Progeroid syndromes: probing the molecular basis of aging?

A Review of Telomere Attrition in Cancer and Aging: Current Molecular Insights and Future Therapeutic Approaches

BACKGROUND/OBJECTIVES

As cells divide, telomeres shorten through a phenomenon known as telomere attrition, which leads to unavoidable senescence of cells. Unprotected DNA exponentially increases the odds of mutations, which can evolve into premature aging disorders and tumorigenesis. There has been growing academic and clinical interest in exploring this duality and developing optimal therapeutic strategies to combat telomere attrition in aging and cellular immortality in cancer. The purpose of this review is to provide an updated overview of telomere biology and therapeutic tactics to address aging and cancer.

METHODS

We used the Rayyan platform to review the PubMed database and examined the ClinicalTrial.gov registry to gain insight into clinical trials and their results.

RESULTS

Cancer cells activate telomerase or utilize alternative lengthening of telomeres to escape telomere shortening, leading to near immortality. Contrarily, normal cells experience telomeric erosion, contributing to premature aging disorders, such as Werner syndrome and Hutchinson-Gilford Progeria, and (2) aging-related diseases, such as neurodegenerative and cardiovascular diseases.

CONCLUSIONS

The literature presents several promising therapeutic approaches to potentially balance telomere maintenance in aging and shortening in cancer. This review highlights gaps in knowledge and points to the potential of these optimal interventions in preclinical and clinical studies to inform future research in cancer and aging.

Skeletal phenotypes and molecular mechanisms in aging mice

Aging is an inevitable physiological process, often accompanied by age-related bone loss and subsequent bone-related diseases that pose serious health risks. Research on skeletal diseases caused by aging in humans is challenging due to lengthy study durations, difficulties in sampling, regional variability, and substantial investment. Consequently, mice are preferred for such studies due to their similar motor system structure and function to humans, ease of handling and care, low cost, and short generation time. In this review, we present a comprehensive overview of the characteristics, limitations, applicability, bone phenotypes, and treatment methods in naturally aging mice and prematurely aging mouse models (including SAMP6, POLG mutant, LMNA, SIRT6, ZMPSTE24, TFAM, ERCC1, WERNER, and KL/KL-deficient mice). We also summarize the molecular mechanisms of these aging mouse models, including cellular DNA damage response, senescence-related secretory phenotype, telomere shortening, oxidative stress, bone marrow mesenchymal stem cell (BMSC) abnormalities, and mitochondrial dysfunction. Overall, this review aims to enhance our understanding of the pathogenesis of aging-related bone diseases.

The Interplay between Oxidative Stress and the Nuclear Lamina Contributes to Laminopathies and Age-Related Diseases

Oxidative stress is a physiological condition that arises when there is an imbalance between the production of reactive oxygen species (ROS) and the ability of cells to neutralize them. ROS can damage cellular macromolecules, including lipids, proteins, and DNA, leading to cellular senescence and physiological aging. The nuclear lamina (NL) is a meshwork of intermediate filaments that provides structural support to the nucleus and plays crucial roles in various nuclear functions, such as DNA replication and transcription. Emerging evidence suggests that oxidative stress disrupts the integrity and function of the NL, leading to dysregulation of gene expression, DNA damage, and cellular senescence. This review highlights the current understanding of the interplay between oxidative stress and the NL, along with its implications for human health. Specifically, elucidation of the mechanisms underlying the interplay between oxidative stress and the NL is essential for the development of effective treatments for laminopathies and age-related diseases.

