A cytogenetic investigation of DNA rereplication after hydroxyurea treatment: implications for gene amplification

Although the mechanisms leading to gene amplification are poorly understood, it has recently been proposed that the initial event of amplification is the rereplication of a variable, but relatively large, amount of the genome within a single cell cycle. We sought evidence for rereplication of DNA as a basis for gene amplification through two cytogenetic techniques: differential staining for sister-chromatid exchange analysis and premature chromosome condensation. Synchronized Chinese hamster ovary cells were incubated continuously with bromodeoxyuridine and treated with hydroxyurea (HU) when cells were approximately 2 h into the S phase. After 6 h exposure to HU, the drug was removed and at 3 h intervals thereafter metaphase cells were collected and the chromosomes were stained by the fluorescence-plus-Giemsa procedure. No staining patterns consistent with rereplication of DNA were observed. Since HU causes cytogenetic damage, the premature chromosome condensation technique was used to determine the kinetics of chromosome damage after removal of HU. Extensive G₂ chromosome damage within 1 h after removal of HU from the medium was found, although cesium chloride gradient analysis showed that there was no rereplication of DNA during this time. Contrary to a previous report, these results provide no evidence that incubation of cells with HU during S phase induces rereplication of DNA within a single cell cycle. The results observed are consistent with the hypothesis that drug-induced aberrations and the subsequent abnormal segregation of chromosomal fragments are the first steps in the process that leads to gene amplification in drug-treated mammalian cells.