Potentiation of Temozolomide Cytotoxicity by Inhibition of DNA Polymerase Is Accentuated by BRCA2 Mutation

Department of Therapeutic Radiology, Yale School of Medicine, New Haven, Connecticut 06520-8040, USA.
Cancer Research (Impact Factor: 9.33). 12/2009; 70(1):409-17. DOI: 10.1158/0008-5472.CAN-09-1353
Source: PubMed

ABSTRACT Base excision repair (BER) plays a critical role in the repair of bases damaged by oxidative metabolism or alkylating agents, such as those commonly used in cancer therapy. Incomplete BER generates intermediates that require activation of homology-dependent DNA repair to resolve. We investigated the effects of lithocholic acid (LCA), an inhibitor of the key BER enzyme DNA polymerase beta (pol beta), in cells deficient in expression of the homology-dependent repair factor BRCA2. In vitro studies show that LCA suppresses the DNA polymerase and 5'-deoxyribose phosphate lyase activities of DNA pol beta by preventing the formation of a stable pol beta-DNA complex, reducing BER effectiveness. Cytotoxicity assays based on colony formation revealed that LCA exhibits synergism with the alkylating agent temozolomide, which engages BER through DNA methylation, and that the degree of synergism is increased in cells lacking functional BRCA2. BRCA2-deficient cells also showed heightened susceptibility to both LCA and temozolomide individually. The potentiation of temozolomide cytotoxicity by LCA owes to the conversion of single-stranded DNA breaks generated through incomplete BER of methylated nucleotides into double-stranded breaks during DNA replication, as indicated by gammaH2AX immunofluorescence. Death seems to be induced in cotreated cells through an accumulation of persistent double-stranded DNA breaks. Mutations of the BRCA2 gene have been extensively characterized and are present in various cancers, implying that inhibition of BER may offer a means to augment tumor selectivity in the use of conventional cancer therapies.

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Available from: Peter Glazer, Mar 30, 2014
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    • "Since the discovery that cells with defects in the BRCA genes are selectively killed by the inhibition of PARP, PARP inhibitors have rapidly made their way into the clinic [11] [12]. In addition, we and others have shown that the Base Excision Repair (BER) pathway is important for the response to the chemotherapeutic agent Temozolomide (TMZ) [13] [14] [15] and TMZ potentiation by targeting the essential BER protein DNA polymerase ß (Polß) is enhanced in BRCA-deficient tumor cells [16], suggesting that defects in BER might also influence response to select agents. As we learn more about the changes in the DNA repair status of tumor cells and the effect of these DNA repair mutations on protein function and the cellular response to genotoxins such as radiation and chemotherapy, we become poised to exploit these defects to our advantage. "
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