Function of Apollo (SNM1B) at telomere highlighted by a splice variant identified in a patient with Hoyeraal-Hreidarsson syndrome

Roles of chromatin and genome instability in cellular senescence and their relevance to ageing and related diseases

Ageing is a complex biological process in which a gradual decline in physiological fitness increases susceptibility to diseases such as neurodegenerative disorders and cancer. Cellular senescence, a state of irreversible cell-growth arrest accompanied by functional deterioration, has emerged as a pivotal driver of ageing. In this Review, we discuss how heterochromatin loss, telomere attrition and DNA damage contribute to cellular senescence, ageing and age-related diseases by eliciting genome instability, innate immunity and inflammation. We also discuss how emerging therapeutic strategies could restore heterochromatin stability, maintain telomere integrity and boost the DNA repair capacity, and thus counteract cellular senescence and ageing-associated pathologies. Finally, we outline current research challenges and future directions aimed at better comprehending and delaying ageing.

Long-telomeropathy is associated with tumor predisposition syndrome

Telomeres protect chromosomal integrity, and telomere length (TL) is influenced by environmental and genetic factors. While short-telomeres are linked to rare telomeropathies, this study explored the hypothesis that a "long-telomeropathy" is associated with a cancer-predisposing syndrome. Using genomic and health data from 113,861 individuals, a trans-ancestry polygenic risk score for TL (PRS TL ) was developed. A phenome-wide association study (PheWAS) identified 65 tumor traits linked to elevated PRS TL . Using this result, a trans-ancestry phenotype risk score for a long-TL (PheRS LTL ) was develop and validated. Rare variant analyses revealed 13 genes associated with PheRS LTL . Individuals who were carriers of these rare variants had a predisposition for long-TL validating original hypothesis. Most of these genes were new to both cancer and telomere biology. In conclusion, this study identified a novel tumor-predisposing syndrome shaped by both common and rare genetic variants, broadening the understanding of telomeropathies to those with a predisposition for long telomeres.

Inherited Telomere Biology Disorders: Pathophysiology, Clinical Presentation, Diagnostics, and Treatment

Background

Telomeres are the end-capping structures of all eukaryotic chromosomes thereby protecting the genome from damage and degradation. During the aging process, telomeres shorten continuously with each cell division until critically short telomeres prevent further proliferation whereby cells undergo terminal differentiation, senescence, or apoptosis. Premature aging due to critically short telomere length (TL) can also result from pathogenic germline variants in the telomerase complex or related genes that typically counteract replicative telomere shortening in germline and certain somatic cell populations, e.g., hematopoetic stem cells. Inherited diseases that result in altered telomere maintenance are summarized under the term telomere biology disorder (TBD).

Summary

Since TL both reflects but more importantly restricts the replicative capacity of various human tissues, a sufficient telomere reserve is particularly important in cells with high proliferative activity (e.g., hematopoiesis, immune cells, intestinal cells, liver, lung, and skin). Consequently, altered telomere maintenance as observed in TBDs typically results in premature replicative cellular exhaustion in the respective organ systems eventually leading to life-threatening complications such as bone marrow failure (BMF), pulmonary fibrosis, and liver cirrhosis.

Key Messages

The recognition of a potential congenital origin in approximately 10% of adult patients with clinical BMF is of utmost importance for the proper diagnosis, appropriate patient and family counseling, to prevent the use of inefficient treatment and to avoid therapy-related toxicities including appropriate donor selection when patients have to undergo stem cell transplantation from related donors. This review summarizes the current state of knowledge about TBDs with particular focus on the clinical manifestation patterns in children (termed early onset TBD) compared to adults (late-onset TBD) including typical treatment- and disease course-related complications as well as their prognosis and adequate therapy. Thereby, it aims to raise awareness for a disease group that is currently still highly underdiagnosed particularly when it first manifests itself in adulthood.

DNA-PK controls Apollo's access to leading-end telomeres

The complex formed by Ku70/80 and DNA-PKcs (DNA-PK) promotes the synapsis and the joining of double strand breaks (DSBs) during canonical non-homologous end joining (c-NHEJ). In c-NHEJ during V(D)J recombination, DNA-PK promotes the processing of the ends and the opening of the DNA hairpins by recruiting and/or activating the nuclease Artemis/DCLRE1C/SNM1C. Paradoxically, DNA-PK is also required to prevent the fusions of newly replicated leading-end telomeres. Here, we describe the role for DNA-PK in controlling Apollo/DCLRE1B/SNM1B, the nuclease that resects leading-end telomeres. We show that the telomeric function of Apollo requires DNA-PKcs's kinase activity and the binding of Apollo to DNA-PK. Furthermore, AlphaFold-Multimer predicts that Apollo's nuclease domain has extensive additional interactions with DNA-PKcs, and comparison to the cryo-EM structure of Artemis bound to DNA-PK phosphorylated on the ABCDE/Thr2609 cluster suggests that DNA-PK can similarly grant Apollo access to the DNA end. In agreement, the telomeric function of DNA-PK requires the ABCDE/Thr2609 cluster. These data reveal that resection of leading-end telomeres is regulated by DNA-PK through its binding to Apollo and its (auto)phosphorylation-dependent positioning of Apollo at the DNA end, analogous but not identical to DNA-PK dependent regulation of Artemis at hairpins.

DCLRE1B/Apollo germline mutations associated with renal cell carcinoma impair telomere protection

Hereditary renal cell carcinoma (RCC) is caused by germline mutations in a subset of genes, including VHL, MET, FLCN, and FH. However, many familial RCC cases do not harbor mutations in the known predisposition genes. Using Whole Exome Sequencing, we identified two germline missense variants in the DCLRE1B/Apollo gene (ApolloN246I and ApolloY273H) in two unrelated families with several RCC cases. Apollo encodes an exonuclease involved in DNA Damage Response and Repair (DDRR) and telomere integrity. We characterized these two functions in the human renal epithelial cell line HKC8. The decrease or inhibition of Apollo expression sensitizes these cells to DNA interstrand crosslink damage (ICLs). HKC8 Apollo-/- cells appear defective in the DDRR and present an accumulation of telomere damage. Wild-type and mutated Apollo forms could interact with TRF2, a shelterin protein involved in telomere protection. However, only ApolloWT can rescue the telomere damage in HKC8 Apollo-/- cells. Our results strongly suggest that ApolloN246I and ApolloY273H are loss-of-function mutants that cause impaired telomere integrity and could lead to genomic instability. Altogether, our results suggest that mutations in Apollo could induce renal oncogenesis.

