Abstract 210: Targeting the mitochondrial DNA polymerase gamma with 2’3’-dideoxycytidine as a novel therapeutic strategy for acute myeloid leukemia
Authors: Liyanage Sanduni , Hurren Rose , Wang Xiaoming , Maclean Neil , Laposa Rebecca , Schimmer Aaron 
Published: July 2016

A subset of AML cells display unique mitochondrial characteristics such as increased mitochondrial DNA (mtDNA), mt mass, and sensitivity to inhibition of mtDNA transcription and protein translation. Given these unique mitochondrial characteristics, we evaluated the effect of inhibiting mtDNA replication by targeting the mtDNA polymerase gamma (POLG). POLG is a nuclear-encoded gene that replicates and repairs mtDNA. POLG mRNA is increased in leukemia cell lines and a subset of AML patients.

We evaluated the preclinical efficacy of targeting POLG using the nucleoside analog 2’,3’-dideoxycytidine (ddC), an FDA-approved anti-retroviral drug that cross-reacts with POLG.

Treatment with ddC at 500nM depleted mtDNA by &gt;90%, decreased mtDNA-encoded COXI and COXII proteins and basal oxygen consumption, and induced apoptosis in AML cell lines (OCI-AML2, TEX, K562). ddC further decreased cell viability in a subset of primary AML cells, while normal peripheral blood stem cells (PBSC’s) were resistant to ddC in vitro.

Next, we assessed the preclinical efficacy and toxicity of ddC in an OCI-AML2 xenograft model of human AML. Treatment with 35mg/kg of i.p. ddC induced tumor regression and decreased tumor mass by &gt;75% compared to vehicle. No toxicity was observed including changes in liver enzymes or organ histology. We also observed reductions in COXI and COXII protein in tumors but not liver excised from treated mice.

We next investigated the mechanism for preferential sensitivity to ddC in AML cells. A two-fold greater decrease in mtDNA content was observed in AML cells treated with ddC compared to PBSC’s, in part explaining the heightened sensitivity. However, the increased sensitivity to ddC in AML was not due to increased mtDNA turnover as mtDNA depletion after ethidium bromide treatment was comparable between AML and PBSC’s.

Therefore, we investigated the role of ddC uptake and metabolism. ddC is transported by nucleoside transporters (ENT’s, CNT’s) and activated by cytoplasmic nucleoside kinases to ddC-triphosphate (ddC-TP). Analysis of Haferlach et. al. (2010) dataset demonstrated that a subset of AML cells have higher mRNA expression of hENT2 and hCNT1, and the nucleoside diphosphate kinases NME1,2,3,5 compared to normal hematopoietic cells. Using mass spectrometry, we observed higher levels of ddC and ddCTP in sensitive AML cells compared to resistant cells. Lastly, knockdown of the deoxycytidine kinase rescued ddC cytotoxicity in TEX cells.

In summary, targeting POLG with ddC displays anti-leukemic activity in both in vitro and in vivo models of AML. The preferential toxicity of ddC is due, in part, to increased rates of uptake and activation in AML along as well as their increased sensitivity to inhibition of oxidative phosphorylation. Thus, POLG inhibitors such as ddC may have clinical utility in a subset of AML.

Citation Format: Sanduni Liyanage, Rose Hurren, Xiaoming Wang, Neil Maclean, Rebecca Laposa, Aaron Schimmer. Targeting the mitochondrial DNA polymerase gamma with 2’3’-dideoxycytidine as a novel therapeutic strategy for acute myeloid leukemia. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 210.