The wide-ranging clinical implications of the short telomere syndromes

There is an increasing number of inherited disorders in which excessive telomere shortening underlies the molecular defect, with dyskeratosis congenita (DC) being the archetypal short telomere syndrome. DC is classically described as a mucocutaneous triad of oral leukoplakia, nail dystrophy and abnormal skin pigmentation. However, excessive telomere shortening can affect almost any organ system, so the clinical manifestations are protean, including developmental delay, cerebellar hypoplasia, exudative retinopathy, aplastic anaemia, acute myeloid leukaemia, idiopathic pulmonary fibrosis, idiopathic hepatic cirrhosis, head and neck cancer and dental abnormalities, and may be multi-systemic. Undiagnosed patients may be seen by essentially any medical subspecialist. Correct diagnosis is important to ensure appropriate management, and for initiating investigations to identify affected family members. Treatment is often supportive, with transplantation offering cure for pulmonary fibrosis or bone marrow failure. Higher rates of mortality and morbidity with transplantation often require regimen alterations, underscoring the need for correct diagnosis. Short telomeres result from mutations in genes essential for telomere maintenance (e.g. genes encoding subunits of the telomerase enzyme complex). Disease severity reflects not only the severity of the defect, but also the inheritance of short telomeres, giving rise to incomplete penetrance and genetic anticipation. Attendees of the inaugural Australian Short Telomere Syndrome Conference were updated on the current scientific and clinical understanding of these disorders, and discussed the best approach for management of these patients in the Australian context. This review will include recommendations from the conference and aims to increase awareness of short telomere disorders.

Upregulation of survivin during immortalization of nontransformed human fibroblasts transduced with telomerase reverse transcriptase

These investigations demonstrate that expression of the inhibitor of apoptosis family member, survivin, is dramatically increased during immortalization of nontransformed human fibroblasts that were transduced with telomerase reverse transcriptase (hTERT). Expression of survivin in immortalized fibroblasts peaked during G(2)/M phase of the cell cycle. However, the upregulation of survivin was dissociated from the rate of proliferation and proportion of G(2)/M cells. Depletion of survivin from immortal fibroblasts increased sensitivity to stress-induced apoptosis and resulted in an accumulation of cells with 4N DNA content. Conversely, overexpression of survivin in mortal fibroblasts conferred resistance to apoptosis. In contrast, very low levels of survivin in proliferating parental fibroblasts had no bearing on sensitivity to apoptosis. The upregulation of survivin did not appear to be a direct consequence of hTERT transduction. However, repression of hTERT resulted in the rapid downregulation of survivin in telomerase-immortalized fibroblasts and tumor cell lines, but not in cells immortalized via an Alternative Lengthening of Telomeres mechanism. These results have important therapeutic implications, as telomerase and survivin are both broadly expressed in human cancers. Selection during the immortalization process for cells expressing high levels of survivin may account for the abundance of survivin in diverse tumor types.

Identification of candidate alternative lengthening of telomeres genes by methionine restriction and RNA interference

Telomerase-negative cancer cells can maintain their telomeres by a recombination-mediated alternative lengthening of telomeres (ALT) process. We reported previously that sequestration of MRE11/RAD50/NBS1 complexes represses ALT-mediated telomere length maintenance, and suppresses formation of ALT-associated promyelocytic leukemia (PML) bodies (APBs). APBs are PML bodies containing telomeric DNA and telomere-binding proteins, and are observed only in a small fraction of cells within asynchronously dividing ALT-positive cell populations. Here, we report that methionine restriction caused a reversible arrest in G0/G1 phase of the cell cycle and reversible induction of APB formation in most cells within an ALT-positive population. We combined methionine restriction with RNA interference to test whether the following proteins are required for APB formation: PML body-associated proteins, PML and Sp100; telomere-associated proteins, TRF1, TRF2, TIN2 and RAP1; and DNA repair proteins, MRE11, RAD50, NBS1 and 53BP1. APB formation was not decreased by depletion of Sp100 (as reported previously) or of 53BP1, although 53BP1 partially colocalizes with APBs. Depletion of the other proteins suppressed APB formation. Because of the close linkage between ALT-mediated telomere maintenance and ability to form APBs, the eight proteins identified by this screen as being required for APB formation are also likely to be required for the ALT mechanism.

Mammalian stanniocalcins and cancer

Stanniocalcin (STC) is a glycoprotein hormone that is secreted by the corpuscle of Stannius, an endocrine gland of bony fish, and is involved in calcium and phosphate homeostasis. The related mammalian proteins, STC1 and STC2, are expressed in a wide variety of tissues. The ovaries have the highest level of STC1, and this increases during pregnancy and lactation. STC1 is present in breast ductal epithelium, and its expression is induced by BRCA1, a tumor suppressor gene that has an important role in breast and ovarian cancer. The expression of STC2 is induced by estrogen, and there is a positive correlation between the level of expression of estrogen receptor and expression of both STC1 and STC2 in breast cancer. This article reviews the data currently available regarding the mammalian STCs, and discusses the roles they may play in normal physiology and in breast and other cancers.

Alternative lengthening of telomeres and survival in patients with glioblastoma multiforme

Despite advances in the molecular pathogenesis of glioblastoma multiforme, no reliable prognostic markers have been identified. We analysed telomerase activity and telomere lengths in glioblastoma multiformes from 77 patients. 19 patients (25%) had tumours with the alternative-lengthening-of-telomere (ALT) phenotype. Median survival for patients with this phenotype was 542 days (95% CI 114-970) compared with 247 days (224-270) for glioblastoma multiformes with normal telomeres (p=0.0003). Cox's regression analysis showed that this association is independent of age. In patients with non-ALT tumours, telomerase activity did not affect survival (median 287 [199-375] vs 236 [230-242] days, p=0.275). We conclude that ALT is a prognostic indicator for patients with glioblastoma multiforme.