Absence of premature senescence in Werner's syndrome keratinocytes

Werner's syndrome (WS) is an autosomal recessive genetic disorder caused by loss of function mutation in wrn and is a useful model of premature in vivo ageing. Cellular senescence is a plausible causal mechanism of mammalian ageing and, at the cellular level, WS fibroblasts show premature senescence resulting from a combination of telomeric attrition and replication fork stalling. Over 90% of WS fibroblast cultures achieve <20 population doublings (PD) in vitro compared to wild type human fibroblast cultures. It has been proposed that some cell types, capable of proliferation, will fail to show a premature senescence phenotype in response to wrn mutations. To test this hypothesis, human dermal keratinocytes (derived from both WS and wild type patients) were cultured long term. WS Keratinocytes showed a replicative lifespan in excess of 100 population doublings but maintained functional growth arrest mechanisms based on p16 and p53. The karyotype of the cells was superficially normal and the cultures retained markers characteristic of keratinocyte holoclones (stem cells) including p63 expression and telomerase activity. Accordingly we conclude that, in contrast to WS fibroblasts, WS keratinocytes do not demonstrate slow growth rates or features of premature senescence. These findings suggest that the epidermis is among the tissue types that do not display symptoms of premature ageing caused by loss of function of wrn. This is in support that Werner's syndrome is a segmental progeroid syndrome.

Cellular senescence: from growth arrest to immunogenic conversion

Cellular senescence was first reported in human fibroblasts as a state of stable in vitro growth arrest following extended culture. Since that initial observation, a variety of other phenotypic characteristics have been shown to co-associate with irreversible cell cycle exit in senescent fibroblasts. These include (1) a pro-inflammatory secretory response, (2) the up-regulation of immune ligands, (3) altered responses to apoptotic stimuli and (4) promiscuous gene expression (stochastic activation of genes possibly as a result of chromatin remodeling). Many features associated with senescent fibroblasts appear to promote conversion to an immunogenic phenotype that facilitates self-elimination by the immune system. Pro-inflammatory cytokines can attract and activate immune cells, the presentation of membrane bound immune ligands allows for specific recognition and promiscuous gene expression may function to generate an array of tissue restricted proteins that could subsequently be processed into peptides for presentation via MHC molecules. However, the phenotypes of senescent cells from different tissues and species are often assumed to be broadly similar to those seen in senescent human fibroblasts, but the data show a more complex picture in which the growth arrest mechanism, tissue of origin and species can all radically modulate this basic pattern. Furthermore, well-established triggers of cell senescence are often associated with a DNA damage response (DDR), but this may not be a universal feature of senescent cells. As such, we discuss the role of DNA damage in regulating an immunogenic response in senescent cells, in addition to discussing less established "atypical" senescent states that may occur independent of DNA damage.

Telomere structure and telomerase in health and disease (review)

Telomerase is the enzyme responsible for maintenance of the length of telomeres by addition of guanine-rich repetitive sequences. Telomerase activity is exhibited in gametes and stem and tumor cells. In human somatic cells, proliferation potential is strictly limited and senescence follows approximately 50–70 cell divisions. In most tumor cells, on the contrary, replication potential is unlimited. The key role in this process of the system of the telomere length maintenance with involvement of telomerase is still poorly studied. Undoubtedly, DNA polymerase is not capable of completely copying DNA at the very ends of chromosomes; therefore, approximately 50 nucleotides are lost during each cell cycle, which results in gradual telomere length shortening. Critically short telomeres cause senescence, following crisis and cell death. However, in tumor cells the system of telomere length maintenance is activated. Much work has been done regarding the complex telomere/telomerase as a unique target, highly specific in cancer cells. Telomeres have additional proteins that regulate the binding of telomerase. Telomerase, also associates with a number of proteins forming the sheltering complex having a central role in telomerase activity. This review focuses on the structure and function of the telomere/telomerase complex and its altered behavior leading to disease, mainly cancer. Although telomerase therapeutics are not approved yet for clinical use, we can assume that based on the promising in vitro and in vivo results and successful clinical trials, it can be predicted that telomerase therapeutics will be utilized soon in the combat against malignancies and degenerative diseases. The active search for modulators is justified, because the telomere/telomerase system is an extremely promising target offering possibilities to decrease or increase the viability of the cell for therapeutic purposes.

Ageing and its implications

Ageing processes are defined as those that increase the susceptibility of individuals, as they grow older, to the factors that eventually lead to death. It is a complex multi-factorial process, where several factors may interact simultaneously and may operate at many levels of functional organization. The heterogeneity of ageing phenotype among individuals of the same species and differences in longevity among species are due to the contribution of both genetic and environmental factors in shaping the life span. The various theories of ageing and their proposed roles are discussed in this review.

