Cell-type-specific consequences of nucleotide excision repair deficiencies: Embryonic stem cells versus fibroblasts.

Pluripotent embryonic stem cells (ES cells) are the precursors of all different cell types comprising the organism. Since persistent DNA damage in this cell type might lead to mutations that cause huge malformations in the developing organism, genome caretaking is of prime importance. We first compared the sensitivity of wild type mouse embryonic fibroblasts (MEFs) and ES cells for various genotoxic agents and show that ES cells are more sensitive to treatment with UV-light, gamma-rays and mitomycin C than MEFs. We next investigated the contribution of the transcription-coupled (TC-NER) and global genome (GG-NER) sub-pathways of nucleotide excision repair (NER) in protection of ES cells, using cells from mouse models for the NER disorders xeroderma pigmentosum (XP) and Cockayne syndrome (CS). TC-NER-deficient Csb(-/-) and GG-NER/TC-NER-defective Xpa(-/-) MEFs are hypersensitive to UV, whereas GG-NER-deficient Xpc(-/-) MEFs attribute intermediate UV sensitivity. The observed UV-hypersensitivity in Csb(-/-) and Xpa(-/-) MEFs correlates with increased apoptosis. In contrast, Xpa(-/-) and Xpc(-/-) ES cells are highly UV-sensitive, while a Csb deficiency only causes a mild increase in UV-sensitivity. Surprisingly, a UV-induced hyperapoptotic response is mainly observed in Xpa(-/-) ES cells, suggesting a different mechanism of apoptosis induction in ES cells, mainly triggered by damage in the global genome rather than in transcribed genes (as in MEFs). Moreover, we show a pronounced S-phase delay in Xpa(-/-) and Xpc(-/-) ES cells, which might well function as a safeguard mechanism for heavily damaged ES cells in case the apoptotic response fails. Although Xpa(-/-) and Xpc(-/-) ES cells are totally NER-defective or GG-NER-deficient respectively, mutation induction upon UV is similar compared to wild type ES cells indicating that the observed apoptotic and cell cycle responses are indeed sufficient to protect against proliferation of damaged cells. In conclusion, we show a double safeguard mechanism in ES cells against NER-type of damages, which mainly relies on damage detection in the global genome.

Tissue specific mutagenic and carcinogenic responses in NER defective mouse models.

Several mouse models with defects in genes encoding components of the nucleotide excision repair (NER) pathway have been developed. In NER two different sub-pathways are known, i.e. transcription-coupled repair (TC-NER) and global-genome repair (GG-NER). A defect in one particular NER protein can lead to a (partial) defect in GG-NER, TC-NER or both. GG-NER defects in mice predispose to cancer, both spontaneous as well as UV-induced. As such these models (Xpa, Xpc and Xpe) recapitulate the human xeroderma pigmentosum (XP) syndrome. Defects in TC-NER in humans are associated with Cockayne syndrome (CS), a disease not linked to tumor development. Mice with TC-NER defects (Csa and Csb) are - except for the skin - not susceptible to develop (carcinogen-induced) tumors. Some NER factors, i.e. XPB, XPD, XPF, XPG and ERCC1 have functions outside NER, like transcription initiation and inter-strand crosslink repair. Deficiencies in these processes in mice lead to very severe phenotypes, like trichothiodystrophy (TTD) or a combination of XP and CS. In most cases these animals have a (very) short life span, display segmental progeria, but do not develop tumors. Here we will overview the available NER-related mouse models and will discuss their phenotypes in terms of (chemical-induced) tissue-specific tumor development, mutagenesis and premature aging features.

DNA damage triggers nucleotide excision repair-dependent monoubiquitylation of histone H2A.

Chromatin changes within the context of DNA repair remain largely obscure. Here we show that DNA damage induces monoubiquitylation of histone H2A in the vicinity of DNA lesions. Ultraviolet (UV)-induced monoubiquitylation of H2A is dependent on functional nucleotide excision repair and occurs after incision of the damaged strand. The ubiquitin ligase Ring2 is required for the DNA damage-induced H2A ubiquitylation. UV-induced ubiquitylation of H2A is dependent on the DNA damage signaling kinase ATR (ATM- and Rad3-related) but not the related kinase ATM (ataxia telangiectasia-mutated). Although the response coincides with phosphorylation of variant histone H2AX, H2AX was not required for H2A ubiquitylation. Together our data show that monoubiquitylation of H2A forms part of the cellular response to UV damage and suggest a role of this modification in DNA repair-induced chromatin remodeling.

Transcriptome analysis reveals cyclobutane pyrimidine dimers as a major source of UV-induced DNA breaks.

Photolyase transgenic mice have opened new avenues to improve our understanding of the cytotoxic effects of ultraviolet (UV) light on skin by providing a means to selectively remove either cyclobutane pyrimidine dimers (CPDs) or pyrimidine (6-4) pyrimidone photoproducts. Here, we have taken a genomics approach to delineate pathways through which CPDs might contribute to the harmful effects of UV exposure. We show that CPDs, rather than other DNA lesions or damaged macromolecules, comprise the principal mediator of the cellular transcriptional response to UV. The most prominent pathway induced by CPDs is that associated with DNA double-strand break (DSB) signalling and repair. Moreover, we show that CPDs provoke accumulation of gamma-H2AX, P53bp1 and Rad51 foci as well as an increase in the amount of DSBs, which coincides with accumulation of cells in S phase. Thus, conversion of unrepaired CPD lesions into DNA breaks during DNA replication may comprise one of the principal instigators of UV-mediated cytotoxicity.

