c-Cbl-mediated neddylation antagonizes ubiquitination and degradation of the TGF-ß type II receptor.

Transforming growth factor ß (TGF-ß) is a potent antiproliferative factor in multiple types of cells. Deregulation of TGF-ß signaling is associated with the development of many cancers, including leukemia, though the molecular mechanisms are largely unclear. Here, we show that Casitas B-lineage lymphoma (c-Cbl), a known proto-oncogene encoding an ubiquitin E3 ligase, promotes TGF-ß signaling by neddylating and stabilizing the type II receptor (TßRII). Knockout of c-Cbl decreases the TßRII protein level and desensitizes hematopoietic stem or progenitor cells to TGF-ß stimulation, while c-Cbl overexpression stabilizes TßRII and sensitizes leukemia cells to TGF-ß. c-Cbl conjugates neural precursor cell-expressed, developmentally downregulated 8 (NEDD8), a ubiquitin-like protein, to TßRII at Lys556 and Lys567. Neddylation of TßRII promotes its endocytosis to EEA1-positive early endosomes while preventing its endocytosis to caveolin-positive compartments, therefore inhibiting TßRII ubiquitination and degradation. We have also identified a neddylation-activity-defective c-Cbl mutation from leukemia patients, implying a link between aberrant TßRII neddylation and leukemia development.

Tankyrases maintain homeostasis of intestinal epithelium by preventing cell death.

Lgr5+ intestinal stem cells are crucial for fast homeostatic renewal of intestinal epithelium and Wnt/ß-catenin signaling plays an essential role in this process by sustaining stem cell self-renewal. The poly(ADP-ribose) polymerases tankyrases (TNKSs) mediate protein poly-ADP-ribosylation and are involved in multiple cellular processes such as Wnt signaling regulation, mitotic progression and telomere maintenance. However, little is known about the physiological function of TNKSs in epithelium homeostasis regulation. Here, using Villin-creERT2;Tnks1-/-;Tnks2fl/fl (DKO) mice, we observed that loss of TNKSs causes a rapid decrease of Lgr5+ intestinal stem cells and magnified apoptosis in small intestinal crypts, leading to intestine degeneration and increased mouse mortality. Consistently, deletion of Tnks or blockage of TNKS activity with the inhibitor XAV939 significantly inhibits the growth of intestinal organoids. We further showed that the Wnt signaling agonist CHIR99021 sustains the growth of DKO organoids, and XAV939 does not cause growth retardation of Apc-/- organoids. Consistent with the promoting function of TNKSs in Wnt signaling, Wnt/ß-catenin signaling is significantly decreased with stabilized Axin in DKO crypts. Together, our findings unravel the essential role of TNKSs-mediated protein parsylation in small intestinal homeostasis by modulating Wnt/ß-catenin signaling.

T-cell development is regulated by the coordinated function of proximal and distal Lck promoters active at different developmental stages.

Expression of Lck, a T-cell lineage-specific tyrosine kinase critical for T-cell development and activation, can be mediated by either proximal or distal lck promoter. We generated BAC transgenic mice in which BAC lck promoter was deleted and bred these transgenes to an Lck knockout background. Lck-PROX mice, in which only the proximal promoter is functional, have maximal Lck protein and normal thymic development through CD4- CD8- double negative (DN) and CD4+ CD8+ double positive (DP) stages, but undetectable Lck later in development and reduced mature single positive thymocytes. In contrast, Lck-DIST mice, in which only distal promoter was functional, are deficient in Lck protein in DN and DP thymocytes and severely defective in early T-cell development, with a block at the DN3-DN4 beta checkpoint equivalent to complete Lck knockouts. The ability of the proximal lck promoter to support thymic development is independent of Fyn; while, in contrast, the distal lck promoter alone is completely unable to support development in the absence of Fyn. Notably, normal thymocyte development is restored by presence of both proximal and distal promoters, even when independently expressed on different lck genes. These results define distinct and complementary requirements for proximal and distal lck promoters during T-cell development.

