Project Summary/Abstract The aberrant incorporation or retention of ribonucleic acids (RNAs) in the genome is a common cause of genomic instability, rendering it susceptible to hydrolysis and downstream mutagenesis. The enzyme RNase H2 is one of the primary mechanisms protecting against this destabilization of the genome by removing these genome- embedded RNAs. Our lab recently uncovered a novel mechanism of regulation of RNase H2, by uncovering that replication termination factor 2 (RTF2) is involved in localizing and regulating the levels of RNase H2 at the replication fork. Further elucidation of this interaction is required to understand the basic biology behind the regulation and function of how RNase H2 prevents genome instability. Interestingly, copy number loss of RNase H2 is commonly found in Chronic Lymphocytic Leukemia (CLL), in over 30% of cases. In my preliminary work, I have developed various cellular models in which RNase H2 and RTF2 can be depleted, and I have expressed and purified recombinant RNase H2 and RTF2, allowing for both in vivo studies of which RNase H2 activities are regulated by RTF2 and in vitro studies of their interaction. Furthermore, I have developed a novel assay allowing a quantitative analysis of ribonucleotide incorporation in the genomes of human cells. This assay will be used to study the regulation of RNase H2 by RTF2, and will be assessed in its applicability to predict CLL responses to PARP-inhibitors. The direct mechanism behind tumor progression in the loss of RNase H2 has not been studied. In this proposal, building on my above preliminary work, I will test the hypothesis that RTF2 interacts directly with and regulates the activities of RNase H2 at the replication fork and examine the mechanism behind how loss of RNase H2 compromises genomic stability and leads to tumor progression. I am an MD/PhD student at the Weill Cornell/Rockefeller/Sloan Kettering Tri-Institutional Program, where I am in the laboratory of Dr. Agata Smogorzewska at The Rockefeller University. My long-term goal is to become a physician scientist, practicing as a hematologist-oncologist as well as running an independent basic science lab as an academic university hospital. The plan outlined in this proposal, along with the support and mentorship of Dr. Agata Smogorzewska, my thesis research committee, and the Tri-Institutional MD-PhD program will help me achieve these career aspirations.