Conditional telomerase induction causes proliferation of hair follicle stem cells

Increased copy number of the TERT and TERC telomerase subunit genes in cancer cells

Telomerase is a ribonucleoprotein enzyme complex that adds telomeric repeats to the ends of chromosomes. The core telomerase components are the telomerase reverse transcriptase (TERT) catalytic subunit, and the telomerase RNA (TR) template subunit. In most cancers, telomerase is expressed at levels that are substantially higher than in normal cells. A known consequence of telomerase up-regulation which is considered to play a critical role in oncogenesis is maintenance of telomere length, and thus evasion by cancer cells of the normal limits on proliferation that are associated with the steady decrease in telomere length that accompanies proliferation of normal cells. It has also been suggested that telomerase up-regulation confers other advantages on cancer cells independent of its enzymatic activity. The mechanisms responsible for up-regulation of telomerase in cancer are incompletely understood. Here we review evidence suggesting that this frequently results from increased copy number of the genes encoding telomerase components. The TERT gene is located at human chromosome band 5p15.33, and the telomerase RNA component (TERC) gene that encodes TR is at 3q26.3. Chromosomal gains and gene amplifications involving chromosome arms 5p and 3q are among the most frequent in human tumors. Increased TERT and TERC gene dosage has been detected frequently in a variety of human cancers, and clonal evolution of cells with increased TERT or TERC copy number has been observed, suggesting a growth advantage in cells with increased TERT or TERC gene dosage.

Oxidative stress causes telomere damage in Fanconi anaemia cells - a possible predisposition for malignant transformation

Fanconi anaemia (FA) is an autosomal recessive and X-linked disease characterized by severe genetic instability and increased incidence of cancer. One explanation for this instability may be the cellular hypersensitivity to oxidative stress leading to chromosomal breaks. This study explored the possible oxidative damage to telomeres of FA lymphocyte cell line, HSC536/N, and its possible effect on telomere function. We postulated that combination of oxidative damage with overexpression of telomerase may provide a possible model for malignant transformation in FA. The cells were grown in the presence of telomerase inhibitor and exposed for 1 month to H(2)O(2) combined with various antioxidants. This exposure caused shortening of telomere length and damage to the telomere single stranded overhang, which was prevented by several oxidants. This shortening was associated with development of severe telomere dysfunction. Control cells did not exhibit this sensitivity to H(2)O(2). Telomere dysfunction did not evoke damage response in FA cells, in contrast to normal P53 upregulation in control cells. Reconstitution of telomerase activity protected FA telomeres from further oxidative damage. These results suggest a scenario in which oxidative stress causes telomere shortening and ensuing telomere dysfunction may form the basis for malignant transformation in FA cells. Upregulation of telomerase activity in sporadic FA cells may perpetuate that process, thus explaining the malignant character of FA cells in vivo.

Identification of ATPases pontin and reptin as telomerase components essential for holoenzyme assembly

Telomerase is a multisubunit ribonucleoprotein (RNP) complex that adds telomere repeats to the ends of chromosomes. Three essential telomerase components have been identified thus far: the telomerase reverse transcriptase (TERT), the telomerase RNA component (TERC), and the TERC-binding protein dyskerin. Few other proteins are known to be required for human telomerase function, limiting our understanding of both telomerase regulation and mechanisms of telomerase action. Here, we identify the ATPases pontin and reptin as telomerase components through affinity purification of TERT from human cells. Pontin interacts directly with both TERT and dyskerin, and the amount of TERT bound to pontin and reptin peaks in S phase, evidence for cell-cycle-dependent regulation of TERT. Depletion of pontin and reptin markedly impairs telomerase RNP accumulation, indicating an essential role in telomerase assembly. These findings reveal an unanticipated requirement for additional enzymes in telomerase biogenesis and suggest alternative approaches for inhibiting telomerase in cancer.

A systems biology approach to anatomic diversity of skin

Human skin exhibits exquisite site-specific morphologies and functions. How are these site-specific differences specified during development, maintained in adult homeostasis, and potentially perturbed by disease processes? Here, we review progress in understanding the anatomic patterning of fibroblasts, a major constituent cell type of the dermis and key participant in epithelial-mesenchymal interactions. The gene expression programs of human fibroblasts largely reflect the superimposition of three gene expression profiles that demarcate the fibroblast's position relative to three developmental axes. The HOX family of homeodomain transcription factors is implicated in specifying site-specific transcriptional programs. The use of gene, tiling, and tissue microarrays together gives a comprehensive view of the gene regulation involved in patterning the skin.

Telomerase does not counteract telomere shortening but protects mitochondrial function under oxidative stress

Telomerase is a ribonucleoprotein that counteracts telomere shortening and can immortalise human cells. There is also evidence for a telomere-independent survival function of telomerase. However, its mechanism is not understood. We show here that TERT, the catalytic subunit of human telomerase, protects human fibroblasts against oxidative stress. While TERT maintains telomere length under standard conditions, telomeres under increased stress shorten as fast as in cells without active telomerase. This is because TERT is reversibly excluded from the nucleus under stress in a dose- and time-dependent manner. Extranuclear telomerase colocalises with mitochondria. In TERT-overexpressing cells, mtDNA is protected, mitochondrial membrane potential is increased and mitochondrial superoxide production and cell peroxide levels are decreased, all indicating improved mitochondrial function and diminished retrograde response. We propose protection of mitochondria under mild stress as a novel function of TERT.

Introducing a single-cell-derived human mesenchymal stem cell line expressing hTERT after lentiviral gene transfer

Human mesenchymal stem cells (hMSCs) can be readily isolated from bone marrow and differentiate into multiple tissues, making them a promising target for future cell and gene therapy applications. The low frequency of hMSCs in bone marrow necessitates their isolation and expansion in vitro prior to clinical use, but due to senescence-associated growth arrest during culture, limited cell numbers can be generated. The lifespan of hMSCs has been extended by ectopic expression of human telomerase reverse transcriptase (hTERT) using retroviral vectors. Since malignant transformation was observed in hMSCs and retroviral vectors cause insertional mutagenesis, we ectopically expressed hTERT using lentiviral gene transfer. Single-cell-derived hMSC clones expressing hTERT did not show malignant transformation in vitro and in vivo after extended culture periods. There were no changes observed in the expression of tumour suppressor genes and karyotype. Cultured hMSCs lack telomerase activity, but it was significantly increased by ectopic expression of hTERT. HTERT expression prevented hMSC senescence and the cells showed significantly higher and unlimited proliferation capacity. Even after an extended culture period, hMSCs expressing hTERT preserved their stem cells character as shown by osteogenic, adipogenic and chon-drogenic differentiation. In summary, extending the lifespan of human mesenchymal stem cells by ectopic expression of hTERT using lentiviral gene transfer may be an attractive and safe way to generate appropriate cell numbers for cell and gene therapy applications.

