Xeroderma pigmentosum variant (XP-V) is an inherited disorder resulting in hypersensitivity to the cytotoxic, mutagenic, and carcinogenic effects of UV light. There is evidence suggesting that XP-V cells carry a defect in the replication of UV-induced DNA damage, leading to mutations in genes, e.g., proto-oncogenes and tumor suppressor genes, of exposed skin cells. Using an in vitro assay to quantitatively evaluate replication of the most prevalent UV-derived DNA lesion, the cis,syn-thymine dimer (T◊T), we have recently found that a T◊T located on the leading strand can be bypassed by a bona fide human replication fork but can also induce fork uncoupling with selective synthesis of the undamaged lagging strand (D. Svoboda and J-M. Vos, Proc. Natl. Acad. Sci. USA, 92: 11975–11979, 1995). We now report the application and further refinement of this sensitive assay to the replication of a T◊T-containing template by XP-V cell-free extracts. In comparison to normal controls, a 10–26-fold deficiency in the bypass replication of T◊T was observed in XP-V cell extracts. In contrast, the disease extracts were as competent as controls for replication of the undamaged TT plasmid and for leading T◊T-induced fork uncoupling. Besides mismatch repair and nucleotide excision repair, the bypass replication defect of XP-V may represent a novel category of hereditary mutator phenotypes affecting DNA damage processing.


This work was supported by grants from the National Cancer Institute and the American Cancer Society (to J-M. H. V.).

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