Impaired genome maintenance suppresses the growth hormone--insulin-like growth factor 1 axis in mice with Cockayne syndrome

ERCC1-XPF cooperates with CTCF and cohesin to facilitate the developmental silencing of imprinted genes

Inborn defects in DNA repair are associated with complex developmental disorders whose causal mechanisms are poorly understood. Using an in vivo biotinylation tagging approach in mice, we show that the nucleotide excision repair (NER) structure-specific endonuclease ERCC1-XPF complex interacts with the insulator binding protein CTCF, the cohesin subunits SMC1A and SMC3 and with MBD2; the factors co-localize with ATRX at the promoters and control regions (ICRs) of imprinted genes during postnatal hepatic development. Loss of Ercc1 or exposure to MMC triggers the localization of CTCF to heterochromatin, the dissociation of the CTCF-cohesin complex and ATRX from promoters and ICRs, altered histone marks and the aberrant developmental expression of imprinted genes without altering DNA methylation. We propose that ERCC1-XPF cooperates with CTCF and cohesin to facilitate the developmental silencing of imprinted genes and that persistent DNA damage triggers chromatin changes that affect gene expression programs associated with NER disorders.

Epithelial magnesium transport by TRPM6 is essential for prenatal development and adult survival

Mg²⁺ regulates many physiological processes and signalling pathways. However, little is known about the mechanisms underlying the organismal balance of Mg²⁺. Capitalizing on a set of newly generated mouse models, we provide an integrated mechanistic model of the regulation of organismal Mg²⁺ balance during prenatal development and in adult mice by the ion channel TRPM6. We show that TRPM6 activity in the placenta and yolk sac is essential for embryonic development. In adult mice, TRPM6 is required in the intestine to maintain organismal Mg²⁺ balance, but is dispensable in the kidney. Trpm6 inactivation in adult mice leads to a shortened lifespan, growth deficit and metabolic alterations indicative of impaired energy balance. Dietary Mg²⁺ supplementation not only rescues all phenotypes displayed by Trpm6-deficient adult mice, but also may extend the lifespan of wildtype mice. Hence, maintenance of organismal Mg²⁺ balance by TRPM6 is crucial for prenatal development and survival to adulthood.

Modulation of inflammation and disease tolerance by DNA damage response pathways

The accurate replication and repair of DNA is central to organismal survival. This process is challenged by the many factors that can change genetic information such as replication errors and direct damage to the DNA molecule by chemical and physical agents. DNA damage can also result from microorganism invasion as an integral step of their life cycle or as collateral damage from host defense mechanisms against pathogens. Here we review the complex crosstalk of DNA damage response and immune response pathways that might be evolutionarily connected and argue that DNA damage response pathways can be explored therapeutically to induce disease tolerance through the activation of tissue damage control processes. Such approach may constitute the missing pillar in the treatment of critical illnesses caused by multiple organ failure, such as sepsis and septic shock.

Restricted diet delays accelerated ageing and genomic stress in DNA-repair-deficient mice

Mice deficient in the DNA excision-repair gene Ercc1 (Ercc1^∆/-) show numerous accelerated ageing features that limit their lifespan to 4-6 months. They also exhibit a 'survival response', which suppresses growth and enhances cellular maintenance. Such a response resembles the anti-ageing response induced by dietary restriction (also known as caloric restriction). Here we report that a dietary restriction of 30% tripled the median and maximal remaining lifespans of these progeroid mice, strongly retarding numerous aspects of accelerated ageing. Mice undergoing dietary restriction retained 50% more neurons and maintained full motor function far beyond the lifespan of mice fed ad libitum. Other DNA-repair-deficient, progeroid Xpg^-/- (also known as Ercc5^-/-) mice, a model of Cockayne syndrome, responded similarly. The dietary restriction response in Ercc1^∆/- mice closely resembled the effects of dietary restriction in wild-type animals. Notably, liver tissue from Ercc1^∆/- mice fed ad libitum showed preferential extinction of the expression of long genes, a phenomenon we also observed in several tissues ageing normally. This is consistent with the accumulation of stochastic, transcription-blocking lesions that affect long genes more than short ones. Dietary restriction largely prevented this declining transcriptional output and reduced the number of γH2AX DNA damage foci, indicating that dietary restriction preserves genome function by alleviating DNA damage. Our findings establish the Ercc1^∆/- mouse as a powerful model organism for health-sustaining interventions, reveal potential for reducing endogenous DNA damage, facilitate a better understanding of the molecular mechanism of dietary restriction and suggest a role for counterintuitive dietary-restriction-like therapy for human progeroid genome instability syndromes and possibly neurodegeneration in general.

Systemic DNA damage responses in aging and diseases

The genome is constantly attacked by a variety of genotoxic insults. The causal role for DNA damage in aging and cancer is exemplified by genetic defects in DNA repair that underlie a broad spectrum of acute and chronic human disorders that are characterized by developmental abnormalities, premature aging, and cancer predisposition. The disease symptoms are typically tissue-specific with uncertain genotype-phenotype correlation. The cellular DNA damage response (DDR) has been extensively investigated ever since yeast geneticists discovered DNA damage checkpoint mechanisms, several decades ago. In recent years, it has become apparent that not only cell-autonomous but also systemic DNA damage responses determine the outcome of genome instability in organisms. Understanding the mechanisms of non-cell-autonomous DNA damage responses will provide important new insights into the role of genome instability in human aging and a host of diseases including cancer and might better explain the complex phenotypes caused by genome instability.

