Hoyeraal-Hreidarsson Syndrome Research Articles

Germline RTEL1 Variants in Telomere Biology Disorders.

Rare germline variation in regulator of telomere elongation helicase 1 (RTEL1) is associated with telomere biology disorders (TBDs). Biallelic RTEL1 variants result in childhood onset dyskeratosis congenita and Hoyeraal-Hreidarsson syndrome whereas heterozygous individuals usually present later in life with pulmonary fibrosis or bone marrow failure. We compiled all TBD-associated RTEL1 variants in the literature and assessed phenotypes and outcomes of 44 individuals from 14 families with mono- or biallelic RTEL1 variants enrolled in clinical trial NCT00027274. Variants were classified by adapting ACMG-AMP guidelines using clinical information, telomere length, and variant allele frequency data. Compared with heterozygotes, individuals with biallelic RTEL1 variants had an earlier age at diagnosis (median age 35.5 vs. 5.1 years, p < 0.01) and worse overall survival (median age 66.5 vs. 22.9 years, p < 0.001). There were 257 unique RTEL1 variants reported in 47 publications, and 209 had a gnomAD minor allele frequency <1%. Only 38.3% (80/209) met pathogenic/likely pathogenic criteria. Notably, 8 of 209 reported disease-associated variants were benign or likely benign and the rest were variants of uncertain significance. Given the considerable differences in outcomes of TBDs associated with RTEL1 germline variants and the extent of variation in the gene, systematic functional studies and standardization of variant curation are urgently needed to inform clinical management.

A Zebrafish Model of Parn-Deficiency Reveals Conserved Mechanisms Underlying Marrow Failure and Opportunities for Therapeutic Amelioration of Hematopoiesis

Dyskeratosis congenita (DC) is a rare inherited bone marrow failure syndrome (IBMFS) and a telomere biology disorder that is characterized by physical malformations such as nail dystrophy, oral leukoplakia, reticular hyperpigmentation, multilineage cytopenia and immunodeficiency. Definitive treatment for the hematological complications is hematopoietic stem cell transplantation (HSCT), which is associated with high risk of organ toxicity and engraftment failure in this population. Further, DC patients are prone to developing myelodysplastic syndrome (MDS) and solid tumors. Germline mutations in the poly(A)-specific ribonuclease (PARN) gene are associated with Hoyeraal-Hreidarsson syndrome, the most severe form of DC. PARN encodes a deadenylase enzyme involved in the 3'-end processing of various RNAs including the telomerase RNA component (TERC). The clinical heterogeneity of phenotypes and the paucity of primary human samples necessitate the use of animal models such as zebrafish ( Danio rerio), given their highly conserved genetic program, to elucidate underlying biology. Zebrafish have a single PARN ortholog with 64% sequence identity at the amino acid level. Using CRISPR-Cas9 mutagenesis, we generated a 1.3 kb deletion in the zebrafish parn gene, predicted to result in premature protein truncation. To measure the hematopoietic output of the parn mutants, we performed whole-mount in situ hybridization (WISH) for key hematopoietic lineage markers. At 48 hours post-fertilization (hpf), we found a reduction in lcp1 + total leukocytes, mpx + neutrophils and hbbe3 + mature erythrocytes. Impaired erythroid differentiation was accompanied by an accumulation of gata1 + erythroid progenitors. This finding was further corroborated by o-dianisidine staining, showing reduced hemoglobinized erythrocytes. Thus, erythrocyte and leukocyte differentiation are compromised in our zebrafish parn mutant, phenocopying the multilineage cytopenia observed in DC patients. Flow cytometry of the whole kidney marrow (WKM, human bone marrow equivalent) at 12 months of age revealed an increase in hematopoietic stem and progenitor cells in the parn mutants. Giemsa staining of WKM touchpreps further identified an increase in myeloid-biased progenitors. Next, we investigated the consequences of Parn loss on telomerase activity. Using the telomere repeat amplification protocol, we found reduced telomerase activity in the WKMs of parn mutant zebrafish. Using 3' rapid amplification of cDNA ends, we observed longer ends in the terc amplicons of the parn mutants, reminiscent of defective deadenylation of human TERC RNA in the setting of PARN mutations. Exogenous thymidine supplementation has been shown to support telomere elongation in human cells. Following treatment of zebrafish embryos with thymidine nucleoside, we observed an improvement in erythrocyte numbers in the parn mutants by o-dianisidine staining at 48 hpf. Collectively, our preliminary findings suggest that telomerase-dependent mechanisms may contribute to the erythropenia seen in parn-mutant zebrafish. In summary, we generated a robust zebrafish model carrying a germline parn mutation that phenocopies the molecular and hematologic features seen in DC patients. Our model holds promise as a new in vivo tool for investigating DC-related disease mechanisms and as a platform for preclinical therapeutic screening.

