Normal human oral keratinocytes demonstrate abnormal DNA end joining activity during replicative senescence

Repair of DNA double-strand breaks (DSBs) is critical for the maintenance of cellular genetic integrity. DSBs are repaired by cellular end joining activity, which could proceed with varying degrees of accuracy. Abnormal end joining may lead to an accumulation of mutations and contribute to genetic instability and cellular aging. In the present study, we compared the efficiency and accuracy of end joining activities in exponentially replicating and senescing normal human oral keratinocytes (NHOK). We developed an in vitro end joining assay utilizing a plasmid linearized with a unique EcoR I or EcoR V restriction site. The efficiency of end joining was determined by PCR with primers that could amplify the fragment containing the end joining site. The accuracy of end joining was assessed by determining whether the original EcoR I site was restored after end joining. Both replicating and senescing cultures of NHOK yielded a similar level of end joining efficiency, which was noted by the similar intensity of PCR amplification. However, the frequency of end joining errors was significantly elevated in NHOK during replicative senescence. Senescing NHOK could thus accumulate abnormal end joining products, which might contribute to cellular aging and cancer.