A common mutational pattern in Cockayne syndrome patients from xeroderma pigmentosum group G: implications for a second XPG function

Base Excision Repair: Mechanisms and Impact in Biology, Disease, and Medicine

Base excision repair (BER) corrects forms of oxidative, deamination, alkylation, and abasic single-base damage that appear to have minimal effects on the helix. Since its discovery in 1974, the field has grown in several facets: mechanisms, biology and physiology, understanding deficiencies and human disease, and using BER genes as potential inhibitory targets to develop therapeutics. Within its segregation of short nucleotide (SN-) and long patch (LP-), there are currently six known global mechanisms, with emerging work in transcription- and replication-associated BER. Knockouts (KOs) of BER genes in mouse models showed that single glycosylase knockout had minimal phenotypic impact, but the effects were clearly seen in double knockouts. However, KOs of downstream enzymes showed critical impact on the health and survival of mice. BER gene deficiency contributes to cancer, inflammation, aging, and neurodegenerative disorders. Medicinal targets are being developed for single or combinatorial therapies, but only PARP and APE1 have yet to reach the clinical stage.

COFS type 3 in an Indian family with antenatally detected arthrogryposis

Fetal akinesia and contractures can be caused by mutations in various genes that lead to overlapping phenotypes with contractures, rocker bottom feet, cerebellar hypoplasia, ventriculomegaly, growth retardation, pulmonary hypoplasia, cystic hygroma and cleft palate in various combinations. Cerebro-oculo-facio-skeletal (COFS) syndrome is a condition resulting from defects in DNA repair pathway, and genes involved include ERCC1 (COFS), ERCC2 (XPD), ERCC5(XPG), and ERCC6 (CSB). It is a severe disorder presenting in fetal or neonatal period with microcephaly, arthrogryposis, prominent nose, and kyphoscoliosis, and leads to early death in childhood. We report a baby with antenatally identified arthrogryposis in which the homozygous pathogenic variant in exon 8 was identified in ERCC5 gene, by targeted next generation sequencing. This was predicted to cause premature chain termination in the protein. ERCC5 gene is mainly implicated in xeroderma pigmentosum, sometimes in COFS syndrome.

Human XPG nuclease structure, assembly, and activities with insights for neurodegeneration and cancer from pathogenic mutations

DNA repair is essential to life and to avoidance of genome instability and cancer. Xeroderma pigmentosum group G (XPG) protein acts in multiple DNA repair pathways, both as an active enzyme and as a scaffold for coordinating with other repair proteins. We present here the structure of the catalytic domain responsible for its DNA binding and nuclease activity. Our analysis provides structure-based hypotheses for how XPG recognizes its bubble DNA substrate and predictions of the structural impacts of XPG disease mutations associated with two phenotypically distinct diseases: xeroderma pigmentosum (XP, skin cancer prone) or Cockayne syndrome (XP/CS, severe progressive developmental defects).

Xeroderma pigmentosum group G (XPG) protein is both a functional partner in multiple DNA damage responses (DDR) and a pathway coordinator and structure-specific endonuclease in nucleotide excision repair (NER). Different mutations in the XPG gene ERCC5 lead to either of two distinct human diseases: Cancer-prone xeroderma pigmentosum (XP-G) or the fatal neurodevelopmental disorder Cockayne syndrome (XP-G/CS). To address the enigmatic structural mechanism for these differing disease phenotypes and for XPG’s role in multiple DDRs, here we determined the crystal structure of human XPG catalytic domain (XPGcat), revealing XPG-specific features for its activities and regulation. Furthermore, XPG DNA binding elements conserved with FEN1 superfamily members enable insights on DNA interactions. Notably, all but one of the known pathogenic point mutations map to XPGcat, and both XP-G and XP-G/CS mutations destabilize XPG and reduce its cellular protein levels. Mapping the distinct mutation classes provides structure-based predictions for disease phenotypes: Residues mutated in XP-G are positioned to reduce local stability and NER activity, whereas residues mutated in XP-G/CS have implied long-range structural defects that would likely disrupt stability of the whole protein, and thus interfere with its functional interactions. Combined data from crystallography, biochemistry, small angle X-ray scattering, and electron microscopy unveil an XPG homodimer that binds, unstacks, and sculpts duplex DNA at internal unpaired regions (bubbles) into strongly bent structures, and suggest how XPG complexes may bind both NER bubble junctions and replication forks. Collective results support XPG scaffolding and DNA sculpting functions in multiple DDR processes to maintain genome stability.

DNA damage and transcription stress cause ATP-mediated redesign of metabolism and potentiation of anti-oxidant buffering

Accumulation of DNA lesions causing transcription stress is associated with natural and accelerated aging and culminates with profound metabolic alterations. Our understanding of the mechanisms governing metabolic redesign upon genomic instability, however, is highly rudimentary. Using Ercc1-defective mice and Xpg knock-out mice, we demonstrate that combined defects in transcription-coupled DNA repair (TCR) and in nucleotide excision repair (NER) directly affect bioenergetics due to declined transcription, leading to increased ATP levels. This in turn inhibits glycolysis allosterically and favors glucose rerouting through the pentose phosphate shunt, eventually enhancing production of NADPH-reducing equivalents. In NER/TCR-defective mutants, augmented NADPH is not counterbalanced by increased production of pro-oxidants and thus pentose phosphate potentiation culminates in an over-reduced redox state. Skin fibroblasts from the TCR disease Cockayne syndrome confirm results in animal models. Overall, these findings unravel a mechanism connecting DNA damage and transcriptional stress to metabolic redesign and protective antioxidant defenses.

ERCC1 is involved in a number of DNA repair pathways including nucleotide excision repair. Here the authors showed that reduced transcription in Ercc1-deficient mouse livers and cells increases ATP levels, suppressing glycolysis and rerouting glucose into the pentose phosphate shunt that generates reductive stress.

Frequent retrotransposition of endogenous genes in ERCC2-deficient cells derived from a patient with xeroderma pigmentosum

BACKGROUND

Retrotransposition of protein-coding genes is thought to occur due to the existence of numerous processed pseudogenes in both animals and plants. Unlike retrotransposons including Alu and LINE-1, direct evidence of such retrotransposition events has not been reported to date. Even if such an event occurs in a somatic cell, it is almost impossible to detect it using bulk of cells as a sample. Single-cell analyses or other techniques are needed.

METHODS

In order to examine genetic stability of stem cells, we have established induced pluripotent stem cell (iPSC) lines from several patients with DNA repair-deficiency disorders, such as ataxia telangiectasia and xeroderma pigmentosum, along with healthy controls. Performing whole-exome sequencing analyses of these parental and iPSC lines, we compiled somatic mutations accumulated by the deficiency of DNA repair mechanisms. Whereas most somatic mutations cannot be detected in bulk, cell reprogramming enabled us to observe all the somatic mutations which had occurred in the cell line. Patterns of somatic mutations should be distinctive depending on which DNA repair gene is impaired.

