Mechanisms and small-molecule targeting of SWI/SNF activity in neuroblastoma

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Abstract Neuroblastoma (NB) is an aggressive pediatric malignancy originating from cells of neural crest origin. NB is among the most frequent pediatric solid tumors, with 37% of patients diagnosed as infants. Patients with high- risk NB have a five-year survival rate of ~50% despite intensive therapy that can cause several negative outcomes, including low body weight, hearing loss and nerve damage, death, or other sequelae. To improve treatment for these pediatric patients, new approaches are needed. SMARCA4 (BRG1), the primary ATPase of SWI/SNF complexes, has been identified as an oncogenic dependency for NB, with frequent gene amplifications in advanced disease. Unfortunately, the suitability of targeting SWI/SNF ATPase activity and the mechanisms of its inhibition in NB have remained poorly understood. We have employed new fast-acting tools to target SMARCA4 in our preliminary data, which revealed that its inactivation induces profound loss of viability of NB cells independently of MYCN or other cytogenetic features. We have found that inactivation of SMARCA4 induces cell death near the G1-S boundary, consistent with replication stress or cell-cycle dysregulation. In this proposal, we seek to: (1) Map the direct cell cycle-specific effects of SWI/SNF complexes towards DNA accessibility, transcription, and cell death; (2) Identify the mechanisms of cell cycle deregulation and replication stress induced by SWI/SNF inhibition; and (3) Identify tumor-specific synergistic drug combinations in vitro and validate their efficacy in vivo. We will employ a panel of drugs currently used clinically to treat high-risk NB, to examine therapeutic synergy with SWI/SNF inhibition in 2D cells, 3D spheroids, and examine their combination in a rigorous immune-competent mouse model of neuroblastoma. Our strategy to identify combination therapies will be informed by a counter-screen with human neural crest and peripheral neurons, to identify drug combinations that show minimal toxicity to non-tumor cells. Our studies will identify the principal pathways influenced by SWI/SNF inhibition in NB related to replication stress and cell cycle dysregulation. Our findings will furthermore enable identification and validation of therapies that potentiate SWI/SNF blockade in vivo. Because small-molecule SWI/SNF inhibitors are well tolerated by mice, our translational studies represent an important preclinical step that may lead to clinical trials for the 50% of unresponsive high-risk NB patients.