Shelterin complex and associated factors at human telomeres

The processes regulating telomere function have major impacts on fundamental issues in human cancer biology. First, active telomere maintenance is almost always required for full oncogenic transformation of human cells, through cellular immortalization by endowment of an infinite replicative potential. Second, the attrition that telomeres undergo upon replication is responsible for the finite replicative life span of cells in culture, a process called senescence, which is of paramount importance for tumor suppression in vivo. The process of telomere-based senescence is intimately coupled to the induction of a DNA damage response emanating from telomeres, which can be elicited by both the ATM and ATR dependent pathways. At telomeres, the shelterin complex is constituted by a group of six proteins which assembles quantitatively along the telomere tract, and imparts both telomere maintenance and telomere protection. Shelterin is known to regulate the action of telomerase, and to prevent inappropriate DNA damage responses at chromosome ends, mostly through inhibition of ATM and ATR. The roles of shelterin have increasingly been associated with transient interactions with downstream factors that are not associated quantitatively or stably with telomeres. Here, some of the important known interactions between shelterin and these associated factors and their interplay to mediate telomere functions are reviewed.

Accumulation of (5'S)-8,5'-cyclo-2'-deoxyadenosine in organs of Cockayne syndrome complementation group B gene knockout mice

Cockayne syndrome (CS) is a human genetic disorder characterized by sensitivity to UV radiation, neurodegeneration, premature aging among other phenotypes. CS complementation group B (CS-B) gene (csb) encodes the CSB protein (CSB) that is involved in base excision repair of a number of oxidatively induced lesions in genomic DNA in vivo. We hypothesized that CSB may also play a role in cellular repair of the DNA helix-distorting tandem lesion (5'S)-8,5'-cyclo-2'-deoxyadenosine (S-cdA). Among many DNA lesions, S-cdA is unique in that it represents a concomitant damage to both the sugar and base moieties of the same nucleoside. Because of the presence of the C8-C5' covalent bond, S-cdA is repaired by nucleotide excision repair unlike most of other oxidatively induced lesions in DNA, which are subject to base excision repair. To test our hypothesis, we isolated genomic DNA from brain, kidney and liver of wild type and csb knockout (csb(-/-)) mice. Animals were not exposed to any exogenous oxidative stress before the experiment. DNA samples were analysed by liquid chromatography/mass spectrometry with isotope-dilution. Statistically greater background levels of S-cdA were observed in all three organs of csb(-/-) mice than in those of wild type mice. These results suggest the in vivo accumulation of S-cdA in genomic DNA due to lack of its repair in csb(-/-) mice. Thus, this study provides, for the first time, the evidence that CSB plays a role in the repair of the DNA helix-distorting tandem lesion S-cdA. Accumulation of unrepaired S-cdA in vivo may contribute to the pathology associated with CS.

DNA double-strand breaks: a potential causative factor for mammalian aging?

Aging is a pleiotropic and stochastic process influenced by both genetics and environment. As a result the fundamental underlying causes of aging are controversial and likely diverse. Genome maintenance and in particular the repair of DNA damage is critical to ensure longevity needed for reproduction and as a consequence imperfections or defects in maintaining the genome may contribute to aging. There are many forms of DNA damage with double-strand breaks (DSBs) being the most toxic. Here we discuss DNA DSBs as a potential causative factor for aging including factors that generate DNA DSBs, pathways that repair DNA DSBs, consequences of faulty or failed DSB repair and how these consequences may lead to age-dependent decline in fitness. At the end we compare mouse models of premature aging that are defective for repairing either DSBs or UV light-induced lesions. Based on these comparisons we believe the basic mechanisms responsible for their aging phenotypes are fundamentally different demonstrating the complex and pleiotropic nature of this process.

