Influence of low doses of BrdU and estimation of spontaneous SCE in CHO chromosomes: three-way differential staining and an immunoperoxidase method.

The influence of low doses of 5-bromodeoxyuridine (BrdU) on the occurrence of sister chromatid exchanges (SCEs) during the first cell cycle, when unsubstituted DNA templates replicate in the presence of the halogenated nucleoside (SCE1) has been assessed in third mitosis (M3) Chinese hamster ovary (CHO) cells showing three-way differential (TWD) staining. In addition, lower concentrations of BrdU, not detectable by Giemsa staining, have been tested by a high resolution immunoperoxidase method (anti-BrdU monoclonal antibody) and SCEs were scored in second mitosis (M2) cells. Our findings was a dose-response curve for SCE1 that allows an estimated mean spontaneous yield of 1.32/cell per cell cycle by extrapolation to zero concentration of BrdU. On the other hand, when the total SCE frequency corresponding to the first and second rounds of replication (SCE1 + SCE2) found in M3 chromosomes was compared with the yield of SCEs scored in M2 cells grown in BrdU at doses lower than 1 microM no further reduction was achieved. This seems to indicate that SCEs can occur spontaneously in this cell line, though the estimated frequency is higher than that reported in vivo.