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

Abstract

Xeroderma pigmentosum variant (XPV) patients have normal DNA excision repair, yet are predisposed to develop sunlightinduced 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) H, 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 H, an abnormally error-prone polymerase substitutes. In vitro replication studies of Pol I 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 I 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 I, 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 UVinduced 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 V50% that of the parent. These data strongly support the hypothesis that in cells lacking Pol H, Pol I 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]

DNA polymerase ζ cooperates with polymerases κ and ι in translesion DNA synthesis across pyrimidine photodimers in cells from XPV patients

“…2B). Similarly, knocking-down the expression of POLI had no effect on UV sensitivity, which was different from several (but not all) previous reports (13)(14)(15)22). In contrast, knocking-down the expression of POLK increased UV sensitivity up to 3.5-5-fold, and REV3L by up to 33-to 39-fold (Fig.…”

“…reports (13)(14)(15)22). As far as mutagenesis is concerned, we saw essentially no effect on mutagenic TLS across a TT CPD when POLI was knocked-down.…”

“…As far as mutagenesis is concerned, we saw essentially no effect on mutagenic TLS across a TT CPD when POLI was knocked-down. Previous studies reported conflicting results of either decreased, altered or unchanged UV mutagenesis in cells deficient in pol (13)(14)(15)35). Like in the case of pol, this may reflect differences in methodologies or cell types.…”

“…The UV hypermutability is explained by the activity of a back-up DNA polymerase that performs TLS across CPD with lower efficiency and higher error-frequency. Although there is evidence that pol is involved in TLS across CPD in XPV cells (13)(14)(15), additional polymerases may be involved, and the picture is far from being complete. This is an important issue because these polymerases are likely to be driving sunlight-induced skin carcinogenesis in XPV patients.…”

p53 suppression overwhelms DNA polymerase η deficiency in determining the cellular UV DNA damage response

“…Since XP-V cells lack Pol η, they must employ other mechanisms aside from Pol η-dependent translesional synthesis to negotiate blocks to DNA replication, such as those posed by UV lesions in DNA. These alternative mechanisms involve the use of alternative polymerases such as Pol ι [16] or recombination between sister chomatids [17]. Several lines of evidence suggest that p53 plays a major role in mediating downstream events associated with S phase arrest from UV damage [18], especially in XP-V cells with protracted S phase arrest due to Pol η deficiency [19].…”