Abstract 2382: Synthetic lethal killing of RAD54B-deficient colorectal cancer cells by targeting SOD1
Authors: Sajesh Babu V. , McManus Kirk 
Published: May 2015

Abstract
Synthetic lethality is a rare genetic interaction that results when two independently viable mutations occur within the same organism/cell and results in cell death. Synthetic lethality is thus a rational approach to identify drug targets that can specifically kill cancer cells harboring somatic mutations in specific genes. RAD54B encodes a protein involved in homologous recombination repair whose expression is normally required to maintain genome integrity. RAD54B is somatically altered in ∼4% of the colorectal cancers and numerous other tumor types including breast, prostate and lung. Accordingly, a synthetic lethal approach designed to uncover novel drug targets that selectively exploit and kill cancer cells harboring defects in RAD54B is highly warranted.
In budding yeast, Rad54B is synthetic lethal with Superoxide dismutase 1 (Sod1). We hypothesized that silencing or targeting SOD1 would result in specific killing of RAD54B-deficient cells in a human cancer context. Using a combination of cross-species gene approaches, RNAi and high-content imaging we identified and validated a synthetic lethal interaction between RAD54B and SOD1 in colorectal cancer cells. We demonstrated that silencing SOD1 resulted in specific synthetic lethal killing of RAD54B-deficient colorectal cancer cells while RAD54B-proficient colorectal cancer cells remained viable. Additionally, chemical compounds (e.g. ATTM and 2ME2) that induce reactive oxygen species phenocopied the synthetic lethal interactions observed using RNAi-based approaches. In fact, RAD54B-deficient cells were &gt;10-fold more sensitive to these chemicals when compared to RAD54B-proficient cells. SOD1 is an enzyme responsible for maintaining the levels of superoxide radicals within tolerable limits in cells. We reasoned that targeting SOD1 would lead to excessive superoxide anions, leading to DNA double-strand break and render RAD54B-deficient cells amenable to synthetic lethal killing. To determine if defects in DNA double-strand break repair occurred in these cells, semi-quantitative imaging microscopy was performed and we confirmed the persistence of two surrogate markers of DNA damage, namely γ-H2A.X and 53BP1, following treatment with either ATTM or 2ME2 relative to controls. Finally, we show that apoptosis as reflected by an increase in cleaved Caspase 3, has a key role in the synthetic lethal killing of the RAD54B-deficient cells relative to controls.
Together these results indicate that RAD54B and SOD1 are synthetic lethal interactors, and further identify SOD1 as a novel candidate therapeutic target. The pharmacological targeting of SOD1 has implications beyond the colorectal cancer context employed above as RAD54B is altered in many tumor types.
Citation Format: Babu V. Sajesh, Kirk McManus. Synthetic lethal killing of RAD54B-deficient colorectal cancer cells by targeting SOD1. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2382. doi:10.1158/1538-7445.AM2014-2382