Repair of radiation-induced DNA strand breaks does not occur preferentially in transcriptionally active DNA.

Certain DNA base lesions induced by ionizing radiation or oxidative stress are repaired faster from the transcribed strand of active genes compared to the genome overall. In this study, it was investigated whether radiation-induced DNA strand breaks are preferentially repaired in active genes compared to the genome as a whole in CHO cells. The alkaline unwinding technique coupled to slot-blot hybridization with specific DNA probes was used to study the induction and repair of DNA strand breaks in defined DNA sequences. Results using this technique showed a linear dose response for the formation of radiation-induced DNA strand breaks in the dihydrofolate reductase (DHFR) gene. Furthermore, the half-life of radiation-induced strand breaks was less than 5 min in the DHFR gene, in the ribosomal genes, and in the genome as a whole. These results suggest that the repair of DNA strand breaks is fast and uniform in the genome of mammalian cells.     

Deficient repair of the transcribed strand of active genes in Cockayne's syndrome cells.

Removal of ultraviolet light induced cyclobutane pyrimidine dimers (CPD) from active and inactive genes was analyzed in cells derived from patients suffering from the hereditary disease Cockayne's syndrome (CS) using strand specific probes. The results indicate that the defect in CS cells affects two levels of repair of lesions in active genes. Firstly, CS cells are deficient in selective repair of the transcribed strand of active genes. In these cells the rate and efficiency of repair of CPD are equal for the transcribed and the nontranscribed strand of the active ADA and DHFR genes. In normal cells on the other hand, the transcribed strand of these genes is repaired faster than the nontranscribed strand. However, the nontranscribed strand is still repaired more efficiently than the inactive 754 gene and the gene coding for coagulation factor IX. Secondly, the repair level of active genes in CS cells exceeds that of inactive loci but is slower than the nontranscribed strand of active genes in normal cells. Our results support the model that CS cells lack a factor which is involved in targeting repair enzymes specifically towards DNA damage located in (potentially) active DNA.