Regulation of T cell development by c-Cbl: essential role of Lck.

A canonical pre-TCR/TCR signaling pathway critical for thymic T cell development involves sequential phosphorylation and signaling through Lck, Zap70, Lat and Slp76. However, we and others have previously reported that genomic deletion of c-Cbl (Cbl) partially or completely reverses the defects in thymic development in mice deficient in Zap70, Slp76, Lat or Vav1, indicating the presence of alternative pathways normally suppressed by Cbl. To further elucidate pre-TCR/TCR signaling pathways involved in thymic development, we characterized the effect of Cbl inactivation on developmental and signaling defects in mice deficient in proximal signaling molecules Lck and Zap70. Inactivation of Cbl partially reversed defective T cell development in Zap70 (-/-) mice and reversed defects in phosphorylation of Erk, Plc-γ1, Vav1 and Akt, in TCR-stimulated Cbl (-/-) Zap70 (-/-) thymocytes. Recent reports identified an essential role of Lck in associating with CD4 and CD8 coreceptors and mediating the requirement for MHC restriction in TCR recognition. Since TCR recognition has been shown to be MHC-restricted in Cbl (-/-) mice, it was of interest to determine whether the requirement for Lck remained unmodified by Cbl deletion. Indeed, in contrast to the effect of Cbl inactivation in partially or fully bypassing requirements for other TCR signaling components, inactivation of Cbl did not reverse either defective T cell development or defective phosphorylation of TCR signaling molecules in Lck (-/-) mice. Thus, Lck, which plays a unique role in enforcing MHC restriction, is essential for thymic development in presence or absence of Cbl, ensuring MHC restriction of T cells derived from either pathway.

Telomere length is inherited with resetting of the telomere set-point.

We have studied models of telomerase haploinsufficiency in humans and mice to analyze regulation of telomere length and the significance of "set points" in inheritance of telomere length. In three families with clinical syndromes associated with short telomeres resulting from haploinsufficient mutations in TERT, the gene encoding telomerase reverse transcriptase, we asked whether restoration of normal genotypes in offspring of affected individuals would elongate inherited short telomeres. Telomeres were shorter than normal in some but not all genotypically normal offspring of telomerase-mutant parents or grandparents. Analysis of these findings was complicated by heterogeneity of telomere length among individuals, as well as by the admixing of telomeres inherited from affected parents with those inherited from unaffected ("wild-type" TERT) parents. To understand further the inheritance of telomere length, we established a shortened-telomere mouse model. When Tert(+/-) heterozygous mice were successively cross-bred through 17 generations, telomere length shortened progressively. The late-generation Tert(+/-) mice were intercrossed to produce genotypically wild-type Tert(+/+) mice, for which telomere length was characterized. Strikingly, telomere length in these Tert(+/+) mice was not longer than that of their Tert(+/-) parents. Moreover, when successive crosses were carried out among these short-telomere Tert(+/+) offspring mice, telomere length was stable, with no elongation up to six generations. This breeding strategy therefore has established a mouse strain, B6.ST (short telomeres), with C57BL/6 genotype and stable short telomeres. These findings suggest that the set point of telomere lengths of offspring is determined by the telomere lengths of their parents in the presence of normal expression of telomerase.

Loss of Dact1 disrupts planar cell polarity signaling by altering dishevelled activity and leads to posterior malformation in mice.

