Oxidative damage-induced PCNA complex formation is efficient in xeroderma pigmentosum group A but reduced in Cockayne syndrome group B cells

Proliferating cell nuclear antigen (PCNA), a processivity factor for DNA po lymerases delta and epsilon, is essential for both DNA replication and repa ir. PCNA is required in the resynthesis step of nucleotide excision repair (NER). After UV irradiation, PCNA translocates into an insoluble protein co mplex, most likely associated with the nuclear matrix. It has not previousl y been investigated in vivo whether PCNA complex formation also takes place after oxidative stress. in this study, we have examined the involvement of PCNA in the repair of oxidative DNA damage. PCNA complex formation was stu died in normal human cells after treatment with hydrogen peroxide, which ge nerates a variety of oxidative DNA lesions. PCNA was detected by two assays , immunofluorescence and western blot analyses. We observed that PCNA redis tributes from a soluble to a DNA-bound form during the repair of oxidative DNA damage. PCNA complex formation was analyzed in two human natural mutant cell lines defective in DNA repair: xeroderma pigmentosum group A (XP-A) a nd Cockayne syndrome group B (CS-B). XP-A cells are defective in overall ge nome NER while CS-B cells are defective only in the preferential repair of active genes. Immunofluorescent detection of PCNA complex formation was sim ilar in normal and XP-A cells, but was reduced in CS-B cells. Consistent wi th this observation, western blot analysis in CS-B cells showed a reduction in the ratio of PCNA relocated as compared to normal and XP-A cells. The e fficient PCNA complex formation observed in XP-A cells following oxidative damage suggests that formation of PCNA-dependent repair foci may not requir e the XPA gene product. The reduced PCNA complex formation observed in CS-B cells suggests that these cells are defective in the processing of oxidati ve DNA damage.