DNA-PK and the TRF2 iDDR inhibit MRN-initiated resection at leading-end telomeres

Telomeres replicated by leading-strand synthesis lack the 3' overhang required for telomere protection. Surprisingly, resection of these blunt telomeres is initiated by the telomere-specific 5' exonuclease Apollo rather than the Mre11-Rad50-Nbs1 (MRN) complex, the nuclease that acts at DNA breaks. Without Apollo, leading-end telomeres undergo fusion, which, as demonstrated here, is mediated by alternative end joining. Here, we show that DNA-PK and TRF2 coordinate the repression of MRN at blunt mouse telomeres. DNA-PK represses an MRN-dependent long-range resection, while the endonuclease activity of MRN-CtIP, which could cleave DNA-PK off of blunt telomere ends, is inhibited in vitro and in vivo by the iDDR of TRF2. AlphaFold-Multimer predicts a conserved association of the iDDR with Rad50, potentially interfering with CtIP binding and MRN endonuclease activation. We propose that repression of MRN-mediated resection is a conserved aspect of telomere maintenance and represents an ancient feature of DNA-PK and the iDDR.

Inherited Reticulate Pigmentary Disorders

Reticulate pigmentary disorders (RPDs) are a group of inherited and acquired skin conditions characterized by hyperpigmented and/or hypopigmented macules. Inherited RPDs include dyschromatosis symmetrica hereditaria (DSH), dyschromatosis universalis hereditaria (DUH), reticulate acropigmentation of Kitamura (RAK), Dowling-Degos disease (DDD), dyskeratosis congenita (DKC), Naegeli-Franceschetti-Jadassohn syndrome (NFJS), dermatopathia pigmentosa reticularis (DPR), and X-linked reticulate pigmentary disorder. Although reticulate pattern of pigmentation is a common characteristic of this spectrum of disorders, the distribution of pigmentation varies among these disorders, and there may be clinical manifestations beyond pigmentation. DSH, DUH, and RAK are mostly reported in East Asian ethnicities. DDD is more common in Caucasians, although it is also reported in Asian countries. Other RPDs show no racial predilection. This article reviews the clinical, histological, and genetic variations of inherited RPDs.

Genetic regulation of RNA splicing in human pancreatic islets

BACKGROUND

Non-coding genetic variants that influence gene transcription in pancreatic islets play a major role in the susceptibility to type 2 diabetes (T2D), and likely also contribute to type 1 diabetes (T1D) risk. For many loci, however, the mechanisms through which non-coding variants influence diabetes susceptibility are unknown.

RESULTS

We examine splicing QTLs (sQTLs) in pancreatic islets from 399 human donors and observe that common genetic variation has a widespread influence on the splicing of genes with established roles in islet biology and diabetes. In parallel, we profile expression QTLs (eQTLs) and use transcriptome-wide association as well as genetic co-localization studies to assign islet sQTLs or eQTLs to T2D and T1D susceptibility signals, many of which lack candidate effector genes. This analysis reveals biologically plausible mechanisms, including the association of T2D with an sQTL that creates a nonsense isoform in ERO1B, a regulator of ER-stress and proinsulin biosynthesis. The expanded list of T2D risk effector genes reveals overrepresented pathways, including regulators of G-protein-mediated cAMP production. The analysis of sQTLs also reveals candidate effector genes for T1D susceptibility such as DCLRE1B, a senescence regulator, and lncRNA MEG3.

CONCLUSIONS

These data expose widespread effects of common genetic variants on RNA splicing in pancreatic islets. The results support a role for splicing variation in diabetes susceptibility, and offer a new set of genetic targets with potential therapeutic benefit.

Inherited human Apollo deficiency causes severe bone marrow failure and developmental defects

Inherited bone marrow failure syndromes (IBMFSs) are a group of disorders typified by impaired production of 1 or several blood cell types. The telomere biology disorders dyskeratosis congenita (DC) and its severe variant, Høyeraal-Hreidarsson (HH) syndrome, are rare IBMFSs characterized by bone marrow failure, developmental defects, and various premature aging complications associated with critically short telomeres. We identified biallelic variants in the gene encoding the 5'-to-3' DNA exonuclease Apollo/SNM1B in 3 unrelated patients presenting with a DC/HH phenotype consisting of early-onset hypocellular bone marrow failure, B and NK lymphopenia, developmental anomalies, microcephaly, and/or intrauterine growth retardation. All 3 patients carry a homozygous or compound heterozygous (in combination with a null allele) missense variant affecting the same residue L142 (L142F or L142S) located in the catalytic domain of Apollo. Apollo-deficient cells from patients exhibited spontaneous chromosome instability and impaired DNA repair that was complemented by CRISPR/Cas9-mediated gene correction. Furthermore, patients' cells showed signs of telomere fragility that were not associated with global reduction of telomere length. Unlike patients' cells, human Apollo KO HT1080 cell lines showed strong telomere dysfunction accompanied by excessive telomere shortening, suggesting that the L142S and L142F Apollo variants are hypomorphic. Collectively, these findings define human Apollo as a genome caretaker and identify biallelic Apollo variants as a genetic cause of a hitherto unrecognized severe IBMFS that combines clinical hallmarks of DC/HH with normal telomere length.

Genetic determinants of telomere length from 109,122 ancestrally diverse whole-genome sequences in TOPMed

Genetic studies on telomere length are important for understanding age-related diseases. Prior GWAS for leukocyte TL have been limited to European and Asian populations. Here, we report the first sequencing-based association study for TL across ancestrally-diverse individuals (European, African, Asian and Hispanic/Latino) from the NHLBI Trans-Omics for Precision Medicine (TOPMed) program. We used whole genome sequencing (WGS) of whole blood for variant genotype calling and the bioinformatic estimation of telomere length in n=109,122 individuals. We identified 59 sentinel variants (p-value <5×10-9) in 36 loci associated with telomere length, including 20 newly associated loci (13 were replicated in external datasets). There was little evidence of effect size heterogeneity across populations. Fine-mapping at OBFC1 indicated the independent signals colocalized with cell-type specific eQTLs for OBFC1 (STN1). Using a multi-variant gene-based approach, we identified two genes newly implicated in telomere length, DCLRE1B (SNM1B) and PARN. In PheWAS, we demonstrated our TL polygenic trait scores (PTS) were associated with increased risk of cancer-related phenotypes.

