TIN2 protein dyskeratosis congenita missense mutants are defective in association with telomerase

Functional genomics for curation of variants in telomere biology disorder associated genes: A systematic review

PURPOSE

Patients with an underlying telomere biology disorder (TBD) have variable clinical presentations, and they can be challenging to diagnose clinically. A genomic diagnosis for patients presenting with TBD is vital for optimal treatment. Unfortunately, many variants identified during diagnostic testing are variants of uncertain significance. This complicates management decisions, delays treatment, and risks nonuptake of potentially curative therapies. Improved application of functional genomic evidence may reduce variants of uncertain significance classifications.

METHODS

We systematically searched the literature for published functional assays interrogating TBD gene variants. When possible, established likely benign/benign and likely pathogenic/pathogenic variants were used to estimate the assay sensitivity, specificity, positive predictive value, negative predictive value, and odds of pathogenicity.

RESULTS

In total, 3131 articles were screened and 151 met inclusion criteria. Sufficient data to enable a PS3/BS3 recommendation were available for TERT variants only. We recommend that PS3 and BS3 can be applied at a moderate and supportive level, respectively. PS3/BS3 application was limited by a lack of assay standardization and limited inclusion of benign variants.

CONCLUSION

Further assay standardization and assessment of benign variants are required for optimal use of the PS3/BS3 criterion for TBD gene variant classification.

A de novo TINF2, R282C Mutation in a Case of Dyskeratosis Congenital Founded by Next-Generation Sequencing

Background

Dyskeratosis congenita (DC), an inherited and rare disease prevalent in males, is clinically manifested by reticulate hyperpigmentation, nail dystrophy, and leukoplakia. DC is associated with the increased risk of malignancy and other potentially lethal complications such as bone marrow failure, as well as lung and liver diseases. Mutations in 19 genes were found to be correlated with DC. Herein, we report a 12-year-old boy carrying a de novo mutation in TINF2 gene.

Methods

Whole exome sequencing (WES) was performed on DNA sample of the proband, and the variant was investigated in the family by Sanger sequencing. Population and bioinformatics analysis were performed.

Results

The NM_ 001099274.3(TINF2): c.844C>T (p.Arg282Cys) mutation was found by WES.

Conclusion

There was no history of the disease in the family, and the variant was classified as a de novo mutation.

A Rare Heterozygous TINF2 Deletional Frameshift Mutation in a Chinese Pedigree With a Spectrum of TBDs Phenotypes

Telomere biology disorders (TBDs) induced by TINF2 mutations manifest clinically with a spectrum of phenotypes, from silent carriers to a set of overlapping conditions. A rare TINF2 frameshift mutation (c.591delG) encoding a truncated mutant TIN2 protein (p.W198fs) was identified in a 6-years-and-3-month-old Chinese girl with neuroblastoma (NB) by next generation sequencing and confirmed by Sanger sequencing. To explore the possible implications of TINF2 mutations in TBDs development, the TINF2 mutant was transfected into the human embryonic kidney (HEK) 293T cells, and mRNA expression of the shelterin complex components as well as the cellular distribution of mutant TIN2 were examined. The TINF2 mutation was phenotypically associated with short stature in the proband, nail dystrophy and spotted hypopigmentation in her mother, and psoriasis in her older brother. I-TASSER modeling analysis revealed conformational changes of the mutant TIN2 protein and loss of pivotal domains downstream of the 198th amino acid. Additionally, mRNA expression of the shelterin components was downregulated, and TIN2 mutant protein expression was reduced in HEK293T cells transfected with mutant TINF2. Furthermore, instead of being restricted to the nucleus, the mutant TIN2 was identified in both the cytoplasm and the nucleus. The TINF2 gene mutation might impair the function of the shelterin complex and the telomere maintenance mechanisms, both of which are involved in the development of TBDs. TBDs have been associated with increased cancer risk. To the best of our knowledge, this is the first report of NB in patients with TBDs. The relationship between the TINF2 mutation and NB may need to further study.

de novo TINF2 C.845G>A: Pathogenic Variant in Patient with Dyskeratosis Congenita

Dyskeratosis congenita (DC) is a clinically and genetically heterogeneous, multisystem inherited syndrome with a very high risk for bone marrow failure (BMF) and cancer predisposition. The classical clinical form of DC is characterized by abnormal skin pigmentation, nail dystrophy, and oral leukoplakia. Bone marrow failure is considered to be an important and major complication of DC and the leading cause of death which develops in around 85% of cases. A number of genes involved in telomere maintenance are associated with DC, such as genes that encode the components of the telomerase complex (TERT, DKC1, TERC, NOP10, and NHP2), T-loop assembly protein (RTEL1), telomere capping (CTC1), telomere shelterin complex (TINF2), and telomerase trafficking protein (TCAB1). Mutations in TINF2 have been reported in 11–20% of all patients with DC and have been associated with bone marrow failure. Here we report on a 19-month old boy with very early presentation of bone marrow failure as a first clinical manifestation of DC. Upon first admission, the patient presented with thrombocytopenia and macrocytic anemia. Soon after, his blood counts deteriorated with the development of pancytopenia and aplastic anemia. Four months later, he developed nail dystrophy and skin hyperpigmentation. A de novo heterozygous pathogenic variant c.845G>A, p.(Arg282His) was located in exon 6 of TINF2 gene and was identified via clinical exome sequencing. The findings confirmed the diagnosis of DC. This is the first case with DC due to TINF2 pathogenic variant reported in North Macedonia.

Differential impact of a dyskeratosis congenita mutation in TPP1 on mouse hematopoiesis and germline

A TPP1 mutation known to cause telomere shortening and bone marrow failure in humans recapitulates telomere loss but results in severe germline defects in mice without impacting murine hematopoiesis.

Telomerase extends chromosome ends in somatic and germline stem cells to ensure continued proliferation. Mutations in genes critical for telomerase function result in telomeropathies such as dyskeratosis congenita, frequently resulting in spontaneous bone marrow failure. A dyskeratosis congenita mutation in TPP1 (K170∆) that specifically compromises telomerase recruitment to telomeres is a valuable tool to evaluate telomerase-dependent telomere length maintenance in mice. We used CRISPR-Cas9 to generate a mouse knocked in for the equivalent of the TPP1 K170∆ mutation (TPP1 K82∆) and investigated both its hematopoietic and germline compartments in unprecedented detail. TPP1 K82∆ caused progressive telomere erosion with increasing generation number but did not induce steady-state hematopoietic defects. Strikingly, K82∆ caused mouse infertility, consistent with gross morphological defects in the testis and sperm, the appearance of dysfunctional seminiferous tubules, and a decrease in germ cells. Intriguingly, both TPP1 K82∆ mice and previously characterized telomerase knockout mice show no spontaneous bone marrow failure but rather succumb to infertility at steady-state. We speculate that telomere length maintenance contributes differently to the evolutionary fitness of humans and mice.

