- Cancer > Hematology
- Medical Oncology
Honors & Awards
Elected Member, American Society for Clinical Investigation (2008)
Fellow, American Association for the Advancement of Science (AAAS) (2008)
Boards, Advisory Committees, Professional Organizations
Editorial Board, Stem Cells (2006 - Present)
Editorial Board, Molecular Cancer Research (2013 - Present)
Medical Education:Columbia University (1995) NY
Residency:Massachusetts General Hospital (1997) MA
Fellowship:Dana-Farber Cancer Institute (2000) MA
Ph.D., Columbia University, Microbiology (1995)
M.D., Columbia University (1995)
A.B., Princeton University, Chemistry (1986)
Current Research and Scholarly Interests
Telomeres, the nucleotide repeats that cap the ends of eukaryotic chromosomes, serve critical roles in promoting cell viability and in maintaining chromosomal stability. In humans, telomeres shorten progressively with cell division and aging because DNA polymerase cannot fully replicate the extreme ends of chromosomes. Critical telomere shortening and loss of the protective telomere capping function in cell culture initiates senescence and crisis responses that profoundly alter chromosome stability, cell cycle progression and survival. Expression of telomerase, the reverse transcriptase that synthesizes telomere repeats, is sufficient to lengthen and stabilize telomeres, thus enabling cells to proliferate in an unlimited fashion. Telomerase is expressed in stem cells and progenitor cells in self-renewing tissues, is downregulated with differentiation and upregulated in the vast majority of human cancers. In the Artandi lab, we are interested in unraveling the molecular and cellular mechanisms according to which telomeres and telomerase modulate stem cell function and carcinogenesis.
TERT and STEM CELLS
Telomerase is comprised of two subunits: TERT, the telomerase reverse transcriptase, and TERC, the telomerase RNA component. In stem cell and progenitor cell compartments, TERT serves a critical role in maintaining telomere length and function to support tissue homeostasis. However, TERT serves an additional function in stem cells, distinct from its role in telomere lengthening and we are actively studying this new role. We have devised new means of identifying telomerase-expressing cells in vivo and we are investigating the location and function of these cells in diverse tissues.
TISSUE REGENERATION AND AGING
Aging in humans and other mammals is associated with impaired proliferative responses in settings of stress, suggesting that altered stem cell function may underlie certain aspects of aging. We are interested in understanding how stem cells self-renew and differentiate and how TERT modulates stem cell function. One major limitation to this understanding is the inability to identify telomerase-positive cells in vivo. We have developed new approaches to solve this problem and are investigating telomerase-positive cells in vivo.
TELOMERASE TRAFFICKING AND ASSEMBLY
Telomerase is a large RNP with complex regulation in human cells. Using IP-MS approaches, we identified a critical new component of the telomerase holoenzyme, TCAB1. TCAB1 is essential for guiding the trafficking of telomerase to Cajal bodies within the nucleus and also to chromosome ends. We seek to understand in molecular detail how telomerase interacts with telomeres and adds telomere repeats in human cells.
TELOMERASE AND DISEASE
Germline mutations in telomerase components underlie several seemingly unrelated disease states, including the bone marrow failure syndrome dyskeratosis congenita, idiopathic pulmonary fibrosis, aplasic anemia and cirrhosis. We are using iPS cell-based approaches to study the mechanisms at play in these diseases with the goal of reversing the life-threatening phenotypes in these patients.
- Current Issues in Aging
GENE 221 (Spr)
Independent Studies (13)
- Directed Reading in Biochemistry
BIOC 299 (Aut, Win, Spr)
- Directed Reading in Cancer Biology
CBIO 299 (Aut, Win, Spr, Sum)
- Directed Reading in Medicine
MED 299 (Aut, Win, Spr, Sum)
- Early Clinical Experience in Medicine
MED 280 (Aut, Win, Spr, Sum)
- Graduate Research
CBIO 399 (Aut, Win, Spr, Sum)
- Graduate Research
MED 399 (Aut, Win, Spr, Sum)
- Graduate Research and Special Advanced Work
BIOC 399 (Aut, Win, Spr, Sum)
- Medical Scholars Research
BIOC 370 (Aut, Win, Spr)
- Medical Scholars Research
MED 370 (Aut, Win, Spr, Sum)
- Out-of-Department Advanced Research Laboratory in Experimental Biology
BIO 199X (Sum)
- The Teaching of Biochemistry
BIOC 221 (Aut, Win)
- Undergraduate Research
BIOC 199 (Aut, Win, Spr)
- Undergraduate Research
MED 199 (Aut, Win, Spr, Sum)
- Directed Reading in Biochemistry
- Prior Year Courses
Understanding telomere diseases through analysis of patient-derived iPS cells
CURRENT OPINION IN GENETICS & DEVELOPMENT
2013; 23 (5): 526-533
A unique characteristic of tissue stem cells is the ability to self-renew, a process that enables the life-long maintenance of many organs. Stem cell self-renewal is dependent in part on the synthesis of telomere repeats by the enzyme telomerase. Defects in telomerase and in genes in the telomere maintenance pathway result in diverse disease states, including dyskeratosis congenita, pulmonary fibrosis, aplastic anemia, liver cirrhosis and cancer. Many of these disease states share a tissue failure phenotype, such as loss of bone marrow cells or failure of pulmonary epithelium, suggesting that stem cell dysfunction is a common pathophysiological mechanism underlying these telomere diseases. Studies of telomere diseases in undifferentiated iPS cells have provided a quantitative relationship between the magnitude of biochemical defects in the telomerase pathway and disease severity in patients, thereby establishing a clear correlation between genotype and phenotype in telomere disease states. Modeling telomere diseases in iPS cells has also revealed diverse underlying disease mechanisms, including reduced telomerase catalytic activity, diminished assembly of the telomerase holoenzyme and impaired trafficking of the enzyme within the nucleus. These studies highlight the need for therapies tailored to the underlying biochemical defect in each class of patients.
