A genetic mosaic mouse model illuminates the pre-malignant progression of basal-like breast cancer.

Basal-like breast cancer (BLBC) is highly aggressive, often characterized by BRCA1 and p53 deficiency. Although conventional mouse models enabled the investigation of BLBC at malignant stages, its initiation and pre-malignant progression remain under-studied. Here, we leveraged a mouse genetic system known as Mosaic Analysis with Double Markers (MADM) to study BLBC initiation by generating rare GFP+ Brca1, p53-deficient mammary cells alongside RFP+ wildtype sibling cells. After confirming the close resemblance of mammary tumors arising in this model to human BLBC at both transcriptomic and genomic levels, we focused our studies on the pre-malignant progression of BLBC. Initiated GFP+ mutant cells showed a stepwise pre-malignant progression trajectory from focal expansion to hyper-alveolarization and then to micro-invasion. Furthermore, despite morphological similarities to alveoli, hyper-alveolarized structures actually originate from ductal cells based on twin-spot analysis of GFP-RFP sibling cells. Finally, luminal-to-basal transition occurred exclusively in cells that have progressed to micro-invasive lesions. Taken together, our MADM model provides excellent spatiotemporal resolution to illuminate the pre-malignant progression of BLBC, and should enable future studies on early detection and cancer prevention for this devastating cancer.