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

## TL;DR

This study shows that focal deletions in the BRD4 gene help cancer cells avoid toxicity from gene overexpression, supporting tumor growth.

## Contribution

The first experimental evidence of a recurrent deletion reducing gene overexpression toxicity in cancer.

## Key findings

- Focal deletions in BRD4 regulatory regions are found in breast, ovarian, and endometrial cancers.
- CRISPR-Cas9 models show BRD4 deletions rescue cells from overexpression toxicity.
- Fine-tuned BRD4 expression is essential for sustained tumor proliferation.

## Abstract

Recent work by the ICGC-PCAWG consortium identified recurrent focal deletions in the BRD4 gene, decreasing expression despite increased copy number. We show that these focal deletions occur in the context of cyclin E1 amplification in breast, ovarian, and endometrial cancers, and serve to disrupt BRD4 regulatory regions and gene expression across isoforms. We analyze open reading frame screen data and find that overexpression of BRD4 long (BRD4-L) and short isoform BRD4-S(a) impairs cell growth across cell lines. We confirm these results in OVSAHO ovarian cancer cells, where the overexpression of BRD4 isoforms significantly reduces tumor growth. Next, we mimic BRD4 focal deletions using CRISPR-Cas9 technology and show that these focal deletions rescue ovarian cancer cells from toxicity associated with BRD4 overexpression, suggesting that BRD4 levels must be fine-tuned for cancer cell proliferation. Our study provides experimental evidence for the first recurrent deletion reducing toxicity in cancer, expanding the landscape of cancer progression mechanisms.

•Focal deletions of BRD4 regulatory regions are found in multiple cancers•BRD4 deletions normalize BRD4 expression in tumors with larger chromosome 19 gains•In a cell line model, focal BRD4 deletions rescue gene overexpression toxicity•Fine-tuned levels of BRD4 expression are required for sustained tumor proliferation

Focal deletions of BRD4 regulatory regions are found in multiple cancers

BRD4 deletions normalize BRD4 expression in tumors with larger chromosome 19 gains

In a cell line model, focal BRD4 deletions rescue gene overexpression toxicity

Fine-tuned levels of BRD4 expression are required for sustained tumor proliferation

Wala et al. identify recurrent focal BRD4 deletions in tumors harboring larger amplifications, suggesting that these deletions may serve to limit toxic BRD4 overexpression. A CRISPR-Cas9 cell line model validates that fine-tuned BRD4 expression is required for cancer cell growth. Together, these support the first recurrent deletion reducing toxicity in cancer.