Abstract B023: SMARCAL1 is a novel candidate therapeutic target for ALT-positive osteosarcoma

Abstract The DNA translocase SMARCAL1 has been characterized by our laboratory and others as a DNA replication-associated factor that promotes the remodeling of stalled replication forks to facilitate the repair of DNA damage and the restart of DNA synthesis. Recent genome-wide analyses of CRISPR-Cas9 loss-of-function screens in pediatric cancer cell lines have suggested an essential function for SMARCAL1 in a subset of pediatric tumors (Dharia NV et al., Nature Genetics, 2021). In agreement with these observations, pan-cancer analysis of dependency scores in over 1,000 cell lines from the DepMap project revealed an important role for SMARCAL1 in tumors of mesenchymal origin and especially in osteosarcoma. We found that SMARCAL1 dependency in osteosarcoma is selectively associated with lack of telomerase expression. In the absence of telomerase, cancer cells mostly rely on a recombination-based mechanism, known as ALT (Alternative Lengthening of Telomeres), for the maintenance of their telomere length. Surprisingly, by comparing genome-wide CRISPR screening data of known ALT-positive vs ALT-negative osteosarcoma cell lines, we found that SMARCAL1 is among the genes that are most selectively essential in ALT-positive tumors. We validated in multiple cellular models a strict correlation and functional relationship between ALT status and SMARCAL1 dependency. In line with previous findings (Cox KE et al., Cell Reports, 2016), we observed that SMARCAL1 localizes to a subset of telomeres in ALT-positive cells and loss of SMARCAL1 induces accumulation of telomere damage. SMARCAL1 enzymatic activity and its RPA-binding function contribute to suppress DNA damage in ALT-positive cells. Furthermore, our studies suggest that SMARCAL1 protects telomere stability in ALT-positive cancer cells by limiting PRIMPOL-mediated restart of stalled replication forks. In addition, SMARCAL1 deficiency enhances cell-intrinsic immune response and anti-tumor immunity. Collectively, our studies have uncovered SMARCAL1 as a novel candidate therapeutic target for the treatment of ALT-positive cancer patients. In these patients, SMARCAL1 inhibition may have a dual beneficial effect by promoting selective cancer cell toxicity combined with stimulation of anti-tumor immunity and enhancement of immunotherapy response. Citation Format: Angelo Taglialatela, Xiao Chen, Zahra Fatema Khan, Filemon Dela Cruz, Dua Shivi, Dillon Oswald, Azeroglu Benura, Eros Lazzerini Denchi, Min Jaewon, Andrew L. Kung, Alberto Ciccia. SMARCAL1 is a novel candidate therapeutic target for ALT-positive osteosarcoma [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr B023.