Unraveling the role of stromal disruption in aggressive breast cancer etiology and outcomes.

BACKGROUND Aggressive (typically high-grade) breast cancers (BCs) remain major contributors to BC-related mortality globally. The tissue changes underpinning their etiology and outcomes, however, remain poorly characterized. METHODS Spatially resolved machine-learning algorithms were used to characterize "stromal disruption" as a morphological metric of reduced/altered extracellular matrix and increased immune, inflammatory, and/or wound response-related processes in normal, benign breast disease (BBD), and invasive hematoxylin and eosin (H&E)-stained breast tissues. Associations of stromal disruption with BC etiologic factors were assessed among 4023 healthy breast tissue donors, its impact on BC incidence was assessed among 974 BBD patients in a nested case-control study, while its prognostic associations were assessed in 4 BC patient cohorts (n = 4223). RESULTS Epidemiologic risk factors for aggressive BC, including younger age, multiparity, Black race, obesity, and family history, demonstrated strong associations with increasing stromal disruption in H&E sections prior to tumor development. Substantial stromal disruption in BBD H&E was associated with ∼4-fold increased risk of aggressive (high-grade) BC and ∼3 years shorter latency from BBD to BC diagnosis, independently of BBD histology. Across BC cohorts, stromal disruption in H&E was associated with aggressive (mostly high-grade) tumor phenotypes and with markedly poor prognosis among ER-positive patients, irrespective of histology. The immunobiology of stromal disruption reflected heightened innate (CD68+), adaptive (CD3+CD4+, CD3+CD8+), immunoregulatory (CD3+CD4+FOXP3+), immune escape (PD1+PDL1+), endothelial (CD31+), and myofibroblast (α-SMA+) marker expression. CONCLUSION Our findings highlight the active stromal role in aggressive BC etiology and outcomes, opening possibilities for readily identifying high-risk women across the BC continuum that may benefit from stroma-centric preventative or therapeutic strategies.