Repair of telomeric DNA prior to replicative senescence

Genetic mechanisms of aging in plants: What can we learn from them?

Plants hold all records in longevity. Their aging is a complex process. In the presented review, we analyzed published data on various aspects of plant aging with focus on any inferences that could shed a light on aging in animals and help to fight it in human. Plant aging can be caused by many factors, such as telomere depletion, genomic instability, loss of proteostasis, changes in intercellular interaction, desynchronosis, autophagy misregulation, epigenetic changes and others. Plants have developed a number of mechanisms to increase lifespan. Among these mechanisms are gene duplication ("genetic backup"), the active work of telomerases, abundance of meristematic cells, capacity of maintaining the meristems permanently active and continuous activity of phytohormones. Plant aging usually occurs throughout the whole perennial life, but could be also seasonal senescence. Study of causes for seasonal aging can also help to uncover the mechanisms of plant longevity. The influence of different factors such as microbiome communities, glycation, alternative oxidase activity, mitochondrial dysfunction on plant longevity was also reviewed. Adaptive mechanisms of long-lived plants are considered. Further comparative study of the mechanisms underlying longevity of plants is necessary. This will allow us to reach a potentially new level of understanding of the aging process of plants.

Telomere Length and Oxidative Stress and Its Relation with Metabolic Syndrome Components in the Aging

Simple Summary

A link between telomere length and some age-related diseases has been identified, including metabolic syndrome. So far, there is no mechanism to explain the origin or cause of telomere shortening in this syndrome; however, oxidative stress is a constant factor. Therefore, we reviewed scientific evidence that supported the association between oxidative stress and telomere length dynamics, also examining how each of the metabolic syndrome components individually affects the length. In this regard, there is strong scientific evidence that an increase in the number of metabolic syndrome components is associated with a shorter telomere length, oxidative damage at the lipid and DNA level, and inflammation, as well as its other components, such as obesity, hyperglycemia, and hypertension, while for dyslipidemia, there is a little more discrepancy. The difficulty for the correct treatment of metabolic syndrome lies in its multifactorial nature. Hence, there is a need to carry out more studies on healthy lifestyles during aging to prevent and reduce oxidative damage and telomere wear during aging, and consequently the progression of chronic degenerative diseases, thus improving the living conditions of older people.

Abstract

A great amount of scientific evidence supports that Oxidative Stress (OxS) can contribute to telomeric attrition and also plays an important role in the development of certain age-related diseases, among them the metabolic syndrome (MetS), which is characterised by clinical and biochemical alterations such as obesity, dyslipidaemia, arterial hypertension, hyperglycaemia, and insulin resistance, all of which are considered as risk factors for type 2 diabetes mellitus (T2DM) and cardiovascular diseases, which are associated in turn with an increase of OxS. In this sense, we review scientific evidence that supports the association between OxS with telomere length (TL) dynamics and the relationship with MetS components in aging. It was analysed whether each MetS component affects the telomere length separately or if they all affect it together. Likewise, this review provides a summary of the structure and function of telomeres and telomerase, the mechanisms of telomeric DNA repair, how telomere length may influence the fate of cells or be linked to inflammation and the development of age-related diseases, and finally, how the lifestyles can affect telomere length.

Caveolin-1, a master regulator of cellular senescence

Cellular senescence is a feature of most somatic cells. It is characterized by an irreversible cell cycle arrest and by the ability to secrete a plethora of mediators of inflammation and growth factors, which can alter the senescent cell's microenvironment. Senescent cells accumulate in tissues over time and contribute to both aging and the development of age-associated diseases. Senescent cells have antagonistic pleiotropic roles in cancer. Given the inability of senescent cells to proliferate, cellular senescence is a powerful tumor suppressor mechanism in young individuals. However, accumulation of senescent stromal cells during aging can fuel cancer cell growth in virtue of their capacity to release factors that stimulate cell proliferation. Caveolin-1 is a structural protein component of caveolae, invaginations of the plasma membrane involved in a variety of cellular processes, including signal transduction. Mounting evidence over the last 10-15 years has demonstrated a central role of caveolin-1 in the development of a senescent phenotype and the regulation of both the anti-tumorigenic and pro-tumorigenic properties of cellular senescence. In this review, we discuss the cellular mechanisms and functions of caveolin-1 in the context of cellular senescence and their relevance to the biology of cancer.

Association between antidiabetic agents use and leukocyte telomere shortening rates in patients with type 2 diabetes

Telomere length and telomere shortening rate (TSR) are accepted indicators of aging in cross-sectional population studies. This study aimed to investigate the potential influence of common antidiabetic agents on telomere length and TSR in patients with type 2 diabetes mellitus (T2DM). Leukocyte telomere length was measured through terminal restriction fragment analysis, and TSR was calculated in 388 T2DM patients. Depending on whether or not they received antidiabetic medication, patients were first divided into a treatment group and a nontreatment group. Treated patients were further subdivided into an acarbose-free group (patients taking antidiabetic agents without acarbose) and an acarbose group (patients using acarbose for more than 3 months). Results showed that untreated patients had higher TSRs than patients on antidiabetic drugs. Interestingly, patients in the acarbose group had significantly higher TSRs than patients in the acarbose-free group. Compared to the nontreatment group, the acarbose group showed better glycemic control of HbA1c, but the TSR was also higher. Our results suggest that antidiabetic treatments without acarbose can slow aging. By contrast, acarbose may accelerate biological aging in patients with T2DM, independently of glycemic control.

Anti-Mullerian hormone levels in female cancer patients of reproductive age in Indonesia: A cross-sectional study

Background: Efforts in reproductive preservation for cancer patients have become one of the important aspects of cancer management. In fact, decline in reproductive function is known to occur after exposure to anti-cancer treatments. Measuring anti-Müllerian hormone (AMH) levels is known to be the best parameter in predicting ovarian reserves, which indicates reproductive function. In total, 68% of cancer survivors of reproductive age who underwent anti-cancer treatments suffer from infertility. Meanwhile, ovarian reserves also decrease with increasing age. There is ongoing debate on whether the ovarian reserves of cancer patients could be reduced long before exposure to anti-cancer therapy. Therefore, it is important to know whether ovarian reserves in cancer patients decrease before or after anti-cancer therapy. This can help predict the reproductive function in such cases and the effectiveness of ovarian preservation efforts. Methods: A cross-sectional study was conducted, comparing the AMH levels of 44 female cancer patients of reproductive age before cancer therapy, to 44 non-cancer patients of reproductive age (age matched) . The biological ages from both groups were adjusted using the Indonesian Kalkulator of Oocytes. Results: The median age in both groups was 28 years old. The AMH levels in the cancer group were found to be significantly lower in contrast to those in the non-cancer group (1.11 [0.08-4.65] ng/ml vs. 3.99 [1.19- 8.7]; p- value <0.001). Therefore, the biological age in the cancer group was 10 years older than that of the non-cancer group, indicating that ovarian aging occurs earlier in cancer patients. Conclusions: AMH levels of cancer patients of reproductive age were already reduced before cancer therapy, given an older biological age, in contrast to that of the non-cancer patients. Proper counseling and implementation of fertility-preserving methods is highly recommended in this group of patients.

