Abstract 4317: Inhibiting the mitochondrial DNA polymerase gamma (POLG) with 2′,3′-dideoxycytidine reduces oxidative phosphorylation and increases apoptosis in acute myeloid leukemia (AML) cells
Authors: Liyanage Sanduni , Hurren Rose , Laposa Rebecca , Schimmer Aaron 
Published: May 2015

Mitochondria contain their own genome and translation machinery, which are required for aerobic energy production through oxidative phosphorylation. A subset of AML patients are sensitive to inhibitors of mitochondrial translation, likely due to increased mitochondrial mass, increased mitochondrial DNA (mtDNA) and reliance on oxidative phosphorylation (Skrtic et. al., Cancer Cell, 20:674, 2011). In the current study we addressed a complementary approach of inhibiting mtDNA replication by targeting the mitochondrial DNA polymerase gamma (POLG). POLG is encoded by the nuclear genome and is the only known DNA polymerase in mammalian mitochondria. POLG indirectly controls electron transport chain (ETC) function since it replicates the mitochondrial genome that encodes 13 proteins essential for ETC activity.

We explored the impact of inhibiting POLG in AML cells with the nucleoside analogue reverse transcriptase inhibitor 2′,3′-dideoxycytidine (ddC), an FDA-approved antiviral drug that demonstrates off-target inhibition of POLG. In OCI-AML2 and TEX leukemia cells, treatment with 0.2-2µM ddC reduced levels of mtDNA in a dose- and time-dependent manner, with &gt;90% depletion of mtDNA after 6 days of treatment with 0.2µM ddC (p &lt;0.01). mtDNA depletion resulted in decreased protein expression of the mtDNA-encoded cytochrome c oxidases (COX) 1 and 2, that form the catalytic core of ETC complex IV. In contrast, levels of COX4, a nuclear-encoded subunit of the same respiratory complex were unaltered by ddC. Likewise, 0.2 and 2µM ddC reduced mRNA transcript levels of all mtDNA-encoded proteins. ddC (2 µM, 3 days) also decreased the activity of respiratory chain complex IV by greater than 50%.

Importantly, AML cells appear to have a large reserve in their mtDNA content; in both TEX and OCI-AML2 cells, mtDNA depletion down to 5% of control was required for levels of mtDNA-encoded transcripts or proteins to be significantly reduced. ddC treatments (2µM, 6-10 days) that depleted mtDNA beyond the above threshold reduced the proliferation (p&lt;0.05) and increased apoptosis (p&lt;0.01) of OCI-AML2 and TEX cells. In addition, 2µM ddC diminished the basal oxygen consumption rate (p&lt;0.05), a measure of oxidative phosphorylation in OCI-AML2 cells and TEX cells without altering mitochondrial mass.

In summary, treatment of AML cells with ddC depletes mtDNA, decreases mitochondrial bioenergetics and causes cell kill. However, AML cells have large reserves in their mtDNA content and can withstand depletion of up to 95% of their mtDNA without loss of oxidative metabolism.

Citation Format: Sanduni Liyanage, Rose Hurren, Rebecca Laposa, Aaron Schimmer. Inhibiting the mitochondrial DNA polymerase gamma (POLG) with 2′,3′-dideoxycytidine reduces oxidative phosphorylation and increases apoptosis in acute myeloid leukemia (AML) cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4317. doi:10.1158/1538-7445.AM2014-4317