Using mouse models to learn about mitochrondial DNA point mutations in ageing and disease

Abstract

Mitochondrial DNA (mtDNA) point mutations clonally expand and result in focal respiratory chain deficiency in tissues of patients with mitochondrial disease and to a lesser extent in normal human ageing. The consequences of mtDNA point mutations upon cellular processes and the mechanisms involved in mitochondrial disease heterogeneity and progression as well as age-related tissue dysfunction, however still remain unknown. The aim of this thesis was to further our understanding of clonally expanded mtDNA point mutations in ageing and the mechanisms associated with disease heterogeneity, using mice that harbour mtDNA point mutations. Characterisation of mitochondrial dysfunction in the PolgA+/mut mouse colon provided evidence for a conserved mechanism for the clonal expansion of somatic mtDNA point mutations by random genetic drift, without any selective constraints, resulting in age-related respiratory chain deficiency in colonic crypts of both PolgA+/mut mice and ageing humans. Preliminary data examining the gene expression profile of respiratory chain deficient colonic crypts revealed potential alterations in processes concerning: the cell cycle and proliferation; DNA maintenance and repair; cell adhesion and tight junction formation; the adaptive immune response and energy metabolism. Such changes hold a number of potentially interesting associations with the development of colorectal cancer and the pathogenesis of inflammatory bowel disease. Using histochemical and mtDNA genome analysis, the m.5024C>T tRNAAla mouse was shown to be a fairly good model of a pathogenic mt-tRNA point mutation in disease. Results demonstrated that the m.5024C>T mutation was pathogenic; mice displayed phenotypic evidence of respiratory chain deficiency; aged mice displayed evidence of disease progression; different tissues showed different biochemical thresholds and there was a selective loss of the mutation in actively dividing cells. These observations have important implications for the role of mtDNA point mutations in ageing and potential mechanisms that govern mitochondrial disease presentation, progression and transmission.