Mitochondrial DNA mutations and breast tumorigenesis

Breast cancer is a heterogeneous disease and genetic factors play an important role in its genesis. Although mutations in tumor suppressors and oncogenes encoded by the nuclear genome are known to play a critical role in breast tumorigenesis, the contribution of the mitochondrial genome to this process is unclear. Like the nuclear genome, the mitochondrial genome also encodes proteins critical for mitochondrion functions such as oxidative phosphorylation (OXPHOS), which is known to be defective in cancer including breast cancer. Mitochondrial DNA (mtDNA) is more susceptible to mutations due to limited repair mechanisms compared to nuclear DNA (nDNA). Thus changes in mitochondrial genes could also contribute to the development of breast cancer. In this review we discuss mtDNA mutations that affect OXPHOS. Continuous acquisition of mtDNA mutations and selection of advantageous mutations ultimately leads to generation of cells that propagate uncontrollably to form tumors. Since irreversible damage to OXPHOS leads to a shift in energy metabolism towards enhanced aerobic glycolysis in most cancers, mutations in mtDNA represent an early event during breast tumorigenesis, and thus may serve as potential biomarkers for early detection and prognosis of breast cancer. Because mtDNA mutations lead to defective OXPHOS, development of agents that target OXPHOS will provide specificity for preventative and therapeutic agents against breast cancer with minimal toxicity.     

POLB: A new role of DNA polymerase beta in mitochondrial base excision repair

The mitochondrial genome is a matrilineally inherited DNA that encodes numerous essential subunits of the respiratory chain in all metazoans. As such mitochondrial DNA (mtDNA) sequence integrity is vital to organismal survival, but it has a limited cadre of DNA repair activities, primarily base excision repair (BER). We have known that the mtDNA is significantly oxidized by both endogenous and exogenous sources, but this does not lead to the expected preferential formation of transversion mutations, which suggest a robust base excision repair (BER) system. This year, two different groups reported compelling evidence that what was believed to be exclusively nuclear DNA repair polymerase, POLB, is located in the mitochondria and plays a significant role in mitochondrial BER, mtDNA integrity and mitochondrial function. In this commentary, we review the findings and highlight remaining questions for the field.     

Depletion of mitochondrial DNA and enzyme in estrogen-induced hamster kidney tumors: a rodent model of hormonal carcinogenesis

Mitochondrial DNA (mtDNA) encodes for 13 polypeptides critical for normal functioning of the electron transport chain and damage to mtDNA has been associated with aging, and implicated in several disease processes. Although damage to mtDNA is being implicated in mutagenesis and carcinogenesis, there are limited studies demonstrating the role and extent of mtDNA damage in human or rodent cancers. Using serial dilution and competitive polymerase chain reaction analysis, we have quantitated the amount of total mtDNA and analyzed the extent of mtDNA damage in estrogen-induced and estrogen-dependent hamster kidney tumors. The hamster kidney tumor model is a useful and widely investigated rodent model of hormonal carcinogenesis, which shares several characteristics with human breast and uterine cancers, and point to a common mechanistic pathway. Our data indicate a significant decrease in the copy number of total mtDNA and the activity of a nuclear-encoded mitochondrial enzyme citrate synthase in hamster kidney tumors compared to age-matched controls. Since there are several hundred mitochondria in a cell and each mitochondrion has multiple copies of mtDNA, a very small percentage of somatic deletion mutation may not be enough to result in a decreased capacity of the mitochondrial genome. However, a significant increase in deletion mutations or a decrease in the mtDNA copy number can result in a decreased oxidative phosphorylation capacity of the mitochondria and decreased energetics, and thus increased susceptibility to the disease process. Therefore, estrogen-induced hamster kidney tumor model can be a useful rodent model of carcinogenesis to understand the role of mtDNA damage in cancer progression and development.     

