Title: Mechanism of the DNA Tabrizi
Funder: Medical Research Council

Abstract: 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 a fatal autosomal dominant neurodegenerative disease caused by a CAG repeat expansion. The repeat is unstable and prone to expansion with age in the most affected tissues, particularly the striatum, and the rate of this somatic instability (SI) correlates with disease course. However, the molecular mechanism underlying SI is not yet fully understood. Our GWAS studies have identified genetic variation in DNA damage response (DDR) proteins such as FAN1 and MSH3 that modify the course of HD. We recently found that FAN1 protectively restrains CAG expansion in cell models, whereas MSH3 likely promotes expansion. We have demonstrated that genetic variation and reduced expression of MSH3 decreases somatic expansion, delays onset and slow progression in HD. We also showed that these HD genetic modifiers also influence the course of other repeat expansion diseases, such as the spinocerebellar ataxias (SCA) and myotonic dystrophy type 1 (DM1). In collaboration with Dr Thalassinos (UCL Institue of Structural Biology) we are investigating the effect of these variants on protein structure, and interactions using mass spectrometry. GWAS also implicate a wider network of DDR proteins, particularly those involved in mismatch repair (MMR) and known to be involved in SI in HD mouse models, such as EXO1, PCNA, MLH1, PMS2, LIG1 and MLH3. We hypothesise that expanded CAG repeats form abnormal secondary DNA structures that are recognised as lesions, prompting the DDR to attempt repair. In cell models we are investigating how genetic variation in FAN1 and MSH3 influences somatic expansion. We developed human U20S cells stably expressing disease-modifying FAN1 variants and a pathogenic HTT exon 1, discovering that FAN1 restrains CAG expansion in a dose-dependent manner. We are currently investigating the functional interactions between FAN1 and mismatch repair (MMR) proteins such as MLH1 and MSH3. We have generated several patient-derived lymphoblastoid and induced pluripotent stem cell (iPSC) lines that show robust CAG repeat expansion in culture, and used them to demonstrate that FAN1 knockdown in medium spiny neurons (MSN) accelerates repeat expansion. We are using CRISPR to introduce disease-modifying MSH3 and FAN1 variation, as well as knockouts of each gene, to investigate the mechanism by which they influence SI in MSNs. In collaboration with Dr Balmus (DRI Cambridge), we will dissect the specific roles of individual proteins in SI using a CRISPR screen for targeted deletion of DDR components in iPSCs and differentiated cortical neurons. The identified modifiers will be potential candidates for biomarkers and therapeutic targets for repeat expansion diseases. Finally, we are working with several industry partners to develop different approaches to targeting the DDR and in particular lowering MSH3 expression. MSH3 is a particularly attractive therapeutic target as its loss is well tolerated in vivo and will work with the Bates lab to explore the therapeutic potential of modulating DDR proteins in mouse models of HD.


Research activity codes: 2.1 BIOLOGICAL AND ENDOGENOUS FACTORS;5.1 PHARMACEUTICALS
Health categories: NEUROLOGICAL