Gene-specific DNA repair and steady state transcription of the MDR1 gene in human tumor cell lines.

We have explored the relationship between DNA repair and transcription in vivo. A gene-specific repair assay has been employed to study removal of ultraviolet light-induced cyclobutane pyrimidine dimers in the MDR1 gene at different levels of MDR1 mRNA expression. The parental human adenocarcinoma cell line, KB-3-1, has very low levels of MDR1 mRNA expression, but its multidrug resistant derivatives KB-8-5 and KB-C1 have 42-fold and 3800-fold increases in MDR1 mRNA expression, respectively. In the KB-3-1 cell line that has a low level of MDR1 mRNA expression, we find a low level of MDR1 gene-specific repair and inefficient repair of the transcribed strand of the gene. In the KB-8-5 cell line that has a modest increase in MDR1 mRNA expression, we find only a minor increase in dimer repair in the MDR1 gene. Here, the repair in the transcribed strand is not significantly higher than that in the KB-3-1 cell line. However, in the KB-C1 derivative, where there is a 3800-fold increase in the level of MDR1 mRNA expression, we find a substantial increase in the level of dimer repair in the MDR1 gene. In addition, the MDR1 transcribed strand repair is markedly more efficient than the repair in the nontranscribed strand. Our data suggest that the rate of transcription in the MDR1 gene must be substantially increased before there is any measurable effect on DNA repair. Repair in the housekeeping gene, dihydrofolate reductase (DHFR), was similar in all three tumor cell lines. Repair in its transcribed strand was markedly lower than previously reported in normal human fibroblasts. We suspect that these human HeLa-derived tumor cell lines have deficient gene-specific DNA repair. This may be an important aspect of their malignant phenotype.