Role of DNA polymerases η, ι and ζ in UV resistance and UV-induced mutagenesis in a human cell line

Genes coding for DNA polymerases η, ι and ζ, or for both Pol η and Pol ι have been inactivated by homologous recombination in the Burkitt's lymphoma BL2 cell line, thus providing for the first time the total suppression of these enzymes in a human context. The UV sensitivities and UV-induced mutagenesis on an irradiated shuttle vector have been analyzed for these deficient cell lines. The double Pol η/ι deficient cell line was more UV sensitive than the Pol η-deficient cell line and mutation hotspots specific to the Pol η-deficient context appeared to require the presence of Pol ι, thus strengthening the view that Pol ι is involved in UV damage translesion synthesis and UV-induced mutagenesis. A role for Pol ζ in a damage repair process at late replicative stages is reported, which may explain the drastic UV-sensitivity phenotype observed when this polymerase is absent. A specific mutation pattern was observed for the UV-irradiated shuttle vector transfected in Pol ζ-deficient cell lines, which, in contrast to mutagenesis at the HPRT locus previously reported, strikingly resembled mutations observed in UV-induced skin cancers in humans. Finally, a Pol η PIP-box mutant (without its PCNA binding domain) could completely restore the UV resistance in a Pol η deficient cell line, in the absence of UV-induced foci, suggesting, as observed for Pol ι in a Pol η-deficient background, that TLS may occur without the accumulation of microscopically visible repair factories.