Project Summary The goal of the proposed five-year training program is the development of the applicant's independent research career as an academic pediatric oncologist focused on the etiology of genetic mutations in childhood cancer. The applicant completed pediatrics residency training, combined fellowship training in pediatric hematology-oncology and infectious diseases, and has joined the faculty at The Children's Hospital of Philadelphia (CHOP) with a joint role in the Divisions of Oncology and Infectious Diseases. The candidate's goals for the near future are to develop and refine the essential skills that will be required for a successful career as an independent investigator. She specifically seeks to gain expertise in mouse modeling, cancer genomics, and DNA replication in order to augment her research skills. Her long-term goal is to investigate sources of somatic mutation in order to improve diagnostic and therapeutic options for childhood cancer. The mentors for this award include Dr. Matthew Weitzman, an internationally recognized leader in the field of virus- host interactions and genome integrity, and Dr. John Maris, an eminent physician-scientist who has led the field of genetic discovery and targeted therapeutics for childhood cancer. To add depth and breadth to the scientific and career guidance of the applicant, a Mentoring Committee is composed of scientists and physician-scientists from diverse and complementary fields. Dr. Green will benefit from the unparalleled resources and mentorship available at both CHOP and the University of Pennsylvania. The proposed research focuses on the role of the APOBEC3 family of cytosine deaminases in promoting genome instability. These enzymes function within the innate immune system by mutating viral genomes to limit infection. Through off-target activity APOBEC3 enzymes have the capacity to mutate cellular DNA and have been implicated as the source of specific mutational patterns identified in cancer genome sequences. This proposal will identify the molecular and biological impact of APOBEC3 interactions with hematopoietic cells. In this proposal, single-stranded DNA substrates susceptible to APOBEC3 deamination will be investigated, as will the impact of these events on critical cellular processes such as DNA replication and repair. The proposed studies will generate and utilize a mouse model to evaluate the in vivo impact of APOBEC3 activity on hematopoietic lineage development, genome integrity, and malignancy. Preliminary data suggest that DNA damage responses are critical to protection of the genome from accumulation of APOBEC3- induced mutations. Using genetic and chemical inhibitors of DNA damage response factors in leukemia cell lines with active APOBEC3 enzymes, resulting DNA damage and cytotoxicity will be examined. These studies have the potential to uncover synthetic lethal interactions that may be exploited for therapeutic targeting of hematologic malignancies in which APOBEC3 enzymes are active.