title: Chromosomal organization of the human dihydrofolate reductase genes: dispersion, selective amplification, and a novel form of polymorphism.
abstract: The human dihydrofolate reductase (DHFR; tetrahydrofolate dehydrogenase; 5,6,7,8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) gene family includes a functional gene (hDHFR) and at least four intronless genes. Three intronless genes (hDHFR-psi 2, hDHFR-psi 3, and hDHFR-psi 4) are identifiable as pseudogenes because of DNA sequence divergence from the functional gene with introns, while one intronless gene (hDHFR-psi 1) is completely homologous to the coding sequences of the functional gene. Analysis of genomic DNA from two panels of somatic human-rodent cell hybrids with specific molecular probes provide insight into the chromosomal organization and assignment of these genes. The five genes are dispersed in that each one is found on a different chromosome. The functional gene hDHFR has been assigned to chromosome 5, and one pseudogene (hDHFR-psi 4), to chromosome 3. In a human cell line (HeLa) that was selected for methotrexate resistance, the functional locus became amplified, while there was no amplification of the four intronless pseudogenes. hDHFR-psi 1 was found to be present in DNA of some individuals and absent from DNA of others, consistent with a recent evolutionary origin of this gene originally suggested by its sequence identity to the coding portions of the functional gene. The presence or absence of this intronless pseudogene represents a previously unreported form of DNA polymorphism.
mesh_terms: D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D002874:Chromosome Mapping; D002905:Chromosomes, Human, 4-5; D006224:Cricetinae; D004262:DNA Restriction Enzymes; D005784:Gene Amplification; D005796:Genes; D006801:Humans; D006822:Hybrid Cells / Q000201:enzymology; D008214:Lymphocytes / Q000201:enzymology; D051379:Mice; D011110:Polymorphism, Genetic; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*
chemical_list: D013762:Tetrahydrofolate Dehydrogenase; D004262:DNA Restriction Enzymes