Wnt signaling plays a key role in embryogenesis and cancer development. Dvl (Dishevelled) is a central mediator for both the canonical and noncanonical Wnt pathways. Dact1 (Dapper1, Dpr1), a Dvl interactor, has been shown to negatively modulate Wnt signaling by promoting lysosomal degradation of Dvl. Here we report that Dact1-deficient mice have multiple physiological defects that resemble the human neonate disease congenital caudal regression syndrome, including caudal vertebrae agenesis, anorectal malformation, renal agenesis/dysplasia, fused kidneys, and loss of bladder. These urogenital defects can be traced to impaired hindgut formation starting at embryonic day 8.25. Examination of morphological changes and Wnt target gene expression revealed that the planar cell polarity (PCP) signaling is deregulated, whereas the canonical Wnt/beta-catenin pathway is largely unaffected in mutant embryos. Consistently, the activity of the PCP signal mediators Rho GTPase and c-Jun N-terminal kinase is altered in Dact1(-/-) mouse embryonic fibroblasts. We further observed alterations in the protein level and the cellular distribution of Dvl in the primitive streak of mutant embryos. An increased amount of Dvl2 tends to be accumulated in the cortical regions of the cells, especially at the primitive streak ectoderm close to the posterior endoderm that lately forms the hindgut diverticulum. Together, these data suggest that Dact1 may regulate vertebrate PCP by controlling the level and the cellular localization of Dvl protein.

Inactivation of c-Cbl reverses neonatal lethality and T cell developmental arrest of SLP-76-deficient mice.

c-Cbl is an adaptor protein that negatively regulates signal transduction events involved in thymic-positive selection. To further characterize the function of c-Cbl in T cell development, we analyzed the effect of c-Cbl inactivation in mice deficient in the scaffolding molecule SLP-76. SLP-76-deficient mice show a high frequency of neonatal lethality; and in surviving mice, T cell development is blocked at the DN3 stage. Inactivation of c-cbl completely reversed the neonatal lethality seen in SLP-76-deficient mice and partially reversed the T cell development arrest in these mice. SLP-76(-/-) Cbl(-/-) mice exhibited marked expansion of polarized T helper type (Th)1 and Th2 cell peripheral CD4(+) T cells, lymphoid infiltrates of parenchymal organs, and premature death. This rescue of T cell development is T cell receptor dependent because it does not occur in recombination activating gene 2(-/-) SLP-76(-/-) Cbl(-/-) triple knockout mice. Analysis of the signal transduction properties of SLP-76(-/-) Cbl(-/-) T cells reveals a novel SLP-76- and linker for activation of T cells-independent pathway of extracellular signal-regulated kinase activation, which is normally down-regulated by c-Cbl.

Generation and characterization of telomere length maintenance in tankyrase 2-deficient mice.

Telomere length and function are crucial factors that determine the capacity for cell proliferation and survival, mediate cellular senescence, and play a role in malignant transformation in eukaryotic systems. The telomere length of a specific mammalian species is maintained within a given range by the action of telomerase and telomere-associated proteins. TRF1 is a telomere-associated protein that inhibits telomere elongation by its binding to telomere repeats, preventing access to telomerase. Human TRF1 interacts with tankyrase 1 and tankyrase 2 proteins, two related members of the tankyrase family shown to have poly(ADP-ribose) polymerase activity. Human tankyrase 1 is reported to ADP-ribosylate TRF1 and to down-regulate the telomeric repeat binding activity of TRF1, resulting in telomerase-dependent telomere elongation. Human tankyrase 2 is proposed to have activity similar to that of tankyrase 1, although tankyrase 2 function has been less extensively characterized. In the present study, we have assessed the in vivo function of mouse tankyrase 2 by germ line gene inactivation and show that inactivation of tankyrase 2 does not result in detectable alteration in telomere length when monitored through multiple generations of breeding. This finding suggests that either mouse tankyrases 1 and 2 have redundant functions in telomere length maintenance or that mouse tankyrase 2 differs from human tankyrase 2 in its role in telomere length maintenance. Tankyrase 2 deficiency did result in a significant decrease in body weight sustained through at least the first year of life, most marked in male mice, suggesting that tankyrase 2 functions in potentially telomerase-independent pathways to affect overall development and/or metabolism.

DNA Damage Activates TGF-ß Signaling via ATM-c-Cbl-Mediated Stabilization of the Type II Receptor TßRII.

