Reduced hematopoietic reserves in DNA interstrand crosslink repair-deficient Ercc1-/- mice

Cell autonomous and nonautonomous mechanisms drive hematopoietic stem/progenitor cell loss in the absence of DNA repair

Daily, cells incur tens of thousands of DNA lesions caused by endogenous processes. Due to their long-lived nature, adult stem cells may be particularly susceptible to the negative impact of this constant genotoxic stress. Indeed, in murine models of DNA repair deficiencies, there is accumulation of DNA damage in hematopoietic stem cells and premature loss of function. Herein, we demonstrate that mice expressing reduced levels of ERCC1-XPF DNA repair endonuclease (Ercc1-/Δ mice) spontaneously display a progressive decline in the number and function of hematopoietic stem/progenitor cells (HSPCs). This was accompanied by increased cell death, expression of senescence markers, reactive oxygen species, and DNA damage in HSPC populations, illustrating cell autonomous mechanisms that contribute to loss of function. In addition, the bone marrow microenvironment of Ercc1-/Δ mice was not permissive for the engraftment of transplanted normal stem cells. Bones from Ercc1-/Δ mice displayed excessive osteoclastic activity, which alters the microenvironment in a way that is unfavorable to HSPC maintenance. This was accompanied by increased proinflammatory cytokines in the bone marrow of Ercc1-/Δ mice. These data provide novel evidence that spontaneous, endogenous DNA damage, if not repaired, promotes progressive attrition of adult stem cells via both cell autonomous and nonautonomous mechanisms.

Molecular pathogenesis and clinical management of Fanconi anemia

Fanconi anemia (FA) is a rare genetic disorder associated with a high frequency of hematological abnormalities and congenital anomalies. Based on multilateral efforts from basic scientists and clinicians, significant advances in our knowledge of FA have been made in recent years. Here we review the clinical features, the diagnostic criteria, and the current and future therapies of FA and describe the current understanding of the molecular basis of the disease.

Meta-analysis of gene expression in the mouse liver reveals biomarkers associated with inflammation increased early during aging

Aging is associated with a loss of cellular homeostasis, a decline in physiological function and an increase in various pathologies. Employing a meta-analysis, hepatic gene expression profiles from four independent mouse aging studies were interrogated. There was little overlap in the number of genes or canonical pathways perturbed, suggesting that independent study-specific factors may play a significant role in determining age-dependent gene expression. However, 43 genes were consistently altered during aging in three or four of these studies, including those that (1) exhibited progressively increased expression starting from 12 months of age, (2) exhibited similar expression changes in models of progeria at young ages and dampened or no changes in old longevity mouse models, (3) were associated with inflammatory tertiary lymphoid neogenesis (TLN) associated with formation of ectopic lymphoid structures observed in chronically inflamed tissues, and (4) overlapped with genes perturbed by aging in brain, muscle, and lung. Surprisingly, around half of the genes altered by aging in wild-type mice exhibited similar expression changes in adult long-lived mice compared to wild-type controls, including those associated with intermediary metabolism and feminization of the male-dependent gene expression pattern. Genes unique to aging in wild-type mice included those linked to TLN.

Loss of ercc1 results in a time- and dose-dependent reduction of proliferating early hematopoietic progenitors

The endonuclease complex Ercc1/Xpf is involved in interstrand crosslink repair and functions downstream of the Fanconi pathway. Loss of Ercc1 causes hematopoietic defects similar to those seen in Fanconi Anemia. Ercc1^−/− mice die 3-4 weeks after birth, which prevents long-term follow up of the hematopoietic compartment. We used alternative Ercc1 mouse models to examine the effect of low or absent Ercc1 activity on hematopoiesis. Tie2-Cre-driven deletion of a floxed Ercc1 allele was efficient (>80%) in fetal liver hematopoietic cells. Hematopoietic stem and progenitor cells (HSPCs) with a deleted allele were maintained in mice up to 1 year of age when harboring a wt allele, but were progressively outcompeted when the deleted allele was combined with a knockout allele. Mice with a minimal Ercc1 activity expressed by 1 or 2 hypomorphic Ercc1 alleles have an extended life expectancy, which allows analysis of HSPCs at 10 and 20 weeks of age. The HSPC compartment was affected in all Ercc1-deficient models. Actively proliferating multipotent progenitors were most affected as were myeloid and erythroid clonogenic progenitors. In conclusion, lack of Ercc1 results in a severe competitive disadvantage of HSPCs and is most deleterious in proliferating progenitor cells.

Genotoxic stress and activation of novel DNA repair enzymes in human endothelial cells and in the retinas and kidneys of streptozotocin diabetic rats

BACKGROUND

Mammalian excision repair cross-complementing 1 (ERCC1) and ERCC4 (a.k.a xeroderma pigmentosum complementation group F) are nucleotide excision repair enzymes, which excise the 5' end of damaged DNA. ERCC1 and ERCC4 have an interactive relationship with poly (adenosine diphosphate ribose) polymerase (PARP). We studied the role of ERCC1 and ERCC4 in glucose-induced extracellular matrix protein production in human endothelial cells and in the retinas and kidneys of streptozotocin diabetic rats.

METHODS

Human umbilical vein endothelial cells were grown with low (5 mM) and high glucose (25 mM). The cells were subjected to ERCC1 and ERCC4 small interfering RNA transfections, PARP blocker (3-aminobenzamide, ABA) and p300 blocker (curcumin). Retinas and kidneys from 1-month-old streptozotocin diabetic rats with or without treatment with curcumin and ABA were examined. Cells and tissues were studied for oxidative stress markers, fibronectin, ERCC1 and ERCC4, PARP and p300 mRNA. Western blot of nuclear proteins was performed.

