Sensitivity to DNA-damaging agents and mutation induction by UV light in UV-sensitive CHO cells

Incidence of DNA repair deficiency disorders in western Europe: Xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy

Laboratory diagnosis for DNA repair diseases has been performed in western Europe from the early seventies for xeroderma pigmentosum (XP) and from the mid-eighties for Cockayne syndrome (CS) and trichothiodystrophy (TTD). The combined data from the DNA repair diagnostic centres in France, (West) Germany, Italy, the Netherlands and the United Kingdom have been investigated for three groups of diseases: XP (including XP-variant), CS (including XP/CS complex) and TTD. Incidences in western Europe were for the first time established at 2.3 per million livebirths for XP, 2.7 per million for CS and 1.2 per million for TTD. As immigrant populations were disproportionately represented in the patients' groups, incidences were also established for the autochthonic western European population at: 0.9 per million for XP, 1.8 per million for CS and 1.1 per million for TTD. Perhaps contrary to general conceptions, compared to XP the incidence of CS appears to be somewhat higher and the incidence of TTD to be quite similar in the native West-European population.

Cell proliferation and DNA breaks are involved in ultraviolet light-induced apoptosis in nucleotide excision repair-deficient Chinese hamster cells

UV light targets both membrane receptors and nuclear DNA, thus evoking signals triggering apoptosis. Although receptor-mediated apoptosis has been extensively investigated, the role of DNA damage in apoptosis is less clear. To analyze the importance of DNA damage induced by UV-C light in apoptosis, we compared nucleotide excision repair (NER)-deficient Chinese hamster ovary cells (lines 27-1 and 43-3B mutated for the repair genes ERCC3 and ERCC1, respectively) with the corresponding DNA repair-proficient fibroblasts (CHO-9 and ERCC1 complemented 43-3B cells). NER-deficient cells were hypersensitive as to the induction of apoptosis, indicating that apoptosis induced by UV-C light is due to unrepaired DNA base damage. Unrepaired lesions, however, do not activate the apoptotic pathway directly because apoptosis upon UV-C irradiation requires DNA replication and cell proliferation. It is also shown that in NER-deficient cells unrepaired lesions are converted into DNA double-strand breaks (DSBs) and chromosomal aberrations by a replication-dependent process that precedes apoptosis. We therefore propose that DSBs arising from replication of DNA containing nonrepaired lesions act as an ultimate trigger of UV-C-induced apoptosis. Induction of apoptosis by UV-C light was related to decline in the expression level of Bcl-2 and activation of caspases. Decline of Bcl-2 and subsequent apoptosis might also be caused, at least in part, by UV-C-induced blockage of transcription, which was more pronounced in NER-deficient than in wild-type cells. This is in line with experiments with actinomycin D, which provoked Bcl-2 decline and apoptosis. UV-C-induced apoptosis due to nonrepaired DNA lesions, replication-dependent formation of DSBs, and activation of the mitochondrial damage pathway is independent of functional p53 for which the cells are mutated.

Ultraviolet light-induced DNA damage triggers apoptosis in nucleotide excision repair-deficient cells via Bcl-2 decline and caspase-3/-8 activation

Ultraviolet (UV) light is a potent mutagenic and genotoxic agent. Whereas DNA damage induced by UV light is known to be responsible for UV-induced genotoxicity, its role in triggering apoptosis is still unclear. We addressed this issue by comparing nucleotide excision repair (NER) deficient 27-1 and 43-3B Chinese hamster (CHO) cells with the corresponding wild-type and ERCC-1 complemented cells. It is shown that NER deficient cells are dramatically hypersensitive to UV-C induced apoptosis, indicating that DNA damage is the major stimulus for the apoptotic response. Apoptosis triggered by UV-C induced DNA damage is related to caspase- and proteosome-dependent degradation of Bcl-2 protein. The expression of other members of the Bcl-2 family such as Bax, Bcl-x(L) and Bak were not affected. Bcl-2 decline is causally involved in UV-C induced apoptosis since overexpression of Bcl-2 protected NER deficient cells against apoptosis. We also demonstrate that caspase-8, caspase-9 and caspase-3 are activated and PARP is cleaved in response to unrepaired UV-C induced DNA damage. Caspase-8 activation occurred independently of CD95 receptor activation since CD95R/FasR and CD95L/FasL were not altered in expression, and transfection of transdominant negative FADD failed to block apoptosis. Overall, the data demonstrate that UV-C induced non-repaired DNA damage triggers apoptosis in NER deficient fibroblasts involving components of the intrinsic mitochondrial damage pathway.

