Title: Using genetic modifiers to investigate the molecular mechanisms of repeat expansion in Huntington’s disease
Funder: The Academy of Medical Sciences

Abstract: Aims and Objectives: The overarching objective is to use new insights from human genetics to understand, on a molecular level, how CAG repeat expansions occur in Huntington’s disease (HD). There are two specific aims: (1) To identify and characterise key proteins in the repeat expansion reaction by utilising a stem cell model of HD; (2) To begin developing a cell-free CAG repeat expansion assay for detailed exploration of repeat expansion mechanisms Research Proposal: HD is characterised by progressive motor, psychiatric and cognitive abnormalities over 10-30 years leading to premature death. It is caused by an expanded CAG trinucleotide in the HTT gene, with longer repeat tracts associated with earlier onset disease. Recent human genetic studies have indicated that DNA repair genes such as MSH3 and MLH1 can significantly reduce the age at onset of HD, possibly through driving somatic CAG repeat expansion in the brains of HD gene carriers. Therefore, preventing somatic CAG expansion is a novel and compelling therapeutic goal. However, molecular details of repeat expansion mechanisms are lacking: this project takes two approaches to filling the gap. First, we will take a novel HD stem cell model of CAG expansion and use immunoprecipitation of DNA repair factors to identify the DNA and proteins to which they are bound. Second, we will generate a plasmid-based substrate containing CAG repeats and start to develop a novel, rapid, cell-free assay system of repeat expansion. Both parts of the project will accelerate understanding of repeat biology and potential therapeutic lead development.

Lay abstract: Huntington’s disease (HD) is a degenerative disorder of the brain characterised by progressive involuntary movements, psychiatric problems, and eventually dementia. It affects about 1 in 8,000 people in the UK and is untreatable, leading to premature death after 10-20 years of debilitating decline. HD is an inherited condition caused by an abnormally long stretch of consecutive ‘CAG’ repeats in the DNA. The more repeats you have, the earlier in life the disease starts- but there is considerable variation. Repeats also expand further in the brains of people with HD, and this process probably causes earlier onset of disease symptoms. It is driven by factors like DNA repair proteins MSH3 and MLH1 but is not well understood. If we could understand the biochemical reactions that cause CAG repeats to expand, we could develop new drugs to target the process and potentially slow onset and progression of HD. This project takes two approaches to understanding repeat biology. First, we will use cells with CAG tract of 109 that expands as cells are grown, making the cells a good model system of repeat expansion. We will use specific antibodies to pull out MSH3 and MLH1 from cells and then analyse which proteins and DNAs are bound to them to give insight into expansion mechanisms. Secondly we will use extracts from these cells on an artificial DNA with CAG repeats to develop a rapid test of expansions that we hope will speed up research and aid therapeutic development.

Research activity codes: 1.1 NORMAL BIOLOGICAL DEVELOPMENT AND FUNCTIONING;2.1 BIOLOGICAL AND ENDOGENOUS FACTORS
Health categories: NEUROLOGICAL