Anti-Mullerian hormone levels in female cancer patients of reproductive age in Indonesia: A cross-sectional study

Background: Efforts in reproductive preservation for cancer patients have become one of the important aspects of cancer management. In fact, decline in reproductive function is known to occur after exposure to anti-cancer treatments. Measuring anti-Müllerian hormone (AMH) levels is known to be the best parameter in predicting ovarian reserves, which indicates reproductive function. In total, 68% of cancer survivors of reproductive age who underwent anti-cancer treatments suffer from infertility. Meanwhile, ovarian reserves also decrease with increasing age. There is ongoing debate on whether the ovarian reserves of cancer patients could be reduced long before exposure to anti-cancer therapy. Therefore, it is important to know whether ovarian reserves in cancer patients decrease before or after anti-cancer therapy. This can help predict the reproductive function in such cases and the effectiveness of ovarian preservation efforts. Methods: A cross-sectional study was conducted, comparing the AMH levels of 44 female cancer patients of reproductive age before cancer therapy, to 44 non-cancer patients of reproductive age (age matched) . The biological ages from both groups were adjusted using the Indonesian Kalkulator of Oocytes. Results: The median age in both groups was 28 years old. The AMH levels in the cancer group were found to be significantly lower in contrast to those in the non-cancer group (1.11 [0.08-4.65] ng/ml vs. 3.99 [1.19- 8.7]; p- value <0.001). Therefore, the biological age in the cancer group was 10 years older than that of the non-cancer group, indicating that ovarian aging occurs earlier in cancer patients. Conclusions: AMH levels of cancer patients of reproductive age were already reduced before cancer therapy, given an older biological age, in contrast to that of the non-cancer patients. Proper counseling and implementation of fertility-preserving methods is highly recommended in this group of patients.

Anti-Mullerian hormone levels in female cancer patients of reproductive age in Indonesia: A cross-sectional study

Background: Efforts in reproductive preservation for cancer patients have become one of the important aspects of cancer management. In fact, decline in reproductive function is known to occur after exposure to anti-cancer treatments. Measuring anti-Müllerian hormone (AMH) levels is known to be the best parameter in predicting ovarian reserves, which indicates reproductive function. In total, 68% of cancer survivors of reproductive age who underwent anti-cancer treatments suffer from infertility. Meanwhile, ovarian reserves also decrease with increasing age. There is ongoing debate on whether the ovarian reserves of cancer patients could be reduced long before exposure to anti-cancer therapy. Therefore, it is important to know whether ovarian reserves in cancer patients decrease before or after anti-cancer therapy. This can help predict the reproductive function in such cases and the effectiveness of ovarian preservation efforts. Methods: A cross-sectional study was conducted, comparing the AMH levels of 44 female cancer patients of reproductive age before cancer therapy, to 44 non-cancer patients of reproductive age (age matched) . The AMH was determined from blood.The biological ages from both groups were adjusted using the Indonesian Kalkulator of Oocytes. Results: The median age in both groups was 28 years old. The AMH levels in the blood of the cancer group were found to be significantly lower in contrast to those in the non-cancer group (1.11 [0.08-4.65] ng/ml vs. 3.99 [1.19- 8.7]; p- value <0.001). Therefore, the biological age in the cancer group was 10 years older than that of the non-cancer group, indicating that ovarian aging occurs earlier in cancer patients. Conclusions: AMH levels of cancer patients of reproductive age were already reduced before cancer therapy, given an older biological age, in contrast to that of the non-cancer patients. Proper counseling and implementation of fertility-preserving methods is highly recommended in this group of patients.

Association of BRCA Mutations and Anti-müllerian Hormone Level in Young Breast Cancer Patients

Background: Several preclinical and clinical studies have suggested that BRCA-mutation carriers may have decreased ovarian reserve. However, data in this area are limited and inconsistent, especially in young breast cancer patients. Objective: This study evaluated the association between BRCA mutation status and serum anti-Müllerian hormone (AMH) level in young, reproductive-aged patients with breast cancer. Materials and Methods: Patients ≤ 40 years of age with breast cancer and who had known BRCA status and baseline serum AMH level at Samsung Medical Center, Seoul, Korea, were considered for inclusion. A total of 52 BRCA mutation carriers (27 BRCA1 and 25 BRCA2) and 264 non-carriers were selected for analyses. The serum level of AMH was compared according to presence of a BRCA mutation, and linear and logistic regression analyses were performed to evaluate the association between BRCA mutation and serum AMH level. Results: No difference was found in clinical characteristics between BRCA-mutation carriers and non-carriers. Subjects with any BRCA mutation had a significantly lower median AMH than those without a mutation (2.60 vs. 3.85 ng/mL, 32% reduction, P = 0.004). Linear regression analysis showed a significant negative association between BRCA mutation and AMH level. In addition, logistic regression demonstrated non-significantly increased odds of mutation carriers having AMH < 1.2 ng/mL. However, no difference was found between BRCA1/2 mutations. Conclusions: Breast cancer patients with BRCA mutation have significantly lower serum AMH level. Fertility preservation should be considered more aggressively in young breast cancer patients with BRCA mutation.

Low-dose levels of bisphenol A inhibit telomerase via ER/GPR30-ERK signalling, impair DNA integrity and reduce cell proliferation in primary PBMC

Controversy exists about the human health risk of environmental exposure to bisphenol A (BPA). Telomerase activity is emerging both as biomarker and contributing factor for age-related diseases. The effects of BPA exposure at 1–1000 nM on telomerase, DNA integrity and cell proliferation were investigated in PBMC from human donors. Telomerase activity was determined by TRAP-ELISA assay and mRNA expression by qRT-PCR. Mechanistic studies were carried out on the ER/GPR30-ERK pathway using specific inhibitors/antagonists, the comet assay to quantify DNA damage and flow cytometry for cell proliferation. 24 h BPA exposure inhibited telomerase in a non-monotonic pattern with a peak inhibition of 32% at 1 nM (p ≤ 0.01). A significant telomerase inhibition was evident at 1 h after exposure with a minimum at 6 h. Elevated levels of DNA damage frequency and decrease in cell proliferation were evident upon long-term exposure. The results further demonstrate that BPA triggered rapidly an ER/GPR30-ERK transduction pathway that leads to decreased telomerase activity in human PBMC. This is the first study to demonstrate adverse impact of BPA at levels of current human exposure on telomerase in normal cells, mediated by ER/GPR30-ERK. The results suggest a potentially harmful influence of BPA on immune cells and should be addressed in future studies.

Telomere Biology of Human Hematopoietic Stem Cells

BACKGROUND

Telomeres are protein DNA structures present at the ends of chromosomes and are essential for genetic stability and cell replication. Telomerase is the enzyme complex that maintains telomere integrity. Hematopoietic stem cells express telomerase and contain long telomeres, which become shorter as cells differentiate and mature. The extent of telomere shortening and the level of telomerase activity often correlate with the presence and severity of some hematopoietic diseases.

METHODS

The fundamentals of telomeres and telomerase are reviewed, and the telomere biology of human hematopoietic cells is discussed.

RESULTS

Telomere length and telomerase activity are important in the self-renewal of hematopoietic stem cells. Changes within these compartments affect both normal hematopoietic cells and the generation of hematopoietic disease. Telomere length provides information pertaining to the proliferative history and potential of a hematopoietic cell.

CONCLUSIONS

The role of telomerase and telomeres within the hematopoietic compartment needs further clarification. Advances in our knowledge in this field may improve clinical outcomes for the treatment of hematologic disease.

Association of BRCA1 Mutations with Impaired Ovarian Reserve: Connection Between Infertility and Breast/Ovarian Cancer Risk

PURPOSE

Mutations in the BRCA1/2 genes are associated with breast and ovarian cancer susceptibility. Recent studies have suggested that the BRCA mutation might be associated with occult primary ovarian insufficiency. To evaluate fertility, several studies have validated anti-Mullerian hormone (AMH) as a direct biomarker for ovarian aging and it is considered a quantitative marker of ovarian reserve. We hypothesize that BRCA1 gene mutations will be negatively associated with AMH levels.

