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Patients with Xeroderma pigmentosum and defective DNA excision repair can be distinguished as a rapid (r-XP) and slow (s-XP) complementing variety. When fused with normal cells, fibroblasts from the r-XP are complemented rapidly and in the absence of protein synthesis while those from the s-XP are complemented slowly by a process partly, but not entirely, dependent on protein synthesis. Heterokaryons with different ratios of r-XP to s-XP nuclei (i.e. 1:1–5 and 1–5:1) and control heterokaryons containing one normal and 1–5 r- or s-XP nuclei show that if cell fusion and incubation is conducted in medium preventing protein synthesis, the r-XP cells do not complement the 8-XP partner at all and, conversely, that the latter is not as effective as normal cells at complementing the r-XP partner. On the contrary, if protein synthesis is permitted, the 2 types of XP cells complement each other in a gene dose-dependent manner and to an extent similar to that observed in the control heterokaryons. These findings indicate that the r- and 8-XP varieties are caused by mutations at different loci and suggest that the products of these loci interact to produce a functional unit which is present in normal control cells but absent in the XP strains. The relationship between the complementation groups described here and those already reported in the literature is being investigated.

Patients with the light-sensitive disease xeroderma pigmentosum (XP) are genetically heterogeneous. Most patients are defective in the excision repair of u.v.-induced DNA damage while some, the XP-variants, seem proficient in excision repair but replicate u.v.-damaged DNA with difficulty. The former have so far been assigned to 5 complementation groups (A-E) of which B and E are extremely rare. Information on the nature of the defect in XP is contradictory. Studies based on the sedimentation of DNA in alkaline gradients suggest that patients of complementation group A can perform the first step of excision repair (‘incision’) while others, based on the elution by alkali of the DNA of cells lysed on filters, suggest that they cannot. We have therefore investigated the repair of DNA damaged by u.v. irradiation using a method based on a precisely defined theoretical approach. A single break in one of the strands of a double-stranded and supercoiled circle of DNA releases supercoiling and reduces its rate of sedimentation in sucrose gradients. Structures resembling nuclei (‘nucleoids’) containing superhelical DNA may be obtained from human lymphocytes and their rate of sedimentation is influenced by the integrity of their DNA. Analysis of the sedimentation of nucleoids from normal and XP lymphocytes in sucrose gradients containing different concentrations of ethidium indicates that their DNA is similarly supercoiled. The lymphocytes of patients of complementation groups A, C and D repair damage induced by γ-rays just like controls. However, they cannot repair normally DNA damaged by u.v. irradiation; the cells are defective in the first step of excision repair. The XP-variant and XP-heterozygous cells repaired u.v.- and γ-ray-induced damage like controls. Some problems regarding the nature of the defect in XP are discussed.