The longest telomeres: a general signature of adult stem cell compartments

Identification of adult stem cells and their location (niches) is of great relevance for regenerative medicine. However, stem cell niches are still poorly defined in most adult tissues. Here, we show that the longest telomeres are a general feature of adult stem cell compartments. Using confocal telomere quantitative fluorescence in situ hybridization (telomapping), we find gradients of telomere length within tissues, with the longest telomeres mapping to the known stem cell compartments. In mouse hair follicles, we show that cells with the longest telomeres map to the known stem cell compartments, colocalize with stem cell markers, and behave as stem cells upon treatment with mitogenic stimuli. Using K15-EGFP reporter mice, which mark hair follicle stem cells, we show that GFP-positive cells have the longest telomeres. The stem cell compartments in small intestine, testis, cornea, and brain of the mouse are also enriched in cells with the longest telomeres. This constitutes the description of a novel general property of adult stem cell compartments. Finally, we make the novel finding that telomeres shorten with age in different mouse stem cell compartments, which parallels a decline in stem cell functionality, suggesting that telomere loss may contribute to stem cell dysfunction with age.

Telomerase activity is required for bleomycin-induced pulmonary fibrosis in mice

In addition to its well-known expression in the germline and in cells of certain cancers, telomerase activity is induced in lung fibrosis, although its role in this process is unknown. To identify the pathogenetic importance of telomerase in lung fibrosis, we examined the effects of telomerase reverse transcriptase (TERT) deficiency in a murine model of pulmonary injury. TERT-deficient mice showed significantly reduced lung fibrosis following bleomycin (BLM) insult. This was accompanied by a significant reduction in expression of lung alpha-SMA, a marker of myofibroblast differentiation. Furthermore, lung fibroblasts isolated from BLM-treated TERT-deficient mice showed significantly decreased proliferation and increased apoptosis rates compared with cells isolated from control mice. Transplantation of WT BM into TERT-deficient mice restored BLM-induced lung telomerase activity and fibrosis to WT levels. Conversely, transplantation of BM from TERT-deficient mice into WT recipients resulted in reduced telomerase activity and fibrosis. These findings suggest that induction of telomerase in injured lungs may be caused by BM-derived cells, which appear to play an important role in pulmonary fibrosis. Moreover, TERT induction is associated with increased survival of lung fibroblasts, which favors the development of fibrosis instead of injury resolution.

Immortalization of neural precursors when telomerase is overexpressed in embryonal carcinomas and stem cells

The DNA repair enzyme telomerase maintains chromosome stability by ensuring that telomeres regenerate each time the cell divides, protecting chromosome ends. During onset of neuroectodermal differentiation in P19 embryonal carcinoma (EC) cells three independent techniques (Southern blotting, Q-FISH, and Q-PCR) revealed a catastrophic reduction in telomere length in nestin-expressing neuronal precursors even though telomerase activity remained high. Overexpressing telomerase protein (mTERT) prevented telomere collapse and the neuroepithelial precursors produced continued to divide, but deaggregated and died. Addition of FGF-2 prevented deaggregation, protected the precursors from the apoptotic event that normally accompanies onset of terminal neuronal differentiation, allowed them to evade senescence, and enabled completion of morphological differentiation. Similarly, primary embryonic stem (ES) cells overexpressing mTERT also initiated neuroectodermal differentiation efficiently, acquiring markers of neuronal precursors and mature neurons. ES precursors are normally cultured with FGF-2, and overexpression of mTERT alone was sufficient to allow them to evade senescence. However, when FGF-2 was removed in order for differentiation to be completed most neural precursors underwent apoptosis indicating that in ES cells mTERT is not sufficient allow terminal differentiation of ES neural precursors in vitro. The results demonstrate that telomerase can potentiate the transition between pluripotent stem cell and committed neuron in both EC and ES cells.

Therapeutic implications of leukemic stem cell pathways

An emerging concept in cancer biology is that a rare population of cancer stem cells exists among the heterogeneous cell mass that constitutes a tumor. This concept is best understood in human myeloid leukemia. Normal and malignant hematopoietic stem cell functions are defined by a common set of critical stemness genes that regulate self-renewal and developmental pathways. Several stemness factors, such as Notch or telomerase, show differential activation in normal hematopoietic versus leukemia stem cells. These differences could be exploited therapeutically even with drugs that are already in clinical use for the treatment of leukemia. The translation of novel and existing leukemic stem cell-directed therapies into clinical practice, however, will require changes in clinical trial design and the inclusion of stem cell biomarkers as correlative end points.

TERT promotes cellular and organismal survival independently of telomerase activity

The expression level of the telomerase catalytic subunit (telomerase reverse transcriptase, TERT) positively correlates with cell survival after exposure to several lethal stresses. However, whether the protective role of TERT is independent of telomerase activity has not yet been clearly explored. Here, we genetically evaluated the protective roles of both TERT and telomerase activity against cell death induced by staurosporine (STS) and N-methyl-D-aspartic acid (NMDA). First generation (G1) TERT-deficient mouse embryonic fibroblasts (MEFs) displayed an increased sensitivity to STS, while TERT transgenic MEFs were more resistant to STS-induced apoptosis than wild-type. Deletion of the telomerase RNA component (TERC) failed to alter the sensitivity of TERT transgenic MEFs to STS treatment. Similarly, NMDA-induced excitotoxic cell death of primary neurons was suppressed by TERT, but not by TERC both in vitro and in vivo. Specifically, NMDA accelerated death of TERT-deficient mice, while TERT transgenic mice showed enhanced survival when compared with wild-type littermates after administration of NMDA. In addition, the transgenic expression of TERT protected motor neurons from apoptosis induced by sciatic nerve axotomy. These results indicate that telomerase activity is not essential for the protective function of TERT. This telomerase activity-independent TERT function may contribute to cancer development and aging independently of telomere lengthening.