METHODS

We evaluated 124 women aged 18-45 years participating in the Northwestern Ovarian Cancer Early Detection and Prevention Program. Patients with a history of cancer, ovarian surgery, or exposure to chemotherapy were excluded. Linear and logistic regression modeling were performed to evaluate the association between AMH levels, age, and BRCA1 mutation. In logistic models, the outcome 'low AMH' was defined as AMH <0.05 ng/mL. Logistic regression models were used to adjust for other factors, including body mass index (BMI), duration of birth control (BC), smoking, gravidity, and parity.

RESULTS

Women with the BRCA1 mutation had a significant decline in AMH with age (p = 0.0011). BRCA1-positive women >35 years had 10 times the odds of a low AMH (<0.5 ng/mL) compared with women ≤35 years. With adjustment for BMI, duration of BC, smoking, gravidity, parity, and age >35, BRCA1 was still strongly associated with a low AMH (p = 0.037).

CONCLUSION

Women >35 with the BRCA1 mutation have a lower AMH, and hence ovarian reserve, than women without a BRCA mutation. Therefore, young adults with the BRCA1 mutation should be counseled regarding this potential decrease in ovarian reserve.

Identification of telomere dysfunction in Friedreich ataxia

BACKGROUND

Friedreich ataxia (FRDA) is a progressive inherited neurodegenerative disorder caused by mutation of the FXN gene, resulting in decreased frataxin expression, mitochondrial dysfunction and oxidative stress. A recent study has identified shorter telomeres in FRDA patient leukocytes as a possible disease biomarker.

RESULTS

Here we aimed to investigate both telomere structure and function in FRDA cells. Our results confirmed telomere shortening in FRDA patient leukocytes and identified similar telomere shortening in FRDA patient autopsy cerebellar tissues. However, FRDA fibroblasts showed significantly longer telomeres at early passage, occurring in the absence of telomerase activity, but with activation of an alternative lengthening of telomeres (ALT)-like mechanism. These cells also showed accelerated telomere shortening as population doubling increases. Furthermore, telomere dysfunction-induced foci (TIF) analysis revealed that FRDA fibroblasts have dysfunctional telomeres.

CONCLUSIONS

Our finding of dysfunctional telomeres in FRDA cells provides further insight into FRDA molecular disease mechanisms, which may have implications for future FRDA therapy.

Oxidative stress-induced inhibition of Sirt1 by caveolin-1 promotes p53-dependent premature senescence and stimulates the secretion of interleukin 6 (IL-6)

Oxidative stress can induce premature cellular senescence. Senescent cells secrete various growth factors and cytokines, such as IL-6, that can signal to the tumor microenvironment and promote cancer cell growth. Sirtuin 1 (Sirt1) is a class III histone deacetylase that regulates a variety of physiological processes, including senescence. We found that caveolin-1, a structural protein component of caveolar membranes, is a direct binding partner of Sirt1, as shown by the binding of the scaffolding domain of caveolin-1 (amino acids 82-101) to the caveolin-binding domain of Sirt1 (amino acids 310-317). Our data show that oxidative stress promotes the sequestration of Sirt1 into caveolar membranes and the interaction of Sirt1 with caveolin-1, which lead to inhibition of Sirt1 activity. Reactive oxygen species stimulation promotes acetylation of p53 and premature senescence in wild-type but not caveolin-1 null mouse embryonic fibroblasts (MEFs). Either down-regulation of Sirt1 expression or re-expression of caveolin-1 in caveolin-1 null MEFs restores reactive oxygen species-induced acetylation of p53 and premature senescence. In addition, overexpression of caveolin-1 induces stress induced premature senescence in p53 wild-type but not p53 knockout MEFs. Phosphorylation of caveolin-1 on tyrosine 14 promotes the sequestration of Sirt1 into caveolar membranes and activates p53/senescence signaling. We also identified IL-6 as a caveolin-1-specific cytokine that is secreted by senescent fibroblasts following the caveolin-1-mediated inhibition of Sirt1. The caveolin-1-mediated secretion of IL-6 by senescent fibroblasts stimulates the growth of cancer cells. Therefore, by inhibiting Sirt1, caveolin-1 links free radicals to the activation of the p53/senescence pathway and the protumorigenic properties of IL-6.

BRCA mutation carriers do not have compromised ovarian reserve

Controversy exists about the impact of BRCA1/2 mutations on female fertility. Previous studies are small or based on indirect parameters (eg, self-reported infertility), which depend on additional factors unrelated to true fertility potential. Most of the previous studies did not use strict fertility markers.

OBJECTIVE

The aim of this study is to evaluate the relation between carrying a BRCA1/2 mutation and fertility using the level of anti-müllerian hormone (AMH), which has been previously shown to be an accurate marker of ovarian reserve and fertility potential.

PATIENTS AND METHODS

Forty-one healthy BRCA1/2 mutation carriers, aged 26 to 40 years, attending a multidisciplinary breast and ovarian cancer surveillance clinic, were tested for AMH levels using a 2-site ELISA. Levels were compared with those of our general population and with well-established normograms of the general population.

RESULTS

The mean age of carriers was 33.2 years (26-39 years; SD, 3.99 years). The mean parity of carriers was 1.97 (0-7; SD, 1.49). All women carried at least 1 Ashkenazi Jewish founder mutation. The AMH levels for most carriers were in the reference range, 2.71 ± 0.59 ng/mL (approximately 50th percentile of normograms). These levels were similar to those in the control group, in which the AMH levels were 2.02 ± 0.12 ng/mL (P = 0.27).

CONCLUSIONS

The AMH levels of healthy BRCA1/2 mutation carriers are similar to those of noncarrier women matched for age; therefore, their ovarian reserve is comparable. This is the only study, to the best of our knowledge, that directly examines ovarian reserve in a relatively large group of carriers with an accurate marker. The results of this study may possibly give reassurance to female carriers concerning fertility potential.

Proteomic, cellular, and network analyses reveal new DUSP3 interactions with nucleolar proteins in HeLa cells

DUSP3 (or Vaccinia virus phosphatase VH1-related; VHR) is a small dual-specificity phosphatase known to dephosphorylate c-Jun N-terminal kinases and extracellular signal-regulated kinases. In human cervical cancer cells, DUSP3 is overexpressed, localizes preferentially to the nucleus, and plays a key role in cellular proliferation and senescence triggering. Other DUSP3 functions are still unknown, as illustrated by recent and unpublished results from our group showing that this enzyme mediates DNA damage response or repair processes. In this study, we sought to identify new interactions between DUSP3 and proteins directly or indirectly involved in or correlated with its biological roles in HeLa cells exposed to gamma or UV radiation. By using GST-DUSP as bait, we pulled down interacting proteins and identified them by LC-MS/MS. Of the 46 proteins obtained, six hits were extensively validated by immune techniques; the proteins Nucleophosmin, HnRNP C1/C2, and Nucleolin were the most promising targets found to directly interact with DUSP3. We then analyzed the DUSP3 interactomes using physical protein-protein interaction networks using our hits as the seed list. The validated hits as well as unvalidated hits fluctuated on the DUSP3 interactomes of HeLa cells, independent of the time post radiation, which confirmed our proteomic and experimental data and clearly showed the proximity of DUSP3 to proteins involved in processes intimately related to DNA repair and senescence, such as Ku70 and Tert, via interactions with nucleolar proteins, which were identified in this study, that regulate DNA/RNA structure and functions.