Adaptive stress response in segmental progeria resembles long-lived dwarfism and calorie restriction in mice

How congenital defects causing genome instability can result in the pleiotropic symptoms reminiscent of aging but in a segmental and accelerated fashion remains largely unknown. Most segmental progerias are associated with accelerated fibroblast senescence, suggesting that cellular senescence is a likely contributing mechanism. Contrary to expectations, neither accelerated senescence nor acute oxidative stress hypersensitivity was detected in primary fibroblast or erythroblast cultures from multiple progeroid mouse models for defects in the nucleotide excision DNA repair pathway, which share premature aging features including postnatal growth retardation, cerebellar ataxia, and death before weaning. Instead, we report a prominent phenotypic overlap with long-lived dwarfism and calorie restriction during postnatal development (2 wk of age), including reduced size, reduced body temperature, hypoglycemia, and perturbation of the growth hormone/insulin-like growth factor 1 neuroendocrine axis. These symptoms were also present at 2 wk of age in a novel progeroid nucleotide excision repair-deficient mouse model (XPD^G602D/R722W/XPA^−/−) that survived weaning with high penetrance. However, despite persistent cachectic dwarfism, blood glucose and serum insulin-like growth factor 1 levels returned to normal by 10 wk, with hypoglycemia reappearing near premature death at 5 mo of age. These data strongly suggest changes in energy metabolism as part of an adaptive response during the stressful period of postnatal growth. Interestingly, a similar perturbation of the postnatal growth axis was not detected in another progeroid mouse model, the double-strand DNA break repair deficient Ku80^−/− mouse. Specific (but not all) types of genome instability may thus engage a conserved response to stress that evolved to cope with environmental pressures such as food shortage.

Oxidative damage to cellular components, including fats, proteins, and DNA, is an inevitable consequence of cellular energy use and may underlie both normal and pathological aging. Calorie restriction delays the aging process and extends lifespan in a number of lower organisms including rodents. Inborn defects in the postnatal growth axis resulting in dwarfism can also extend lifespan. Both may function via overlapping pathways impacting on energy metabolism. Here, we report a novel DNA repair-deficient mouse model with symptoms of the related premature aging disorders Cockayne syndrome and trichothiodystrophy, namely reduced fat deposits, neurological dysfunction, failure to thrive, and reduced lifespan. Surprisingly, we also observed traits usually associated with extended longevity as found in calorie restriction and dwarfism, including reduced blood sugar and reduced insulin-like growth factor-1. These characteristics were present at 2 wk of age, that is, during the period of rapid postnatal development, but returned to normal by sexual maturation at 10 wk. Furthermore, they were absent altogether in another premature aging mouse model with a distinct DNA repair defect. Specific types of unrepaired DNA damage may thus elicit a preservative organismal response affecting energy metabolism that is similar to the one that evolved to cope with the stress of food shortage.

What can progeroid syndromes tell us about human aging?

Human genetic diseases that resemble accelerated aging provide useful models for gerontologists. They combine known single-gene mutations with deficits in selected tissues that are reminiscent of changes seen during normal aging. Here, we describe recent progress toward linking molecular and cellular changes with the phenotype seen in two of these disorders. One in particular, Werner syndrome, provides evidence to support the hypothesis that the senescence of somatic cells may be a causal agent of normal aging.

Critical periods in human growth and their relationship to diseases of aging

It has long been recognized that there are "critical periods" during mammalian development when exposure to specific environmental stimuli are required in order to elicit the normal development of particular anatomical structures or their normal functioning. The responses of the organism to these stimuli depend on a specific level of anatomical maturation and a state of rapid anatomical and/or functional change. This discussion of critical periods in growth is not confined to the classic definition of a narrow time frame of development during which a particular environmental threshold or limit must exist for normal growth and function to ensue. Using both auxological and epidemiological approaches, we suggest a lifespan perspective which encompasses accumulating and interacting risks that are manifest from prenatal life onward. By understanding the process of growth development, and by scrutinizing the growth process, early variations that lead to later disease can be identified. Here we review a significant amount of the evidence that links exposure during growth to later morbidity and mortality. The fetus appears to respond to insults during the prenatal period through the process of "programming," which has short-term survival advantages but may have a long-term disadvantage in that it is associated with cardiovascular disease, hypertension, type II diabetes, and later obesity. Low birth weight combined with rapid postnatal growth during infancy also appears to be associated, for instance, with later childhood and adult sequelae in terms of glucose tolerance and obesity. Independent of birth weight, the timing of adiposity rebound during mid-childhood also predicts later obesity. The timing, magnitude, and duration of adolescent growth and maturationare associated with critical body composition changes, including the normal acquisition of body fat and bone mineralization. In particular, the acquisition of appropriate peak bone mass is critical in determining the later risk of osteoporosis. A putative causal mechanism linking early growth variation to later chronic disease risk through telomeric attrition is discussed. The obligatory loss of telomeric DNA with each cell division serves as a mitotic clock and marks the rate of growth and repair processes in the cell. Although much more work is required, existing studies support the notion that telomere shortening is not only a clock of cellular division, but also marks relative growth rate, as well as contributing to common degenerative processes of aging through its impact on cellular senescence.