Structure and Function of SNM1 Family Nucleases

Three human nucleases, SNM1A, SNM1B/Apollo, and SNM1C/Artemis, belong to the SNM1 gene family. These nucleases are involved in various cellular functions, including homologous recombination, nonhomologous end-joining, cell cycle regulation, and telomere maintenance. These three proteins share a similar catalytic domain, which is characterized as a fused metallo-β-lactamase and a CPSF-Artemis-SNM1-PSO2 domain. SNM1A and SNM1B/Apollo are exonucleases, whereas SNM1C/Artemis is an endonuclease. This review contains a summary of recent research on SNM1's cellular and biochemical functions, as well as structural biology studies. In addition, protein structure prediction by the artificial intelligence program AlphaFold provides a different view of the proteins' non-catalytic domain features, which may be used in combination with current results from X-ray crystallography and cryo-EM to understand their mechanism more clearly.

UFMylation of MRE11 is essential for telomere length maintenance and hematopoietic stem cell survival

Ubiquitin-fold modifier 1 (UFM1) is involved in neural and erythroid development, yet its biological roles in these processes are unknown. Here, we generated zebrafish models deficient in Ufm1 and Ufl1 that exhibited telomere shortening associated with developmental delay, impaired hematopoiesis and premature aging. We further report that HeLa cells lacking UFL1 have instability of telomeres replicated by leading-strand synthesis. We uncover that MRE11 UFMylation is necessary for the recruitment of the phosphatase PP1-α leading to dephosphorylation of NBS1. In the absence of UFMylation, NBS1 remains phosphorylated, thereby reducing MRN recruitment to telomeres. The absence of MRN at telomeres favors the formation of the TRF2-Apollo/SNM1 complex consistent with the loss of leading telomeres. These results suggest that MRE11-UFMylation may serve as module to recruit PP1-α. Last, zebrafish expressing Mre11 that cannot be UFMylated phenocopy Ufm1-deficient zebrafish, demonstrating that UFMylation of MRE11 is a previously undescribed evolutionarily conserved mechanisms regulating telomere length.

A phosphate binding pocket is a key determinant of exo- versus endo-nucleolytic activity in the SNM1 nuclease family

The SNM1 nucleases which help maintain genome integrity are members of the metallo-β-lactamase (MBL) structural superfamily. Their conserved MBL-β-CASP-fold SNM1 core provides a molecular scaffold forming an active site which coordinates the metal ions required for catalysis. The features that determine SNM1 endo- versus exonuclease activity, and which control substrate selectivity and binding are poorly understood. We describe a structure of SNM1B/Apollo with two nucleotides bound to its active site, resembling the product state of its exonuclease reaction. The structure enables definition of key SNM1B residues that form contacts with DNA and identifies a 5' phosphate binding pocket, which we demonstrate is important in catalysis and which has a key role in determining endo- versus exonucleolytic activity across the SNM1 family. We probed the capacity of SNM1B to digest past sites of common endogenous DNA lesions and find that base modifications planar to the nucleobase can be accommodated due to the open architecture of the active site, but lesions axial to the plane of the nucleobase are not well tolerated due to constriction around the altered base. We propose that SNM1B/Apollo might employ its activity to help remove common oxidative lesions from telomeres.

The Power of Stress: The Telo-Hormesis Hypothesis

Adaptative response to stress is a strategy conserved across evolution to promote survival. In this context, the groundbreaking findings of Miroslav Radman on the adaptative value of changing mutation rates opened new avenues in our understanding of stress response. Inspired by this work, we explore here the putative beneficial effects of changing the ends of eukaryotic chromosomes, the telomeres, in response to stress. We first summarize basic principles in telomere biology and then describe how various types of stress can alter telomere structure and functions. Finally, we discuss the hypothesis of stress-induced telomere signaling with hormetic effects.

POT1-TPP1 telomere length regulation and disease

Telomeres are DNA repeats at the ends of linear chromosomes and are replicated by telomerase, a ribonucleoprotein reverse transcriptase. Telomere length regulation and chromosome end capping are essential for genome stability and are mediated primarily by the shelterin and CST complexes. POT1-TPP1, a subunit of shelterin, binds the telomeric overhang, suppresses ATR-dependent DNA damage response, and recruits telomerase to telomeres for DNA replication. POT1 localization to telomeres and chromosome end protection requires its interaction with TPP1. Therefore, the POT1-TPP1 complex is critical to telomere maintenance and full telomerase processivity. The aim of this mini-review is to summarize recent POT1-TPP1 structural studies and discuss how the complex contributes to telomere length regulation. In addition, we review how disruption of POT1-TPP1 function leads to human disease.

Defects in t6A tRNA modification due to GON7 and YRDC mutations lead to Galloway-Mowat syndrome

N⁶-threonyl-carbamoylation of adenosine 37 of ANN-type tRNAs (t⁶A) is a universal modification essential for translational accuracy and efficiency. The t⁶A pathway uses two sequentially acting enzymes, YRDC and OSGEP, the latter being a subunit of the multiprotein KEOPS complex. We recently identified mutations in genes encoding four out of the five KEOPS subunits in children with Galloway-Mowat syndrome (GAMOS), a clinically heterogeneous autosomal recessive disease characterized by early-onset steroid-resistant nephrotic syndrome and microcephaly. Here we show that mutations in YRDC cause an extremely severe form of GAMOS whereas mutations in GON7, encoding the fifth KEOPS subunit, lead to a milder form of the disease. The crystal structure of the GON7/LAGE3/OSGEP subcomplex shows that the intrinsically disordered GON7 protein becomes partially structured upon binding to LAGE3. The structure and cellular characterization of GON7 suggest its involvement in the cellular stability and quaternary arrangement of the KEOPS complex.

The biosynthesis of N⁶-threonylcarbamoylated adenosine 37 in tRNA (t⁶A) involves the YRDC enzyme and the KEOPS complex. Here, the authors report mutations in YRDC and the KEOPS component GON7 in Galloway-Mowat syndrome and determine the crystal structure of a GON7-containg subcomplex that suggests a role in KEOPS complex stability.