Tissue-Specific Suppression of Thyroid Hormone Signaling in Various Mouse Models of Aging

DNA damage contributes to the process of aging, as underscored by premature aging syndromes caused by defective DNA repair. Thyroid state changes during aging, but underlying mechanisms remain elusive. Since thyroid hormone (TH) is a key regulator of metabolism, changes in TH signaling have widespread effects. Here, we reveal a significant common transcriptomic signature in livers from hypothyroid mice, DNA repair-deficient mice with severe (Csbm/m/Xpa-/-) or intermediate (Ercc1-/Δ-7) progeria and naturally aged mice. A strong induction of TH-inactivating deiodinase D3 and decrease of TH-activating D1 activities are observed in Csbm/m/Xpa-/- livers. Similar findings are noticed in Ercc1-/Δ-7, in naturally aged animals and in wild-type mice exposed to a chronic subtoxic dose of DNA-damaging agents. In contrast, TH signaling in muscle, heart and brain appears unaltered. These data show a strong suppression of TH signaling in specific peripheral organs in premature and normal aging, probably lowering metabolism, while other tissues appear to preserve metabolism. D3-mediated TH inactivation is unexpected, given its expression mainly in fetal tissues. Our studies highlight the importance of DNA damage as the underlying mechanism of changes in thyroid state. Tissue-specific regulation of deiodinase activities, ensuring diminished TH signaling, may contribute importantly to the protective metabolic response in aging.

Animal models of aging research: implications for human aging and age-related diseases

Aging is characterized by an increasing morbidity and functional decline that eventually results in the death of an organism. Aging is the largest risk factor for numerous human diseases, and understanding the aging process may thereby facilitate the development of new treatments for age-associated diseases. The use of humans in aging research is complicated by many factors, including ethical issues; environmental and social factors; and perhaps most importantly, their long natural life span. Although cellular models of human disease provide valuable mechanistic information, they are limited in that they may not replicate the in vivo biology. Almost all organisms age, and thus animal models can be useful for studying aging. Herein, we review some of the major models currently used in aging research and discuss their benefits and pitfalls, including interventions known to extend life span and health span. Finally, we conclude by discussing the future of animal models in aging research.

Cockayne syndrome-derived neurons display reduced synapse density and altered neural network synchrony

Cockayne syndrome (CS) is a rare genetic disorder in which 80% of cases are caused by mutations in the Excision Repair Cross-Complementation group 6 gene (ERCC6). The encoded ERCC6 protein is more commonly referred to as Cockayne Syndrome B protein (CSB). Classical symptoms of CS patients include failure to thrive and a severe neuropathology characterized by microcephaly, hypomyelination, calcification and neuronal loss. Modeling the neurological aspect of this disease has proven difficult since murine models fail to mirror classical neurological symptoms. Therefore, a robust human in vitro cellular model would advance our fundamental understanding of the disease and reveal potential therapeutic targets. Herein, we successfully derived functional CS neural networks from human CS induced pluripotent stem cells (iPSCs) providing a new tool to facilitate studying this devastating disease. We identified dysregulation of the Growth Hormone/Insulin-like Growth Factor-1 (GH/IGF-1) pathway as well as pathways related to synapse formation, maintenance and neuronal differentiation in CSB neurons using unbiased RNA-seq gene expression analyses. Moreover, when compared to unaffected controls, CSB-deficient neural networks displayed altered electrophysiological activity, including decreased synchrony, and reduced synapse density. Collectively, our work reveals that CSB is required for normal neuronal function and we have established an alternative to previously available models to further study neural-specific aspects of CS.

Genome Integrity in Aging: Human Syndromes, Mouse Models, and Therapeutic Options

Human syndromes and mouse mutants that exhibit accelerated but bona fide aging in multiple organs and tissues have been invaluable for the identification of nine denominators of aging: telomere attrition, genome instability, epigenetic alterations, mitochondrial dysfunction, deregulated nutrient sensing, altered intercellular communication, loss of proteostasis, cellular senescence and adult stem cell exhaustion. However, whether and how these instigators of aging interrelate or whether they have one root cause is currently largely unknown. Rare human progeroid syndromes and corresponding mouse mutants with resolved genetic defects highlight the dominant importance of genome maintenance for aging. A second class of aging-related disorders reveals a cross connection with metabolism. As genome maintenance and metabolism are closely interconnected, they may constitute the main underlying biology of aging. This review focuses on the role of genome stability in aging, its crosstalk with metabolism, and options for nutritional and/or pharmaceutical interventions that delay age-related pathology.

Quality control mechanisms in cellular and systemic DNA damage responses

The maintenance of the genome is of pivotal importance for the functional integrity of cells and tissues. The gradual accumulation of DNA damage is thought to contribute to the functional decline of tissues and organs with ageing. Defects in multiple genome maintenance systems cause human disorders characterized by cancer susceptibility, developmental failure, and premature ageing. The complex pathological consequences of genome instability are insufficiently explained by cell-autonomous DNA damage responses (DDR) alone. Quality control pathways play an important role in DNA repair and cellular DDR pathways. Recent years have revealed non-cell autonomous effects of DNA damage that impact the physiological adaptations during ageing. We will discuss the role of quality assurance pathways in cell-autonomous and systemic responses to genome instability.

Genome Instability in Development and Aging: Insights from Nucleotide Excision Repair in Humans, Mice, and Worms

DNA damage causally contributes to aging and cancer. Congenital defects in nucleotide excision repair (NER) lead to distinct cancer-prone and premature aging syndromes. The genetics of NER mutations have provided important insights into the distinct consequences of genome instability. Recent work in mice and C. elegans has shed new light on the mechanisms through which developing and aging animals respond to persistent DNA damage. The various NER mouse mutants have served as important disease models for Xeroderma pigmentosum (XP), Cockayne syndrome (CS), and trichothiodystrophy (TTD), while the traceable genetics of C. elegans have allowed the mechanistic delineation of the distinct outcomes of genome instability in metazoan development and aging. Intriguingly, highly conserved longevity assurance mechanisms respond to transcription-blocking DNA lesions in mammals as well as in worms and counteract the detrimental consequences of persistent DNA damage. The insulin-like growth factor signaling (IIS) effector transcription factor DAF-16 could indeed overcome DNA damage-driven developmental growth delay and functional deterioration even when DNA damage persists. Longevity assurance mechanisms might thus delay DNA damage-driven aging by raising the threshold when accumulating DNA damage becomes detrimental for physiological tissue functioning.