Primary fibroblasts of Cockayne syndrome patients are defective in cellular repair of 8-hydroxyguanine and 8-hydroxyadenine resulting from oxidative stress

Cockayne syndrome (CS) is a genetic human disease with clinical symptoms that include neurodegeneration and premature aging. The disease is caused by the disruption of CSA, CSB, or some types of xeroderma pigmentosum genes. It is known that the CSB protein coded by the CS group B gene plays a role in the repair of 8-hydroxyguanine (8-OH-Gua) in transcription-coupled and non-strand discriminating modes. Recently we reported a defect of CSB mutant cells in the repair of another oxidatively modified lesion 8-hydroxyadenine (8-OH-Ade). We show here that primary fibroblasts from CS patients lack the ability to efficiently repair these particular types of oxidatively induced DNA damages. Primary fibroblasts of 11 CS patients and 6 control individuals were exposed to 2 Gy of ionizing radiation to induce oxidative DNA damage and allowed to repair the damage. DNA from cells was analyzed using liquid chromatography/isotope dilution mass spectrometry to measure the biologically important lesions 8-OH-Gua and 8-OH-Ade. After irradiation, no significant change in background levels of 8-OH-Gua and 8-OH-Ade was observed in control human cells, indicating their complete cellular repair. In contrast, cells from CS patients accumulated significant amounts of these lesions, providing evidence for a lack of DNA repair. This was supported by the observation that incision of 8-OH-Gua- or 8-OH-Ade-containing oligodeoxynucleotides by whole cell extracts of fibroblasts from CS patients was deficient compared to control individuals. This study suggests that the cells from CS patients accumulate oxidatively induced specific DNA base lesions, especially after oxidative stress. A deficiency in cellular repair of oxidative DNA damage might contribute to developmental defects in CS patients.

Functional crosstalk between hOgg1 and the helicase domain of Cockayne syndrome group B protein

We have previously reported that the Cockayne syndrome group B gene product (CSB) contributes to base excision repair (BER) of 8-hydroxyguanine (8-OH-Gua) and the importance of motifs V and VI of the putative helicase domains of CSB in BER of 8-OH-Gua. To further elucidate the function of CSB in BER, we investigated its role in the pathway involving human 8-OH-Gua glycosylase/apurinic lyase (hOgg1). Depletion of CSB protein with anti-CSB antibody reduced the 8-OH-Gua incision rate of wild type cell extracts but not of CSB null and motif VI mutant cell extracts, suggesting a direct contribution of CSB to the catalytic process of 8-OH-Gua incision and the importance of its motif VI in this pathway. Introduction of recombinant purified CSB partially complemented the depletion of CSB as shown by the recovery of the incision activity. This complementation could not fully recover the deficiency of the incision activity in WCE from CS-B null and mutant cell lines, suggesting that some additional factor(s) are necessary for the full activity. Electrophoretic mobility shift assays (EMSAs) showed a defect in binding of CSB null and motif VI mutant cell extracts to 8-OH-Gua-containing oligonucleotides. We detected less hOgg1 transcript and protein in the cell extracts from CS-B null and mutant cells, suggesting hOgg1 may be the missing component. Pull-down of hOgg1 by histidine-tagged CSB and co-localization of those two proteins after gamma-radiation indicated their co-existence in vivo, particularly under cellular stress. However, we did not detect any functional and physical interaction between purified CSB and hOgg1 by incision, gel shift and yeast two-hybrid assays, suggesting that even though hOgg1 and CSB might be in a common protein complex, they may not interact directly. We conclude that CSB functions in the catalysis of 8-OH-Gua BER and in the maintenance of efficient hOgg1 expression, and that motif VI of the putative helicase domain of CSB is crucial in these functions.

