Mitochondrial Bioenergetics and Colon Cancer

Funded by the National Cancer Institute 

​ABSTRACT:

Relative to matched normal tissue, recent large scale sequencing efforts indicate that colorectal cancer (CRC) tumors are specifically enriched in loss of function mutations in mitochondrial DNA (mtDNA); however, the significance and functional consequences of high mtDNA mutational burden in CRC remains unknown. Given that mtDNA encodes critical subunits in 4 out of the 5 complexes of the electron transport chain (ETC), high mtDNA mutational burden suggests that CRC incidence and/or pathogenesis is dependent on disruptions in mitochondrial respiration. On the contrary, impairing tumor mitochondrial respiration, via pharmacological ETC inhibitors or deletion of genes required for the function of the respiratory complexes, blunts tumor growth across many tumor types, including CRC. Together, these seemingly contradictory data sets highlight an intriguing paradox: how do accumulated mtDNA mutations support CRC tumorigenesis, if mitochondrial oxidative metabolism is inherently required for tumors to grow? Generating targeted and efficient CRC therapeutics is dependent on answering this question. In preliminary studies using purified human CRC mitochondria we confirmed that, relative to matched normal, CRC tumors have more mtDNA mutations and discovered that functional bioenergetic deficiencies exclusively localize to mitochondrial complex I, with 100% (12/12) of clinical CRC tumors displaying partial loss-of-function in complex I activity. To model human CRC bioenergetic deficiencies in the mouse, we reduced complex I activity by 50% in the colon using tissue-specific deletion of the complex I accessory subunit NDUFS4. Partial complex I inhibition increased both tumor number and size following CRC initiation with AOM/DSS and induced a pronounced growth advantage in tumor-derived organoids. These results demonstrate that partial complex I loss of function provides a growth advantage sufficient to accelerate CRC outgrowth. Surprisingly, despite it's role as the initiating complex of the ETC, complex I deficient CRC tumors respired normally; although at the expense of increased matrix NADH/NAD+. Additional bioenergetic analysis revealed that CRC tumors circumvent NADH/NAD+ hyper-reduction to sustain mitochondrial oxidative metabolism by uncoupling respiration from ATP synthesis. Thus, our preliminary data indicate that CRC mitochondria exhibit unique metabolic rewiring that allows respiration to proceed despite partial complex I inhibition. The goal of this project is to test the hypothesis that partial complex I deficiency induces increased NADH/NAD+ and/or respiratory uncoupling that accelerates CRC growth. Successful completion of this project will establish the mechanisms by which complex I deficiency accelerates CRC. Given that complex I genes are the most frequently mutated mtDNA genes across tumor types, these findings will provide broader insight on the interplay between mitochondrial function and cancer.