Selective DNA repair in active genes.

My colleagues and I have discovered intragenomic heterogeneity in DNA repair in mammalian cells. Consequences of unrepaired DNA damage depend upon the precise location of the damage with respect to relevant genes. It is therefore important to understand rules governing accessibility of specific DNA sequences in chromatin to damage and repair. The efficiency of removal of pyrimidine dimers has been mapped in the active dihydrofolate reductase (DHFR) gene in Chinese hamster ovary (CHO) cells. Repair within the gene was shown to be much more efficient than that in silent downstream sequences or in the genome overall. Preferential repair of active and essential genes such as DHFR may account for the fact that rodent cells are as UV-resistant as human cells in spite of their much lower overall repair efficiencies. In repair proficient human cells the rate of repair in the DHFR gene is greater than that in the overall genome or in non-transcribed alpha DNA sequences. The efficiency of removal of pyrimidine dimers is much higher in the transcribed than the non-transcribed DNA strands of the DHFR gene in both CHO and human cells. An excision-repair complex may be directly coupled to the transcription machinery to ensure early removal of transcription-blocking lesions in active genes. Sequences in the active c-abl protooncogene are repaired much more efficiently than are sequences containing the inactive c-mos protooncogene in Swiss mouse 3T3 cells. Tissue specific and cell specific differences in the coordinate regulation of protooncogene expression and DNA repair may account for corresponding differences in the carcinogenic response. Efficient replicative bypass of persisting psoralen monoadducts, but not interstrand crosslinks, was demonstrated in the human DHFR gene. It is likely that most bulky lesions in mammalina DNA, other than crosslinks, do not pose insurmountable problems for replication in vivo, but they must be removed from essential transcribed sequences to maintain cellular viability.