Abstract 6602: Targeting the SWI/SNF complex in POU2F-POU2AF transcription factor-driven malignancies
Authors: Xiao Lanbo , He Tongchen , Klingbeil Olaf , Eleanor Young , Wu Xiaoli , Qiao Yuanyuan , Parolia Abhijit , Eyunni Sanjana , Mannan Rahul , Mahapatra Somnath , Kim NamHoon , Zheng Heng , Su Fengyun , Cao Xuhong , Samajdar Susanta , Ramachandra Murali , Vakoc Christopher R. , Chinnaiyan Arul M. 
Published: March 2024

Abstract
Background: Small cell lung cancer (SCLC) is a rapidly progressing subtype of lung cancer with a high growth rate and early metastasis propensity, often resulting in a more advanced disease stage at diagnosis. A recent transcriptome analysis of human SCLC tumors revealed that SCLC could be characterized by the expression pattern of certain transcription factors (TFs), including ASCL1 (achaete-scute family bHLH transcription factor 1) and POU2F3 (POU domain class 2 transcription factor 3), exemplifying SCLC as a TF-driven malignancy. ASCL1-driven SCLC (SCLC-A) manifests a neuroendocrine phenotype, while POU2F3-driven SCLC (SCLC-P) is characterized as a tuft-cell-like variant. Recent studies revealed that POU domain class 2 transcription factors uniquely rely on coactivators to achieve their lineage-defining functions in the tuft cell and B cell lineages. In tuft-cell-like SCLC cells, the coactivators of POU2F3 (POU2AF2 and POU2AF3) endow POU2F3 with a critical transactivation domain by forming a master regulator complex, which supports enhancer-mediated cancer-promoting gene activation in SCLC-P cells. This indicates a potential therapeutic vulnerability of tuft-cell-like SCLC whereby strategies aimed at blocking POU2F3-POU2AF2/3 function may lead to clinical benefit.
Methods: We conducted a domain-targeted CRISPR screen to identify druggable targets for SCLC-P, followed by pharmacological validation in preclinical SCLC models. Multi-omics techniques, including ATAC-seq, ChIP-seq, RNA-seq, and RIME, were employed to elucidate the SWI/SNF complex's regulatory influence on the POU2F-POU2AF axis.
Results: Here we identified that the POU2F3 molecular subtype of SCLC (SCLC-P) exhibits an exquisite dependence on the activity of the SWI/SNF epigenetic complex. SCLC-P cell lines were sensitive to low nanomolar levels of a SWI/SNF ATPase degrader when compared to other molecular subtypes of SCLC. Co-factors of POU2F were found to interact with components of the SWI/SNF complex. The POU2F3 transcription factor complex is evicted from chromatin upon SWI/SNF ATPase degradation. A novel, orally bioavailable SWI/SNF ATPase PROTAC degrader demonstrated preferential efficacy in SCLC-P relative to the SCLC-A subtype. The SWI/SNF ATPase PROTAC degrader did not alter normal tuft cell numbers in lung or colon, nor did it exhibit toxicity in mice. B cell malignancies which displayed a dependency on the POU2F co-factor, POU2AF1, were also remarkably sensitive to SWI/SNF ATPase degradation. In a POU2AF1-dependent, disseminated murine model of multiple myeloma, AU-24118 had an enhanced survival benefit as compared to pomalidomide, an approved treatment for multiple myeloma.
Conclusions: Taken together, our studies suggest that POU2F-POU2AF driven malignancies have an intrinsic dependence on the SWI/SNF complex representing a striking therapeutic vulnerability.
Citation Format: Lanbo Xiao, Tongchen He, Olaf Klingbeil, Young Eleanor, Xiaoli Wu, Yuanyuan Qiao, Abhijit Parolia, Sanjana Eyunni, Rahul Mannan, Somnath Mahapatra, NamHoon Kim, Heng Zheng, Fengyun Su, Xuhong Cao, Susanta Samajdar, Murali Ramachandra, Christopher R. Vakoc, Arul M. Chinnaiyan. Targeting the SWI/SNF complex in POU2F-POU2AF transcription factor-driven malignancies [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 6602.