TPP1 mutagenesis screens unravel shelterin interfaces and functions in hematopoiesis

Telomerase catalyzes chromosome end replication in stem cells and other long-lived cells. Mutations in telomerase or telomere-related genes result in diseases known as telomeropathies. Telomerase is recruited to chromosome ends by the ACD/TPP1 protein (TPP1 hereafter), a component of the shelterin complex that protects chromosome ends from unwanted end joining. TPP1 facilitates end protection by binding shelterin proteins POT1 and TIN2. TPP1 variants have been associated with telomeropathies but remain poorly characterized in vivo. Disease variants and mutagenesis scans provide efficient avenues to interrogate the distinct physiological roles of TPP1. Here, we conduct mutagenesis in the TIN2- and POT1-binding domains of TPP1 to discover mutations that dissect TPP1's functions. Our results extend current structural data to reveal that the TPP1-TIN2 interface is more extensive than previously thought and highlight the robustness of the POT1-TPP1 interface. Introduction of separation-of-function mutants alongside known TPP1 telomeropathy mutations in mouse hematopoietic stem cells (mHSCs) lacking endogenous TPP1 demonstrated a clear phenotypic demarcation. TIN2- and POT1-binding mutants were unable to rescue mHSC failure resulting from end deprotection. In contrast, TPP1 telomeropathy mutations sustained mHSC viability, consistent with their selectively impacting end replication. These results highlight the power of scanning mutagenesis in revealing structural interfaces and dissecting multifunctional genes.

Molecular mechanisms of telomere biology disorders

Genetic mutations that affect telomerase function or telomere maintenance result in a variety of diseases collectively called telomeropathies. This wide spectrum of disorders, which include dyskeratosis congenita, pulmonary fibrosis, and aplastic anemia, is characterized by severely short telomeres, often resulting in hematopoietic stem cell failure in the most severe cases. Recent work has focused on understanding the molecular basis of these diseases. Mutations in the catalytic TERT and TR subunits of telomerase compromise activity, while others, such as those found in the telomeric protein TPP1, reduce the recruitment of telomerase to the telomere. Mutant telomerase-associated proteins TCAB1 and dyskerin and the telomerase RNA maturation component poly(A)-specific ribonuclease affect the maturation and stability of telomerase. In contrast, disease-associated mutations in either CTC1 or RTEL1 are more broadly associated with telomere replication defects. Yet even with the recent surge in studies decoding the mechanisms underlying these diseases, a significant proportion of dyskeratosis congenita mutations remain uncharacterized or poorly understood. Here we review the current understanding of the molecular basis of telomeropathies and highlight experimental data that illustrate how genetic mutations drive telomere shortening and dysfunction in these patients. This review connects insights from both clinical and molecular studies to create a comprehensive view of the underlying mechanisms that drive these diseases. Through this, we emphasize recent advances in therapeutics and pinpoint disease-associated variants that remain poorly defined in their mechanism of action. Finally, we suggest future avenues of research that will deepen our understanding of telomere biology and telomere-related disease.

Dyskeratosis congenita

Dyskeratosis congenita (DC) is a genetic syndrome with progressive multisystem involvement classically characterized by the clinical triad of oral leukoplakia, nail dystrophy, and reticular hyperpigmentation. Frequent complications are bone marrow failure, increased rate of malignancy, lung and liver diseases. DC results from an anomalous progressive shortening of telomeres resulting in DNA replication problems inducing replicative senescence. We report a death due to DC in a 16-year-old male with bone marrow failure and multiple organ dysfunction. At autopsy, nail dystrophy and skin hypopigmentation were observed. Gross and microscopic examinations of the internal organs showed cardiac hypertrophy, multiple lung consolidations and prominent interstitial fibrosis, liver cirrhosis, and fibrosis. Multiple foci of extramedullary hematopoiesis were identified, including on the epidural surface of the dura, that is an infrequent location, mimicking a focal area of epidural hemorrhage. Only a few autopsy studies about DC are reported in the literature. Further research should be done to understand the pathophysiology of the disease and its complications.

Regulation of human telomerase in homeostasis and disease

Telomerase is a ribonucleoprotein complex, the catalytic core of which includes the telomerase reverse transcriptase (TERT) and the non-coding human telomerase RNA (hTR), which serves as a template for the addition of telomeric repeats to chromosome ends. Telomerase expression is restricted in humans to certain cell types, and telomerase levels are tightly controlled in normal conditions. Increased levels of telomerase are found in the vast majority of human cancers, and we have recently begun to understand the mechanisms by which cancer cells increase telomerase activity. Conversely, germline mutations in telomerase-relevant genes that decrease telomerase function cause a range of genetic disorders, including dyskeratosis congenita, idiopathic pulmonary fibrosis and bone marrow failure. In this Review, we discuss the transcriptional regulation of human TERT, hTR processing, assembly of the telomerase complex, the cellular localization of telomerase and its recruitment to telomeres, and the regulation of telomerase activity. We also discuss the disease relevance of each of these steps of telomerase biogenesis.

Structural Features of Nucleoprotein CST/Shelterin Complex Involved in the Telomere Maintenance and Its Association with Disease Mutations

Telomere comprises the ends of eukaryotic linear chromosomes and is composed of G-rich (TTAGGG) tandem repeats which play an important role in maintaining genome stability, premature aging and onsets of many diseases. Majority of the telomere are replicated by conventional DNA replication, and only the last bit of the lagging strand is synthesized by telomerase (a reverse transcriptase). In addition to replication, telomere maintenance is principally carried out by two key complexes known as shelterin (TRF1, TRF2, TIN2, RAP1, POT1, and TPP1) and CST (CDC13/CTC1, STN1, and TEN1). Shelterin protects the telomere from DNA damage response (DDR) and regulates telomere length by telomerase; while, CST govern the extension of telomere by telomerase and C strand fill-in synthesis. We have investigated both structural and biochemical features of shelterin and CST complexes to get a clear understanding of their importance in the telomere maintenance. Further, we have analyzed ~115 clinically important mutations in both of the complexes. Association of such mutations with specific cellular fault unveils the importance of shelterin and CST complexes in the maintenance of genome stability. A possibility of targeting shelterin and CST by small molecule inhibitors is further investigated towards the therapeutic management of associated diseases. Overall, this review provides a possible direction to understand the mechanisms of telomere borne diseases, and their therapeutic intervention.

TIN2 Functions with TPP1/POT1 To Stimulate Telomerase Processivity

TIN2 is an important regulator of telomere length, and mutations in TINF2, the gene encoding TIN2, cause short-telomere syndromes. While the genetics underscore the importance of TIN2, the mechanism through which TIN2 regulates telomere length remains unclear. Here, we tested the effects of human TIN2 on telomerase activity. We identified a new isoform in human cells, TIN2M, that is expressed at levels similar to those of previously studied TIN2 isoforms. All three TIN2 isoforms localized to and maintained telomere integrity in vivo, and localization was not disrupted by telomere syndrome mutations. Using direct telomerase activity assays, we discovered that TIN2 stimulated telomerase processivity in vitro All of the TIN2 isoforms stimulated telomerase to similar extents. Mutations in the TPP1 TEL patch abrogated this stimulation, suggesting that TIN2 functions with TPP1/POT1 to stimulate telomerase processivity. We conclude from our data and previously published work that TIN2/TPP1/POT1 is a functional shelterin subcomplex.