DNA repair mechanisms in cancer development and therapy

DNA damage has been long recognized as causal factor for cancer development. When erroneous DNA repair leads to mutations or chromosomal aberrations affecting oncogenes and tumor suppressor genes, cells undergo malignant transformation resulting in cancerous growth. Genetic defects can predispose to cancer: mutations in distinct DNA repair systems elevate the susceptibility to various cancer types. However, DNA damage not only comprises a root cause for cancer development but also continues to provide an important avenue for chemo- and radiotherapy. Since the beginning of cancer therapy, genotoxic agents that trigger DNA damage checkpoints have been applied to halt the growth and trigger the apoptotic demise of cancer cells. We provide an overview about the involvement of DNA repair systems in cancer prevention and the classes of genotoxins that are commonly used for the treatment of cancer. A better understanding of the roles and interactions of the highly complex DNA repair machineries will lead to important improvements in cancer therapy.

MARK-AGE biomarkers of ageing

Many candidate biomarkers of human ageing have been proposed in the scientific literature but in all cases their variability in cross-sectional studies is considerable, and therefore no single measurement has proven to serve a useful marker to determine, on its own, biological age. A plausible reason for this is the intrinsic multi-causal and multi-system nature of the ageing process. The recently completed MARK-AGE study was a large-scale integrated project supported by the European Commission. The major aim of this project was to conduct a population study comprising about 3200 subjects in order to identify a set of biomarkers of ageing which, as a combination of parameters with appropriate weighting, would measure biological age better than any marker in isolation.

DAF-16/FOXO and EGL-27/GATA promote developmental growth in response to persistent somatic DNA damage

Genome maintenance defects cause complex disease phenotypes characterized by developmental failure, cancer susceptibility, and premature aging. It remains poorly understood how DNA damage responses function during organismal development and maintain tissue functionality when DNA damage accumulates with aging. Here we show that the FoxO transcription factor DAF-16 is activated in response to DNA damage during development while the DNA damage responsiveness of DAF-16 declines with aging. We find that in contrast to its established role in mediating starvation arrest, DAF-16 alleviates DNA damage-induced developmental arrest and even in the absence of DNA repair promotes developmental growth and enhances somatic tissue functionality. We demonstrate that the GATA transcription factor EGL-27 co-regulates DAF-16 target genes in response to DNA damage and together with DAF-16 promotes developmental growth. We propose that EGL-27/GATA activity specifies DAF-16 mediated DNA damage responses to enable developmental progression and to prolong tissue functioning when DNA damage persists.

A C. elegans homolog of the Cockayne syndrome complementation group A gene

Cockayne syndrome (CS) is a debilitating and complex disorder that results from inherited mutations in the CS complementation genes A and B, CSA and CSB. The links between the molecular functions of the CS genes and the complex pathophysiology of CS are as of yet poorly understood and are the subject of intense debate. While mouse models reflect the complexity of CS, studies on simpler genetic models might shed new light on the consequences of CS mutations. Here we describe a functional homolog of the human CSA gene in Caenorhabditis elegans. Similar to its human counterpart, mutations in the nematode csa-1 gene lead to developmental growth defects as a consequence of DNA lesions.

Aag-initiated base excision repair promotes ischemia reperfusion injury in liver, brain, and kidney

Inflammation is accompanied by the release of highly reactive oxygen and nitrogen species (RONS) that damage DNA, among other cellular molecules. Base excision repair (BER) is initiated by DNA glycosylases and is crucial in repairing RONS-induced DNA damage; the alkyladenine DNA glycosylase (Aag/Mpg) excises several DNA base lesions induced by the inflammation-associated RONS release that accompanies ischemia reperfusion (I/R). Using mouse I/R models we demonstrate that Aag(-/-) mice are significantly protected against, rather than sensitized to, I/R injury, and that such protection is observed across three different organs. Following I/R in liver, kidney, and brain, Aag(-/-) mice display decreased hepatocyte death, cerebral infarction, and renal injury relative to wild-type. We infer that in wild-type mice, Aag excises damaged DNA bases to generate potentially toxic abasic sites that in turn generate highly toxic DNA strand breaks that trigger poly(ADP-ribose) polymerase (Parp) hyperactivation, cellular bioenergetics failure, and necrosis; indeed, steady-state levels of abasic sites and nuclear PAR polymers were significantly more elevated in wild-type vs. Aag(-/-) liver after I/R. This increase in PAR polymers was accompanied by depletion of intracellular NAD and ATP levels plus the translocation and extracellular release of the high-mobility group box 1 (Hmgb1) nuclear protein, activating the sterile inflammatory response. We thus demonstrate the detrimental effects of Aag-initiated BER during I/R and sterile inflammation, and present a novel target for controlling I/R-induced injury.

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.

Aging: not all DNA damage is equal

Recent advances have identified accumulation of DNA damage as a major driver of aging. However, there are numerous kinds of DNA lesions each with their own characteristics and cellular outcome, which highly depends on cellular context: proliferation (cell cycle), differentiation, propensity for survival/death, cell condition and systemic hormonal and immunological parameters. In addition, DNA damage is strongly influenced by cellular metabolism, anti-oxidant status and exogenous factors, consistent with the multi-factorial nature of aging. Notably, DNA lesions interfering with replication have very different outcomes compared to transcription. These considerations provide a conceptual framework in which different types of DNA damage and their setting contribute to the aging process in differential manners.

