Mechanism of the DNA damage response in Huntington’s disease pathogenesis and relevance for therapeutics

The work was based on statistically robust genetic evidence focused on two very interesting GWAS hits implicated in repeat instability, discovered through significant contributions by the investigators. MSH3 protects against and FAN1 worsens CAG repeat expansion, and the programme aimed to determine whether these molecules may therefore play a role in HD progression. The applicants would also investigate the role of other DNA damage response proteins. The reviewers noted that it is well-known that dysfunction of DDR proteins can result in a variety of rare neurodegenerative disorders and therefore insights discovered into the pathophysiology of HD may be applicable to other neurodegenerative disorders.

The UK Dementia Research Institute (UK DRI) is an initiative funded by the Medical Research Council, Alzheimer's Society and Alzheimer's Research UK. Funding details for UK DRI programmes will be added in 2019.

Huntington’s disease (HD) is caused by the expansion of a CAG repeat tract in exon 1 of the HTT gene, and early pathogenic events that are proximal to the mutation have recently been identified. Somatic expansion of the CAG repeat in specific brain regions and peripheral tissues occurs with disease progression in both HD patients and mouse models. It has been known for many years that ablation of specific mismatch repair proteins completely ablates somatic CAG repeat expansion, and the identification of these same mismatch repeat genes as genetic modifiers of HD, by the Tabrizi lab and others, has brought DNA repair and somatic expansion sharply into focus. At the same time, we have found that the incomplete splicing of the huntingtin gene (HTT) generates a small transcript (Httexon1) encoding an exon 1 HTT protein, and that the extent to which this occurs is CAG repeat length dependent. Therefore, we have identified a mechanism by which genetic modifiers of HD are linked directly to the production of what is known to be a highly pathogenic protein.
We have optimized our method for measuring CAG repeat expansions in mouse tissue, are generating a panel of quantitative qPCR assays to measure all HTT transcripts and have established a multiplex quantigene assay that can be used to measure relative HTT expression levels directly in tissue lysates. We have established a TR-FRET assay that measures the levels of the exon 1 HTT protein and are testing whether this is more sensitive if developed for AlphaLISA, MSD, Singulex or Simoa technologies. Once established we shall determine the relationship between somatic expansion, incomplete splicing, the exon 1 HTT protein and HTT aggregation in brain regions and peripheral tissues during the course of disease in the zQ175 knock-in HD model. To complement the work of the Tabrizi lab, we plan to use mouse models to investigate the extent to which targeting somatic CAG expansion or lowering the levels of the Httexon1 transcript might have therapeutic benefit. We are attempting to import a mismatch repair line that can be used to ablate CAG repeat expansion in a well-characterized mouse model of HD, to determine the maximum benefit of this approach, and set a baseline against which all future therapeutic approaches targeting DNA repair can be measured. We shall also be able to compare this to approaches directly targeting HTT transcripts as we have a number of reagents (ASOs, siRNAs and U1 adaptors), at various stages of development, that can be used to lower the levels of the Httexon1 and the full length HTT transcript.