The role of telomere-binding modulators in pluripotent stem cells

Pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs), ESCs derived by somatic cell nuclear transfer (ntESCs), and induced pluripotent stem cells (iPSCs) have unlimited capacity for self-renewal and pluripotency and can give rise to all types of somatic cells. In order to maintain their self-renewal and pluripotency, PSCs need to preserve their telomere length and homeostasis. In recent years, increasing studies have shown that telomere reprogramming is essential for stem cell pluripotency maintenance and its induced pluripotency process. Telomere-associated proteins are not only required for telomere maintenance in both stem cells, their extra-telomeric functions have also been found to be critical as well. Here, we will discuss how telomeres and telomere-associated factors participate and regulate the maintenance of stem cell pluripotency.

The C-Terminal Extension Unique to the Long Isoform of the Shelterin Component TIN2 Enhances Its Interaction with TRF2 in a Phosphorylation- and Dyskeratosis Congenita Cluster-Dependent Fashion

TIN2 is central to the shelterin complex, linking the telomeric proteins TRF1 and TRF2 with TPP1/POT1. Mutations in TINF2, which encodes TIN2, that are found in dyskeratosis congenita (DC) result in very short telomeres and cluster in a region shared by the two TIN2 isoforms, TIN2S (short) and TIN2L (long). Here we show that TIN2L, but not TIN2S, is phosphorylated. TRF2 interacts more with TIN2L than TIN2S, and both the DC cluster and phosphorylation promote this enhanced interaction. The binding of TIN2L, but not TIN2S, is affected by TRF2-F120, which is also required for TRF2's interaction with end processing factors such as Apollo. Conversely, TRF1 interacts more with TIN2S than with TIN2L. A DC-associated mutation further reduces TIN2L-TRF1, but not TIN2S-TRF1, interaction. Cells overexpressing TIN2L or phosphomimetic TIN2L are permissive to telomere elongation, whereas cells overexpressing TIN2S or phosphodead TIN2L are not. Telomere lengths are unchanged in cell lines in which TIN2L expression has been eliminated by clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-mediated mutation. These results indicate that TIN2 isoforms are biochemically and functionally distinguishable and that shelterin composition could be fundamentally altered in patients with TINF2 mutations.

How long does telomerase extend telomeres? Regulation of telomerase release and telomere length homeostasis

Telomerase, the enzyme that replenishes telomeres, is essential for most eukaryotes to maintain their generations. Telomere length homeostasis is achieved via a balance between telomere lengthening by telomerase, and erosion over successive cell divisions. Impaired telomerase regulation leads to shortened telomeres and can cause defects in tissue maintenance. Telomeric DNA is composed of a repetitive sequence, which recruits the protective protein complex, shelterin. Shelterin, together with chromatin remodelling proteins, shapes the heterochromatic structure at the telomere and protects chromosome ends. Shelterin also provides a foothold for telomerase to be recruited and facilitates telomere extension. Such mechanisms of telomere recruitment and activation are conserved from unicellular eukaryotes to humans, with the rate of telomere extension playing an important role in determining the length maintained. Telomerase can be processive, adding multiple telomeric repeats before dissociating. However, a question remains: how does telomerase determine the number of repeats to add? In this review, I will discuss about how telomerase can monitor telomere extension using fission yeast as a model. I propose a model whereby the accumulation of the Pot1 complex on the synthesised telomere single-strand counteracts retention of telomerase via chromatin proteins and the similar system may be conserved in mammals.

Shwachman-Diamond Syndrome Protein SBDS Maintains Human Telomeres by Regulating Telomerase Recruitment

Shwachman-Diamond syndrome (SDS) is a rare pediatric disease characterized by various systemic disorders, including hematopoietic dysfunction. The mutation of Shwachman-Bodian-Diamond syndrome (SBDS) gene has been proposed to be a major causative reason for SDS. Although SBDS patients were reported to have shorter telomere length in granulocytes, the underlying mechanism is still unclear. Here we provide data to elucidate the role of SBDS in telomere protection. We demonstrate that SBDS deficiency leads to telomere shortening. We found that overexpression of disease-associated SBDS mutants or knockdown of SBDS hampered the recruitment of telomerase onto telomeres, while the overall reverse transcriptase activity of telomerase remained unaffected. Moreover, we show that SBDS could specifically bind to TPP1 during the S phase of cell cycle, likely functioning as a stabilizer for TPP1-telomerase interaction. Our findings suggest that SBDS is a telomere-protecting protein that participates in regulating telomerase recruitment.

Dysfunctional telomeres and hematological disorders

Telomere biology disorders, which are characterized by telomerase activity haploinsufficiency and accelerated telomere shortening, most commonly manifest as degenerative diseases. Tissues with high rates of cell turnover, such as those in the hematopoietic system, are particularly vulnerable to defects in telomere maintenance genes that eventually culminate in bone marrow (BM) failure syndromes, in which the BM cannot produce sufficient new blood cells. Here, we review how telomere defects induce degenerative phenotypes across multiple organs, with particular focus on how they impact the hematopoietic stem and progenitor compartment and affect hematopoietic stem cell (HSC) self-renewal and differentiation. We also discuss how both the increased risk of myelodysplastic syndromes and other hematological malignancies that is associated with telomere disorders and the discovery of cancer-associated somatic mutations in the shelterin components challenge the conventional interpretation that telomere defects are cancer-protective rather than cancer-promoting.

Acute telomerase components depletion triggers oxidative stress as an early event previous to telomeric shortening

Loss of function of dyskerin (DKC1), NOP10 and TIN2 are responsible for different inheritance patterns of Dyskeratosis congenita (DC; ORPHA1775). They are key components of telomerase (DKC1 and NOP10) and shelterin (TIN2), and play an important role in telomere homeostasis. They participate in several fundamental cellular processes by contributing to Dyskeratosis congenita through mechanisms that are not fully understood. Presence of oxidative stress was postulated to result from telomerase ablation. However, the resulting disturbed redox status can promote telomere attrition by generating a vicious circle, which promotes cellular senescence. This fact prompted us to study if acute loss of DKC1, NOP10 and TINF2 can promote redox disequilibrium as an early event when telomere shortening has not yet taken place. We generated siRNA-mediated (DKC1, NOP10 and TINF2) cell lines by RNA interference, which was confirmed by mRNA and protein expression analyses. No telomere shortening occurred in any silenced cell line. Depletion of H/ACA ribonucleoproteins DKC1 and NOP10 diminished telomerase activity via TERC down-regulation, and produced alterations in pseudouridylation and ribosomal biogenesis. An increase in the GSSG/GSH ratio, carbonylated proteins and oxidized peroxiredoxin-6 was observed, in addition to MnSOD and TRX1 overexpression in the siRNA DC cells. Likewise, high PARylation levels and high PARP1 protein expression were detected. In contrast, the silenced TINF2 cells did not alter any evaluated oxidative stress marker. Altogether these findings lead us to conclude that loss of DKC1 and NOP10 functions induces oxidative stress in a telomere shortening independent manner.