RESULTS

ERCC1 and ERCC4 messenger RNA and protein levels were higher in high glucose than in low glucose, along with increasing oxidative stress and augmented p300 and fibronectin production. ABA, curcumin, ERCC1 and ERCC4 silencing reduced such upregulations and oxidative stress. Similar changes were seen in the kidneys and retinas of diabetic rats. ABA and curcumin treatment significantly reduced such changes.

CONCLUSIONS

These data indicate that glucose-induced ERCC1 and ERCC4 upregulation leads to increased fibronectin production via a p300-dependent pathway in umbilical endothelial cells, as well as in the retina and in the kidneys of streptozotocin diabetic rats. ERCC1 and ERCC4 may play important roles in the development of diabetic retinopathy and nephropathy.

A mouse model of accelerated liver aging caused by a defect in DNA repair

The liver changes with age, leading to an impaired ability to respond to hepatic insults and increased incidence of liver disease in the elderly. Therefore, there is critical need for rapid model systems to study aging-related liver changes. One potential opportunity is murine models of human progerias or diseases of accelerated aging. Ercc1(-/Δ) mice model a rare human progeroid syndrome caused by inherited defects in DNA repair. To determine whether hepatic changes that occur with normal aging occur prematurely in Ercc1(-/Δ) mice, we systematically compared liver from 5-month-old progeroid Ercc1(-/Δ) mice to old (24-36-month-old) wild-type (WT) mice. Both displayed areas of necrosis, foci of hepatocellular degeneration, and acute inflammation. Loss of hepatic architecture, fibrosis, steatosis, pseudocapillarization, and anisokaryosis were more dramatic in Ercc1(-/Δ) mice than in old WT mice. Liver enzymes were significantly elevated in serum of Ercc1(-/Δ) mice and old WT mice, whereas albumin was reduced, demonstrating liver damage and dysfunction. The regenerative capacity of Ercc1(-/Δ) liver after partial hepatectomy was significantly reduced. There was evidence of increased oxidative damage in Ercc1(-/Δ) and old WT liver, including lipofuscin, lipid hydroperoxides and acrolein, as well as increased hepatocellular senescence. There was a highly significant correlation in genome-wide transcriptional changes between old WT and 16-week-old, but not 5-week-old, Ercc1(-/Δ) mice, emphasizing that the Ercc1(-/Δ) mice acquire an aging profile in early adulthood.

CONCLUSION

There are strong functional, regulatory, and histopathological parallels between accelerated aging driven by a DNA repair defect and normal aging. This supports a role for DNA damage in driving aging and validates a murine model for rapidly testing hypotheses about causes and treatment for aging-related hepatic changes.

The Fanconi anaemia pathway orchestrates incisions at sites of crosslinked DNA

Fanconi anaemia (FA) is a rare, autosomal recessive, genetically complex, DNA repair deficiency syndrome in man. Patients with FA exhibit a heterogeneous spectrum of clinical features. The most significant and consistent phenotypic characteristics are stem cell loss, causing progressive bone marrow failure and sterility, diverse developmental abnormalities and a profound predisposition to neoplasia. To date, 15 genes have been identified, biallelic disruption of any one of which results in this clinically defined syndrome. It is now apparent that all 15 gene products act in a common process to maintain genome stability. At the molecular level, a fundamental defect in DNA repair underlies this complex phenotype. Cells derived from FA patients spontaneously accumulate broken chromosomes and exhibit a marked sensitivity to DNA-damaging chemotherapeutic agents. Despite complementation analysis defining many components of the FA DNA repair pathway, no direct link to DNA metabolism was established until recently. First, it is now evident that the FA pathway is required to make incisions at the site of damaged DNA. Second, a specific component of the FA pathway has been identified that regulates nucleases previously implicated in DNA interstrand crosslink repair. Taken together, these data provide genetic and biochemical evidence that the FA pathway is a bona fide DNA repair pathway that directly mediates DNA repair transactions, thereby elucidating the specific molecular defect in human Fanconi anaemia.

Regulation of DNA repair by ubiquitylation

Cellular DNA repair is a frontline system that is responsible for maintaining genome integrity and thus preventing premature aging and cancer by repairing DNA lesions and strand breaks caused by endogenous and exogenous mutagens. However, it is also the principal cellular system in cancer cells that counteracts the killing effect of the major cancer treatments, e.g. chemotherapy and ionizing radiation. Although it is clear that an individual's DNA repair capacity varies, the mechanisms involved in the regulation of repair systems that are responsible for such variations are only just emerging. This knowledge gap is impeding the finding of new cancer therapy targets and the development of novel treatment strategies. In recent years the vital role of post-translational modifications of DNA repair proteins, including ubiquitylation and phosphorylation, has been uncovered. This review will cover recent progress in our understanding of the role of ubiquitylation in the regulation of DNA repair.

Broad segmental progeroid changes in short-lived Ercc1(-/Δ7) mice

Genome maintenance is considered a prime longevity assurance mechanism as apparent from many progeroid human syndromes that are caused by genome maintenance defects. The ERCC1 protein is involved in three genome maintenance systems: nucleotide excision repair, interstrand cross-link repair, and homologous recombination. Here we describe in-life and post-mortem observations for a hypomorphic Ercc1 variant, Ercc1(-/Δ7), which is hemizygous for a single truncated Ercc1 allele, encoding a protein lacking the last seven amino acids. Ercc1(-/Δ7) mice were much smaller and median life span was markedly reduced compared to wild-type siblings: 20 and 118 weeks, respectively. Multiple signs and symptoms of aging were found to occur at an accelerated rate in the Ercc1(-/Δ7) mice as compared to wild-type controls, including a decline in weight of both whole body and various organs, numerous histopathological lesions, and immune parameters. Together they define a segmental progeroid phenotype of the Ercc1(-/Δ7) mouse model.

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.