Telomere, aging and age-related diseases

Aging is an inevitable biological process that affects most living organisms. The process of aging is regulated at the level of the organism, as well as at the level of tissues and cells. Despite the enormous consequences associated with the aging process, relatively little systematic effort has been expended on the scientific understanding of this important life process. Many theories have been proposed to explain the aging process, the centerpiece of which is molecular damage. Located at the ends of eukaryotic chromosomes and synthesized by telomerase, telomeres maintain the stabilization of chromosomes. Thus, the loss of telomeres may lead to DNA damage. The relationship between cellular senescence and telomere shortening is well established. Furthermore, telomere attrition occurs with age, and is proposed to be a fundamental factor in the aging process. Here, we review the contemporary literatures to explore the current views on the correlation of telomere loss and telomerase action with aging and age-related diseases.

Frequency of premature menopause in women who carry a BRCA1 or BRCA2 mutation

OBJECTIVE

To evaluate the impact of carrying a BRCA1 or BRCA2 mutation on the probability of experiencing premature natural menopause.

DESIGN

Observational study.

SETTING

Patients in an academic research environment.

PATIENT(S)

Women who carry a BRCA1 or BRCA2 mutation (case subjects) and women who do not carry a mutation (control subjects).

INTERVENTION(S)

Survey about reproductive history administered on study entry and every 2 years thereafter.

MAIN OUTCOME MEASURE(S)

The impact of carrying a BRCA mutation on age at menopause and other factors, including parity, age at first birth, age at last birth, and self-reported fertility.

RESULT(S)

A total of 908 matched pairs were identified. The mean age at natural menopause was 48.8 years for BRCA1 carriers, 49.2 years for BRCA2 carriers, and 50.3 years for control subjects. Women who carried a BRCA mutation had parity similar to noncarriers and were as likely as noncarriers to have a child after age 35 years. Similar proportions reported a history of fertility problems (12.5% vs. 13.7%) and use of fertility medication (6.0% vs. 7.0%).

CONCLUSION(S)

Women who carry a BRCA mutation experience menopause earlier, on average, than women who do not have a mutation, but the difference is small and does not appear to affect fertility.

Collapse of telomere homeostasis in hematopoietic cells caused by heterozygous mutations in telomerase genes

Telomerase activity is readily detectable in extracts from human hematopoietic stem and progenitor cells, but appears unable to maintain telomere length with proliferation in vitro and with age in vivo. We performed a detailed study of the telomere length by flow FISH analysis in leukocytes from 835 healthy individuals and 60 individuals with reduced telomerase activity. Healthy individuals showed a broad range in average telomere length in granulocytes and lymphocytes at any given age. The average telomere length declined with age at a rate that differed between age-specific breakpoints and between cell types. Gender differences between leukocyte telomere lengths were observed for all cell subsets studied; interestingly, this trend could already be detected at birth. Heterozygous carriers for mutations in either the telomerase reverse transcriptase (hTERT) or the telomerase RNA template (hTERC) gene displayed striking and comparable telomere length deficits. Further, non-carrier relatives of such heterozygous individuals had somewhat shorter leukocyte telomere lengths than expected; this difference was most profound for granulocytes. Failure to maintain telomere homeostasis as a result of partial telomerase deficiency is thought to trigger cell senescence or cell death, eventually causing tissue failure syndromes. Our data are consistent with these statements and suggest that the likelihood of similar processes occurring in normal individuals increases with age. Our work highlights the essential role of telomerase in the hematopoietic system and supports the notion that telomerase levels in hematopoietic cells, while limiting and unable to prevent overall telomere shortening, are nevertheless crucial to maintain telomere homeostasis with age.

Human blood cells all originate from a common precursor, the hematopoietic stem cell. Telomerase, the enzyme responsible for adding telomere repeats to chromosome ends, is active in human hematopoietic stem cells but appears unable to maintain a constant telomere length with age. We first document the telomere length of different blood cell subsets from 835 healthy individuals between birth and 100 years, to delineate the normal rate of telomere attrition with age. Telomere lengths of blood cells were found to be slightly longer in women than in men, from birth and throughout life. We then compared this reference data to the telomere length in similar blood cell subsets from individuals with reduced telomerase activity as a result of a mutation in one of the genes encoding telomerase and from their direct relatives. Strikingly short telomeres were found in telomerase-deficient individuals, consistent with their cellular pathology and disease susceptibility, and somewhat shorter telomeres than expected were found in cells of relatives with normal telomerase maintenance. Our data can be used as a reference for blood cell telomere length in studies of normal and accelerated aging.

Caveolin-1, cellular senescence and age-related diseases

According to the "free radical theory" of aging, normal aging occurs as the result of tissue damages inflicted by reactive oxygen species (ROS) when ROS production exceeds the antioxidant capacity of the cell. ROS induce cellular dysfunctions such as stress-induced premature senescence (SIPS), which is believed to contribute to normal organismal aging and play a role in age-related diseases. Consistent with this hypothesis, increased oxidative damage of DNA, proteins, and lipids have been reported in aged animals and senescent cells accumulate in vivo with advancing age. Caveolin-1 acts as a scaffolding protein that concentrates and functionally regulates signaling molecules. Recently, great progress has been made toward understanding of the role of caveolin-1 in stress-induced premature senescence. Data show that caveolin-mediated signaling may contribute to explain, at the molecular level, how oxidative stress promotes the deleterious effects of cellular senescence such as aging and age-related diseases. In this review, we discuss the cellular mechanisms and functions of caveolin-1 in the context of SIPS and their relevance to the biology of aging.

SegMine workflows for semantic microarray data analysis in Orange4WS

Background

In experimental data analysis, bioinformatics researchers increasingly rely on tools that enable the composition and reuse of scientific workflows. The utility of current bioinformatics workflow environments can be significantly increased by offering advanced data mining services as workflow components. Such services can support, for instance, knowledge discovery from diverse distributed data and knowledge sources (such as GO, KEGG, PubMed, and experimental databases). Specifically, cutting-edge data analysis approaches, such as semantic data mining, link discovery, and visualization, have not yet been made available to researchers investigating complex biological datasets.

Results

We present a new methodology, SegMine, for semantic analysis of microarray data by exploiting general biological knowledge, and a new workflow environment, Orange4WS, with integrated support for web services in which the SegMine methodology is implemented. The SegMine methodology consists of two main steps. First, the semantic subgroup discovery algorithm is used to construct elaborate rules that identify enriched gene sets. Then, a link discovery service is used for the creation and visualization of new biological hypotheses. The utility of SegMine, implemented as a set of workflows in Orange4WS, is demonstrated in two microarray data analysis applications. In the analysis of senescence in human stem cells, the use of SegMine resulted in three novel research hypotheses that could improve understanding of the underlying mechanisms of senescence and identification of candidate marker genes.

Conclusions

Compared to the available data analysis systems, SegMine offers improved hypothesis generation and data interpretation for bioinformatics in an easy-to-use integrated workflow environment.

Fertility in women with BRCA mutations: a case-control study

OBJECTIVE

To investigate whether or not fertility is reduced in women carrying mutations in the BRCA genes (BRCA1 and BRCA2), compared with noncarrier family members.

DESIGN

Matched case-control study.

SETTING

Academic.

PATIENT(S)

A total of 2,254 BRCA carriers and 764 noncarrier controls who were from the same families as the carriers, but tested negative for the BRCA mutation.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

History of fertility problems (yes/no) and prior use of fertility medications (yes/no) in cases and controls, obtained through administered questionnaire.

RESULT(S)

There was no difference in mean parity between carriers (1.9) and noncarriers (1.9).

