title: Overproduction of DNA polymerase eta does not raise the spontaneous mutation rate in diploid human fibroblasts.
abstract: Telomerase-immortalized lines of diploid xeroderma pigmentosum variant (XP-V) fibroblasts (XP115LO and XP4BE) were complemented for constitutive or regulated expression of wild-type human DNA polymerase eta (hpol eta). The ectopic gene was expressed from a retroviral LTR at a population average of 34- to 59-fold above the endogenous (mutated) mRNA and high levels of hpol eta were detected by immunoblotting. The POLH cDNA was also cloned downstream from an ecdysone-regulated promoter and transduced into the same recipient cells. Abundance of the wild-type mRNA increased approximately 10-fold by addition of ponasterone to the culture medium. Complemented cell lines acquired normal resistance to the cytotoxic effects of UVC, even in the presence of 1mM caffeine. They also tolerated higher levels of UVC-induced template lesions during nascent DNA elongation when compared to normal fibroblasts (NHF). UVC-induced mutation frequencies at the hypoxanthine-guanine phosphoribosyl transferase (HPRT) locus were measured in the XP115LO+XPV cell line overproducing hpol eta constitutively (E. Bassett, N.M. King, M.F. Bryant, S. Hector, L. Pendyala, S.G. Chaney, M. Cordeiro-Stone, The role of DNA polymerase eta in translesion synthesis past platinum-DNA adducts in human fibroblasts, Cancer Res. 64 (2004) 6469-6475). Induced mutation frequencies were significantly reduced, even below those observed in NHF; however, the average mutation frequency in untreated cultures was about three-fold higher than in the isogenic vector-control cell line. In this study, spontaneous HPRT mutation frequencies were measured at regular intervals, as isogenic fibroblasts either lacking or overproducing hpol eta were expanded for 100 population doublings. The mutation rates estimated from these results were not significantly increased in XP115LO cells expressing abnormal levels of hpol eta, relative to the cells lacking this specialized polymerase. These findings suggest that diploid human fibroblasts with normal DNA repair capacities and intact checkpoints are well protected against the potential mutagenic outcome of overproducing hpol eta, while still benefiting from accurate translesion synthesis of UV-induced pyrimidine dimers.
authors.affiliation: Department of Pathology and Laboratory Medicine, University of North Carolina School of Medicine, Chapel Hill, NC 27599-7525, USA.
authors.forename: Nicole M
authors.initials: NM
authors.lastname: King
authors.forename: Nana
authors.initials: N
authors.lastname: Nikolaishvili-Feinberg
authors.forename: Miriam F
authors.initials: MF
authors.lastname: Bryant
authors.forename: Douglas D
authors.initials: DD
authors.lastname: Luche
authors.forename: Timothy P
authors.initials: TP
authors.lastname: Heffernan
authors.forename: Dennis A
authors.initials: DA
authors.lastname: Simpson
authors.forename: Fumio
authors.initials: F
authors.lastname: Hanaoka
authors.forename: William K
authors.initials: WK
authors.lastname: Kaufmann
authors.forename: Marila
authors.initials: M
authors.lastname: Cordeiro-Stone
mesh_terms: D015153:Blotting, Western; D002110:Caffeine / Q000494:pharmacology; D002461:Cell Line, Transformed; D004260:DNA Repair; D004259:DNA-Directed DNA Polymerase / Q000235:genetics / Q000378:metabolism*; D004171:Diploidy; D004441:Ecdysterone / Q000031:analogs & derivatives / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism / Q000473:pathology / Q000528:radiation effects; D016368:Frameshift Mutation; D018628:Gene Dosage; D005816:Genetic Complementation Test; D014644:Genetic Variation; D006801:Humans; D007041:Hypoxanthine Phosphoribosyltransferase / Q000235:genetics; D007700:Kinetics; D012333:RNA, Messenger / Q000187:drug effects; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014466:Ultraviolet Rays; D014983:Xeroderma Pigmentosum / Q000201:enzymology / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D000099270:Y-Family DNA Polymerases
chemical_list: D002110:Caffeine; D004259:DNA-Directed DNA Polymerase; D004441:Ecdysterone; D007041:Hypoxanthine Phosphoribosyltransferase; D012333:RNA, Messenger; C008999:ponasterone A; C112338:Rad30 protein; D000099270:Y-Family DNA Polymerases