Promoting longevity by maintaining metabolic and proliferative homeostasis

Aging is characterized by a widespread loss of homeostasis in biological systems. An important part of this decline is caused by age-related deregulation of regulatory processes that coordinate cellular responses to changing environmental conditions, maintaining cell and tissue function. Studies in genetically accessible model organisms have made significant progress in elucidating the function of such regulatory processes and the consequences of their deregulation for tissue function and longevity. Here, we review such studies, focusing on the characterization of processes that maintain metabolic and proliferative homeostasis in the fruitfly Drosophila melanogaster. The primary regulatory axis addressed in these studies is the interaction between signaling pathways that govern the response to oxidative stress, and signaling pathways that regulate cellular metabolism and growth. The interaction between these pathways has important consequences for animal physiology, and its deregulation in the aging organism is a major cause for increased mortality. Importantly, protocols to tune such interactions genetically to improve homeostasis and extend lifespan have been established by work in flies. This includes modulation of signaling pathway activity in specific tissues, including adipose tissue and insulin-producing tissues, as well as in specific cell types, such as stem cells of the fly intestine.

The thyroid hormone receptor β induces DNA damage and premature senescence

There is increasing evidence that the thyroid hormone (TH) receptors (THRs) can play a role in aging, cancer and degenerative diseases. In this paper, we demonstrate that binding of TH T3 (triiodothyronine) to THRB induces senescence and deoxyribonucleic acid (DNA) damage in cultured cells and in tissues of young hyperthyroid mice. T3 induces a rapid activation of ATM (ataxia telangiectasia mutated)/PRKAA (adenosine monophosphate-activated protein kinase) signal transduction and recruitment of the NRF1 (nuclear respiratory factor 1) and THRB to the promoters of genes with a key role on mitochondrial respiration. Increased respiration leads to production of mitochondrial reactive oxygen species, which in turn causes oxidative stress and DNA double-strand breaks and triggers a DNA damage response that ultimately leads to premature senescence of susceptible cells. Our findings provide a mechanism for integrating metabolic effects of THs with the tumor suppressor activity of THRB, the effect of thyroidal status on longevity, and the occurrence of tissue damage in hyperthyroidism.

Impact of genomic damage and ageing on stem cell function

Impairment of stem cell function contributes to the progressive deterioration of tissue maintenance and repair with ageing. Evidence is mounting that age-dependent accumulation of DNA damage in both stem cells and cells that comprise the stem cell microenvironment are partly responsible for stem cell dysfunction with ageing. Here, we review the impact of the various types of DNA damage that accumulate with ageing on stem cell functionality, as well as the development of cancer. We discuss DNA-damage-induced cell intrinsic and extrinsic alterations that influence these processes, and review recent advances in understanding systemic adjustments to DNA damage and how they affect stem cells.

Systemic DNA damage responses: organismal adaptations to genome instability

DNA damage checkpoints are important tumor-suppressor mechanisms that halt cell cycle progression to allow time for DNA repair, or induce senescence and apoptosis to remove damaged cells permanently. Non-cell-autonomous DNA damage responses activate the innate immune system in multiple metazoan species. These responses not only enable clearance of damaged cells and contribute to tissue remodeling and regeneration but can also result in chronic inflammation and tissue damage. Germline DNA damage-induced systemic stress resistance (GDISR) is mediated by an ancestral innate immune response and results in organismal adjustments to the presence of damaged cells. We discuss GDISR as an organismal DNA damage checkpoint mechanism through which elevated somatic endurance can extend reproductive lifespan when germ cells require extended time for restoring genome stability.

Role of Ca/CaN/NFAT signaling in IL-4 expression by splenic lymphocytes exposed to phthalate (2-ethylhexyl) ester in spleen lymphocytes

The aims of present study were to investigate the effect of phthalate (2-ethylhexyl) ester (DEHP) and mono-(2-ethylhexyl) phthalate (MEHP) on Th1/Th2 balance signaling for interleukin 4 (IL-4) expression in splenic lymphocytes, and contribution of MEHP to any hypothesized changes in vitro. Primary splenic lymphocytes were exposed to DEHP/MEHP. ELISA and Western blotting were used to detect proteins. Confocal-microscopy was used to examine nuclear translocation. Nuclear factor of activated T cells (NFAT) DNA binding activity was examined by electrophoretic mobility-shift assay. DEHP significantly increased IL-4 and interferon gamma (IFN-γ) level, and reduced Th1/Th2 ratio (reflected by IFN-γ/IL-4) with 5 μg/L Concanavalin A (ConA) treatment. While MEHP reduced Th1/Th2 ratio (represented by IFN-γ/IL-6). IL-4 mRNA was significantly increased by DEHP but not by MEHP after PMA and Ion treatment. DEHP significantly inhibited NFATp protein in cytosol and nucleus. DEHP augmented nuclear translocation of NFATc in transfected EL4 cells and NFAT DNA-binding activity. DEHP-mediated enhancement of calcium-dependent phosphatase calcineurin (CaN) protein, and NFAT and IL-4 expression were abrogated by calcium antagonist verapamil and CaN inhibitor tarcolimus. Ca(2+)/calmodulin antagonist chlorpromazine significantly suppressed IL-4 and CaN production with no NFAT mRNA change. Our study suggests that DEHP and MEHP impact Th1/Th2 balance by modulating different cytokines. DEHP-affected IL-4 expression through Ca/CaN/NFAT signaling pathway, but no effect was discovered for MEHP.