Polycomb group proteins in hematopoietic stem cell aging and malignancies

Protection of the transcriptional "stemness" network is important to maintain a healthy hematopoietic stem cells (HSCs) compartment during the lifetime of the organism. Recent evidence shows that fundamental changes in the epigenetic status of HSCs might be one of the driving forces behind many age-related HSC changes and might pave the way for HSC malignant transformation and subsequent leukemia development, the incidence of which increases exponentially with age. Polycomb group (PcG) proteins are key epigenetic regulators of HSC cellular fate decisions and are often found to be misregulated in human hematopoietic malignancies. In this review, we speculate that PcG proteins balance HSC aging against the risk of developing cancer, since a disturbance in PcG genes and proteins affects several important cellular processes such as cell fate decisions, senescence, apoptosis, and DNA damage repair.

Congenital bone marrow failure in DNA-PKcs mutant mice associated with deficiencies in DNA repair

The nonhomologous end-joining (NHEJ) pathway is essential for radioresistance and lymphocyte-specific V(D)J (variable [diversity] joining) recombination. Defects in NHEJ also impair hematopoietic stem cell (HSC) activity with age but do not affect the initial establishment of HSC reserves. In this paper, we report that, in contrast to deoxyribonucleic acid (DNA)-dependent protein kinase catalytic subunit (DNA-PKcs)-null mice, knockin mice with the DNA-PKcs(3A/3A) allele, which codes for three alanine substitutions at the mouse Thr2605 phosphorylation cluster, die prematurely because of congenital bone marrow failure. Impaired proliferation of DNA-PKcs(3A/3A) HSCs is caused by excessive DNA damage and p53-dependent apoptosis. In addition, increased apoptosis in the intestinal crypt and epidermal hyperpigmentation indicate the presence of elevated genotoxic stress and p53 activation. Analysis of embryonic fibroblasts further reveals that DNA-PKcs(3A/3A) cells are hypersensitive to DNA cross-linking agents and are defective in both homologous recombination and the Fanconi anemia DNA damage response pathways. We conclude that phosphorylation of DNA-PKcs is essential for the normal activation of multiple DNA repair pathways, which in turn is critical for the maintenance of diverse populations of tissue stem cells in mice.

Accelerated aging of intervertebral discs in a mouse model of progeria

Intervertebral disc degeneration (IDD) is a common and debilitating disorder that results in reduced flexibility of the spine, pain, and reduced mobility. Risk factors for IDD include age, genetic predisposition, injury, and other environmental factors such as smoking. Loss of proteoglycans (PGs) contributes to IDD with advancing age. Currently there is a lack of a model for rapid investigation of disc aging and evaluation of therapeutic interventions. Here we examined progression of disc aging in a murine model of a human progeroid syndrome caused by deficiency of the DNA repair endonuclease, ERCC1-XPF (Ercc1(-/Δ) mice). The ERCC1-deficient mice showed loss of disc height and degenerative structural changes in their vertebral bodies similar to those reported for old rodents. Compared to their wild-type littermates, Ercc1(-/Δ) mice also exhibit other age-related IDD characteristics, including premature loss of disc PG, reduced matrix PG synthesis, and enhanced apoptosis and cell senescence. Finally, the onset of age-associated disc pathologies was further accelerated in Ercc1(-/Δ) mice following chronic treatment with the chemotherapeutic agent mechlorethamine. These results demonstrate that Ercc1(-/Δ) mice represent an accurate and rapid model of disc aging and provide novel evidence that DNA damage negatively impacts PG synthesis.

Hematopoietic stem cell defects in mice with deficiency of Fancd2 or Usp1

Fanconi anemia (FA) is a human genetic disease characterized by a DNA repair defect and progressive bone marrow failure. Central events in the FA pathway are the monoubiquitination of the Fancd2 protein and the removal of ubiquitin by the deubiquitinating enzyme, Usp1. Here, we have investigated the role of Fancd2 and Usp1 in the maintenance and function of murine hematopoietic stem cells (HSCs). Bone marrow from Fancd2-/- mice and Usp1-/- mice exhibited marked hematopoietic defects. A decreased frequency of the HSC populations including Lin-Sca-1+Kit+ cells and cells enriched for dormant HSCs expressing signaling lymphocyte activation molecule (SLAM) markers, was observed in the bone marrow of Fancd2-deficient mice. In addition, bone marrow from Fancd2-/- mice contained significantly reduced frequencies of late-developing cobblestone area-forming cell activity in vitro compared to the bone marrow from wild-type mice. Furthermore, Fancd2-deficient and Usp1-deficient bone marrow had defective long-term in vivo repopulating ability. Collectively, our data reveal novel functions of Fancd2 and Usp1 in maintaining the bone marrow HSC compartment and suggest that FA pathway disruption may account for bone marrow failure in FA patients.

Fancd2-/- mice have hematopoietic defects that can be partially corrected by resveratrol

Progressive bone marrow failure is a major cause of morbidity and mortality in human Fanconi Anemia patients. In an effort to develop a Fanconi Anemia murine model to study bone marrow failure, we found that Fancd2(-/-) mice have readily measurable hematopoietic defects. Fancd2 deficiency was associated with a significant decline in the size of the c-Kit(+)Sca-1(+)Lineage(-) (KSL) pool and reduced stem cell repopulation and spleen colony-forming capacity. Fancd2(-/-) KSL cells showed an abnormal cell cycle status and loss of quiescence. In addition, the supportive function of the marrow microenvironment was compromised in Fancd2(-/-) mice. Treatment with Sirt1-mimetic and the antioxidant drug, resveratrol, maintained Fancd2(-/-) KSL cells in quiescence, improved the marrow microenvironment, partially corrected the abnormal cell cycle status, and significantly improved the spleen colony-forming capacity of Fancd2(-/-) bone marrow cells. We conclude that Fancd2(-/-) mice have readily quantifiable hematopoietic defects, and that this model is well suited for pharmacologic screening studies.

