LARGE MUTAGENIC LESIONS ARE INDUCED BY PHOTODYNAMIC THERAPY IN MURINEL5178Y LYMPHOBLASTS

Mutagenic lesions at the thymidine kinase locus (tk) in mouse lymphoma L5178Y (LY) cells treated with red light and either Photofrin (PF) or chloroaluminum phthalocyanine (AlPc) as the photosensitizer were comp ared in the relatively photodynamic therapy (PDT)-sensitive strain LY- R16 and the relatively resistant strains LY-S1 and LY-SR1. Southern bl ot analysis revealed that 92% (36/39) of the PDT-induced thymidine kin ase (TK-/-) mutants of strains LY-R16 and LY-SR1 lost the entire activ e tk allele. (Strain LY-S1 lacks a known tk polymorphism and has not b een analyzed for loss of the active tk allele.) A decrease in galactok inase (GK) activity in the TK-/- mutants has been taken as an indicati on that the mutagenic lesion extends from the tk gene to the closely l inked galactokinase gene (gk). Using PF as the photosensitizer, GK act ivity was decreased in 45% of the LY-R16 mutants and in 22% of the LY- S1 and LY-SR1 mutants. With photoactivated AlPc, 59% of the TK-/- muta nts of strains LY-S1 and LY-SR1 showed GK inactivation. (LY-R16 mutant s were not analyzed because of the low LY-R16 mutant frequency induced by PDT with AlPc.) Thus, many of the TK-/- mutants of LY cells induce d by PDT with either PF or AlPc harbor multilocus lesions.