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Transient silencing of DKC1 and NOP10 genes produce oxidative stress.

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Cells depleted of DKC1 and NOP10 are susceptible to DNA damage.

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Acute DKC1 and NOP10 depletion disrupts RNA maturation.

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Oxidative stress is an early event previous to telomere shortening.

Telomerase Regulation from Beginning to the End

The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts to its maturity as a complex and function at its site of action, emphasizing new developments and how they contribute to the foundational knowledge of telomerase and telomere biology.

Multi-step coordination of telomerase recruitment in fission yeast through two coupled telomere-telomerase interfaces

Tightly controlled recruitment of telomerase, a low-abundance enzyme, to telomeres is essential for regulated telomere synthesis. Recent studies in human cells revealed that a patch of amino acids in the shelterin component TPP1, called the TEL-patch, is essential for recruiting telomerase to telomeres. However, how TEL-patch—telomerase interaction integrates into the overall orchestration of telomerase regulation at telomeres is unclear. In fission yeast, Tel1^ATM/Rad3^ATR-mediated phosphorylation of shelterin component Ccq1 during late S phase is involved in telomerase recruitment through promoting the binding of Ccq1 to a telomerase accessory protein Est1. Here, we identify the TEL-patch in Tpz1^TPP1, mutations of which lead to decreased telomeric association of telomerase, similar to the phosphorylation-defective Ccq1. Furthermore, we find that telomerase action at telomeres requires formation and resolution of an intermediate state, in which the cell cycle-dependent Ccq1-Est1 interaction is coupled to the TEL-patch—Trt1 interaction, to achieve temporally regulated telomerase elongation of telomeres.

DOI:http://dx.doi.org/10.7554/eLife.15470.001

The genetic blueprints for animals, plants and fungi are mostly contained within long strands of DNA and packaged into more compact thread-like structures called chromosomes. As such, most cells need to duplicate their chromosomes before they divide so that the two new cells each get a complete set of genetic instructions.

The machinery that copies DNA is unable to make it to the very ends of the chromosomes. Instead, an enzyme called telomerase adds new DNA to the chromosome ends to prevent them becoming too short. Problems with this process can cause serious issues, such as cell death or cancer, and so the activity of telomerase is carefully controlled. Other proteins guide telomerase to the ends of the chromosome only after the rest of the DNA has been copied. However, scientists do not know exactly how cells correctly time the arrival of telomerase.

A group of proteins called shelterin protects the chromosome ends, and studies with human cells have shown that telomerase attaches to a specific patch on one of shelterin proteins, called the TEL-patch, to begin its work. Now, Hu, Liu et al. have identified a similar TEL-patch in a shelterin protein from a type of yeast called fission yeast; this patch is also needed to attach telomerase to the chromosome ends. Further experiments with this yeast then showed that telomerase only arrives at the ends of the chromosomes after two parallel interaction interfaces have formed. Importantly, one of these interactions only takes place after most of the chromosomes are have been copied. As such, this “two-pronged interaction” mechanism ensures that the telomerase enzyme arrives at the end of the chromosomes at the right time.

Other similarities between human and fission yeast chromosome ends make it plausible that a comparable process controls the timing of telomerase attachment in human cells. However, more studies will be needed to confirm if this is the case.

DOI:http://dx.doi.org/10.7554/eLife.15470.002

The shelterin complex and hematopoiesis

Mammalian chromosomes terminate in stretches of repetitive telomeric DNA that act as buffers to avoid loss of essential genetic information during end-replication. A multiprotein complex known as shelterin prevents recognition of telomeric sequences as sites of DNA damage. Telomere erosion contributes to human diseases ranging from BM failure to premature aging syndromes and cancer. The role of shelterin telomere protection is less understood. Mutations in genes encoding the shelterin proteins TRF1-interacting nuclear factor 2 (TIN2) and adrenocortical dysplasia homolog (ACD) were identified in dyskeratosis congenita, a syndrome characterized by somatic stem cell dysfunction in multiple organs leading to BM failure and other pleiotropic manifestations. Here, we introduce the biochemical features and in vivo effects of individual shelterin proteins, discuss shelterin functions in hematopoiesis, and review emerging knowledge implicating the shelterin complex in hematological disorders.

Control of telomerase action at human telomeres

Recent progress has greatly increased the understanding of telomere-bound shelterin proteins and the telomerase holoenzyme, predominantly as separate complexes. Pioneering studies have begun to investigate the requirements for shelterin-telomerase interaction. From this vantage point, focusing on human cells, we review and discuss models for how telomerase and shelterin subunits coordinate to achieve balanced telomere-length homeostasis.

Hypothesis: Paralog Formation from Progenitor Proteins and Paralog Mutagenesis Spur the Rapid Evolution of Telomere Binding Proteins

Through elegant studies in fungal cells and complex organisms, we propose a unifying paradigm for the rapid evolution of telomere binding proteins (TBPs) that associate with either (or both) telomeric DNA and telomeric proteins. TBPs protect and regulate telomere structure and function. Four critical factors are involved. First, TBPs that commonly bind to telomeric DNA include the c-Myb binding proteins, OB-fold single-stranded binding proteins, and G-G base paired Hoogsteen structure (G4) binding proteins. Each contributes independently or, in some cases, cooperatively, to provide a minimum level of telomere function. As a result of these minimal requirements and the great abundance of homologs of these motifs in the proteome, DNA telomere-binding activity may be generated more easily than expected. Second, telomere dysfunction gives rise to genome instability, through the elevation of recombination rates, genome ploidy, and the frequency of gene mutations. The formation of paralogs that diverge from their progenitor proteins ultimately can form a high frequency of altered TBPs with altered functions. Third, TBPs that assemble into complexes (e.g., mammalian shelterin) derive benefits from the novel emergent functions. Fourth, a limiting factor in the evolution of TBP complexes is the formation of mutually compatible interaction surfaces amongst the TBPs. These factors may have different degrees of importance in the evolution of different phyla, illustrated by the apparently simpler telomeres in complex plants. Selective pressures that can utilize the mechanisms of paralog formation and mutagenesis to drive TBP evolution along routes dependent on the requisite physiologic changes.