Telomeres and telomerase: three decades of progress

Many recent advances have emerged in the telomere and telomerase fields. This Timeline article highlights the key advances that have expanded our views on the mechanistic underpinnings of telomeres and telomerase and their roles in ageing and disease. Three decades ago, the classic view was that telomeres protected the natural ends of linear chromosomes and that telomerase was a specific telomere-terminal transferase necessary for the replication of chromosome ends in single-celled organisms. While this concept is still correct, many diverse fields associated with telomeres and telomerase have substantially matured. These areas include the discovery of most of the key molecular components of telomerase, implications for limits to cellular replication, identification and characterization of human genetic disorders that result in premature telomere shortening, the concept that inhibiting telomerase might be a successful therapeutic strategy and roles for telomeres in regulating gene expression. We discuss progress in these areas and conclude with challenges and unanswered questions in the field.

SNM1B/Apollo in the DNA damage response and telomere maintenance

hSNM1B/Apollo is a member of the highly conserved β-CASP subgroup within the MBL superfamily of proteins. It interacts with several DNA repair proteins and functions within the Fanconi anemia pathway in response to DNA interstrand crosslinks. As a shelterin accessory protein, hSNM1B/Apollo is also vital for the generation and maintenance of telomeric overhangs. In this review, we will summarize studies on hSNM1B/Apollo's function, including its contribution to DNA damage signaling, replication fork maintenance, control of topological stress and telomere protection. Furthermore, we will highlight recent studies illustrating hSNM1B/Apollo's putative role in human disease.

Heterozygous RTEL1 variants in bone marrow failure and myeloid neoplasms

Biallelic germline mutations in RTEL1 (regulator of telomere elongation helicase 1) result in pathologic telomere erosion and cause dyskeratosis congenita. However, the role of RTEL1 mutations in other bone marrow failure (BMF) syndromes and myeloid neoplasms, and the contribution of monoallelic RTEL1 mutations to disease development are not well defined. We screened 516 patients for germline mutations in telomere-associated genes by next-generation sequencing in 2 independent cohorts; one constituting unselected patients with idiopathic BMF, unexplained cytopenia, or myeloid neoplasms (n = 457) and a second cohort comprising selected patients on the basis of the suspicion of constitutional/familial BMF (n = 59). Twenty-three RTEL1 variants were identified in 27 unrelated patients from both cohorts: 7 variants were likely pathogenic, 13 were of uncertain significance, and 3 were likely benign. Likely pathogenic RTEL1 variants were identified in 9 unrelated patients (7 heterozygous and 2 biallelic). Most patients were suspected to have constitutional BMF, which included aplastic anemia (AA), unexplained cytopenia, hypoplastic myelodysplastic syndrome, and macrocytosis with hypocellular bone marrow. In the other 18 patients, RTEL1 variants were likely benign or of uncertain significance. Telomeres were short in 21 patients (78%), and 3' telomeric overhangs were significantly eroded in 4. In summary, heterozygous RTEL1 variants were associated with marrow failure, and telomere length measurement alone may not identify patients with telomere dysfunction carrying RTEL1 variants. Pathogenicity assessment of heterozygous RTEL1 variants relied on a combination of clinical, computational, and functional data required to avoid misinterpretation of common variants.

The DNA damage response at dysfunctional telomeres, and at interstitial and subtelomeric DNA double-strand breaks

In mammals, DNA double-strand breaks (DSBs) are primarily repaired by classical non-homologous end joining (C-NHEJ), although homologous recombination repair and alternative NHEJ (A-NHEJ), which involve DSB processing, can also occur. These pathways are tightly regulated to maintain chromosome integrity. The ends of chromosomes, called telomeres, contain telomeric DNA that forms a cap structure in cooperation with telomeric proteins to prevent the activation of the DNA damage response and chromosome fusion at chromosome termini. Telomeres and subtelomeric regions are poor substrates for DNA replication; therefore, regions near telomeres are prone to replication fork stalling and chromosome breakage. Moreover, DSBs near telomeres are poorly repaired. As a result, when DSBs occur near telomeres in normal cells, the cells stop proliferating, while in cancer cells, subtelomeric DSBs induce rearrangements due to the absence of cell cycle checkpoints. The sensitivity of subtelomeric regions to DSBs is due to the improper regulation of processing, because although C-NHEJ is functional at subtelomeric DSBs, excessive processing results in an increased frequency of large deletions and chromosome rearrangements involving A-NHEJ.

Mutations in KEOPS-complex genes cause nephrotic syndrome with primary microcephaly

Galloway-Mowat syndrome (GAMOS) is an autosomal-recessive disease characterized by the combination of early-onset nephrotic syndrome (SRNS) and microcephaly with brain anomalies. Here we identified recessive mutations in OSGEP, TP53RK, TPRKB, and LAGE3, genes encoding the four subunits of the KEOPS complex, in 37 individuals from 32 families with GAMOS. CRISPR-Cas9 knockout in zebrafish and mice recapitulated the human phenotype of primary microcephaly and resulted in early lethality. Knockdown of OSGEP, TP53RK, or TPRKB inhibited cell proliferation, which human mutations did not rescue. Furthermore, knockdown of these genes impaired protein translation, caused endoplasmic reticulum stress, activated DNA-damage-response signaling, and ultimately induced apoptosis. Knockdown of OSGEP or TP53RK induced defects in the actin cytoskeleton and decreased the migration rate of human podocytes, an established intermediate phenotype of SRNS. We thus identified four new monogenic causes of GAMOS, describe a link between KEOPS function and human disease, and delineate potential pathogenic mechanisms.

DNA Replication Origins and Fork Progression at Mammalian Telomeres

Telomeres are essential chromosomal regions that prevent critical shortening of linear chromosomes and genomic instability in eukaryotic cells. The bulk of telomeric DNA is replicated by semi-conservative DNA replication in the same way as the rest of the genome. However, recent findings revealed that replication of telomeric repeats is a potential cause of chromosomal instability, because DNA replication through telomeres is challenged by the repetitive telomeric sequences and specific structures that hamper the replication fork. In this review, we summarize current understanding of the mechanisms by which telomeres are faithfully and safely replicated in mammalian cells. Various telomere-associated proteins ensure efficient telomere replication at different steps, such as licensing of replication origins, passage of replication forks, proper fork restart after replication stress, and dissolution of post-replicative structures. In particular, shelterin proteins have central roles in the control of telomere replication. Through physical interactions, accessory proteins are recruited to maintain telomere integrity during DNA replication. Dormant replication origins and/or homology-directed repair may rescue inappropriate fork stalling or collapse that can cause defects in telomere structure and functions.

