Data from Evidence that in Xeroderma Pigmentosum Variant Cells, which Lack DNA Polymerase η, DNA Polymerase ι Causes the Very High Frequency and Unique Spectrum of UV-Induced Mutations

Xeroderma pigmentosum variant (XPV) patients have normal DNA excision repair, yet are predisposed to develop sunlight-induced cancer. They exhibit a 25-fold higher than normal frequency of UV-induced mutations and very unusual kinds (spectrum), mainly transversions. The primary defect in XPV cells is the lack of functional DNA polymerase (Pol) η, the translesion synthesis DNA polymerase that readily inserts adenine nucleotides opposite photoproducts involving thymine. The high frequency and striking difference in kinds of UV-induced mutations in XPV cells strongly suggest that, in the absence of Pol η, an abnormally error-prone polymerase substitutes. In vitro replication studies of Pol ι show that it replicates past 5′T-T3′ and 5′T-U3′ cyclobutane pyrimidine dimers, incorporating G or T nucleotides opposite the 3′ nucleotide. To test the hypothesis that Pol ι causes the high frequency and abnormal spectrum of UV-induced mutations in XPV cells, we identified an unlimited lifespan XPV cell line expressing two forms of Pol ι, whose frequency of UV-induced mutations is twice that of XPV cells expressing one form. We eliminated expression of one form and compared the parental cells and derivatives for the frequency and kinds of UV-induced mutations. All exhibited similar sensitivity to the cytotoxicity of UV_{(254 nm)}, and the kinds of mutations induced were identical, but the frequency of mutations induced in the derivatives was reduced to ≤50% that of the parent. These data strongly support the hypothesis that in cells lacking Pol η, Pol ι is responsible for the high frequency and abnormal spectrum of UV-induced mutations, and ultimately their malignant transformation. [Cancer Res 2007;67(7):3018–26]