Premalignant Development of BRCA-1-associated Ovarian & Breast Cancer

Understanding the premalignant development of cancer is fundamental for minimizing cancer incidence among individuals at risk. Women with germline BRCA1 mutations face a remarkably high risk for ovarian and breast cancers. However, few options are available to reduce cancer risk for these mutation carriers. Further investigation of the critical alterations along the premalignant development of cancer is essential for designing rational preventions. In this thesis, I leveraged a unique genetic mosaic tool (Mosaic Analysis with Double Markers, MADM) to establish mouse models that allow clonal tracking of the entire premalignant development of cancer. The cellular alterations along the premalignant development of BRCA1-mutated ovarian and breast cancers were interrogated.

With a MADM-based mouse model for BRCA1-mutated ovarian cancer, I delineated the progressive trajectory from an initiated cell to premalignant lesions through clonal analysis and spatial profiling. I found that the initial mutant cells went through dichotomous clonal expansion —only a rare subset of clones progressed, whereas others stalled immediately upon acquiring oncogenic mutations. Spatial profiling and marker staining revealed that the expanded clones are less differentiated, more proliferative, and bi-potential, indicative of stem/progenitor-like cells as their founders. Oncogenic mutations conferred cells in the expanded clones with sustained proliferation and unbalanced differentiation toward a primitive cell fate that led to long-term clonal persistence, representing premalignant progression toward precursor cancer lesions. This study warrants further investigations into the mechanisms of how oncogenic mutations extend proliferation, augment survival, and bias differentiation, which may ultimately lead to novel approaches for effective cancer preventions.

With a MADM-based mouse model for BRCA1-mutated breast cancer, I performed a spatial-temporal tracing of the early cancer genesis and revealed multiple intermediate stages before tumor formation, including focal expansion of mutant cells, hyper-alveologenesis of mammary ducts, and formation of disorganized cell mass. I found the earliest cancer progression event—mutant cell focal expansion- occurred specifically in the ductal, but not in the alveolar region. Further progression of mutant cells led to ductal hyper-alveolarization, the earliest morphological abnormality that resembles those ducts in early pregnancy. Afterward, mutant cells progressed to a disorganized cell mass that lost the ductal organization and showed a partial luminal-to-basal transition state. Full-blown tumors formed shortly thereafter. This study delineated a set of ordered cellular alterations along the cancer premalignant development, which should pave the way for the future success of cancer early detection and prevention.

Collectively, the studies presented in this thesis established new MADM-based mouse models for BRCA1-mutated ovarian and breast cancers, which present a powerful tool for studying the premalignant development of cancer. The cellular alterations that were characterized provide critical insights and targets for designing novel approaches for cancer early detection and prevention in the future. More broadly, this research scheme, which interrogates the premalignant development of cancer through in vivo clonal tracing with MADM-based mouse models, can be applied to all other cancer types to search critical progression events for cancer early detection and prevention.