DNA-REPAIR IN THE ENDOGENOUS AND EPISOMAL AMPLIFIED C-MYC ONCOGENE LOCI IN HUMAN TUMOR-CELLS

We have studied the repair of u.v.-induced cyclobutane pyrimidine dine rs (CPDs) in amplified c-myr: oncogene loci in human colon cancer cell s to better understand the relationship between chromatin structure, t ranscription and DNA repair. To assess the variation in DNA repair in the same gene whether located in a chromosomal site or in a extra-chro mosomal site, we have quantitated the efficiency of excision repair af ter 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 i s localized in double minute chromosomes (DM). Our results indicate th at the repair is less efficient in c-myc amplicons organized as double minute chromosomes than in the endogenous c-myc amplicons, The episom al gene is not repaired with the same efficiency as when it is intrach romosomal. This may reflect differences in chromatin structure. An adv antage of this biological system is that the cells possess two differe nt alleles of the c-myc gene, one that is active and another which is inactive. We have studied the relationship between DNA repair and tran scriptional activity in the c-myc locus by measuring the efficiency of excision repair after u.v. exposure in the normal and rearranged alle les of the c-myc gene. Surprisingly, the c-myc gene is repaired with s imilar efficiency in the highly transcribed allele as in the poorly ex pressed allele. However, u.v. damage is selectively removed from the t ranscribed strand of the active c-myc allele, but DNA repair is not st rand specific in the non-expressed c-myc allele.