Mechanisms involved in the accumulation of mitochrondrial DNA defects following anti-HIV therapy

Abstract

HIV-infected patients present with a range of pathologies that have been associated with mitochondrial toxicity, and more specifically, induced through exposure to the main class of antiretrovirals used in anti-HIV therapy (HAART), nucleoside analogue reverse transcriptase inhibitors, or NRTIs. It has recently been found that such patients have an excess of mitochondrial DNA mutations when exposed to NRTIs; however, the underlying pathophysiological process of this remains undetermined. To elaborate upon this further, a range of molecular approaches were developed to study the behaviour of mitochondrial DNA mutations, both large scale deletions and single point mutations, in both tissue samples and in vitro. High throughput, ultra-deep DNA sequencing technology was also utilised to characterise mitochondrial DNA mutation burden in detail. Here I present data from physiological samples of HIV-infected individuals receiving NRTIs indicating an increased level of point mutation heteroplasmy and the level of the common mitochondrial deletion, compared to HIV-infected individuals who are NRTI treatment naïve. The mechanisms of this mutation level increase are elucidated through in vitro studies indicating that deletions accumulate through a size dependent mechanism during exposure and a point mutation level increase is mediated through a bottleneck mechanism. The possibility of NRTI de novo mutagenesis is refuted through the use of next generation sequencing technologies; the data also further supports a bottleneck mechanism of increased point mutation level. I also refute the idea that compounds associated with mitochondrial biogenesis can reduce mitochondrial depletion during NRTI exposure and that there appears to be no genetic predisposition to pathologies in sub-Saharan African individuals. I therefore conclude that there is an accelerated clonal expansion of pre-existing mitochondrial DNA mutations through NRTI exposure, which is mediated by a size dependent replication bias for deletions, and a molecular bottleneck effect which accelerates drift for point mutations. These data suggest an acceleration of normal cellular ageing through mitochondrial mechanisms in HIV-infected individuals.