title: Why Cockayne syndrome patients do not get cancer despite their DNA repair deficiency.
abstract: Cockayne syndrome (CS) and xeroderma pigmentosum (XP) are human photosensitive diseases with mutations in the nucleotide excision repair (NER) pathway, which repairs DNA damage from UV exposure. CS is mutated in the transcription-coupled repair (TCR) branch of the NER pathway and exhibits developmental and neurological pathologies. The XP-C group of XP patients have mutations in the global genome repair (GGR) branch of the NER pathway and have a very high incidence of UV-induced skin cancer. Cultured cells from both diseases have similar sensitivity to UV-induced cytotoxicity, but CS patients have never been reported to develop cancer, although they often exhibit photosensitivity. Because cancers are associated with increased mutations, especially when initiated by DNA damage, we examined UV-induced mutagenesis in both XP-C and CS cells, using duplex sequencing for high-sensitivity mutation detection. Duplex sequencing detects rare mutagenic events, independent of selection and in multiple loci, enabling examination of all mutations rather than just those that confer major changes to a specific protein. We found telomerase-positive normal and CS-B cells had increased background mutation frequencies that decreased upon irradiation, purging the population of subclonal variants. Primary XP-C cells had increased UV-induced mutation frequencies compared with normal cells, consistent with their GGR deficiency. CS cells, in contrast, had normal levels of mutagenesis despite their TCR deficiency. The lack of elevated UV-induced mutagenesis in CS cells reveals that their TCR deficiency, although increasing cytotoxicity, is not mutagenic. Therefore the absence of cancer in CS patients results from the absence of UV-induced mutagenesis rather than from enhanced lethality.
authors.affiliation: Department of Pathology, University of Washington, Seattle, WA 98195;
authors.forename: Kate S
authors.initials: KS
authors.lastname: Reid-Bayliss
authors.affiliation: Department of Dermatology, University of California, San Francisco, CA 94143;
authors.forename: Sarah T
authors.initials: ST
authors.lastname: Arron
authors.affiliation: Department of Pathology, University of Washington, Seattle, WA 98195;
authors.forename: Lawrence A
authors.initials: LA
authors.lastname: Loeb
authors.affiliation: California Pacific Medical Center, San Francisco, CA 94107.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Bezrookove
authors.affiliation: Department of Dermatology, University of California, San Francisco, CA 94143; james.cleaver@ucsf.edu.
authors.forename: James E
authors.initials: JE
authors.lastname: Cleaver
mesh_terms: D003057:Cockayne Syndrome / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D004247:DNA / Q000737:chemistry* / Q000378:metabolism; D053903:DNA Breaks, Double-Stranded; D004260:DNA Repair; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism / Q000528:radiation effects; D064368:Healthy Volunteers; D006801:Humans; D015603:Keratinocytes / Q000166:cytology / Q000378:metabolism; D009154:Mutation; D061251:Primary Cell Culture; D017422:Sequence Analysis, DNA; D012878:Skin Neoplasms / Q000209:etiology / Q000235:genetics / Q000473:pathology / Q000517:prevention & control; D014466:Ultraviolet Rays / Q000009:adverse effects*; D014983:Xeroderma Pigmentosum / Q000235:genetics* / Q000378:metabolism / Q000473:pathology
chemical_list: D004247:DNA
keywords: RNA pol II; apoptosis; dipyrimidines; mutagenesis; transcription arrest