The Microenvironment in DCIS and Its Role in Disease Progression.

Ductal carcinoma in situ (DCIS) accounts for ~20% of all breast cancer diagnoses but whilst known to be a precursor of invasive breast cancer (IBC), evidence suggests only one in six patients will ever progress. A key challenge is to distinguish between those lesions that will progress and those that will remain indolent. Molecular analyses of neoplastic epithelial cells have not identified consistent differences between lesions that progressed and those that did not, and this has focused attention on the tumour microenvironment (ME).The DCIS ME is unique, complex and dynamic. Myoepithelial cells form the wall of the ductal-lobular tree and exhibit broad tumour suppressor functions. However, in DCIS they acquire phenotypic changes that bestow them with tumour promoter properties, an important evolution since they act as the primary barrier for invasion. Changes in the peri-ductal stromal environment also arise in DCIS, including transformation of fibroblasts into cancer-associated fibroblasts (CAFs). CAFs orchestrate other changes in the stroma, including the physical structure of the extracellular matrix (ECM) through altered protein synthesis, as well as release of a plethora of factors including proteases, cytokines and chemokines that remodel the ECM. CAFs can also modulate the immune ME as well as impact on tumour cell signalling pathways. The heterogeneity of CAFs, including recognition of anti-tumourigenic populations, is becoming evident, as well as heterogeneity of immune cells and the interplay between these and the adipocyte and vascular compartments. Knowledge of the impact of these changes is more advanced in IBC but evidence is starting to accumulate for a role in DCIS. Detailed in vitro, in vivo and tissue studies focusing on the interplay between DCIS epithelial cells and the ME should help to define features that can better predict DCIS behaviour.