Molecular analysis by electron microscopy of the removal of psoralen-photoinduced DNA cross-links in normal and Fanconi's anemia fibroblasts

The induction and fate of psoralen-photoinduced DNA interstrand cross-links in the genome of Fanconi's anemia (FA) fibroblasts of complementation groups A and B, and of normal human fibroblasts, were investigated by quantitative analysis of totally denatured DNA fragments visualized by electron microscopy. 8-Methoxypsoralen (5 x 10(-5) M) interstrand cross-links were induced as a function of the near ultraviolet light dose. With time of postexposure incubation, a fraction of interstrand cross-links disappeared in all cell lines. However, 24 h after treatment, this removal was significantly lower in the two FA group A cell lines examined (34-39%) than in the FA group B and normal cell lines (43-53 and 47-57%, respectively). These data indicate that FA cells are at least able to recognize and incise interstrand cross-links, as normal cells do, although group A cells seem somewhat hampered in this process. This is in accord with data obtained on the same cell lines using another biochemical assay (D. Papadopoulo, D. Averbeck, and E. Moustacchi. Mutat. Res., DNA Repair Rep., 184: 271-280, 1987). Since the fate of cross-links in FA constituted a controversial matter, it is important to stress that two different methodologies applied to genetically well defined cell lines led to the same conclusions.