View details for DOI 10.1016/j.gde.2013.07.006
View details for Web of Science ID 000324970600005
View details for PubMedID 23993228
TPP1 OB-Fold Domain Controls Telomere Maintenance by Recruiting Telomerase to Chromosome Ends
2012; 150 (3): 481-494
Telomere synthesis in cancer cells and stem cells involves trafficking of telomerase to Cajal bodies, and telomerase is thought to be recruited to telomeres through interactions with telomere-binding proteins. Here, we show that the OB-fold domain of the telomere-binding protein TPP1 recruits telomerase to telomeres through an association with the telomerase reverse transcriptase TERT. When tethered away from telomeres and other telomere-binding proteins, the TPP1 OB-fold domain is sufficient to recruit telomerase to a heterologous chromatin locus. Expression of a minimal TPP1 OB-fold inhibits telomere maintenance by blocking access of telomerase to its cognate binding site at telomeres. We identify amino acids required for the TPP1-telomerase interaction, including specific loop residues within the TPP1 OB-fold domain and individual residues within TERT, some of which are mutated in a subset of pulmonary fibrosis patients. These data define a potential interface for telomerase-TPP1 interaction required for telomere maintenance and implicate defective telomerase recruitment in telomerase-related disease.
View details for DOI 10.1016/j.cell.2012.07.012
View details for Web of Science ID 000307301400009
View details for PubMedID 22863003
Reversible cell-cycle entry in adult kidney podocytes through regulated control of telomerase and Wnt signaling
2012; 18 (1): 111-119
Mechanisms of epithelial cell renewal remain poorly understood in the mammalian kidney, particularly in the glomerulus, a site of cellular damage in chronic kidney disease. Within the glomerulus, podocytes--differentiated epithelial cells crucial for filtration--are thought to lack substantial capacity for regeneration. Here we show that podocytes rapidly lose differentiation markers and enter the cell cycle in adult mice in which the telomerase protein component TERT is conditionally expressed. Transgenic TERT expression in mice induces marked upregulation of Wnt signaling and disrupts glomerular structure, resulting in a collapsing glomerulopathy resembling those in human disease, including HIV-associated nephropathy (HIVAN). Human and mouse HIVAN kidneys show increased expression of TERT and activation of Wnt signaling, indicating that these are general features of collapsing glomerulopathies. Silencing transgenic TERT expression or inhibiting Wnt signaling through systemic expression of the Wnt inhibitor Dkk1 in either TERT transgenic mice or in a mouse model of HIVAN results in marked normalization of podocytes, including rapid cell-cycle exit, re-expression of differentiation markers and improved filtration barrier function. These data reveal an unexpected capacity of podocytes to reversibly enter the cell cycle, suggest that podocyte renewal may contribute to glomerular homeostasis and implicate the telomerase and Wnt-β-catenin pathways in podocyte proliferation and disease.
View details for DOI 10.1038/nm.2550
View details for Web of Science ID 000299018600036
Genomic Maps of Long Noncoding RNA Occupancy Reveal Principles of RNA-Chromatin Interactions
2011; 44 (4): 667-678
Long noncoding RNAs (lncRNAs) are key regulators of chromatin state, yet the nature and sites of RNA-chromatin interaction are mostly unknown. Here we introduce Chromatin Isolation by RNA Purification (ChIRP), where tiling oligonucleotides retrieve specific lncRNAs with bound protein and DNA sequences, which are enumerated by deep sequencing. ChIRP-seq of three lncRNAs reveal that RNA occupancy sites in the genome are focal, sequence-specific, and numerous. Drosophila roX2 RNA occupies male X-linked gene bodies with increasing tendency toward the 3' end, peaking at CES sites. Human telomerase RNA TERC occupies telomeres and Wnt pathway genes. HOTAIR lncRNA preferentially occupies a GA-rich DNA motif to nucleate broad domains of Polycomb occupancy and histone H3 lysine 27 trimethylation. HOTAIR occupancy occurs independently of EZH2, suggesting the order of RNA guidance of Polycomb occupancy. ChIRP-seq is generally applicable to illuminate the intersection of RNA and chromatin with newfound precision genome wide.
