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 processing of psoralen photolesions, interstrand cross-links (CL) and monoadducts (MA), leaves a specific molecular signature in the mutational events produced at an endogenous locus, HPRT. Human lymphoblasts were treated by 4,5',8-trimethylpsoralen (Me3Pso) in association with a double irradiation protocol (365 plus 365 nm) which allows us to increase the 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 mutations induced by the same psoralen in combination with a single irradiation of either 365 nm (induction of MA and a low proportion of CL) or 405 nm (producing almost exclusively MA). In all treatment conditions, base substitutions constitute the major type of Me3Pso photoinduced mutations. The majority of base substitutions involve a T residue preferably within a 5'-TpA sequence which corresponds to the favoured sites of psoralen photoadducts. In other words, the Me3Pso photolesions induce at the endogenous HPRT locus a high specific signature. Moreover, base substitutions have been essentially found in the non-transcribed strand of the HPRT gene suggesting that the psoralen photolesions are preferentially removed from the transcribed strand. In spite of the considerable difference between the proportion of lesions of both types (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.