Activation of both the DNA damage response (DDR) and transforming growth factor ß (TGF-ß) signaling induces growth arrest of most cell types. However, it is unclear whether the DDR activates TGF-ß signaling that in turn contributes to cell growth arrest. Here, we show that in response to DNA damage, ataxia telangiectasia mutated (ATM) stabilizes the TGF-ß type II receptor (TßRII) and thus enhancement of TGF-ß signaling. Mechanistically, ATM phosphorylates and stabilizes c-Cbl, which promotes TßRII neddylation and prevents its ubiquitination-dependent degradation. Consistently, DNA damage enhances the interaction among ATM, c-Cbl, and TßRII. The ATM-c-Cbl-TßRII axis plays a pivotal role in intestinal regeneration after X-ray-induced DNA damage in mouse models. Therefore, ATM not only mediates the canonical DDR pathway but also activates TGF-ß signaling by stabilizing TßRII. The double brake system ensures full cell-cycle arrest, allowing efficient DNA damage repair and avoiding passage of the damaged genome to the daughter cells.

Telomere-associated protein TIN2 is essential for early embryonic development through a telomerase-independent pathway.

TIN2 is a negative regulator of telomere elongation that interacts with telomeric DNA repeat binding factor 1 (TRF1) and affects telomere length by a telomerase-dependent mechanism. Here we show that inactivation of the mouse TRF1-interacting protein 2 (TIN2) gene results in early embryonic lethality. We further observed that the embryonic lethality of TIN2 mutant mice was not affected by inactivation of the telomerase reverse transcriptase gene, indicating that embryonic lethality is not the result of telomerase-dependent changes in telomere length or function. Our findings suggest that TIN2 has a role independent of telomere length regulation that is essential for embryonic development and cell viability.

Expression of telomerase RNA template, but not telomerase reverse transcriptase, is limiting for telomere length maintenance in vivo.

Telomerase consists of two essential components, the telomerase RNA template (TR) and telomerase reverse transcriptase (TERT). The haplo-insufficiency of TR was recently shown to cause one form of human dyskeratosis congenita, an inherited disease marked by abnormal telomere shortening. Consistent with this finding, we recently reported that mice heterozygous for inactivation of mouse TR exhibit a similar haplo-insufficiency and are deficient in the ability to elongate telomeres in vivo. To further assess the genetic regulation of telomerase activity, we have compared the abilities of TR-deficient and TERT-deficient mice to maintain or elongate telomeres in interspecies crosses. Homozygous TERT knockout mice had no telomerase activity and failed to maintain telomere length. In contrast, TERT(+/-) heterozygotes had no detectable defect in telomere elongation compared to wild-type controls, whereas TR(+/-) heterozygotes were deficient in telomere elongation. Levels of TERT mRNA in heterozygous mice were one-third to one-half the levels expressed in wild-type mice, similar to the reductions in telomerase RNA observed in TR heterozygotes. These findings indicate that both TR and TERT are essential for telomere maintenance and elongation but that gene copy number and transcriptional regulation of TR, but not TERT, are limiting for telomerase activity under the in vivo conditions analyzed.

Co-stimulatory function in primary germinal center responses: CD40 and B7 are required on distinct antigen-presenting cells.

T cell-dependent germinal center (GC) responses require coordinated interactions of T cells with two antigen-presenting cell (APC) populations, B cells and dendritic cells (DCs), in the presence of B7- and CD40-dependent co-stimulatory pathways. Contrary to the prevailing paradigm, we found unique cellular requirements for B7 and CD40 expression in primary GC responses to vaccine immunization with protein antigen and adjuvant: B7 was required on DCs but was not required on B cells, whereas CD40 was required on B cells but not on DCs in the generation of antigen-specific follicular helper T cells, antigen-specific GC B cells, and high-affinity class-switched antibody production. There was, in fact, no requirement for coexpression of B7 and CD40 on the same cell in these responses. Our findings support a substantially revised model for co-stimulatory function in the primary GC response, with crucial and distinct contributions of B7- and CD40-dependent pathways expressed by different APC populations and with important implications for understanding how to optimize vaccine responses or limit autoimmunity.