View details for DOI 10.1016/j.molcel.2011.08.027
View details for Web of Science ID 000297387800017
View details for PubMedID 21963238
In Situ Genetic Correction of the Sickle Cell Anemia Mutation in Human Induced Pluripotent Stem Cells Using Engineered Zinc Finger Nucleases
2011; 29 (11): 1717-1726
The combination of induced pluripotent stem cell (iPSC) technology and targeted gene modification by homologous recombination (HR) represents a promising new approach to generate genetically corrected, patient-derived cells that could be used for autologous transplantation therapies. This strategy has several potential advantages over conventional gene therapy including eliminating the need for immunosuppression, avoiding the risk of insertional mutagenesis by therapeutic vectors, and maintaining expression of the corrected gene by endogenous control elements rather than a constitutive promoter. However, gene targeting in human pluripotent cells has remained challenging and inefficient. Recently, engineered zinc finger nucleases (ZFNs) have been shown to substantially increase HR frequencies in human iPSCs, raising the prospect of using this technology to correct disease causing mutations. Here, we describe the generation of iPSC lines from sickle cell anemia patients and in situ correction of the disease causing mutation using three ZFN pairs made by the publicly available oligomerized pool engineering method (OPEN). Gene-corrected cells retained full pluripotency and a normal karyotype following removal of reprogramming factor and drug-resistance genes. By testing various conditions, we also demonstrated that HR events in human iPSCs can occur as far as 82 bps from a ZFN-induced break. Our approach delineates a roadmap for using ZFNs made by an open-source method to achieve efficient, transgene-free correction of monogenic disease mutations in patient-derived iPSCs. Our results provide an important proof of principle that ZFNs can be used to produce gene-corrected human iPSCs that could be used for therapeutic applications.
View details for DOI 10.1002/stem.718
View details for Web of Science ID 000296565500009
View details for PubMedID 21898685
Telomere shortening and loss of self-renewal in dyskeratosis congenita induced pluripotent stem cells
2011; 474 (7351): 399-?
The differentiation of patient-derived induced pluripotent stem cells (iPSCs) to committed fates such as neurons, muscle and liver is a powerful approach for understanding key parameters of human development and disease. Whether undifferentiated iPSCs themselves can be used to probe disease mechanisms is uncertain. Dyskeratosis congenita is characterized by defective maintenance of blood, pulmonary tissue and epidermal tissues and is caused by mutations in genes controlling telomere homeostasis. Short telomeres, a hallmark of dyskeratosis congenita, impair tissue stem cell function in mouse models, indicating that a tissue stem cell defect may underlie the pathophysiology of dyskeratosis congenita. Here we show that even in the undifferentiated state, iPSCs from dyskeratosis congenita patients harbour the precise biochemical defects characteristic of each form of the disease and that the magnitude of the telomere maintenance defect in iPSCs correlates with clinical severity. In iPSCs from patients with heterozygous mutations in TERT, the telomerase reverse transcriptase, a 50% reduction in telomerase levels blunts the natural telomere elongation that accompanies reprogramming. In contrast, mutation of dyskerin (DKC1) in X-linked dyskeratosis congenita severely impairs telomerase activity by blocking telomerase assembly and disrupts telomere elongation during reprogramming. In iPSCs from a form of dyskeratosis congenita caused by mutations in TCAB1 (also known as WRAP53), telomerase catalytic activity is unperturbed, yet the ability of telomerase to lengthen telomeres is abrogated, because telomerase mislocalizes from Cajal bodies to nucleoli within the iPSCs. Extended culture of DKC1-mutant iPSCs leads to progressive telomere shortening and eventual loss of self-renewal, indicating that a similar process occurs in tissue stem cells in dyskeratosis congenita patients. These findings in iPSCs from dyskeratosis congenita patients reveal that undifferentiated iPSCs accurately recapitulate features of a human stem cell disease and may serve as a cell-culture-based system for the development of targeted therapeutics.
View details for DOI 10.1038/nature10084
View details for Web of Science ID 000291647100050
View details for PubMedID 21602826
- TRAPping telomerase within the intestinal stem cell niche EMBO JOURNAL 2011; 30 (6): 986-987
Disruption of telomerase trafficking by TCAB1 mutation causes dyskeratosis congenita
GENES & DEVELOPMENT
2011; 25 (1): 11-16
Dyskeratosis congenita (DC) is a genetic disorder of defective tissue maintenance and cancer predisposition caused by short telomeres and impaired stem cell function. Telomerase mutations are thought to precipitate DC by reducing either the catalytic activity or the overall levels of the telomerase complex. However, the underlying genetic mutations and the mechanisms of telomere shortening remain unknown for as many as 50% of DC patients, who lack mutations in genes controlling telomere homeostasis. Here, we show that disruption of telomerase trafficking accounts for unknown cases of DC. We identify DC patients with missense mutations in TCAB1, a telomerase holoenzyme protein that facilitates trafficking of telomerase to Cajal bodies. Compound heterozygous mutations in TCAB1 disrupt telomerase localization to Cajal bodies, resulting in misdirection of telomerase RNA to nucleoli, which prevents telomerase from elongating telomeres. Our findings establish telomerase mislocalization as a novel cause of DC, and suggest that telomerase trafficking defects may contribute more broadly to the pathogenesis of telomere-related disease.