Coexistence of alternative lengthening of telomeres and telomerase in hTERT-transfected GM847 cells

It has been shown previously that some immortalized human cells maintain their telomeres in the absence of significant levels of telomerase activity by a mechanism referred to as alternative lengthening of telomeres (ALT). Cells utilizing ALT have telomeres of very heterogeneous length, ranging from very short to very long. Here we report the effect of telomerase expression in the ALT cell line GM847. Expression of exogenous hTERT in GM847 (GM847/hTERT) cells resulted in lengthening of the shortest telomeres; this is the first evidence that expression of hTERT in ALT cells can induce telomerase that is active at the telomere. However, rapid fluctuation in telomere length still occurred in the GM847/hTERT cells after more than 100 population doublings. Very long telomeres and ALT-associated promyelocytic leukemia (PML) bodies continued to be generated, indicating that telomerase activity induced by exogenous hTERT did not abolish the ALT mechanism. In contrast, when the GM847 cell line was fused with two different telomerase-positive tumor cell lines, the ALT phenotype was repressed in each case. These hybrid cells were telomerase positive, and the telomeres decreased in length, very rapidly at first and then at the rate seen in telomerase-negative normal cells. Additionally, ALT-associated PML bodies disappeared. After the telomeres had shortened sufficiently, they were maintained at a stable length by telomerase. Together these data indicate that the telomerase-positive cells contain a factor that represses the ALT mechanism but that this factor is unlikely to be telomerase. Further, the transfection data indicate that ALT and telomerase can coexist in the same cells.

Pex19p dampens the p19ARF-p53-p21WAF1 tumor suppressor pathway

We isolated a 33-kDa protein, Pex19p/HK33/HsPXF, as a p19ARF-binding protein in a yeast two-hybrid screen. We demonstrate here that Pex19p interacts with p19ARF in the cell cytoplasm and excludes p19ARF from the nucleus, leading to a concurrent inactivation of p53 function. Down-regulation of Pex19p by its antisense expression resulted in increased levels of p19ARF, increased p53 function, and a p53/p21WAF1-mediated senescence-like cell cycle arrest. The data demonstrated a novel mechanism of down-regulation of the p19ARF-p53 pathway.

Effects of simian virus 40 T-antigens on normal human mammary epithelial cells reveal evidence for spontaneous alterations in addition to loss of p16(INK4a) expression

Under standard culture conditions, normal human mammary epithelial cells (HMECs) divide a limited number of times before proliferation ceases in a growth-arrested state referred to as selection. Cells that have undergone spontaneous loss of p16(INK4a) expression due to hypermethylation of the p16(INK4a) CpG island emerge from selection and proliferate for an extended, but limited, period before senescence. Here we show, as expected, that selection was bypassed by expression of SV40 large T-antigen proteins containing an intact pRb-binding domain in preselection cells. These cells were immortalized with high efficiency (seven of nine separate cultures). Also as expected, postselection cells were immortalized by expression of the human papillomavirus-16 E6 oncoprotein (four of four cultures), which inactivates p53 protein. In contrast, we found that expression of SV40 large T-antigen protein, which also inactivates p53, was poorly maintained in postselection cultures due to its growth-suppressive effects; consequently, these cells became immortalized at low efficiency (one of 11 cultures). Reexpression of p16(INK4a) in postselection HMECs by the demethylating agent, 5-azacytidine, or transfection of a p16(INK4a) expression plasmid did not restore the ability of these cells to undergo SV40-induced transformation. Postselection HMECs are a widely used in vitro model system, but these observations indicate they have undergone changes in gene expression in addition to loss of p16(INK4a) expression.

Alternative lengthening of telomeres in human cells

Activation of a telomere maintenance mechanism appears to be essential for immortalization. In most human tumors and tumor cell lines, the telomere maintenance mechanism involves the activity of telomerase, a reverse transcriptase holoenzyme that synthesizes telomeric repeat DNA. In some cases, telomere maintenance occurs in the absence of telomerase activity by a mechanism referred to as alternative lengthening of telomeres (ALT). The development of telomere-targeted anticancer therapies will be facilitated by an understanding of the molecular mechanisms of ALT and of the means whereby ALT is repressed in normal cells.

Transcriptional inactivation of p53 by deletions and single amino acid changes in mouse mot-1 protein

Mouse mortalin proteins, mot-1 and mot-2, differ by only two amino acid residues in their C-terminus. In previous studies we showed that they differ in their subcellular distributions and interactions with the tumor suppressor protein, p53. By using mot-1 deletion mutants and amino acid substitution constructs, we report here that inability of mot-1 to affect p53 activity in vivo is dependent on the presence of both of the unique mot-1 amino acids and all three of the predicted hsp70, EF hand, and leucine zipper motif regions. The two proteins and their single amino acid mutants showed different mobilities on SDS-polyacrylamide gel presenting an evidence for their different secondary structures. Taken together, the data suggest that each of the two differing amino acids between mot-1 and mot-2 is an important determinant of their secondary structures and in vivo activities.

Selective toxicity of MKT-077 to cancer cells is mediated by its binding to the hsp70 family protein mot-2 and reactivation of p53 function

MKT-077, a cationic rhodacyanine dye analogue has been under preclinical cancer therapeutical trials because of its selective toxicity to cancer cells. Its cellular targets and mechanism of action remain poorly understood. Here we report that MKT-077 binds to an hsp70 family member, mortalin (mot-2), and abrogates its interactions with the tumor suppressor protein, p53. In cancer cells, but not in normal cells, MKT-077 induced release of wild-type p53 from cytoplasmically sequestered p53-mot-2 complexes and rescued its transcriptional activation function. Thus, MKT-077 may be particularly useful for therapy of cancers with wild-type p53.

