Ultraviolet light-induced sister chromatid exchanges in xeroderma pigmentosum and in Cockayne's syndrome lymphocyte cell lines.

Sister chromatid exchanges (SCE's) were studied in lymphocyte cell lines derived from 11 patients with xeroderma pigmentosum (XP) and 2 patients with Cockayne's syndrome (CS), autosomal recessive diseases character ized by sun sensitivity in vivo and by an abnormal sensitiv ity of the patient's cells to the killing effects of ultraviolet light (UV) in vitro. XP patients have DNA repair defects and develop numerous tumors on sun-exposed skin. CS patients do not develop such tumors, and the cause of the sensitivity of their cells to UV is not known. Reciprocal SCE's were evaluated by fluorescence microscopy follow ing staining of the chromosomes with the bisbenzimidazole dye 33258 Hoechst after the cells had passed through two phases of DNA synthesis in the presence of 5-bromodeoxyuridine. In the absence of UV irradiation, the XP and CS cells had the same frequency of "spontaneous" SCE's as did normal cells. UV irradiation resulted in significant increases in the SCE's in the normal, XP, and CS cells. The number of UV-induced SCE's in the XP variant's cells was not greater than the number in the normal cells. The number of UV-induced SCE's in the XP Complementation Group B and E cells and in the CS cells, while higher than the number in the normal cells, was not significantly greater at the 0.05 confidence level. However, all the XP Complementation Group A, C, and D cells tested had significantly greater numbers of UV-induced SCE's than did the normal cells. There were significant differences in the number of UV-induced SCE's among cells from unre lated XP Group C patients, while cells from 3 XP Group C siblings had almost identical UV-induced responses. The distribution of spontaneous and UV-induced SCE's among chromosomes of normal and XP cells was determined, and in all lines there was a significantly greater frequency of observable SCE's per chromosome length in the longer chromosomes than in the shorter chromosomes. Our studies indicate that cells from certain excision-deficient XP patients have abnormally increased numbers of UVinduced SCE's. Thus, XP is another disease characterized by excessive numbers of cancers and by abnormally high frequencies of SCE's. INTRODUCTION SCE's have recently become of great interest in cancer 1Present address: Dermatology Department, College of Physicians and Surgeons, Columbia University, 630 West 168th Street, New York. N. Y. 10032. 2To whom requests for reprints should be addressed, at Dermatology Branch, Building 10, Room 12N258, NIH, Bethesda, Md. 20014. Received December 14, 1977; accepted February 20, 1978. biology with the development of improved and accurate methods for their study (26, 35) and with the recognition that SCE's3 occur with an abnormally high frequency in certain conditions predisposed to cancer. For example, there are abnormally high frequencies of "spontaneous" SCE's in phytohemagglutinin-stimulated cultured leuko cytes (9) from patients with Bloom's syndrome (18) and from patients with histories of inorganic trivalent arsenical ingestion followed by development of skin cancer (8). Abnormally increased numbers of SCE's are found in XP fibroblasts exposed in culture to chemical carcinogens (12, 46). XP is a rare autosomal recessive disease characterized by sun sensitivity and excessive tumor formation on sunlightexposed areas of skin (38). XP cells are unable to perform normal repair (13, 38) of DNA damage induced by UV and certain chemical carcinogens. XP was the first human disease shown to be caused by defective DNA repair (11). There are currently 5 forms of XP in which DNA excision repair is deficient, and these forms are classified into XP Complementation Groups A to E (25). In the sixth form of XP, the variant form, excision repair appears normal, but there is a marked impairment in postreplication repair (27). While there is a normal frequency of spontaneous SCE's in cultured XP fibroblasts (22, 45) and in XP peripheral blood leukocyte cultures stimulated by phytohemagglutinin (43), there can be markedly increased frequencies after cultured XP cells have been irradiated with UV. This abnormal increase in UV-induced SCE's has been reported to occur in XP leukocytes stimulated with phytohemagglutinin (Ref. 6, note added in proof; Ref. 43), in XP fibroblasts (16) and in LCL's (10) derived from XP patients' Epstein-Barr virustransformed peripheral blood lymphocytes. We now report in detail on our study (10) of UV-induced SCE's in LCL's derived from XP patients representing each of the 6 forms of XP. In addition, we report our findings on UV-induced SCE's in LCL's derived from patients with CS, a form of cachectic dwarfism characterized by acute sensi tivity without any known predisposition to sunlight-induced carcinogenesis (19). CS is of particular interest because 1 XP patient also has CS (38, 47) and because CS cells (3, 4, 41, 42), like XP cells (3), are more readily killed by UV than are normal cells. MATERIALS AND METHODS Origin of the Control Donor LCL's. Six control donor LCL's were studied. Each was derived from the peripheral 3The abbreviations used are: SCE, sister chromatid exchange; XP, xeroderma pigmentosum; LCL, lymphocyte cell line; CS, Cockayne's syn drome; RPMI, Rosewell Park Memorial Institute.