View details for DOI 10.1101/gad.2006411
View details for Web of Science ID 000285870300002
View details for PubMedID 21205863
Short Telomeres and Stem Cell Exhaustion Model Duchenne Muscular Dystrophy in mdx/mTR Mice
2010; 143 (7): 1059-1071
In Duchenne muscular dystrophy (DMD), dystrophin mutation leads to progressive lethal skeletal muscle degeneration. For unknown reasons, dystrophin deficiency does not recapitulate DMD in mice (mdx), which have mild skeletal muscle defects and potent regenerative capacity. We postulated that human DMD progression is a consequence of loss of functional muscle stem cells (MuSC), and the mild mouse mdx phenotype results from greater MuSC reserve fueled by longer telomeres. We report that mdx mice lacking the RNA component of telomerase (mdx/mTR) have shortened telomeres in muscle cells and severe muscular dystrophy that progressively worsens with age. Muscle wasting severity parallels a decline in MuSC regenerative capacity and is ameliorated histologically by transplantation of wild-type MuSC. These data show that DMD progression results, in part, from a cell-autonomous failure of MuSC to maintain the damage-repair cycle initiated by dystrophin deficiency. The essential role of MuSC function has therapeutic implications for DMD.
View details for DOI 10.1016/j.cell.2010.11.039
View details for Web of Science ID 000285625400005
View details for PubMedID 21145579
Shared molecular mechanisms regulate multiple catenin proteins: canonical Wnt signals and components modulate p120-catenin isoform-1 and additional p120 subfamily members
JOURNAL OF CELL SCIENCE
2010; 123 (24): 4351-4365
Wnt signaling pathways have fundamental roles in animal development and tumor progression. Here, employing Xenopus embryos and mammalian cell lines, we report that the degradation machinery of the canonical Wnt pathway modulates p120-catenin protein stability through mechanisms shared with those regulating β-catenin. For example, in common with β-catenin, exogenous expression of destruction complex components, such as GSK3β and axin, promotes degradation of p120-catenin. Again in parallel with β-catenin, reduction of canonical Wnt signals upon depletion of LRP5 and LRP6 results in p120-catenin degradation. At the primary sequence level, we resolved conserved GSK3β phosphorylation sites in the amino-terminal region of p120-catenin present exclusively in isoform-1. Point-mutagenesis of these residues inhibited the association of destruction complex components, such as those involved in ubiquitylation, resulting in stabilization of p120-catenin. Functionally, in line with predictions, p120 stabilization increased its signaling activity in the context of the p120-Kaiso pathway. Importantly, we found that two additional p120-catenin family members, ARVCF-catenin and δ-catenin, associate with axin and are degraded in its presence. Thus, as supported using gain- and loss-of-function approaches in embryo and cell line systems, canonical Wnt signals appear poised to have an impact upon a breadth of catenin biology in vertebrate development and, possibly, human cancers.
View details for DOI 10.1242/jcs.067199
View details for Web of Science ID 000284837100017
View details for PubMedID 21098636
Telomeres and telomerase in cancer
2010; 31 (1): 9-18
Myriad genetic and epigenetic alterations are required to drive normal cells toward malignant transformation. These somatic events commandeer many signaling pathways that cooperate to endow aspiring cancer cells with a full range of biological capabilities needed to grow, disseminate and ultimately kill its host. Cancer genomes are highly rearranged and are characterized by complex translocations and regional copy number alterations that target loci harboring cancer-relevant genes. Efforts to uncover the underlying mechanisms driving genome instability in cancer have revealed a prominent role for telomeres. Telomeres are nucleoprotein structures that protect the ends of eukaryotic chromosomes and are particularly vulnerable due to progressive shortening during each round of DNA replication and, thus, a lifetime of tissue renewal places the organism at risk for increasing chromosomal instability. Indeed, telomere erosion has been documented in aging tissues and hyperproliferative disease states-conditions strongly associated with increased cancer risk. Telomere dysfunction can produce the opposing pathophysiological states of degenerative aging or cancer with the specific outcome dictated by the integrity of DNA damage checkpoint responses. In most advanced cancers, telomerase is reactivated and serves to maintain telomere length and emerging data have also documented the capacity of telomerase to directly regulate cancer-promoting pathways. This review covers the role of telomeres and telomerase in the biology of normal tissue stem/progenitor cells and in the development of cancer.
View details for DOI 10.1093/carcin/bgp268
View details for Web of Science ID 000273493100002
View details for PubMedID 19887512
Reverse Transcribing the Code for Chromosome Stability
2009; 36 (5): 715-719
The linearity of eukaryotic chromosomes presents challenges to cells, as the presence of DNA "ends" poses problems for the DNA replication machinery and the cell's damage response systems. This year's Nobel Prize in Physiology or Medicine recognized groundbreaking studies establishing the telomere field as a crucial area of biomedical research.
View details for DOI 10.1016/j.molcel.2009.11.030
View details for Web of Science ID 000272965400001
View details for PubMedID 20005831
Stem Cell Aging and Aberrant Differentiation within the Niche
CELL STEM CELL
2009; 5 (1): 6-8
Stem cells age, but the underlying mechanisms remain unclear. In a recent issue of Cell, Inomata and colleagues (2009) show that DNA damage, a prime suspect in stem cell aging, causes graying and loss of melanocyte stem cells by inducing premature differentiation, without inducing apoptosis or senescence.
