# Protocol to Establish Estrogen Receptor-Negative Heterozygous BRCA1 Organoids

## TL;DR

This paper describes a protocol to create breast organoids with a BRCA1 mutation and no estrogen receptors, helping study how cancer develops in these cells.

## Contribution

The novel contribution is a protocol for generating ER/PR/HER2-negative breast organoids with a BRCA1 germline mutation.

## Key findings

- The organoids are unresponsive to estrogen and express the stem/progenitor marker CD44.
- The organoids contain outgrowths resembling mature ductal and lobular mammary gland units.

## Abstract

Cancer development in BRCA1 carriers is a multi-step process, which is triggered by several factors and mechanisms that are not clearly understood. Most BRCA1 carriers develop triple-negative breast cancer (TNBC)—estrogen receptor (ER)-, progesterone receptor (PR)-, and HER2 -negative cancers—which originates from ER/PR/HER2-negative breast progenitor cells. Due to a lack of ER/PR/HER2-negative cell models with BRCA mutations, the processes inducing cancer development in BRCA carriers have not been comprehensively studied. Thus, studies characterizing ER/PR/HER2-negative cells carrying a BRCA1 germline mutation are needed to gain more in-depth knowledge about the steps leading to cancer initiation in BRCA1 carriers. To study the cancer development in these patients, we established a protocol for the generation of human ER/PR/HER2-negative breast organoids carrying a BRCA1 germline mutation. We confirmed that these organoids are unresponsive to estrogen, can self-renew, and express the stem/progenitor marker CD44. In addition, we observed that these organoids contain outgrowths that resemble the mature ductal and lobular units of the mammary gland, thus making it a suitable model system to study how cancer develops in ER/PR/HER2-negative mammary cells that carry a BRCA1 germline mutation.