CONCLUSION(S)

This study, which is the first epidemiologic study to specifically investigate the effect of carrying a BRCA mutation on parity and fertility, suggests that there is likely little or no effect of the BRCA gene mutation on fertility. To our knowledge previous epidemiologic studies have not investigated the effect of carrying a BRCA mutation on parity or fertility. However, we expect that if fertility differences exist, they would be toward the limits of reproductive life, thus larger studies with age-stratified analyses are needed to definitively answer this question.

Genome-health nutrigenomics and nutrigenetics: nutritional requirements or ‘nutriomes’ for chromosomal stability and telomere maintenance at the individual level

It is becoming increasingly evident that (a) risk for developmental and degenerative disease increases with more DNA damage, which in turn is dependent on nutritional status, and (b) the optimal concentration of micronutrients for prevention of genome damage is also dependent on genetic polymorphisms that alter the function of genes involved directly or indirectly in the uptake and metabolism of micronutrients required for DNA repair and DNA replication. The development of dietary patterns, functional foods and supplements that are designed to improve genome-health maintenance in individuals with specific genetic backgrounds may provide an important contribution to an optimum health strategy based on the diagnosis and individualised nutritional prevention of genome damage, i.e. genome health clinics. The present review summarises some of the recent knowledge relating to micronutrients that are associated with chromosomal stability and provides some initial insights into the likely nutritional factors that may be expected to have an impact on the maintenance of telomeres. It is evident that developing effective strategies for defining nutrient doses and combinations or ‘nutriomes’ for genome-health maintenance at the individual level is essential for further progress in this research field.

Enhancement of extra chromosomal recombination in somatic cells by affecting the ratio of homologous recombination (HR) to non-homologous end joining (NHEJ)

Advancements in somatic cell gene targeting have been slow due to the finite lifespan of somatic cells and the overall inefficiency of homologous recombination. The rate of homologous recombination is determined by mechanisms of DNA repair, and by the balance between homologous recombination (HR) and non-homologous end joining (NHEJ). A plasmid-to-plasmid, extra chromosomal recombination system was used to study the effects of the manipulation of molecules involved in NHEJ (Mre11, Ku70/80, and p53) on HR/NHEJ ratios. In addition, the effect of telomerase expression, cell synchrony, and DNA nuclear delivery was examined. While a mutant Mre11 and an anti-Ku aptamer did not significantly affect the rate of NHEJ or HR, transient expression of a p53 mutant increased overall HR/NHEJ by 2.5 fold. However, expression of the mutant p53 resulted in increased aneuploidy of the cultured cells. Additionally, we found no relationship between telomerase expression and changes in HR/NHEJ. In contrast, cell synchrony by thymidine incorporation did not induce chromosomal abnormalities, and increased the ratio of HR/NHEJ 5-fold by reducing the overall rate of NHEJ. Overall our results show that attempts at reducing NHEJ by use of Mre11 or anti-Ku aptamers were unsuccessful. Cell synchrony via thymidine incorporation, however, does increase the ratio of HR/NHEJ and this indicates that this approach may be of use to facilitate targeting in somatic cells by reducing the numbers of colonies that need to be analyzed before a HR is identified.

Telomerase with mutated catalytic motifs has dominant negative effects on telomerase activity and inhibits cell growth

Telomerase catalytic subunit (TERT) seems a key factor controlling telomerase activity, telomere length, and cell growth. To further address this issue, we forced expression of a catalytically inactive mutant human TERT (hTERT) in hTERT-immortalised sheep fibroblasts to examine its effects. Expression of mutant hTERT compromised telomerase activity reconstituted by wild-type hTERT in a manner directly attributable to mutant hTERT expression level. High levels of mutant hTERT expression inhibited cell growth with a subset of cells entering replicative senescence. Furthermore, significant telomere attrition was evident in two of three clones with high levels of mutant hTERT expression. Our findings are consistent with the notion that hTERT homodimers are necessarily required to form a functional telomerase complex at the telomere substrate. We also highlight the requirement of a more thorough understanding of telomerase- and telomere-associated factors to understand fully the interplay that governs telomere homeostasis in vitro and in vivo.

Role of Telomerase in Hematopoietic Stem Cells

Studies in hematopoietic stem cell (HSC) biology are often focused on "self-renewal" and differentiation. Implicit in the word self-renewal is that the two daughter cells generated by a self-renewal division are identical to the parental cell. Strictly speaking, this is not possible because DNA is continuously damaged and repaired by DNA-repair mechanisms that are not 100% efficient. It is important to note that the efficiency of DNA repair varies greatly among different stem cell types. For example, embryonic stem cells are quite resistant to DNA damage and maintain the length of telomere repeats on serial passage, whereas HSCs are quite sensitive to DNA damage and less able to maintain telomere length. Most likely, differences between stem cell types in DNA repair and telomere maintenance pathways coevolved with cell mass, turnover, reproductive strategy, and life span. This idea has given rise to the notion that many aspects of normal aging could primarily reflect limitations in DNA repair and telomere-maintenance pathways in the (stem) cells of the soma. In humans, levels of telomerase in HSCs are under extremely tight control, as is illustrated by the marrow failure in patients with (mild) telomerase deficiencies. Here, the role of telomerase in human HSC biology is reviewed, and it is proposed that telomerase has an important role in the repair of G-rich DNA.

The significance of telomeric aggregates in the interphase nuclei of tumor cells

Telomeres are TTAGGG repetitive motifs found at the ends of vertebrate chromosomes. In humans, telomeres are protected by shelterin, a complex of six proteins (de Lange [2005] Genes Dev. 19: 2100-2110). Since (Müller [1938] Collecting Net. 13: 181-198; McClintock [1941] Genetics 26: 234-282), their function in maintaining chromosome stability has been intensively studied. This interest, especially in cancer biology, stems from the fact that telomere dysfunction is linked to genomic instability and tumorigenesis (Gisselsson et al. [2001] Proc. Natl. Acad. Sci. USA 98: 12683-12688; Deng et al. [2003] Genes Chromosomes Cancer 37: 92-97; DePinho and Polyak [2004] Nat. Genetics 36: 932-934; Meeker et al. [2004] Clin. Cancer Res. 10: 3317-3326). In the present overview, we will discuss the role of telomeres in genome stability, recent findings on three-dimensional (3D) changes of telomeres in tumor interphase nuclei, and outline future avenues of research.

The concept of telomeric non-reciprocal recombination (TENOR) applied to human fibroblasts grown in serial cultures: concordance with genealogical data

Since the discovery of the limited life span of human fibroblasts some 50 years ago, many genealogical studies have been undertaken to describe growth kinetics of fibroblasts in serial cultures by their individual division behavior. It is now accepted that proliferation capacities of human fibroblasts strongly depend on their telomere lengths and integrity. Telomeres shorten with each replication round, and there is a direct correlation between cell division capacity and telomere lengths; that is, the consumption of disposable telomeric DNA repeats during cell divisions progresses until critically short telomeres determining the replicative senescence of the cells are present. Recently, we have suggested that telomeres in fibroblasts can also become elongated during DNA replication by telomeric non-reciprocal recombination (TENOR). Here we discuss genealogical data collected over the last decades as well as more recent findings on the telomere-driven replicative senescence process, and we summarize both to give an integrated picture.

