Abstract 267: Targeting the alt-NHEJ DNA repair pathway selectively sensitizes KRAS-mutant cancer cells to chemotherapy
Authors: Haehnel Patricia S. , Swoboda Sarah , Lehmann Nadine , Rosigkeit Sebastian , Gothe Hernike , Sasca Danial I. , Strand Dennis , Theobald Matthias , Roukos Vassilis , Bockamp Ernesto , Kindler Thomas 
Published: August 2019

Abstract
Genomic instability is considered as a hallmark of human cancer. Whereas the oncogene-induced emergence of genetic lesions caused by increased levels of reactive oxygen species (ROS) and/or replicative stress is well characterised, the impact of defined driver mutations on individual DNA damage response and repair pathways is poorly understood. Recently, we have shown that oncogenic Kirsten-rat sarcoma (KRAS) dysregulates the repair of DNA double-strand breaks (DSB) and shifts the balance from canonical non-homologous end-joining (c-NHEJ) towards the highly error-prone alternative-NHEJ (alt-NHEJ) repair pathway by upregulation of XRCC1, PARP1 and DNA ligase 3α (Lig3α).
In this study, we wanted to investigate, whether KRAS-mutant (KRASmut) lung cancer cells rely on alt-NHEJ to repair genotoxic stress-induced DSBs. In addition, we addressed the question whether targeting essential component of the alt-NHEJ pathways sensitises KRASmut cells toward genotoxic agents.
The presence of oncogenic KRAS associates with increased expression of essential components of the error-prone alt-NHEJ pathway and causes enhanced alt-NHEJ activity compared to KRAS-wild-type (KRASwt) lung cancer cells as revealed by in vivo host cell reactivation assays. Pharmacologic inhibition of PARP, which has been shown to promote end joining by alt-NHEJ, resulted in increased apoptosis upon combined treatment with standard genotoxic agents in KRASmut cells but not KRASwt cells. Moreover, mice bearing tumor xenografts derived from KRASmut lung cancer cell lines but not KRASwt tumors experienced attenuated tumor growth and longer overall survival upon treatment with the PARP-inhibitor Olaparib and chemotherapy compared to single agent therapy, respectively. Similar results were obtained using a genetically modified mouse model of inducible, KRASmut-dependent lung cancer. Mice bearing KRASmut lung tumors showed a significantly reduced tumor burden when treated with the combination therapy of Olaparib and the topoisomerase II inhibitor VP-16 compared to animals treated only with single agents alone. Mechanistically, the DNA damage sensor protein KU70 is trapped in the cytoplasm in a KRASmut-dependent manner. siRNA-mediated knockdown of KRAS caused a shift of KU70 into the nucleus allowing access of KU70 to DSBs and to initiate DNA repair via the c-NHEJ repair pathway.
Our data provide evidence for a novel synthetic lethal interaction between oncogenic KRAS and DNA damage repair. Targeting components of the alt-NHEJ represents a so far not recognized therapeutic strategy to induce synthetic vulnerability in cells harbouring otherwise non-druggable KRAS mutations.
Citation Format: Patricia S. Haehnel, Sarah Swoboda, Nadine Lehmann, Sebastian Rosigkeit, Hernike Gothe, Danial I. Sasca, Dennis Strand, Matthias Theobald, Vassilis Roukos, Ernesto Bockamp, Thomas Kindler. Targeting the alt-NHEJ DNA repair pathway selectively sensitizes KRAS-mutant cancer cells to chemotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 267.