Telomere maintenance by recombination in human cells

Telomeres of eukaryotic chromosomes contain many tandem repeats of a G-rich sequence (for example, TTAGGG in vertebrates). In most normal human cells, telomeres shorten with each cell division, and it is proposed that this limits the number of times these cells can replicate. Telomeres may be maintained in germline cells, and in many immortalized cells and cancers, by the telomerase holoenzyme (first discovered in the ciliate Tetrahymena), which uses an RNA subunit as template for synthesis of telomeric DNA by the reverse transcriptase catalytic subunit. Some immortalized human cell lines and some tumours maintain their telomeres in the absence of any detectable telomerase activity by a mechanism referred to as alternative lengthening of telomeres (ALT). Here we show that DNA sequences are copied from telomere to telomere in an immortalized human ALT cell line, indicating that ALT occurs by means of homologous recombination and copy switching.

Identification of a novel human mitochondrial D-loop RNA species which exhibits upregulated expression following cellular immortalization

We report the identification and characterization of a novel human mitochondrial RNA species approximately 0.47 kb long that is transcribed from the mtDNA L-strand and is derived from the D-loop. Its expression increases when human cells become immortal, a key event in tumorigenesis. The RNA is therefore designated IDL (Immortalization-associated D-Loop). Sequence and hybrid cell analyses suggest that the increased level of IDL RNA in immortal cells is due to a recessive change, possibly in the activity of a trans-acting factor that controls IDL RNA expression.

The hTERTalpha splice variant is a dominant negative inhibitor of telomerase activity

The telomerase catalytic subunit (hTERT) is an essential component of the holoenzyme complex that adds telomeric repeats to the ends of human chromosomes. Maintenance of telomeres by telomerase or another mechanism is required for cell immortalization, and loss of telomeric DNA has been proposed as a trigger for cellular senescence. Available evidence suggests that regulation of telomerase activity primarily depends on transcriptional control of hTERT. However, several human tissues as well as some normal cell strains have been shown to express low levels of hTERT mRNA even though they lack telomerase activity. We have previously identified six splice variants of hTERT, including a "deletion" variant (hTERTalpha) that is missing conserved residues from the catalytic core of the protein. Several of the deletion variants have been detected in normal and developing human tissues. We now show that hTERTalpha inhibits endogenous telomerase activity, which results in telomere shortening and chromosome end-to-end fusions. Telomerase inhibition induced a senescence-like state in HT1080 cells and apoptosis in a jejunal fibroblast cell line. These results suggest a possible role for hTERT splice variants in the regulation of telomerase activity.

Stanniocalcin 1 and 2 are secreted as phosphoproteins from human fibrosarcoma cells

Stanniocalcin 1 (STC1) and stanniocalcin 2 (STC2) are two recently identified mammalian peptide hormones. STC1 plays a role in calcium and phosphate homoeostasis, while the role of STC2 is unknown. We examined a human fibrosarcoma cell line, HT1080, that has high steady-state STC1 and STC2 mRNA levels, to determine whether these proteins are secreted. Following incubation of HT1080 cells with (32)P, labelled STC1 and STC2 were found to be secreted into the medium. STC1 was phosphorylated in vitro by protein kinase C (PKC). In vitro and in vivo phosphorylation both occurred exclusively on serine and the phosphopeptide maps were similar, suggesting that PKC might be the in vivo kinase. STC2 was phosphorylated in vitro by casein kinase II (CK2), in vitro and in vivo phosphorylation were exclusively on serine and the phosphopeptide maps were indistinguishable. Phosphorylation of STC2 in intact cells resulted from the action of an ecto-protein kinase, since exogenous STC2 was phosphorylated by HT1080 cells and no phosphorylated STC2 was detectable inside the cells. The ectokinase activity was abolished by heparin and GTP could substitute for ATP as the phosphate donor, indicative of an ecto-CK2-like activity. The in vitro CK2 phosphorylation site was shown by matrix-assisted laser-desorption ionization-time-of-flight MS to be a single serine located between Ser-285 and Ser-298 in the C-terminal region of STC2. This is the first report of the secretion of STC1 or STC2 from mammalian cells. We conclude that these human fibrosarcoma cells express both STC1 and STC2 as secreted phosphoproteins in vivo, with STC2 being phosphorylated by an ecto-CK2-like enzyme.

Structurally and functionally distinct mouse hsp70 family members Mot-1 and Mot-2 proteins are encoded by two alleles

The mouse mortalin proteins Mot-1 and Mot-2 differ by two amino acids in their carboxy-terminus. These proteins are differentially localized in the cell cytoplasm and have contrasting biological activities. The genetic relationship between Mot-1 and Mot-2 was deciphered by mouse family analyses. Mot-1 and Mot-2 segregated in F1 and F2 progeny, providing direct evidence that the two proteins are encoded by two alleles.

The distribution of stanniocalcin 1 protein in fetal mouse tissues suggests a role in bone and muscle development

We previously isolated a mammalian gene STC1 that encodes a glycoprotein related to stanniocalcin (STC), a fish hormone that plays a major role in calcium homeostasis. However, the mammalian STC1 gene is expressed in a variety of adult tissues in contrast to fish where STC is expressed only in one unique gland, the corpuscles of Stannius. This suggested that STC1 may have wider autocrine/paracrine functions in mammals. In the present study, using immunocytochemistry, we showed that STC1 protein is localized in the developing bone and muscle of the mouse fetus. During endochondral bone formation, STC1 is found principally in prechondrocytes and prehypertrophic chondrocytes. During intramembranous bone formation STC1 is present in the mesenchyme that is about to undergo ossification. STC1 is also found in the myocardiocytes of the developing heart and at all stages of differentiation from myoblasts to myotube formation in developing skeletal muscle. The specific localization of STC1 to chondrocytes and muscle cells suggests a role for this protein in chondrogenic and myogenic differentiation.