Robust DNA Damage Response and Elevated Reactive Oxygen Species in TINF2-Mutated Dyskeratosis Congenita Cells

Dyskeratosis Congenita (DC) is an inherited multisystem premature aging disorder with characteristic skin and mucosal findings as well as a predisposition to cancer and bone marrow failure. DC arises due to gene mutations associated with the telomerase complex or telomere maintenance, resulting in critically shortened telomeres. The pathogenesis of DC, as well as several congenital bone marrow failure (BMF) syndromes, converges on the DNA damage response (DDR) pathway and subsequent elevation of reactive oxygen species (ROS). Historically, DC patients have had poor outcomes following bone marrow transplantation (BMT), perhaps as a consequence of an underlying DNA hypersensitivity to cytotoxic agents. Previously, we demonstrated an activated DDR and increased ROS, augmented by chemotherapy and radiation, in somatic cells isolated from DC patients with a mutation in the RNA component of telomerase, TERC. The current study was undertaken to determine whether previous findings related to ROS and DDR in TERC patients’ cells could be extended to other DC mutations. Of particular interest was whether an antioxidant approach could counter increased ROS and decrease DC pathologies. To test this, we examined lymphocytes from DC patients from different DC mutations (TERT, TINF2, and TERC) for the presence of an active DDR and increased ROS. All DC mutations led to increased steady-state p53 (2-fold to 10-fold) and ROS (1.5-fold to 2-fold). Upon exposure to ionizing radiation (XRT), DC cells increased in both DDR and ROS to a significant degree. Exposing DC cells to hydrogen peroxide also revealed that DC cells maintain a significant oxidant burden compared to controls (1.5-fold to 3-fold). DC cell culture supplemented with N-acetylcysteine, or alternatively grown in low oxygen, afforded significant proliferative benefits (proliferation: maximum 2-fold increase; NAC: 5-fold p53 decrease; low oxygen: maximum 3.5-fold p53 decrease). Together, our data supports a mechanism whereby telomerase deficiency and subsequent shortened telomeres initiate a DDR and create a pro-oxidant environment, especially in cells carrying the TINF2 mutations. Finally, the ameliorative effects of antioxidants in vitro suggest this could translate to therapeutic benefits in DC patients.

Cold-inducible RNA-binding protein CIRP/hnRNP A18 regulates telomerase activity in a temperature-dependent manner

The telomerase is responsible for adding telomeric repeats to chromosomal ends and consists of the reverse transcriptase TERT and the RNA subunit TERC. The expression and activity of the telomerase are tightly regulated, and aberrant activation of the telomerase has been observed in >85% of human cancers. To better understand telomerase regulation, we performed immunoprecipitations coupled with mass spectrometry (IP-MS) and identified cold inducible RNA-binding protein (CIRP or hnRNP A18) as a telomerase-interacting factor. We have found that CIRP is necessary to maintain telomerase activities at both 32°C and 37°C. Furthermore, inhibition of CIRP by CRISPR-Cas9 or siRNA knockdown led to reduced telomerase activities and shortened telomere length, suggesting an important role of CIRP in telomere maintenance. We also provide evidence here that CIRP associates with the active telomerase complex through direct binding of TERC and regulates Cajal body localization of the telomerase. In addition, CIRP regulates the level of TERT mRNAs. At the lower temperature, TERT mRNA is upregulated in a CIRP-dependent manner to compensate for reduced telomerase activities. Taken together, these findings highlight the dual roles that CIRP plays in regulating TERT and TERC, and reveal a new class of telomerase modulators in response to hypothermia conditions.

Molecular basis of telomere dysfunction in human genetic diseases

Mutations in genes encoding proteins required for telomere structure, replication, repair and length maintenance are associated with several debilitating human genetic disorders. These complex telomere biology disorders (TBDs) give rise to critically short telomeres that affect the homeostasis of multiple organs. Furthermore, genome instability is often a hallmark of telomere syndromes, which are associated with increased cancer risk. Here, we summarize the molecular causes and cellular consequences of disease-causing mutations associated with telomere dysfunction.

The molecular genetics of the telomere biology disorders

The importance of telomere function for human health is exemplified by a collection of Mendelian disorders referred to as the telomere biology disorders (TBDs), telomeropathies, or syndromes of telomere shortening. Collectively, the TBDs cover a spectrum of conditions from multisystem disease presenting in infancy to isolated disease presentations in adulthood, most notably idiopathic pulmonary fibrosis. Eleven genes have been found mutated in the TBDs to date, each of which is linked to some aspect of telomere maintenance. This review summarizes the molecular defects that result from mutations in these genes, highlighting recent advances, including the addition of PARN to the TBD gene family and the discovery of heterozygous mutations in RTEL1 as a cause of familial pulmonary fibrosis.

Practice parameter for the diagnosis and management of primary immunodeficiency

The American Academy of Allergy, Asthma & Immunology (AAAAI) and the American College of Allergy, Asthma & Immunology (ACAAI) have jointly accepted responsibility for establishing the "Practice parameter for the diagnosis and management of primary immunodeficiency." This is a complete and comprehensive document at the current time. The medical environment is a changing environment, and not all recommendations will be appropriate for all patients. Because this document incorporated the efforts of many participants, no single individual, including those who served on the Joint Task Force, is authorized to provide an official AAAAI or ACAAI interpretation of these practice parameters. Any request for information about or an interpretation of these practice parameters by the AAAAI or ACAAI should be directed to the Executive Offices of the AAAAI, the ACAAI, and the Joint Council of Allergy, Asthma & Immunology. These parameters are not designed for use by pharmaceutical companies in drug promotion.

The Shelterin TIN2 Subunit Mediates Recruitment of Telomerase to Telomeres

Dyskeratosis Congenita (DC) is a heritable multi-system disorder caused by abnormally short telomeres. Clinically diagnosed by the mucocutaneous symptoms, DC patients are at high risk for bone marrow failure, pulmonary fibrosis, and multiple types of cancers. We have recapitulated the most common DC-causing mutation in the shelterin component TIN2 by introducing a TIN2-R282H mutation into cultured telomerase-positive human cells via a knock-in approach. The resulting heterozygous TIN2-R282H mutation does not perturb occupancy of other shelterin components on telomeres, result in activation of telomeric DNA damage signaling or exhibit other characteristics indicative of a telomere deprotection defect. Using a novel assay that monitors the frequency and extension rate of telomerase activity at individual telomeres, we show instead that telomerase elongates telomeres at a reduced frequency in TIN2-R282H heterozygous cells; this recruitment defect is further corroborated by examining the effect of this mutation on telomerase-telomere co-localization. These observations suggest a direct role for TIN2 in mediating telomere length through telomerase, separable from its role in telomere protection.

The shelterin complex protects telomeres from being processed by the DNA damage repair machinery, and also regulates telomerase access and activity at telomeres. The only shelterin subunit known to promote telomerase function is TPP1, which mediates telomerase recruitment to telomeres and stimulates telomerase processivity. Mutations in shelterin components cause Dyskeratosis Congenita (DC) and related disease syndromes due to the inability to maintain telomere homeostasis. In this study, we have identified TIN2-R282H, the most common DC-causing mutation in shelterin subunit TIN2, as a separation-of-function mutant which impairs telomerase recruitment to telomeres, but not chromosome end protection. The telomerase recruitment defect conferred by TIN2-R282H is likely through a mechanism independent of TIN2’s role in anchoring TPP1 at telomeres, since TPP1 localization to telomeres is unaffected by the mutation.

