Xeroderma pigmentosum variant heterozygotes show reduced levels of recovery of replicative DNA synthesis in the presence of caffeine after ultraviolet irradiation

Patients with xeroderma pigmentosum variant show clinical photosensitivity, skin neoplasias induced by ultraviolet light, and defective postreplicatio n repair, but normal nucleotide excision repair. We recently reported an al ternative, simple method for the diagnosis of xeroderma pigmentosum variant that measures by autoradiography three cellular markers for DNA repair aft er ultraviolet irradiation: unscheduled DNA synthesis, recovery of RNA synt hesis, and recovery of replicative DNA synthesis. Among hereditary photosen sitive disorders, including other xeroderma pigmentosum groups, Cockayne sy ndrome, and a newly established ultraviolet-sensitive syndrome, only xerode rma pigmentosum variant cells exhibited normal unscheduled DNA synthesis, n ormal recovery of RNA synthesis, but reduced recovery of replicative DNA sy nthesis (51 +/- 6% the rate relative to normal controls). This reduction of recovery of replicative DNA synthesis was enhanced in the presence of a no ntoxic level of caffeine to 36 +/- 5%. In this study we assess the cellular markers in two independent families that included two photosensitive patie nts that were identified as xeroderma pigmentosum variant. Cells from heter ozygotic parents showed normal levels of unscheduled DNA synthesis, recover y of RNA synthesis, and recovery of replicative DNA synthesis, but reduced rates of recovery of replicative DNA synthesis in the presence of 1 mM caff eine (53 +/- 8% relative to the normal control). Furthermore, with a colony -forming assay, the cells showed normal survival by ultraviolet without caf feine, but slightly reduced survival by ultraviolet with 1 mM caffeine pres ent. In one family, we confirmed inheritance of two heterozygous mis-sense mutations. One mutation is an A -->G transition at nucleotide 1840 that gen erates a K535E mis-sense mutation. Another mutation is an A -->C transversi on at nucleotide 2003 that generates a K589 mis-sense mutation. Each of the se mutations were absent in 52 unrelated Japanese individuals. These result s suggest that xeroderma pigmentosum variant heterozygotes can be identifie d by their sensitivity to ultraviolet irradiation in the presence of nontox ic levels of caffeine.