Abstract 2335: Targeting the DNA repair as well as survival functions of MCL-1 enhances cancer cell killing
Authors: Mattoo Abid , Jessup J Milburn Milburn , Pandita Tej K. 
Published: July 2017

MCL-1 is a a pro-survival BCL2 protein family member which is over-expressed in drug resistant cancer cells. Our purpose was to assess whether MCL-1 increases genomic instability along with its anti-apoptotic function in cancer cells. Depletion of MCL-1 by treating human small cell lung cancer (SCLC) lines with immunotoxin sensitizes these cells to increased killing by BH3 mimetic - Navitoclax (ABT-263) both in vitro and in mice xenograft models. These SCLC cell lines are resistant to both single agent Immunotoxin or ABT-263. In addition, inhibition of NANOG or NANOGP8 by lentivrus delivered shRNA depletes MCL-1 in human colorectal cancer cells and sensitizes these cells to increased cell killing by venetaclox (ABT-199). This increased killing depended on loss of MCL-1, was caspase-dependent and reversed by re-expression of MCL-1. Depletion of MCL-1 in cancer cells , increases cell sensitivity to ionizing radiation (IR) induced death, which was reversed by expression of MCL-1. In response to IR, MCL-1 depleted cells showed reduced survival even at radiation doses less than 6 Gy, however an increase in caspases 3/7 activity was observed at 6 Gy and above, suggesting that apoptosis is activated only when MCL-1 depleted cells are irradiated with higher IR doses. In order to explain the decreased survival post- irradiation the genomic instability and DNA repair pathways were analyzed in MCL-1 depleted cells. Post-irradiation MCL-1 depleted cells exhibited increased genomic instability as measured by increase in chromosome aberrations at different phases of the cell cycle. The increase in aberrations were significantly higher in G2 and S- phases of the cell cycle suggesting defect in Homologous recombination repair pathway (HR). Moreover, the MCL-1 depleted cells show decreased gamma-H2AX foci at earlier time points (30 and 90 minutes) post-irradiation indicating defect in DNA Damage response (DDR) and higher residual foci at 360 minutes, indicating defective DNA repair. These observations were further confirmed by decreased phosphorylation of ATR suggesting defect in DNA Damage response and higher levels of residual 53BP1 and RIF1 foci in MCL-1 depleted cells , confirming DNA DSB repair by homologous recombination (HR) was compromised. Consistent with this model, MCL-1 depleted cells exhibited a reduced frequency of IR-induced MRE11, BRCA1, RPA and Rad51 foci formation, decreased DNA end resection and decreased HR repair in the DR-GFP DSB repair model. Similarly, after Hydoxyurea (HU) induction of stalled replication forks in MCL-1 depleted cells there was a decreased ability to subsequently restart DNA synthesis, which is normally dependent upon HR mediated resolution of collapsed forks. In summary, MCL-1 is an important therapeutic target in cancer cells and its depletion increases cell killing by either increase in apoptosis or suppression of HR and increased replication stress.

Citation Format: Abid Mattoo, J Milburn Milburn Jessup, Tej K. Pandita. Targeting the DNA repair as well as survival functions of MCL-1 enhances cancer cell killing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2335. doi:10.1158/1538-7445.AM2017-2335