EWS–FLI1 increases transcription to cause R-loops and block BRCA1 repair in Ewing sarcoma

The EWS–FLI1 fusion protein, expressed in Ewing sarcoma, increases global transcription, causes accumulation of R loops and replication stress, and impairs BRCA1-mediated repair. Ewing sarcoma's sensitivity to DNA damage Ewing sarcoma, a paediatric cancer, is characterized by the presence of cells that contain an oncogene that is a fusion between the EWSR1 gene and another gene. Ewing sarcoma cells are highly sensitive to genotoxins—substances that cause DNA damage. Alexander Bishop and colleagues show that these cells have increased replication stress and elevated levels of the three-stranded nucleic acid structures known as R-loops. The cells are also deficient in their ability to repair damaged DNA, which is likely to be due to the interaction between the BRCA1 protein and the transcriptional machinery. In normal cells, these effects are counteracted by wild-type EWSR1, which suppresses R-loop formation and facilitates repair after DNA damage. Ewing sarcoma is an aggressive paediatric cancer of the bone and soft tissue. It results from a chromosomal translocation, predominantly t(11;22)(q24:q12), that fuses the N-terminal transactivation domain of the constitutively expressed EWSR1 protein with the C-terminal DNA binding domain of the rarely expressed FLI1 protein 1 . Ewing sarcoma is highly sensitive to genotoxic agents such as etoposide, but the underlying molecular basis of this sensitivity is unclear. Here we show that Ewing sarcoma cells display alterations in regulation of damage-induced transcription, accumulation of R-loops and increased replication stress. In addition, homologous recombination is impaired in Ewing sarcoma owing to an enriched interaction between BRCA1 and the elongating transcription machinery. Finally, we uncover a role for EWSR1 in the transcriptional response to damage, suppressing R-loops and promoting homologous recombination. Our findings improve the current understanding of EWSR1 function, elucidate the mechanistic basis of the sensitivity of Ewing sarcoma to chemotherapy (including PARP1 inhibitors) and highlight a class of BRCA-deficient-like tumours.