Recent extensive genomic studies of human ovarian carcinomas have described a broad repertoire of common alterations. Unfortunately, utilization of this information is compromised because the originating cell types have not been determined. While some investigators believe that these malignancies arise from the ovarian surface epithelium (OSE), others favor cells of the tubal (Fallopian) epithelium (TE). We identified a stem cell niche located near the junction between the OSE, mesothelium and TE (Flesken-Nikitin et al., Nature 495: 241–245, 2013). Using a mouse model we showed that high-grade serous ovarian carcinoma (HGSOC) may originate from this stem cell niche after inactivation of tumor suppressor genes p53 and Rb, consistent with frequent alterations of their pathways in the same type of human ovarian cancer. OSE stem cells are involved in regeneration of OSE but not TE. More recently we identified another pool of stem/progenitor cells located at the TE/mesothelium junction area (TE-SC), which may serve as an alternative cellular HGSOC origin. It remains unknown if either of these recently identified stem cell populations are susceptible to transforming mutations in genes typically associated with other types of ovarian carcinomas. Furthermore, the presence of similar stem cell niches in humans has not been definitively proven. We propose to address these issues by joint efforts of three laboratories located at Cornell University and Weill Cornell Medical College. Dr. Alexander Nikitin will extend his studies aimed at comparative testing of OSE-SC and TE-SC susceptibility to malignant transformation in the native settings. He will also determine if elevated activity of p53/miR-34/MET/WNT network in normal stem cells may explain their propensity to malignant transformation. Dr. Lora Hedrick Ellenson will test the transforming potential of OSE-SC and TE-SC by using conditional inactivation of the Pten and Apc genes, which are involved in the endometrioid type of ovarian carcinoma. Working together, the Nikitin and Ellenson laboratories will isolate and characterize putative adult human stem cells for OSE and TE and test their transformation potential. Dr. John Schimenti will identify gene mutations critical for malignant transformation of OSE and TE stem and differentiated cells by performing CRISPR-based sensitized in vivo genomic screening. We expect that our proposed research will significantly improve our understanding of ovarian carcinoma pathogenesis and lead to identification of early critical drivers of this malignancy. Our findings also will stimulate targeted searches for preneoplastic/early neoplastic lesions in the areas of transition between OSE, mesothelium and TE in humans.