Nuclear matrix associated DNA is preferentially repaired in normal human fibroblasts, exposed to a low dose of ultraviolet light but not in Cockayne's syndrome fibroblasts.

In this study we addressed the questions as to whether repair is confined to the nuclear matrix compartment, analogous to replication and transcription and how repair events are distributed in DNA loops associated with the nuclear matrix. Pulse labelling of ultraviolet (254 nm) irradiated confluent human fibroblasts revealed that repair was preferentially located in nuclear matrix associated DNA in cells exposed to 5 J/m2. However, in cells exposed to 30 J/m2 repair approached a random distribution. The non-random distribution of repair label at 5 J/m2 was most pronounced directly after irradiation and gradually changed to a more random distribution within two hours after treatment. The results of pulse-chase experiments exclude the possibility of transient binding of repair sites to the matrix and favour the model of preferential repair of DNA sequences permanently associated with the nuclear matrix. Pronounced differences in distribution pattern of repair events in DNA loops were found among normal and UV-sensitive cell lines exposed to 5 J/m2. Repair in nuclear matrix associated DNA was 1.7 fold more efficient than in loop DNA in normal and xeroderma pigmentosum group D cells and over 3 fold in xeroderma pigmentosum group C cells. In Cockayne's syndrome fibroblasts repair in nuclear matrix DNA was found to be 2 fold less efficient than in loop DNA. This heterogeneity in distribution of repair correlates well with preferential removal of pyrimidine dimers from transcriptionally active DNA in normal and xeroderma pigmentosum group C cells and its absence in Cockayne's syndrome cells as recently reported by Mayne et al., 1988. The results suggest that Cockayne's syndrome cells have a defect in excision of UV-damage from transcriptionally active genes located proximal to the nuclear matrix. Xeroderma pigmentosum group C cells may possess a defect in DNA repair associated with chromatin regions outside transcriptionally active DNA.