A nonsense mutation in the DNA repair factor Hebo causes mild bone marrow failure and microcephaly

de Villartay et al. describe a patient with a DNA repair factor mutation that leads to an increased sensitivity to DNA-damaging agents and, ultimately, to mild bone marrow failure and microcephaly.

Inherited bone marrow failure syndromes are human conditions in which one or several cell lineages of the hemopoietic system are affected. They are present at birth or may develop progressively. They are sometimes accompanied by other developmental anomalies. Three main molecular causes have been recognized to result in bone marrow failure syndromes: (1) defects in the Fanconi anemia (FA)/BRCA DNA repair pathway, (2) defects in telomere maintenance, and (3) abnormal ribosome biogenesis. We analyzed a patient with mild bone marrow failure and microcephaly who did not present with the typical FA phenotype. Cells from this patient showed increased sensitivity to ionizing radiations and phleomycin, attesting to a probable DNA double strand break (dsb) repair defect. Linkage analysis and whole exome sequencing revealed a homozygous nonsense mutation in the ERCC6L2 gene. We identified a new ERCC6L2 alternative transcript encoding the DNA repair factor Hebo, which is critical for complementation of the patient’s DNAdsb repair defect. Sequence analysis revealed three structured regions within Hebo: a TUDOR domain, an adenosine triphosphatase domain, and a new domain, HEBO, specifically present in Hebo direct orthologues. Hebo is ubiquitously expressed, localized in the nucleus, and rapidly recruited to DNAdsb’s in an NBS1-dependent manner.

Complex interactions between the DNA-damage response and mammalian telomeres

Natural chromosome ends resemble double-stranded DNA breaks, but they do not activate a damage response in healthy cells. Telomeres therefore have evolved to solve the 'end-protection problem' by inhibiting multiple DNA damage-response pathways. During the past decade, the view of telomeres has progressed from simple caps that hide chromosome ends to complex machineries that have an active role in organizing the genome. Here we focus on mammalian telomeres and summarize and interpret recent discoveries in detail, focusing on how repair pathways are inhibited, how resection and replication are controlled and how these mechanisms govern cell fate during senescence, crisis and transformation.

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.

The Chemical Biology of Human Metallo-β-Lactamase Fold Proteins

The αββα metallo β-lactamase (MBL) fold (MBLf) was first observed in bacterial enzymes that catalyze the hydrolysis of almost all β-lactam antibiotics, but is now known to be widely distributed. The MBL core protein fold is present in human enzymes with diverse biological roles, including cell detoxification pathways and enabling resistance to clinically important anticancer medicines. Human (h)MBLf enzymes can bind metals, including zinc and iron ions, and catalyze a range of chemically interesting reactions, including both redox (e.g., ETHE1) and hydrolytic processes (e.g., Glyoxalase II, SNM1 nucleases, and CPSF73). With a view to promoting basic research on MBLf enzymes and their medicinal targeting, here we summarize current knowledge of the mechanisms and roles of these important molecules.

MBLs are mono- or di-zinc ion-dependent hydrolases that enable bacterial resistance to almost all β-lactam antibiotics.

The αββα MBL core fold is widely distributed and supports a range of catalytic activities, including redox reactions.

hMBL proteins are a small family of approximately 18 zinc- and iron-dependent proteins with roles in metabolism and/or detoxification and nucleic acid modification.

In a notable parallel with the role of bacterial MBLs in antibiotic resistance, some hMBLf enzymes enable resistance to chemotherapy drugs, such as cisplatin and mitomycin C.

The structures of the SNM1A and SNM1B/Apollo nuclease domains reveal a potential basis for their distinct DNA processing activities

The human SNM1A and SNM1B/Apollo proteins are members of an extended family of eukaryotic nuclease containing a motif related to the prokaryotic metallo-β-lactamase (MBL) fold. SNM1A is a key exonuclease during replication-dependent and transcription-coupled interstrand crosslink repair, while SNM1B/Apollo is required for maintaining telomeric overhangs. Here, we report the crystal structures of SNM1A and SNM1B at 2.16 Å. While both proteins contain a typical MBL-β-CASP domain, a region of positive charge surrounds the active site of SNM1A, which is absent in SNM1B and explains the greater apparent processivity of SNM1A. The structures of both proteins also reveal a putative, wide DNA-binding groove. Extensive mutagenesis of this groove, coupled with detailed biochemical analysis, identified residues that did not impact on SNM1A catalytic activity, but drastically reduced its processivity. Moreover, we identified a key role for this groove for efficient digestion past DNA interstrand crosslinks, facilitating the key DNA repair reaction catalysed by SNM1A. Together, the architecture and dimensions of this groove, coupled to the surrounding region of high positive charge, explain the remarkable ability of SNM1A to accommodate and efficiently digest highly distorted DNA substrates, such as those containing DNA lesions.

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.

Molecular mechanisms of ageing and related diseases

Human and other multicellular life species age, and ageing processes become dominant during the late phase of life. Recent studies challenge this dogma, suggesting that ageing does not occur in some animal species. In mammals, cell replicative senescence occurs as early as before birth (i.e. in embryos) under physiological conditions. How the molecular machinery operates and why ageing cells dominate under some circumstances are intriguing questions. Recent studies show that cell ageing involves extensive cellular remodelling, including telomere attrition, heterochromatin formation, endoplasmic reticulum stress, mitochondrial disorders and lysosome processing organelles and chromatins. This article provides an update on the molecular mechanisms underlying the ageing of various cell types, the newly described developmental and programmed replicative senescence and the critical roles of cellular organelles and effectors in Parkinson's disease, diabetes, hypertension and dyskeratosis congenita.