RESULTS

The comparison revealed that deficiency of ATM and XPA preferentially gives rise to indels and single-nucleotide substitutions, respectively. On the other hand, deficiency of ERCC2 caused not only single-nucleotide mutations but also many retrotranspositions of endogenous genes, which were readily identified by examining removal of introns in whole-exome sequencing. Although the number was limited, those events were also detected in healthy control samples.

CONCLUSIONS

The present study exploits clonality of iPSCs to unveil somatic mutation sets that are usually hidden in bulk cell analysis. Whole-exome sequencing analysis facilitated the detection of retrotransposition mutations. The results suggest that retrotranspositions of human endogenous genes are more frequent than expected in somatic cells and that ERCC2 plays a defensive role against transposition of endogenous and exogenous DNA fragments.

Identification of a ERCC5 c.2333T>C (L778P) Variant in Two Tunisian Siblings With Mild Xeroderma Pigmentosum Phenotype

Xeroderma pigmentosum (XP) is a rare autosomal recessive disorder due to a defect in the nucleotide excision repair (NER) DNA repair pathway, characterized by severe sunburn development of freckles, premature skin aging, and susceptibility to develop cancers at an average age of eight. XP is an example of accelerated photo-aging. It is a genetically and clinically heterogeneous disease. Eight complementation groups have been described worldwide. In Tunisia, five groups have been already identified. In this work, we investigated the genetic etiology in a family with an atypically mild XP phenotype. Two Tunisian siblings born from first-degree consanguineous parents underwent clinical examination in the dermatology department of the Charles Nicolle Hospital on the basis of acute sunburn reaction and mild neurological disorders. Blood samples were collected from two affected siblings after written informed consent. As all mutations reported in Tunisia have been excluded using Sanger sequencing, we carried out mutational analysis through a targeted panel of gene sequencing using the Agilent HaloPlex target enrichment system. Our clinical study shows, in both patients, the presence of achromic macula in sun exposed area with dermatological feature suggestive of Xeroderma pigmentosum disease. No developmental and neurological disorders were observed except mild intellectual disability. Genetic investigation shows that both patients were carriers of an homozygous T to C transition at the nucleotide position c.2333, causing the leucine to proline amino acid change at the position 778 (p.Leu778Pro) of the ERCC5 gene, and resulting in an XP-G phenotype. The same variation was previously reported at the heterozygous state in a patient cell line in Europe, for which no clinical data were available and was suggested to confer an XP/CS phenotype based on functional tests. This study contributes to further characterization of the mutation spectrum of XP in consanguineous Tunisian families and is potentially helpful for early diagnosis. It also indicates that the genotype-phenotype correlation is not always coherent for patients with mild clinical features. These data therefore suggest that targeted NGS is a highly informative diagnostic strategy, which can be used for XP molecular etiology determination.

The capacity of oocytes for DNA repair

Female fertility and offspring health are critically dependent on the maintenance of an adequate supply of high-quality oocytes. Like somatic cells, oocytes are subject to a variety of different types of DNA damage arising from endogenous cellular processes and exposure to exogenous genotoxic stressors. While the repair of intentionally induced DNA double strand breaks in gametes during meiotic recombination is well characterised, less is known about the ability of oocytes to repair pathological DNA damage and the relative contribution of DNA repair to oocyte quality is not well defined. This review will discuss emerging data suggesting that oocytes are in fact capable of efficient DNA repair and that DNA repair may be an important mechanism for ensuring female fertility, as well as the transmission of high-quality genetic material to subsequent generations.

Sunlight damage to cellular DNA: Focus on oxidatively generated lesions

The routine and often unavoidable exposure to solar ultraviolet (UV) radiation makes it one of the most significant environmental DNA-damaging agents to which humans are exposed. Sunlight, specifically UVB and UVA, triggers various types of DNA damage. Although sunlight, mainly UVB, is necessary for the production of vitamin D, which is necessary for human health, DNA damage may have several deleterious consequences, such as cell death, mutagenesis, photoaging and cancer. UVA and UVB photons can be directly absorbed not only by DNA, which results in lesions, but also by the chromophores that are present in skin cells. This process leads to the formation of reactive oxygen species, which may indirectly cause DNA damage. Despite many decades of investigation, the discrimination among the consequences of these different types of lesions is not clear. However, human cells have complex systems to avoid the deleterious effects of the reactive species produced by sunlight. These systems include antioxidants, that protect DNA, and mechanisms of DNA damage repair and tolerance. Genetic defects in these mechanisms that have clear harmful effects in the exposed skin are found in several human syndromes. The best known of these is xeroderma pigmentosum (XP), whose patients are defective in the nucleotide excision repair (NER) and translesion synthesis (TLS) pathways. These patients are mainly affected due to UV-induced pyrimidine dimers, but there is growing evidence that XP cells are also defective in the protection against other types of lesions, including oxidized DNA bases. This raises a question regarding the relative roles of the various forms of sunlight-induced DNA damage on skin carcinogenesis and photoaging. Therefore, knowledge of what occurs in XP patients may still bring important contributions to the understanding of the biological impact of sunlight-induced deleterious effects on the skin cells.

Deep phenotyping of 89 xeroderma pigmentosum patients reveals unexpected heterogeneity dependent on the precise molecular defect

Xeroderma pigmentosum (XP) is a rare DNA repair disorder characterized by increased susceptibility to UV radiation (UVR)-induced skin pigmentation, skin cancers, ocular surface disease, and, in some patients, sunburn and neurological degeneration. Genetically, it is assigned to eight complementation groups (XP-A to -G and variant). For the last 5 y, the UK national multidisciplinary XP service has provided follow-up for 89 XP patients, representing most of the XP patients in the United Kingdom. Causative mutations, DNA repair levels, and more than 60 clinical variables relating to dermatology, ophthalmology, and neurology have been measured, using scoring systems to categorize disease severity. This deep phenotyping has revealed unanticipated heterogeneity of clinical features, between and within complementation groups. Skin cancer is most common in XP-C, XP-E, and XP-V patients, previously considered to be the milder groups based on cellular analyses. These patients have normal sunburn reactions and are therefore diagnosed later and are less likely to adhere to UVR protection. XP-C patients are specifically hypersensitive to ocular damage, and XP-F and XP-G patients appear to be much less susceptible to skin cancer than other XP groups. Within XP groups, different mutations confer susceptibility or resistance to neurological damage. Our findings on this large cohort of XP patients under long-term follow-up reveal that XP is more heterogeneous than has previously been appreciated. Our data now enable provision of personalized prognostic information and management advice for each XP patient, as well as providing new insights into the functions of the XP proteins.

