THE MUTAGENIC PROCESSING OF PSORALEN PHOTOLESIONS LEAVES A HIGHLY SPECIFIC SIGNATURE AT AN ENDOGENOUS HUMAN LOCUS

To assess the role of a given genotoxic agent in the etiology of human cancers, it is useful to establish the mutational specificity of this agent. The aim of this study was to investigate whether the processin g of psoralen photolesions, interstrand cross-links (CL) and monoadduc ts (MA), leaves a specific molecular signature in the mutational event s produced at an endogenous locus, HPRT. Human lymphoblasts were treat ed by 4,5',8-trimethylpsoralen (Me(3)Pso) in association with a double irradiation protocol (365 plus 365 nm) which allows us to increase th e proportion of CL for a given constant number of total photoadducts. The molecular spectrum of mutations at the HPRT locus induced in these conditions was compared to the previously reported spectra of mutatio ns induced by the same psoralen in combination with a single irradiati on of either 365 nm (induction of MA and a low proportion of CL) or 40 5 nm (producing almost exclusively MA). In all treatment conditions, b ase substitutions constitute the major type of Me(3)Pso photoinduced m utations. The majority of base substitutions involve a T residue prefe rably within a 5'-TpA sequence which corresponds to the favoured sites of psoralen photoadducts. In other words, the Me(3)Pso photolesions i nduce at the endogenous HPRT locus a highly specific signature. Moreov er, base substitutions have been essentially found in the non-transcri bed strand of the HPRT gene suggesting that the psoralen photolesions are preferentially removed from the transcribed strand. In spite of th e considerable difference between the proportion of lesions of both ty pes (CL or MA) induced in different treatment conditions, the kind of mutations and their sequence distribution are similar suggesting that the mutagenic processing of psoralen CL and MA is similar at least for the steps resulting in base substitutions. (C) 1995 Academic Press Li mited