Characterization and impact of novel regulators of mitochondrial DNA integrity on tumourigenesis

Mitochondria are the known as the power houses of the cell, such that they generate the energy required for cellular processes. Consequently, aberrations in mitochondrial function or in mitochondrial DNA can have direct implications on the pathogenesis of disease including cancer. To date, although many mutations in mitochondria DNA have been associated with cancer, there is little known about the exact way in which mitochondrial DNA mutations can give rise to tumours. To further investigate this, we analysed 230 genes involved in repairing DNA to see if they could influence the levels of mitochondrial DNA, when they were silenced. Interestingly, we identified 3 genes which were previously associated with cancer, that can regulate mitochondrial DNA levels. We also observed upon depletion of these genes, changes in expression of some of the proteins involved in the energy-generating pathways in the cell. Therefore suggesting that these genes can also alter the function of mitochondria. In this proposal, we aim to investigate how these identified genes can regulate mitochondrial DNA integrity and determine how this can affect how mitochondria function. We will also look at patient tumour samples to determine the level of mitochondrial DNA in the absence of some of these genes.

Mitochondrial defects and mitochondrial genomic instability have long been suspected to play an important role in the development and progression of cancer but to date little direct evidence for a functional role of these defects have been elucidated. Mutations in nuclear genes have been shown to give rise to mitochondrial DNA mutations but the mechanism behind this is unclear. To this end, we carried out an siRNA screen to identify novel regulators of mitochondrial DNA stability. We identified a novel role for the nuclear DNA repair and cancer associated genes, MLH1, PMS1 and MeCP2 in maintenance of mitochondrial DNA integrity. Loss of each of these genes results in decreased levels of the mitochondrial genes, Mt-Cot1, Mt-ND2 and Mt-ATPase6. Further analysis has revealed that deficiency in these genes increases susceptibility to oxidative stress and results in differential expression of complexes in the oxidative phosphorylation pathway. Therefore, given the increasing evidence suggesting an involvement of mtDNA stability in the transformation process, we aim to elucidate the mechanism behind this maintenance of mitochondrial DNA, determine the functional implications and elucidate its impact on carcinogenesis.

This proposal investigates both basic and translational biology and therefore has obvious clinical impact.

Firstly, data generated from this proposal will provide further understanding of the role of MLH1, PMS1 and MeCP2 in cancer. Therefore in the longer term, this knowledge may impact on human health and welfare.

Secondly, a more translational impact based on this research proposal is that a reduction in mitochondrial DNA levels may prove to be a good biomarker for firstly MLH1, PMS1 and MeCP2 deficiency and secondly, potentially for a biomarker for response to mitochondria-targeted agents such as Menadione. A reduction in mtDNA levels in a patient tumour sample may suggest susceptibility to an increase in mitochondrial oxidative damage, as we observe for MLH1 deficiency. Therefore, this would have a direct impact on MLH1, PMS1 and MeCP2 deficient patients.

Lastly, in this proposal we will also analyze MeCP2 deficiency in Rett syndrome associated mutations with respect to its role in regulation of mitochondrial DNA levels. Therefore if we observe an association, this data will have translational implications for the treatment and health care of Rett syndrome patients.