MUTAGENESIS BY 8-METHOXYPSORALEN AND 5-METHYLANGELICIN PHOTOADDUCTS IN MOUSE FIBROBLASTS - MUTATIONS AT CROSS-LINKABLE SITES INDUCED BY MONOADDUCTS AS WELL AS CROSS-LINKS

Psoralens are used clinically in the treatment of several skin disease s, including psoriasis, vitiligo, and cutaneous T cell lymphoma. Howev er, psoralen treatment has been associated with an increased risk of s quamous cell carcinoma of the skin. To elucidate molecular events that may play a role in the psoralen-related carcinogenesis, we examined p soralen-induced mutagenesis in a mouse fibroblast cell line carrying a recoverable, chromosomally integrated lambda phage shuttle vector. Us ing the supF gene as a mutation reporter gene, we determined the spect rum of mutations induced by photoactivation of 8-methoxypsoralen and o f 5-methylangelicin. Both psoralens generated predominately T:A to A:T and some T:A to G:C transversions. Most of the mutations occurred at either 5' TpA or 5' ApT sites, both of which are conducive to interstr and cross-link formation. However, 5-methylangelicin produces only mon oadducts, whereas 8-methoxypsoralen generated 20% cross-links and 80% monoadducts under the conditions of our experiments, as measured by di rect HPLC analysis of the DNA from the treated cells. Although most of the mutations occurred at potentially cross-linkable sites, these res ults implicate monoadducts, as well as cross-links, as critical premut agenic lesions in psoralen-treated mammalian cells. These findings may help in the identification of carcinogenic changes induced by psorale n, and they may aid in the improved design of psoralen-based treatment regimens in the future.