Xeroderma pigmentosum and skin cancer.

The hypersensitivity of DNA repair deficient xeroderma pigmentosum (XP) patients to solar irradiation results in the development of high levels of squamous and basal cell carcinomas as well as malignant melanomas in early childhood. Indeed, XP presents a unique model for analysing the effects of unrepaired DNA lesions in skin carcinogenesis. The skin cancer predisposition, observed in XP patients, is due to the mutator gene activity of XP cells which lead to high levels of UV specific modifications of crucial regulatory genes in skin cells leading to Cancer. Thus, the high levels of UV specific mutations, seen in oncogenes and tumor suppressor genes, which have been characterized in XP tumors, clearly demonstrate the major role of the UV component of sunlight in skin cancer development. The UV specific C to T and the tandem CC to TT UV signature transition mutations found in XP tumors are located at bipyrimidine sequences, the preferred UV targets in DNA. The same UV specific alterations are seen in key regulatory genes in sporadic skin cancers but at lower frequencies than those found in XP tumors.