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, 25 (2), 179-85

Newly Identified CHO ERCC3/XPB Mutations and Phenotype Characterization

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Newly Identified CHO ERCC3/XPB Mutations and Phenotype Characterization

Ivana Rybanská et al. Mutagenesis.

Abstract

Nucleotide excision repair (NER) is a complex multistage process involving many interacting gene products to repair a wide range of DNA lesions. Genetic defects in NER cause human hereditary diseases including xeroderma pigmentosum (XP), Cockayne syndrome (CS), trichothiodystrophy and a combined XP/CS overlapping symptom. One key gene product associated with all these disorders is the excision repair cross-complementing 3/xeroderma pigmentosum B (ERCC3/XPB) DNA helicase, a subunit of the transcription factor IIH complex. ERCC3 is involved in initiation of basal transcription and global genome repair as well as in transcription-coupled repair (TCR). The hamster ERCC3 gene shows high degree of homology with the human ERCC3/XPB gene. We identified new mutations in the Chinese hamster ovary cell ERCC3 gene and characterized the role of hamster ERCC3 protein in DNA repair of ultraviolet (UV)-induced and oxidative DNA damage. All but one newly described mutations are located in the protein C-terminal region around the last intron-exon boundary. Due to protein truncations or frameshifts, they lack amino acid Ser751, phosphorylation of which prevents the 5' incision of the UV-induced lesion during NER. Thus, despite the various locations of the mutations, their phenotypes are similar. All ercc3 mutants are extremely sensitive to UV-C light and lack recovery of RNA synthesis (RRS), confirming a defect in TCR of UV-induced damage. Their limited global genome NER capacity averages approximately 8%. We detected modest sensitivity of ercc3 mutants to the photosensitizer Ro19-8022, which primarily introduces 8-oxoguanine lesions into DNA. Ro19-8022-induced damage interfered with RRS, and some of the ercc3 mutants had delayed kinetics. All ercc3 mutants showed efficient base excision repair (BER). Thus, the positions of the mutations have no effect on the sensitivity to, and repair of, Ro19-8022-induced DNA damage, suggesting that the ERCC3 protein is not involved in BER.

Figures

Fig. 1
Fig. 1
Cytotoxic effects of RO (Panel A) and UV-C (Panel B) on the wild-type parental cell line AA8 and the ercc3 mutants. The survival curves show averages of at least three independent experiments, and error bars represent standard deviation.
Fig. 2
Fig. 2
RRS. Cells were treated and pulse labelled with [3H]uridine for 30 min and the data normalized to the values of untreated cells. Each data point represents the average of at least three independent experiments; error bars represent the SD. (A) Treatment with RO (0.1 μM) plus visible light; (B) 10 J/m2 UV-C.
Fig. 3
Fig. 3
The level of DNA damage in parental and ercc3 mutant lines. (A) Cells were treated with 0.05 μM RO and remaining DNA damage was estimated by Fpg endonuclease modified comet assay 4 h after treatment. (B) Cells were irradiated with 0.25 J/m2 and remaining DNA damage was estimated by T4 endoV modified comet assay 24 h after UV irradiation. The numbers on the top of columns represent the percentage of repair. (***) Denotes P < 0.001 significance of difference between mutant cell line and corresponding wild-type cells.

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