Host: The Japan Radiation Research Society
To investigate how the XPA mutations influence the temporal and spatial interactions between NER proteins and DNA damage, detergent-insoluble PCNA was visualized in XP-A cells using micropore UV irradiation combined with fluorescent antibody labeling. Interestingly, the PCNA foci appeared at UV damage sites as late as 9 h and 24 h post-irradiation, even though XP-A cells could not repair CPDs and 6-4PPs at all. The following explanations may underlie the delayed recruitment of PCNA to DNA damage sites. Firstly, it might be due to DNA repair that occurs normally in XP-A cells. Defects in NER do not affect the global genome repair of oxidative DNA damage or single strand breaks, which are minor types of DNA lesions caused by UV. Secondly, it might relate to DNA fragmentation resulting from apoptosis. The UV irradiation level (100 J per m2) used in this study produces a high density of DNA damage at localized nuclear areas and might cause apoptotic DNA breaks, which are possibly associated with PCNA dependent repair. Thirdly, it might reflect blocked or delayed DNA replication. When partially irradiated cells enter the S phase, DNA replication in damaged nuclear areas may be slowed down more than in undamaged areas because of the block by DNA damage. Finally, it might be attributable to an incomplete or stalled NER of CPDs or 6-4PPs. If the lack of functional XPA interrupts the normal NER process and keeps it stalled for more than several hours, other NER proteins including PCNA might be recruited slowly to the sites of DNA damage. Our results support the last explanation.