Lifespan extension by dietary intervention in a mouse model of Cockayne syndrome uncouples early postnatal development from segmental progeria

Cockayne syndrome (CS) is a rare autosomal recessive segmental progeria characterized by growth failure, lipodystrophy, neurological abnormalities, and photosensitivity, but without skin cancer predisposition. Cockayne syndrome life expectancy ranges from 5 to 16 years for the two most severe forms (types II and I, respectively). Mouse models of CS have thus far been of limited value due to either very mild phenotypes, or premature death during postnatal development prior to weaning. The cause of death in severe CS models is unknown, but has been attributed to extremely rapid aging. Here, we found that providing mutant pups with soft food from as late as postnatal day 14 allowed survival past weaning with high penetrance independent of dietary macronutrient balance in a novel CS model (Csa(-/-) | Xpa(-/-)). Survival past weaning revealed a number of CS-like symptoms including small size, progressive loss of adiposity, and neurological symptoms, with a maximum lifespan of 19 weeks. Our results caution against interpretation of death before weaning as premature aging, and at the same time provide a valuable new tool for understanding mechanisms of progressive CS-related progeroid symptoms including lipodystrophy and neurodysfunction.

Forkhead box(O) in control of reactive oxygen species and genomic stability to ensure healthy lifespan

SIGNIFICANCE

Transcription factors of the Forkhead box O class (FOXOs) are associated with lifespan and play a role in age-related diseases. FOXOs, therefore, serve as a paradigm for developing an understanding as to how age-related diseases, such as cancer and diabetes interconnect with lifespan. Understanding the regulatory inputs on FOXO may reveal how changes in these regulatory signaling pathways affect disease and lifespan.

RECENT ADVANCES

Numerous regulators of FOXO have now been described and a clear and evolutionary conserved role has emerged for phosphoinositide-3 kinase/protein kinase B (also known as c-Akt or AKT) signaling and c-jun N-terminal kinase signaling. Analysis of FOXO function in the context of these signaling pathways has shown the importance of FOXO-mediated transcriptional regulation on cell cycle progression and other cell fates, such as cell metabolism, stress resistance, and apoptosis in mediating disease and lifespan.

CRITICAL ISSUES

Persistent DNA damage is also tightly linked to disease and aging; yet, data on a possible link between DNA damage and FOXO have been limited. Here, we discuss possible connections between FOXO and the DNA damage response in the context of the broader role of connecting lifespan and disease.

FUTURE DIRECTIONS

Understanding the role of lifespan in diseases onset may provide unique and generic possibilities to intervene in disease processes to ensure a healthy lifespan.

Active transcriptomic and proteomic reprogramming in the C. elegans nucleotide excision repair mutant xpa-1

Oxidative stress promotes human aging and contributes to common neurodegenerative diseases. Endogenous DNA damage induced by oxidative stress is believed to be an important promoter of neurodegenerative diseases. Although a large amount of evidence correlates a reduced DNA repair capacity with aging and neurodegenerative disease, there is little direct evidence of causality. Moreover, the contribution of oxidative DNA damage to the aging process is poorly understood. We have used the nematode Caenorhabditis elegans to study the contribution of oxidative DNA damage and repair to aging. C. elegans is particularly well suited to tackle this problem because it has a minimum complexity DNA repair system, which enables us to circumvent the important limitation presented by the extensive redundancy of DNA repair enzymes in mammals.

BACH2: a marker of DNA damage and ageing

DNA damage and ageing share expression changes involving alterations in many aspects of metabolism, suppression of growth and upregulation of defence and genome maintenance systems. "Omics" technologies have permitted large-scale parallel measurements covering global cellular constituents and aided the identification of specific response pathways that change during ageing and after DNA damage. We have set out to identify genes with highly conserved response patterns through meta-analysis of mRNA expression datasets collected during natural ageing and accelerated ageing caused by a Transcription-Coupled Nucleotide Excision Repair (TC-NER) defect in a diverse set of organs and tissues in mice, and from in vitro UV-induced DNA damage in a variety of murine cells. The identified set of genes that show similar expression patterns in response to organ ageing (accelerated and normal), and endogenously and exogenously induced DNA damage, consists of genes involved in anti-oxidant systems and includes the transcription factor Bach2 as one of the most consistent markers. BACH2 was originally identified as a partner of the small Maf proteins and antagonist of the NRF2 anti-oxidant defence pathway and has been implicated in B-cell differentiation and immune system homeostasis. Although BACH2 has never before been associated with UV-induced damage or ageing, it shows a strong downregulation in both conditions. We have characterized the dynamics of Bach2 expression in response to DNA damage and show that it is a highly sensitive responder to transcription-blocking DNA lesions. Gene expression profiling using Affymetrix microarray analysis after siRNA-mediated silencing of Bach2 identified cell cycle and transcription regulation as the most significantly altered processes consistent with a function as transcription factor affecting proliferation.

Misregulation of an adaptive metabolic response contributes to the age-related disruption of lipid homeostasis in Drosophila

Loss of metabolic homeostasis is a hallmark of aging and is commonly characterized by the deregulation of adaptive signaling interactions that coordinate energy metabolism with dietary changes. The mechanisms driving age-related changes in these adaptive responses remain unclear. Here, we characterize the deregulation of an adaptive metabolic response and the development of metabolic dysfunction in the aging intestine of Drosophila. We find that activation of the insulin-responsive transcription factor Foxo in intestinal enterocytes is required to inhibit the expression of evolutionarily conserved lipases as part of a metabolic response to dietary changes. This adaptive mechanism becomes chronically activated in the aging intestine, mediated by changes in Jun-N-terminal kinase (JNK) signaling. Age-related chronic JNK/Foxo activation in enterocytes is deleterious, leading to sustained repression of intestinal lipase expression and the disruption of lipid homeostasis. Changes in the regulation of Foxo-mediated adaptive responses thus contribute to the age-associated breakdown of metabolic homeostasis.

