Metabolic processing of cyclobutyl pyrimidine dimers and (6-4) photoproducts in UV-treated human cells. Evidence for distinct excision-repair pathways.

A new nuclease digestion assay was developed to elucidate the human excision-repair system operating on cyclobutyl pyrimidine dimers and (6-4) photoproducts. We analyzed lesions that accumulated in excised oligonucleotide fragments during incubation of UV-treated cultured fibroblasts. (6-4) photoproducts were removed intact, whereas excised cyclobutyl dimers often contained ruptured interpyrimidine phosphodiester bonds, raising the possibility that the intradimer backbone-cleavage reaction may help promote the bypass of unexcised dimers by the DNA replication or RNA transcription machinery. Cell strains representing eight different inherited forms of the cancer-prone skin disease xeroderma pigmentosum (XP) were generally found to exhibit characteristic abilities to excise the two classes of photolesions, ranging from total deficiency in groups A and G to normal proficiency in the variant. The capacity of any given XP group to act on one class of photoproducts in no way predicted its ability to act on the other. Finally, in those XP strains displaying significant levels of dimer removal, the ratio of intact-versus-modified dimers was normal, implying that rupture of the intradimer backbone linkage occurs independently of subsequent excision-repair reactions. Our data indicate that cyclobutyl dimers and (6-4) photoproducts are processed by distinct nucleotide-excision-repair pathways in human cells.