Exploring the Neurodegenerative Phenotype in Huntington’s Disease, Using Transgenic Mouse Models, Behavioural Assays and Omics; Toward a Combination Therapy Approach

Huntington’s disease (HD) is a monogenic neurodegenerative disorder characterised by progressive motor, cognitive and psychiatric symptoms. HD is caused by a mutation in the huntingtin (HTT) gene that results in an expanded polyglutamine tract within exon 1 of the protein. The normal biological functions of wild-type HTT and the toxic effects of mutant HTT (mHTT) are not completely understood. However, clinical trials for HTT-lowering therapies are underway and have encountered issues with efficacy. The work presented in this thesis aims to provide insight into the underlying biology of mHTT and to identify a suitable pharmacological drug for use in combination with HTT-lowering approaches. First, an antisense oligonucleotide (ASO) approach to lower mHTT in the R6/1 mouse model of HD was tested and optimal treatment strategy for behavioural improvement determined using behavioural assays. Then, the effect of mHTT-lowering on downstream protein expression was explored using LC-MS/MS proteomics. Next, the suitability of metformin as a combination therapy was tested using behavioural and proteomic signatures in response to treatment in the Yac128 mouse model of HD. Finally, combination of metformin and ASO-mediated HTT-lowering was tested in the R6/1 mouse model of HD. Targeting mHTT exon 1 with an ASO in the R6/1 mouse improved motor and cognitive symptoms. ASO treatment at the pre-symptomatic stage, compared to the prodromal stage, induced a more robust mHTT-lowering effect and augmented observed phenotypical improvements in the motor and cognitive domains. Early mHTT-lowering in the R6/1 mouse had a restorative effect on the dysregulated HD proteome in several key brain regions and plasma. Synaptic structure and function, the protein quality control system, protein translation and metabolism were identified as key pathways to target with a candidate combination therapy. Metformin was initially tested in the Yac128 mouse model of HD, where it rescued an extradimensional shift deficit, innate avoidance behaviour and recognition memory. Proteomic signatures from key brain regions showed metformin also acted on synaptic structure and function and protein translation pathways. However, when tested in the R6/1 mouse in combination with ASO-mediated mHTT-lowering, no synergistic effect was found. Overall, this work has provided novel insights into the biology of HTT-lowering and will guide future combination therapy approaches for the treatment of HD.