E3 ubiquitin ligase Cbl-b suppresses proallergic T cell development and allergic airway inflammation.

E3 ubiquitin ligase Cbl-b has emerged as a gatekeeper that controls the activation threshold of the T cell antigen receptor and maintains the balance between tolerance and autoimmunity. Here, we report that the loss of Cbl-b facilitates T helper 2 (Th2) and Th9 cell differentiation in vitro. In a mouse model of asthma, the absence of Cbl-b results in severe airway inflammation and stronger Th2 and Th9 responses. Mechanistically, Cbl-b selectively associates with Stat6 upon IL-4 ligation and targets Stat6 for ubiquitination and degradation. These processes are heightened in the presence of T cell receptor (TCR)/CD28 costimulation. Furthermore, we identify K108 and K398 as Stat6 ubiquitination sites. Intriguingly, introducing Stat6 deficiency into Cblb(-/-) mice abrogates hyper-Th2 responses but only partially attenuates Th9 responses. Therefore, our data reveal a function for Cbl-b in the regulation of Th2 and Th9 cell differentiation.

Exon 1 disruption alters tissue-specific expression of mouse p53 and results in selective development of B cell lymphomas.

p53 is a tumor suppressor gene mutated in >50% of human cancers, while p53 deficiency in mice results in cancers and accelerated mortality. Thymic T cell lymphoma is the most common malignancy in p53-deficient mice, making it difficult to study the role of p53 in other malignancies. To overcome this limitation, we attempted to generate mice with a reversible p53 knockout (p53(rev/rev)) by inserting a floxed transcriptional stop into the first exon of p53, anticipating that this would allow tissue-specific Cre-mediated expression of p53. Contrary to expectations, functional p53 protein was expressed in the thymus and multiple other tissues of p53(rev/rev) mice in the absence of Cre, whereas B cells expressed p53 protein only in the presence of B cell-specific CD19-Cre. In the absence of Cre, 76% of p53(rev/rev) mice developed splenic marginal zone B cell lymphomas, indicating sensitivity of this B cell subset to transformation caused by p53 deficiency. 5'-RACE identified p53 mRNA transcribed from a novel start site utilized in thymocytes but not normal B cells or B cell lymphomas from p53(rev/rev) mice. The p53(rev/rev) mouse thus demonstrates an effect of p53 deficiency in development of splenic marginal zone lymphomas and provides a model for study of p53-deficient human B cell lymphomas.

Cbl enforces an SLP76-dependent signaling pathway for T cell differentiation.

A signaling pathway involving ZAP-70, LAT, and SLP76 has been regarded as essential for receptor-driven T cell development and activation. Consistent with this model, mice deficient in SLP76 have a complete block at the double negative 3 stage of T cell development. Recently, however, it has been reported that inactivation of Cbl, a ubiquitin-protein isopeptide ligase, partially rescues T cell development in SLP76-deficient mice. To probe the influence of Cbl on domain-specific SLP76 functions, we reconstituted SLP76(-/-) Cbl(-/-) mice with Slp76 transgenes bearing mutations in each of three functional domains of SLP76 as follows: Y3F, in which the amino-terminal tyrosine residues of SLP76 were mutated, eliminating sites of SLP76 interaction with Vav, Nck, and Itk; Delta20, in which 20 amino acids in the proline-rich region of SLP76 were deleted, removing a binding site for Gads; and RK, in which arginine 448 of SLP76 was replaced by lysine, abolishing function of the Src homology 2 domain. Although each of these transgenes has been shown to partially rescue T cell development in SLP76(-/-) mice, we report here that Cbl inactivation completely reverses the severe double negative 3 developmental block that occurs in SLP76-deficient mice expressing the Y3F transgene (Y3F mice) and partially rescues the defect in positive selection in T cell receptor transgenic Y3F mice, but in contrast fails to rescue thymic development of SLP76-deficient mice expressing the Delta20 or RK transgene. Rescue in SLP76(-/-)Cbl(-/-)Y3F double-positive thymocytes is associated with enhanced tyrosine phosphorylation of signaling molecules, including Lck, Vav, PLC-gamma1, and ERKs, but not Itk, in response to T cell receptor stimulation. Thus, our data demonstrate that Cbl suppresses activation of a bypass signaling pathway and thereby enforces SLP76 dependence of early T cell development.

