Impact of mtDNA Mutations and Transfer on Tumor Growth Dynamics

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PROJECT SUMMARY/ABSTRACT Beyond generating ATP for cellular processes, mitochondria also provide metabolites and signaling factors that orchestrate cell survival, proliferation, and metabolism. It has been known since the 1930s that most cancer cells rely mainly on glycolysis even in aerobic conditions. Mitochondria nonetheless remain essential for metabolic plasticity, the generation of reactive oxygen species (ROS), and nucleotide biosynthesis, all factors that contribute to metastasis and poor outcomes in cancer. The direct impact of mitochondrial dysfunction, such as mutations in the mitochondrial genome (mtDNA), has been difficult to study without experimental tools for editing mtDNA into desired sequences. It remains unknown whether differences in the mtDNA sequence are sufficient to alter the growth kinetics of a tumor, which is a fundamental lack of understanding with potential clinical implications. Furthermore, it is unclear whether tumor cells with deleterious mtDNA mutations will mitigate an impairment in respiration over the natural course of tumor growth, during an epithelial-to- mesenchymal transition, and with metastasis. Cancer cells lacking endogenous mtDNA (ρ0) have previously been shown to restore oxidative metabolism by acquiring mitochondria by organelle transfer from non- malignant cells in the tumor microenvironment. However, this highly artificial system relies upon extreme selective pressure in ρ0 cancer cells that do not exist in nature. Whether the native mtDNA status in mtDNA- resident (ρ+) tumor cells changes with tumor progression has not been determined. To address this key question, it is necessary to generate tumor cells with different known mtDNA mutations in an isogenic nuclear background. Members of my thesis lab developed an in vitro method for permanently transferring isolated mitochondria into ρ0 recipient cells of choice, enabling generation of cancer clones with specific mtDNA mutations and identical nuclear genomes. In Aim 1 studies, I will use B16 mouse melanoma and 4T1 mammary cancer cells as the nuclear backgrounds and generate cancer cells with a panel of wild- type and mutant mtDNA sequences. I will quantify how these mutations alter their metabolism, ROS levels, and invasive capacities in vitro. In Aim 2, I will inject these cells, along with ρ+ and ρ0 controls, into mice subcutaneously and quantify how mtDNA mutations affect kinetics of tumor growth and metastasis. In Aim 3, I will purify tumor cells from local and disseminated disease and analyze changes in mtDNA sequences and tumor cell function following metastasis. Due to differences in respiratory capacities and metabolic profiles between cells containing different mtDNA, I anticipate growth rates and rates of metastasis will vary between lines even before potential mitochondrial acquisition from the normal microenvironment. Understanding the dependence of cancer cells on their mtDNA and whether mtDNA is exchanged in a ρ+ tumor cell setting will provide fresh insight into tumor heterogeneity and evolution in cancer metabolism as a whole.