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Ultraviolet Light-induced Sister Chromatid Exchanges in Xeroderma Pigmentosum and in Cockayne's Syndrome Lymphocyte Cell Lines

Dermatology Branch [A. D. A., J. H. R.], Biometry Branch [R. E. T.], and Medicine Branch [W-S. C., J. W-P.], National Cancer Institute, NIH, Bethesda, Maryland 20014

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 characterized by sun sensitivity in vivo and by an abnormal sensitivity 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 following 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 unrelated 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 UV-induced SCE's. Thus, XP is another disease characterized by excessive numbers of cancers and by abnormally high frequencies of SCE's.

¹ Present address: Dermatology Department, College of Physicians and Surgeons, Columbia University, 630 West 168th Street, New York, N. Y. 10032.

² To whom requests for reprints should be addressed, at Dermatology Branch, Building 10, Room 12N258, NIH, Bethesda, Md. 20014.

Received 12/14/77. Accepted 2/20/78.

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