title: DNA repair in the endogenous and episomal amplified c-myc oncogene loci in human tumor cells.
abstract: We have studied the repair of u.v.-induced cyclobutane pyrimidine dimers (CPDs) in amplified c-myc oncogene loci in human colon cancer cells to better understand the relationship between chromatin structure, transcription and DNA repair. To assess the variation in DNA repair in the same gene whether located in a chromosomal site or in a extra-chromosomal site, we have quantitated the efficiency of excision repair after u.v. exposure in the endogenous and episomal c-myc genes isolated from COLO320HSR and DM cells. In the HSR cells, c-myc is localized in a homogeneously staining region (HSR), and in the DM cells, the gene is localized in double minute chromosomes (DM). Our results indicate that the repair is less efficient in c-myc amplicons organized as double minute chromosomes than in the endogenous c-myc amplicons. The episomal gene is not repaired with the same efficiency as when it is intrachromosomal. This may reflect differences in chromatin structure. An advantage of this biological system is that the cells possess two different alleles of the c-myc gene, one that is active and another which is inactive. We have studied the relationship between DNA repair and transcriptional activity in the c-myc locus by measuring the efficiency of excision repair after u.v. exposure in the normal and rearranged alleles of the c-myc gene. Surprisingly, the c-myc gene is repaired with similar efficiency in the highly transcribed allele as in the poorly expressed allele. However, u.v. damage is selectively removed from the transcribed strand of the active c-myc allele, but DNA repair is not strand specific in the non-expressed c-myc allele.
authors.affiliation: Laboratory of Molecular Genetics, National Institute on Aging, NIH, Baltimore, Maryland 21224, USA.
authors.forename: F
authors.initials: F
authors.lastname: Larminat
authors.forename: E J
authors.initials: EJ
authors.lastname: Beecham
authors.forename: C J
authors.initials: CJ
authors.lastname: Link
authors.forename: A
authors.initials: A
authors.lastname: May
authors.forename: V A
authors.initials: VA
authors.lastname: Bohr
mesh_terms: D002874:Chromosome Mapping; D004260:DNA Repair; D005784:Gene Amplification; D016259:Genes, myc; D006801:Humans; D009369:Neoplasms / Q000235:genetics*; D010957:Plasmids; D011740:Pyrimidine Dimers / Q000378:metabolism; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
chemical_list: D011740:Pyrimidine Dimers; D013762:Tetrahydrofolate Dehydrogenase