Impact of Ku80 in genomic stability, cancer and aging

Double strand lesions are most dangerous since single unrepaired double strand lesion could kill the cell. Most times, unrepaired double stand lesions would lead to genomic instability and tumor formation. In mammals, nonhomologous end joining (NHEJ) and homologous recombination (HR) is the most important pathways for repairing DNA double-strand breaks (DSBs). Ku70 and Ku80 form heterodimer and are essential for NHEJ. Ku80-mutants are reported to exhibit early aging with very low cancer levels while only Ku70-mutants are reported to exhibit leaky scid, and high incidence of thymic lymphomas. This dissertation focuses on Ku80's function in molecular level besides NHEJ and their implication on ageing and cancer.; The phenotypic difference between Ku80-mutant and Ku70-mutant mice could be explained by two possible reasons. First, either Ku70 or Ku80 function outside the Ku heterodimer such that deletion of one is not identical to deletion of the other. Second, divergent genetic backgrounds/environments influence phenotype. To distinguish between these possibilities, the Ku70 and Ku80 mutations were crossed together to generate Ku70-, Ku80- and double-mutant mice in the same genetic background raised in the same environment. We show these three cohorts have identical phenotypes most similar to the one reported for Ku80-mutant mice. These cohorts exhibit low cancer incidence and early aging but do not exhibit leaky scid or aganglionosis. Thus, divergent genetic backgrounds and/or environments likely account for these different phenotypes.; The fundamental question raised from Ku80-mutant mice phenotype is: Does only NHEJ deficiency cause ageing phenotypes or other functions of Ku80 also contributes to it. The free radical theory of aging proposes that by-products of oxygen metabolism, called reactive oxygen species (ROS), damage macromolecules that ultimately lead to age-related decline. Mammalian aging models suggest that DNA is a target for ROS-induced damage since most of these models are defective in DNA repair. I showed that deletion of Ku80, but not another NHEJ protein, DNA Ligase IV (Lig4), decreases resistance to ROS and alkylating agents that generate base lesions predominately corrected by BER. Most of these agents fail to induce a DSB repair response as measured by gamma-H2AX foci; thus, these lesions are likely restricted to a single DNA strand. Ku80-deletion impairs both single nucleotide and long patch BER, and the BER deficiency can be rescued by overexpression of specific BER components. Thus, Ku80 deletion disables BER and that could contribute to the early aging phenotype exhibited by Ku80-mutant mice.