p53 Pathway Inactivation Drives SMARCB1-deficient p53-wildtype Epithelioid Sarcoma Onset Indicating Therapeutic Vulnerability Through MDM2 Inhibition.

Loss of the gene SMARCB1 drives the development of malignant rhabdoid tumors, epithelioid sarcomas, and other malignancies. The SMARCB1 protein is a core component of the SWI/SNF (SWItch/Sucrose Non-Fermentable) family of chromatin remodeling complexes, which are important regulators of gene expression and cell differentiation. Here, we use CRISPR-Cas9 to create germline smarcb1 loss of function in zebrafish. We demonstrate that the combination of smarcb1 deficiency with mutant p53 results in the development of epithelioid sarcomas, angiosarcomas, and carcinomas of the thyroid and colon. Although human epithelioid sarcomas do not frequently harbor p53 mutations, smarcb1-deficient tumors in zebrafish were only observed following disruption of p53, indicating that p53 signaling in human tumors might be attenuated through alternative mechanisms, such as MDM2-mediated proteasomal degradation of p53. To leverage this possibility for the treatment of human epithelioid sarcoma, we tested small molecule-mediated disruption of the p53-MDM2 interaction, which stabilized p53 protein leading to p53-pathway reactivation, cell-cycle arrest, and increased apoptosis. Moreover, we found that MDM2 inhibition and the topoisomerase II inhibitor doxorubicin synergize in targeting epithelioid sarcoma cell viability. This could be especially relevant for patients with epithelioid sarcoma because doxorubicin represents the current gold standard for their clinical treatment. Our results therefore warrant reactivating p53 protein in SMARCB1-deficient, p53-wildtype epithelioid sarcomas using combined doxorubicin and MDM2 inhibitor therapy.