[Study on common fragile sites in chorionic villi cells cultured in vitro]

The chorionic cells cultured in vitro were treated with the inducing factors of fragile sites. These factors could lead to an unbalance of the dNTP pools and thus induce expression of both common fragile sites and Fra(X). Excess guanosine (250 mg/l) or thymidine (250 mg/l) was added to the culture medium 24 hours before harvest. MEM-FA medium was added 48 hrs before harvest and some of these cases were added with guanosine (250 mg/l) 24 hrs before harvest. After harvest the slides were stained with Giemsa staining. Chromosome gaps and breaks were observed and recorded. Then slides were destained and G-banded to confirm the chromosome region of breaks. The expression frequencies of the common fragile site 3p14 and others in villi cells were observed. Both guanosine and thymidine had strong inducing effect on common fragile sites. Guanosine inhibited the cell growth more apparently than thymidine. After guanosine being used in MEM-FA medium, the weaker inducing effect was shown. The possible reason is that the lack of folic acid could block the synthesis of guanosine, so the unbalance of dNTP pools caused by excess guanosine tend to be balanced. Because of the counteracting effect between MEM-FA medium and guanosine or thymidine, it is suggested that these two nucleosides should not be used with MEM-FA medium together in inducing Fra(X).