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DNA repair in human cells: in Cockayne syndrome cells rejoining of DNA strands is impaired.

Fibroblasts from patients with Cockayne Syndrome (CS) are hypersensitive to UV light. DNA repair was analyzed in these cells by sedimentation behaviour of DNA nucleoids in sucrose gradients and compared to normal control cells. The initiation of repair, the incision of the DNA strand next to the UV lesion appeared to be normal. The rejoining of DNA stretches, however, is retarded in CS cells. DNA repair synthesis of UV damages was measured by autoradiography of [14C]thymidine incorporation into resting cells. Up to 4 h the DNA repair synthesis was comparable with normal cells. From 4 to 7 h the incorporation of radioactive precursors declined in CS cells. Besides a defective DNA polymerase this could be due to accelerated excorporation of radioactive nucleotides as a consequence of delayed ligation. In ligation the enzyme itself could be affected as well as its activation by ADP-ribosylation. Nicotine adenine dinucleotide (NAD+) is needed for the ADP ribosylation process. The cellular NAD+ content, however, was found to be the same in normal and in CS fibroblasts. Increase of the extracellular NAD+ supply accelerated the rejoining of UV damaged DNA in CS cells.