Molecular Biology of Breast Cancer

Breast cancer is a common disease affecting approximately 1 in 10 women at some time in their lives. Moreover, it is not a single disease but is characterised by distinct pathological types with different clinical outcomes. In the past decade, considerable progress has been made in understanding these different cancer subtypes at the molecular level. In particular, global gene expression studies have identified five predominant subtypes that have unique expression signatures. Unlike many other cancers, where a hierarchy of genetic mutations that leads to tumorigenesis has been identified, breast tumours have diverse genetic mutations that affect a variety of signalling pathways. The recent discovery of micro ribonucleic acids (microRNAs) and their role in tumour metastasis has opened up a whole new area of research. Although some highly successful treatments have been developed, the challenge for the future is the development of individualised therapies that are specific to each patient's tumour type. Key Concepts: Breast cancer is a heterogeneous disease characterised by distinct pathological types with different outcomes. Breast tumours have diverse genetic mutations that affect a variety of signalling pathways. Breast tumours can be classified into different subgroups according to their molecular expression profiles. It is not clear whether specific molecular pathways establish the subtype of breast cancer or whether different cell types become transformed and give rise to each tumour subtype. Breast cancer patients would benefit from individual therapeutic regimens designed to treat their particular type of cancer. Keywords: mammary gland; breast cancer; cell of origin; molecular classification; individualised therapy