Mice with DNA repair gene Ercc1 deficiency in a neural crest lineage are a model for late-onset Hirschsprung disease

The Ercc1 gene is essential for nucleotide excision repair and is also important in recombination repair and the repair of interstrand crosslinks. We have previously used a floxed Ercc1 allele with a keratinocyte-specific Cre recombinase transgene to inactivate Ercc1 in the epidermal layer of the skin and so generate a mouse model for UV-induced non-melanoma skin cancer. Now, in an attempt to generate a model for UV-induced melanoma, we have used the floxed Ercc1 allele in combination with a Cre transgene under the control of the tyrosinase gene promoter to produce mice with Ercc1-deficient melanocytes that are hypersensitive to UV irradiation. These animals developed normally, but died when 4-6 months old with severe colonic obstruction. Melanocytes are derived from the neural crest and the tyrosinase promoter is also expressed in additional neural crest-derived lineages, including the progenitors of the parasympathetic nervous system that innervates the gastrointestinal tract and controls gut peristalsis. A functional enteric nervous system developed in floxed Ercc1 mice with the tyrosinase Cre transgene, but was found to have degenerated in the colons of affected mice. We suggest that accumulating unrepaired endogenous DNA damage in the Ercc1-deficient colonic parasympathetic ganglia leads to the degeneration of this network and results in a colonic obstructive disorder that resembles late-onset Hirschsprung disease in man.

Mechanisms of stem cell self-renewal

Self-renewal is the process by which stem cells divide to make more stem cells, perpetuating the stem cell pool throughout life. Self-renewal is division with maintenance of the undifferentiated state. This requires cell cycle control and often maintenance of multipotency or pluripotency, depending on the stem cell. Self-renewal programs involve networks that balance proto-oncogenes (promoting self-renewal), gate-keeping tumor suppressors (limiting self-renewal), and care-taking tumor suppressors (maintaining genomic integrity). These cell-intrinsic mechanisms are regulated by cell-extrinsic signals from the niche, the microenvironment that maintains stem cells and regulates their function in tissues. In response to changing tissue demands, stem cells undergo changes in cell cycle status and developmental potential over time, requiring different self-renewal programs at different stages of life. Reduced stem cell function and tissue regenerative capacity during aging are caused by changes in self-renewal programs that augment tumor suppression. Cancer arises from mutations that inappropriately activate self-renewal programs.

How the fanconi anemia pathway guards the genome

Fanconi Anemia (FA) is an inherited genomic instability disorder, caused by mutations in genes regulating replication-dependent removal of interstrand DNA crosslinks. The Fanconi Anemia pathway is thought to coordinate a complex mechanism that enlists elements of three classic DNA repair pathways, namely homologous recombination, nucleotide excision repair, and mutagenic translesion synthesis, in response to genotoxic insults. To this end, the Fanconi Anemia pathway employs a unique nuclear protein complex that ubiquitinates FANCD2 and FANCI, leading to formation of DNA repair structures. Lack of obvious enzymatic activities among most FA members has made it challenging to unravel its precise modus operandi. Here we review the current understanding of how the Fanconi Anemia pathway components participate in DNA repair and discuss the mechanisms that regulate this pathway to ensure timely, efficient, and correct restoration of chromosomal integrity.

Aging in the lympho-hematopoietic stem cell compartment

Cells of the immune system are progeny of a single primitive cell type, the hematopoietic stem cell (HSC). Aging in most strains of mice is associated with a reduction in HSC frequency and a reduction in HSC function. Aged HSCs demonstrate reduced differentiation toward the lymphoid lineage, and this might be a relevant factor influencing immunosenescence. The molecular mechanisms of HSC aging need to be determined in more detail, but current studies have identified, among others, a role for telomere dysfunction in inducing cell intrinsic checkpoints and environmental alterations, which both skews and reduces stem cell differentiation and function. Reverting or ameliorating aging of HSCs might be a crucial step to restoring immuno-competence in the elderly.

Effects of aging on hematopoietic stem and progenitor cells

Although relationships between cellular and organismal aging are not well understood, several studies describe age-related changes in hematopoietic stem cells (HSCs) with functional consequences for the hematopoietic system. Importantly, aged hematopoietic stem and progenitor cells (HSPCs) differ from their younger counterparts in functional capacity, the complement of proteins on the cell surface, transcriptional activity, and genome integrity. These changes, however, are likely the result of a combination of cell-intrinsic and microenvironment-derived influences. Evolving views of the composition of the HSC compartment suggest that changes in HSCs may reflect the effects of the aging process on individual HSCs or a shift in the clonal composition of the HSC pool with age.

Altered hematopoiesis in mice lacking DNA polymerase mu is due to inefficient double-strand break repair

Polymerase mu (Polμ) is an error-prone, DNA-directed DNA polymerase that participates in non-homologous end-joining (NHEJ) repair. In vivo, Polμ deficiency results in impaired Vκ-Jκ recombination and altered somatic hypermutation and centroblast development. In Polμ^−/− mice, hematopoietic development was defective in several peripheral and bone marrow (BM) cell populations, with about a 40% decrease in BM cell number that affected several hematopoietic lineages. Hematopoietic progenitors were reduced both in number and in expansion potential. The observed phenotype correlates with a reduced efficiency in DNA double-strand break (DSB) repair in hematopoietic tissue. Whole-body γ-irradiation revealed that Polμ also plays a role in DSB repair in non-hematopoietic tissues. Our results show that Polμ function is required for physiological hematopoietic development with an important role in maintaining early progenitor cell homeostasis and genetic stability in hematopoietic and non-hematopoietic tissues.