DNA damage leads to progressive replicative decline but extends the life span of long-lived mutant animals

Human-nucleotide-excision repair (NER) deficiency leads to different developmental and segmental progeroid symptoms of which the pathogenesis is only partially understood. To understand the biological impact of accumulating spontaneous DNA damage, we studied the phenotypic consequences of DNA-repair deficiency in Caenorhabditis elegans. We find that DNA damage accumulation does not decrease the adult life span of post-mitotic tissue. Surprisingly, loss of functional ERCC-1/XPF even further extends the life span of long-lived daf-2 mutants, likely through an adaptive activation of stress signaling. Contrariwise, NER deficiency leads to a striking transgenerational decline in replicative capacity and viability of proliferating cells. DNA damage accumulation induces severe, stochastic impairment of development and growth, which is most pronounced in NER mutants that are also impaired in their response to ionizing radiation and inter-strand crosslinks. These results suggest that multiple DNA-repair pathways can protect against replicative decline and indicate that there might be a direct link between the severity of symptoms and the level of DNA-repair deficiency in patients.

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.

Prolonged lifespan with enhanced exploratory behavior in mice overexpressing the oxidized nucleoside triphosphatase hMTH1

The contribution that oxidative damage to DNA and/or RNA makes to the aging process remains undefined. In this study, we used the hMTH1-Tg mouse model to investigate how oxidative damage to nucleic acids affects aging. hMTH1-Tg mice express high levels of the hMTH1 hydrolase that degrades 8-oxodGTP and 8-oxoGTP and excludes 8-oxoguanine from both DNA and RNA. Compared to wild-type animals, hMTH1-overexpressing mice have significantly lower steady-state levels of 8-oxoguanine in both nuclear and mitochondrial DNA of several organs, including the brain. hMTH1 overexpression prevents the age-dependent accumulation of DNA 8-oxoguanine that occurs in wild-type mice. These lower levels of oxidized guanines are associated with increased longevity and hMTH1-Tg animals live significantly longer than their wild-type littermates. Neither lipid oxidation nor overall antioxidant status is significantly affected by hMTH1 overexpression. At the cellular level, neurospheres derived from adult hMTH1-Tg neural progenitor cells display increased proliferative capacity and primary fibroblasts from hMTH1-Tg embryos do not undergo overt senescence in vitro. The significantly lower levels of oxidized DNA/RNA in transgenic animals are associated with behavioral changes. These mice show reduced anxiety and enhanced investigation of environmental and social cues. Longevity conferred by overexpression of a single nucleotide hydrolase in hMTH1-Tg animals is an example of lifespan extension associated with healthy aging. It provides a link between aging and oxidative damage to nucleic acids.

UVSSA and USP7, a new couple in transcription-coupled DNA repair

Transcription-coupled nucleotide excision repair (TC-NER) specifically removes transcription-blocking lesions from our genome. Defects in this pathway are associated with two human disorders: Cockayne syndrome (CS) and UV-sensitive syndrome (UV^SS). Despite a similar cellular defect in the UV DNA damage response, patients with these syndromes exhibit strikingly distinct symptoms; CS patients display severe developmental, neurological, and premature aging features, whereas the phenotype of UV^SS patients is mostly restricted to UV hypersensitivity. The exact molecular mechanism behind these clinical differences is still unknown; however, they might be explained by additional functions of CS proteins beyond TC-NER. A short overview of the current hypotheses addressing possible molecular mechanisms and the proteins involved are presented in this review. In addition, we will focus on two new players involved in TC-NER which were recently identified: UV-stimulated scaffold protein A (UVSSA) and ubiquitin-specific protease 7 (USP7). UVSSA has been found to be the causative gene for UV^SS and, together with USP7, is implicated in regulating TC-NER activity. We will discuss the function of UVSSA and USP7 and how the discovery of these proteins contributes to a better understanding of the molecular mechanisms underlying the clinical differences between UV^SS and the more severe CS.

Disruption of TTDA results in complete nucleotide excision repair deficiency and embryonic lethality

The ten-subunit transcription factor IIH (TFIIH) plays a crucial role in transcription and nucleotide excision repair (NER). Inactivating mutations in the smallest 8-kDa TFB5/TTDA subunit cause the neurodevelopmental progeroid repair syndrome trichothiodystrophy A (TTD-A). Previous studies have shown that TTDA is the only TFIIH subunit that appears not to be essential for NER, transcription, or viability. We studied the consequences of TTDA inactivation by generating a Ttda knock-out (Ttda^−/−) mouse-model resembling TTD-A patients. Unexpectedly, Ttda^−/− mice were embryonic lethal. However, in contrast to full disruption of all other TFIIH subunits, viability of Ttda^−/− cells was not affected. Surprisingly, Ttda^−/− cells were completely NER deficient, contrary to the incomplete NER deficiency of TTD-A patient-derived cells. We further showed that TTD-A patient mutations only partially inactivate TTDA function, explaining the relatively mild repair phenotype of TTD-A cells. Moreover, Ttda^−/− cells were also highly sensitive to oxidizing agents. These findings reveal an essential role of TTDA for life, nucleotide excision repair, and oxidative DNA damage repair and identify Ttda^−/− cells as a unique class of TFIIH mutants.