A key role for telomerase reverse transcriptase unit in modulating human embryonic stem cell proliferation, cell cycle dynamics, and in vitro differentiation

Embryonic stem cells (ESC) are a unique cell population with the ability to self-renew and differentiate into all three germ layers. Human ESC express the telomerase reverse transcriptase (TERT) gene and the telomerase RNA (TR) and show telomerase activity, but TERT, TR, and telomerase are all downregulated during the differentiation process. To examine the role of telomerase in human ESC self-renewal and differentiation, we modulated the expression of TERT. Upregulation of TERT and increased telomerase activity enhanced the proliferation and colony-forming ability of human ESC, as well as increasing the S phase of the cell cycle at the expense of a reduced G1 phase. Upregulation of TERT expression was associated with increases in CYCLIN D1 and CDC6 expression, as well as hyperphosphorylation of RB. The differentiated progeny of control ESC showed shortening of telomeric DNA as a result of loss of telomerase activity. In contrast, the differentiated cells from TERT-overexpressing ESC maintained high telomerase activity and accumulated lower concentrations of peroxides than wild-type cells, implying greater resistance to oxidative stress. Although the TERT-overexpressing human ESC are able to form teratoma composed of three germ layers in vivo, their in vitro differentiation to all primitive and embryonic lineages was suppressed. In contrast, downregulation of TERT resulted in reduced ESC proliferation, increased G1, and reduced S phase. Most importantly, downregulation of TERT caused loss of pluripotency and human ESC differentiation to extraembryonic and embryonic lineages. Our results indicate for the first time an important role for TERT in the maintenance of human ESC pluripotency, cell cycle regulation, and in vitro differentiation capacity.

Lysine-specific demethylase 1 (LSD1) Is required for the transcriptional repression of the telomerase reverse transcriptase (hTERT) gene

Background

Lysine-specific demethylase 1 (LSD1), catalysing demethylation of mono- and di-methylated histone H3-K4 or K9, exhibits diverse transcriptional activities by mediating chromatin reconfiguration. The telomerase reverse transcriptase (hTERT) gene, encoding an essential component for telomerase activity that is involved in cellular immortalization and transformation, is silent in most normal human cells while activated in up to 90% of human cancers. It remains to be defined how exactly the transcriptional activation of the hTERT gene occurs during the oncogenic process.

Methodology/Principal Findings

In the present study, we determined the effect of LSD1 on hTERT transcription. In normal human fibroblasts with a tight hTERT repression, a pharmacological inhibition of LSD1 led to a weak hTERT expression, and a robust induction of hTERT mRNA was observed when LSD1 and histone deacetylases (HDACs) were both inhibited. Small interference RNA-mediated depletion of both LSD1 and CoREST, a co-repressor in HDAC-containing complexes, synergistically activated hTERT transcription. In cancer cells, inhibition of LSD1 activity or knocking-down of its expression led to significant increases in levels of hTERT mRNA and telomerase activity. Chromatin immunoprecipitation assay showed that LSD1 occupied the hTERT proximal promoter, and its depletion resulted in elevated di-methylation of histone H3-K4 accompanied by increased H3 acetylation locally in cancer cells. Moreover, during the differentiation of leukemic HL60 cells, the decreased hTERT expression was accompanied by the LSD1 recruitment to the hTERT promoter.

Conclusions/Significance

LSD1 represses hTERT transcription via demethylating H3-K4 in normal and cancerous cells, and together with HDACs, participates in the establishment of a stable repression state of the hTERT gene in normal or differentiated malignant cells. The findings contribute to better understandings of hTERT/telomerase regulation, which may be implicated in the development of therapeutic strategies for telomerase dysregulation-associated human diseases including cancers.

Human cancer cells harbor T-stumps, a distinct class of extremely short telomeres

Using a modified single telomere length analysis protocol (STELA) to clone and examine the sequence composition of individual human XpYp telomeres, we discovered a distinct class of extremely short telomeres in human cancer cells with active telomerase. We name them "t-stumps," to distinguish them from the well-regulated longer bulk telomeres. T-stumps contained arrangements of telomeric repeat variants and a minimal run of seven canonical telomeric TTAGGG repeats, but all could bind at least one TRF1 or TRF2 in vitro. The abundance of t-stumps was unaffected by ATM alteration but could be changed by manipulating telomerase catalytic subunit (hTERT) levels in cancer cells. We propose that in the setting of active telomerase and compromised checkpoints characteristic of human cancer cells, t-stumps define the minimal telomeric unit that can still be protected by a TRF1/TRF2-capping complex and, further, that hTERT (or telomerase) may have a role in protecting t-stumps.

TERT promotes epithelial proliferation through transcriptional control of a Myc- and Wnt-related developmental program

Telomerase serves a critical role in stem cell function and tissue homeostasis. This role depends on its ability to synthesize telomere repeats in a manner dependent on the reverse transcriptase (RT) function of its protein component telomerase RT (TERT), as well as on a novel pathway whose mechanism is poorly understood. Here, we use a TERT mutant lacking RT function (TERT^ci) to study the mechanism of TERT action in mammalian skin, an ideal tissue for studying progenitor cell biology. We show that TERT^ci retains the full activities of wild-type TERT in enhancing keratinocyte proliferation in skin and in activating resting hair follicle stem cells, which triggers initiation of a new hair follicle growth phase and promotes hair synthesis. To understand the nature of this RT-independent function for TERT, we studied the genome-wide transcriptional response to acute changes in TERT levels in mouse skin. We find that TERT facilitates activation of progenitor cells in the skin and hair follicle by triggering a rapid change in gene expression that significantly overlaps the program controlling natural hair follicle cycling in wild-type mice. Statistical comparisons to other microarray gene sets using pattern-matching algorithms revealed that the TERT transcriptional response strongly resembles those mediated by Myc and Wnt, two proteins intimately associated with stem cell function and cancer. These data show that TERT controls tissue progenitor cells via transcriptional regulation of a developmental program converging on the Myc and Wnt pathways.

