Abstract 5268: Targeting DHX9 triggers interferon response and replication stress in small cell lung cancer

Abstract Small cell lung cancer (SCLC) is the most lethal type of lung cancer. The uniqueness of this tumor consists of an initial exquisite response to chemotherapy. However, at relapse, which occurs nearly in all patients, the tumor is resistant to all available therapies, causing a premature death of the patient. Despite the addition of immune checkpoint blockade (ICB) therapy to standard chemotherapy, response rates are modest and only a very small fraction of SCLC patients responds to these therapies because of immunologically “cold” microenvironment. Activating innate immunity in cancer cells through cytoplasmic nucleic acid sensing pathways, a phenomenon known as “viral mimicry”, has emerged as an effective strategy to convert immunologically “cold” tumors into “hot”. Here, through a curated CRISPR-based screen of RNA Helicases, we identified DExD/H-box helicase 9 (DHX9) as a potent repressor of double-stranded RNA (dsRNA) in SCLC cells. Depletion of DHX9 induced accumulation of cytoplasmic dsRNA, which mainly derived from repetitive sequences of short interspersed nuclear elements (SINEs) and long interspersed nuclear elements (LINEs), and triggered tumor-intrinsic innate immunity. Intriguingly, ablating DHX9 also induced the aberrant accumulation of R-loops (DNA/RNA hybrids), which resulted in an increase of DNA damage-derived cytoplasmic DNA and replication stress in tumor cells, selectively killing them. In vivo, DHX9 deletion in SCLC tumor cells promoted a decrease in tumor growth while inducing a more immunogenic tumor microenvironment (TME), invigorating responsiveness to immune checkpoint blockade (ICB). These findings suggest that DHX9 is a crucial repressor of tumor-intrinsic innate immunity and replication stress, and represents a promising and unexplored target for SCLC and other “cold” tumor types where replication stress and genomic instability contribute to pathology. Citation Format: Takahiko Murayama, Jun Nakayama, Kathy Q. Cai, Kenichi Miyata, Andrey Efimov, Yinfei Tan, Yan Zhou, Kerry S. Campbell, Yibin Yang, Siddharth Balachandran, Israel Cañadas. Targeting DHX9 triggers interferon response and replication stress in small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5268.