Increased UV resistance of a xeroderma pigmentosum revertant cell line is correlated with selective repair of the transcribed strand of an expressed gene.

Abstract

A UV-resistant revertant (XP129) of a xeroderma pigmentosum group A cell line has been reported to be totally deficient in repair of cyclobutane pyrimidine dimers (CPDs) but proficient in repair of 6-4 photoproducts. This finding has been interpreted to mean that CPDs play no role in cell killing by UV. We have analyzed the fine structure of repair of CPDs in the dihydrofolate reductase gene in the revertant. In this essential, active gene, we observe that repair of the transcribed strand is as efficient as that in normal, repair-proficient human cells, but repair of the nontranscribed strand is not. Within 4 h after UV at 7.5 J/m2, over 50% of the CPDs were removed, and by 8 h, 80%o of the CPDs were removed. In contrast, there was essentially no removal from the nontranscribed strand even by 24 h. Our results demonstrate that overall repair measurements can be misleading, and they support the hypothesis that removal of CPDs from the transcribed strands of expressed genes is essential for UV resistance.

Repair by human cell extracts of single (6-4) and cyclobutane thymine-thymine photoproducts in DNA.

“…For example, cyclobutane pyrimidine dimers appear to be better tolerated in cellular genomes than (6-4) photoproducts, and cells can complete DNA replication with a large number of dimers still present (30,41). In addition, cyclobutane dimers are preferentially removed from transcribed genes (42). In the present analysis many complications of in vivo studies have been eliminated by comparing UV photoproducts in isolation under identical conditions.…”

DNA damage by anti-cancer agents resolved at the nucleotide level of a single copy gene: evidence for a novel binding site for cisplatin in cells

“…DNA repair is known to be heterogeneous, it has been found that active genes are often repaired more efficiently than inactive genes, (29)(30)(31) and that the transcribed strand is repaired more rapidly than the nontranscribed strand (27,32,33). These studies have mainly been performed with Southern blotting based methods which investigate damage at the level of the whole gene (10-20 kb).…”

Mutagenic Lesions in Photocarcinogenesis: The Fate of Pyrimidine Photoproducts in Repair‐Deficient Disorders

“…This mutation appears to reduce the capacity for repair of cyclobutane dimers specifically in inactive regions of chromatin (14,15). We wanted to investigate the intracellular function of XPA by development of an inducible gene expression system with which variations of XPA expression could be related to DNA repair and cell survival.…”