Telomere shortening is associated with malformation in p53-deficient mice after irradiation during specific stages of development

The natural ends of linear chromosomes, the telomeres, recruit specific proteins in the formation of protective caps that preserve the integrity of the genome. Unprotected chromosomes induce DNA damage checkpoint cascades and ultimately lead to senescence both in mouse and man in a p53 dependent manner and initial telomere length setting therefore determines the proliferative capacity of each cell. Yet, only little information is available on telomere biology during embryonic development. We have previously shown that the p53 gene plays a crucial role in the development of malformations (exencephaly, gastroschisis, polydactyly, cleft palate and dwarfism) in control and irradiated mouse embryos. Here, we investigated telomere biology and the outcome of radiation exposure in wild type (p53+/+) and p53-mutant (p53+/-- and--/--) C57BL mouse foetuses irradiated at three different developmental stages. We show that telomeres are significantly shorter in malformed foetuses as compared to normal counterparts. In addition, our results indicate that the observed telomere attrition is primarily associated with p53-deficiency but is also modulated by irradiation, more specifically during the gastrulation and organogenesis stages. In conclusion, we formulate a hypothesis in which telomere shortening is linked to the absence of p53 in mouse foetuses and that when, in the presence of shorter telomeres, these foetuses are irradiated, the chance for the occurrence of developmental defects increases substantially.

Deficient DNA repair in the human progeroid disorder, Werner syndrome

The study of how DNA repair mechanisms change with aging is central to our understanding of the aging process. Here, I review the molecular functions of a key aging protein, Werner protein (WRN), which is deficient in the premature aging disorder, Werner syndrome (WS). This protein plays a significant role in DNA repair, particularly in base excision repair and in recombination. WRN may be a key regulatory factor in these processes and may also play a role in coordinating them. WRN belongs to the RecQ helicase family of proteins, often referred to as the guardians of the genome. These proteins appear to integrate with the more classic DNA repair pathways and proteins.

The Genome Health Clinic and Genome Health Nutrigenomics concepts: diagnosis and nutritional treatment of genome and epigenome damage on an individual basis

The evidence of a direct link between increased genome/epigenome damage and elevated risk for adverse health outcomes during the various stages of life, such as infertility, foetal development and cancer is becoming increasingly stronger. The latter is briefly reviewed against a background of evidence indicating that genome and epigenome damage biomarkers, in the absence of overt exposure of genotoxins, are themselves sensitive indicators of deficiency in micronutrients required as cofactors or as components of DNA repair enzymes, for maintenance methylation of CpG sequences and prevention of DNA oxidation and/or uracil incorporation into DNA. The latter is illustrated with cross-sectional and dietary intervention data obtained using the micronucleus assay and other efficient biomarkers for diagnosing genome and/or epigenome instability. The concept of recommended dietary allowances for genome stability and how this could be achieved is discussed. The 'Genome Health Nutrigenomics' concept is also introduced to define and focus attention on the specialized research area of how diet impacts on genome stability and how genotype determines nutritional requirements for genome health maintenance. The review concludes with a vision for a paradigm shift in disease prevention strategy based on the diagnosis and nutritional treatment of genome/epigenome damage on an individual basis, i.e. The Genome Health Clinic.

Repair of cyclobutane pyrimidine dimers or dimethylsulfate damage in DNA is identical in normal or telomerase-immortalized human skin fibroblasts

The progression of a normal cell to senescence in vivo and in vitro is accompanied by a reduction in the length of the telomeres, the chromosome capping segments at the end of each linkage group. However, overexpression of the reverse transcriptase subunit (HTERT) of the ribonucleoprotein telomerase restores telomere length and delays cellular senescence. Although some data exist in the literature with respect to survival, no molecular data have shown that DNA repair in telomerase-immortalized cells is normal. Several telomerase-immortalized human skin fibroblast cell lines were constructed from a primary human fibroblast cell line. The primary line and the telomerase-immortalized cell lines were treated with either ultraviolet (UV) radiation or dimethylsulfate (DMS). UV radiation principally produces cyclobutane pyrimidine dimers that are repaired by nucleotide excision repair, whereas DMS introduces mainly N-methylpurines repaired by base excision repair. Here, we show that repair of both types of damage in the telomerase-immortalized human skin fibroblast cell lines is identical to repair observed in normal skin fibroblasts. Thus, telomerase expression and consequent immortalization of skin fibroblasts do not alter nucleotide or base excision repair in human cells.

Telomere loss, senescence, and genetic instability in CD4+ T lymphocytes overexpressing hTERT

Little is known about the long-term consequences of overexpression of the human telomerase reverse transcriptase (hTERT) gene in T lymphocytes. To address this issue, we transduced polyclonal as well as clonally derived populations of naive and memory CD44 T cells from 2 healthy donors (aged 24 and 34 years) with retroviral vectors encoding green fluorescence protein (GFP) and hTERT (GFP-hTERT) or GFP alone. After transduction, cells were sorted on the basis of GFP expression and cultured in vitro until senescence. T cells transduced with hTERT exhibited high stable telomerase activity throughout the culture period. Relative to GFP controls, minor changes in overall gene expression were observed yet the proliferative lifespan of the hTERT-transduced populations was significantly increased and the rate of telomere loss was lower. Nevertheless, hTERT-transduced cells showed progressive telomere loss and had shorter telomeres at senescence than controls (2.3 +/- 0.3 kilobase [kb] versus 3.4 +/- 0.1 kb). Furthermore, a population of cells with 4N DNA consisting of binucleated cells with connected nuclei emerged in the hTERT-transduced cells prior to senescence. We conclude that overexpression of hTERT in CD4+ T cells provides a proliferative advantage independent of the average telomere length but does not prevent eventual genetic instability and replicative senescence.

Tissue-specific changes of DNA repair protein Ku and mtHSP70 in aging rats and their retardation by caloric restriction

To provide an improved understanding of the molecular basis of the aging process, it is necessary to measure biological age on a tissue-specific basis. The role of DNA damage has emerged as a significant mechanism for determination of life span, and DNA repair genes and stress-response genes are also implicated in the aging process. In the present study, we investigated the changes of DNA-PK activity, especially Ku activity, in the various tissues including kidney, lung, testis and liver during aging and its correlation with mtHSP70 expression. We showed that the modulation of Ku activity during the aging process was highly tissue-specific as shown with highly impaired Ku activity in testis and unaffected Ku activity in liver with age, and the level of Ku70 or Ku80 was differentially expressed in each aging tissue. We found also that age-associated alteration of Ku70/80 was prevented or not prevented by caloric restriction (CR) in a tissue-specific manner. Age-related decline in Ku70 during the aging process was associated with the increase of mtHSP70, which could play a role as a predictive marker for aging related to Ku regulation, and CR retarded aging-induced mtHSP70.

Sister chromatid separation at human telomeric regions

Telomeres are nucleoprotein complexes located at chromosome ends, vital for preserving chromosomal integrity. Telomeric DNA shortens with progressive rounds of cell division, culminating in replicative senescence. Previously we have reported, on the basis of fluorescent in situ hybridization, that several human telomeric regions display solitary signals (singlets) in metaphase cells of presenescent fibroblasts, in comparison to other genomic regions that hybridize as twin signals (doublets). In the current study, we show that an additional 12 out of 12 telomeric regions examined also display metaphase singlet signals in pre-senescent cells, and that excess telomere-metaphase singlets also occur in earlier passage cells harvested from elderly individuals. In cancer cell lines expressing telomerase and in pre-senescent fibroblasts ectopically expressing hTERT, this phenomenon is abrogated. Confocal microscope image analysis showed that the telomere metaphase singlets represent regions that have replicated but not separated; this is presumably because of persistent cohesion. The introduction of mutations that interfere with the normal dissolution of cohesion at the metaphase to anaphase transition induced the cut (chromosomes untimely torn) phenotype in early passage fibroblasts, with predominantly telomeric rather than centromeric DNA, present on the chromatin bridges between the daughter nuclei. These results suggest that telomeric regions in animal cells may potentially be sites of persistent cohesion, and that this cohesion may be the basis for an observed excess of fluorescent in situ hybridization metaphase singlets at telomeres. Persistent cohesion at telomeres may be associated with attempted DNA repair or chromosomal abnormalities, which have been described in pre-senescent cells.