View details for DOI 10.1016/j.stem.2009.06.006
View details for Web of Science ID 000267879200004
View details for PubMedID 19570507
Telomerase modulates Wnt signalling by association with target gene chromatin
2009; 460 (7251): 66-U77
Stem cells are controlled, in part, by genetic pathways frequently dysregulated during human tumorigenesis. Either stimulation of Wnt/beta-catenin signalling or overexpression of telomerase is sufficient to activate quiescent epidermal stem cells in vivo, although the mechanisms by which telomerase exerts these effects are not understood. Here we show that telomerase directly modulates Wnt/beta-catenin signalling by serving as a cofactor in a beta-catenin transcriptional complex. The telomerase protein component TERT (telomerase reverse transcriptase) interacts with BRG1 (also called SMARCA4), a SWI/SNF-related chromatin remodelling protein, and activates Wnt-dependent reporters in cultured cells and in vivo. TERT serves an essential role in formation of the anterior-posterior axis in Xenopus laevis embryos, and this defect in Wnt signalling manifests as homeotic transformations in the vertebrae of Tert(-/-) mice. Chromatin immunoprecipitation of the endogenous TERT protein from mouse gastrointestinal tract shows that TERT physically occupies gene promoters of Wnt-dependent genes. These data reveal an unanticipated role for telomerase as a transcriptional modulator of the Wnt/beta-catenin signalling pathway.
View details for DOI 10.1038/nature08137
View details for Web of Science ID 000267545200030
View details for PubMedID 19571879
Telomere Uncapping, Chromosomes, and Carcinomas
2009; 15 (6): 455-457
Data from mouse models and from human cancers have supported the idea that telomere shortening leads to chromosomal instability and epithelial carcinogenesis. In this issue of Cancer Cell, Else et al. demonstrate that telomere uncapping-altering a protein that protects chromosome ends without shortening telomeres-also results in epithelial cancers.
View details for DOI 10.1016/j.ccr.2009.05.006
View details for Web of Science ID 000266686500001
View details for PubMedID 19477422
TCAB1 Driving telomerase to Cajal bodies
2009; 8 (9): 1329-1331
Telomerase supports the proliferation of progenitor cells and tumor cells by adding telomere repeats to chromosome ends. The low abundance and restricted expression pattern of telomerase have limited our knowledge of this important enzyme. A new telomerase protein, TCAB1, sheds light on the pathway that governs telomerase holoenzyme assembly and function in vivo. TCAB1 is a component of active telomerase and is required for the telomerase holoenzyme to accumulate in Cajal bodies and to elongate telomeres. These findings provide important new insights into how telomerase functions in cancer and in stem cell biology.
View details for Web of Science ID 000266114600016
View details for PubMedID 19342896
A Human Telomerase Holoenzyme Protein Required for Cajal Body Localization and Telomere Synthesis
2009; 323 (5914): 644-648
Telomerase is a ribonucleoprotein (RNP) complex that synthesizes telomere repeats in tissue progenitor cells and cancer cells. Active human telomerase consists of at least three principal subunits, including the telomerase reverse transcriptase, the telomerase RNA (TERC), and dyskerin. Here, we identify a holoenzyme subunit, TCAB1 (telomerase Cajal body protein 1), that is notably enriched in Cajal bodies, nuclear sites of RNP processing that are important for telomerase function. TCAB1 associates with active telomerase enzyme, established telomerase components, and small Cajal body RNAs that are involved in modifying splicing RNAs. Depletion of TCAB1 by using RNA interference prevents TERC from associating with Cajal bodies, disrupts telomerase-telomere association, and abrogates telomere synthesis by telomerase. Thus, TCAB1 controls telomerase trafficking and is required for telomere synthesis in human cancer cells.
View details for DOI 10.1126/science.1165357
View details for Web of Science ID 000262862800046
View details for PubMedID 19179534
Telomerase-dependent and -independent chromosome healing in mouse embryonic stem cells
2008; 7 (8): 1233-1249
Telomeres play an important role in protecting the ends of chromosomes and preventing chromosome fusion. We have previously demonstrated that double-strand breaks near telomeres in mammalian cells result in either the addition of a new telomere at the site of the break, termed chromosome healing, or sister chromatid fusion that initiates chromosome instability. In the present study, we have investigated the role of telomerase in chromosome healing and the importance of chromosome healing in preventing chromosome instability. In embryonic stem cell lines that are wild type for the catalytic subunit of telomerase (TERT), chromosome healing at I-SceI-induced double-strand breaks near telomeres accounted for 22 of 35 rearrangements, with the new telomeres added directly at the site of the break in all but one instance. In contrast, in two TERT-knockout embryonic stem cell lines, chromosome healing accounted for only 1 of 62 rearrangements, with a 23 bp insertion at the site of the sole chromosome-healing event. However, in a third TERT-knockout embryonic stem cell line, 10PTKO-A, chromosome healing was a common event that accounted for 20 of 34 rearrangements. Although this chromosome healing also occurred at the I-SceI site, differences in the microhomology at the site of telomere addition demonstrated that the mechanism was distinct from that in wild-type embryonic stem cell lines. In addition, the newly added telomeres in 10PTKO-A shortened with time in culture, eventually resulting in either telomere elongation through a telomerase-independent mechanism or loss of the subtelomeric plasmid sequences entirely. The combined results demonstrate that chromosome healing can occur through both telomerase-dependent and -independent mechanisms, and that although both mechanisms can prevent degradation and sister chromatid fusion, neither mechanism is efficient enough to prevent sister chromatid fusion from occurring in many cells experiencing double-strand breaks near telomeres.
View details for DOI 10.1016/j.dnarep.2008.04.004
View details for Web of Science ID 000258259000006
View details for PubMedID 18502190
Identification of ATPases pontin and reptin as telomerase components essential for holoenzyme assembly
2008; 132 (6): 945-957
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.