Mitochondrial DNA Polymerase POLG1 Disease Mutations and Germline Variants Promote Tumorigenic Properties

Germline mutations in mitochondrial DNA polymerase gamma (POLG1) induce mitochondrial DNA (mtDNA) mutations, depletion, and decrease oxidative phosphorylation. Earlier, we identified somatic mutations in POLG1 and the contribution of these mutations in human cancer. However, a role for germline variations in POLG1 in human cancers is unknown. In this study, we examined a role for disease associated germline variants of POLG1, POLG1 gene expression, copy number variation and regulation in human cancers. We analyzed the mutations, expression and copy number variation in POLG1 in several cancer databases and validated the analyses in primary breast tumors and breast cancer cell lines. We discovered 5-aza-2''-deoxycytidine led epigenetic regulation of POLG1, mtDNA-encoded genes and increased mitochondrial respiration. We conducted comprehensive race based bioinformatics analyses of POLG1 gene in more than 33,000 European-Americans and 5,000 African-Americans. We identified a mitochondrial disease causing missense variation in polymerase domain of POLG1 protein at amino acid 1143 (E1143G) to be 25 times more prevalent in European-Americans (allele frequency 0.03777) when compared to African-American (allele frequency 0.00151) population. We identified T251I and P587L missense variations in exonuclease and linker region of POLG1 also to be more prevalent in European-Americans. Expression of these variants increased glucose consumption, decreased ATP production and increased matrigel invasion. Interestingly, conditional expression of these variants revealed that matrigel invasion properties conferred by these germline variants were reversible suggesting a role of epigenetic regulators. Indeed, we identified a set of miRNA whose expression was reversible after variant expression was turned off. Together, our studies demonstrate altered genetic and epigenetic regulation of POLG1 in human cancers and suggest a role for POLG1 germline variants in promoting tumorigenic properties.     

Decreased mitochondrial DNA mutagenesis in human colorectal cancer

Genome instability is regarded as a hallmark of cancer. Human tumors frequently carry clonally expanded mutations in their mitochondrial DNA (mtDNA), some of which may drive cancer progression and metastasis. The high prevalence of clonal mutations in tumor mtDNA has commonly led to the assumption that the mitochondrial genome in cancer is genetically unstable, yet this hypothesis has not been experimentally tested. In this study, we directly measured the frequency of non-clonal (random) de novo single base substitutions in the mtDNA of human colorectal cancers. Remarkably, tumor tissue exhibited a decreased prevalence of these mutations relative to adjacent non-tumor tissue. The difference in mutation burden was attributable to a reduction in C:G to T:A transitions, which are associated with oxidative damage. We demonstrate that the lower random mutation frequency in tumor tissue was also coupled with a shift in glucose metabolism from oxidative phosphorylation to anaerobic glycolysis, as compared to non-neoplastic colon. Together these findings raise the intriguing possibility that fidelity of mitochondrial genome is, in fact, increased in cancer as a result of a decrease in reactive oxygen species-mediated mtDNA damage.     

Mitochondrial Genetic Control of Assembly and Function of Complex I in Mammalian Cells

Sixteen years ago, we demonstrated, by immunological and biochemical approaches, that seven subunits of complex I are encoded in mitochondrial DNA (mtDNA) and synthesized on mitochondrial ribosomes in mammalian cells. More recently, we carried out a biochemical, molecular, and cellular analysis of a mutation in the gene for one of these subunits, ND4, that causes Leber's hereditary optic neuropathy (LHON). We demonstrated that, in cells carrying this mutation, the mtDNA-encoded subunits of complex I are assembled into a complex, but the rate of complex I-dependent respiration is decreased. Subsequently, we isolated several mutants affected in one or another of the mtDNA-encoded subunits of complex I by exposing established cell lines to high concentrations of rotenone. Our analyses of these mtDNA mutations affecting subunits of complex I have shown that at least two of these subunits, ND4 and ND6, are essential for the assembly of the enzyme. ND5 appears to be located at the periphery of the enzyme and, while it is not essential for assembly of the other mtDNA-encoded subunits into a complex, it is essential for complex I activity. In fact, the synthesis of the ND5 polypeptide is rate limiting for the activity of the enzyme.