Exome sequencing identifies mutant TINF2 in a family with pulmonary fibrosis

BACKGROUND

Short telomeres are a common defect in idiopathic pulmonary fibrosis, yet mutations in the telomerase genes account for only a subset of these cases.

METHODS

We identified a family with pulmonary fibrosis, idiopathic infertility, and short telomeres.

RESULTS

Exome sequencing of blood-derived DNA revealed two mutations in the telomere-binding protein TINF2. The first was a 15-base-pair deletion encompassing the exon 6 splice acceptor site, and the second was a missense mutation, Thr284Arg. Haplotype analysis indicated both variants fell on the same allele. However, lung-derived DNA showed predominantly the Thr284Arg allele, indicating that the deletion seen in the blood was acquired and may have a protective advantage because it diminished expression of the missense mutation. This mosaicism may represent functional reversion in telomere syndromes similar to that described for Fanconi anemia. No mutations were identified in over 40 uncharacterized pulmonary fibrosis probands suggesting that mutant TINF2 accounts for a small subset of familial cases. However, similar to affected individuals in this family, we identified a history of male and female infertility preceding the onset of pulmonary fibrosis in 11% of TERT and TR mutation carriers (five of 45).

CONCLUSIONS

Our findings identify TINF2 as a mutant telomere gene in familial pulmonary fibrosis and suggest that infertility may precede the presentation of pulmonary fibrosis in a small subset of adults with telomere syndromes.

Unraveling the pathogenesis of Hoyeraal-Hreidarsson syndrome, a complex telomere biology disorder

Hoyeraal-Hreidarsson (HH) syndrome is a multisystem genetic disorder characterized by very short telomeres and considered a clinically severe variant of dyskeratosis congenita. The main cause of mortality, usually in early childhood, is bone marrow failure. Mutations in several telomere biology genes have been reported to cause HH in about 60% of the HH patients, but the genetic defects in the rest of the patients are still unknown. Understanding the aetiology of HH and its diverse manifestations is challenging because of the complexity of telomere biology and the multiple telomeric and non-telomeric functions played by telomere-associated proteins in processes such as telomere replication, telomere protection, DNA damage response and ribosome and spliceosome assembly. Here we review the known clinical complications, molecular defects and germline mutations associated with HH, and elucidate possible mechanistic explanations and remaining questions in our understanding of the disease.

Genetics of familial melanoma: 20 years after CDKN2A

Twenty years ago, the first familial melanoma susceptibility gene, CDKN2A, was identified. Two years later, another high-penetrance gene, CDK4, was found to be responsible for melanoma development in some families. Progress in identifying new familial melanoma genes was subsequently slow; however, with the advent of next-generation sequencing, a small number of new high-penetrance genes have recently been uncovered. This approach has identified the lineage-specific oncogene MITF as a susceptibility gene both in melanoma families and in the general population, as well as the discovery of telomere maintenance as a key pathway underlying melanoma predisposition. Given these rapid recent advances, this approach seems likely to continue to pay dividends. Here, we review the currently known familial melanoma genes, providing evidence that most additionally confer risk to other cancers, indicating that they are likely general tumour suppressor genes or oncogenes, which has significant implications for surveillance and screening.

Hoyeraal-Hreidarsson syndrome caused by a germline mutation in the TEL patch of the telomere protein TPP1

Germline mutations in telomere biology genes cause dyskeratosis congenita (DC), an inherited bone marrow failure and cancer predisposition syndrome. Hoyeraal-Hreidarsson syndrome (HH) is a clinically severe variant of DC. Using exome sequencing, Kocak et al. identified mutations in ACD (encoding TPP1), a component of the telomeric shelterin complex, in one family affected by HH. Characterization of the mutations revealed that the single-amino-acid deletion affecting the TEL patch surface of the TPP1 protein significantly compromises both telomerase recruitment and processivity.

Germline mutations in telomere biology genes cause dyskeratosis congenita (DC), an inherited bone marrow failure and cancer predisposition syndrome. DC is a clinically heterogeneous disorder diagnosed by the triad of dysplastic nails, abnormal skin pigmentation, and oral leukoplakia; Hoyeraal-Hreidarsson syndrome (HH), a clinically severe variant of DC, also includes cerebellar hypoplasia, immunodeficiency, and intrauterine growth retardation. Approximately 70% of DC cases are associated with a germline mutation in one of nine genes, the products of which are all involved in telomere biology. Using exome sequencing, we identified mutations in Adrenocortical Dysplasia Homolog (ACD) (encoding TPP1), a component of the telomeric shelterin complex, in one family affected by HH. The proband inherited a deletion from his father and a missense mutation from his mother, resulting in extremely short telomeres and a severe clinical phenotype. Characterization of the mutations revealed that the single-amino-acid deletion affecting the TEL patch surface of the TPP1 protein significantly compromises both telomerase recruitment and processivity, while the missense mutation in the TIN2-binding region of TPP1 is not as clearly deleterious to TPP1 function. Our results emphasize the critical roles of the TEL patch in proper stem cell function and demonstrate that TPP1 is the second shelterin component (in addition to TIN2) to be implicated in DC.

A TIN2 dyskeratosis congenita mutation causes telomerase-independent telomere shortening in mice

The progressive bone marrow failure syndrome dyskeratosis congenita (DC) is often caused by mutations in telomerase or the factors involved in telomerase biogenesis and trafficking. However, a subset of DC patients is heterozygous for mutations in the shelterin component TIN2. To determine how the TIN2-DC mutations affect telomere function, we generated mice with the equivalent of the TIN2 K280E DC allele (TIN2(DC)) by gene targeting. Whereas homozygous TIN2(DC/DC) mice were not viable, first-generation TIN2(+/DC) mice were healthy and fertile. In the second and third generations, the TIN2(+/DC) mice developed mild pancytopenia, consistent with hematopoietic dysfunction in DC, as well as diminished fecundity. Bone marrow telomeres of TIN2(+/DC) mice shortened over the generations, and immortalized TIN2(+/DC) mouse embryonic fibroblasts (MEFs) showed telomere shortening with proliferation. Unexpectedly, telomere shortening was accelerated in TIN2(+/DC) mTR(-/-) mice and MEFs compared with TIN2(+/+) mTR(-/-) controls, establishing that the TIN2(DC) telomere maintenance defect was not solely due to diminished telomerase action. The TIN2(DC) allele induced mild ATR kinase signaling at telomeres and a fragile telomere phenotype, suggestive of telomere replication problems. These data suggest that this TIN2-DC mutation could induce telomeric dysfunction phenotypes in telomerase-negative somatic cells and tissues that further exacerbate the telomere maintenance problems in telomerase-positive stem cell compartments.