Conformational plasticity and allosteric communication networks explain Shelterin protein TPP1 binding to human telomerase

The Shelterin complex protein TPP1 interacts with human telomerase (TERT) by means of the TEL-patch region, controlling telomere homeostasis. Aberrations in the TPP1-TERT heterodimer formation might lead to short telomeres and severe diseases like dyskeratosis congenita and Hoyeraal-Hreidarsson syndrome. In the present study, we provide a thorough characterization of the structural properties of the TPP1’s OB-domain by combining data coming from microsecond-long molecular dynamics calculations, time-series analyses, and graph-based networks. Our results show that the TEL-patch conformational freedom is influenced by a network of long-range amino acid communications that together determine the proper TPP1-TERT binding. Furthermore, we reveal that in TPP1 pathological variants Glu169Δ, Lys170Δ and Leu95Gln, the TEL-patch plasticity is reduced, affecting the correct binding to TERT and, in turn, telomere processivity, which eventually leads to accelerated aging of affected cells. Our study provides a structural basis for the design of TPP1-targeting ligands with therapeutic potential against cancer and telomeropathies.

A missense variant in the nuclear localization signal of DKC1 causes Hoyeraal-Hreidarsson syndrome

Hoyeraal-Hreidarsson syndrome (HHS) is the most severe form of dyskeratosis congenita (DC) and is caused by mutations in genes involved in telomere maintenance. Here, we identified male siblings from a family with HHS carrying a hemizygous mutation (c.1345C > G, p.R449G), located in the C-terminal nuclear localization signal (NLS) of the DKC1 gene. These patients exhibit progressive cerebellar hypoplasia, recurrent infections, pancytopenia due to bone marrow failure, and short leukocyte telomere lengths. Single-cell RNA sequencing analysis suggested defects in the NLRP3 inflammasome in monocytes and the activation and maturation of NK cells and B cells. In experiments using induced pluripotent stem cells (iPSCs) from patients, DKC1_R449G iPSCs had short telomere lengths due to reduced levels of human telomerase RNA (hTR) and increased cytosolic proportions of DKC1. Treatment with dihydroquinolizinone RG7834 and 3′deoxyanosine cordycepin rescued telomere length in patient-derived iPSCs. Together, our findings not only provide new insights into immunodeficiency in DC patients but also provide treatment options for telomerase insufficiency disorders.

Resonance assignment and secondary structure of the tandem harmonin homology domains of human RTEL1.

Regulator of telomere elongation helicase 1 (RTEL1) is an Fe-S cluster containing DNA helicase that plays important roles in telomere DNA maintenance, DNA repair, and genomic stability. It is a modular protein comprising an N-terminal helicase domain, two tandem harmonin homology domains 1 & 2 (HHD1 and HHD2), and a C-terminal C4C4 type RING domain. The N-terminal helicase domain disassembles the telomere t/D-loop and unwinds the G-quadruplex via its helicase activity. The C-terminal RING domain interacts with telomere DNA binding protein TRF2 and helps RTEL1 recruitment to the telomere. The tandem HHD1 and HHD2 are characterized as a putative protein-protein interaction domain and have recently been shown to interact with a DNA repair protein SLX4. Several mutations associated with Hoyeraal-Hreidarsson syndrome and pulmonary fibrosis have been found in HHD1 and HHD2 of RTEL1. However, these domains have not been characterized for their structures. We have expressed and purified HHD1 and HHD2 of human RTEL1 for their characterization using solution NMR spectroscopy. Here, we report near complete backbone and sidechain 1H, 13C and 15N chemical shift assignments and secondary structure of the HHD1 and HHD2 domains of human RTEL1.

Hoyeraal-Hreidarsson syndrome with combined immunodeficiency and enterocolitis caused by a DCK1 gene variant

1岁9月龄男性患儿存在胎儿期宫内发育迟缓、小头畸形、小脑发育不良和体格发育迟缓，同时表现为早发的联合免疫缺陷、小肠结肠炎及骨髓衰竭，进而合并严重感染，出现巨细胞病毒感染所致坏死性视网膜综合征及大肠埃希菌败血症。经基因检测分析发现DKC1基因半合子错义变异c.146C>T（p.Thr49Met），该变异遗传自母亲，且该变异为Hoyeraal-Hreidarsson综合征已知的致病性变异。.