Non-catalytic Roles for XPG with BRCA1 and BRCA2 in Homologous Recombination and Genome Stability

XPG is a structure-specific endonuclease required for nucleotide excision repair, and incision-defective XPG mutations cause the skin cancer-prone syndrome xeroderma pigmentosum. Truncating mutations instead cause the neurodevelopmental progeroid disorder Cockayne syndrome, but little is known about how XPG loss results in this devastating disease. We identify XPG as a partner of BRCA1 and BRCA2 in maintaining genomic stability through homologous recombination (HRR). XPG depletion causes DNA double-strand breaks, chromosomal abnormalities, cell-cycle delays, defective HRR, inability to overcome replication fork stalling, and replication stress. XPG directly interacts with BRCA2, RAD51, and PALB2, and XPG depletion reduces their chromatin binding and subsequent RAD51 foci formation. Upstream in HRR, XPG interacts directly with BRCA1. Its depletion causes BRCA1 hyper-phosphorylation and persistent chromatin binding. These unexpected findings establish XPG as an HRR protein with important roles in genome stability and suggest how XPG defects produce severe clinical consequences including cancer and accelerated aging.

Regulation of Transcription Elongation by the XPG-TFIIH Complex Is Implicated in Cockayne Syndrome

XPG is a causative gene underlying the photosensitive disorder xeroderma pigmentosum group G (XP-G) and is involved in nucleotide excision repair. Here, we show that XPG knockdown represses epidermal growth factor (EGF)-induced FOS transcription at the level of transcription elongation with little effect on EGF signal transduction. XPG interacted with transcription elongation factors in concert with TFIIH, suggesting that the XPG-TFIIH complex serves as a transcription elongation factor. The XPG-TFIIH complex was recruited to promoter and coding regions of both EGF-induced (FOS) and housekeeping (EEF1A1) genes. Further, EGF-induced recruitment of RNA polymerase II and TFIIH to FOS was reduced by XPG knockdown. Importantly, EGF-induced FOS transcription was markedly lower in XP-G/Cockayne syndrome (CS) cells expressing truncated XPG than in control cells expressing wild-type (WT) XPG, with less significant decreases in XP-G cells with XPG nuclease domain mutations. In corroboration of this finding, both WT XPG and a missense XPG mutant from an XP-G patient were recruited to FOS upon EGF stimulation, but an XPG mutant mimicking a C-terminal truncation from an XP-G/CS patient was not. These results suggest that the XPG-TFIIH complex is involved in transcription elongation and that defects in this association may partly account for Cockayne syndrome in XP-G/CS patients.

Genetics and molecular biology of brain calcification

Brain calcification is a common neuroimaging finding in patients with neurological, metabolic, or developmental disorders, mitochondrial diseases, infectious diseases, traumatic or toxic history, as well as in otherwise normal older people. Patients with brain calcification may exhibit movement disorders, seizures, cognitive impairment, and a variety of other neurologic and psychiatric symptoms. Brain calcification may also present as a single, isolated neuroimaging finding. When no specific cause is evident, a genetic etiology should be considered. The aim of the review is to highlight clinical disorders associated with brain calcification and provide summary of current knowledge of diagnosis, genetics, and pathogenesis of brain calcification.

Cell-autonomous progeroid changes in conditional mouse models for repair endonuclease XPG deficiency

As part of the Nucleotide Excision Repair (NER) process, the endonuclease XPG is involved in repair of helix-distorting DNA lesions, but the protein has also been implicated in several other DNA repair systems, complicating genotype-phenotype relationship in XPG patients. Defects in XPG can cause either the cancer-prone condition xeroderma pigmentosum (XP) alone, or XP combined with the severe neurodevelopmental disorder Cockayne Syndrome (CS), or the infantile lethal cerebro-oculo-facio-skeletal (COFS) syndrome, characterized by dramatic growth failure, progressive neurodevelopmental abnormalities and greatly reduced life expectancy. Here, we present a novel (conditional) Xpg^−/− mouse model which -in a C57BL6/FVB F1 hybrid genetic background- displays many progeroid features, including cessation of growth, loss of subcutaneous fat, kyphosis, osteoporosis, retinal photoreceptor loss, liver aging, extensive neurodegeneration, and a short lifespan of 4–5 months. We show that deletion of XPG specifically in the liver reproduces the progeroid features in the liver, yet abolishes the effect on growth or lifespan. In addition, specific XPG deletion in neurons and glia of the forebrain creates a progressive neurodegenerative phenotype that shows many characteristics of human XPG deficiency. Our findings therefore exclude that both the liver as well as the neurological phenotype are a secondary consequence of derailment in other cell types, organs or tissues (e.g. vascular abnormalities) and support a cell-autonomous origin caused by the DNA repair defect itself. In addition they allow the dissection of the complex aging process in tissue- and cell-type-specific components. Moreover, our data highlight the critical importance of genetic background in mouse aging studies, establish the Xpg^−/− mouse as a valid model for the severe form of human XPG patients and segmental accelerated aging, and strengthen the link between DNA damage and aging.

Accumulation of DNA damage has been implicated in aging. Many premature aging syndromes are due to defective DNA repair systems. The endonuclease XPG is involved in repair of helix-distorting DNA lesions, and XPG defects cause the cancer-prone condition xeroderma pigmentosum (XP) alone or combined with the severe neurodevelopmental progeroid disorder Cockayne syndrome (CS). Here, we present a novel (conditional) Xpg^−/− mouse model which -in a C57BL6/FVB F1 hybrid background- displays many progressive progeroid features, including early cessation of growth, cachexia, kyphosis, osteoporosis, neurodegeneration, liver aging, retinal degeneration, and reduced lifespan. In a constitutive mutant with a complex phenotype it is difficult to dissect cause and consequence. We have therefore generated liver- and forebrain-specific Xpg mutants and demonstrate that they exhibit progressive anisokaryosis and neurodegeneration, respectively, indicating that a cell-intrinsic repair defect in neurons can account for neuronal degeneration. These findings strengthen the link between DNA damage and the complex process of aging.

XPG is predictive gene of clinical outcome in advanced non-small-cell lung cancer with platinum drug therapy

Polymorphisms in XPG are considered to contribute to the clinical outcome of patients receiving platinum drug chemotherapy. We aimed to investigate the role of five potential SNPs of XPG gene on the response to platinum-based chemotherapy in advanced Chinese NSCLC patients. A total of 451 patients with newly diagnosed and histopathologically confirmed primary NSCLC were consecutively collected. XPG rs2296147, rs4150261, rs17655, rs1047768 and rs2094258 were genotyped by the Taqman real-time polymerase chain reaction (PCR). In our study, we found patients carrying rs1057768 TT genotype had a significantly lower treatment response when compared with the CC genotype (OR=0.38, 95% CI=0.18-0.78). Patients carrying rs1047768 TT genotype showed a significantly short median PFS (11.2 months) and OS (13.6 months) than CC genotype, and the hazard ratios (HR) for PFS and OS were 2.06 (1.01-4.50) and 2.29 (1.21-2.49), respectively. Moreover, we found a significant decreased risk of death from NSCLC among patients carrying the rs2296147 TT genotype when compared with the CC genotype, the HR (95% CI) for OS being 0.50 (0.27-0.95). In conclusion, our study found that polymorphisms in rs1047768 C/T and rs2296147 C/T are associated with response to platinum-based chemotherapy in advanced NSCLC, and XPG polymorphisms could be predictive of prognosis.