The role of senescence and immortalization in carcinogenesis

Normal somatic cells are able to divide only a limited number of times before they become senescent. The occurrence of intratumoral cell death and the need for clonal evolution mean that many more cell divisions are required for tumorigenesis than is possible unless cells breach the senescence proliferation barrier and become immortalized. Senescence may therefore be a major tumor suppressor mechanism. During the past decade the study of senescence and immortalization has entered the mainstream of cancer research. A major reason for the current interest in this subject is the observation that most cancers have an activated telomere maintenance mechanism, a marker of immortalization. It has also been found that some of the most common genetic changes known to occur in cancer have a key role in the immortalization process.

Methylation of the human telomerase gene CpG island

The acquisition of expression of hTERT, the catalytic subunit of the telomerase enzyme, seems to be an essential step in the development of a majority of human tumors. However, little is known about the mechanisms preventing telomerase gene expression in normal and transformed cells that do not express hTERT. Using a methylation-specific PCR-based assay, we have found that the CpG island associated with the hTERT gene is unmethylated in telomerase-negative primary tissues and nonimmortalized cultured cells, indicating that mechanisms independent of DNA methylation are sufficient to prevent hTERT expression. The hTERT CpG island is methylated in many telomerase-negative and telomerase-positive cultured cells and tumors, but the extent of methylation did not correlate with expression of hTERT. Demethylation of DNA with 5-azacytidine in two cell lines induced expression of hTERT, suggesting that DNA methylation can contribute to hTERT repression in some cells. Together, these data show that the hTERT CpG island can undergo cytosine methylation in cultured cells and tumors and that DNA methylation may contribute to the regulation of the hTERT gene, but that CpG island methylation is not responsible for repressing hTERT expression in most telomerase-negative cells.

Constructing immortalized human cell lines

Obtaining an abundant supply of human cells with specific differentiated properties is a key issue in many research studies and biotechnology applications. Recent advances in this area include the finding that forced expression of the enzyme telomerase can greatly increase the proliferative potential of at least some types of human cells, and also the development of methods for culturing human stem cells.

Inhibition of SV40 large T antigen induced apoptosis by small T antigen

It is well established that the expression of simian virus 40 (SV40) early gene products causes oncogenic transformation of rodent cells. An important aspect of this process is the inactivation of the p53 and retinoblastoma (pRb) tumour suppressor proteins through interaction with the SV40 large tumour antigen (LT). In addition, the SV40 small tumour antigen (ST) may enhance LT induced transformation. Here we show that LT induces apoptotic cell death in rat embryo fibroblast (REF) cells and that ST functions to inhibit this effect by a mechanism which is different from other known anti-apoptotic proteins. Mutational analysis of LT indicates that mutants defective in the pRb-binding domain are unable to induce apoptosis whereas LT mutants defective in the p53-binding domain are still competent to induce apoptosis. Thus, interaction between LT and one or more pRb family members must occur for induction of apoptosis and that binding of p53 by LT is insufficient to inhibit LT induced apoptosis in REFs. The data presented herein suggest that the anti-apoptotic function of ST may explain, at least in part, how ST contributes to SV40 early region induced transformation of REF cells.

Telomerase-negative immortalized human cells contain a novel type of promyelocytic leukemia (PML) body

Telomerase-negative immortalized human cells maintain their telomeres by a mechanism known as alternative lengthening of telomeres (ALT). We report here that ALT cells contain a novel promyelocytic leukemia (PML) body (ALT-associated PML body, APB). APBs are large donut-shaped nuclear structures containing PML protein, telomeric DNA, and the telomere binding proteins human telomere repeat binding factors 1 and 2. Immunostaining showed that APBs also contain replication factor A, RAD51, and RAD52, proteins involved in DNA synthesis and recombination. During immortalization, APBs appeared at exactly the same time as activation of ALT. APBs were found in ALT tumors and cell lines but not in mortal cell strains or in telomerase-positive cell lines or tumors.

Identification of a 55-kDa ezrin-related protein that induces cytoskeletal changes and localizes to the nucleolus

Normal and transformed human cells when stained for ezrin, an F-actin-binding ERM (ezrin/radixin/moesin) family protein, revealed a faint and intense immunofluorescence, respectively. Surprisingly, nuclear staining that was assigned to the nucleolus by confocal laser and immunoelectron microscopy was detected in both cell types and was more prominent in normal cells due to the absence of glistering cytoplasmic fluorescence. By Western analysis the nuclear fraction was seen to have a 55-kDa ezrin-reactive protein that did not react to the antibodies raised against the C-terminus of the protein, suggesting that it may correspond to an endogenously cleaved N-terminus of the protein. Transfections of cells with a cDNA encoding full-length ezrin tagged with green fluorescent protein (GFP) at its N-terminus indeed resulted in two GFP-tagged products corresponding to full-length and 55-kDa endogenously cleaved forms. Transfection with a cDNA encoding approximately 55 kDa of the ezrin N-terminus (N-ezrin) showed that it can translocate to the nucleus. N-ezrin transfected cells exhibited irregular cell edges and collapse of actin fibers. Similar changes were seen following microinjection of anti-p81/ezrin antibody, suggesting that N-ezrin may function as a dominant negative competitor of ezrin. These data demonstrate the existence of an N-terminal cleavage form of ezrin that localizes to the nucleolus and that its overexpression induces cytoskeletal changes.

Expression and localisation of stanniocalcin 1 in rat bladder, kidney and ovary

Bony fish use the glycoprotein hormone stanniocalcin (STC) to counteract hypercalcaemia. This is achieved through dual mechanisms involving gill calcium uptake inhibition and stimulation of renal inorganic phosphate reabsorption. Human STC (hSTC-1) shows considerable homology with both rat and mouse STC (mSTC) and their mRNA is expressed in a wide range of tissues. In fish, STC is produced by endocrine glands known as the corpuscles of Stannius but in mammals the widespread expression is suggestive of a paracrine rather than an endocrine role. In order to determine the distribution and strucutral characteristics of hSTC-1, the recombinant protein was expressed in bacteria, purified by metal-ion affinity chromatography, and a study was made of the likely epitopes for raising an antibody. This novel hSTC-1 antibody was used to test the purification protocol. Since the role of mammalian STC is largely unknown, the specific distribution of STC needed to be addressed. To test the specificity of the antibody, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)/Western blotting was undertaken in homogenised rat bladder, ovary and kidney.

