Chromosomal changes without DNA overproduction in hydroxyurea-treated mammalian cells: implications for gene amplification

It has been reported that a 6-h incubation of early S-phase Chinese hamster cells with hydroxyurea promotes DNA overproduction, i.e., replication of DNA a second time within a single cell cycle, and that this could be the basis for gene amplification in drug-treated mammalian cells. When we incubated methotrexate-resistant Chinese hamster cells that were approximately 2 h into the S phase with hydroxyurea for 6 h, DNA that had been replicated before the incubation with hydroxyurea (early S-phase DNA) was replicated again within 11 h after the hydroxyurea treatment. However, incubation with colchicine or Colcemid after hydroxyurea treatment virtually abolished this overreplication, as well as that of the amplified dihydrofolate reductase genes in these cells, indicating that the second replication had occurred in a second cell cycle. Cells collected in the first mitosis after incubation with hydroxyurea never contained overreplicated DNA but did contain abundant chromosome aberrations. Early S-phase DNA replicated again on schedule during the first few hours after mitosis. Asymmetric segregation of chromosome fragments or unequal sister chromatid exchange may be the actual basis for gene amplification in drug-treated mammalian cells.