DNA is under constant attack of various environmental and cellular produced DNA damaging agents. DNA damage hampers normal cell function; however, different DNA repair mechanisms protect our genetic information. Nucleotide Excision Repair is one of the most versatile repair processes, as it removes a large variety of DNA helix-distorting lesions induced by UV light and various chemicals. To remove these lesions, the DNA helix needs to be opened by the transcription/repair factor II H (TFIIH). TFIIH is a multifunctional complex that consists of 10 subunits and plays a fundamental role in opening the DNA helix in both NER and transcription. TTDA, the smallest subunit of TFIIH, was thought to be dispensable for both NER and transcription. However, in this paper, we show for the first time that TTDA is in fact a crucial component of TFIIH for NER. We demonstrate that Ttda^−/− mice are embryonic lethal. We also show that Ttda^−/− mouse cells are the first known viable TFIIH subunit knock-out cells, which are completely NER deficient and sensitive to oxidative agents (showing a new role for TFIIH outside NER and transcription).

Active transcriptomic and proteomic reprogramming in the C. elegans nucleotide excision repair mutant xpa-1

Transcription-blocking oxidative DNA damage is believed to contribute to aging and to underlie activation of oxidative stress responses and down-regulation of insulin-like signaling (ILS) in Nucleotide Excision Repair (NER) deficient mice. Here, we present the first quantitative proteomic description of the Caenorhabditis elegans NER-defective xpa-1 mutant and compare the proteome and transcriptome signatures. Both methods indicated activation of oxidative stress responses, which was substantiated biochemically by a bioenergetic shift involving increased steady-state reactive oxygen species (ROS) and Adenosine triphosphate (ATP) levels. We identify the lesion-detection enzymes of Base Excision Repair (NTH-1) and global genome NER (XPC-1 and DDB-1) as upstream requirements for transcriptomic reprogramming as RNA-interference mediated depletion of these enzymes prevented up-regulation of genes over-expressed in the xpa-1 mutant. The transcription factors SKN-1 and SLR-2, but not DAF-16, were identified as effectors of reprogramming. As shown in human XPA cells, the levels of transcription-blocking 8,5'-cyclo-2'-deoxyadenosine lesions were reduced in the xpa-1 mutant compared to the wild type. Hence, accumulation of cyclopurines is unlikely to be sufficient for reprogramming. Instead, our data support a model where the lesion-detection enzymes NTH-1, XPC-1 and DDB-1 play active roles to generate a genomic stress signal sufficiently strong to result in transcriptomic reprogramming in the xpa-1 mutant.

Atrx deficiency induces telomere dysfunction, endocrine defects, and reduced life span

Human ATRX mutations are associated with cognitive deficits, developmental abnormalities, and cancer. We show that the Atrx-null embryonic mouse brain accumulates replicative damage at telomeres and pericentromeric heterochromatin, which is exacerbated by loss of p53 and linked to ATM activation. ATRX-deficient neuroprogenitors exhibited higher incidence of telomere fusions and increased sensitivity to replication stress-inducing drugs. Treatment of Atrx-null neuroprogenitors with the G-quadruplex (G4) ligand telomestatin increased DNA damage, indicating that ATRX likely aids in the replication of telomeric G4-DNA structures. Unexpectedly, mutant mice displayed reduced growth, shortened life span, lordokyphosis, cataracts, heart enlargement, and hypoglycemia, as well as reduction of mineral bone density, trabecular bone content, and subcutaneous fat. We show that a subset of these defects can be attributed to loss of ATRX in the embryonic anterior pituitary that resulted in low circulating levels of thyroxine and IGF-1. Our findings suggest that loss of ATRX increases DNA damage locally in the forebrain and anterior pituitary and causes tissue attrition and other systemic defects similar to those seen in aging.

From laboratory tests to functional characterisation of Cockayne syndrome

The significant progress made over the last few years on the pathogenesis of Cockayne syndrome (CS) greatly improved our knowledge on several aspects crucial for development and ageing, demonstrating that this disorder, even if rare, represents a valuable tool to clarify key aspects of human health. Primary cells from patients have been instrumental to elucidate the multiple roles of CS proteins and to approach the dissection of the complex interplay between repair and transcription that is central to the CS clinical phenotype. Here we discuss the results of the cellular assays applied for confirmation of the clinical diagnosis as well as the results of genetic and molecular studies in DNA repair defective patients. Furthermore, we provide a general overview of recent in vivo and in vitro studies indicating that both CSA and CSB proteins are involved in distinct aspects of the cellular responses to UV and oxidative stress, transcription and regulation of gene expression, chromatin remodelling, redox balance and cellular bioenergetics. In light of the literature data, we will finally discuss how inactivation of specific functional roles of CS proteins may differentially affect the phenotype, thus explaining the wide range in type and severity of symptoms reported in CS patients.

Cockayne syndrome group B (CSB) protein: at the crossroads of transcriptional networks

Cockayne syndrome (CS) is a rare genetic disorder characterized by a variety of growth and developmental defects, photosensitivity, cachectic dwarfism, hearing loss, skeletal abnormalities, progressive neurological degeneration, and premature aging. CS arises due to mutations in the CSA and CSB genes. Both gene products are required for the transcription-coupled (TC) branch of the nucleotide excision repair (NER) pathway, however, the severe phenotype of CS patients is hard to reconcile with a sole defect in TC-NER. Studies using cells from patients and mouse models have shown that the CSB protein is involved in a variety of cellular pathways and plays a major role in the cellular response to stress. CSB has been shown to regulate processes such as the transcriptional recovery after DNA damage, the p53 transcriptional response, the response to hypoxia, the response to insulin-like growth factor-1 (IGF-1), transactivation of nuclear receptors, transcription of housekeeping genes and the transcription of rDNA. Some of these processes are also affected in combined XP/CS patients. These new advances in the function(s) of CSB shed light onto the etiology of the clinical features observed in CS patients and could potentially open therapeutic avenues for these patients in the future. Moreover, the study of CS could further our knowledge of the aging process.