Repression of an alternative mechanism for lengthening of telomeres in somatic cell hybrids

Some immortalized cell lines maintain their telomeres in the absence of detectable telomerase activity by an alternative (ALT) mechanism. To study how telomere maintenance is controlled in ALT cells, we have fused an ALT cell line GM847 (SV40 immortalized human skin fibroblasts) with normal fibroblasts or with telomerase positive immortal human cell lines and have examined their proliferative potential and telomere dynamics. The telomeres in ALT cells are characteristically very heterogeneous in length, ranging from very short to very long. The ALT x normal hybrids underwent a rapid reduction in telomeric DNA and entered a senescence-like state. Immortal segregants rapidly reverted to the ALT telomere phenotype. Fusion of ALT cells to telomerase-positive immortal cells in the same immortalization complementation group resulted in hybrids that appeared immortal and also exhibited repression of the ALT telomere phenotype. In these hybrids, which were all telomerase-positive, we observed an initial rapid loss of most long telomeres, followed either by gradual loss of the remaining long telomeres at a rate similar to the rate of telomere shortening in normal telomerase-negative cells, or by maintenance of shortened telomeres. These data indicate the existence of a mechanism of rapid telomere deletion in human cells. They also demonstrate that normal cells and at least some telomerase-positive immortal cells contain repressors of the ALT telomere phenotype.

p16(INK4a) and the control of cellular proliferative life span

Normal somatic cells have a limited proliferative capacity in vitro: after a finite number of cell divisions they eventually enter a non-proliferative state referred to as senescence. Senescence is thought to be a major tumor suppressor mechanism, and many cancers contain cells that have escaped from senescence and become immortalized. The role of telomerase activation in immortalization is currently attracting considerable attention, but immortalization is often associated with other changes including loss of normal function of the tumor suppressor locus, INK4a/ARF. Two proteins, p16(INK4a) and p14(ARF), are encoded by this locus. Here we focus on p16(INK4a) and review accumulating evidence that loss of p16(INK4a) function may be involved in escape from the normal limits on cellular proliferative life span.

Cloning and characterization of a novel gene, striamin, that interacts with the tumor suppressor protein p53

Expression analysis of a novel cDNA isolated from immortal murine fibroblasts revealed a single transcript of 3.0 kilobase pairs that was highly expressed in mouse and human striated muscle and in mouse heart. The gene has therefore been named striamin. Its expression was confined to skeletal muscle types with a fast glycolytic (2B) contractile phenotype. It was also detected in C2C12 mouse myoblasts and was down-regulated during in vitro myogenesis. The cDNA has a single open reading frame encoding a predicted 16.8-kDa protein of 149 amino acids with no homology to known proteins. Microinjection and transfection of green fluorescence protein-tagged striamin demonstrated that it localizes to the nucleus. Coimmunoprecipitations revealed that it can interact with p53 (a positive marker for myoblast differentiation) in vivo and in vitro. Furthermore, it repressed p53 activity in p53-mediated reporter assays. Fluorescence in situ hybridization with a mouse P1 genomic clone localized the gene to chromosome 12C3, which is syntenic to human chromosome 14q21-22.

Telomere maintenance mechanisms and cellular immortalization

Immortal cell populations are able to proliferate indefinitely. Immortalization is associated with activation of processes that compensate for the telomeric shortening that accompanies cell division in normal somatic cells. In many immortal cell lines, telomere maintenance is provided by the action of the ribonucleoprotein enzyme complex, telomerase. Some immortal cell lines have undetectable or very low levels of telomerase activity and there is evidence that these cells maintain their telomeres by an alternative mechanism.

Downregulation of metallothionein-IIA expression occurs at immortalization

Metallothioneins (MTs) may modulate a variety of cellular processes by regulating the activity of zinc-binding proteins. These proteins have been implicated in cell growth regulation, and their expression is abnormal in some tumors. In particular, MT-IIA is expressed 27-fold less in human colorectal tumors and tumor cell lines compared with normal tissue (Zhang et al., 1997). Here we demonstrate that MT-IIA downregulation occurs when human cells become immortal, a key event in tumorigenesis. After immortalization MT-IIA expression remains inducible but the basal activity of the MT-IIA promoter is decreased. MT-IIA downregulation at immortalization is one of the most common immortalization-related changes identified to date, suggesting that MT-IIA has a role in this process.

A reassessment of the telomere hypothesis of senescence

According to the telomere hypothesis of senescence, the telomeric shortening that accompanies the replication of normal somatic cells acts as the mitotic clock that eventually results in their permanent exit from the cell cycle. Although evidence consistent with the telomere hypothesis continues to accumulate, on the basis of recent findings it is suggested that instead of a single clock mechanism there are multiple inducers of senescence.

Genes involved in the control of cellular proliferative potential

Evidence that control of cellular proliferative potential may be linked to telomere length, along with data indicating that other factors may also be involved, will be reviewed. According to the telomere hypothesis of senescence, the sequential loss of telomeric repeat DNA that occurs during the replication of normal somatic cells eventually dictates the onset of the permanently nonreplicative state known as senescence. Many immortalized cells express telomerase, a ribonucleoprotein enzyme that replaces the telomeric DNA that would otherwise be lost due to replication. However, some immortalized human cells may avoid telomeric shortening without using telomerase. The mechanism involved is currently unknown, but other eukaryotes are able to replace telomeric DNA through (1) recombination and copy switching or (2) retrotransposition. Human fibroblasts that lose p53 function proliferate a limited number of times beyond the population-doubling level at which their normal counterparts become senescent. Lack of functional retinoblastoma (Rb) protein (or equivalent events, such as loss of p16INK4 function, resulting in abrogation of Rb regulatory activity) also permits a temporary extension of proliferative potential. The p53 and pRb effects are additive, indicating that they exert their control on proliferative potential separately. The temporary life span extension associated with loss of p53 and/or Rb pathway function is accompanied by continued telomere shortening. The proliferation arrest that eventually ensues in p53-minus cells or in p53-minus/Rb-minus cells may be regarded as terminal proliferation arrest states serving as a backup to senescence. p53-minus/Rb-minus cells cannot proliferate further unless they acquire the ability to prevent telomeric shortening. Somatic cell hybridization and microcell-mediated chromosome transfer experiments indicate that immortalization involves the loss of function of other, as yet unidentified, genes; some of these may normally repress telomerase expression in somatic cells.

