In the funded parent grant 1R01 CA256199-01A1 entitled “premalignant chronology of cell-state variability in basal-like breast cancer”, we proposed to comprehensively and progressively analyze gene expression profiles of mutant cells during the premalignant stage of basal-like breast cancer to understand how cell state plasticity could lead to tumor progression. Basal-like carcinoma is a rapidly-progressing and highly variable subtype of breast cancer that arises spontaneously (often in African Americans) or in genetically predisposed women. Such tumors are believed to arise from the functional loss of the BRCA1 and TP53 tumor suppressors in uncommitted basoluminal progenitors of the breast. However, it has been challenging to dissect the origins of the disease for lack of appropriate tools, making it difficult to conceive of how the cell-state variability of premalignant mutants gives rise to basal-like breast cancer. The current application deploys a novel genetically engineered mouse model called mosaic analysis of double markers (MADM), which randomly deletes murine Brca1–Trp53 in transit-amplifying progenitors of the mammary gland. In MADM, stochastic deletion is genetically defined by coexpression of GFP, allowing locally expanded premalignant lesions to be visualized within the gland before the onset of basal-like disease. The aims of the parent R01 are to use transcriptomic analysis: 1) To define shared premalignant trajectories of basoluminal diversification triggered by BRCA1– TP53 deficiency in mice and humans. 2) To deconvolve the immune heterogeneities that are locally paired with specific premalignant ecosystems for basal-like breast cancer. 3) To functionally validate cell states important for progression by using genetic, pharmacologic, and paracrine perturbations that homogenize intrinsic or extrinsic variability of premalignant cells ex vivo. Co-PIs Janes and Zong are thought leaders in their respective fields of intratumor cell-state heterogeneity and genetically engineered mouse modeling with a multi-year track record of collaboration. Together with a pair of senior clinicians, the team is poised to have a significant overall impact on our understanding of basal-like breast tumorigenesis. The project proposed by the supplement candidate is distinct from but perfectly complements with the experiments proposed in the parent R01: while the parent R01 focuses on delineating cell state plasticity based on gene expression profiles, her project approaches the problem from phenotype-guided functional interventions. In Specific Aim 1, she plans to investigate the involvement of aberrant hormonal signaling during premalignant progression and test potential treatment strategies. In Specific Aim 2, she will investigate whether premalignant hyperproliferation can be exploited using PARPi treatment. As a whole, these complementary experiments should not only allow us to dive more deeply into key mechanisms of premalignant progression of bas...