Abstract 5495: ALT NHEJ is a therapeutic target in hormone therapy resistant ER/PR/HER+ and ER/PR/HER- breast cancers

Approximately 75% of breast cancers are estrogen and/or progesterone positive (ER/PR+) and can be treated in the clinic with anti-hormone therapy; however, resistance is an increasing problem. ER/PR negative (−) breast cancers, on the other hand, have few treatment options. Therefore, novel therapies are required for therapy resistant ER/PR+ and ER/PR- breast cancers. Breast cancers have abnormal responses to the repair of DNA double strand breaks (DSBs) leading to the generation of structural chromosomal abnormalities and genomic instability. DSBs are repaired by at least 2 pathways: homologous recombination (HR) that performs error-free repair and non-homologous end-joining (NHEJ) that is error-prone. An alternative (ALT) NHEJ pathway has been identified that is associated with the generation of chromosomal translocations, deletions and genomic instability. We have recently shown that with respect to non-tumorigenic MCF10A, ER/PR+ MCF7 breast cancer cells express reduced levels of DNA ligase IV, a component of the classic (C) NHEJ pathway, whereas, expression of DNA ligase IIIα, a component of ALT NHEJ, is increased. To determine whether abnormal ALT NHEJ may be a target for therapy in hormone resistant ER/PR+ and ER/PR- patients, we examined steady state levels of C- and ALT-NHEJ proteins in tamoxifen (TAM 1,2,3) and aromatase (LTLT) resistant cell lines derived from MCF7 cells and ER/PR- cells (MDAMB231, SKBR3). Only ER/PR- cells show further reductions in DNA ligase IV and Ku70 compared with MCF10A. Both hormone resistant ER/PR+ and ER/PR- cells show further increases in ALT NHEJ proteins DNA ligase IIIα, poly- (ADP-ribose) polymerase (PARP1) and ALT NHEJ activity as measured by established NHEJ assays compared to controls. Since PARP1 inhibitors have been shown to specifically kill cells with defects in DSB repair and sporadic breast cancer cells have upregulated PARP1 and DNA ligase IIIα, we examined the effect of PARP1 inhibitor ABT888 in ER/PR+ and ER/PR- cells in the presence or absence of a DNA ligase I/III inhibitor. While either inhibitor alone had little effect on cell survival, both therapy resistant ER/PR+ and ER/PR- cells are highly sensitive to a combination of DNA repair inhibitors, acting synergistically. This treatment also decreases ALT NHEJ repair, demonstrating specific targeting of this pathway. Importantly, MCF10A is insensitive to these inhibitors. Finally, siRNA knockdown of C NHEJ protein Ku70 in hormone therapy sensitive ER/PR+ MCF7 increases ALT NHEJ repair activity and sensitivity to the combination of DNA repair inhibitors. Intriguingly, MCF7 cells with increased ALT NHEJ activity also acquire resistance to hormone therapy. Thus, ALT NHEJ appears to be linked to resistance to hormone therapy and is a therapeutic target. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5495. doi:10.1158/1538-7445.AM2011-5495