Transcription-independent repair of nitrogen mustard-induced N-alkylpurines in the c-myc gene in Burkitt's lymphoma CA46 cells.

Recently, it has been demonstrated that N-alkylpurines produced by nitrogen mustard are excised more rapidly from actively transcribing genes compared to non-coding regions of the overall genome. Such studies have suggested that transcriptional activity is a determinant of the rate of removal of these DNA lesions. We have examined the removal of nitrogen mustard-induced N-alkylpurines in the actively transcribed/translocated and transcriptionally repressed native alleles of the c-myc gene in Burkitt's lymphoma, CA46 cells. Burkitt's lymphoma cells, exhibiting a c-myc translocation that can be distinguished from the native allele by Southern blotting, provide a useful model system in which to explore regulatory elements that govern DNA repair in transcriptionally active genes. Northern analysis verified the selective allelic expression of the translocated c-myc gene in CA46 cells. At the drug exposure examined, nitrogen mustard produced a similar level of N-alkylpurines in the two alleles of c-myc. Also, the kinetics of lesion repair from both c-myc alleles over a 24 h repair incubation period was of the same order of magnitude: approximately 34% and approximately 25% of nitrogen mustard-induced N-alkylpurines were removed by 8 h; approximately 72% and approximately 66% of nitrogen mustard lesions were removed by 24 h from the untranslocated and translocated alleles respectively. The untranslocated allele did not become transcriptionally activated upon drug treatment and nitrogen mustard produced a suppression of c-myc message levels from the translocated allele. Therefore, our results suggest that the rate of repair of nitrogen mustard-induced N-alkylpurines is independent of transcriptional activity of the c-myc gene in Burkitt's lymphoma. These findings are discussed in terms of the current views about the mechanisms of gene-specific repair.