title: Novel submicroscopic extrachromosomal elements containing amplified genes in human cells.
abstract: In previous work, several methotrexate (MTX)-resistant variants were isolated frm the human cell line HeLa BU25, which exhibited a high degree of dihydrofolate (DHFR) gene amplification (estimated to be 250- to 300-fold). These variants did not contain any chromosome with a homogeneously staining region (HSR) and exhibited only a small average number of minute chromosomes per cell: these two types of karyotypic abnormalities generally accompany selective gene amplification. We now report that structures containing amplified DHFR genes in one of these variants (HeLa BU25-10B3) can be isolated by pulsed-field gradient or field-inversion gel electrophoresis as homogeneous DNA molecules of approximately 650 kilobases (kb). Electron microscopy of metaphase spreads from these cells reveals chromatin fibres with a similar DNA content, which are probably related to the above elements. These represent a novel type of extrachromosomal structures in mammalian cells.
mesh_terms: D004351:Drug Resistance; D005111:Extrachromosomal Inheritance; D005784:Gene Amplification; D005796:Genes; D014644:Genetic Variation; D006367:HeLa Cells / Q000201:enzymology / Q000648:ultrastructure; D006801:Humans; D008727:Methotrexate / Q000494:pharmacology; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*
chemical_list: D013762:Tetrahydrofolate Dehydrogenase; D008727:Methotrexate