Ultraviolet-induced mutations in Cockayne syndrome cells are primarily caused by cyclobutane dimer photoproducts while repair of other photoproducts is normal.

Abstract

We compared the contribution to mutagenesis in Cockayne syndrome (CS) cells of the major class of UV photoproducts, the cyclobutane pyrimidine dimer, to that of other DNA photoproducts by using the mutagenesis shuttle vector pZ189. Lymphoblastoid cell lines from the DNA repairdeficient disorders CS and xeroderma pigmentosum (XP) and a normal line were transfected with UV-treated pZ189. Cyclobutane dimers were selectively removed before transfection by photoreactivation (PR), leaving nondimer photoproducts intact. After UV exposure and replication in CS and XP cells, plasmid survival was abnormally reduced and mutation frequency was abnormally elevated. After PR, plasmid survival increased and mutation frequency in CS cells decreased to normal levels but remained abnormal in XP cells. Sequence analysis of >200 mutant plasmids showed that with CS cells a major mutational hot spot was caused by unrepaired cyclobutane dimers. These data indicate that with both CS and XP cyclobutane dimers are major photoproducts generating reduced plasmid survival and increased mutation frequency. However, unlike XP, CS cells are proficient in repair of nondimer photoproducts. Since XP but not CS patients have a high frequency of UV-induced skin cancers, our data suggest that prevention of UV-induced skin cancers is associated with proficient repair of nondimer photoproducts.

Why Cockayne syndrome patients do not get cancer despite their DNA repair deficiency

“…2B). These results are consistent with previous reports of UV-induced mutagenesis in reporter genes (11,(13)(14)(15) and validate our use of duplex sequencing for the detection of UV-induced mutagenesis. We chose to carry out subsequent experiments using UVC because UVB and UVC produce similar mutagenic photoproducts.…”

“…Similar studies in CS cells, however, failed to show an increase in UV-induced mutations in HPRT, T-cell receptor, or glycophorin A gene loci (13). In contrast, an episomal plasmid (pZ189), irradiated with UV and passed through CS cells, showed increased levels of mutations (14,15). The limitations of these methodologies include the small number of potential gene loci suitable for drug selection and the possibility that episomal vectors may not fully induce the DNAdamage response of whole cells, thereby resulting in a high mutation frequency that is not representative of mutagenesis in chromosomal loci.…”

Nucleotide excision repair and cancer

“…Finally, differences in repair capacity of different types of UV-induced DNA photoproducts may play a role in tumorigenesis. CS cells can repair 6-4 photoproducts normally but not cyclobutane pyrimidine dimers (Parris and Kraemer 1993). This holds also true for TTD patient cells with a defect in the XPD gene (Itin et al 2001).…”

Molecular cloning and characterization of Saccharomyces cerevisiae RAD28, the yeast homolog of the human Cockayne syndrome A (CSA) gene

“…CS cells have been reported to be hypermutable after exposure to UV radiation (39). Additionally, both XP and CS cells were shown to be defective in mutagenesis observed in normal cells following their transfection with triplex-forming oligonucleotides (61).…”