Tankyrase 1 and tankyrase 2 are essential but redundant for mouse embryonic development.

Tankyrases are proteins with poly(ADP-ribose) polymerase activity. Human tankyrases post-translationally modify multiple proteins involved in processes including maintenance of telomere length, sister telomere association, and trafficking of glut4-containing vesicles. To date, however, little is known about in vivo functions for tankyrases. We recently reported that body size was significantly reduced in mice deficient for tankyrase 2, but that these mice otherwise appeared developmentally normal. In the present study, we report generation of tankyrase 1-deficient and tankyrase 1 and 2 double-deficient mice, and use of these mutant strains to systematically assess candidate functions of tankyrase 1 and tankyrase 2 in vivo. No defects were observed in development, telomere length maintenance, or cell cycle regulation in tankyrase 1 or tankyrase 2 knockout mice. In contrast to viability and normal development of mice singly deficient in either tankyrase, deficiency in both tankyrase 1 and tankyrase 2 results in embryonic lethality by day 10, indicating that there is substantial redundancy between tankyrase 1 and tankyrase 2, but that tankyrase function is essential for embryonic development.

Both telomeric and non-telomeric DNA damage are determinants of mammalian cellular senescence.

BACKGROUND: Cellular senescence is a state reached by normal mammalian cells after a finite number of cell divisions and is characterized by morphological and physiological changes including terminal cell-cycle arrest. The limits on cell division imposed by senescence may play an important role in both organismal aging and in preventing tumorigenesis. Cellular senescence and organismal aging are both accompanied by increased DNA damage, seen as the formation of gamma-H2AX foci (gamma-foci), which may be found on uncapped telomeres or at non-telomeric sites of DNA damage. However, the relative importance of telomere- and non-telomere-associated DNA damage to inducing senescence has never been demonstrated. Here we present a new approach to determine accurately the chromosomal location of gamma-foci and quantify the number of telomeric versus non-telomeric gamma-foci associated with senescence in both human and mouse cells. This approach enables researchers to obtain accurate values and to avoid various possible misestimates inherent in earlier methods. RESULTS: Using combined immunofluorescence and telomere fluorescence in situ hybridization on metaphase chromosomes, we show that human cellular senescence is not solely determined by telomeric DNA damage. In addition, mouse cellular senescence is not solely determined by non-telomeric DNA damage. By comparing cells from different generations of telomerase-null mice with human cells, we show that cells from late generation telomerase-null mice, which have substantially short telomeres, contain mostly telomeric gamma-foci. Most notably, we report that, as human and mouse cells approach senescence, all cells exhibit similar numbers of total gamma-foci per cell, irrespective of chromosomal locations. CONCLUSION: Our results suggest that the chromosome location of senescence-related gamma-foci is determined by the telomere length rather than species differences per se. In addition, our data indicate that both telomeric and non-telomeric DNA damage responses play equivalent roles in signaling the initiation of cellular senescence and organismal aging. These data have important implications in the study of mechanisms to induce or delay cellular senescence in different species.

Redundancy in B cell developmental pathways: c-Cbl inactivation rescues early B cell development through a B cell linker protein-independent pathway.