Hypomorphic PCNA mutation underlies a human DNA repair disorder

Numerous human disorders, including Cockayne syndrome, UV-sensitive syndrome, xeroderma pigmentosum, and trichothiodystrophy, result from the mutation of genes encoding molecules important for nucleotide excision repair. Here, we describe a syndrome in which the cardinal clinical features include short stature, hearing loss, premature aging, telangiectasia, neurodegeneration, and photosensitivity, resulting from a homozygous missense (p.Ser228Ile) sequence alteration of the proliferating cell nuclear antigen (PCNA). PCNA is a highly conserved sliding clamp protein essential for DNA replication and repair. Due to this fundamental role, mutations in PCNA that profoundly impair protein function would be incompatible with life. Interestingly, while the p.Ser228Ile alteration appeared to have no effect on protein levels or DNA replication, patient cells exhibited marked abnormalities in response to UV irradiation, displaying substantial reductions in both UV survival and RNA synthesis recovery. The p.Ser228Ile change also profoundly altered PCNA's interaction with Flap endonuclease 1 and DNA Ligase 1, DNA metabolism enzymes. Together, our findings detail a mutation of PCNA in humans associated with a neurodegenerative phenotype, displaying clinical and molecular features common to other DNA repair disorders, which we showed to be attributable to a hypomorphic amino acid alteration.

A novel homozygous ERCC5 truncating mutation in a family with prenatal arthrogryposis--further evidence of genotype-phenotype correlation

We report on a family with five fetuses conceived to first cousin parents presenting with abnormal ultrasound findings including contractures and microcephaly. Cerebellar hypoplasia and ventriculomegaly were also present in two and fetal edema developed in the one fetus that survived beyond 24 weeks of gestation. Linkage studies of 15 members of the family, including four affecteds, were undertaken followed by exome sequencing of one affected individual and their parents. Analysis of exome data was restricted to the 9.3 Mb largest shared region of homozygosity identified by linkage; a single novel homozygous mutation in the proband that was heterozygous in the parents (ERCC5 c.2766dupA, p.Leu923ThrfsX7) was identified. This segregated with disease. ERCC5 is a component of the nucleotide excision repair machinery and biallelic mutations in the gene have previously been associated with xeroderma pigmentosum (group G), Cockayne syndrome and the more severe cerebrooculofacioskeletal syndrome. The phenotype in the family we report on is consistent with a severe manifestation of cerebrooculofacioskeletal syndrome. Our data broaden the reported clinical spectrum of ERCC5 mutations and provide further evidence of genotype-phenotype correlation with truncating mutations being associated with severe phenotypes. They also demonstrate the molecular diagnostic power of a combined approach of linkage studies and exome sequencing in families with rare, genetically heterogeneous disorders and a well described pedigree.

Yeast RAD2, a homolog of human XPG, plays a key role in the regulation of the cell cycle and actin dynamics

Mutations in the human XPG gene cause Cockayne syndrome (CS) and xeroderma pigmentosum (XP). Transcription defects have been suggested as the fundamental cause of CS; however, defining CS as a transcription syndrome is inconclusive. In particular, the function of XPG in transcription has not been clearly demonstrated. Here, we provide evidence for the involvement of RAD2, the Saccharomyces cerevisiae counterpart of XPG, in cell cycle regulation and efficient actin assembly following ultraviolet irradiation. RAD2 C-terminal deletion, which resembles the XPG mutation found in XPG/CS cells, caused cell growth arrest, the cell cycle stalling, a defective α-factor response, shortened lifespan, cell polarity defect, and misregulated actin-dynamics after DNA damage. Overexpression of the C-terminal 65 amino acids of Rad2p was sufficient to induce hyper-cell polarization. In addition, RAD2 genetically interacts with TPM1 during cell polarization. These results provide insights into the role of RAD2 in post-UV irradiation cell cycle regulation and actin assembly, which may be an underlying cause of XPG/CS.

Mammalian transcription-coupled excision repair

Transcriptional arrest caused by DNA damage is detrimental for cells and organisms as it impinges on gene expression and thereby on cell growth and survival. To alleviate transcriptional arrest, cells trigger a transcription-dependent genome surveillance pathway, termed transcription-coupled nucleotide excision repair (TC-NER) that ensures rapid removal of such transcription-impeding DNA lesions and prevents persistent stalling of transcription. Defective TC-NER is causatively linked to Cockayne syndrome, a rare severe genetic disorder with multisystem abnormalities that results in patients' death in early adulthood. Here we review recent data on how damage-arrested transcription is actively coupled to TC-NER in mammals and discuss new emerging models concerning the role of TC-NER-specific factors in this process.

Oxidative DNA damage and nucleotide excision repair

SIGNIFICANCE

Oxidative DNA damage is repaired by multiple, overlapping DNA repair pathways. Accumulating evidence supports the hypothesis that nucleotide excision repair (NER), besides base excision repair (BER), is also involved in neutralizing oxidative DNA damage.

RECENT ADVANCES

NER includes two distinct sub-pathways: transcription-coupled NER (TC-NER) and global genome repair (GG-NER). The CSA and CSB proteins initiate the onset of TC-NER. Recent findings show that not only CSB, but also CSA is involved in the repair of oxidative DNA lesions, in the nucleus as well as in mitochondria. The XPG protein is also of importance for the removal of oxidative DNA lesions, as it may enhance the initial step of BER. Substantial evidence exists that support a role for XPC in NER and BER. XPC deficiency not only results in decreased repair of oxidative lesions, but has also been linked to disturbed redox homeostasis.

CRITICAL ISSUES

The role of NER proteins in the regulation of the cellular response to oxidative (mitochondrial and nuclear) DNA damage may be the underlying mechanism of the pathology of accelerated aging in Cockayne syndrome patients, a driving force for internal cancer development in XP-A and XP-C patients, and a contributor to the mixed exhibited phenotypes of XP-G patients.

FUTURE DIRECTIONS

Accumulating evidence indicates that DNA repair factors can be involved in multiple DNA repair pathways. However, the distinct detailed mechanism and consequences of these additional functions remain to be elucidated and can possibly shine a light on clinically related issues.