TRF2 and lamin A/C interact to facilitate the functional organization of chromosome ends

Telomeres protect the ends of linear genomes, and the gradual loss of telomeres is associated with cellular ageing. Telomere protection involves the insertion of the 3' overhang facilitated by telomere repeat-binding factor 2 (TRF2) into telomeric DNA, forming t-loops. We present evidence suggesting that t-loops can also form at interstitial telomeric sequences in a TRF2-dependent manner, forming an interstitial t-loop (ITL). We demonstrate that TRF2 association with interstitial telomeric sequences is stabilized by co-localization with A-type lamins (lamin A/C). We also find that lamin A/C interacts with TRF2 and that reduction in levels of lamin A/C or mutations in LMNA that cause an autosomal dominant premature ageing disorder--Hutchinson Gilford Progeria Syndrome (HGPS)-lead to reduced ITL formation and telomere loss. We propose that cellular and organismal ageing are intertwined through the effects of the interaction between TRF2 and lamin A/C on chromosome structure.

Cell biology of disease: Telomeropathies: an emerging spectrum disorder

A constellation of related genetic diseases are caused by defects in the telomere maintenance machinery. These disorders, often referred to as telomeropathies, share symptoms and molecular mechanisms, and mounting evidence indicates they are points along a spectrum of disease. Several new causes of these disorders have been recently discovered, and a number of related syndromes may be unrecognized telomeropathies. Progress in the clinical understanding of telomeropathies has in turn driven progress in the basic science of telomere biology. In addition, the pattern of genetic anticipation in some telomeropathies generates thought-provoking questions about the way telomere length impacts the course of these diseases.

Hoyeraal-Hreidarsson syndrome with a DKC1 mutation identified by whole-exome sequencing

BACKGROUND

Hoyeraal-Hreidarsson syndrome is a severe multisystem disorder that is characterized by bone-marrow failure, intrauterine growth retardation, microcephaly, immunodeficiency, and cerebellar atrophy. This rare disease shares clinical features with dyskeratosis congenita and, together, they are recognized as a group of disorders caused by telomere dysfunction. As the genetic background of dyskeratosis congenita or Hoyeraal-Hreidarsson syndrome has expanded rapidly, multiple causative genes and inheritance patterns pose a great challenge to their genetic diagnosis.

CASE PRESENTATION

A 3-month-old boy was referred for head titubation and tremulous movements of the trunk. Multiple petechiae also developed on his face and trunk at the age of 5 months. Extensive evaluation, including brain magnetic resonance imaging, hematologic tests, and bone-marrow evaluation, revealed cerebellar atrophy and aplastic anemia. His elder brother exhibited a similar clinical presentation and died from sepsis after hematopoietic stem cell transplantation. Although skin pigmentation or nail dystrophy was not evident, Hoyeraal-Hreidarsson syndrome was suggested as a differential diagnosis. Instead of the conventional gene-specific approach with Sanger sequencing, we used whole-exome sequencing for the genetic diagnosis of this patient with possible Hoyeraal-Hreidarsson syndrome and successfully identified a missense mutation (c.146C>T, p.Thr49Me) in DKC1.

CONCLUSION

This case suggests that whole-exome sequencing is particularly useful for the genetic diagnosis of extremely rare diseases with genetic heterogeneity, although there are many limitations, including cost and uneven or suboptimal coverage, to the application of this method as a routine genetic diagnosis.

Modern management of primary T-cell immunodeficiencies

The study of human T-cell PIDs with Mendelian inheritance has enabled the molecular characterization of important key functions and pathways in T-cell biology. In most cases, T-cell PIDs become apparent as combined T- and B-cell deficiencies. Severe combined immunodeficiencies (SCIDs) are characterized by a complete lack of T-cell development and, in some cases, a developmental block in other lymphoid lineages and manifest within the first year of life. Combined immunodeficiency syndromes (CIDs) result from hypomorphic mutations in typical SCID associated genes or from partial defects of T-cell development and manifest later in childhood by increased susceptibility to infection often combined with disturbances in immune homeostasis, e.g., autoimmunity and increased incidence in lymphoproliferation. The discovery of mutations and characterization of the cellular changes that underlie lymphocyte defects and immune dysregulation have led to novel, specific, successful therapies for severe diseases which are often fatal if left untreated. Over the last few years, impressive progress has been made in understanding the disease mechanisms of T-cell immunodeficiencies and in improving the long-term outcomes of potentially curative treatments, including gene therapy.

Isolation of chromatin from dysfunctional telomeres reveals an important role for Ring1b in NHEJ-mediated chromosome fusions

When telomeres become critically short, DNA damage response factors are recruited at chromosome ends, initiating a cellular response to DNA damage. We performed proteomic isolation of chromatin fragments (PICh) in order to define changes in chromatin composition that occur upon onset of acute telomere dysfunction triggered by depletion of the telomere-associated factor TRF2. This unbiased purification of telomere-associated proteins in functional or dysfunctional conditions revealed the dynamic changes in chromatin composition that take place at telomeres upon DNA damage induction. On the basis of our results, we describe a critical role for the polycomb group protein Ring1b in nonhomologous end-joining (NHEJ)-mediated end-to-end chromosome fusions. We show that cells with reduced levels of Ring1b have a reduced ability to repair uncapped telomeric chromatin. Our data represent an unbiased isolation of chromatin undergoing DNA damage and are a valuable resource to map the changes in chromatin composition in response to DNA damage activation.

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.

Human telomeres and telomere biology disorders

Telomeres consist of long nucleotide repeats and a protein complex at chromosome ends essential for chromosome stability. Telomeres shorten with each cell division and thus are markers of cellular age. Dyskeratosis congenita (DC) is a cancer-prone inherited bone marrow failure syndrome caused by germ-line mutations in key telomere biology genes that result in extremely short telomeres. The triad of nail dysplasia, abnormal skin pigmentation, and oral leukoplakia is diagnostic of DC but highly variable. Patients with DC may also have but numerous other medical problems, including pulmonary fibrosis, liver abnormalities, avascular necrosis of the hips, and stenosis of the esophagus, lacrimal ducts, and/or urethra. All modes of inheritance have been reported in DC and de novo mutations are common. Broad phenotypic heterogeneity occurs within DC. Clinically severe variants of DC are Hoyeraal-Hreidarsson syndrome and Revesz syndrome. Coats plus syndrome joined the spectrum of DC with the discovery that it is caused by mutations in a telomere-capping gene. Less clinically severe variants, such as subsets of apparently isolated aplastic anemia or pulmonary fibrosis, have also been recognized. These patients may not have the DC-associated mucocutaneous triad or complicated medical features, but they do have the same underlying genetic etiology. This has led to the use of the descriptive term telomere biology disorder (TBD). This chapter will review the connection between telomere biology and human disease through the examples of DC and its related TBDs.