Stem cells and progenitor cells within a tissue are required to maintain tissue homeostasis and to repair tissues after injury by giving rise to differentiated daughter cells. Many progenitor cells express telomerase, a reverse transcriptase enzyme that adds DNA repeats to telomeres, the protective structures that cap chromosome ends. Telomere addition by telomerase is important for normal progenitor cell function and is crucial for enabling cancer cells to divide an unlimited number of times. In addition to its telomere-lengthening function, telomerase reverse transcriptase (TERT) can directly activate quiescent epidermal stem cells. However, the mechanism underlying this novel function for TERT is still not understood. In this study, we demonstrate that the catalytic activity of TERT is dispensable for its ability to activate tissue progenitor cells in vivo. Furthermore, using gene microarrays, we show that TERT controls a developmental program that overlaps the natural transcriptional program of hair follicle cycling in mouse skin. Using pattern-matching algorithms, we find that the TERT-controlled genetic program significantly resembles programs regulated by Myc and Wnt, two pathways critical for stem cell function and tumorigenesis. This paper reveals critical new insights into novel mechanisms of non-telomerase functions of TERT, identifying TERT as a developmental regulator linked to control of transcriptional responses.

Telomerase reverses epidermal hair follicle stem cell defects and loss of long-term survival associated with critically short telomeres

Organ homeostasis and organismal survival are related to the ability of stem cells to sustain tissue regeneration. As a consequence of accelerated telomerase shortening, telomerase-deficient mice show defective tissue regeneration and premature death. This suggests a direct impact of telomere length and telomerase activity on stem cell biology. We recently found that short telomeres impair the ability of epidermal stem cells to mobilize out of the hair follicle (HF) niche, resulting in impaired skin and hair growth and in the suppression of epidermal stem cell proliferative capacity in vitro. Here, we demonstrate that telomerase reintroduction in mice with critically short telomeres is sufficient to correct epidermal HF stem cell defects. Additionally, telomerase reintroduction into these mice results in a normal life span by preventing degenerative pathologies in the absence of increased tumorigenesis.

Telomerase and its potential for therapeutic intervention

Telomerase and telomeres are attractive targets for anticancer therapy. This is supported by the fact that the majority of human cancers express the enzyme telomerase which is essential to maintain their telomere length and thus, to ensure indefinite cell proliferation--a hallmark of cancer. Tumours have relatively shorter telomeres compared to normal cell types, opening the possibility that human cancers may be considerably more susceptible to killing by agents that inhibit telomere replication than normal cells. Advances in the understanding of the regulation of telomerase activity and the telomere structure, as well as the identification of telomerase and telomere associated binding proteins have opened new avenues for therapeutic intervention. Here, we review telomere and telomerase biology and the various approaches which have been developed to inhibit the telomere/telomerase complex over the past decade. They include inhibitors of the enzyme catalytic subunit and RNA component, agents that target telomeres, telomerase vaccines and drugs targeting binding proteins. The emerging role of telomerase in cancer stem cells and the implications for cancer therapy are also discussed.

Telomere length, stem cells and aging

Telomere shortening occurs concomitant with organismal aging, and it is accelerated in the context of human diseases associated with mutations in telomerase, such as some cases of dyskeratosis congenita, idiopathic pulmonary fibrosis and aplastic anemia. People with these diseases, as well as Terc-deficient mice, show decreased lifespan coincidental with a premature loss of tissue renewal, which suggests that telomerase is rate-limiting for tissue homeostasis and organismal survival. These findings have gained special relevance as they suggest that telomerase activity and telomere length can directly affect the ability of stem cells to regenerate tissues. If this is true, stem cell dysfunction provoked by telomere shortening may be one of the mechanisms responsible for organismal aging in both humans and mice. Here, we will review the current evidence linking telomere shortening to aging and stem cell dysfunction.

Assessing ageing of individual T lymphocytes: mission impossible?

Effector T lymphocytes are the progeny of a limited number of antigen-specific precursor cells and it has been estimated that clonotypic human T cells may expand million fold on their way reaching high cell numbers that are sufficient for immune protection. Moreover, memory T cell responses are characterized by repetitive expansion of antigen-specific T cell clonotypes, and limitations in the proliferative capacity could lead to immune senescence. Because telomeres progressively shorten as a function of cell division, telomere length is a powerful indicator of the replicative in vivo history of human T lymphocytes. In this review, we summarize observations made over the last decade on telomere length dynamics of well-defined T cell populations derived from healthy donors and patients with infectious disease or cancer. We focus on T cell differentiation, T cell ageing, and natural and vaccine induced immune responses. We also discuss the scientific evidence for in vivo replicative senescence of antigen-specific T cells, and evaluate the available methods for measuring telomere lengths and telomerase activity, and their potential and limitations to increase our understanding of T cell physiology.

Cellular senescence in cancer and aging

Cellular senescence, a state of irreversible growth arrest, can be triggered by multiple mechanisms including telomere shortening, the epigenetic derepression of the INK4a/ARF locus, and DNA damage. Together these mechanisms limit excessive or aberrant cellular proliferation, and so the state of senescence protects against the development of cancer. Recent evidence suggests that cellular senescence also may be involved in aging.

Visualizing the dynamics of p21(Waf1/Cip1) cyclin-dependent kinase inhibitor expression in living animals

Although the role of p21(Waf1/Cip1) gene expression is well documented in various cell culture studies, its in vivo roles are poorly understood. To gain further insight into the role of p21(Waf1/Cip1) gene expression in vivo, we attempted to visualize the dynamics of p21(Waf1/Cip1) gene expression in living animals. In this study, we established a transgenic mice line (p21-p-luc) expressing the firefly luciferase under the control of the p21(Waf1/Cip1) gene promoter. In conjunction with a noninvasive bioluminescent imaging technique, p21-p-luc mice enabled us to monitor the endogenous p21(Waf1/Cip1) gene expression in vivo. By monitoring and quantifying the p21(Waf1/Cip1) gene expression repeatedly in the same mouse throughout its entire lifespan, we were able to unveil the dynamics of p21(Waf1/Cip1) gene expression in the aging process. We also applied this system to chemically induced skin carcinogenesis and found that the levels of p21(Waf1/Cip1) gene expression rise dramatically in benign skin papillomas, suggesting that p21(Waf1/Cip1) plays a preventative role(s) in skin tumor formation. Surprisingly, moreover, we found that the level of p21(Waf1/Cip1) expression strikingly increased in the hair bulb and oscillated with a 3-week period correlating with hair follicle cycle progression. Notably, this was accompanied by the expression of p63 but not p53. This approach, together with the analysis of p21(Waf1/Cip1) knockout mice, has uncovered a novel role for the p21(Waf1/Cip1) gene in hair development. These data illustrate the unique utility of bioluminescence imaging in advancing our understanding of the timing and, hence, likely roles of specific gene expression in higher eukaryotes.

