Person:

Tao, Luwei

Summary Lineage tracing using Cre/lox transgenic mice provides a powerful tool for studying normal mammary epithelial cell (MEC) development and the cellular origins of mammary tumors under physiological settings. However, generation of new transgenic mice for lineage-tracing purposes is often time consuming. Here, we report a lineage-tracing tool for MECs based on intraductal injection of lineage-specific Cre-expressing adenovirus (Ad-Cre). Using well-characterized promoters for Keratin 8 and Keratin 14, we generated lineage-specific Ad-Cre lines for luminal and basal MECs, respectively. By pulse-chase lineage tracing using these Ad-Cre lines, we showed that luminal and basal lineages are largely self-sustained and that IRS1 and IRS2 are essential for maintaining the basal lineage; we also showed that heterogeneous mammary tumors can be induced from luminal MECs in mice carrying the Etv6-NTRK3 fusion gene. Overall, we validated the Ad-Cre system as a promising and efficient tool for fate mapping of normal and malignant cells in adult tissues.

Most breast cancers may have a luminal origin. TP53 is one of the most frequently mutated genes in breast cancers. However, how p53 deficiency contributes to breast tumorigenesis from luminal cells remains elusive. Here we report that induced p53 loss in Krt8+ mammary luminal cells leads to their clonal expansion without directly affecting their luminal identity. All induced mice develop mammary tumours with 9qA1 (Yap1) and/or 6qA2 (Met) amplification(s). These tumours exhibit a mammary stem cell (MaSC)-like expression signature and most closely resemble claudin-low breast cancer. Thus, although p53 does not directly control the luminal fate, its loss facilitates acquisition of MaSC-like properties by luminal cells and predisposes them to development of mammary tumours with loss of luminal identity. Our data also suggest that claudin-low breast cancer can develop from luminal cells, possibly via a basal-like intermediate state, although further study using a different luminal promoter is needed to fully support this conclusion.

Summary It has been shown that the mammary luminal lineage could be maintained by luminal stem cells or long-lived progenitors, but their identity and role in breast cancer remain largely elusive. By lineage analysis using Wap-Cre mice, we found that, in nulliparous females, mammary epithelial cells (MECs) genetically marked by Wap-Cre represented a subpopulation of CD61+ luminal progenitors independent of ovarian hormones for their maintenance. Using a pulse-chase lineage-tracing approach based on Wap-Cre adenovirus (Ad-Wap-Cre), we found that Ad-Wap-Cre-marked nulliparous MECs were enriched with CD61+ alveolar progenitors (APs) that gave rise to CD61− alveolar luminal cells during pregnancy/lactation and could maintain themselves long term. When transformed by different oncogenes, they could serve as cells of origin of heterogeneous mammary tumors. Thus, our study revealed a type of long-lived AP within the luminal lineage that may serve as the cellular origin of multiple breast cancer subtypes.