Abstract 5709: Transcriptional circuitry of NKX2-1 and SOX1 defines a previously unrecognized lineage subtype of small cell lung cancer

Abstract Introduction: The current molecular classification of small cell lung cancer (SCLC) based on expression of four lineage transcription factors, SCLC-A (ASCL1), SCLC-N (NEUROD1), SCLC-P (POU2F3), and SCLC-Y (YAP1) still leaves its major subtype SCLC-A as a large heterogeneous group, necessitating more precise characterization of lineage subclasses. Experimental procedure: To refine the current SCLC classification and to identify specific lineage features of the SCLC subtypes, we performed unsupervised hierarchical clustering of H3K27ac profiles on transcriptional regulators from 25 SCLC cell lines and determined the epigenomic features for each cluster. Functional significance of the transcriptional lineage regulators for the identified cluster was evaluated by cell growth, apoptosis and xenograft using CRISPR-Cas9-mediated deletion. The specific cistromic profiles by ChIP-seq and its functional transcriptional partners using co-immunoprecipitation followed by mass spectrometry were determined to reveal their functional output in the identified subtype. Rb1fl/flTrp53fl/fl and Rb1fl/flTrp53fl/flNkx2-1fl/fl genetic engineered mouse model were generated to explore the function of Nkx2-1 in tumor initiation and differentiation. H3K27ac profiles were further analyzed to reveal 6 human SCLC specimen and 20 mice tumors epigenomic landscapes. Summary: We identified previously uncharacterized epigenomic sub-clusters of the major SCLC-A subtype, named SCLC-A1 and SCLC-A2. SCLC-A1 was characterized by the presence of a super-enhancer at the NKX2-1 locus, which was observed in human SCLC specimens and a murine SCLC model. We found NKX2-1, a dual lung and neural lineage factor, is uniquely relevant in SCLC-A1, where it maintains neural lineage rather than pulmonary epithelial identity. We further found maintenance of this neural identity in SCLC-A1 is mediated by collaborative transcriptional activity with another neuronal transcriptional factor SOX1. ? Conclusions: We comprehensively describe an additional epigenomic heterogeneity of the major SCLC-A subtype, and define SCLC-A1 subtype by the core regulatory circuitry representing NKX2-1 and SOX1 super-enhancers and their functional collaborations to maintain a neuronal linage state. Citation Format: ranran kong, Ayushi S. Patel, Takashi Sato, Seungyeul Yoo, Abhilasha Sinha, Yang Tian, Feng Jiang, Charles A. Powell, Eric Snyder, Jiantao Jiang, Shaomin Li, Hideo Watanabe. Transcriptional circuitry of NKX2-1 and SOX1 defines a previously unrecognized lineage subtype of small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5709.