The study of DNA damage has broad relevance to human pathophysiology with its involvement in birth defects, cancer, premature aging syndromes, and certain neurologic disorders. Single strand breaks (SSBs) are among the most common form of endogenous DNA damage. Here we will describe mechanisms by which SSBs threaten genome integrity in mitotic and post-mitotic cells. If they are not resolved in a timely manner, SSBs can derail passing replication forks converting them to toxic double strand breaks (DSBs). These so called “collapsed forks” arise spontaneously in every S phase and are believed to be the primary physiological trigger of homologous recombination. We will describe a recent study using the CRISPR/Cas9 nicking enzymes to examine the interaction of the human replication machinery with SSBs. We combine direct analyses of DNA end structures at sites of DNA nick-induced fork collapse with detailed mapping of repair factor binding to establish a comprehensive high-resolution view of how replication-coupled DSBs are generated, processed, and repaired. Our recent studies have also identified unexpected high levels of single strand breakage in neurons, which arise during metabolic processes intrinsic to neuronal development, differentiation, and maintenance. The source of this “programmed” DNA breakage in neurons will be described, and how this physiological process can be corrupted to drive neurodegenerative diseases and neurotoxicity after chemotherapy. Through these basic research efforts, we hope to identify vulnerabilities specific to cancer cells and to discover mechanisms underpinning chemotherapy-induced neurotoxities, which may contribute to new preventive and treatment strategies.

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