Frequency of ultraviolet light-induced mutations is higher in xeroderma pigmentosum variant cells than in normal human cells

Abstract

PATIENTS with the inherited disease, xeroderma pigmentosum (XP), are subject to multiple carcinomas of the skin on areas exposed to sunlight¹. Fibroblasts from the skin of the majority of these patients (classical XP) have been shown to be deficient in excision repair of lesions induced in DNA by ultraviolet radiation^1–3. One group of patients, however, has been designated ‘XP variants’ because, although they suffer the clinical manifestations of the disease, they carry on normal excision repair of such ultraviolet-light damage to DNA^4–6. Lehmann et al.⁶ reported that cells from such XP variants are abnormally slow in converting initially low molecular weight DNA, synthesised after ultraviolet irradiation, into high molecular weight DNA similar in size to that produced in unirradiated cells. Although these authors suggest that such abnormal DNA replication might explain why such patients are susceptible to cancer of the skin, they do not propose any mechanism. If the somatic cell mutation hypothesis on the origin of cancer, first suggested by Boveri⁷, is correct one would expect the frequency of mutations induced by ultraviolet light to be higher in cells derived from skin biopsies from both classical and variant XP patients than in cells from normal persons. To test this hypothesis, we have carried out a series of quantitative investigations comparing the frequency of ultraviolet-light induced mutations to azaguanine resistance in normal human skin fibroblasts with that found in various strains derived from classical and variant XPs. We have found that cells derived from both kinds of XP patients indeed show much higher frequencies than normal cells. The data comparing two classical XP strains with normal cells have been published^8,9. Here, we present data obtained with variant strain, XP4BE.