STUDY OF THE RELATIONSHIPS BETWEEN COMMON FRAGILE SITES, CHROMOSOME BREAKAGES AND SISTER-CHROMATID EXCHANGES

This paper reports the results of an investigation into the relationsh ip between common fragile sites and sister chromatid exchanges (SCE). Human Ieukocyte cultures were grown in two different media, one comple te (RPMI 1640) and one deficient in folic acid and thymidine (199M). S ome of the cultures were treated with DAPI, a non-intercalating compou nd which binds preferentially to the AT bases of DNA and is capable of inducing fragile sites, Bromodeoxyuridine (BrdU) was added to all the cultures for SCE analysis. Chromomycin A(3) was used for mapping lesi ons and SCEs by R-banding. A total of 400 cells was examined. The main results show that: BrdU, probably by re-equilibrating the unbalanced nucleotide pool of the 199 culture medium, interferes with the synergi sm between this culture medium and DAPI in inducing the expression of fragile sites; the SCE frequency per cell is not increased by DAPI in both culture media, therefore this compound does not seem to cause any damage to the DNA and seems merely to act by inhibiting the normal co ndensation of a subset of fragile sites that possess DAPI-specific bas e sequences; even in the absence of chromosomal lesions, the fragile s ites are significantly preferred as SCE sites to nonfragile sites, whe reas in the presence of a lesion, both fragile and non-fragile sites h ave the same likelihood of undergoing SCE. All this indicates that the presence of a lesion strongly favours SCE formation and that common f ragile sites are probably chromosome regions preferentially damaged du ring the S phase.