Aldous, S. G. | bioRxiv Science

bioRxiv2025

Lowering the HTT1a transcript as an effective therapy for Huntingtons disease

Lowering the levels of HTT transcripts has been a major focus of therapeutic development for Huntingtons disease (HD), but which transcript should be lowered? HD is caused by a CAG repeat expansion in exon 1 of the HTT gene, and the rate of somatic expansion of this CAG repeat throughout life is now known to drive the age of onset and rate of disease progression. As the CAG repeat expands, the extent to which the HTT mRNA is alternatively processed to generate the HTT1a transcript and highly aggregation-prone and pathogenic HTT1a protein increases. Several HTT-lowering modalities have entered clinical trials that either target both HTT and HTT1a together, or full-length HTT alone. We have developed siRNAs that target the Htt1a mouse transcript (634/486) and used these, together with a potent Htt-targeting siRNA (10150) to compare the efficacy of lowering either full-length Htt or Htt1a. zQ175 and wild-type mice were treated with 10150 or 634/486 alongside control groups at 2 months of age with treatment to 6 or 10 months, or at 6 months with treatment to 10 months. The siRNA potency and durability were most effective in the hippocampus. Whilst both strategies showed benefits, despite the greater potency of 10150, targeting Htt1a was more effective at delaying HTT aggregation and transcriptional dysregulation than targeting full-length Htt. These data support HTT-lowering strategies that are designed to target the HTT1a transcript, either alone, or together with lowering full-length HTT. One Sentence SummaryLowering HTT1a transcript levels delays the onset of molecular and neuropathological phenotypes in a knock-in mouse model of Huntingtons disease.

neuroscience↗

bioRxiv2023

A CAG repeat threshold for therapeutics targeting somatic instability in Huntington's disease

The Huntingtons disease mutation is a CAG repeat expansion in the huntingtin gene that results in an expanded polyglutamine tract in the huntingtin protein. The CAG repeat is unstable, and expansions of hundreds of CAGs have been detected in Huntingtons disease post-mortem brains. The age of disease onset can be predicted partially from the length of the CAG repeat as measured in blood. Onset age is also determined by genetic modifiers, which in six cases involve variation in DNA mismatch repair pathways genes. Knocking-out specific mismatch repair genes in mouse models of Huntingtons disease prevents somatic CAG repeat expansion. Taken together, these results have led to the hypothesis that somatic CAG repeat expansion in Huntingtons disease brains is required for pathogenesis. Therefore, the pathogenic repeat threshold in brain is longer than (CAG)40, as measured in blood, and is currently unknown. The mismatch repair gene MSH3 has become a major focus for therapeutic development, as unlike other mismatch repair genes, nullizygosity for MSH3 does not cause malignancies associated with mismatch repair deficiency. Potential treatments targeting MSH3 currently under development include gene therapy, biologics and small molecules, which will be assessed for efficacy in mouse models of Huntingtons disease. The zQ175 knock-in model carries a mutation of approximately (CAG)185 and develops early molecular and pathological phenotypes that have been extensively characterised. Therefore, we crossed the mutant huntingtin allele onto heterozygous and homozygous Msh3 knock-out backgrounds to determine the maximum benefit of targeting Msh3 in this model. Ablation of Msh3 prevented somatic expansion throughout the brain and periphery, and reduction of Msh3 by 50% decreased the rate of expansion. This had no effect on the deposition of huntingtin aggregation in the nuclei of striatal neurons, nor on the dysregulated striatal transcriptional profile. This contrasts with ablating Msh3 in knock-in models with shorter CAG repeat expansions. Therefore, further expansion of a (CAG)185 repeat in striatal neurons does not accelerate the onset of molecular and neuropathological phenotypes. It is striking that highly expanded CAG repeats of a similar size in humans cause disease onset before 2 years of age, indicating that somatic CAG repeat expansion in the brain is not required for pathogenesis. Given that the trajectory for somatic CAG expansion in the brains of Huntingtons disease mutation carriers is unknown, our study underlines the importance of administering treatments targeting somatic instability as early as possible.

neuroscience↗