Characterization of three XPG-defective patients identifies three missense mutations that impair repair and transcription

Only 16 XPG-defective patients with 20 different mutations have been described. The current hypothesis is that missense mutations impair repair (xeroderma pigmentosum (XP) symptoms), whereas truncating mutations impair both repair and transcription (XP and Cockayne syndrome (CS) symptoms). We identified three cell lines of XPG-defective patients (XP40GO, XP72MA, and XP165MA). Patients' fibroblasts showed a reduced post-UVC cell survival. The reduced repair capability, assessed by host cell reactivation, could be complemented by XPG cDNA. XPG mRNA expression of XP165MA, XP72MA, and XP40GO was 83%, 97%, and 82.5%, respectively, compared with normal fibroblasts. XP165MA was homozygous for a p.G805R mutation; XP72MA and XP40GO were both compound heterozygous (p.W814S and p.E727X, and p.L778P and p.Q150X, respectively). Allele-specific complementation analysis of these five mutations revealed that p.L778P and p.W814S retained considerable residual repair activity. In line with the severe XP/CS phenotypes of XP72MA and XP165MA, even the missense mutations failed to interact with the transcription factor IIH subunits XPD and to some extent cdk7 in coimmunoprecipitation assays. Immunofluorescence techniques revealed that the mutations destabilized early recruitment of XP proteins to localized photodamage and delayed their redistribution in vivo. Thus, we identified three XPG missense mutations in the I-region of XPG that impaired repair and transcription and resulted in severe XP/CS.

Expression of XPG protein in human normal and tumor tissues

XPG (Xeroderma pigmentosum group G complementing factor) is a protein associated with DNA repair and transcription. Point mutations in ERCC5, the gene coding for XPG, cause the cancer-prone disorder xeroderma pigmentosum (XP) while truncation mutations give rise to individuals with the combined clinical features of XP and Cockayne syndrome. Polymorphisms of ERCC5 or alterations in XPG mRNA expression were also associated to an increase risk of different cancers types and to prognosis of cancer patients. However, the expression of XPG protein in different normal or tumor human tissues is not well known. In the present work, we have validated an immunohistochemistry (IHC) assay for detection of expression levels of XPG protein in FFPE human tissue samples. We have also tested this IHC assay in different normal and tumor human tissues. On a microarray containing 28 normal cores, positive staining was observed in 60% of the samples. The highest staining was detected in adrenal gland, breast, colon, heart, kidney, thyroid and tongue. In tumors, positive staining was observed in 9 of 10 breast cancer samples and in all 5 ovarian cancer and 5 sarcomas samples. Subcellular localization was predominantly nuclear. The use of this validated methodology would help to interpret the role of XPG in tumorogenesis and its use as a possible prognostic or predictive factor.

Sirt1 suppresses RNA synthesis after UV irradiation in combined xeroderma pigmentosum group D/Cockayne syndrome (XP-D/CS) cells

Specific mutations in the XPD subunit of transcription factor IIH result in combined xeroderma pigmentosum (XP)/Cockayne syndrome (CS), a severe DNA repair disorder characterized at the cellular level by a transcriptional arrest following UV irradiation. This transcriptional arrest has always been thought to be the result of faulty transcription-coupled repair. In the present study, we showed that, following UV irradiation, XP-D/CS cells displayed a gross transcriptional dysregulation compared with "pure" XP-D cells or WT cells. Furthermore, global RNA-sequencing analysis showed that XP-D/CS cells repressed the majority of genes after UV, whereas pure XP-D cells did not. By using housekeeping genes as a model, we demonstrated that XP-D/CS cells were unable to reassemble these gene promoters and thus to restart transcription after UV irradiation. Furthermore, we found that the repression of these promoters in XP-D/CS cells was not a simple consequence of deficient repair but rather an active heterochromatinization process mediated by the histone deacetylase Sirt1. Indeed, RNA-sequencing analysis showed that inhibition of and/or silencing of Sirt1 changed the chromatin environment at these promoters and restored the transcription of a large portion of the repressed genes in XP-D/CS cells after UV irradiation. Our work demonstrates that a significant part of the transcriptional arrest displayed by XP-D/CS cells arises as a result of an active repression process and not simply as a result of a DNA repair deficiency. This dysregulation of Sirt1 function that results in transcriptional repression may be the cause of various severe clinical features in patients with XP-D/CS that cannot be explained by a DNA repair defect.

Polymorphisms in the XPG gene and risk of gastric cancer in Chinese populations

DNA repair genes play an important role in maintaining stability and integrity of genomic DNA. Polymorphisms in nucleotide excision repair genes may cause variations in DNA repair capacity phenotype and thus contribute to cancer risk. In this case-control study of 1,125 gastric cancer cases and 1,196 cancer-free controls, we investigated the association between three functional single nucleotide polymorphisms (SNPs, rs2296147T > C, rs2094258C > T and rs873601G > A) in the xeroderma pigmentosum group G (XPG) gene and gastric cancer risk. We used the Taqman assays to genotype these three SNPs and logistic regression models to estimate odds ratios (ORs) and 95% confidence intervals (95% CIs). We found that only the rs873601A variant genotypes were associated with a significant higher risk for gastric adenocarcinoma (adjusted OR = 1.30, 95% CI = 1.03-1.64 for AA vs. GG and adjusted OR = 1.23, 95% CI = 1.01-1.49 for AA vs. GG/AG). Stratification analysis indicated that this risk was more pronounced in subgroups of older age (>59 years), males, ever-smokers, and patients with NGCA. All these were not found for the other two SNPs (rs2296147T > C and rs2094258C > T). We then performed expression analysis using gastric cancer adjacent normal tissues from 141 patients and found that the A variant allele was associated with non-significantly reduced expression of XPG mRNA (P(trend) = 0.107). Further analysis using mRNA expression data from the HapMap suggested that the A allele was associated with significantly reduced expression of XPG mRNA in normal cell lines for 45 Chinese (P(trend) = 0.003) as well as for 261 subjects with different ethnicities (P(trend) = 0.001). These support the hypothesis that functional XPG variants may contribute to the risk of gastric cancer. Larger studies with different ethnic populations are warranted to validate our findings.

Lack of recognition by global-genome nucleotide excision repair accounts for the high mutagenicity and persistence of aristolactam-DNA adducts

Exposure to aristolochic acid (AA), a component of Aristolochia plants used in herbal remedies, is associated with chronic kidney disease and urothelial carcinomas of the upper urinary tract. Following metabolic activation, AA reacts with dA and dG residues in DNA to form aristolactam (AL)-DNA adducts. These mutagenic lesions generate a unique TP53 mutation spectrum, dominated by A:T to T:A transversions with mutations at dA residues located almost exclusively on the non-transcribed strand. We determined the level of AL-dA adducts in human fibroblasts treated with AA to determine if this marked strand bias could be accounted for by selective resistance to global-genome nucleotide excision repair (GG-NER). AL-dA adduct levels were elevated in cells deficient in GG-NER and transcription-coupled NER, but not in XPC cell lines lacking GG-NER only. In vitro, plasmids containing a single AL-dA adduct were resistant to the early recognition and incision steps of NER. Additionally, the NER damage sensor, XPC-RAD23B, failed to specifically bind to AL-DNA adducts. However, placing AL-dA in mismatched sequences promotes XPC-RAD23B binding and renders this adduct susceptible to NER, suggesting that specific structural features of this adduct prevent processing by NER. We conclude that AL-dA adducts are not recognized by GG-NER, explaining their high mutagenicity and persistence in target tissues.

