Investigation of the involvement of mitochondrial DNA variants in cardiometabolic disease : the SABPA study

Abstract

Mitochondria are intricately involved in cell homeostatic and adaptive stress signalling pathways, and play a central role in cell differentiation, proliferation and death. Consequently, mitochondrial dysfunction has been implicated in a vast number of rare and common disease phenotypes. Mitochondrial DNA (mtDNA) encoded for 13 protein sub-units essential to mitochondrial function, as well as two subunits for mt-rRNA and 22 mt-tRNA molecules, required to translate and transcribe these proteins. As such, mtDNA variation, which could directly cause alterations in mitochondrial function and downstream processes, has been investigated as a possible risk factor in disease susceptibility, onset and progression. In this thesis, the role of mtDNA variation in cardiometabolic disease (CMD) is investigated. When mtDNA variation in common complex diseases such as CMD and other late onset and degenerative disorders are investigated, several approaches have been used to date, most notably the haplogroup association method. However, studies using these traditional methods have been plagued by inconsistencies, difficulties in replicating findings in other cohorts/populations, and contradicting reports. It is therefore clear that alternative approaches are needed in the field. A novel approach, the MutPred adjusted load hypothesis, is introduced in this thesis. This novel approach makes use of the MutPred scoring system to assign pathogenicity scores to non-synonymous mtDNA variants, which are then used to calculate a mutational load, a single statistical metric. The cumulative effect of several mildly deleterious variants can thus be measured in disease, using parametric statistical analyses. This new approach together with other more classic approaches were applied in a bi-ethnic South African cohort (N = 363) in this thesis. In addition, transmitochondrial cytoplasmic hybrids (cybrid) cells were utilised to investigate the impact of MutPred mutational loads on mitochondrial function. Using several investigative approaches, no significant associations between mtDNA variants and hypertension, hyperglycaemia, or indicators of inflammation and oxidative stress could be found (P > 0.05). However, in a preliminary study done in cybrid cells, several classifications of mtDNA variation, including MutPred mutational loads (P < 0.01), mtDNA variants with low MutPred scores (P < 0.00001), and relative mtDNA copy number (P < 0.00001) were shown to be significantly correlated with mitochondrial respiration rates. Further studies, investigating the underlying mechanisms of these relationships are warranted. Thus, while a role for MutPred mutational loads in CMD could not be found in the current cohort, a role for mtDNA variants in mitochondrial function in cybrid cells was found. In addition, it was demonstrated that the MutPred adjusted load hypothesis approach delivers more statistical power to studies when compared to haplogroup association studies, making it suitable for use even in moderately sizes cohorts. This approach should find wide application in the field, being especially useful for cohorts from multiple locations or with a variety of mtDNA lineages, where the traditional haplogroup association method has failed