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Replication of Mitochondrial DNA (mtDNA) requires adequate nucleotide pools from the mitochondria and cytoplasm to support DNA biosynthesis. mtDNA is copied mainly by Mitochondrial DNA Polymerase Gamma (POLG) and replication factors. Additionally, mtDNA biosynthesis is also dependent on nucleotide pools derived from cytoplasmic pathways. Acute Myeloid Leukemia (AML) cells have an increased mitochondrial biosynthesis and reliance on oxidative phosphorylation compared to normal Hematopoietic Cells (HSCs).
In an article published ahead of print in Blood on 10th March 2017, Sanduni U. Liyanage and colleagues from the University of Toronto, Canada, discussed their data, which investigated the activity of cytoplasmic nucleotide salvage pathway in AML and its effect on the mtDNA biosynthesis.
In summary, AML cells have elevated cytidine nucleoside kinase pathway activity, which supports mtDNA biosynthesis. Leveraging this “biological vulnerability”, AML cells preferentially activated ddC, which inhibited mtDNA replication, oxidative phosphorylation, and induced anti-AML effects.
Mitochondrial DNA (mtDNA) biosynthesis requires replication factors and adequate nucleotide pools from the mitochondria and cytoplasm. We performed gene expression profiling analysis of 542 human AML samples and identified 55% with upregulated mtDNA biosynthesis pathway expression compared to normal hematopoietic cells. Genes that support mitochondrial nucleotide pools, including mitochondrial nucleotide transporters and a subset of cytoplasmic nucleoside kinases, were also increased in AML compared to normal hematopoietic samples. Knockdown of cytoplasmic nucleoside kinases reduced mtDNA levels in AML cells, demonstrating their contribution in maintaining mtDNA. To assess cytoplasmic nucleoside kinase pathway activity, we employed a nucleoside analog 2'3'-dideoxycytidine (ddC), which is phosphorylated to the activated anti-metabolite, 2'3'-dideoxycytidine triphosphate (ddCTP) by cytoplasmic nucleoside kinases. ddC is a selective inhibitor of the mitochondrial DNA polymerase, POLG. ddC was preferentially activated in AML cells compared to normal hematopoietic progenitor cells. ddC treatment inhibited mtDNA replication, oxidative phosphorylation, and induced cytotoxicity in a panel of AML cell lines. Furthermore, ddC preferentially inhibited mtDNA replication in a subset of primary human leukemia cells and selectively targeted leukemia cells while sparing normal progenitors cells. In animal models of human AML, treatment with ddC decreased mtDNA, electron transport chain proteins, and induced tumor regression without toxicity. ddC also targeted leukemic stem cells in secondary AML xenotransplantation assays. Thus, AML cells have increased cytidine nucleoside kinase activity that regulates mtDNA biogenesis and can be leveraged to selectively target oxidative phosphorylation in AML.
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