Induction of fragile sites by fluorodeoxyuridine and caffeine accompanies with misincorpolation of endogenous uridine nucleotide into DNA of feline fibroblasts

Fluorodeoxyuridine, an inhibitor of thymidylate synthetase, is known to ind uce chromosomal fragile sites. The drug treatment may cause deprivation of intracellular thymidine nucleotide pool followed by a serious imbalance of deoxynucleotide pool. Though the stress is probably related to the inductio n of folate-sensitive fragile sites, the exact machanism is still to be inv estigated. The present study has been carried out to test the possibility t hat the fragile sites are originated, at least in part, from incorpolated u racil residues. The incorpolated uracil residue can be detected by a novel assay for abasic sites after treatment with uracil-DNA N-glycosylase (UDG). About 2.7 abasic sites per 10(4) nucleotides were detected in the DNA extr acted from feline fibroblasts after the treatment with FUdR and caffeine. B y digesting the DNA with UDG prior to the assay, significant increase in th e number of abasic sites were observed. These results indicate that the lar ge amount of uracil residues are present in the feline fibroblast DNA under the condition which induces chromosomal fragile sites.