Double-strand breaks (DSB) in DNA are a highly deleterious type of genetic damage, potentially causing genomic rearrangements or cell death if unrepaired. DSB can be triggered by environmental factors (such as electromagnetic radiation or clastogenic chemicals) or normal cell metabolism. The main mechanism of DSB repair in mammals is thought to be the non-homologous end-joining (NHEJ) pathway. Our article describes how DNA polymerase mu (Polμ), a recently identified component of the NHEJ machinery, is required for hematopoiesis—the process that generates and maintains the correct balance of the millions of blood cells needed to sustain life and defend against infection. Hematopoietic stem cells (HSC) divide asymmetrically, yielding another HSC and a progenitor cell. These progenitors proliferate and differentiate, their progeny eventually generating mature blood cells. In mice in which Polμ is genetically eliminated, we found that hematopoietic progenitors proliferate slowly and are functionally impaired. The incidence of DSB in hematopoietic cells from these mice is increased, suggesting that reduced DNA repair may be the cause of the hematopoietic defects. DNA damage was also increased in tissues unrelated to hematopoiesis, including liver, kidney, lung, and mouse embryonic fibroblasts. Thus, these results demonstrate that Polμ plays an important role in general DSB repair in many cell lineages.

Strategies for future histocompatible stem cell therapy

Stem cell therapy based on the safe and unlimited self-renewal of human pluripotent stem cells is envisioned for future use in tissue or organ replacement after injury or disease. A gradual decline of regenerative capacity has been documented among the adult stem cell population in some body organs during the aging process. Recent progress in human somatic cell nuclear transfer and inducible pluripotent stem cell technologies has shown that patient-derived nuclei or somatic cells can be reprogrammed in vitro to become pluripotent stem cells, from which the three germ layer lineages can be generated, genetically identical to the recipient. Once differentiation protocols and culture conditions can be defined and optimized, patient-histocompatible pluripotent stem cells could be directed towards virtually every cell type in the human body. Harnessing this capability to enrich for given cells within a developmental lineage, would facilitate the transplantation of organ/tissue-specific adult stem cells or terminally differentiated somatic cells to improve the function of diseased organs or tissues in an individual. Here, we present an overview of various experimental cell therapy technologies based on the use of patient-histocompatible stem cells, the pending issues needed to be dealt with before clinical trials can be initiated, evidence for the loss and/or aging of the stem cell pool and some of the possible uses of human pluripotent stem cell-derivatives aimed at curing disease and improving health.

Impaired spermatogenesis and elevated spontaneous tumorigenesis in xeroderma pigmentosum group A gene (Xpa)-deficient mice

We have reported that xeroderma pigmentosum group A (Xpa) gene-knockout mice [Xpa (-/-) mice] are deficient in nucleotide excision repair (NER) and highly sensitive to UV-induced skin carcinogenesis. Although xeroderma pigmentosum group A patients show growth retardation, immature sexual development, and neurological abnormalities as well as a high incidence of UV-induced skin tumors, Xpa (-/-) mice were physiologically and behaviorally normal. In the present study, we kept Xpa (-/-) mice for 2 years under specific pathogen-free (SPF) conditions and found that the testis diminished in an age-dependent manner, and degenerating seminiferous tubules and no spermatozoa were detected in the 24-month-old Xpa (-/-) mice. In addition, a higher incidence of spontaneous tumorigenesis was observed in the 24-month-old Xpa (-/-) mice compared to Xpa (+/+) controls. Xpa (-/-) mice provide a useful model for investigating the aging and internal tumor formation in XPA patients.

Stem cells and the rate of living

The "rate-of-living theory" is an ancient explanation of longevity which holds that aging occurs due to the exhaustion of some finite substance-breaths, heartbeats, etc. While this theory as originally conceived has been debunked, new work (Ruzankina et al. [2007], in this issue of Cell Stem Cell) suggests that mammals in fact do have a finite number of stem cell replications per life.

Structural and functional relationships of the XPF/MUS81 family of proteins

Proteins belonging to the XPF/MUS81 family play important roles in the repair of DNA lesions caused by UV-light or DNA cross-linking agents. Most eukaryotes have four family members that assemble into two distinct heterodimeric complexes, XPF-ERCC1 and MUS81-EME1. Each complex contains one catalytic and one noncatalytic subunit and exhibits endonuclease activity with a variety of 3'-flap or fork DNA structures. The catalytic subunits share a characteristic core containing an excision repair cross complementation group 4 (ERCC4) nuclease domain and a tandem helix-hairpin-helix (HhH)(2) domain. Diverged domains are present in the noncatalytic subunits and may be required for substrate targeting. Vertebrates possess two additional family members, FANCM and Fanconi anemia-associated protein 24 kDa (FAAP24), which possess inactive nuclease domains. Instead, FANCM contains a functional Superfamily 2 (SF2) helicase domain that is required for DNA translocation. Determining how these enzymes recognize specific DNA substrates and promote key repair reactions is an important challenge for the future.

ERCC1-XPF endonuclease facilitates DNA double-strand break repair

ERCC1-XPF endonuclease is required for nucleotide excision repair (NER) of helix-distorting DNA lesions. However, mutations in ERCC1 or XPF in humans or mice cause a more severe phenotype than absence of NER, prompting a search for novel repair activities of the nuclease. In Saccharomyces cerevisiae, orthologs of ERCC1-XPF (Rad10-Rad1) participate in the repair of double-strand breaks (DSBs). Rad10-Rad1 contributes to two error-prone DSB repair pathways: microhomology-mediated end joining (a Ku86-independent mechanism) and single-strand annealing. To determine if ERCC1-XPF participates in DSB repair in mammals, mutant cells and mice were screened for sensitivity to gamma irradiation. ERCC1-XPF-deficient fibroblasts were hypersensitive to gamma irradiation, and gammaH2AX foci, a marker of DSBs, persisted in irradiated mutant cells, consistent with a defect in DSB repair. Mutant mice were also hypersensitive to irradiation, establishing an essential role for ERCC1-XPF in protecting against DSBs in vivo. Mice defective in both ERCC1-XPF and Ku86 were not viable. However, Ercc1(-/-) Ku86(-/-) fibroblasts were hypersensitive to gamma irradiation compared to single mutants and accumulated significantly greater chromosomal aberrations. Finally, in vitro repair of DSBs with 3' overhangs led to large deletions in the absence of ERCC1-XPF. These data support the conclusion that, as in yeast, ERCC1-XPF facilitates DSB repair via an end-joining mechanism that is Ku86 independent.