Physiological consequences of defects in ERCC1-XPF DNA repair endonuclease

ERCC1-XPF is a structure-specific endonuclease required for nucleotide excision repair, interstrand crosslink repair, and the repair of some double-strand breaks. Mutations in ERCC1 or XPF cause xeroderma pigmentosum, XFE progeroid syndrome or cerebro-oculo-facio-skeletal syndrome, characterized by increased risk of cancer, accelerated aging and severe developmental abnormalities, respectively. This review provides a comprehensive overview of the health impact of ERCC1-XPF deficiency, based on these rare diseases and mouse models of them. This offers an understanding of the tremendous health impact of DNA damage derived from environmental and endogenous sources.

Regulation of endonuclease activity in human nucleotide excision repair

Nucleotide excision repair (NER) is a DNA repair pathway that is responsible for removing a variety of lesions caused by harmful UV light, chemical carcinogens, and environmental mutagens from DNA. NER involves the concerted action of over 30 proteins that sequentially recognize a lesion, excise it in the form of an oligonucleotide, and fill in the resulting gap by repair synthesis. ERCC1-XPF and XPG are structure-specific endonucleases responsible for carrying out the incisions 5' and 3' to the damage respectively, culminating in the release of the damaged oligonucleotide. This review focuses on the recent work that led to a greater understanding of how the activities of ERCC1-XPF and XPG are regulated in NER to prevent unwanted cuts in DNA or the persistence of gaps after incision that could result in harmful, cytotoxic DNA structures.

The DNA repair endonuclease XPG interacts directly and functionally with the WRN helicase defective in Werner syndrome

XPG is a structure-specific endonuclease required for nucleotide excision repair (NER). XPG incision defects result in the cancer-prone syndrome xeroderma pigmentosum, whereas truncating mutations of XPG cause the severe postnatal progeroid developmental disorder Cockayne syndrome. We show that XPG interacts directly with WRN protein, which is defective in the premature aging disorder Werner syndrome, and that the two proteins undergo similar subnuclear redistribution in S phase and colocalize in nuclear foci. The co-localization was observed in mid- to late S phase, when WRN moves from nucleoli to nuclear foci that have been shown to contain both protein markers of stalled replication forks and telomeric proteins. We mapped the interaction between XPG and WRN to the C-terminal domains of each, and show that interaction with the C-terminal domain of XPG strongly stimulates WRN helicase activity. WRN also possesses a competing DNA single-strand annealing activity that, combined with unwinding, has been shown to coordinate regression of model replication forks to form Holliday junction/chicken foot intermediate structures. We tested whether XPG stimulated WRN annealing activity, and found that XPG itself has intrinsic strand annealing activity that requires the unstructured R- and C-terminal domains but not the conserved catalytic core or endonuclease activity. Annealing by XPG is cooperative, rather than additive, with WRN annealing. Taken together, our results suggest a novel function for XPG in S phase that is, at least in part, performed coordinately with WRN, and which may contribute to the severity of the phenotypes that occur upon loss of XPG.

ERCC6 founder mutation identified in Finnish patients with COFS syndrome

Cerebro-oculo-facio-skeletal (COFS) syndrome is an autosomal recessive disorder characterized by microcephaly, congenital cataracts, facial dysmorphism, neurogenic arthrogryposis, growth failure and severe psychomotor retardation. We report a large consanguineous pedigree from northern Finland with six individuals belonging into four different sibships and affected with typical COFS syndrome phenotype. Two deceased patients have been published previously in 1982 as the first cases exhibiting cerebral calcifications typical for this disorder. Two living and one of the deceased patients were all shown to possess a novel homozygous mutation in the ERCC6 [Cockayne syndrome B (CSB)] gene, thereby confirming the diagnosis on molecular genetic level even for the earlier published cases. Genealogical investigation showed a common ancestor living in a northeastern village in Finland in the 18th century for all six patients implying a founder effect.

ERCC5/XPG, ERCC1, and BRCA1 gene status and clinical benefit of trabectedin in patients with soft tissue sarcoma

BACKGROUND

The objective of this study was to determine whether specific single nucleotide polymorphisms (SNPs) from nucleotide excision repair (NER) and homologous recombination (HR) DNA repair pathways are associated with sensitivity to trabectedin in patients with soft tissue sarcoma (STS).

METHODS

The authors analyzed excision repair cross-complementation group 5/xeroderma pigmentosum group G (ERCC5/XPG) (NER), excision repair cross-complementation group 1 (ERCC1) (NER), and breast cancer 1 (BRCA1) (HR) SNPs and messenger RNA expression levels in tumor specimens from 113 patients with advanced STS who were enrolled in previously published phase 2 trials or in a compassionate-use program. The 6-month progression-free rate (PFR), progression-free survival (PFS), and overall survival (OS) were analyzed according to ERCC5, ERCC1, and BRCA1 status using log-rank tests.

RESULTS

High expression of the common allele (aspartic acid at codon 1104) of ERCC5, high expression of ERCC1, and BRCA1 haplotype were associated significantly with improved PFR, PFS, and OS. The ERCC1 thymine-to-cytosine (T→C) SNP at codon 19007 and BRCA1 expression were not associated with outcome. On univariate analysis, tumor histology, favorable NER status (high expression of common ERCC5 and/or high ERCC1 expression status), and favorable BRCA1 haplotype (at least 1 triple-adenine plus guanine [AAAG] allele) were the sole variables associated significantly with PFS and OS.

CONCLUSIONS

In the current study, ERCC5, ERCC1, and BRCA1 status represented a potential DNA repair signature that could be used for the prediction of clinical response to trabectedin in patients with STS.

Dissociation of CAK from core TFIIH reveals a functional link between XP-G/CS and the TFIIH disassembly state

Transcription factor II H (TFIIH) is comprised of core TFIIH and Cdk-activating kinase (CAK) complexes. Here, we investigated the molecular and cellular manifestation of the TFIIH compositional changes by XPG truncation mutations. We showed that both core TFIIH and CAK are rapidly recruited to damage sites in repair-proficient cells. Chromatin immunoprecipitation against TFIIH and CAK components revealed a physical engagement of CAK in nucleotide excision repair (NER). While XPD recruitment to DNA damage was normal, CAK was not recruited in severe XP-G and XP-G/CS cells, indicating that the associations of CAK and XPD to core TFIIH are differentially affected. A CAK inhibition approach showed that CAK activity is not required for assembling pre-incision machinery in vivo or for removing genomic photolesions. Instead, CAK is involved in Ser5-phosphorylation and UV-induced degradation of RNA polymerase II. The CAK inhibition impaired transcription from undamaged and UV-damaged reporter, and partially decreased transcription of p53-dependent genes. The overall results demonstrated that a) XP-G/CS mutations affect the disassembly state of TFIIH resulting in the dissociation of CAK, but not XPD from core TFIIH, and b) CAK activity is not essential for global genomic repair but involved in general transcription and damage-induced RNA polymerase II degradation.

