MOLECULAR-SPECTRUM OF MUTATIONS INDUCED AT THE HPRT LOCUS BY A CROSS-LINKING AGENT IN HUMAN CELL-LINES WITH DIFFERENT REPAIR CAPACITIES

Molecular characterization of mutations photoinduced by a cross-linkin g agent, 4,5',8-trimethylpsoralen (Me3PSO), in normal human lymphoblas ts was conducted in parallel with lymphoblasts derived from Fanconi an emia patients. Such cells have been previously described to be impaire d in repair of psoralen photolesions. The endogenous HPRT locus was us ed as a target gene. The treatment of cells with Me3PSO in combination with 365 nm irradiation leads to the formation of interstrand cross-l inks, and specific monoadducts. Our analysis revealed that the mutagen ic processing of Me3PSO photoadducts in normal human cells results ess entially in base substitutions (84%). These are localized to sequences shown previously to be favored for the formation of Me3PSO monoadduct s. The mutagenic processing of the same lesions in Fanconi anemia cell s results in fewer base substitutions (22%), with deletions (66%) bein g the predominant class of mutation. In contrast to prokaryotic system s, frameshifts are poorly represented among Me3PSO induced mutations i n human cells. In spite of important differences between the kinds of mutations observed in the two cell lines, our analysis reveals similar ities in the type of base substitutions and their sequence distributio n. In both normal and Fanconi anemia cell lines mutations, mostly targ eted on thymine residues, are preferentially located on the non-transc ribed strand.