Tissue-specific accelerated aging in nucleotide excision repair deficiency

Nucleotide excision repair (NER) is a multi-step DNA repair mechanism that removes helix-distorting modified nucleotides from the genome. NER is divided into two subpathways depending on the location of DNA damage in the genome and how it is first detected. Global genome NER identifies and repairs DNA lesions throughout the genome. This subpathway of NER primarily protects against the accumulation of mutations in the genome. Transcription-coupled (TC)-NER rapidly repairs lesions in the transcribed strand of DNA that block transcription by RNA polymerase II. TC-NER prevents cell death in response to stalled transcription. Defects in NER cause three distinct human diseases: xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy. Each of these syndromes is characterized by premature onset of pathologies that overlap with those associated with old age in humans. This reveals the contribution of DNA damage to multiple age-related diseases. Tissues affected include the skin, eye, bone marrow, nervous system and endocrine axis. This review emphasizes accelerated aging associated with xeroderma pigmentosum and discusses the cause of these pathologies, either mutation accumulation or cell death as a consequence of failure to repair DNA damage.

Stem cells and aging in the hematopoietic system

The effector cells of the blood have limited lifetimes and must be replenished continuously throughout life from a small reserve of hematopoietic stem cells (HSCs) in the bone marrow. Although serial bone marrow transplantation experiments in mice suggest that the replicative potential of HSCs is finite, there is little evidence that replicative senescence causes depletion of the stem cell pool during the normal lifespan of either mouse or man. Studies conducted in murine genetic models defective in DNA repair, intracellular ROS management, and telomere maintenance indicate that all these pathways are critical to the longevity and stress response of the stem cell pool. With age, HSCs show an increased propensity to differentiate towards myeloid rather than lymphoid lineages, which may contribute to the decline in lymphopoiesis that attends aging. Challenges for the future include assessing the significance of 'lineage skewing' to immune dysfunction, and investigating the role of epigenetic dysregulation in HSC aging.

How does suppression of IGF-1 signaling by DNA damage affect aging and longevity?

Long-lived animals have evolved a robust set of defenses to maintain genomic integrity over their entire lifespan. The DNA damage response and DNA repair pathways are critical pillars of organismal defenses, minimizing somatic mutations in both post-mitotic and mitotic cells. These genomic maintenance systems not only prevent the premature emergence of cancers but may also maintain normal tissue function and organismal longevity. Genetic defects in a number of DNA repair and DNA damage response genes often leads to a dramatic increase in cancer incidence; in other cases, premature aging or progeroid syndromes may be induced. In this review, we discuss two recent reports of two nucleotide excision repair-deficient models that exhibit dramatic premature aging and shortened longevity. The DNA repair defects were also associated with a significant inhibition of the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis, an endocrine signaling pathway shown to influence aging and longevity in both vertebrates and invertebrates. Potential mechanisms of how DNA damage might affect IGF-1 signaling and aging are discussed, with a particular emphasis on the role of such signaling alterations in the adult tissue stem cell compartments.

The role of DNA damage repair in aging of adult stem cells

DNA repair maintains genomic stability and the loss of DNA repair capacity results in genetic instability that may lead to a decline of cellular function. Adult stem cells are extremely important in the long-term maintenance of tissues throughout life. They regenerate and renew tissues in response to damage and replace senescent terminally differentiated cells that no longer function. Oxidative stress, toxic byproducts, reduced mitochondrial function and external exposures all damage DNA through base modification or mis-incorporation and result in DNA damage. As in most cells, this damage may limit the survival of the stem cell population affecting tissue regeneration and even longevity. This review examines the hypothesis that an age-related loss of DNA damage repair pathways poses a significant threat to stem cell survival and longevity. Normal stem cells appear to have strict control of gene expression and DNA replication whereas stem cells with loss of DNA repair may have altered patterns of proliferation, quiescence and differentiation. Furthermore, stem cells with loss of DNA repair may be susceptible to malignant transformation either directly or through the emergence of cancer-prone stem cells. Human diseases and animal models of loss of DNA repair provide longitudinal analysis of DNA repair processes in stem cell populations and may provide links to the physiology of aging.

Contributions of DNA interstrand cross-links to aging of cells and organisms

Impaired DNA damage repair, especially deficient transcription-coupled nucleotide excision repair, leads to segmental progeroid syndromes in human patients as well as in rodent models. Furthermore, DNA double-strand break signalling has been pinpointed as a key inducer of cellular senescence. Several recent findings suggest that another DNA repair pathway, interstrand cross-link (ICL) repair, might also contribute to cell and organism aging. Therefore, we summarize and discuss here that (i) systemic administration of anti-cancer chemotherapeutics, in many cases DNA cross-linking drugs, induces premature progeroid frailty in long-term survivors; (ii) that ICL-inducing 8-methoxy-psoralen/UVA phototherapy leads to signs of premature skin aging as prominent long-term side effect and (iii) that mutated factors involved in ICL repair like ERCC1/XPF, the Fanconi anaemia proteins, WRN and SNEV lead to reduced replicative life span in vitro and segmental progeroid syndromes in vivo. However, since ICL-inducing drugs cause damage different from ICL and since all currently known ICL repair factors work in more than one pathway, further work will be needed to dissect the actual contribution of ICL damage to aging.

Stem cell ageing: does it happen and can we intervene?