New RAB3GAP1 mutations in patients with Warburg Micro Syndrome from different ethnic backgrounds and a possible founder effect in the Danish

Warburg Micro Syndrome is a rare, autosomal recessive syndrome characterized by microcephaly, microphthalmia, microcornia, congenital cataracts, optic atrophy, cortical dysplasia, in particular corpus callosum hypoplasia, severe mental retardation, spastic diplegia, and hypogonadism. We have found five new mutations in the RAB3GAP1 gene in seven patients with suspected Micro Syndrome from families with Turkish, Palestinian, Danish, and Guatemalan backgrounds. A thorough clinical investigation of the patients has allowed the delineation of symptoms that are consistently present in the patients and may aid the differential diagnosis of Micro Syndrome for patients in the future. All patients had postnatal microcephaly, micropthalmia, microcornia, bilateral congenital cataracts, short palpebral fissures, optic atrophy, severe mental retardation, and congenital hypotonia with subsequent spasticity. Only one patient had microcephaly at birth, highlighting the fact that congenital microcephaly is not a consistent feature of Micro syndrome. Analysis of the brain magnetic resonance imagings (MRIs) revealed a consistent pattern of polymicrogyria in the frontal and parietal lobes, wide sylvian fissures, a thin hypoplastic corpus callosum, and increased subdural spaces. All patients were homozygous for the mutations detected and all mutations were predicted to result in a truncated RAB3GAP1 protein. The analysis of nine polymorphic markers flanking the RAB3GAP1 gene showed that the mutation c.1410C>A (p.Tyr470X), for which a Danish patient was homozygous, occurred on a haplotype that is shared by the unrelated heterozygous parents of the patient. This suggests a possible founder effect for this mutation in the Danish population.

Hoeijmakers JH, Vermeulen W, J. Jaspers NG, Schärer OD, Niedernhofer LJ. Mislocalization of XPF-ERCC1 nuclease contributes to reduced DNA repair in XP-F patients

Xeroderma pigmentosum (XP) is caused by defects in the nucleotide excision repair (NER) pathway. NER removes helix-distorting DNA lesions, such as UV-induced photodimers, from the genome. Patients suffering from XP exhibit exquisite sun sensitivity, high incidence of skin cancer, and in some cases neurodegeneration. The severity of XP varies tremendously depending upon which NER gene is mutated and how severely the mutation affects DNA repair capacity. XPF-ERCC1 is a structure-specific endonuclease essential for incising the damaged strand of DNA in NER. Missense mutations in XPF can result not only in XP, but also XPF-ERCC1 (XFE) progeroid syndrome, a disease of accelerated aging. In an attempt to determine how mutations in XPF can lead to such diverse symptoms, the effects of a progeria-causing mutation (XPF(R153P)) were compared to an XP-causing mutation (XPF(R799W)) in vitro and in vivo. Recombinant XPF harboring either mutation was purified in a complex with ERCC1 and tested for its ability to incise a stem-loop structure in vitro. Both mutant complexes nicked the substrate indicating that neither mutation obviates catalytic activity of the nuclease. Surprisingly, differential immunostaining and fractionation of cells from an XFE progeroid patient revealed that XPF-ERCC1 is abundant in the cytoplasm. This was confirmed by fluorescent detection of XPF(R153P)-YFP expressed in Xpf mutant cells. In addition, microinjection of XPF(R153P)-ERCC1 into the nucleus of XPF-deficient human cells restored nucleotide excision repair of UV-induced DNA damage. Intriguingly, in all XPF mutant cell lines examined, XPF-ERCC1 was detected in the cytoplasm of a fraction of cells. This demonstrates that at least part of the DNA repair defect and symptoms associated with mutations in XPF are due to mislocalization of XPF-ERCC1 into the cytoplasm of cells, likely due to protein misfolding. Analysis of these patient cells therefore reveals a novel mechanism to potentially regulate a cell's capacity for DNA repair: by manipulating nuclear localization of XPF-ERCC1.

Lithium chloride protects retinal neurocytes from nutrient deprivation by promoting DNA non-homologous end-joining

Lithium chloride is a therapeutic agent for treatment of bipolar affective disorders. Increasing numbers of studies have indicated that lithium has neuroprotective effects. However, the molecular mechanisms underlying the actions of lithium have not been fully elucidated. This study aimed to investigate whether lithium chloride produces neuroprotective function by improving DNA repair pathway in retinal neurocyte. In vitro, the primary cultured retinal neurocytes (85.7% are MAP-2 positive cells) were treated with lithium chloride, then cultured with serum-free media to simulate the nutrient deprived state resulting from ischemic insult. The neurite outgrowth of the cultured cells increased significantly in a dose-dependent manner when exposed to different levels of lithium chloride. Genomic DNA electrophoresis demonstrated greater DNA integrity of retinal neurocytes when treated with lithium chloride as compared to the control. Moreover, mRNA and protein levels of Ligase IV (involved in DNA non-homologous end-joining (NHEJ) pathway) in retinal neurocytes increased with lithium chloride. The end joining activity assay was performed to determine the role of lithium on NHEJ in the presence of extract from retinal neurocytes. The rejoining levels in retinal neurocytes treated with lithium were significantly increased as compared to the control. Furthermore, XRCC4, the Ligase IV partner, and the transcriptional factor, CREB and CTCF, were up-regulated in retinal cells after treating with 1.0mM lithium chloride. Therefore, our data suggest that lithium chloride protects the retinal neural cells from nutrient deprivation in vitro, which may be similar to the mechanism of cell death in glaucoma. The improvement in DNA repair pathway involving in Ligase IV might have an important role in lithium neuroprotection. This study provides new insights into the neural protective mechanisms of lithium chloride.

Nucleotide excision repair deficient mouse models and neurological disease

Nucleotide excision repair (NER) is a highly conserved mechanism to remove helix-distorting DNA base damage. A major substrate for NER is DNA damage caused by environmental genotoxins, most notably ultraviolet radiation. Xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy are three human diseases caused by inherited defects in NER. The symptoms and severity of these diseases vary dramatically, ranging from profound developmental delay to cancer predisposition and accelerated aging. All three syndromes include neurological disease, indicating an important role for NER in protecting against spontaneous DNA damage as well. To study the pathophysiology caused by DNA damage, numerous mouse models of NER-deficiency were generated by knocking-out genes required for NER or knocking-in disease-causing human mutations. This review explores the utility of these mouse models to study neurological disease caused by NER-deficiency.

Photoprotection of bacterial-derived melanin against ultraviolet A-induced cell death and its potential application as an active sunscreen

BACKGROUND

The increase in the incidence of non-melanoma skin tumours, photoaging, and immunosuppression demand for more effective sunscreen on ultraviolet A (UVA) irradiation.

