Recurrent breakpoints in the BRD4 locus reduce toxicity associated with gene amplification

Structural variants (SVs) represent a mechanism by which cancer cells activate oncogenes or disrupt the function of genes with tumor suppressor roles. A recent study by the PCAWG Consortium investigated structural variants in 30 tumor types, identifying focal rearrangements in the oncogenic BRD4 gene in ovarian, endometrial and breast cancers. These rearrangements resulted in decreased BRD4 expression despite increased copy number, suggesting a novel mechanism to finetune gene over-expression. In this study, we show that focal deletions of BRD4 disrupt genomic regulatory regions and impact gene isoform expression in breast and ovarian tumors when compared to their expression across normal tissues. To determine the functional impact of these concomitant amplification and focal deletion events, we first leveraged open-reading-frame (ORF) screen data from 16 cancer cell lines, where we observed that overexpression of BRD4-long and BRD4-short isoforms is toxic for cancer cell growth. We confirmed these results in OVSAHO ovarian cancer cells, where overexpression of both isoforms significantly reduced tumor growth. Next, we mimicked the focal deletions occurring in BRD4 regulatory regions by CRISPR-Cas9 technology, and observed that their depletion functionally ablates tumor cell growth. We finally show that these focal deletions rescue ovarian carcinoma cells from the toxicity effects associated with gene overexpression, suggesting that global BRD4 gene expression levels must be fine tuned to ensure proper cancer cell proliferation. Our study provides experimental evidence for BRD4 deletions constituting the first example of a driver SV alteration reducing toxicity in cancer, therefore expanding the landscape of cancer progression mechanisms.