Repair in ribosomal RNA genes is deficient in xeroderma pigmentosum group C and in Cockayne's syndrome cells.

Previous studies have demonstrated transcription-coupled DNA repair in mammalian genes transcribed by RNA polymerase II but not in ribosomal RNA genes (rDNA), which are transcribed by RNA polymerase I. The removal of UV-induced cyclobutane pyrimidine dimers (CPD) from rDNA in repair-proficient human cells has been shown to be slow but detectable and apparently not coupled to transcription. We studied the induction and removal of CPD from rDNA in cultured cells from two repair-deficient human disorders. Primary xeroderma pigmentosum complementation group C (XP-C) cells, whether proliferating or nondividing, removed no CPD from either rDNA strand in 24 h post-UV, a result which supports earlier conclusions that XP-C cells lack the general, transcription-independent pathway of nucleotide excision repair. We also observed lower than normal repair of rDNA in Cockayne's syndrome (CS) cells from complementation groups A and B. In agreement with previous findings, the repair of both strands of the RNA polymerase II-transcribed dihydrofolate reductase gene was also deficient relative to that of normal cells. This strongly suggests that the defect in CS cells is not limited to a deficiency in a transcription-repair coupling factor. Rather, the defect may interfere with the ability of repair proteins to gain access to all expressed genes.