Different effects of CSA and CSB deficiency on sensitivity to oxidative DNA damage.

Mutations in the CSA and CSB genes cause Cockayne syndrome, a rare inherited disorder characterized by UV sensitivity, severe neurological abnormalities, and progeriod symptoms. Both gene products function in the transcription-coupled repair (TCR) subpathway of nucleotide excision repair (NER), providing the cell with a mechanism to remove transcription-blocking lesions from the transcribed strands of actively transcribed genes. Besides a function in TCR of NER lesions, a role of CSB in (transcription-coupled) repair of oxidative DNA damage has been suggested. In this study we used mouse models to compare the effect of a CSA or a CSB defect on oxidative DNA damage sensitivity at the levels of the cell and the intact organism. In contrast to CSB(-/-) mouse embryonic fibroblasts (MEFs), CSA(-/-) MEFs are not hypersensitive to gamma-ray or paraquat treatment. Similar results were obtained for keratinocytes. In contrast, both CSB(-/-) and CSA(-/-) embryonic stem cells show slight gamma-ray sensitivity. Finally, CSB(-/-) but not CSA(-/-) mice fed with food containing di(2-ethylhexyl)phthalate (causing elevated levels of oxidative DNA damage in the liver) show weight reduction. These findings not only uncover a clear difference in oxidative DNA damage sensitivity between CSA- and CSB-deficient cell lines and mice but also show that sensitivity to oxidative DNA damage is not a uniform characteristic of Cockayne syndrome. This difference in the DNA damage response between CSA- and CSB-deficient cells is unexpected, since until now no consistent differences between CSA and CSB patients have been reported. We suggest that the CSA and CSB proteins in part perform separate roles in different DNA damage response pathways.

Mutation of the mouse Rad17 gene leads to embryonic lethality and reveals a role in DNA damage-dependent recombination.

Genetic defects in DNA repair mechanisms and cell cycle checkpoint (CCC) genes result in increased genomic instability and cancer predisposition. Discovery of mammalian homologs of yeast CCC genes suggests conservation of checkpoint mechanisms between yeast and mammals. However, the role of many CCC genes in higher eukaryotes remains elusive. Here, we report that targeted deletion of an N-terminal part of mRad17, the mouse homolog of the Schizosaccharomyces pombe Rad17 checkpoint clamp-loader component, resulted in embryonic lethality during early/mid-gestation. In contrast to mouse embryos, embryonic stem (ES) cells, isolated from mRad17(5'Delta/5'Delta) embryos, produced truncated mRad17 and were viable. These cells displayed hypersensitivity to various DNA-damaging agents. Surprisingly, mRad17(5'Delta/5'Delta) ES cells were able to arrest cell cycle progression upon induction of DNA damage. However, they displayed impaired homologous recombination as evidenced by a strongly reduced gene targeting efficiency. In addition to a possible role in DNA damage-induced CCC, based on sequence homology, our results indicate that mRad17 has a function in DNA damage-dependent recombination that may be responsible for the sensitivity to DNA-damaging agents.

Cell type-specific hypersensitivity to oxidative damage in CSB and XPA mice.

Mutations in the CSB gene cause Cockayne syndrome (CS), a rare inherited disorder, characterized by UV-sensitivity, severe neurodevelopmental and progeroid symptoms. CSB functions in the transcription-coupled repair (TCR) sub-pathway of nucleotide excision repair (NER), responsible for the removal of UV-induced and other helix-distorting lesions from the transcribed strand of active genes. Several lines of evidence support the notion that the CSB TCR defect extends to other non-NER type transcription-blocking lesions, notably various kinds of oxidative damage, which may provide an explanation for part of the severe CS phenotype. We used genetically defined mouse models to examine the relationship between the CSB defect and sensitivity to oxidative damage in different cell types and at the level of the intact organism. The main conclusions are: (1) CSB(-/-) mouse embryo fibroblasts (MEFs) exhibit a clear hypersensitivity to ionizing radiation, extending the findings in genetically heterogeneous human CSB fibroblasts to another species. (2) CSB(-/-) MEFs are highly sensitive to paraquat, strongly indicating that the increased cytotoxicity is due to oxidative damage. (3) The hypersenstivity is independent of genetic background and directly related to the CSB defect and is not observed in totally NER-deficient XPA MEFs. (4) Wild type embryonic stem (ES) cells display an increased sensitivity to ionizing radiation compared to fibroblasts. Surprisingly, the CSB deficiency has only a very minor additional effect on ES cell sensitivity to oxidative damage and is comparable to that of an XPA defect, indicating cell type-specific differences in the contribution of TCR and NER to cellular survival. (5) Similar to ES cells, CSB and XPA mice both display a minor sensitivity to whole-body X-ray exposure. This suggests that the response of an intact organism to radiation is largely determined by the sensitivity of stem cells, rather than differentiated cells. These findings establish the role of transcription-coupled repair in resistance to oxidative damage and reveal a cell- and organ-specific impact of this repair pathway to the clinical phenotype of CS and XP.