Abstract 6143: Harnessing antiviral innate immunity regulated by <i>BRCA1 p</i>seudogene RNA in preclinical cancer models

Background: Antiviral innate immune responses hold intrinsic anticancer benefits by promoting antitumor immunity and thereby increasing the efficacy of chemotherapy and immunotherapy. They mediate essential antitumor responses through several mechanisms, including stimulation of cytokine production and activation of cytotoxic immune cells. Both cancer cells and virus-infected cells threaten the host by expressing damage-associated molecular patterns and by evading control mechanisms of host immune surveillance. Interestingly, current evidence suggests that not only exogenous non-self-RNA but also host-derived pseudogene RNAs regulate antiviral innate immunity. Methods: Expression of the BRCA1 pseudogene (BRCA1P1)-long noncoding RNA (lncRNA) was measured in 28 different cell lines using qRT-PCR and inhibited using CRISR-Cas9 genome editing tools or antisense oligonucleotides (ASOs). In patient samples, BRCA1P1-lncRNA was detected by BaseScope RNA-In Situ Hybridization. For phagocytosis assays, pHrodo™ red-positive cells were determined in cancer cells co-cultured with macrophage cells. Patient-derived organoids (PDOs) were established from primary and metastatic breast tumors and transfected with BRCA1P1-specific ASOs using electroporation. In vivo, mice were humanized with human PBMC and engrafted with breast cancer cells +/- BRCA1P1-knockout. Immune infiltration into tumors was measured by flow cytometry and immunohistochemistry. Results: We discovered the regulation of viral mimicry and antiviral immunity by transcripts of BRCA1P1, which serve as immunoregulatory RNAs in pan-cancer cells and preclinical models of human cancers. BRCA1P1 is expressed in 13 types of human (solid and blood) cancers, with elevated expression in cancer cells compared to normal cells. Depletion of BRCA1P1 induced antiviral gene expression, stimulated apoptosis, and increased sensitivity to doxorubicin in various cancer cells, with no apoptotic effect on non-malignant cells. Subsequently, the increase in antiviral responses triggered macrophage-induced phagocytosis of BRCA1P1-depleted cancer cells. Mechanistically, BRCA1P1 regulated NF-κB-driven antiviral gene expression in the nuclei of cancer cells. In PDOs of breast tumors, BRCA1P1 depletion significantly inhibited growth of primary and metastatic tumor PDOs. Lastly, in a humanized mouse model of breast cancer, loss of BRCA1P1 stimulated cytokine expression and immune cell infiltration into tumors, and reduced tumor growth. Conclusion: This study identifies an important role for BRCA1P1 in regulating innate immune defense mechanisms and antitumor responses in pan-cancers and preclinical models of human tumors. This mechanism of antitumor viral immunity regulated by a host-derived pseudogene RNA may guide the development of novel therapies to boost antitumor immune responses in various human cancers. Citation Format: Yoo-Jane Han, Jing Zhang, Sulin Wu, Galina Khramtsova, Daniel Peiffer, Olufunmilayo I. Olopade. Harnessing antiviral innate immunity regulated by BRCA1 pseudogene RNA in preclinical cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6143.