The shortest telomeres drive karyotype evolution in transformed cells

Maintenance of telomeres is essential for chromosome stability. In the absence of telomerase, telomeres shorten with cell division until they approach a stability threshold, at which point cells enter senescence. When senescence-signaling pathways are inactive, further telomere shortening leads to chromosome instability characterized by telomeric fusions and breakage-fusion-bridge (BFB) cycles. Since the distribution of telomere lengths among chromosome extremities is heterogeneous, we wondered about the impact of such variability on the stability of particular chromosome arms. We correlated the initial length of individual telomeres in telomerase-negative-transformed cells with the stability of the corresponding chromosome arms during the precrisis period. We show that arms carrying the shortest telomeres are the first to become unstable and this instability affects the chromosome homologues with shorter telomeres almost exclusively. The analysis of several postcrisis cell populations, which had stabilized their telomeres by re-expressing telomerase, showed that the karyotypic outcome is strongly influenced by the initial telomere length heterogeneity. The timing of telomerase re-expression also seems to play a role in limiting the extent of karyotypic changes, probably by reducing the frequency of telomeric fusions and hence BFB. Since the distribution of telomere lengths within somatic cells is proper to every individual, our results predict that the risk for a particular chromosome arm of becoming unstable early in tumorigenesis will differ between individuals and contribute directly to the heterogeneity of chromosome aberrations found in tumors.

Enhanced telomere shortening in transformed lymphoblasts from patients with X linked dyskeratosis

AIM

Dyskeratosis congenita (DC) is characterised by the failure of those tissues that are rapidly dividing in the adult, particularly the skin, mucosae, and haemopoietic system. The X linked form of the disease is caused by mutations of the DKC1 gene, which encodes dyskerin, a protein that is necessary for the function of telomerase. Cultured DC lymphoblastoid cells are characterised by a reduced expansion of the cell population because of the progressive increase in apoptosis compared with the number of cell divisions. This report aimed to verify whether this is caused by a defect in telomerase function.

METHODS

Variations in telomere length over time were evaluated in two cultured lymphoblastoid cell lines derived from patients with X linked DC and control cells derived from a non-affected individual. In addition, the effect of inhibiting poly (ADP-ribose) polymerase (PARP), which is involved in the cellular response to excessive telomere shortening, was assessed. One DC cell line and the control cells were treated with the specific PARP inhibitor 1,5-dihydroxyquinoline (IQ).

RESULTS

In DC cells the increase in cell death was associated with progressive telomere shortening, and this was not seen in the control cells. Treatment with IQ delayed the increase of apoptosis in DC cells.

CONCLUSIONS

These observations indicate that the reduced expansion that characterises cultured cells obtained from patients with X linked DC is caused by premature telomere shortening.

Telomerase levels control the lifespan of human T lymphocytes

The loss of telomeric DNA with each cell division contributes to the limited replicative lifespan of human T lymphocytes. Although telomerase is transiently expressed in T lymphocytes upon activation, it is insufficient to confer immortality. We have previously shown that immortalization of human CD8+ T lymphocytes can be achieved by ectopic expression of the human telomerase reverse transcriptase (hTERT) gene, which encodes for the catalytic component of the telomerase complex. To study the role of endogenous hTERT in the lifespan of human T cells, we blocked endogenous hTERT expression by ectopic expression of dominant-negative (DN) hTERT. Cells expressing DN-hTERT had a decreased lifespan and showed cytogenetic abnormalities, including chromosome ends without detectable telomeric DNA as well as chromosome fusions. These results indicate that while endogenous hTERT cannot prevent overall telomere shortening, it has a major influence on the longevity of human T cells. Furthermore, we show that up-regulation of hTERT in T cells upon activation decreases over time in culture. Long-term-cultured T cells also show a decreased expression of c-myc upon activation, resulting in less c-myc-induced transcription of hTERT. Moreover, memory T cells, which have expanded in vivo upon antigen encounter, expressed a lower level of hTERT upon activation than naive cells from the same donor. The observed inverse correlation between telomerase levels and replicative history suggests that telomerase levels in T cells are limiting and increasingly insufficient to sustain their proliferation.

Mouse and human cells versus oxygen

Mice and humans are at opposite ends of the mammalian spectrum of longevity. A major question in biology is whether this difference can be accounted for by differences in the properties of cells from these two species. A new publication from Judith Campisi's lab reports that human cells in culture are more resistant than mouse cells to the damaging effects of 20% oxygen. The greater burden of DNA damage sustained by mouse cells causes them to rapidly enter a phase of culture in which most cells enter permanent growth arrest (replicative senescence). However, some mouse cells usually escape from senescence and then grow into an immortal cell line. This never happens in human fibroblast cell cultures. Human cells also eventually enter replicative senescence in culture, but this phenomenon is caused by shortening of telomeres and not by DNA damage of the type responsible for mouse cell senescence. Human fibroblasts never spontaneously escape from senescence. This Perspective reviews differences between mouse and human cells that could account for these differences in behavior. Some evidence indicates that human cells are generally more resistant than mouse cells to oxidative damage to DNA, but more needs to be done to confirm this finding and to understand the underlying mechanisms. Whether or not there are differences in the amount of DNA damage caused by oxygen or in the early phase of repair, there may be important differences in the later consequences of DNA damage. Mouse cells appear to be able to continue to divide with DNA damage that has not been repaired or has been misrepaired, and becomes fixed in the form of chromosomal abnormalities. The checkpoints that cause cells to stop dividing when chromosomes develop abnormalities (aberrations or shortened telomeres) appear to operate more efficiently in human cells. Much more work is needed to understand the basis for these differences and the implications for aging and cancer.

Telomeres in hematopoietic stem cells

Hematopoietic stem cells have an impressive regenerative potential, strikingly illustrated in transplantation experiments using limited number of cells. In mice, serial transplantation experiments suggest that individual hematopoietic cells are capable of extensive self-renewal and that any possible limitations in the replicative potential of individual hematopoietic stem cells are not affecting normal blood cell formation. The situation with human hematopoietic stem cells is less clear. Unlike the situation in the mouse, the telomere length in nucleated human blood cells shows a remarkable decline with age. Furthermore, even partial telomerase deficiency in humans typically results in marrow failure, whereas complete lack of telomerase is tolerated up to several generations in the mouse. The decline in telomere length in human leukocytes with age follows a cubic function and is much higher in lymphocytes than in granulocytes. This finding suggests that, under normal circumstances, telomere loss is more likely to compromise the function of lymphocytes than the function of hematopoietic stem cells. To reconcile differences in telomere biology between man and mice, it has been proposed that telomere shortening evolved as a tumor suppressor mechanism in long-lived species that may not exist in shorter-lived mammals. According to this model, telomeres in human cells are intimately involved in signaling cell cycle progression and cell division. Most likely, a minimum number of telomere repeats is required at each telomere to prevent activation of a "telomere checkpoint" and allow cell cycle progression. Telomere length measurements appear useful to distinguish between depletion and exhaustion of hematopoietic stem cells as a cause of marrow failure.