Inherited mutations in the helicase RTEL1 cause telomere dysfunction and Hoyeraal-Hreidarsson syndrome

Telomeres repress the DNA damage response at the natural chromosome ends to prevent cell-cycle arrest and maintain genome stability. Telomeres are elongated by telomerase in a tightly regulated manner to ensure a sufficient number of cell divisions throughout life, yet prevent unlimited cell division and cancer development. Hoyeraal-Hreidarsson syndrome (HHS) is characterized by accelerated telomere shortening and a broad range of pathologies, including bone marrow failure, immunodeficiency, and developmental defects. HHS-causing mutations have previously been found in telomerase and the shelterin component telomeric repeat binding factor 1 (TRF1)-interacting nuclear factor 2 (TIN2). We identified by whole-genome exome sequencing compound heterozygous mutations in four siblings affected with HHS, in the gene encoding the regulator of telomere elongation helicase 1 (RTEL1). Rtel1 was identified in mouse by its genetic association with telomere length. However, its mechanism of action and whether it regulates telomere length in human remained unknown. Lymphoblastoid cell lines obtained from a patient and from the healthy parents carrying heterozygous RTEL1 mutations displayed telomere shortening, fragility and fusion, and growth defects in culture. Ectopic expression of WT RTEL1 suppressed the telomere shortening and growth defect, confirming the causal role of the RTEL1 mutations in HHS and demonstrating the essential function of human RTEL1 in telomere protection and elongation. Finally, we show that human RTEL1 interacts with the shelterin protein TRF1, providing a potential recruitment mechanism of RTEL1 to telomeres.

Cell cycle regulated phosphorylation of the telomere-associated protein TIN2

The protein TIN2 is a member of telomere-binding protein complex that serves to cap and protect mammalian chromosome ends. As a number of proteins in this complex are phosphorylated in a cell cycle-dependent manner, we investigated whether TIN2 is modified by phosphorylation as well. We performed phospho-proteomic analysis of human TIN2, and identified two phosphorylated residues, serines 295 and 330. We demonstrated that both these sites were phosphorylated during mitosis in human cells, as detected by Phos-tag reagent and phosphorylation-specific antibodies. Phosphorylation of serines 295 and 330 appeared to be mediated, at least in part, by the mitotic kinase RSK2. Specifically, phosphorylation of TIN2 at both these residues was increased upon expression of RSK2 and reduced by an inhibitor of the RSK family of kinases. Moreover, RSK2 phosphorylated TIN2 in vitro. The identification of these specifically timed post-translational events during the cell cycle suggests a potential mitotic regulation of TIN2 by phosphorylation.

Telomere dysfunction and tumor suppression responses in dyskeratosis congenita: balancing cancer and tissue renewal impairment

Dyskeratosis congenita (DC) encompasses a large spectrum of diseases and clinical manifestations generally related to premature aging, including bone marrow failure and cancer predisposition. The major risk factor for DC is to carry germline telomere-related mutations - in telomerase or telomere shelterin genes - which results in premature telomere dysfunction, thus increasing the risk of premature aging impairments. Despite the advances that have been accomplished in DC research, the molecular aspects underlying the phenotypic variability of the disease remain poorly understood. Here different aspects of telomere biology, concerning adult stem cells senescence, tumor suppression and cancer are considered in the context of DC, resulting in two translational models: late onset of DC symptoms in telomere-related mutations carriers is a potential indicator of increased cancer risk and differences in tumor suppression capacities among the genetic subgroups are (at least partial) causes of different clinical manifestations of the disease. The limitations of both models are presented, and further experiments for their validation, as well as clinical implications, are discussed.

Finding the end: recruitment of telomerase to telomeres

Telomeres, the ends of linear eukaryotic chromosomes, are characterized by the presence of multiple repeats of a short DNA sequence. This telomeric DNA is protected from illicit repair by telomere-associated proteins, which in mammals form the shelterin complex. Replicative polymerases are unable to synthesize DNA at the extreme ends of chromosomes, but in unicellular eukaryotes such as yeast and in mammalian germ cells and stem cells, telomere length is maintained by a ribonucleoprotein enzyme known as telomerase. Recent work has provided insights into the mechanisms of telomerase recruitment to telomeres, highlighting the contribution of telomere-associated proteins, including TPP1 in humans, Ccq1 in Schizosaccharomyces pombe and Cdc13 and Ku70-Ku80 in Saccharomyces cerevisiae.

Telomeres-structure, function, and regulation

In mammals, maintenance of the linear chromosome ends (or telomeres) involves faithful replication of genetic materials and protection against DNA damage signals, to ensure genome stability and integrity. These tasks are carried out by the telomerase holoenzyme and a unique nucleoprotein structure in which an array of telomere-associated proteins bind to telomeric DNA to form special protein/DNA complexes. The telomerase complex, which is comprised of telomeric reverse transcriptase (TERT), telomeric RNA component (TERC), and other assistant factors, is responsible for adding telomeric repeats to the ends of chromosomes. Without proper telomere maintenance, telomere length will shorten with successive round of DNA replication due to the so-called end replication problem. Aberrant regulation of telomeric proteins and/or telomerase may lead to abnormalities that can result in diseases such as dyskeratosis congenita (DC) and cancers. Understanding the mechanisms that regulate telomere homeostasis and the factors that contribute to telomere dysfunction should aid us in developing diagnostic and therapeutic tools for these diseases.

Specificity requirements for human telomere protein interaction with telomerase holoenzyme

Human telomeres are maintained by the enzyme telomerase, which uses a template within its integral RNA subunit (hTR) and telomerase reverse transcriptase protein (TERT) to accomplish the synthesis of single-stranded DNA repeats. Many questions remain unresolved about the cellular regulation of telomerase subunits and the fully assembled telomerase holoenzyme, including the basis for the specificity of binding and acting on telomeres. Previous studies have revealed that the telomere protein TPP1 is necessary for stable TERT and hTR association with telomeres in vivo. Here, we expand the biochemical characterization and understanding of TPP1 interaction with TERT and the catalytically active telomerase holoenzyme. Using extracts from human cells, we show that TPP1 interacts sequence-specifically with TERT when TERT is assembled into holoenzyme context. In holoenzyme context, the TERT N-terminal domain mediates a TPP1 interaction. Assays of stable subunit complexes purified after their cellular assembly suggest that other telomere proteins do not necessarily influence TPP1 association with telomerase holoenzyme or alter its impact on elongation processivity. We show that a domain of recombinant TPP1 comprised of an oligonucleotide/oligosaccharide binding fold recapitulates the full-length protein interaction specificity for the TERT N-terminal domain assembled into telomerase holoenzyme. By global analysis of TPP1 side chain requirements for holoenzyme association, we demonstrate a selective requirement for the amino acids in one surface-exposed protein loop. Our results reveal the biochemical determinants of a sequence-specific TPP1-TERT interaction in human cells, with implications for the mechanisms of TPP1 function in recruiting telomerase subunits to telomeres and in promoting telomere elongation.