The cockayne syndrome group B gene product is involved in cellular repair of 8-hydroxyadenine in DNA

Cockayne syndrome (CS) is a human disease characterized by sensitivity to sunlight, severe neurological abnormalities, and accelerated aging. CS has two complementation groups, CS-A and CS-B. The CSB gene encodes the CSB protein with 1493 amino acids. We previously reported that the CSB protein is involved in cellular repair of 8-hydroxyguanine, an abundant lesion in oxidatively damaged DNA and that the putative helicase motif V/VI of the CSB may play a role in this process. The present study investigated the role of the CSB protein in cellular repair of 8-hydroxyadenine (8-OH-Ade), another abundant lesion in oxidatively damaged DNA. Extracts of CS-B-null cells and mutant cells with site-directed mutation in the motif VI of the putative helicase domain incised 8-hydroxyadenine in vitro less efficiently than wild type cells. Furthermore, CS-B-null and motif VI mutant cells accumulated more 8-hydroxyadenine in their genomic DNA than wild type cells after exposure to gamma-radiation at doses of 2 or 5 Gy. These results suggest that the CSB protein contributes to cellular repair of 8-OH-Ade and that the motif VI of the putative helicase domain of CSB is required for this activity.

Targeting assay to study the cis functions of human telomeric proteins: evidence for inhibition of telomerase by TRF1 and for activation of telomere degradation by TRF2

We investigated the control of telomere length by the human telomeric proteins TRF1 and TRF2. To this end, we established telomerase-positive cell lines in which the targeting of these telomeric proteins to specific telomeres could be induced. We demonstrate that their targeting leads to telomere shortening. This indicates that these proteins act in cis to repress telomere elongation. Inhibition of telomerase activity by a modified oligonucleotide did not further increase the pace of telomere erosion caused by TRF1 targeting, suggesting that telomerase itself is the target of TRF1 regulation. In contrast, TRF2 targeting and telomerase inhibition have additive effects. The possibility that TRF2 can activate a telomeric degradation pathway was directly tested in human primary cells that do not express telomerase. In these cells, overexpression of full-length TRF2 leads to an increased rate of telomere shortening.

The Cockayne Syndrome group B gene product is involved in general genome base excision repair of 8-hydroxyguanine in DNA

Cockayne Syndrome (CS) is a human genetic disorder with two complementation groups, CS-A and CS-B. The CSB gene product is involved in transcription-coupled repair of DNA damage but may participate in other pathways of DNA metabolism. The present study investigated the role of different conserved helicase motifs of CSB in base excision repair. Stably transformed human cell lines with site-directed CSB mutations in different motifs within its putative helicase domain were established. We find that CSB null and helicase motif V and VI mutants had greater sensitivity than wild type cells to gamma-radiation. Whole cell extracts from CSB null and motif V/VI mutants had lower activity of 8-hydroxyguanine incision in DNA than wild type cells. Also, 8-hydroxyguanine accumulated more in CSB null and motif VI mutant cells than in wild type cells after exposure to gamma-radiation. We conclude that a deficiency in general genome base excision repair of selective modified DNA base(s) might contribute to CS pathogenesis. Furthermore, whereas the disruption of helicase motifs V or VI results in a CSB phenotype, mutations in other helicase motifs do not cause this effect. The biological functions of CSB in different DNA repair pathways may be mediated by distinct functional motifs of the protein.

