XP is caused by a defect in the ability of cells to deal with damage induced in DNA by ultraviolet light. In most cases of XP, the defect is in the process of nucleotide excision repair, a multi-step pathway, whereby the damage is recognised and subsequently cut out of the DNA. The resulting gap is then repaired (repair synthesis). However, NER is normal in XPV cells as Polη, the product of the POLHgene is not involved in this process. Polη is a specialised DNA polymerase, which is able to replicate DNA past unrepaired UV-induced DNA damage efficiently and accurately. In the absence of polη, another DNA polymerase replicates past the damage, but less efficiently and less accurately. This results in more mutations on exposure of the skin to sunlight and the skin hypersensitivity results. Clinical diagnosis can be confirmed, either by molecular analysis of the POLHgene and/or by analysing the response to UV light in cultured skin fibroblasts. XPV cells are specifically sensitised to UV light by the inclusion of caffeine in growth medium after UV irradiation. Under these conditions XPV cells accumulate in the S phase of the cell cycle and this can be measured using flow cytometry. Other tests for sensitivity to UV+caffeine can also be employed.
POLH