Telomere dysfunction and hematologic disorders

Aplastic anemia is a disease in which the hematopoietic stem cell fails to adequately produce peripheral blood cells, causing pancytopenia. In some cases of acquired aplastic anemia and in inherited type of aplastic anemia, dyskeratosis congenita, telomere biology gene mutations and telomere shortening are etiologic. Telomere erosion hampers the ability of hematopoietic stem and progenitor cells to adequately replicate, clinically resulting in bone marrow failure. Additionally, telomerase mutations and short telomeres are genetic risk factors for the development of some hematologic cancers, including myelodysplastic syndrome, acute myeloid leukemia, and chronic lymphocytic leukemia.

Updates on the biology and management of dyskeratosis congenita and related telomere biology disorders

Dyskeratosis congenita (DC) is a cancer-prone inherited bone marrow failure syndrome caused by aberrant telomere biology. The mucocutaneous triad of nail dysplasia, abnormal skin pigmentation and oral leukoplakia is diagnostic, but is not always present; DC can also be diagnosed by the presence of very short leukocyte telomeres. Patients with DC are at high risk of bone marrow failure, pulmonary fibrosis, liver disease, cancer and other medical problems. Germline mutations in one of nine genes associated with telomere maintenance are present in approximately 60% of patients. DC is one among the group of clinically and biologically related telomere biology disorders, including Hoyeraal-Hreidarsson syndrome, Revesz syndrome, Coats plus (also known as cranioretinal microangiopathy with calcifications and cysts) and subsets of aplastic anemia, pulmonary fibrosis, nonalcoholic and noninfectious liver disease and leukemia. The authors review the pathobiology that connects DC and the related telomere biology disorders, methods of diagnosis and management modalities.

Comprehensive repertoire of foldable regions within whole genomes

In order to get a comprehensive repertoire of foldable domains within whole proteomes, including orphan domains, we developed a novel procedure, called SEG-HCA. From only the information of a single amino acid sequence, SEG-HCA automatically delineates segments possessing high densities in hydrophobic clusters, as defined by Hydrophobic Cluster Analysis (HCA). These hydrophobic clusters mainly correspond to regular secondary structures, which together form structured or foldable regions. Genome-wide analyses revealed that SEG-HCA is opposite of disorder predictors, both addressing distinct structural states. Interestingly, there is however an overlap between the two predictions, including small segments of disordered sequences, which undergo coupled folding and binding. SEG-HCA thus gives access to these specific domains, which are generally poorly represented in domain databases. Comparison of the whole set of SEG-HCA predictions with the Conserved Domain Database (CDD) also highlighted a wide proportion of predicted large (length >50 amino acids) segments, which are CDD orphan. These orphan sequences may either correspond to highly divergent members of already known families or belong to new families of domains. Their comprehensive description thus opens new avenues to investigate new functional and/or structural features, which remained so far uncovered. Altogether, the data described here provide new insights into the protein architecture and organization throughout the three kingdoms of life.

Spontaneous or induced folding into a specific 3D structure is a key property of proteins to perform their biological functions. Folded 3D structures of proteins perform specific functions, including interactions with other proteins. Intrinsically disordered regions also mediate interaction, gaining structure only when bound to a target protein. In both cases, hydrophobicity generally plays a major role in the protein segment “foldability”. Here, we developed an original procedure to identify foldable segments from only the information of a single amino acid sequence and to explore protein structures at a proteomic scale. Our approach goes beyond the simple consideration of mean hydrophobicity, by including the secondary structure information through the use of a two-dimensional transposition of the sequence. The developed procedure, combined with disorder predictors, may facilitate the specific identification of small segments that undergo coupled folding and binding. Combined with the analysis of specific domain databases, it also highlights orphan foldable segments, which remain yet uncharacterized.

Genomic characterization of the inherited bone marrow failure syndromes

The inherited bone marrow failure syndromes (IBMFS) are a set of clinically related yet heterogeneous disorders in which at least one hematopoietic cell lineage is significantly reduced. Many of the IBMFS have notably increased cancer risks, as well as other physical findings. Highly penetrant germline mutations in key pathways, such as DNA repair, telomere biology, or ribosomal biogenesis, are causative of Fanconi anemia (FA), dyskeratosis congenita (DC), and Diamond-Blackfan anemia (DBA), respectively. Next-generation sequencing (NGS) generally refers to high-throughput, large-scale sequencing technologies and is being used more frequently to understand disease etiology. In the IBMFS, NGS has facilitated the discovery of germline mutations that cause thrombocytopenia absent radii syndrome (TAR), a subset of DC and DBA, and other uncharacterized, but related, disorders. Panels of large numbers of genes are being used to molecularly characterize patients with IBMFS, such as FA and DBA. NGS is also accelerating the discovery of the genetic etiology of previously unclassified IBMFS. In this review, we will highlight recent studies that have employed NGS to ascertain the genetic etiology of IBMFS, namely, FA, DC, DBA, and TAR, and discuss the translational utility of these findings.

New features on Pso2 protein family in DNA interstrand cross-link repair and in the maintenance of genomic integrity in Saccharomyces cerevisiae

Pso2 protein, a member of the highly conserved metallo-β-lactamase (MBL) super family of nucleases, plays a central role in interstrand crosslink repair (ICL) in yeast. Pso2 protein is the founder member of a distinct group within the MBL superfamily, called β-CASP family. Three mammalian orthologs of this protein that act on DNA were identified: SNM1A, SNM1B/Apollo and SNM1C/Artemis. Yeast Pso2 and all three mammalian orthologs proteins have been shown to possess nuclease activity. Besides Pso2, ICL repair involves proteins of several DNA repair pathways. Over the last years, new homologs for human proteins have been identified in yeast. In this review, we will focus on studies clarifying the function of Pso2 protein during ICL repair in yeast, emphasizing the contribution of Brazilian research groups in this topic. New sub-pathways in the mechanisms of ICL repair, such as recently identified conserved Fanconi Anemia pathway in yeast as well as a contribution of non-homologous end joining are discussed.