View details for DOI 10.1016/j.cell.2008.01.019
View details for Web of Science ID 000254273600014
View details for PubMedID 18358808
Telomere uncapping in progenitor cells with critical telomere shortening is coupled to S-phase progression in vivo
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2007; 104 (45): 17747-17752
Telomeres protect chromosome ends and serve as a substrate for telomerase, a reverse transcriptase that adds DNA repeats to the telomere terminus. In the absence of telomerase, telomeres progressively shorten, ultimately leading to telomere uncapping, a structural change at the telomere that activates DNA damage responses and leads to ligation of chromosome ends. Telomere uncapping has been implicated in aging and cancer, yet the precise mechanism of uncapping and its relationship to cell cycle remain to be defined. Here, we show that telomeres uncap in an S-phase-dependent manner in gastrointestinal progenitors of TERT(-/-) mice. We develop an in vivo assay that allows a quantitative kinetic assessment of telomere dysfunction-induced apoptosis and its relationship to cell cycle. By exploiting the mathematical relationship between rates of generation and clearance of apoptotic cells, we show that 86.2 +/- 8.8% of apoptotic gastrointestinal cells undergo programmed cell death either late in S-phase or in G2. Apoptosis is primarily triggered via a signaling cascade from newly uncapped telomeres to the tumor suppressor p53, rather than by chromosome fusion-bridge breakage, because mitotic blockade did not alter the rate of newly generated apoptotic bodies. These data support a model in which rapidly dividing progenitor cells within a tissue with short telomeres are vulnerable to telomere uncapping during or shortly after telomere replication.
View details for DOI 10.1073/pnas.0706485104
View details for Web of Science ID 000250897600038
View details for PubMedID 17965232
DNA-dependent protein kinase catalytic subunit is not required for dysfunctional telomere fusion and checkpoint response in the telomerase-deficient mouse
MOLECULAR AND CELLULAR BIOLOGY
2007; 27 (6): 2253-2265
Telomeres are key structural elements for the protection and maintenance of linear chromosomes, and they function to prevent recognition of chromosomal ends as DNA double-stranded breaks. Loss of telomere capping function brought about by telomerase deficiency and gradual erosion of telomere ends or by experimental disruption of higher-order telomere structure culminates in the fusion of defective telomeres and/or the activation of DNA damage checkpoints. Previous work has implicated the nonhomologous end-joining (NHEJ) DNA repair pathway as a critical mediator of these biological processes. Here, employing the telomerase-deficient mouse model, we tested whether the NHEJ component DNA-dependent protein kinase catalytic subunit (DNA-PKcs) was required for fusion of eroded/dysfunctional telomere ends and the telomere checkpoint responses. In late-generation mTerc(-/-) DNA-PKcs(-/-) cells and tissues, chromosomal end-to-end fusions and anaphase bridges were readily evident. Notably, nullizygosity for DNA Ligase4 (Lig4)--an additional crucial NHEJ component--was also permissive for chromosome fusions in mTerc(-/-) cells, indicating that, in contrast to results seen with experimental disruption of telomere structure, telomere dysfunction in the context of gradual telomere erosion can engage additional DNA repair pathways. Furthermore, we found that DNA-PKcs deficiency does not reduce apoptosis, tissue atrophy, or p53 activation in late-generation mTerc(-/-) tissues but rather moderately exacerbates germ cell apoptosis and testicular degeneration. Thus, our studies indicate that the NHEJ components, DNA-PKcs and Lig4, are not required for fusion of critically shortened telomeric ends and that DNA-PKcs is not required for sensing and executing the telomere checkpoint response, findings consistent with the consensus view of the limited role of DNA-PKcs in DNA damage signaling in general.
View details for DOI 10.1128/MCB.01354-06
View details for Web of Science ID 000245003600023
View details for PubMedID 17145779
Aging, graying and loss of melanocyte stem cells
STEM CELL REVIEWS
2007; 3 (3): 212-217
Hair graying is one of the prototypical signs of human aging. Maintenance of hair pigmentation is dependent on the presence and functionality of melanocytes, neural crest derived cells which synthesize pigment for growing hair. The melanocytes, themselves, are maintained by a small number of stem cells which reside in the bulge region of the hair follicle. The recent characterization of the melanocyte lineage during aging has significantly accelerated our understanding of how age-related changes in the melanocyte stem cell compartment contribute to hair graying. This review will discuss our current understanding of hair graying, drawing on evidence from human and mouse studies, and consider the contribution of melanocyte stem cells to this process. Furthermore, using the melanocyte lineage as an example, it will discuss common theories of tissue and stem cell aging.
View details for DOI 10.1007/s12015-007-0028-0
View details for Web of Science ID 000249929800004
View details for PubMedID 17917134
Telomeres, telomerase, and human disease.
New England journal of medicine
2006; 355 (12): 1195-1197
View details for PubMedID 16990382
Telomerase flies the coop: the telomerase RNA component as a viral-encoded oncogene
JOURNAL OF EXPERIMENTAL MEDICINE
2006; 203 (5): 1143-1145
Telomerase, the enzyme that elongates our telomeres, is crucial for cancer development based on extensive analyses of human cells, human cancers, and mouse models. New data now suggest that a viral telomerase RNA gene encoded by Marek's disease virus (MDV), an oncogenic herpesvirus of chickens, promotes tumor formation. These findings highlight the importance of telomerase in cancer and raise new questions regarding the mechanisms by which the telomerase RNA component supports tumorigenesis.