Mitochondrial localization of telomeric protein TIN2 links telomere regulation to metabolic control

Both mitochondria, which are metabolic powerhouses, and telomeres, which help maintain genomic stability, have been implicated in cancer and aging. However, the signaling events that connect these two cellular structures remain poorly understood. Here, we report that the canonical telomeric protein TIN2 is also a regulator of metabolism. TIN2 is recruited to telomeres and associates with multiple telomere regulators including TPP1. TPP1 interacts with TIN2 N terminus, which contains overlapping mitochondrial and telomeric targeting sequences, and controls TIN2 localization. We have found that TIN2 is posttranslationally processed in mitochondria and regulates mitochondrial oxidative phosphorylation. Reducing TIN2 expression by RNAi knockdown inhibited glycolysis and reactive oxygen species (ROS) production and enhanced ATP levels and oxygen consumption in cancer cells. These results suggest a link between telomeric proteins and metabolic control, providing an additional mechanism by which telomeric proteins regulate cancer and aging.

Haploinsufficiency and telomere length homeostasis

In humans, autosomal dominant or X-linked disease can arise through a phenomenon termed haploinsufficiency, where one remaining wild-type allele is insufficient for function. In model organisms, the impact of heterozygosity can be tested directly with engineered mutant alleles or in a hemizygous state where the expression of one allele is abrogated completely. This review will focus on haploinsufficiency as it relates to telomerase and telomere length maintenance and, citing selected examples in various model organisms, it will discuss how the problem of gene dosage relates to telomere function in normal and diseased states.

Native gel electrophoresis of human telomerase distinguishes active complexes with or without dyskerin

Telomeres, the ends of linear chromosomes, safeguard against genome instability. The enzyme responsible for extension of the telomere 3′ terminus is the ribonucleoprotein telomerase. Whereas telomerase activity can be reconstituted in vitro with only the telomerase RNA (hTR) and telomerase reverse transcriptase (TERT), additional components are required in vivo for enzyme assembly, stability and telomere extension activity. One such associated protein, dyskerin, promotes hTR stability in vivo and is the only component to co-purify with active, endogenous human telomerase. We used oligonucleotide-based affinity purification of hTR followed by native gel electrophoresis and in-gel telomerase activity detection to query the composition of telomerase at different purification stringencies. At low salt concentrations (0.1 M NaCl), affinity-purified telomerase was ‘supershifted’ with an anti-dyskerin antibody, however the association with dyskerin was lost after purification at 0.6 M NaCl, despite the retention of telomerase activity and a comparable yield of hTR. The interaction of purified hTR and dyskerin in vitro displayed a similar salt-sensitive interaction. These results demonstrate that endogenous human telomerase, once assembled and active, does not require dyskerin for catalytic activity. Native gel electrophoresis may prove useful in the characterization of telomerase complexes under various physiological conditions.

The genetics of dyskeratosis congenita

Dyskeratosis congenita (DC) is an inherited bone marrow failure syndrome associated with characteristic mucocutaneous features and a variable series of other somatic abnormalities. The disease is heterogeneous at the genetic and clinical levels. Determination of the genetic basis of DC has established that the disease is caused by a number of genes, all of which encode products involved in telomere maintenance, either as part of telomerase or as part of the shelterin complex that caps and protects telomeres. There is overlap at the genetic and clinical levels with other, more common conditions, including aplastic anemia (AA), pulmonary fibrosis (PF), and liver cirrhosis. Although part of the spectrum of disorders known to be associated with DC, it has emerged that mutations in telomere maintenance genes can lead to the development of AA and PF in the absence of other DC features. Here we discuss the genetics of DC and its relationship to disease presentation.

Essential roles for Pot1b in HSC self-renewal and survival

Maintenance of mammalian telomeres requires both the enzyme telomerase and shelterin, which protect telomeres from inappropriately activating DNA damage response checkpoints. Dyskeratosis congenita is an inherited BM failure syndrome disorder because of defects in telomere maintenance. We have previously shown that deletion of the shelterin component Pot1b in the setting of telomerase haploinsufficiency results in rapid telomere shortening and fatal BM failure in mice, eliciting phenotypes that strongly resemble human syskeratosis congenita. However, it was unclear why BM failure occurred in the setting of Pot1b deletion. In this study, we show that Pot1b plays an essential role in HSC survival. Deletion of Pot1b results in increased apoptosis, leading to severe depletion of the HSC reserve. BM from Pot1b(Δ/Δ) mice cannot compete with BM from wild-type mice to provide multilineage reconstitution, indicating that there is an intrinsic requirement for Pot1b the maintenance of HSC function in vivo. Elimination of the p53-dependent apoptotic function increased HSC survival and significantly extended the lifespan of Pot1b-null mice deficient in telomerase function. Our results document for the first time the essential role of a component of the shelterin complex in the maintenance of HSC and progenitor cell survival.

A role for heterochromatin protein 1γ at human telomeres

Human telomere function is mediated by shelterin, a six-subunit complex that is required for telomere replication, protection, and cohesion. TIN2, the central component of shelterin, has binding sites to three subunits: TRF1, TRF2, and TPP1. Here we identify a fourth partner, heterochromatin protein 1γ (HP1γ), that binds to a conserved canonical HP1-binding motif, PXVXL, in the C-terminal domain of TIN2. We show that HP1γ localizes to telomeres in S phase, where it is required to establish/maintain cohesion. We further demonstrate that the HP1-binding site in TIN2 is required for sister telomere cohesion and can impact telomere length maintenance by telomerase. Remarkably, the PTVML HP1-binding site is embedded in the recently identified cluster of mutations in TIN2 that gives rise to dyskeratosis congenita (DC), an inherited bone marrow failure syndrome caused by defects in telomere maintenance. We show that DC-associated mutations in TIN2 abrogate binding to HP1γ and that DC patient cells are defective in sister telomere cohesion. Our data indicate a novel requirement for HP1γ in the establishment/maintenance of cohesion at human telomeres and, furthermore, may provide insight into the mechanism of pathogenesis in TIN2-mediated DC.

Dyskeratosis congenita as a disorder of telomere maintenance

Since 1998, there have been great advances in our understanding of the pathogenesis of dyskeratosis congenita (DC), a rare inherited bone marrow failure and cancer predisposition syndrome with prominent mucocutaneous abnormalities and features of premature aging. DC is now characterized molecularly by the presence of short age-adjusted telomeres. Mutations in seven genes have been unequivocally associated with DC, each with a role in telomere length maintenance. These observations, combined with knowledge that progressive telomere shortening can impose a proliferative barrier on dividing cells and contribute to chromosome instability, have led to the understanding that extreme telomere shortening drives the clinical features of DC. However, some of the genes implicated in DC encode proteins that are also components of H/ACA-ribonucleoprotein enzymes, which are responsible for the post-translational modification of ribosomal and spliceosomal RNAs, raising the question whether alterations in these activities play a role in the pathogenesis of DC. In addition, recent reports suggest that some cases of DC may not be characterized by short age-adjusted telomeres. This review will highlight our current knowledge of the telomere length defects in DC and the factors involved in its development.