The role of DNA polymerase ι in UV mutational spectra

UVB (280–320 nm) and UVC (200–280 nm) irradiation generate predominantly cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts in DNA. CPDs are thought to be responsible for most of the UV-induced mutations. Thymine–thymine CPDs, and probably also CPDs containing cytosine, are replicated in vivo in a largely accurate manner by a DNA polymerase η (Pol η) dependent process. Pol η is a DNA damage-tolerant and error-prone DNA polymerase encoded by the POLH (XPV) gene in humans. Another member of the Y family of error-prone DNA polymerases is POLI encoding DNA polymerase iota (Pol ι). In order to clarify the specific role of Pol ι in UV mutagenesis, we have used an siRNA knockdown approach in combination with a supF shuttle vector which replicates in mammalian cells, similar as we have previously done for Pol η. Synthetic RNA duplexes were used to efficiently inhibit Pol ι expression in 293T cells. The supF shuttle vector was irradiated with 254 nm UVC and replicated in 293T cells in presence of anti-Pol ι siRNA. Surprisingly, there was a consistent reduction of recovered plasmid from cells with Pol ι knockdown and this was independent of UV irradiation of the plasmid. The supF mutant frequency was unchanged in the siRNA knockdown cells relative to control cells confirming that Pol ι does not play an important role in UV mutagenesis. UV-induced supF mutants were sequenced from siRNA-treated cells and controls. Neither the type of mutations nor their distribution along the supF gene were significantly different between controls and siRNA knockdown cells and were predominantly C to T and CC to TT transitions at dipyrimidine sites. These results show that Pol ι has no significant role in UV lesion bypass and mutagenesis in vivo and provides some initial data suggesting that this polymerase may be involved in replication of extrachromosomal DNA.