driven by HTT CAG length lead directly to death but not via the striatal pathology associated with motor manifestations. Each scenario has important ramifications for the design and testing of potential therapeutics, especially those aimed at preventing or delaying characteristic motor manifestations.

Full sequence of mutant huntingtin 3'-untranslated region and modulation of its gene regulatory activity by endogenous microRNA.

Huntington's disease (HD) is caused by an expanded CAG trinucleotide repeat in the first exon of the huntingtin gene (HTT). Since the entire course of the disease starts from this dominant gain-of-function mutation, lowering total or mutant huntingtin mRNA/protein has emerged as an appealing therapeutic strategy. We reasoned that endogenous mechanisms underlying HTT gene regulation may inform strategies to target the source of the disease. As part of our investigation to understand how the expression of HTT is controlled, we performed (1) complete sequencing analysis for mutant HTT 3'-UTR and (2) unbiased screening assays to identify naturally-occurring miRNAs that could lower the HTT mRNA levels. By sequencing HD families inheriting the major European mutant haplotype, we determined the full sequence of HTT 3'-UTRs of the most frequent mutant (i.e., hap.01) and normal (i.e., hap.08) haplotypes, revealing 5 sites with alternative alleles. In subsequent miRNA activity assays using the full-length hap.01 and hap.08 3'-UTR reporter vectors and follow-up validation experiments, hsa-miR-4324 and hsa-miR-4756-5p significantly reduced HTT 3'-UTR reporter activity and endogenous HTT protein levels. However, those miRNAs did not show strong haplotype-specific effects. Nevertheless, our data highlighting full sequences of HTT 3'-UTR haplotypes, effects of miRNAs on HTT levels, and potential interaction sites provide rationale and promising targets for total and mutant-specific HTT lowering intervention strategies using endogenous and artificial miRNAs, respectively.