Abstract 2051: Inhibiting CDC7 kinase for the treatment of Ewing sarcoma

Abstract

Abstract Ewing Sarcoma is a malignant tumor of the bone and soft tissue that is primarily driven by the fusion protein EWS-FLI1. With no current targeted therapy, treatment options remain limited and toxic. Recent reports have shown that Ewing tumors harbor large amounts of transcription-associated replication stress and that this offers an attractive therapeutic intervention point. Our group has shown that inhibition of the kinase complex DDK (CDC7-DBF4) results in apoptosis and a significant reduction in cell viability in EWS-FLI1-harboring Ewing's sarcoma cell lines. Also, low dose DDK inhibition in Ewing's sarcoma cells caused an S/G2 cell cycle accumulation at 24 hours, followed by re-entry into G1 and appearance of sub-G1 DNA. This suggests a failure to properly progress through S-phase followed by premature mitotic entry and mitotic catastrophe resulting in cell death. To this end, there was a significant increase in the formation of micronuclei and other aberrant mitotic structures upon DDK inhibition in Ewing's cells. Interestingly, there was minimal signs of mitotic accumulation, despite clear evidence of mitotic stress, indicating a failure to properly enforce a G2-M checkpoint. Importantly, these phenotypes were not mimicked with the use of the CDK2 inhibitor Roscotivine, suggesting an origin firing-independent basis for the sensitivity to DDK inhibition. Furthermore, we show that DDK inhibition synergizes with the TOP1 inhibitor Camptothecin and with the PARP1 inhibitor Talazoparib to kill Ewing Sarcoma cells in vitro. We have hypothesized this to be due to DDK's potential role in replication fork instability/restart. Together, these results suggest that Ewing sarcoma cells rely on the activity of DDK to maintain cell viability independently from its role in the regulation of replication origin initiation and suggest that DDK inhibition may prove to be a viable treatment option for patients with Ewing's sarcoma. Citation Format: Jeffrey C. Martin. Inhibiting CDC7 kinase for the treatment of Ewing sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2051.