Arresting transcription and sentencing the cell: the consequences of blocked transcription

Bulky DNA adducts induced by agents like ultraviolet light, cisplatin and oxidative metabolism pose a block to elongation by RNA polymerase II (RNAPII). The arrested RNAPII can initiate the repair of transcription-blocking DNA lesions by transcription-coupled nucleotide excision repair (TC-NER) to permit efficient recovery of mRNA synthesis while widespread sustained transcription blocks lead to apoptosis. Therefore, RNAPII serves as a processive DNA damage sensor that identifies transcription-blocking DNA lesions. Cockayne syndrome (CS) is an autosomal recessive disorder characterized by a complex phenotype that includes clinical photosensitivity, progressive neurological degeneration and premature-aging. CS is associated with defects in TC-NER and the recovery of mRNA synthesis, making CS cells exquisitely sensitive to a variety of DNA damaging agents. These defects in the coupling of repair and transcription appear to underlie some of the complex clinical features of CS. Recent insight into the consequences of blocked transcription and their relationship to CS will be discussed.

Genome maintenance and transcription integrity in aging and disease

DNA damage contributes to cancer development and aging. Congenital syndromes that affect DNA repair processes are characterized by cancer susceptibility, developmental defects, and accelerated aging (Schumacher et al., 2008). DNA damage interferes with DNA metabolism by blocking replication and transcription. DNA polymerase blockage leads to replication arrest and can gives rise to genome instability. Transcription, on the other hand, is an essential process for utilizing the information encoded in the genome. DNA damage that interferes with transcription can lead to apoptosis and cellular senescence. Both processes are powerful tumor suppressors (Bartek and Lukas, 2007). Cellular response mechanisms to stalled RNA polymerase II complexes have only recently started to be uncovered. Transcription-coupled DNA damage responses might thus play important roles for the adjustments to DNA damage accumulation in the aging organism (Garinis et al., 2009). Here we review human disorders that are caused by defects in genome stability to explore the role of DNA damage in aging and disease. We discuss how the nucleotide excision repair system functions at the interface of transcription and repair and conclude with concepts how therapeutic targeting of transcription might be utilized in the treatment of cancer.

Conceptual developments in the causes of Cockayne syndrome

Cockayne syndrome is an autosomal recessive disease that covers a wide range of symptoms, from mild photosensitivity to severe neonatal lethal disorder. The pathology of Cockayne syndrome may be caused by several mechanisms such as a DNA repair deficiency, transcription dysregulation, altered redox balance and mitochondrial dysfunction. Conceivably each of these mechanisms participates during a different stage in life of a Cockayne syndrome patient. Endogenous reactive oxygen is considered as an ultimate cause of DNA damage that contributes to Cockayne syndrome pathology. Here we demonstrate that mitochondrial reactive oxygen does not cause detectable nuclear DNA damage. This observation implies that a significant component of Cockayne syndrome pathology may be due to abnormal mitochondrial function independent of nuclear DNA damage. The source of nuclear DNA damage to central nervous system tissue most likely occurs from extrinsic neurotransmitter signaling.

What role (if any) does the highly conserved CSB-PGBD3 fusion protein play in Cockayne syndrome?

The PGBD3 piggyBac transposon inserted into CSB intron 5 early in the primate lineage. As a result of alternative splicing, the human CSB gene now encodes three proteins: CSB, a CSB-PGBD3 fusion protein that joins the N-terminal CSB domain to the C-terminal PGBD3 transposase domain, and PGBD3 transposase. The fusion protein is as highly conserved as CSB, suggesting that it is advantageous in health; however, expression of the fusion protein in CSB-null cells induces a constitutive interferon (IFN) response. The fusion protein binds in vivo to PGBD3-related MER85 elements, but is also tethered to c-Jun, TEAD1, and CTCF motifs by interactions with the cognate transcription factors. The fusion protein regulates nearby genes from the c-Jun (and to a lesser extent TEAD1 and CTCF) motifs, but not from MER85 elements. We speculate that the fusion protein interferes with CSB-dependent chromatin remodeling, generating double-stranded RNA (dsRNA) that induces an IFN response through endosomal TLR or cytoplasmic RIG-I and/or MDA5 RNA sensors. We suggest that the fusion protein was fixed in primates because an elevated IFN response may help to fight viral infection. We also speculate that an inappropriate IFN response may contribute to the clinical presentation of CS.

Cockayne syndrome b maintains neural precursor function

Neurodevelopmental defects are observed in the hereditary disorder Cockayne syndrome (CS). The gene most frequently mutated in CS, Cockayne Syndrome B (CSB), is required for the repair of bulky DNA adducts in transcribed genes during transcription-coupled nucleotide excision repair. CSB also plays a role in chromatin remodeling and mitochondrial function. The role of CSB in neural development is poorly understood. Here we report that the abundance of neural progenitors is normal in Csb(-/-) mice and the frequency of apoptotic cells in the neurogenic niche of the adult subependymal zone is similar in Csb(-/-) and wild type mice. Both embryonic and adult Csb(-/-) neural precursors exhibited defective self-renewal in the neurosphere assay. In Csb(-/-) neural precursors, self-renewal progressively decreased in serially passaged neurospheres. The data also indicate that Csb and the nucleotide excision repair protein Xpa preserve embryonic neural stem cell self-renewal after UV DNA damage. Although Csb(-/-) neural precursors do not exhibit altered neuronal lineage commitment after low-dose UV (1J/m(2)) in vitro, neurons differentiated in vitro from Csb(-/-) neural precursors that had been irradiated with 1J/m(2) UV exhibited defective neurite outgrowth. These findings identify a function for Csb in neural precursors.