EXECUTIVE SUMMARY The overall goal of this proposal is to investigate the role of chromosomal aberrations (aneuploidies) in determining the aggressiveness of tumors derived from human pluripotent stem cells (hPSCs) and to identify chromosomal events or genes that lead to aggressive hPSC-derived tumors. The underlying hypothesis is that aneuploid hPSCs develop more aggressive tumors than do diploid hPSCs. The hPSC lines to be studied comprise normal diploid human embryonic stem cells (hESCs), hESC derived from aneuploid blastocysts, hESC with chromosomal aberrations resulting from extended time in culture, and diploid and aneuploid human induced pluripotent stem cells (hiPSC). The applicant will use these cell lines to generate teratomas and proposes to assess tumor status using histological and molecular readouts. Molecular profiling of these tumors is expected to reveal clues about the mechanisms that underlie the differences in tumor aggressiveness. Based on these findings, the expression of the best candidate molecules will be manipulated in order to further explore their role in stem cell fate and tumorigenicity. Finally, the applicant will explore the potential for teratoma-specific genes to serve as targets for anti-tumor drugs. The reviewers agreed that understanding how to decrease the likelihood of teratoma formation from transplanted hPSC derivatives is an important problem in the stem cell field and worth studying. However, reviewers pointed out that there is no consensus in the field as to how significant the problem of aneuploidy-driven aggressive tumors will be in the clinic and thought that this aspect of teratoma biology does not represent the highest priority in hESC or hiPSC research. Although the reviewers agreed that pursuing the problem of teratoma formation is important, they were less enthusiastic about the proposed approach and questioned whether it would lead to the identification of specific mechanisms or predictions. The reviewers were uncertain of the overall feasibility of this proposal, which they found difficult to assess due to a briefly described, overly general research plan that appeared unfocused and lacked experimental detail. For example, details were lacking with respect to the methods and histological parameters that would be used to score and characterize tumors. Furthermore, reviewers commented that the rationale for much of the latter aims, such as the decision to focus the search for teratoma-specific proteins on the cell surface rather than cytosolic factors like the one described in the prior work, was not adequately supported by the preliminary data. Moreover, reviewers were not convinced that there is sufficient evidence to suggest that teratomas derived from hPSC necessarily use different survival mechanisms than do hPSC or the desired differentiated progeny, which dampened their enthusiasm for the proposed anti-tumor approaches. While appreciative of the investigator’s unique access to chromosomally abnormal hPSCs, the reviewers criticized the lack of plans to compare multiple hiPSC lines derived from the same individual to better control for the known variability amongst hiPSC lines. In addition, some reviewers questioned the conclusion that hiPSCs display novel aneuploidies compared to hESCs, given the limited number of lines that have been studied and uncertainties about the statistical significance of the studies. The reviewers described the principal investigator as a talented, committed scientist with an outstanding track record of contributions to his/her field. The members of the research team were all thought to be well qualified, and the institution was judged to provide an excellent environment for the proposed studies. In summary, the reviewers felt that this application addressed an important topic but was limited in impact due to numerous ambiguities and uncertainties in the experimental design.