Correction of the copper transport defect of Menkes patient fibroblasts by expression of the Menkes and Wilson ATPases

Menkes' disease is a fatal, X-linked, copper deficiency disorder that results from defective copper efflux from intestinal cells and inadequate copper delivery to other tissues, leading to deficiencies of critical copper-dependent enzymes. Wilson's disease is an autosomally inherited, copper toxicosis disorder resulting from defective biliary excretion of copper, which leads to copper accumulation in the liver. The ATP7A and ATP7B genes that are defective in patients with Menkes' and Wilson's diseases, respectively, encode transmembrane, P-type ATPase proteins (ATP7A or MNK and ATP7B or WND, respectively) that function to translocate copper across cellular membranes. In this study, the cDNAs derived from a normal human ATP7A gene and the murine ATP7B homologue, Atp7b, were separately transfected into an immortalized fibroblast cell line obtained from a Menkes' disease patient. Both MNK and WND expressed from plasmid constructs were able to correct the copper accumulation and copper retention phenotype of these cells. However, the two proteins responded differently to elevated extracellular copper levels. Although MNK showed copper-induced trafficking from the trans-Golgi network to the plasma membrane, in the same cell line the intracellular location of WND did not appear to be affected by elevated copper.

Inactivation of tumor suppressor p53 by mot-2, a hsp70 family member

The mortalin genes, mot-1 and mot-2, are hsp70 family members that were originally cloned from normal and immortal murine cells, respectively. Their proteins differ by only two amino acid residues but exhibit different subcellular localizations, arise from two distinct genes, and have contrasting biological activities. We report here that the two proteins also differ in their interactions with the tumor suppressor protein p53. The pancytosolic mot-1 protein in normal cells did not show colocalization with p53; in contrast, nonpancytosolic mot-2 and p53 overlapped significantly in immortal cells. Transfection of mot-2 but not mot-1 resulted in the repression of p53-mediated transactivation in p53-responsive reporter assays. Inactivation of p53 by mot-2 was supported by the down-regulation of p53-responsive genes p21(WAF-1) and mdm-2 in mot-2-transfected cells only. Furthermore, NIH 3T3 cells transfected with expression plasmid encoding green fluorescent protein-tagged mot-2 but not mot-1 showed an abrogation of nuclear translocation of wild-type p53. These results demonstrate a novel mechanism of p53 inactivation by mot-2 protein.

Differential display of mRNA

Differential display of mRNA (DD) is a technique in which mRNA species expressed by a cell population are reverse transcribed and then amplified by many separate polymerase chain reactions (PCR). PCR primers and conditions are chosen so that any given reaction yields a limited number of amplified cDNA fragments, permitting their visualization as discrete bands following gel electrophoresis. This robust and relatively simple procedure allows identification of genes that are differentially expressed in different cell populations. Here we review DD including some recent modifications, and compare it with other techniques for analyzing differential mRNA expression.

Malignant transformation of NIH3T3 cells by overexpression of mot-2 protein

The murine mortalin genes, mot-1 and mot-2, are members of the hsp70 family of proteins and differ from each other by only two amino acid residues. Mot-1 is expressed in normal cells and has pancytosolic cellular distribution whereas mot-2 is found in the perinuclear region of immortal cells. We report here that a high level of expression of mot-2 protein resulted in malignant transformation of cells as analysed by anchorage independent growth and nude mice assays. A high level of protein expression is attributed to the 900 bp 3' untranslated region of the cDNA which does not have any transforming activity per se. Mortalin cDNA clones isolated from human transformed cells were also found to have transforming activity in similar assays and a high level of expression was apparent in some of the human immortalized cells that showed non-pancytosolic mortalin immunofluorescence. Taken together, the data suggest that nonpancytosolic mortalin may have a role in tumorigenesis.

Loss of p16INK4 expression by methylation is associated with lifespan extension of human mammary epithelial cells

Inactivation of p16INK4 tumor suppressor gene function is frequently observed in breast cancer. We examined p16INK4 expression in human mammary epithelial cell (HMEC) cultures established from four normal donors. Normal HMECs divide a limited number of times before proliferation ceases in a state referred to as selection (or M0). The cell subpopulation that emerges spontaneously from selection undergoes a further limited period of proliferation before senescence. By immunofluorescence and Western blot analysis of four independent cultures, we have shown loss of p16INK4 expression in postselection HMECs. In contrast, p16INK4 was present in both early and late passage fibroblasts from the same individuals. Bisulfite genomic sequencing revealed extensive methylation of the p16INK4 CpG island in post- but not preselection cells. Thus, the extended period of growth observed in postselection HMECs is associated with hypermethylation of the p16INK4 CpG island and loss of p16INK4 expression. Although postselection HMECs are widely considered to be normal, these data indicate that they have sustained an epigenetic alteration.

