Abstract 2540: Identification of factors governing sensitivity of NSCLC cell lines to inhibition of Poly (ADP-Ribose) Polymerase (PARP)

Abstract

Abstract Recently, Poly (ADP Ribose) Polymerase-1 (PARP-1) has been shown to be an important target in subsets of breast and ovarian cancers. PARP inhibitors are emerging as important new anticancer agents, especially in cancers defective in homologous recombination (HR) repair, such as those lacking BRCA1 or BRCA2. To date, the potential role of PARP inhibition in NSCLC has not been extensively studied. BRCA1 or BRCA2 promoter hypermethylation and ATM mutation have been described in primary NSCLCs, alterations that may sensitize cells to PARP inhibition. We have examined a panel of 26 genotypically-defined NSCLC cell lines for sensitivity to the PARP-1 inhibitor AG014699 using colony formation assays. We have identified 5cell lines that are exquisitely sensitive (IC50 ≤ 50nM), 9 cell lines that are moderately sensitive (IC50 ≤ 60-500nM) and 12 cell lines that are resistant (IC50 > 5 µM). To determine whether sensitive cell lines have defective HR compared to resistant cell lines, we examined the formation of Rad51 foci following exposure to PARP inhibition, as well as to gamma-irradiation and cisplatin. Both sensitive and resistant cell lines demonstrated efficient formation of Rad51 foci as well as gamma-H2AX foci in response to DNA damaging treatments, suggesting correct initiation of HR. In resistant cell lines, the repair process following PARP inhibition is completed within 24 hours, with resolution of Rad51 and gamma H2AX foci. No alterations in cell cycle distribution were detected in these cell lines, suggesting that the efficiency of repair allows proliferation to continue largely unabated. However, in sensitive cell lines, these foci persist greater than 30 hours following DNA damaging treatments. These results suggest that although sensitive lines are able to initiate HR repair, they are unable to complete the process, so that Rad51 foci are not resolved. The persistence of DNA damage in these cell lines is confirmed by persistent gamma H2AX focus formation as well as S/G2 arrest, the latter representing intact checkpoint control. We are currently investigating whether PARP inhibitor-sensitive cell lines have a deficiency in one of the helicases or resolvases required for resolution of the complex Holliday junctions formed during HR repair. Our results may define a novel subset of NSCLCs with a late-step defect in HR that can be approached with PARP inhibition as a novel treatment strategy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2540. doi:1538-7445.AM2012-2540