A. van Hoffen

Transcription-coupled repair removes both cyclobutane pyrimidine dimers and 6-4 photoproducts with equal efficiency and in a sequential way from transcribed DNA in xeroderma pigmentosum group C fibroblasts.

We investigated the contribution of the global and the transcription‐coupled nucleotide excision repair pathway to the removal of structurally different DNA lesions. The repair kinetics of UV‐induced cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6‐4) pyrimidone photoproducts (6‐4PPs) were determined in an active and inactive gene in normal human fibroblasts and in xeroderma pigmentosum group C (XP‐C) fibroblasts. Previously we have shown that in normal human cells exposed to a UV dose of 10 J/m2 repair of CPDs takes place via two pathways: global repair and transcription‐coupled repair, the latter being responsible for accelerated repair of CPDs in the transcribed strand of active genes. So far, no clear evidence for transcription‐coupled repair of 6‐4PPs has been presented. Here we demonstrate that 6‐4PPs really form a target for transcription‐coupled repair. In XP‐C cells, exposed to 30 J/m2 and only capable of performing transcription‐coupled repair, CPDs as well as 6‐4PPs are removed selectively and with similar kinetics from the transcribed strand of the adenosine deaminase (ADA) gene. The non‐transcribed strand of the ADA gene and the inactive 754 gene are hardly repaired. In contrast to XP‐C cells, normal cells exposed to 30 J/m2 lack strand‐specific repair of both 6‐4PPs and CPDs, suggesting that transcription‐coupled repair is overruled by global repair, probably due to severe inhibition of transcription at this high UV dose. The much more rapid repair of 6‐4PPs compared with CPDs in normal cells may be related to higher affinity of the global repair system for the former lesion. In XP‐C cells the similarity of the rate of repair of both 6‐4PPs and CPDs in the transcribed strand at 30 J/m2 indicates that transcription‐coupled repair of photolesions takes place in a sequential way. Our results strongly suggest that the significance of transcription‐coupled repair for removal of lesions depends on the type of lesion and on the dose employed.

UItraviolet-Induced Photolesions: Repair and Mutagenesis

There is convincing evidence that the structure of chromatin may influence both the induction and processing of DNA damage within various parts of the genome that exhibit diverse molecular structures and activities. For a variety of lesions it has been shown that nucleotide excision repair (NER) takes place preferentially in transcriptionally active DNA (Mullenders and Smith 1994). Cyclobutane pyrimidine dimers (CPD) induced by ultraviolet (UV) light as well as DNA adducts induced by chemical carcinogens such as benzo(a)pyrene diol epoxide, aflatoxin B1 and psoralen are more rapidly repaired in transcriptionally active housekeeping genes than in inactive tissue specific genes, ribosomal genes, regions of noncoding DNA, or the genome overall (Bohr et al. 1985; Mellon et al. 1986; Venema et al. 1990; Ruven et al. 1993). However, from the analysis of repair of other bulky lesions in specific sequences in both rodent and human cells it has become clear that not all bulky lesions are preferentially repaired in active genes and that there are no simple rules predicting whether a given lesion is repaired preferentially or not (Mullenders and Smith 1994).

Heterogeneity of DNA Repair in Relation to Chromatin Structure

Analysis of repair processes in mammalian cells has largely been focused on induction and repair of DNA damage in the genome overall. In particular the repair of ultraviolet light-induced photoproducts has been intensively studied in a variety of mammalian cells and in most cases UV-induced cytotoxicity can be correlated to the extent of unscheduled DNA synthesis or removal of pyrimidine dimers from the nuclear DNA. For example variation in UV-induced cytotoxicity found both within and between the various complementation groups of the human UV-sensitive disorder xeroderma pigmentosum (XP) generally correlates with the extent of defective excision repair (Kantor and Hull 1984). However, a notable exception to this is found in nondividing XP-cells belonging to complementation group C, which are relatively resistant to the lethal effects of UV (Kantor and Hull 1984; Mayne and Lehmann 1982). Also, in a number of other cases the removal of pyrimidine dimers from the genome overall turned out to be an invalid parameter to predict UV-induced cytotoxicity. Cockayne’s syndrome (CS) is a human disorder characterized at the cellular level by an increased sensitivity to the killing effects of UV-light, but with an apparently normal capacity to perform unscheduled DNA synthesis or to remove pyrimidine dimers (Mayne and Lehmann 1982). The various rodent cell lines consistently exhibit low levels of pyrimidine dimer removal for the genome overall (Van Zeeland et al. 1981) but are equally resistant to lethal effects of UV-light as human cells which are capable of performing fast and efficient repair of pyrimidine dimers.