Identification of a second stanniocalcin cDNA in mouse and human: stanniocalcin 2

It has recently been shown that mammals contain a gene encoding a protein that is related to stanniocalcin (STC), a hormone previously considered to be present only in bony fish where it plays a major role in Ca2+ homeostasis. Here we report the identification in the mouse and human of a cDNA, STC-2, that shows significant similarity to the first mammalian STC (which we now rename STC-1). Northern analysis revealed that mammalian STC-2, like STC-1, is expressed in a wide variety of tissues. In contrast to STC-1, the predicted amino acid sequence of STC-2 contains a cluster of histidine residues in the C-terminal portion of the protein, which suggests that STC-2 may interact with metal ions.

Telomere length dynamics in telomerase-positive immortal human cell populations

It has been proposed that the progressive shortening of telomeres in somatic cells eventually results in senescence. Previous experiments have demonstrated that many immortal cell lines have acquired telomerase activity leading to stabilization of telomere length. Telomere dynamics and telomerase activity were examined in the telomerase-positive immortal cell lines HeLa and 293 and subclones derived from them. A mass culture of HeLa cells had a stable mean telomere length over 60 population doublings (PD) in vitro. Subclones of this culture, however, had a range of mean telomere lengths indicating that telomeric heterogeneity exists within a population with a stable mean telomere length. Some of the subclones lacked detectable telomerase activity soon after isolation but regained it by PD 18, suggesting that at least some of the variation in telomere length can be attributed to variations in telomerase activity levels. 293 subclones also varied in telomere length and telomerase activity. Some telomerase-positive 293 subclones contained long telomeres that gradually shortened, demonstrating that factors other than telomerase also act to modulate telomere length. Fluctuations in telomere length in telomerase-positive immortalized cells may contribute to chromosomal instability and clonal evolution.

Human stanniocalcin (STC): genomic structure, chromosomal localization, and the presence of CAG trinucleotide repeats

Stanniocalcin (STC) is a glycoprotein hormone that is secreted by the corpuscle of Stannius, an endocrine gland of bony fish. It prevents hypercalcemia via mechanisms including inhibition of calcium uptake across the gills. Mammalian homologues have recently been reported but their function is unknown. Here we report the genomic organization and the transcription start site of the human STC gene and the existence of a polymorphic CAG trinucleotide repeat complex within the 5' untranslated region (UTR) of the mRNA and a smaller [CAG]6 repeat in the 3' UTR. As CAG repeats are associated with various human diseases, we used dual-color fluorescence in situ hybridization to localize the STC gene near markers D8S131 and D8S339 on chromosome 8p11.2-p21. STC should be considered a candidate gene for hereditary diseases mapped to this region.

Genetic changes associated with immortalization. A review

Human fibroblasts appear to have three terminal proliferative arrest (TPA) states that act as independent barriers to immortalization. The first of these TPA states is senescence, and several recent studies have shown that abrogation of p53 function permits temporary escape from senescence that ends in a poorly characterized form of arrest (referred to as p53-minus TPA) in which the pRB and p16INK4 genes appear to be involved. Abrogation of the function of both p53 and pRB (or p16INK4) results in continued proliferation until the cells enter crisis. Escape from crisis is always associated with the activation of a telomere maintenance mechanism. We also review evidence for the involvement of other genes in the immortalization process. Immortalization appears to be a complex process involving many genetic changes, not all of which are necessarily related to telomere maintenance.

Isolation of a candidate human telomerase catalytic subunit gene, which reveals complex splicing patterns in different cell types

Telomerase is a multicomponent reverse transcriptase enzyme that adds DNA repeats to the ends of chromosomes using its RNA component as a template for synthesis. Telomerase activity is detected in the germline as well as the majority of tumors and immortal cell lines, and at low levels in several types of normal cells. We have cloned a human gene homologous to a protein from Saccharomyces cerevisiae and Euplotes aediculatus that has reverse transcriptase motifs and is thought to be the catalytic subunit of telomerase in those species. This gene is present in the human genome as a single copy sequence with a dominant transcript of approximately 4 kb in a human colon cancer cell line, LIM1215. The cDNA sequence was determined using clones from a LIM1215 cDNA library and by RT-PCR, cRACE and 3'RACE on mRNA from the same source. We show that the gene is expressed in several normal tissues, telomerase-positive post-crisis (immortal) cell lines and various tumors but is not expressed in the majority of normal tissues analyzed, pre-crisis (non-immortal) cells and telomerase-negative immortal (ALT) cell lines. Multiple products were identified by RT-PCR using primers within the reverse transcriptase domain. Sequencing of these products suggests that they arise by alternative splicing. Strikingly, various tumors, cell lines and even normal tissues (colonic crypt and testis) showed considerable differences in the splicing patterns. Alternative splicing of the telomerase catalytic subunit transcript may be important for the regulation of telomerase activity and may give rise to proteins with different biochemical functions.

Immortalized cells with no detectable telomerase activity. A review

Immortalization of human cells in culture is usually associated with expression of telomerase activity. In some cases, however, no telomerase activity is detectable even though comparison of the terminal restriction fragment (TRF) pattern before and after immortalization shows that lengthening of telomeres has occurred. The extreme heterogeneity in telomere length and the differences in the dynamics of telomere maintenance in telomerase-negative cell lines compared to telomerase-positive cell lines indicate that these cells have utilized one or more alternative mechanisms for lengthening of telomeres (ALT). All telomerase-negative immortalized cell lines examined to date show evidence of ALT activity, consistent with the hypothesis that telomere maintenance either by telomerase or by ALT is required for immortalization. The nature of the ALT mechanism(s) is currently unknown, but studies of telomere dynamics in an ALT cell line containing a marker just proximal to the telomeric sequences show gradual shortening of the telomere followed by rapid elongation. This is consistent with a non-reciprocal recombinational mechanism similar to that found in telomerase-defective mutant yeast strains.