Deficiency in the adaptor protein B cell linker protein (BLNK) results in a substantial but incomplete block in B cell development, suggesting that alternative pathways exist for B lineage differentiation. Another adaptor protein, c-Cbl, plays a negative regulatory role in several BCR-signaling pathways. We therefore investigated the role of c-Cbl during B cell development and addressed the possibility that redundancies in pathways for B cell differentiation could be further revealed by eliminating negative effects mediated by c-Cbl. Strikingly, c-Cbl inactivation reversed a number of the critical defects in early B cell differentiation that are seen in BLNK-deficient mice. c-Cbl(-/-)BLNK(-/-) mice exhibited normalized down-regulation of pre-BCR and CD43, up-regulation of MHC class II, and augmented L chain rearrangement, resulting in a successful transition from pre-B cells to immature B cells. c-Cbl inactivation also reversed the potentially tumor-predisposing hyperproliferative response of BLNK(-/-) pre-B cells to IL-7. Pre-BCR cross-linking induced enhanced and prolonged tyrosine phosphorylation in c-Cbl(-/-)BLNK(-/-) pre-BCR(+) pre-B cells compared with c-Cbl(+/-)BLNK(-/-) cells, including elevated phosphorylation of Lyn, Syk, Btk, and phospholipase C-gamma2. Our studies suggest that some, but not all, pre-BCR-triggered developmental events can be mediated by BLNK-independent pathways that are negatively regulated by c-Cbl, and further suggest that different events during early B cell development require different strength or duration of pre-BCR signaling.

Telomere biology and immune system.

Extract: In the 1960s Hayflick reported that normal human somatic cells have a finite replicative capacity and that, after a limited number of cell divisions, cells no longer divide and enter a state termed senescence. Since this observation was made, considerable research energy has focused on identifying molecular pathways that regulate the proliferative capacity of normal somatic (body) cells. One molecular mechanism that has been implicated in the regulation of somatic cell proliferation is mediated by telomeres -- specialized DNA-protein structures that cap the ends of all linear chromosomes. In mammalian cells, telomeres are composed of hexanucleotide repeats (TTAGGG) and a variety of associated proteins. Although all mammalian telomeres are composed of these (TTAGGG) repeats, telomere length varies substantially between different species. The higher order chromatin structure formed by telomeres functions to protect chromosomes ends from degradation and activation of DNA-repair pathways. In the absence of compensatory mechanisms, telomeres shorten progressively with successive rounds of cell division as a result of incomplete replication of telomeric termini, until they reach a critically short length that is no longer protective. Cells that lack protective telomeres fail to proliferate, and they undergo senescence or apoptosis.

Differential cis-regulation of human versus mouse TERT gene expression in vivo: identification of a human-specific repressive element.

In vivo expression of human telomerase is significantly different from that of mouse telomerase. To assess the basis for this difference, a bacterial artificial chromosome clone containing the entire hTERT (human telomerase reverse transcriptase) gene was introduced in mice. In these transgenic mice, expression of the hTERT transgene was similar to that of endogenous hTERT in humans, rather than endogenous mTERT (mouse telomerase reverse transcriptase). In tissues and cells showing a striking difference in expression levels between hTERT in humans and mTERT in mice (i.e., liver, kidney, lung, uterus, and fibroblasts), expression of the hTERT transgene in transgenic mice was repressed, mimicking hTERT in humans. The transcriptional activity of the hTERT promoter was much lower than that of the mTERT promoter in mouse embryonic fibroblasts or human fibroblasts. Mutational analysis of the hTERT and mTERT promoters revealed that a nonconserved GC-box within the hTERT promoter was responsible for the human-specific repression. These results reveal that a difference in cis-regulation of transcription, rather than transacting transcription factors, is critical to species differences in tissue-specific TERT expression. Our data also suggest that the GC-box-mediated, human-specific mechanism for TERT repression is impaired in human cancers. This study represents a detailed characterization of the functional difference in a gene promoter of mice versus humans and provides not only important insight into species-specific regulation of telomerase and telomeres but also an experimental basis for generating mice humanized for telomerase enzyme and its pattern of expression.