Cancer and ageing: convergent and divergent mechanisms

Cancer and ageing are both fuelled by the accumulation of cellular damage. Consequently, those mechanisms that protect cells from damage simultaneously provide protection against cancer and ageing. By contrast, cancer and longevity require a durable cell proliferation potential and, therefore, those mechanisms that limit indefinite proliferation provide cancer protection but favour ageing. The overall balance between these convergent and divergent mechanisms guarantees fitness and a cancer-free life until late adulthood for most individuals.

The common biology of cancer and ageing

At first glance, cancer and ageing would seem to be unlikely bedfellows. Yet the origins for this improbable union can actually be traced back to a sequence of tragic--and some say unethical--events that unfolded more than half a century ago. Here we review the series of key observations that has led to a complex but growing convergence between our understanding of the biology of ageing and the mechanisms that underlie cancer.

Telomerase core components protect Candida telomeres from aberrant overhang accumulation

Telomerase is a cellular reverse transcriptase that extends one strand (the G-strand) of the telomere terminal repeats. Aside from this role in telomere length maintenance, telomerase has been proposed to serve a protective function at chromosome ends, although this is not well understood mechanistically. Earlier analysis suggests that, in the pathogenic yeast Candida albicans, the catalytic reverse transcriptase subunit of telomerase (TERT/EST2) can protect telomeres against nucleolytic degradation. In this report we demonstrate that the RNA component (TER1) has a similar function; in addition to complete loss of telomerase activity and progressive telomere attrition, the ter1-DeltaDelta strains manifested a dramatic increase in the amount of G-strand overhangs, consistent with aberrant degradation of the complementary C-strand. We also demonstrate that a catalytically incompetent EST2 protein can suppress such overhang accumulation in the est2-DeltaDelta mutant to the same extent as the wild-type protein. Altogether, our data support the notion that the Candida telomerase core components mediate a protective function through a mechanism that is independent of its catalytic activity.

Hair follicle stem cells: walking the maze

The discovery of epithelial stem cells (eSCs) in the bulge region of the outer root sheath of hair follicles in mice and man has encouraged research into utilizing the hair follicle as a therapeutic source of stem cells (SCs) for regenerative medicine, and has called attention to the hair follicle as a highly instructive model system for SC biology. Under physiological circumstances, bulge eSCs serve as cell pool for the cyclic regeneration of the anagen hair bulb, while they can also regenerate the sebaceous gland and the epidermis after injury. More recently, melanocyte SCs, nestin+, mesenchymal and additional, as yet ill-defined "stem cell" populations, have also been identified in or immediately adjacent to the hair follicle epithelium, including in the specialized hair follicle mesenchyme (connective tissue sheath), which is crucial to wound healing. Thus the hair follicle and its adjacent tissue environment contain unipotent, multipotent, and possibly even pluripotent SC populations of different developmental origin. It provides an ideal model system for the study of central issues in SC biology such as plasticity and SC niches, and for the identification of reliable, specific SC markers, which distinguish them from their immediate progeny (e.g. transient amplifying cells). The current review attempts to provide some guidance in this growing maze of hair follicle-associated SCs and their progeny, critically reviews potential or claimed hair follicle SC markers, highlights related differences between murine and human hair follicles, and defines major unanswered questions in this rapidly advancing field.

Prostatic stem cell marker identified by cDNA microarray in mouse

PURPOSE

Identifying prostatic stem cells is important to elucidate the mechanisms by which the prostate develops and control prostate cancer. We recently reported that the proximal region of the mouse prostate contains a population of stem cells. However, to our knowledge the specific marker of stem cells in the proximal region remains unknown.

MATERIALS AND METHODS

We performed cDNA microarray analysis of cells obtained from the proximal region and from the remaining regions in dorsal prostates to identify several candidate stem cell markers. After we focused on 1 candidate among them we confirmed the expression of this candidate gene by reverse transcriptase-polymerase chain reaction analysis and immunohistochemistry. We also investigated the relation between positive cells for this marker and those for telomerase reverse transcriptase. Finally, we investigated the functional potential of prominin positive cells in 3-dimensional culture.

RESULTS

Seven of 4,800 genes analyzed showed proximal/remaining ratios greater than 20. Of these genes we focused on prominin because it is a cell surface marker widely used to identify and isolate stem cells from various organs. We found a prominin positive cell population enriched in the basal cell layer in the proximal region, and the coincidence of prominin and telomerase reverse transcriptase immunostaining. We also found that prominin positive cells gave rise to numerous and large-branched ducts, whereas prominin negative cells formed far fewer such structures in 3-dimensional culture.

CONCLUSIONS

A small population of prominin positive cells in the mouse prostate basal layer of the proximal region represents a stem cell population.

Nucleus pulposus cellular longevity by telomerase gene therapy

STUDY DESIGN

Nonviral transfection of nucleus pulposus cells with a telomerase expression construct to assess the effects on cellular lifespan, function, karyotypic stability, and transformation properties.

OBJECTIVES

To investigate whether telomerase gene therapy can extend the cellular lifespan while retaining functionality of nucleus pulposus cells in a safe manner.

SUMMARY OF BACKGROUND DATA

Degeneration of the intervertebral disc is an age-related condition in which cells responsible for the maintenance and health of the disc deteriorate with age. Telomerase can extend the cellular lifespan and function of other musculoskeletal tissues, such as the heart, bones, and connective tissues. Therefore, extension of the cellular lifespan and matrix production of intervertebral disc cells may have the potential to delay the degeneration process.

METHODS

Ovine nucleus pulposus cells were lipofectamine transfected in vitro with a human telomerase reverse transcriptase (hTERT) expression construct. Cellular lifespan and matrix transcript levels were determined by cumulative population doublings and real-time RT-PCR, respectively. G1-cell cycle checkpoint, p53 functionality, growth of transfected cells in anchorage-independent or serum starvation conditions, and karyotypic analysis were performed.

RESULTS

Transfection was achieved successfully with 340% +/- 7% (mean +/- SD) relative telomerase activity in hTERT-transfected cells. hTERT transfection enabled a 50% extension in mean cellular lifespan and prolonged matrix production of collagen 1 and 2 for more than 282 days. Karyotypic instability was detected but G1-cell cycle checkpoint and p53 was functionally comparable to parental cells with no growth in serum starvation or anchorage-independent conditions.

