Mutational response of Fanconi anaemia cells to shuttle vector site-specific psoralen cross-links.

Fanconi anaemia (FA) is a hereditary tumour-prone disorder. FA cells exposed to DNA crosslinking agents show an increased frequency of chromosome aberrations and of deletion type mutations. The molecular basis of FA presumably is a deficiency in cellular repair of DNA adducts. In this work a shuttle vector plasmid was treated with 8-methoxypsoralen + a split dose of UVA (leading to crosslink induction), and transfected into FA lymphoblasts. The supF gene of the vector showed a mutation frequency similar to that of normal cells; however, the number of base substitutions was relatively low, whereas a high level (50%) of deletions was seen. With both normal and FA cells these deletions varied greatly in size and were randomly distributed within the supF gene. DNA cross-links were also induced using a triple helix forming 22-mer oligonucleotide linked to a psoralen molecule and being complementary to part of supF, leading to a > 30-fold increase of mutations, which were mainly position 167 single-base substitutions and showed a pattern identical to that of the normal cells. This normal response of FA cells to the site-specific DNA cross-links may reflect that not all gene sequences of FA cells are subjected to abnormal DNA repair. Alternatively, it may reflect a lower than normal genome-overall activity of a DNA cross-link repair complex, fully capable of efficiently repairing only molecules carrying relatively few adducts.