ABSTRACT
Ewing sarcoma, an oncofusion-driven primary bone tumor, can occur in the setting of various germline mutations in DNA damage repair pathway genes. We recently reported our discovery of a germline mutation in the DNA damage repair protein BARD1 (BRCA1-associated RING domain-1) in a patient with Ewing sarcoma. BARD1 is recruited to the site of DNA double stranded breaks via the poly(ADP-ribose) polymerase (PARP) protein and plays a critical role in DNA damage response pathways including homologous recombination. PARP inhibitors (PARPi) are effective against Ewing sarcoma cells in vitro, though have demonstrated limited success in clinical trials to date. In order to assess the impact of BARD1 loss on Ewing sarcoma sensitivity to PARP inhibitor therapy, we generated the novel PSaRC318 patient-derived Ewing tumor cell from our patient with a germline BARD1 mutation and then analyzed the response of these cells to PARPi. We demonstrate that PSaRC318 cells are sensitive to PARP inhibition and by testing the effect of BARD1 depletion in additional Ewing sarcoma cell lines, we confirm that loss of BARD1 enhances PARPi sensitivity. In certain malignancies, DNA damage can activate the IRF1 (interferon response factor 1) immunoregulatory pathway, and the activation of this pathway can drive immunosuppression through upregulation of the immune checkpoint protein PD-L1. In order to determine the ability of PARPi to alter Ewing tumor immunoregulation, we evaluated whether PARPi results in upregulation of the IRF1-PDL1 pathway. Indeed, we now demonstrate that PARPi leads to increased PD-L1 expression in Ewing sarcoma. Together, these data thus far suggest that while Ewing tumors harboring germline mutations in DNA damage repair proteins may in respond to PARPi in vitro, in vivo benefit of PARPi may only be demonstrated when counteracting the immunosuppressive effects of DNA damage by concurrently targeting immune checkpoint proteins.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Financial Support This work was supported by Alex’s Lemonade Stand Foundation Young Investigator and Million Mile Award (KMB), the Children’s Cancer Research Fund Emerging Scientist Award (KMB), Hyundai Hope on Wheels Young Investigator Award (KMB), The Morden Foundation (KMB), St. Baldrick’s Fellowship Award (LMM) and K12HD052892-13 (LMM, PI Dermody K12HD052892-13). The University of Pittsburgh holds a Physician-Scientist Institutional Award from the Burroughs Welcome Fund (JDD). Research reported here has been supported by the National Institutes of Health under their award number T32HD071834 “Research Training Program for Pediatric Subspecialty Fellows” PI: Terence S. Dermody, M.D.