CONCLUSIONS

Telomerase can extend the cellular lifespan of nucleus pulposus cells and prolong the production of extracellular matrix. Safety is still unresolved, as karyotypic instability was detected but no loss of contact inhibition, mitogen dependency, or G1-cell cycle checkpoint control was evident.

Telomere uncapping by the G-quadruplex ligand RHPS4 inhibits clonogenic tumour cell growth in vitro and in vivo consistent with a cancer stem cell targeting mechanism

The pentacyclic acridinium methosulfate salt RHPS4 induces the 3'single-stranded guanine-rich telomeric overhang to fold into a G-quadruplex structure. Stabilisation of the latter is incompatible with an attachment of telomerase to the telomere and thus G-quadruplex ligands can effectively inhibit both the catalytic and capping functions of telomerase. In this study, we examined mechanisms underlying telomere uncapping by RHPS4 in uterus carcinoma cells (UXF1138L) with short telomeres and compared the susceptibility of bulk and clonogenic cancer cells to the G-quadruplex ligand. We show that treatment of UXF1138L cells with RHPS4 leads to the displacement of the telomerase catalytic subunit (hTERT) from the nucleus, induction of telomere-initiated DNA-damage signalling and chromosome fusions. We further report that RHPS4 is more potent against cancer cells that grow as colonies in soft agar than cells growing as monolayers. Human cord blood and HEK293T embryonic kidney cell colony forming units, however, were more resistant to RHPS4. RHPS4-treated UXF1138L xenografts had a decreased clonogenicity, showed loss of nuclear hTERT expression and an induction of mitotic abnormalities compared with controls. Although single-agent RHPS4 had limited in vivo efficacy, a combination of RHPS4 with the mitotic spindle poison Taxol caused tumour remissions and further enhancement of telomere dysfunction.

The epigenetic regulation of mammalian telomeres

Increasing evidence indicates that chromatin modifications are important regulators of mammalian telomeres. Telomeres provide well studied paradigms of heterochromatin formation in yeast and flies, and recent studies have shown that mammalian telomeres and subtelomeric regions are also enriched in epigenetic marks that are characteristic of heterochromatin. Furthermore, the abrogation of master epigenetic regulators, such as histone methyltransferases and DNA methyltransferases, correlates with loss of telomere-length control, and telomere shortening to a critical length affects the epigenetic status of telomeres and subtelomeres. These links between epigenetic status and telomere-length regulation provide important new avenues for understanding processes such as cancer development and ageing, which are characterized by telomere-length defects.

Telomere and telomerase in stem cells

Telomeres, guanine-rich tandem DNA repeats of the chromosomal end, provide chromosomal stability, and cellular replication causes their loss. In somatic cells, the activity of telomerase, a reverse transcriptase that can elongate telomeric repeats, is usually diminished after birth so that the telomere length is gradually shortened with cell divisions, and triggers cellular senescence. In embryonic stem cells, telomerase is activated and maintains telomere length and cellular immortality; however, the level of telomerase activity is low or absent in the majority of stem cells regardless of their proliferative capacity. Thus, even in stem cells, except for embryonal stem cells and cancer stem cells, telomere shortening occurs during replicative ageing, possibly at a slower rate than that in normal somatic cells. Recently, the importance of telomere maintenance in human stem cells has been highlighted by studies on dyskeratosis congenital, which is a genetic disorder in the human telomerase component. The regulation of telomere length and telomerase activity is a complex and dynamic process that is tightly linked to cell cycle regulation in human stem cells. Here we review the role of telomeres and telomerase in the function and capacity of the human stem cells.

p53-dependent integration of telomere and growth factor deprivation signals

Ectopically expressed hTERT enables p16(INK4A)(-) human mammary epithelial cells to proliferate in the absence of growth factors, a finding that has led to the hypothesis that hTERT has growth regulatory properties independent of its role in telomere maintenance. We now show that telomerase can alter the growth properties of cells indirectly through its role in telomere maintenance, without altering growth stimulatory pathways. We find that telomere dysfunction, indicated by 53BP1/phosphorylated histone H2AX foci at chromosome ends, is present in robustly proliferating human mammary epithelial cells long before senescence. These foci correlate with increased levels of active p53. Ectopic expression of hTERT reduces the number of foci and the level of active p53, thereby decreasing sensitivity to growth factor depletion, which independently activates p53. The continuous presence of hTERT is not necessary for this effect, indicating that telomere maintenance, rather than the presence of the enzyme itself, is responsible for the increased ability to proliferate in the absence of growth factors. Our findings provide a previously unrecognized mechanistic explanation for the observation that ectopically expressed hTERT conveys growth advantages to cells, without having to postulate nontelomeric functions for the enzyme.

Akt activation induces epidermal hyperplasia and proliferation of epidermal progenitors

Various common signaling pathways maintain tissue stem cells, including Notch and Wnt/beta-catenin signals. Phosphoinositide-3 kinase (PI3K)/Akt signaling regulates the 'stemness' of several stem cells in culture, specifically in maintaining embryonic stem and neural stem cells, and in deriving embryonic germ cells from primordial germ cells. We examined the effect of Akt signaling in epidermal cells in transgenic mice expressing an Akt-Mer fusion protein whose kinase activity was conditionally activated by treatment with 4-hydroxytamoxifen (4OHT). The topical application of 4OHT to adult skin of the transgenic mice induced new hair growth in resting phase follicles. In addition, the mice showed hyperplasia in interfollicular epidermis (IFE) and hair follicles, which was presumably caused by the extensive proliferation of keratinocytes in basal layer of IFE and outer root sheath of hair follicles, respectively. The progenitor cell population increased consistently in 4OHT-treated transgenic mice. Our results show that PI3K/Akt signaling induces epidermal hyperplasia and proliferation of epidermal progenitors.

[Mechanism of cell proliferation--cell cycle, oncogenes, and senescence]

Cell proliferation is regulated through a transition between the G0 phase and cell cycle. We isolated a mammalian temperature-sensitive mutant cell line defective in the function from the G0 phase to cell cycle. Senescent human somatic cells fail to enter into the cell cycle from the G0 phase with stimulation by any growth factor. Telomere shortening was found to be a cause of cellular senescence, and reexpression of telomerase immortalized human somatic cells. Immortalized human somatic cells showed normal phenotypes and were useful not only for basic research but also for clinical and applied fields. The importance of p53 and p21 activation/induction i now well accepted in the signal transduction process from telomere shortening to growth arrest, but the precise mechanism is largely unknown as yet. We found that the MAP kinase cascade and histone acetylase have an important role in the signaling process to express p21. Tumor tissues and cells were found to have strong telomerase activity, while most normal somatic human tissues showed very weak or no activity. Telomerase activity was shown to be a good marker for early tumor diagnosis because significant telomerase activity was detected in very early tumors or even in some precancerous tissues compared with adjacent normal tissues. Telomere/telomerase is a candidate target for cancer chemotherapeutics, and an agent that abrogated telomere functions was found to kill tumor cells effectively by inducing apoptosis whereas it showed no effect on the viability of normal cells.