Telomeres and telomerase: basic science implications for aging

Life expectancy in the United States and other developed nations has increased remarkably over the past century, and continues to increase. However, lifespan has remained relatively unchanged over this period. As life expectancy approaches maximum human lifespan, further increase in life expectancy would only be possible if lifespan could also be increased. Although little is known about the aging process, increasing lifespan and delaying aging are the research challenges of the new century, and have caused intense debate and research activities among biogerontologists. Many theories have been proposed to explain the aging process. However, damage to deoxyribonucleic acid (DNA) is the centerpiece of most of these. Recently telomere shortening has been described to be associated with DNA damage. Located at the ends of eukaryotic chromosomes and synthesized by telomerase, telomeres maintain the length of chromosomes. The loss of telomeres can lead to DNA damage. The association between cellular senescence and telomere shortening in vitro is well established. In the laboratory, telomerase-negative differentiated somatic cells maintain a youthful state, instead of aging, when transfected with vectors encoding telomerase. Many human cancer cells demonstrate high telomerase activity. Evidence is also accumulating that telomere shortening is associated with cellular senescence in vivo. What causes changes in expression of telomerase in different cell types and premature aging syndromes? Does the key to "youthfulness" lie in our ability to control the expression of telomerase? We have reviewed the contemporary literature to find answers to these questions and explore the association between aging, telomeres, and telomerase.

Telomeres, replicative senescence and human ageing

Ageing concerns the extracellular environment and cells that are either post-mitotic or capable of division during life. Primary human cells have a finite division capacity in culture before they enter a state of viable cell cycle arrest termed senescence. Cell division occurs during life in many tissues, either as part of normal tissue function or in response to tissue damage. The accumulation of cells at the end of their replicative lifespan in the elderly might contribute to aged tissue either because of a reduced ability to undergo proliferation or because of the known altered gene-expression patterns of senescent cells. This has been illustrated experimentally using a transgenic telomerase-negative mouse, which shows some premature ageing phenotypes. The mechanism whereby cells count divisions uses the gradual erosion of the ends of chromosomes (telomeres) with cell division caused by the repression of the telomere-maintenance enzyme telomerase in most human cells. Telomere erosion ultimately triggers replicative senescence in many cell types; this can be prevented experimentally by forcibly expressing telomerase. This extends the lifespan of normal human cells and those from progeroid syndromes such as Werner's. Telomere-driven senescence did not evolve to cause ageing, but is instead a by-product of a system devised to provide a tumour-suppression function, a concept that fits well with evolutionary arguments regarding trade-offs between somatic maintenance and reproduction. Work in the future will focus on the development of new animal models to critically address the quantitative significance of this ageing mechanism.

Tubular cell senescence and expression of TGF-beta1 and p21(WAF1/CIP1) in tubulointerstitial fibrosis of aging rats

Kidney aging has been recognized as a chronic process of compromised renal function and structural changes in the tubulointerstitium and glomerulus. Cell senescence is associated with alterations in cell structure and function, including expression of cytokines and structural and regulatory components of extracellular matrix proteins. In this investigation, we tested the hypothesis that senescent renal cells may accumulate in vivo with advancing age. We also evaluated the expression of transforming growth factor (TGF)-beta1 and p21WAF1/CIP1 in aging kidneys. Sprague-Dawley rats at the ages of 3, 12, and 24 months were used for this study. Renal tissues were processed for morphometric and senescence analysis. Expression of TGF-beta1 and p21WAF1/CIP1 was evaluated by Northern or Western blot analysis and immunohistochemistry. Substantial tubulointerstitial injury occurred at the age of 12 months, but significant glomerular structure alteration was observed at the age of 24 months. Tubular cells developed senescence, which was detected by beta-galactosidase staining. This staining increased in frequency and intensity with age. Renal cortices showed a significant increase in the mRNA expression for TGF-beta1 and protein level for p21WAF1/CIP1. The enhanced expression of TGF-beta1 and p21WAF1/CIP1 was localized in the tubulointersititial cells. These data suggest that tubular cells undergo senescence and express increased TGF-beta1 and p21WAF1/CIP1 with advancing age. These age-related cellular and molecular alterations may play an important role in the initiation and/or progression of tubulointerstitial fibrosis and glomerulosclerosis in aging.

