Role of DNA polymerases eta, iota and zeta in UV resistance and UV-induced mutagenesis in a human cell line

Genes coding for DNA polymerases eta, iota and zeta, or for both Pol eta and Pol iota 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 eta/iota deficient cell line was more UV sensitive than the Pol eta-deficient cell line and mutation hotspots specific to the Pol eta-deficient context appeared to require the presence of Pol iota, thus strengthening the view that Pol iota is involved in UV damage translesion synthesis and UV-induced mutagenesis. A role for Pol zeta 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 zeta-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 eta PIP-box mutant (without its PCNA binding domain) could completely restore the UV resistance in a Pol eta deficient cell line, in the absence of UV-induced foci, suggesting, as observed for Pol iota in a Pol eta-deficient background, that TLS may occur without the accumulation of microscopically visible repair factories.