Cutaneous gene therapy

Possibilities of using the skin for somatic gene therapy have been investigated for more than 20 years. Strategies have included both direct gene transfer into the skin and indirect gene transfer utilizing cultured cells as an intermediate step for gene manipulation. Viral as well as nonviral vectors have been used, and both gene addition and gene editing have been performed. Although cutaneous gene therapy has now begun translating into clinical medicine (as seen by the first clinical gene therapy project of an inherited skin disorder) further developments are still required.

Epidermal stem cells of the skin

The skin constantly renews itself throughout adult life, and the hair follicle undergoes a perpetual cycle of growth and degeneration. Stem cells (SCs) residing in the epidermis and hair follicle ensure the maintenance of adult skin homeostasis and hair regeneration, but they also participate in the repair of the epidermis after injuries. We summarize here the current knowledge of epidermal SCs of the adult skin. We discuss their fundamental characteristics, the methods recently designed to isolate these cells, the genes preferentially expressed in the multipotent SC niche, and the signaling pathways involved in SC niche formation, SC maintenance, and activation. Finally, we speculate on how the deregulation of these pathways may lead to cancer formation.

The molecular basis of ageing in stem cells

Ageing is often defined in the context of telomerase activity and telomere length regulation. Most somatic cells have limited replication ability and undergo senescence eventually. Stem cells are unique as they possess more abundant telomerase activity and are able to maintain telomere lengths for a longer period. Embryonic stem cells are particularly resistant to ageing and can be propagated indefinitely. Remarkably, adult somatic cells can be reprogrammed to an ESC-like state by various means including cell fusion, exposure to ESC cell-free extracts, enforced expression of specific molecules, and somatic cell nuclear transfer. Thus, the rejuvenation of an 'aged' state can be effected by the activation of specific key molecules in the cell. Here, we argue that cellular ageing is a reversible process, and this is determined by the balance of biological molecules which directly or indirectly control telomere length and telomerase activity, either through altering gene expression and/or modulating the epigenetic state of the chromatin.

Stem cells, senescence, neosis and self-renewal in cancer

We describe the basic tenets of the current concepts of cancer biology, and review the recent advances on the suppressor role of senescence in tumor growth and the breakdown of this barrier during the origin of tumor growth. Senescence phenotype can be induced by (1) telomere attrition-induced senescence at the end of the cellular mitotic life span (MLS*) and (2) also by replication history-independent, accelerated senescence due to inadvertent activation of oncogenes or by exposure of cells to genotoxins. Tumor suppressor genes p53/pRB/p16INK4A and related senescence checkpoints are involved in effecting the onset of senescence. However, senescence as a tumor suppressor mechanism is a leaky process and senescent cells with mutations or epimutations in these genes escape mitotic catastrophe-induced cell death by becoming polyploid cells. These polyploid giant cells, before they die, give rise to several cells with viable genomes via nuclear budding and asymmetric cytokinesis. This mode of cell division has been termed neosis and the immediate neotic offspring the Raju cells. The latter inherit genomic instability and transiently display stem cell properties in that they differentiate into tumor cells and display extended, but, limited MLS, at the end of which they enter senescent phase and can undergo secondary/tertiary neosis to produce the next generation of Raju cells. Neosis is repeated several times during tumor growth in a non-synchronized fashion, is the mode of origin of resistant tumor growth and contributes to tumor cell heterogeneity and continuity. The main event during neosis appears to be the production of mitotically viable daughter genome after epigenetic modulation from the non-viable polyploid genome of neosis mother cell (NMC). This leads to the growth of resistant tumor cells. Since during neosis, spindle checkpoint is not activated, this may give rise to aneuploidy. Thus, tumor cells also are destined to die due to senescence, but may escape senescence due to mutations or epimutations in the senescent checkpoint pathway. A historical review of neosis-like events is presented and implications of neosis in relation to the current dogmas of cancer biology are discussed. Genesis and repetitive re-genesis of Raju cells with transient "stemness" via neosis are of vital importance to the origin and continuous growth of tumors, a process that appears to be common to all types of tumors. We suggest that unlike current anti-mitotic therapy of cancers, anti-neotic therapy would not cause undesirable side effects. We propose a rational hypothesis for the origin and progression of tumors in which neosis plays a major role in the multistep carcinogenesis in different types of cancers. We define cancers as a single disease of uncontrolled neosis due to failure of senescent checkpoint controls.

Telomeres: Cancer to Human Aging

The cell phenotypes of senescence and crisis operate to circumscribe the proliferative potential of mammalian cells, suggesting that both are capable of operating in vivo to suppress the formation of tumors. The key regulators of these phenotypes are the telomeres, which are located at the ends of chromosomes and operate to protect the chromosomes from end-to-end fusions. Telomere erosion below a certain length can trigger crisis. The relationship between senescence and telomere function is more complex, however: Cell-physiological stresses as well as dysfunction of the complex molecular structures at the ends of telomeric DNA can trigger senescence. Cells can escape senescence by inactivating the Rb and p53 tumor suppressor proteins and can surmount crisis by activating a telomere maintenance mechanism. The resulting cell immortalization is an essential component of the tumorigenic phenotype of human cancer cells. Here we discuss how telomeres are monitored and maintained and how loss of a functional telomere influences biological functions as diverse as aging and carcinogenesis.