Werner's syndrome T lymphocytes display a normal in vitro life-span

Werner's syndrome (WS) is an autosomal recessive disorder displaying many features consistent with accelerated ageing. Fibroblasts from WS patients show a distinct mutator phenotype (characterised by the production of large chromosomal deletions) and a profound reduction in proliferative capacity. The disorder results from a mutation in a novel ReqQ helicase. Recently, we demonstrated that the proliferative defect was corrected by the ectopic expression of telomerase. From these data, we propose that mutations in the wrn gene lead to deletions at or near the telomere which reduce the cells replicative life-span. This hypothesis predicts that cell types which retain the ability to upregulate telomerase as part of their response to a proliferative stimulus would fail to show any significant effect of wrn gene mutations upon life-span. Human T lymphocytes represent a well-characterised example of such a cell type. To test the hypothesis, WS T lymphocytes were cultured until they reached replicative senescence. These cultures displayed life-spans which did not differ significantly from those of normal controls. These findings are consistent with the hypothesis that the effects of wrn mutations on replicative life-span are telomere-mediated.

Physical and functional interaction between p53 and the Werner's syndrome protein

Werner's syndrome is a human autosomal recessive disorder leading to premature aging. The mutations responsible for this disorder have recently been localized to a gene (WRN) encoding a protein that possesses DNA helicase and exonuclease activities. Patients carrying WRN gene mutations exhibit an elevated rate of cancer, accompanied by increased genomic instability. The latter features are also characteristic of the loss of function of p53, a tumor suppressor that is very frequently inactivated in human cancer. Moreover, changes in the activity of p53 have been implicated in the onset of cellular replicative senescence. We report here that the WRN protein can form a specific physical interaction with p53. This interaction involves the carboxyl-terminal part of WRN and the extreme carboxyl terminus of p53, a region that plays an important role in regulating the functional state of p53. A small fraction of WRN can be found in complex with endogenous p53 in nontransfected cells. Overexpression of WRN leads to augmented p53-dependent transcriptional activity and induction of p21(Waf1) protein expression. These findings support the existence of a cross-talk between WRN and p53, which may be important for maintaining genomic integrity and for preventing the accumulation of aberrations that can give rise to premature senescence and cancer.

Drug dosage in the elderly. Is it rational?

Pharmacotherapy in the elderly requires an understanding of the age-dependent changes in function and composition of the body. Aging is characterised by a progressive loss of functional capacities of most if not all organs, a reduction in response to receptor stimulation and homeostatic mechanisms, and a loss of water content and an increase of fat content in the body. The most important pharmacokinetic change in old age is a decrease in the excretory capacity of the kidney; in this regard, the elderly should be considered as renally insufficient patients. The decline in the rate of drug metabolism with advancing age is less marked. In addition, the volume of distribution and the oral bioavailability of drugs may be changed in the elderly compared with younger individuals. Average dosage adjustments for the aged can be derived from simple equations and mean pharmacokinetic parameters from older and younger adults. However, these average dose adjustment factors neglect the large variation in the decline in organ functions among the elderly. Individual dose adjustment factors can be obtained from the drug clearance in a particular patient, where clearance/fractional bioavailability (CL/f) may be calculated from the area under the plasma concentration-time curve (AUC) of the drug in question. Using pharmacokinetic guidelines for dose adjustments, the same plasma drug concentrations result in elderly as in younger adults. However, we are frequently confronted with pharmacodynamic changes in old age which alter the sensitivity to drugs, irrespective of changes in drug disposition. For instance, the sensitivity of the cardiovascular system to beta-adrenergic agonists and antagonists decreases in old age and the incidence of orthostatic episodes in response to drugs that lower blood pressure is increased. The CNS is especially vulnerable in the elderly; agents that affect brain function (anaesthetics, opioids, anticonvulsants, psychotropic drugs) must be used very cautiously in this age group. The increased responsiveness to drugs in the elderly renders the measurement of drug plasma concentrations an attractive method to monitor pharmacotherapy in this age group. Sensitive technology to quantitatively determine plasma drug concentrations is available. However, optimal therapeutic plasma concentrations have not been established for most drugs in the elderly. Investigations concerning drug pharmacokinetic-pharmacodynamic relationships in the aged are an important area of future work in clinical pharmacology.