How will telomeric complex be further contributed to our longevity? - the potential novel biomarkers of telomere complex counteracting both aging and cancer

With the smooth move towards the coming expected clinical reports of anticancer pharmaceutical molecules targeting telomeres and telomerase, and also with the exciting success in the extension of lifespan by regulating telomerase activity without increased onset of oncogenesis in laboratory mouse models (Garcia-Cao et al., 2006; Jaskelioff et al., 2011), we are convinced that targeting telomeres based on telomerase will be a potential approach to conquer both aging and cancer and the idea of longevity seems to be no more mysterious. More interestingly, emerging evidences from clinical research reveal that other telomeric factors, like specific telomeric binding proteins and nonspecific telomere associated proteins also show crucial importance in aging and oncogenesis. This stems from their roles in the stability of telomere structure and in the inhibition of DNA damage response at telomeres. Uncapping these proteins from chromosome ends leads to dramatic telomere loss and telomere dysfunction which is more abrupt than those induced by telomerase inactivation. Abnormal expression of these factors results in developmental failure, aging and even oncogenesis evidenced by several experimental models and clinical cases, indicating telomere specific proteins and its associated proteins have complimentary roles to telomerase in telomere protection and controlling cellular fate. Thus, these telomeric factors might be potential clinical biomarkers for early detection or even therapeutic targets of aging and cancer. Future studies to elucidate how these proteins function in telomere protection might benefit patients suffering aging or cancer who are not sensitive to telomerase mediation.

Human RTEL1 deficiency causes Hoyeraal-Hreidarsson syndrome with short telomeres and genome instability

Hoyeraal-Hreidarsson syndrome (HHS), a severe variant of dyskeratosis congenita (DC), is characterized by early onset bone marrow failure, immunodeficiency and developmental defects. Several factors involved in telomere length maintenance and/or protection are defective in HHS/DC, underlining the relationship between telomere dysfunction and these diseases. By combining whole-genome linkage analysis and exome sequencing, we identified compound heterozygous RTEL1 (regulator of telomere elongation helicase 1) mutations in three patients with HHS from two unrelated families. RTEL1 is a DNA helicase that participates in DNA replication, DNA repair and telomere integrity. We show that, in addition to short telomeres, RTEL1-deficient cells from patients exhibit hallmarks of genome instability, including spontaneous DNA damage, anaphase bridges and telomeric aberrations. Collectively, these results identify RTEL1 as a novel HHS-causing gene and highlight its role as a genomic caretaker in humans.

The gastrointestinal manifestations of telomere-mediated disease

Defects in telomere maintenance genes cause pathological telomere shortening, and manifest in syndromes which have prominent phenotypes in tissues of high turnover: the skin and bone marrow. Because the gastrointestinal (GI) epithelium is highly proliferative, we sought to determine whether telomere syndromes cause GI disease, and to define its prevalence, spectrum, and natural history. We queried subjects in the Johns Hopkins Telomere Syndrome Registry for evidence of luminal GI disease. In sixteen percent of Registry subjects (6 of 38), there was a history of significant GI pathology, and 43 additional cases were identified in the literature. Esophageal stenosis, enteropathy, and enterocolitis were the recurrent findings. In the intestinal mucosa, there was striking villous atrophy, extensive apoptosis, and anaphase bridging pointing to regenerative defects in the epithelial compartment. GI disease was often the first and most severe manifestation of telomere disease in young children. These findings indicate that telomere dysfunction disrupts the epithelial integrity in the human GI tract manifesting in recognizable disease processes. A high index of suspicion should facilitate diagnosis and management.

A homozygous telomerase T-motif variant resulting in markedly reduced repeat addition processivity in siblings with Hoyeraal Hreidarsson syndrome

Hoyeraal Hreidarsson syndrome (HHS) is a form of dyskeratosis congenita (DC) characterized by bone marrow failure, intrauterine growth retardation, developmental delay, microcephaly, cerebellar hypoplasia, immunodeficiency, and extremely short telomeres. As with DC, mutations in genes encoding factors required for telomere maintenance, such as telomerase reverse transcriptase (TERT), have been found in patients with HHS. We describe 2 sibling HHS cases caused by a homozygous mutation (p.T567M) within the TERT T motif. This mutation resulted in a marked reduction in the capacity of telomerase to processively synthesize telomeric repeats, indicating a role for the T motif in this unique aspect of telomerase function. We support this finding by demonstrating defective processivity in the previously reported p.K570N T-motif mutation. The consanguineous, heterozygous p.T567M parents exhibited telomere lengths around the first percentile and no evidence of a DC phenotype. Although heterozygous processivity defects have been associated with familial, adult-onset pulmonary fibrosis, these cases demonstrate the severe clinical and functional impact of biallelic processivity mutations. Thus, despite retaining the capacity to add short stretches of telomeric repeats onto the shortest telomeres, sole expression of telomerase processivity mutants can lead to a profound failure of telomere maintenance and early-onset multisystem disease.

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.

Primary microcephaly, impaired DNA replication, and genomic instability caused by compound heterozygous ATR mutations

Ataxia telangiectasia-mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) kinases are two key regulators of DNA-damage responses (DDR) that are mainly activated in response to DNA double-strand breaks and single-stranded DNA damages, respectively. Seckel syndrome, a rare genetic disorder characterized by a microcephaly and a markedly reduced body size, has been associated with defective ATR-dependent DNA damage signaling. However, the only human genetic ATR defect reported so far is a hypomorphic splicing mutation identified in five related individuals with Seckel syndrome. Here, we report the first case of primary microcephaly with compound heterozygous mutations in ATR: a 540 kb genomic deletion on one allele and a missense mutation leading to splice dysregulation on the other, which ultimately lead to a sharp decrease in ATR expression. DNA combing technology revealed a profound spontaneous alteration of several DNA replication parameters in patient's cells and FISH analyses highlighted the genomic instability caused by ATR deficiency. Collectively, our results emphasize the crucial role for ATR in the control of DNA replication, and reinforce the complementary and nonredundant contributions of ATM and ATR in human cells to face DNA damages and warrant genome integrity.