View details for DOI 10.1084/jem.20060849
View details for Web of Science ID 000237803700003
View details for PubMedID 16682501
Regulation of cellular immortalization and steady-state levels of the telomerase reverse transcriptase through its carboxy-terminal domain
MOLECULAR AND CELLULAR BIOLOGY
2006; 26 (6): 2146-2159
Telomerase maintains cell viability and chromosomal stability through the addition of telomere repeats to chromosome ends. The reactivation of telomerase through the upregulation of TERT, the telomerase protein subunit, is an important step during cancer development, yet TERT protein function and regulation remain incompletely understood. Despite its close sequence similarity to human TERT (hTERT), we find that mouse TERT (mTERT) does not immortalize primary human fibroblasts. Here we exploit these differences in activity to understand TERT protein function by creating chimeric mouse-human TERT proteins. Through the analysis of these chimeric TERT proteins, we find that sequences in the human carboxy-terminal domain are critical for telomere maintenance in human fibroblasts. The substitution of the human carboxy-terminal sequences into the mouse TERT protein is sufficient to confer immortalization and maintenance of telomere length and function. Strikingly, we find that hTERT protein accumulates to markedly higher levels than does mTERT protein and that the sequences governing this difference in protein regulation also reside in the carboxy-terminal domain. These elevated protein levels, which are characteristic of hTERT, are necessary but not sufficient for telomere maintenance because stabilized mTERT mutants cannot immortalize human cells. Thus, the TERT carboxy terminus contains sequences that regulate TERT protein levels and determinants that are required for productive action on telomere ends.
View details for DOI 10.1128/MCB.26.6.2146-2159.2006
View details for Web of Science ID 000235915400012
View details for PubMedID 16507993
Conditional telomerase induction causes proliferation of hair follicle stem cells
2005; 436 (7053): 1048-1052
TERT, the protein component of telomerase, serves to maintain telomere function through the de novo addition of telomere repeats to chromosome ends, and is reactivated in 90% of human cancers. In normal tissues, TERT is expressed in stem cells and in progenitor cells, but its role in these compartments is not fully understood. Here we show that conditional transgenic induction of TERT in mouse skin epithelium causes a rapid transition from telogen (the resting phase of the hair follicle cycle) to anagen (the active phase), thereby facilitating robust hair growth. TERT overexpression promotes this developmental transition by causing proliferation of quiescent, multipotent stem cells in the hair follicle bulge region. This new function for TERT does not require the telomerase RNA component, which encodes the template for telomere addition, and therefore operates through a mechanism independent of its activity in synthesizing telomere repeats. These data indicate that, in addition to its established role in extending telomeres, TERT can promote proliferation of resting stem cells through a non-canonical pathway.
View details for DOI 10.1038/nature03836
View details for Web of Science ID 000231263900057
View details for PubMedID 16107853
Pathways connecting telomeres and p53 in senescence, apoptosis, and cancer
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
2005; 331 (3): 881-890
The ends of eukaryotic chromosomes are protected by specialized structures termed telomeres that serve in part to prevent the chromosome end from activating a DNA damage response. However, this important function for telomeres in chromosome end protection can be lost as telomeres shorten with cell division in culture or in self-renewing tissues with advancing age. Impaired telomere function leads to induction of a DNA damage response and activation of the tumor suppressor protein p53. p53 serves a critical role in enforcing both senescence and apoptotic responses to dysfunctional telomeres. Loss of p53 creates a permissive environment in which critically short telomeres are inappropriately joined to generate chromosomal end-to-end fusions. These fused chromosomes result in cycles of chromosome fusion-bridge-breakage, which can fuel cancer initiation, especially in epithelial tissues, by facilitating changes in gene copy number.
View details for Web of Science ID 000229135900023
View details for PubMedID 15865944
Complex roles for telomeres and telomerase in breast carcinogenesis
BREAST CANCER RESEARCH
2003; 5 (1): 37-41
Telomerase - an enzyme that endows cells with unlimited proliferative potential - is differentially expressed in cancer cells and in normal cells. Although most primary human cells lack telomerase, the enzyme is upregulated in more than 90% of invasive breast cancers. As a result, much of breast cancer development occurs before telomerase is reactivated during a critical transition from a telomerase-negative to a telomerase-positive state. During this transition, the telomere shortening that accompanies cell division may either prevent or facilitate tumorigenesis by activating checkpoints and impairing chromosomal stability. In mature cancers, telomerase probably serves a crucial role in tumor progression and maintenance by stabilizing telomeres and supporting the immortal growth of breast cancer cells.
