Treating the genetic make-up of breast cancer: a new fashion?

Clinical relevance of the triple-negative breast cancer concept: genetic basis and clinical utility of the concept

The beginning of microarray technology in the 1990s and the sequencing of the human genome in 2000 paved the way for the seminal paper of the Stanford group on the molecular portraits of human breast tumours in the same year. They described four distinct breast cancer subtypes, which they called 'luminal', 'basal', 'HER2-positive', and 'normal breast-like', based on unique gene expression patterns. This paper caused a paradigm shift. Breast cancer was no longer hormone receptor-positive or -negative, but rather luminal, basal or HER2-positive. Since then, numerous papers have appeared, trying to further characterise these subtypes on the DNA, RNA and protein level. Other groups have focussed on the epidemiology, prognosis and outcome after therapy of breast cancer patients according to these molecular subtypes. A promising prognostic marker within the subgroup of basal-like breast cancer is an up-regulated immune response, which is associated with favourable outcome. In addition, the majority of basal-like breast cancers harbour traits of a DNA damage repair defect. This feature can be exploited by the use of DNA damaging agents, and first exciting clinical results of the combination of carboplatin, gemcitabine and a poly (ADP-ribose) polymerase 1 (PARP-1) inhibitor have recently been reported. In this review, the molecular characterisation of triple-negative breast cancer, a proxy for basal-like breast cancer, is described and findings have been put into clinical context.

BRCA1-deficient mammary tumor cells are dependent on EZH2 expression and sensitive to Polycomb Repressive Complex 2-inhibitor 3-deazaneplanocin A

Introduction

Treatment of breast cancer is becoming more individualized with the recognition of tumor subgroups that respond differently to available therapies. Breast cancer 1 gene (BRCA1)-deficient tumors are usually of the basal subtype and associated with poor survival rates, highlighting the need for more effective therapy.

Methods

We investigated a mouse model that closely mimics breast cancer arising in BRCA1-mutation carriers to better understand the molecular mechanism of tumor progression and tested whether targeting of the Polycomb-group protein EZH2 would be a putative therapy for BRCA1-deficient tumors.

Results

Gene expression analysis demonstrated that EZH2 is overexpressed in BRCA1-deficient mouse mammary tumors. By immunohistochemistry we show that an increase in EZH2 protein levels is also evident in tumors from BRCA1-mutation carriers. EZH2 is responsible for repression of genes driving differentiation and could thus be involved in the undifferentiated phenotype of these tumors. Importantly, we show that BRCA1-deficient cancer cells are selectively dependent on their elevated EZH2 levels. In addition, a chemical inhibitor of EZH2, 3-deazaneplanocin A (DZNep), is about 20-fold more effective in killing BRCA1-deficient cells compared to BRCA1-proficient mammary tumor cells.

Conclusions

We demonstrate by specific knock-down experiments that EZH2 overexpression is functionally relevant in BRCA1-deficient breast cancer cells. The effectiveness of a small molecule inhibitor indicates that EZH2 is a druggable target. The overexpression of EZH2 in all basal-like breast cancers warrants further investigation of the potential for targeting the genetic make-up of this particular breast cancer type.