Phenotypic heterogeneity in nucleotide excision repair mutants of rodent complementation groups 1 and 4

Rodent ultraviolet light (UV)-sensitive mutant cells in complementation groups (CGs) 1 and 4 normally are known for their extraordinary (approximately 80-100 x) sensitivity to mitomycin C (MMC), although some CG1 mutants with reduced MMC sensitivity were previously reported (Stefanini et al. (1987) Cytotechnology 1, 91). We report here new CG1 and CG4 mutants with only 1.6-10 x wild-type MMC sensitivity despite low unscheduled DNA synthesis (UDS) levels. Mutant UV140, in UV CG4, has approximately 3.8 x the UV sensitivity of parental line AA8, approximately 1.6 x wild-type MMC sensitivity, wild-type X-ray and ethyl methanesulfonate (EMS) sensitivity, and is only slightly (approximately 1.4 x) hypermutable to 8-azaadenine resistance by UV light. It has moderately decreased incision of UV-damaged DNA, has moderately decreased removal of (6-4) photoproducts, and is profoundly deficient in UDS after UV. After UV, it shows abnormally decreased DNA synthesis and persistently decreased RNA synthesis. In addition a cell-free extract of this mutant displays strongly reduced nucleotide excision repair synthesis using DNA treated with N-acetoxy-acetyl-amino-fluorene (AAF). The extract selectively fails to complement extracts of group 1 and 4 mutants consistent with the notion that the affected proteins, ERCC1 and ERCC4, are part of the same complex and that mutations in one subunit also affect the other component. Mutant UV212 is a CG1 mutant with approximately 3.3 x wild-type UV and approximately 5-10 x wild-type MMC sensitivity, with profoundly deficient UDS and hypermutability (approximately 5.8 x) by UV. Mutant UV201, probably in CG1, is only slightly (approximately 1.5 x) UV-sensitive and has near wild-type (1.02X) UV mutability. These unusual group 1 and 4 mutants demonstrate that the unique UV and MMC sensitivity phenotypes displayed by these groups can be separated and support the idea that they are the result of distinct repair functions of the corresponding ERCC1 and ERCC4 genes: nucleotide excision repair for UV lesions and a separate repair pathway for removal of interstrand crosslinks.

The molecular genetics of the incision step in the DNA excision repair process

This review describes the evolution of research into the genetic basis of how different organisms use the process of excision repair to recognize and remove lesions from their cellular DNA. One particular aspect of excision repair, DNA incision, and how it is controlled at the genetic level in bacteriophage, bacteria, S. cerevisae, D. melanogaster, rodent cells and humans is examined. In phage T4, DNA is incised by a DNA glycosylase-AP endonuclease that is coded for by the denV gene. In E. coli, the products of three genes, uvrA, uvrB and uvrC, are required to form the UVRABC excinuclease that cleaves DNA and releases a fragment 12-13 nucleotides long containing the site of damage. In S. cerevisiae, genes complementing five mutants of the RAD3 epistasis group, rad1, rad2, rad3, rad4 and rad10 have been cloned and analyzed. Rodent cells sensitive to a variety of mutagenic agents and deficient in excision repair are being used in molecular studies to identify and clone human repair genes (e.g. ERCC1) capable of complementing mammalian repair defects. Most studies of the human system, however, have been done with cells isolated from patients suffering from the repair defective, cancer-prone disorder, xeroderma pigmentosum, and these cells are now beginning to be characterized at the molecular level. Studies such as these that provide a greater understanding of the genetic basis of DNA repair should also offer new insights into other cellular processes, including genetic recombination, differentiation, mutagenesis, carcinogenesis and aging.

Mutagen detection with a mouse line containing 3 distinct mutations conferring sensitivity

A mouse-cell mutant sensitive to methyl methanesulfonate (MMS), X-rays, ultraviolet light (UV), and crosslinking agents was selected using the replica plating and cell suspension spotting methods. This mutant (XUM1) is a mitomycin C-sensitive derivative of previously reported XU1, a mutant sensitive to MMS, X-rays and UV. Since XU1 is highly susceptible to the lethal effect of 4-nitroquinoline-1-oxide (4NQO), XUM1 is also hypersensitive to 4NQO. Growth inhibition area tests showed that low concentrations of mutagens were detected with the multiple mutagen-sensitive mutant XUM1. Hence XUM1 cells will be useful in detecting with high sensitivity a wide range of mutagens and carcinogens which mimic X-rays, UV and crosslinking agents.