Reassessment of immortalization complementation group D

Previous somatic cell hybridization studies have assigned many human cell lines to one of four complementation groups (A-D) for immortalization. We report here that the A1698DM cell line, which contains selectable markers and has previously been defined as the immortalization group D representative, was derived from T24 cells rather than A1698. A1698DM did not undergo senescence when fused with cell lines assigned to groups A, B, or C. This raises the possibility that this cell line has undergone further evolution and lost multiple putative senescence genes so that it is now unable to complement any, or most, other cell lines for senescence. Cell lines previously assigned to group D may, therefore, be heterogeneous with respect to the genetic changes that resulted in their immortalization. This has important implications for strategies to clone senescence genes based on complementation groups.

Evidence for an alternative mechanism for maintaining telomere length in human tumors and tumor-derived cell lines

The gradual loss of DNA from the ends of telomeres has been implicated in the control of cellular proliferative potential. Telomerase is an enzyme that restores telomeric DNA sequences, and expression of its activity was thought to be essential for the immortalization of human cells, both in vitro and in tumor progression in vivo. Telomerase activity has been detected in 50-100% of tumors of different types, but not in most normal adult somatic tissues. It has also been detected in about 70% of human cell lines immortalized in vitro and in all tumor-derived cell lines examined to date. It has previously been shown that in vitro immortalized telomerase-negative cell lines acquire very long and heterogeneous telomeres in association with immortalization presumably via one or more novel telomere-lengthening mechanisms that we refer to as ALT (alternative lengthening of telomeres). Here we report evidence for the presence of ALT in a subset of tumor-derived cell lines and tumors. The maintenance of telomeres by a mechanism other than telomerase, even in a minority of cancers, has major implications for therapeutic uses of telomerase inhibitors.

Normal telomere maintenance in immortal ataxia telangiectasia cell lines

Telomeres are maintained in germ line cells and immortal cell lines, but shorten with each cell division in most somatic cells. Blood lymphocytes from individuals with ataxia telangiectasia (AT) demonstrate an accelerated rate of telomere shortening and high levels of telomere associations. This accelerated loss of telomeres in somatic cells in AT could be due to either the loss of more telomeric DNA with every cell division or an increased rate of cell division. The gene for AT shares homology with the yeast TEL1 gene, in which mutations result in abnormally shortened telomeres. Thus, mutations in the gene for ataxia telangiectasia may also influence the ability of germ line cells and immortal cell lines to properly maintain telomere homeostasis. To investigate a possible defect of telomere maintenance in AT we have analyzed 8 simian virus 40 (SV40)-immortalized AT cell lines and twelve SV40-immortalized non-AT cell lines for both telomerase activity and telomere length. The results demonstrate that telomere length in AT cells is maintained via telomerase or an alternative (ALT) pathway in a manner indistinguishable from cell lines derived from normal cells. We also investigated telomere dynamics in one telomerase-positive AT cell line by analyzing the changes in the length of a single telomere, and found that this telomere maintained its equilibrium mean length (EML) similar to normal cell lines with stable chromosomes. The combined results show no significant differences between the telomeres of immortal AT and non-AT cell lines, demonstrating that the absence of wild-type ATM does not result in a fundamental defect in telomere maintenance in these cells.

The telomere lengthening mechanism in telomerase-negative immortal human cells does not involve the telomerase RNA subunit

According to the telomere hypothesis of senescence, the progressive shortening of telomeres that occurs upon division of normal somatic cells eventually leads to cellular senescence. The immortalisation of human cells is associated with the acquisition of a telomere maintenance mechanism which is usually dependent upon expression of the enzyme telomerase. About one third of in vitro immortalised human cell lines, however, have no detectable telomerase but contain telomeres that are abnormally long. The nature of the alternative telomere maintenance mechanism (referred to as ALT, for Alternative Lengthening of Telomeres) that must exist in these telomerase-negative cells has not been elucidated. It has previously been shown that abnormal lengthening of yeast telomeres may occur due to mutations in the yeast telomerase RNA gene. That this is not the mechanism of the abnormally long telomeres in ALT cell lines was demonstrated by the finding that seven of seven ALT lines have wild-type human telomerase RNA (hTR) sequence, including a novel polymorphism that is present in 30% of normal individuals. We found that two ALT cell lines have no detectable expression of the hTR gene. This shows that the ALT mechanism in these cell lines is not dependent on hTR. Expression of exogenous hTR via infection of these cells with a recombinant hTR-adenovirus vector did not result in telomerase activity, indicating that their lack of telomerase activity is not due to absence of hTR expression. We conclude that the ALT mechanism is not dependent on the expression of hTR, and does not involve mutations in the hTR sequence.

Telomere dynamics and telomerase activity in in vitro immortalised human cells

This article reviews the current understanding of the involvement of telomerase in in vitro immortalisation of human cells. In vitro immortalisation with DNA tumour viruses or chemicals usually occurs in two phases. The first stage is an extension of lifespan beyond that at which cells would normally senescence, after which the culture enters a period of crisis. The second stage involves the escape from crisis of a rare cell in the culture, which goes on to proliferate indefinitely. The hypothesis that telomere shortening acts as a signal for senescence and crisis, and that cells need to activate telomerase to survive these states, gained support from early studies examining telomere behaviour and telomerase activity in immortalised cell lines. In many cases, telomeres were found to continue to shorten during the phase of extended lifespan, and no telomerase was detectable. Cells which survived crisis had activated telomerase and had stable or lengthened telomerase. However, it is now clear that this model does not apply to all cell lines. Approximately a quarter of in vitro immortalised cell lines so far examined have no detectable telomerase activity, yet have very long and heterogeneous telomeres. These cell lines have acquired a novel mechanism for lengthening their telomeres, named ALT (Alternative Lengthening of Telomeres). The nature of ALT is not yet understood, but may involve non-reciprocal recombination between telomeres. ALT is not merely a phenomenon of in vitro immortalised cell lines, but has also been found in tumours and tumour-derived cell lines. Furthermore, there are a number of cell lines which have been shown to have low levels of telomerase prior to crisis while telomere shortening is still occurring, and the function of these low levels of telomerase activity is unknown.