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DNA Repair in Human Cells Containing Photoadducts of 8-Methoxypsoralen or Angelicin¹

Department of Biological Sciences, Stanford University, Stanford, California 94305

Photoactivated 8-methoxypsoralen (8-MOP) has been proven to be clinically effective for a number of dermatological conditions including lichen planus, mycosis fungoides, and psoriasis. 8-MOP forms two types of covalent photoproducts with DNA, monoadducts, and bifunctional adducts which crosslink the two DNA strands. Angelicin is a congener of 8-MOP which forms only monoadducts.

We have used the combined density and isotopic labeling technique to study repair replication in cultured human fibroblasts treated with either of these compounds and exposed to near-ultraviolet light. In human diploid fibroblasts (WI-38), the time course of repair replication for both compounds is similar. Drug concentration and ultraviolet dose responses are also similar for 8-MOP and angelicin. No repair replication was stimulated by either compound in xeroderma pigmentosum cells from Complementation Group A (XP12BE). These results suggest that repair replication in response to 8-MOP is primarily a response to monoadducts and that the enzymatic pathway for this repair synthesis shares at least one step with the pathway for repair of pyrimidine dimers.

Cross-link persistence in treated cells was assayed by use of the single-strand-specific S₁ nuclease to digest DNA that did not renature readily following heat denaturation. Partial removal of cross-links was observed in normal, xeroderma pigmentosum variant, and Fanconi's anemia fibroblasts, but not in xeroderma pigmentosum Group A cells.

¹ This work was supported by Grant NP-161 from the American Cancer Society and by grants from NIH.

Received 8/16/79. Accepted 11/26/79.

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