Adult stem cells have become the focus of intense research in recent years as a result of their role in the maintenance and repair of tissues. They exert this function through their extensive expansion (self-renewal) and multipotent differentiation capacity. Understanding whether adult stem cells retain this capacity throughout the lifespan of the individual, or undergo a process of ageing resulting in a decreased stem cell pool, is an important area of investigation. Progress in this area has been hampered by lack of suitable models and of appropriate markers and assays to identify stem cells. However, recent data suggest that an understanding of the mechanisms governing stem cell ageing can give insight into the mechanism of tissue ageing and, most importantly, advance our ability to use stem cells in cell and gene therapy strategies.

DNA repair in stem cell maintenance and conversion to cancer stem cells

Genomic stability is essential for cell and organism longevity. Without genomic stability, replication errors and external stress as well as direct forms of DNA damage can induce mutations, which decrease cell survival, cause altered gene expression, and can lead to cellular transformation. All represent the antithesis of maintenance of normal stem cell function. We argue here that genomic stability is essential for stem cell maintenance and longevity. This concept is supported by human diseases associated with premature aging and animal models of DNA damage repair abnormalities all of which lead to abnormalities of stem cell survival. Furthermore, with competitive repopulation, hematopoietic stem cell survival can be assessed in the face of DNA repair defects, and results from these studies support the general conclusion that chemotherapy and other forms of DNA damage lead to stem cell failure syndromes and malignant transformation most commonly along the myeloid and lymphoid pathways. Thus one origin of the cancer stem cell phenotype is the inability to maintain genomic stability among the stem cell population leading to mutational alterations and transformation. Capturing stem cells at this transition point represents an exciting field of discovery possibly leading to early detection and therapeutic interventions.

Telomeres, senescence, and hematopoietic stem cells

The replicative lifespan of normal somatic cells is restricted by the erosion of telomeres, which are protective caps at the ends of linear chromosomes. The loss of telomeres induces antiproliferative signals that eventually lead to cellular senescence. The enzyme complex telomerase can maintain telomeres, but its expression is confined to highly proliferative cells such as stem cells and tumor cells. The immense regenerative capacity of the hematopoietic system is provided by a distinct type of adult stem cell: hematopoietic stem cells (HSCs). Although blood cells have to be produced continuously throughout life, the HSC pool seems not to be spared by aging processes. Indeed, limited expression of telomerase is not sufficient to prevent telomere shortening in these cells, which is thought ultimately to limit their proliferative capacity. In this review, we discuss the relevance of telomere maintenance for the hematopoietic stem cell compartment and consider potential functions of telomerase in this context. We also present possible clinical applications of telomere manipulation in HSCs and new insights affecting the aging of the hematopoietic stem cell pool and replicative exhaustion.

How stem cells age and why this makes us grow old

Recent data suggest that we age, in part, because our self-renewing stem cells grow old as a result of heritable intrinsic events, such as DNA damage, as well as extrinsic forces, such as changes in their supporting niches. Mechanisms that suppress the development of cancer, such as senescence and apoptosis, which rely on telomere shortening and the activities of p53 and p16(INK4a), may also induce an unwanted consequence: a decline in the replicative function of certain stem-cell types with advancing age. This decreased regenerative capacity appears to contribute to some aspects of mammalian ageing, with new findings pointing to a 'stem-cell hypothesis' for human age-associated conditions such as frailty, atherosclerosis and type 2 diabetes.

Current status of excision repair cross complementing-group 1 (ERCC1) in cancer

Cisplatin, carboplatin and oxaliplatin are some of the most widely used anti-cancer agents in solid tumours. The cytotoxicity of platinating agents is directly related to their ability to cause DNA intra-strand crosslinks that trigger a series of intracellular events that ultimately result in cell death. DNA intra-strand crosslinks are processed and repaired by the nucleotide excision repair pathway. It is now clear that nucleotide excision repair (NER) capacity may have a major impact on the emergence of resistance, normal tissue tolerance and patient outcomes. ERCC1 is a key player in NER. In this review, we provide an overview of mammalian NER and then focus on biochemical, structural and pre-clinical aspects of ERCC1. We then present current clinical evidence implicating ERCC1 as a predictive and prognostic marker in cancer. Early evidence also suggests that ERCC1 or the pathways involved in the regulation of ERCC1 expression may be attractive anti-cancer targets. Such agents are expected to potentiate the cytotoxicity of platinating agents and could have a major impact on cancer therapy.

Customizing Cisplatin Based on Quantitative Excision Repair Cross-Complementing 1 mRNA Expression: A Phase III Trial in Non–Small-Cell Lung Cancer

Purpose

Although current treatment options for metastatic non–small-cell lung cancer (NSCLC) rely on cisplatin-based chemotherapy, individualized approaches to therapy may improve response or reduce unnecessary toxicity. Excision repair cross-complementing 1 (ERCC1) has been associated with cisplatin resistance. We hypothesized that assigning cisplatin based on pretreatment ERCC1 mRNA levels would improve response.

Patients and Methods

From August 2001 to October 2005, 444 stage IV NSCLC patients were enrolled. RNA was isolated from pretreatment biopsies, and quantitative real-time reverse transcriptase PCR assays were performed to determine ERCC1 mRNA expression. Patients were randomly assigned in a 1:2 ratio to either the control or genotypic arm before ERCC1 assessment. Patients in the control arm received docetaxel plus cisplatin. In the genotypic arm, patients with low ERCC1 levels received docetaxel plus cisplatin, and those with high levels received docetaxel plus gemcitabine. The primary end point was the overall objective response rate.

Results

Of 444 patients enrolled, 78 (17.6%) went off study before receiving one cycle of chemotherapy, mainly due to insufficient tumor tissue for ERCC1 mRNA assessment. Of the remaining 346 patients assessable for response, objective response was attained by 53 patients (39.3%) in the control arm and 107 patients (50.7%) in the genotypic arm (P = .02).

