Replication Stress Is an Actionable Genetic Vulnerability in Desmoplastic Small Round Cell Tumors

Desmoplastic small round cell tumor (DSRCT) is an aggressive sarcoma subtype that is driven by the EWS-WT1 chimeric transcription factor. The prognosis for DSRCT is poor, and major advances in treating DSRCT have not occurred for over two decades. To identify effective therapeutic approaches to target DSRCT, we conducted a high-throughput drug sensitivity screen in a DSRCT cell line assessing chemosensitivity profiles for 79 small-molecule inhibitors. DSRCT cells were sensitive to PARP inhibitors (PARPi) and ataxia-telangiectasia and Rad3-related inhibitors (ATRi), as monotherapies and in combination. These effects were recapitulated using multiple clinical PARPi and ATRi in three biologically distinct, clinically relevant models of DSRCT, including cell lines, a patient-derived xenograft-derived organoid model, and a cell line-derived xenograft mouse model. Mechanistically, exposure to a combination of PARPi and ATRi caused increased DNA damage, G2-M checkpoint activation, micronuclei accumulation, replication stress, and R-loop formation. EWS-WT1 silencing abrogated these phenotypes and was epistatic with exogenous expression of the R-loop resolution enzyme RNase H1 in reversing sensitivity to PARPi and ATRi monotherapies. The combination of PARPi and ATRi also induced EWS-WT1-dependent cell-autonomous activation of the cyclic GMP-AMP synthase-stimulator of IFN genes innate immune pathway and cell-surface expression of PD-L1. Taken together, these findings point toward a role for EWS-WT1 in generating R-loop-dependent replication stress that leads to a targetable vulnerability, providing a rationale for the clinical assessment of PARPi and ATRi in DSRCT. Significance: EWS-WT1, the unique oncogenic driver of desmoplastic small round cell tumors, confers sensitivity to PARP and ATR inhibitors, supporting the potential of these drugs in treating patients with this aggressive sarcoma subtype.