View details for DOI 10.1186/bcr553
View details for Web of Science ID 000180046700010
View details for PubMedID 12559044
Constitutive telomerase expression promotes mammary carcinomas in aging mice
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2002; 99 (12): 8191-8196
Telomerase is up-regulated in the vast majority of human cancers and serves to halt the progressive telomere shortening that ultimately blocks would-be cancer cells from achieving a full malignant phenotype. In contrast to humans, the laboratory mouse possesses long telomeres and, even in early generation telomerase-deficient mice, the level of telomere reserve is sufficient to avert telomere-based checkpoint responses and to permit full malignant progression. These features in the mouse provide an opportunity to determine whether enforced high-level telomerase activity can serve functions that extend beyond its ability to sustain telomere length and function. Here, we report the generation and characterization of transgenic mice that express the catalytic subunit of telomerase (mTERT) at high levels in a broad variety of tissues. Expression of mTERT conferred increased telomerase enzymatic activity in several tissues, including mammary gland, splenocytes, and cultured mouse embryonic fibroblasts. In mouse embryonic fibroblasts, mTERT overexpression extended telomere lengths but did not prevent culture-induced replicative arrest, thus reinforcing the view that this phenomenon is not related to occult telomere shortening. Robust telomerase activity, however, was associated with the spontaneous development of mammary intraepithelial neoplasia and invasive mammary carcinomas in a significant proportion of aged females. These data indicate that enforced mTERT expression can promote the development of spontaneous cancers even in the setting of ample telomere reserve.
View details for DOI 10.1073/pnas.112515399
View details for Web of Science ID 000176217700071
View details for PubMedID 12034875
Telomere shortening and cell fates in mouse models of neoplasia
TRENDS IN MOLECULAR MEDICINE
2002; 8 (1): 44-47
Cell division in the absence of telomerase leads to telomere shortening that can activate checkpoint responses and impair chromosomal stability. The absence of telomerase in primary human cells and its near universal reactivation in human cancers has highlighted the importance of telomere shortening and telomerase reactivation during tumor development. Data from telomerase-deficient mouse models of cancer have indicated that telomere shortening can exert profoundly different influences on cell fates in developing cancers, limiting tumorigenesis by enhancing cell death or facilitating carcinogenesis by compromising chromosomal stability. These alternate fates depend on the integrity of the p53 pathway and on cell type.
View details for Web of Science ID 000173187000008
View details for PubMedID 11796266
Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice
2000; 406 (6796): 641-645
Aged humans sustain a high rate of epithelial cancers such as carcinomas of the breast and colon, whereas mice carrying common tumour suppressor gene mutations typically develop soft tissue sarcomas and lymphomas. Among the many factors that may contribute to this species variance are differences in telomere length and regulation. Telomeres comprise the nucleoprotein complexes that cap the ends of eukaryotic chromosomes and are maintained by the reverse transcriptase, telomerase. In human cells, insufficient levels of telomerase lead to telomere attrition with cell division in culture and possibly with ageing and tumorigenesis in vivo. In contrast, critical reduction in telomere length is not observed in the mouse owing to promiscuous telomerase expression and long telomeres. Here we provide evidence that telomere attrition in ageing telomerase-deficient p53 mutant mice promotes the development of epithelial cancers by a process of fusion-bridge breakage that leads to the formation of complex non-reciprocal translocations--a classical cytogenetic feature of human carcinomas. Our data suggest a model in which telomere dysfunction brought about by continual epithelial renewal during life generates the massive ploidy changes associated with the development of epithelial cancers.
View details for Web of Science ID 000088653800051
View details for PubMedID 10949306
Mice without telomerase: what can they teach us about human cancer?
2000; 6 (8): 852-855
Unicellular organisms, human cells and mice have provided insights into the processes of senescence, crisis, genomic instability and cancer in humans. Here, Artandi and DePinho discuss how studies in mice have uncovered a complex interplay between the ARF-p53 pathway, genomic instability due to telomere dysfunction, and the suppression or promotion of cancer.
View details for Web of Science ID 000165473800015
View details for PubMedID 10932211
A critical role for telomeres in suppressing and facilitating carcinogenesis
CURRENT OPINION IN GENETICS & DEVELOPMENT
2000; 10 (1): 39-46
Progressive telomere shortening occurs with the division of primary human cells and activates tumor suppressor pathways, triggering senescence and inhibiting tumorigenesis. Loss of p53 function, however, allows continued cell division despite increasing telomere dysfunction and entry into telomere crisis. Recent data suggest that the severe chromosomal instability of telomere crisis promotes secondary genetic changes that facilitate carcinogenesis. Reactivation of telomerase stabilizes telomere ends and allows continued tumor growth.
View details for Web of Science ID 000085533800005
View details for PubMedID 10679392
p53 deficiency rescues the adverse effects of telomere loss and cooperates with telomere dysfunction to accelerate carcinogenesis
1999; 97 (4): 527-538
Maintenance of telomere length and function is critical for the efficient proliferation of eukaryotic cells. Here, we examine the interactions between telomere dysfunction and p53 in cells and organs of telomerase-deficient mice. Coincident with severe telomere shortening and associated genomic instability, p53 is activated, leading to growth arrest and/or apoptosis. Deletion of p53 significantly attenuated the adverse cellular and organismal effects of telomere dysfunction, but only during the earliest stages of genetic crisis. Correspondingly, the loss of telomere function and p53 deficiency cooperated to initiate the transformation process. Together, these studies establish a key role for p53 in the cellular response to telomere dysfunction in both normal and neoplastic cells, question the significance of crisis as a tumor suppressor mechanism, and identify a biologically relevant stage of advanced crisis, termed genetic catastrophe.
View details for Web of Science ID 000080299100013
View details for PubMedID 10338216