SMARCA4 is essential for early-stage tumor development but its loss promotes late-stage cancer progression in small-cell lung cancer

SMARCA4 and other components of the SWI/SNF chromatin remodeling complex have been implicated in various cancers. Yet, its role in small cell lung cancer (SCLC) tumorigenesis remains poorly understood. Genetically engineered mouse models (GEMMs) of SCLC revealed that deletion of Smarca4 significantly decreased tumor development in this model. Pharmacological inhibition of SMARCA4 decreased the proliferation of preneoplastic neuroendocrine (NE) cells. These effects coincided with reduced expression of the lineage-specific transcription factor, ASCL1, suggesting that disruption of the SMARCA4-ASCL1 axis impairs tumor development. However, Smarca4-deficient tumors, albeit smaller than controls, displayed features associated with malignant progression, including variant histology and the loss of NE differentiation. This prompted us to test the functional role of SMARCA4 in established tumor cells that recapitulate late-stage disease. Intriguingly, whilst Smarca4 knockdown in tumor cells failed to affect their proliferative capacity in vitro, Smarca4 knockdown tumors exhibited enhanced growth following subcutaneous transplantation in athymic nude mice. Interestingly, SMARCA4 knockdown significantly reduced expression and cell-surface display of PVR, a ligand for activating natural killer (NK) cells. These results led to an idea that the enhanced tumor formation was partly owing to altered tumor-NK cell interactions mediated by the SMARCA4-PVR axis in tumor cells. These findings suggest that SMARCA4 plays a temporally distinct role in SCLC, supporting early tumorigenesis but potentially functioning as a tumor suppressor in the later stages. The dramatic differences observed when targeting SMARCA4 in distinct disease states emphasize a need to acknowledge how differences in the timing of alterations can drastically alter tumor evolution.