Current developments in the design of onco-retrovirus and lentivirus vector systems for hematopoietic cell gene therapy

Over the past dozen years, the majority of clinical gene therapy trials for inherited genetic diseases and cancer therapy have been performed using murine onco-retrovirus as the gene delivery vector. The earliest systems used were relatively inefficient in both the rates of transduction and expression of the transgene. Formidable obstacles inherent in the cell biology and/or the immunology of the target cell systems limited the efficacy of gene therapy for many target diseases. Development of novel retrovirus gene transfer systems that are in progress have begun to overcome these obstacles. Evidence of this progress is the recent successful functional correction of the immune T and B lymphocyte deficiency in patients with X-linked severe combined immunodeficiency (X-SCID) and adenosine deaminase (ADA)-deficient SCID following onco-retrovirus vector ex vivo transduction of autologous marrow stem cells [Science 296 (2002) 2410; Science 288 (2000) 669; N. Engl. J. Med. 346 (2002) 1185]. These achievements of prolonged clinical benefit from gene therapy were tempered by the finding of insertional mutageneses in two of the treated X-SCID patients [N. Engl. J. Med. 348 (2003) 255].

Stabilization of telomere length and karyotypic stability are directly correlated with the level of hTERT gene expression in primary fibroblasts

Telomere shortening and lack of telomerase activity have been implicated in cellular senescence in human fibroblasts. Expression of the human telomerase (hTERT) gene in sheep fibroblasts reconstitutes telomerase activity and extends their lifespan. However, telomere length is not maintained in all cell lines, even though in vitro telomerase activity is restored in all of them. Cell lines expressing higher levels of hTERT mRNA do not exhibit telomere erosion or genomic instability. By contrast, fibroblasts expressing lower levels of hTERT do exhibit telomere shortening, although the telomeres eventually stabilize at a shorter length. The shorter telomere lengths and the extent of karyotypic abnormalities are both functions of hTERT expression level. We conclude that telomerase activity is required to bypass senescence but is not sufficient to prevent telomere erosion and genomic instability at lower levels of expression.

Mesenchymal cells in the liver--one cell type or two?

The wall of the liver sinusoid is made of highly specialized cells, the hepatic stellate cells (HSC) which together with the sinusoidal endothelial cells represent a loose barrier to the corpusculate part of the blood flowing through the liver. Quiescent stellate cells (quiescent HSC) store Vitamin A; "activated" stellate cells become involved in the reaction to acute or chronic noxae damaging the liver parenchyma. Activated HSC show increased protein synthesis capacity, increased DNA-synthesis and acquire a myofibroblast-like phenotype. Under similar conditions liver myofibroblasts (MF) of the portal field and of the pericentral area may also become "activated" by increasing protein synthesis, DNA synthesis and cell division. They express the fibulin-2 gene and produce large amounts of IL-6. In contrast to "activated" HSC they do not undergo spontaneous apoptosis in vitro and do not express the CD95-ligand gene. So far no definite prove has been found for a "transdifferentiation" of HSC to myofibroblasts. On the contrary an increasing amount of data support the conviction that HSC and MF represent two similar but not identical cell populations the latter being comparable to those of other organs.

Relationship between chromosome fragility, aneuploidy and severity of the haematological disease in Fanconi anaemia

Fanconi anemia (FA) is a chromosome instability syndrome, characterized by progressive pancytopenia and cancer susceptibility. Other cellular features of FA cells are hypersensitivity to DNA cross-linking agents and accelerated telomere shortening. We have quantified overall genome chromosome fragility and euploidy as well as chromosomes 7 and 8 aneuploidy in peripheral blood lymphocytes from a group of FA patients and age-matched controls that were previously measured for telomere length. The haematology of FA samples were also characterized in terms of whole blood cell, neuthrophil and platelet counts, transfusion dependency, requirement of androgens, cortico-steroids or bone marrow transplantation, and the development of bone marrow clonal cytogenetic abnormalities, myelodysplastic syndrome or acute myeloid leukemia. As expected, a high frequency of spontaneous chromosome breaks was observed in FA patients, especially of chromatid-type. No differences in chromosomes 7 and 8 monosomy, polysomy and non-disjunction were detected between FA patients and controls. The same was true for overall genome haploidy or polyploidy. Interestingly, the spontaneous levels of chromosome fragility but not of numerical abnormalities were correlated to the severity of the haematological disease in FA. None of the variables included in the present investigation (chromosome fragility, chromosome numerical abnormalities and haematological status) were correlated to telomere length.

Cellular senescence and tissue aging in vivo

A long-standing controversy concerns the relevance of cellular senescence, defined and observed as a cell culture phenomenon, to tissue aging in vivo. Here the evidence on this topic is reviewed. The main conclusions are as follows. First, telomere shortening, the principal known mediator of cellular senescence, occurs in many human tissues in aging. Second, it is not clear whether this results in cellular senescence or in some other cell fate (e.g., crisis). Third, rodents probably are not appropriate experimental models for these questions, because of important differences in telomere biology between rodent cells and cells from long-lived mammals (e.g., human or bovine cells). Fourth, better and more comprehensive observations on aging human tissues are needed to answer the question of the occurrence of senescent cells in tissues, and new experimental approaches are needed to elucidate the consequences of telomere shortening in tissues in aging.

Stem cells: hype and reality

This update discusses what is known regarding embryonic and adult tissue-derived pluripotent stem cells, including the mechanisms underlying self-renewal without senescence, differentiation in multiple cell types both in vitro and in vivo, and future potential clinical uses of such stem cells. In Section I, Dr. Lansdorp reviews the structure and function of telomerase, the enzyme that restores telomeric ends of chromosomes upon cell division, highly present in embryonic stem cells but not adult stem cells. He discusses the structure and function of telomerase and signaling pathways activated by the enzyme, with special emphasis on normal and leukemic hematopoietic stem cells. In Section II, Dr. Pera reviews the present understanding of mammalian pluripotent embryonic stem cells. He discusses the concept of pluripotentiality in its embryonic context, derivation of stem cells from embryonic or fetal tissue, the basic properties of the stem cells, and methods to produce specific types of differentiated cell from stem cells. He examines the potential applications of stem cells in research and medicine and some of the barriers that must be crossed to achieve these goals. In Section III, Dr. Verfaillie reviews the present understanding of pluripotency of adult stem cells. She discusses the concept of stem cell plasticity, a term used to describe the greater potency described by several investigators of adult tissue-derived stem cells, critically reviews the published studies demonstrating stem cell plasticity, and possible mechanisms underlying such plasticity, and examines the possible role of pluripotent adult stem cells in research and medicine.

DNA-PKcs is critical for telomere capping

The DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is critical for DNA repair via the nonhomologous end joining pathway. Previously, it was reported that bone marrow cells and spontaneously transformed fibroblasts from SCID (severe combined immunodeficiency) mice have defects in telomere maintenance. The genetically defective SCID mouse arose spontaneously from its parental strain CB17. One known genomic alteration in SCID mice is a truncation of the extreme carboxyl terminus of DNA-PKcs, but other as yet unidentified alterations may also exist. We have used a defined system, the DNA-PKcs knockout mouse, to investigate specifically the role DNA-PKcs specifically plays in telomere maintenance. We report that primary mouse embryonic fibroblasts (MEFs) and primary cultured kidney cells from 6-8 month-old DNA-PKcs-deficient mice accumulate a large number of telomere fusions, yet still retain wild-type telomere length. Thus, the phenotype of this defect separates the two-telomere related phenotypes, capping, and length maintenance. DNA-PKcs-deficient MEFs also exhibit elevated levels of chromosome fragments and breaks, which correlate with increased telomere fusions. Based on the high levels of telomere fusions observed in DNA-PKcs deficient cells, we conclude that DNA-PKcs plays an important capping role at the mammalian telomere.