OBJECTIVES

The aim of the study is to evaluate the photoprotective effects of a bacterial-derived melanin against UVA-induced damages in vitro and in vivo. Methods Human fibroblasts were used to assess the role of the bacterial-derived melanin on cell viability against UVA. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and nuclear morphology were employed to evaluate the photoprotection at the cellular level. Fluorometric assays were performed to detect the formation of reactive oxygen species (ROS) in the cells. Evaluations of the bacterial-derived melanin as a sunscreen were measured by transmission test and persistent pigment darkening on human skin.

RESULTS

Bacterial-derived melanin efficiently scavenged ROS in the fibroblasts after UVA irradiation. The cell viability of xeroderma pigmentosum (XP) fibroblast treated with varied doses of melanin increased dramatically in comparison with untreated control and the treated XP fibroblasts became more resistant to UVA-induced apoptosis than normal fibroblasts. Although the relative transmission didn't change too much with different concentration of bacterial-derived melanin, this melanin could keep UVA-irradiated skin from pigment darkening and act as an active sunscreen on skin.

CONCLUSIONS

The bacterial-derived melanin provided significant protection to fibroblast cell and human skin against the UVA radiation. It has the potential to be developed as an active sunscreen for the patients with photosensitivity skin to sun exposure.

Hot topics in DNA repair: the molecular basis for different disease states caused by mutations in TFIIH and XPG

Alterations in genes involved in nucleotide excision repair (NER) are associated with three genetic disorders, xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD). The transcription and repair factor TFIIH is a central component of NER and mutations of its subunits are associated with all three diseases. A recent report provides a molecular basis for how mutations in the NER endonuclease XPG that affect the interaction of TFIIH might give rise to CS features. In cells of XP-G patients with a combined XP and CS phenotype, XPG fails to associate with TFIIH and as a consequence the CAK subunit dissociates from core TFIIH. A simplified, but general model of how various assembly and disassembly states of TFIIH can be invoked to explain different disease states is discussed. Accordingly, defects in specific enzymatic functions typically result in XP, dissociation of the CAK subunit from TFIIH is associated with XP/CS and a more generalized destabilization of TFIIH gives rise to TTD. While this classification provides a useful framework to understand how alterations in TFIIH correlate with disease states, it does not universally apply and relevant exception and alternative explanations are discussed.

Minimal ionizing radiation sensitivity in a large cohort of xeroderma pigmentosum fibroblasts

We have examined our ionizing radiation survival data for 33 xeroderma pigmentosum (XP) primary fibroblast lines and compared the data to that of 53 normal fibroblast lines, 7 Cockayne syndrome (CS) lines, 4 combined XP/CS lines and 8 ataxia-telangiectasia fibroblast lines. Although there are differences in radiosensitivity between cell lines within each class, we have no convincing evidence that XP lines as a group are more sensitive to ionizing radiation than the general population. However, because the XP phenotype may lead to premature ageing, especially of sun-exposed tissues, we would still advocate caution when XP patients come to radiotherapy. Our results confirm the extreme ionizing radiation hypersensitivity of ataxia-telangiectasia; they are also consistent with a tendency for slight hypersensitivity in CS, but not (necessarily) in combined XP/CS.

XPG: its products and biological roles

Xeroderma pigmetosum patients of the complementation group G are rare. One group of XP-G patients displays a rather mild and typical XP phenotype. Mutations in these patients interfere with the function of XPG in the nucleotide excision repair, where it has a structural role in the assembly of the preincision complex and a catalytic role in making the incision 3' to the damaged site in DNA. Another set of XP-G patient is much more severely affected, displaying combined symptoms of xeroderma pigmentosum and Cockayne syndrome, referred to as XP/CS complex. Although the molecular basis leading to the XP/CS complex has not yet been fully established, current evidence suggests that these patients suffer from a mild defect in transcription in addition to a repair defect. Here, the history of how the XPG gene was discovered, the biochemical properties of the XPG protein and the molecular defects found in XP-G patients and mouse models are reviewed.

New insights into the combined Cockayne/xeroderma pigmentosum complex: human XPG protein can function in transcription factor stability

A new study provides evidence supporting a function for XPG protein in maintaining the integrity and function of TFIIH (Ito et al. [2007], this issue of Molecular Cell). This observation likely explains some of the clinical features of individuals with both defective DNA repair and development.

UV-induced apoptosis in XPG-deficient fibroblasts involves activation of CD95 and caspases but not p53

Mildly affected individuals from xeroderma pigmentosum complementation group G (XP-G) possess single amino acid substitutions in the XPG protein that adversely affects its 3' endonuclease function in nucleotide excision repair. More serious mutations in the XPG gene generate truncated or unstable XPG proteins and result in a particularly early and severe form of the combined XP/CS complex. Following UV irradiation, cells from such XP-G/CS patients enter apoptosis more readily than other DNA repair-deficient cells. Here, we explore the mechanisms by which UV triggers the apoptotic cell death program in XP-G and XP-G/CS primary fibroblasts. Activation of the CD95 signalling pathway occurs within minutes and it is the earliest detectable post-UV event in such cells. This is rapidly followed by activation of caspase-8 then caspase-3. Several hours later caspase-9 becomes activated and the mitochondrial membrane potential drops, but without any obvious prior release of cytochrome c. Although p53 accumulates in XPG-deficient cells after UV irradiation, use of RNA interference demonstrates that p53 is not required for their UV-induced apoptotic response. p53 ablation of wild-type fibroblasts reduces MDM2 mRNA levels, inhibits accumulation of the 90kDa/92kDa Mdm2 isoforms, and prevents the nuclear relocalisation of Mdm2 after UV treatment. The same post-UV effects occur in XPG-deficient cells that express normal p53 levels. These results emphasise the importance of the extrinsic apoptotic pathway and aberrant Mdm2 events for the severe UV-induced apoptosis of XPG-deficient primary fibroblasts. XP-G/CS cells constitutively overexpress the pro-apoptotic Bax protein and a long isoform of the E2F1 transcription factor that controls S phase entry, which may prime them to enter apoptosis very readily after UV irradiation.

New insights for understanding the transcription-coupled repair pathway

Transcription-coupled repair (TCR) is a sub-pathway of nucleotide excision repair (NER) able to remove bulky DNA lesions located on the transcribed strands of active genes more rapidly than those located on the non-transcribed genomic DNA. Two recently published reports try to dissect the molecular mechanisms of TCR using simplified in vitro assays. A third report shows in vivo data that confirmed the in vitro ones and extends them to the role of other TCR factors such as those involved in chromatin remodeling. These approaches shed light on the interplay between stalled RNA polymerase II and NER factors necessary for efficient repair. Because severe diseases, such as Cockayne syndrome, are associated with defects or mutations in proteins required for transcription-coupled nucleotide excision repair, complete understanding of this pathway should allow us to understand this disease better and eventually to propose adequate therapies.