title: Amplification of the human dihydrofolate reductase gene via double minutes is initiated by chromosome breaks.
abstract: DNA sequence amplification is one of the most frequent manifestations of genomic instability in human tumors. We have shown previously that amplification of the dihydrofolate reductase (DHFR) gene in Chinese hamster cells is initiated by chromosome breaks, followed by bridge-breakage-fusion cycles that generate large intrachromosomal repeats; these are ultimately trimmed by an unknown process to smaller, more homogenous units manifested as homogenously staining chromosome regions (HSRs). However, in most human tumor cells, amplified DNA sequences are borne on unstable, extrachromosomal double minutes (DMs), which suggests the operation of a different amplification mechanism. In this study, we have isolated a large number of independent methotrexate-resistant human cell lines, all of which contained DHFR-bearing DMs. Surprisingly, all but one of these also had suffered partial or complete loss of one of the parental DHFR-bearing chromosomes. Cells in a few populations displayed what could be transient intermediates in the amplification process, including an initial HSR, its subsequent breakage, the appearance of DHFR-containing fragments, and, finally, DMs. Our studies suggest that HSRs and DMs both are initiated by chromosome breaks, but that cell types differ in how the extra sequences ultimately are processed and/or maintained.
authors.affiliation: Department of Molecular Biotechnology, University of Washington, Seattle, WA 98195-7730; and Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, VA 22908, USA.
authors.forename: M J
authors.initials: MJ
authors.lastname: Singer
authors.forename: L D
authors.initials: LD
authors.lastname: Mesner
authors.forename: C L
authors.initials: CL
authors.lastname: Friedman
authors.forename: B J
authors.initials: BJ
authors.lastname: Trask
authors.forename: J L
authors.initials: JL
authors.lastname: Hamlin
mesh_terms: D002869:Chromosome Aberrations; D002872:Chromosome Deletion; D020223:Chromosome Painting; D002877:Chromosomes, Human / Q000648:ultrastructure; D002895:Chromosomes, Human, Pair 5; D004249:DNA Damage; D004351:Drug Resistance / Q000235:genetics; D005784:Gene Amplification; D006367:HeLa Cells; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D008727:Methotrexate / Q000494:pharmacology; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*
chemical_list: D013762:Tetrahydrofolate Dehydrogenase; D008727:Methotrexate