Changes induced by ultraviolet light in the superhelical dna of lymphocytes from subjects with xeroderma pigmentosum and normal controls

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.