Conclusion

Assessment of ERCC1 mRNA expression in patient tumor tissue is feasible in the clinical setting and predicts response to docetaxel and cisplatin. Additional studies are warranted to optimize methodologies for ERCC1 analysis in small tumor samples and to refine a multibiomarker profile predictive of patient outcome.

Deficiencies in DNA damage repair limit the function of haematopoietic stem cells with age

A diminished capacity to maintain tissue homeostasis is a central physiological characteristic of ageing. As stem cells regulate tissue homeostasis, depletion of stem cell reserves and/or diminished stem cell function have been postulated to contribute to ageing. It has further been suggested that accumulated DNA damage could be a principal mechanism underlying age-dependent stem cell decline. We have tested these hypotheses by examining haematopoietic stem cell reserves and function with age in mice deficient in several genomic maintenance pathways including nucleotide excision repair, telomere maintenance and non-homologous end-joining. Here we show that although deficiencies in these pathways did not deplete stem cell reserves with age, stem cell functional capacity was severely affected under conditions of stress, leading to loss of reconstitution and proliferative potential, diminished self-renewal, increased apoptosis and, ultimately, functional exhaustion. Moreover, we provide evidence that endogenous DNA damage accumulates with age in wild-type stem cells. These data are consistent with DNA damage accrual being a physiological mechanism of stem cell ageing that may contribute to the diminished capacity of aged tissues to return to homeostasis after exposure to acute stress or injury.

First reported patient with human ERCC1 deficiency has cerebro-oculo-facio-skeletal syndrome with a mild defect in nucleotide excision repair and severe developmental failure

Nucleotide excision repair (NER) is a genome caretaker mechanism responsible for removing helix-distorting DNA lesions, most notably ultraviolet photodimers. Inherited defects in NER result in profound photosensitivity and the cancer-prone syndrome xeroderma pigmentosum (XP) or two progeroid syndromes: Cockayne and trichothiodystrophy syndromes. The heterodimer ERCC1-XPF is one of two endonucleases required for NER. Mutations in XPF are associated with mild XP and rarely with progeria. Mutations in ERCC1 have not been reported. Here, we describe the first case of human inherited ERCC1 deficiency. Patient cells showed moderate hypersensitivity to ultraviolet rays and mitomycin C, yet the clinical features were very severe and, unexpectedly, were compatible with a diagnosis of cerebro-oculo-facio-skeletal syndrome. This discovery represents a novel complementation group of patients with defective NER. Further, the clinical severity, coupled with a relatively mild repair defect, suggests novel functions for ERCC1.

Epigenetic control of hematopoietic stem cell aging the case of Ezh2

Hematopoietic stem cells have potent, but not unlimited, selfrenewal potential. The mechanisms that restrict selfrenewal are likely to play a role during aging. Recent data suggest that the regulation of histone modifications by Polycomb group genes may be of crucial relevance to balance selfrenewal and aging. We provide evidence for the involvement of one of these Polycomb group genes, Ezh2, in aging of the hematopoietic stem cell system.

Tissue specific mutagenic and carcinogenic responses in NER defective mouse models

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

Age-related genetic abnormalities: the Achilles' heel for customizing therapy in elderly lung cancer patients

Aging and cancer are closely related, and DNA repair systems, mainly involving the nucleotide excision repair pathway, have an important caretaker function in both processes. More than half of non-small cell lung cancer patients are elderly, and the expression of some genes involved in the mitotic checkpoint, such as BubR1, declines with aging. Cisplatin-based chemotherapy is the standard treatment for advanced non-small cell lung cancer; if performance status is good, both elderly and younger patients can tolerate this treatment equally well. Customized cisplatin treatment, based on reduction of the nucleotide excision repair pathway function, could be an attractive approach, and the assessment of mitotic checkpoint genes can be used for selecting docetaxel treatment. Epidermal growth factor receptor (EGFR) mutations are particularly frequent in elderly lung cancer patients who are never-smokers and constitute an attractive target for treatment with EGFR tyrosine kinase inhibitors.

A new progeroid syndrome reveals that genotoxic stress suppresses the somatotroph axis

XPF-ERCC1 endonuclease is required for repair of helix-distorting DNA lesions and cytotoxic DNA interstrand crosslinks. Mild mutations in XPF cause the cancer-prone syndrome xeroderma pigmentosum. A patient presented with a severe XPF mutation leading to profound crosslink sensitivity and dramatic progeroid symptoms. It is not known how unrepaired DNA damage accelerates ageing or its relevance to natural ageing. Here we show a highly significant correlation between the liver transcriptome of old mice and a mouse model of this progeroid syndrome. Expression data from XPF-ERCC1-deficient mice indicate increased cell death and anti-oxidant defences, a shift towards anabolism and reduced growth hormone/insulin-like growth factor 1 (IGF1) signalling, a known regulator of lifespan. Similar changes are seen in wild-type mice in response to chronic genotoxic stress, caloric restriction, or with ageing. We conclude that unrepaired cytotoxic DNA damage induces a highly conserved metabolic response mediated by the IGF1/insulin pathway, which re-allocates resources from growth to somatic preservation and life extension. This highlights a causal contribution of DNA damage to ageing and demonstrates that ageing and end-of-life fitness are determined both by stochastic damage, which is the cause of functional decline, and genetics, which determines the rates of damage accumulation and decline.

Extended longevity mechanisms in short-lived progeroid mice: identification of a preservative stress response associated with successful aging

Semantic distinctions between "normal" aging, "pathological" aging (or age-related disease) and "premature" aging (otherwise known as segmental progeria) potentially confound important insights into the nature of each of the complex processes. Here we review a recent, unexpected discovery: the presence of longevity-associated characteristics typical of long-lived endocrine-mutant and dietary-restricted animals in short-lived progeroid mice. These data suggest that a subset of symptoms observed in premature aging, and possibly normal aging as well, may be indirect manifestations of a beneficial adaptive stress response to endogenous oxidative damage, rather than a detrimental result of the damage itself.