Assignment of six patients with xeroderma pigmentosum in Hokkaido area to a variant form

Six Japanese patients in Hokkaido area were diagnosed as xeroderma pigmentosum (XP) at two University Hospital; one patient, a 29 years old female (XP2AS) at Asahikawa Medical College, and five patients, namely, a 53 years old male (XP1SA), a 45 years old female (XP2SA), a 46 years old female (XP3SA), a 47 years old male (XP4SA) and a 34 years old male (XP5SA) at Hokkaido University School of Medicine. All these XP patients showed mild skin symptoms and had no apparent neurological abnormalities. To identify genetic complementation groups of these patients, DNA repair capacities in skin fibroblasts derived from the patients were analyzed by ultraviolet light (UV)-induced unscheduled DNA synthesis (UDS), and also by colony-forming abilities of UV-irradiated cells incubated with or without caffeine added in post-UV culture medium. The levels of UDS measured by autoradiography in these XP cells irradiated with 20 J/m2 of UV were 67 to 84% of those of normal human fibroblasts. Although one XP cell strain (XP2AS) showed about 2-fold greater sensitivity to killing by UV than normal cells, UV sensitivities of the rest of 5 XP strains were similar or only slightly higher than those of normal cells. However, UV sensitivities of all the 6 XP cell strains markedly increased by addition of 1 mM caffeine in the post-UV incubation medium, a phenomenon characteristic of XP variant cells. We also analyzed post-replication repair in XP1SA cells by sedimentation of newly-synthesized DNA in alkaline sucrose gradient, and found that this strain is defective in this repair system. From these results, we concluded that all the 6 XP strains studied here belong to XP variants.