Loss of unstably amplified dihydrofolate reductase genes from mouse cells is greatly accelerated by hydroxyurea

Previous work has shown that mammalian cells that carry unstably amplified genes for dihydrofolate reductase (DHFR) gradually lose the amplified DHFR genes when grown in the absence of the DHFR inhibitor methotrexate (MTX). Unstably amplified genes occur on small acentric chromosomes called double minutes (DMs) or even smaller chromatin fragments, in contrast to stably amplified genes, which reside in centromere-containing chromosomes. We have found that the rate of loss of the unstably amplified DHFR genes can be greatly increased by growing the cells in the presence of a nonlethal concentration of hydroxyurea. For example, in one MTX-resistant subline studied, approximate to 90% of the original DHFR gene dosage is lost in 25-30 cell doublings in the absence of MTX. The same degree of loss is achieved, however, in