HPV16-E6 associated hTERT promoter acetylation is E6AP dependent, increased in later passage cells and enhanced by loss of p300

The E6 oncoprotein from high-risk HPV types activates human telomerase reverse transcriptase (hTERT) transcription in human keratinocytes. Studies on how E6 regulates hTERT have implicated E-box or X-box elements in the hTERT promoter (Veldman et al., Proc Natl Acad Sci USA 2003;100:8211-14; Oh et al., J Virol 2001;75:5559-66; Gewin et al., Genes Dev 2004;18:2269-82), but the mechanism of activation by E6 is still controversial and not well defined. Here, we demonstrate that induction of both hTERT expression and telomerase activity by HPV-16 E6 in early passage keratinocytes is associated with acetylation of histone H3 at the hTERT promoter, is dependent on the E6 associated protein (E6AP) and is not exclusively reliant on E-box or X-box elements. Further increases in histone acetylation of the hTERT promoter and hTERT transcriptional activity in E6 expressing cells that had been passaged extensively in culture were found to occur only with the endogenous promoter and not with an exogenously introduced hTERT promoter construct. Telomerase activity at both early and late passages, however, was dependent on E6AP expression, implying a continued reliance on E6 function for telomerase activity. Our results demonstrate that E6 induces hTERT promoter acetylation, but that further increases in telomerase activity and histone acetylation in later passage E6 expressing cells are independent of E6 activation of the core hTERT promoter. We also provide evidence that the transcription factor p300 is a potential repressor of telomerase activation and histone acetylation in the context of E6 expression. These studies give insight into how immortalization by HPV results in upregulation of hTERT and furthers our understanding of how telomerase is activated during the process of malignant transformation.

Basal cells of the human adult airway surface epithelium retain transit-amplifying cell properties

In numerous airway diseases, such as cystic fibrosis, the epithelium is severely damaged and must regenerate to restore its defense functions. Although the human airway epithelial stem cells have not been identified yet, we have suggested recently that epithelial stem/progenitor cells exist among both human fetal basal and suprabasal cell subsets in the tracheal epithelium. In this study, we analyzed the capacity of human adult basal cells isolated from human adult airway tissues to restore a well-differentiated and functional airway epithelium. To this end, we used the human-specific basal cell markers tetraspanin CD151 and tissue factor (TF) to separate positive basal cells from negative columnar cells with a FACSAria cell sorter. Sorted epithelial cells were seeded into epithelium-denuded rat tracheae that were grafted subcutaneously in nude mice and on collagen-coated porous membranes, where they were grown at the air-liquid interface. Sorted basal and columnar populations were also analyzed for their telomerase activity, a specific transit-amplifying cell marker, by the telomeric repeat amplification protocol assay. After cell sorting, the pure and viable CD151/TF-positive basal cell population proliferated on plastic and adhered on epithelium-denuded rat tracheae, as well as on collagen-coated porous membranes, where it was able to restore a fully differentiated mucociliary and functional airway epithelium, whereas viable columnar negative cells did not. Telomerase activity was detected in the CD151/TF-positive basal cell population, but not in CD151/TF-negative columnar cells. These results demonstrate that human adult basal cells are at least airway surface transit-amplifying epithelial cells.

Genetic analysis of myc and telomerase interactions in vivo

Myc is a transcription factor with pleiotropic effects on tumorigenesis which are likely to be mediated by its target genes. A known Myc transcriptional target is the catalytic subunit of telomerase, Tert. However, the contribution of Tert activation to Myc-induced tumorigenesis in vivo remains unknown. In this study, we addressed the role of telomerase in Myc-induced skin papillomatosis by using compound mice with a switchable Myc gene, Inv-MycERTAM mice, in combination with either telomerase deficiency (Terc-/-) or telomerase overexpression (K5-mTert) in the skin. We first demonstrated that Myc activates telomerase in the skin. With Inv-MycERTAM x Terc-/- mice, we further showed that this telomerase activation is partially required to elicit a full hyperplastic Myc-induced response. The presence of critically short telomeres in late-generation Inv-MycERTAM x Terc-/- mice further reduced the skin lesion induced by Myc. On the other hand, telomerase overexpression in the skin of K5-mTert mice augments Myc-induced hyperplasia in the absence of changes in telomere length, suggesting a direct role of telomerase in the Myc protumorigenic response. Taken together, these results highlight telomerase as a mediator of Myc-induced papillomatosis and suggest telomerase as a putative therapeutic target for Myc-dependent lesions.

Regulation of telomerase by human papillomaviruses

The E6 oncoprotein of human papillomaviruses (HPVs) induces telomerase activity in primary human epithelial cells. This activity is dependent on association of E6 with E6AP, a cellular ubiquitin ligase. E6 activates the transcription of hTERT, the catalytic subunit of telomerase. E boxes near the start of hTERT transcription are required for E6; however, acetylated histones are only present in the E6 cells. We identified two isoforms of NFX1, a new binding partner of E6/E6AP. The NFX1- 91 isoform binds to an X-box motif located adjacent to the proximal E box, binds Sin3A and HDACs, repressing hTERT transcription. It preferentially binds E6/E6AP and is targeted for ubiquitin-mediated degradation. The NFX1-123 isoform has the opposite activity, increasing hTERT transcription or translation. This is the first example of viral oncoproteins disrupting regulation of telomerase, a critical event in tumorigenesis.

Somatic stem cells and the origin of cancer

Most human cancers derive from a single cell targeted by genetic and epigenetic alterations that initiate malignant transformation. Progressively, these early cancer cells give rise to different generations of daughter cells that accumulate additional mutations, acting in concert to drive the full neoplastic phenotype. As we have currently deciphered many of the gene pathways disrupted in cancer, our knowledge about the nature of the normal cells susceptible to transformation upon mutation has remained more elusive. Adult stem cells are those that show long-term replicative potential, together with the capacities of self-renewal and multi-lineage differentiation. These stem cell properties are tightly regulated in normal development, yet their alteration may be a critical issue for tumorigenesis. This concept has arisen from the striking degree of similarity noted between somatic stem cells and cancer cells, including the fundamental abilities to self-renew and differentiate. Given these shared attributes, it has been proposed that cancers are caused by transforming mutations occurring in tissue-specific stem cells. This hypothesis has been functionally supported by the observation that among all cancer cells within a particular tumor, only a minute cell fraction has the exclusive potential to regenerate the entire tumor cell population; these cells with stem-like properties have been termed cancer stem cells. Cancer stem cells can originate from mutation in normal somatic stem cells that deregulate their physiological programs. Alternatively, mutations may target more committed progenitor cells or even mature cells, which become reprogrammed to acquire stem-like functions. In any case, mutated genes should promote expansion of stem/progenitor cells, thus increasing their predisposition to cancer development by expanding self-renewal and pluripotency over their normal tendency towards relative quiescency and proper differentiation.