Project Summary Environmental genotoxins such as oxidation agents, alkylating agents, aromatic amines, crosslinking agents, polycyclic aromatic hydrocarbons, and natural toxins induce a full spectrum of DNA lesions including abasic sites, interstrand crosslinks, and bulky DNA base adducts. These environmental genotoxins are found in our waterways, food, industrial and agricultural chemicals, and air pollution and have the potential to induce mutagenesis and genomic instability if genetic lesions are not repaired. Mutagenesis and genomic instability can lead to developmental disorders, aging, and cancers. HMCES is a replication-coupled repair protein that responds to single-strand DNA abasic sites and prevents their cleavage by AP-endonucleases. Abasic sites are a common lesion caused by environmental genotoxins. My preliminary results suggest that HMCES prevents both genomic instability and mutagenesis, and I hypothesize that it promotes a more faithful repair mechanism such as template switching or fork reversal. For the K99-phase of this proposal I will learn to perform short and long-term mutagenesis assays and DNA deep sequencing methods to understand in detail how HMCES prevents mutagenesis and genomic instability in human cells and promotes more error-free repair. This work will create a technical foundation and blueprint for studies (R00) characterizing the strand-specific replication stress response and how strand-specific obstacles and environmental genotoxins contribute to leading and lagging strand mutagenesis. There are core differences between replication on the leading and lagging strands. DNA replication occurs continuously on the leading strand and discontinuously on the lagging strand through a series of repriming events. I hypothesize that strand-specific lesions and obstacles generate a differential replication stress response, and potentiate mutagenesis differently. I will characterize the lagging and leading strand stress responses using unbiased approaches. Further, I will determine the consequences of strand-specific stress and genotoxins on mutagenic strand bias using deep sequencing-based mutagenesis assays. Ultimately, this proposal will advance the environmental toxicology and DNA repair fields leading to paradigm shifting discoveries.