Histopathological Features of Monogenic Inflammatory Bowel Disease: Subanalysis of Systematic Review.

Histopathological Features of Monogenic Inflammatory Bowel Disease: Subanalysis of Systematic Review.

Phylogenetic analysis of Harmonin homology domains

BackgroundHarmonin Homogy Domains (HHD) are recently identified orphan domains of about 70 residues folded in a compact five alpha-helix bundle that proved to be versatile in terms of function, allowing for direct binding to a partner as well as regulating the affinity and specificity of adjacent domains for their own targets. Adding their small size and rather simple fold, HHDs appear as convenient modules to regulate protein–protein interactions in various biological contexts. Surprisingly, only nine HHDs have been detected in six proteins, mainly expressed in sensory neurons.ResultsHere, we built a profile Hidden Markov Model to screen the entire UniProtKB for new HHD-containing proteins. Every hit was manually annotated, using a clustering approach, confirming that only a few proteins contain HHDs. We report the phylogenetic coverage of each protein and build a phylogenetic tree to trace the evolution of HHDs. We suggest that a HHD ancestor is shared with Paired Amphipathic Helices (PAH) domains, a four-helix bundle partially sharing fold and functional properties. We characterized amino-acid sequences of the various HHDs using pairwise BLASTP scoring coupled with community clustering and manually assessed sequence features among each individual family. These sequence features were analyzed using reported structures as well as homology models to highlight structural motifs underlying HHDs fold. We show that functional divergence is carried out by subtle differences in sequences that automatized approaches failed to detect.ConclusionsWe provide the first HHD databases, including sequences and conservation, phylogenic trees and a list of HHD variants found in the auditory system, which are available for the community. This case study highlights surprising phylogenetic properties found in orphan domains and will assist further studies of HHDs. We unveil the implication of HHDs in their various binding interfaces using conservation across families and a new protein–protein surface predictor. Finally, we discussed the functional consequences of three identified pathogenic HHD variants involved in Hoyeraal-Hreidarsson syndrome and of three newly reported pathogenic variants identified in patients suffering from Usher Syndrome.

Brain imaging features of children with Hoyeraal-Hreidarsson syndrome.

ObjectiveThis study aimed to summarize the magnetic resonance imaging (MRI) and computed tomography (CT) features of the central nervous system (CNS) in children with Hoyeraal‐Hreidarsson syndrome.MethodsThe imaging and clinical data of four children diagnosed with Hoyeraal‐Hreidarsson syndrome by clinical and laboratory tests in the Guangzhou Women and Children's Medical Center were gathered and analyzed retrospectively. The clinical manifestations and CNS imaging features of Hoyeraal‐Hreidarsson syndrome were summarized based on our results and a literature review.ResultsOur results showed that delayed development, skin pigmentation, nail/toenail dystrophy, thrombocytopenia, and anemia are the most observed clinical presentations of Hoyeraal‐Hreidarsson syndrome. Important findings on CNS imaging showed that all patients had cerebellar hypoplasia, delayed myelination, hydrocephalus, brain atrophy, and calcification. The gene mutations in all cases were consistent with those of dyskeratosis congenita, including TINF2 mutations in three cases and DKC1 mutations in one case.ConclusionHoyeraal‐Hreidarsson syndrome is a severe variant of dyskeratosis congenita. Both DKC1 and TINF2 mutations can lead to the phenotypes of Hoyeraal‐Hreidarsson syndrome. In our study, CNS imaging revealed that cerebellar hypoplasia has an important diagnostic value for Hoyeraal‐Hreidarsson syndrome while delayed myelination, calcification of the parenchyma, brain atrophy, and hydrocephalus are also important findings on CNS imaging. Combining imaging features with clinical and laboratory indicators can assist the diagnosis of Hoyeraal‐Hreidarsson syndrome.

Genotypic and Phenotypic Spectrum of Dyskeratosis Congenita: Results from the Canadian Inherited Marrow Failure Registry

Introduction: Dyskeratosis congenita (DC) is an inherited bone marrow failure syndrome caused by mutations in one of 13 telomere-related genes, resulting in disruption of normal telomere maintenance; however, about 30% of patients do not have a molecular diagnosis. DC patients are at increased risk for severe bone marrow failure (SBMF), myelodysplastic syndrome (MDS), acute myeloid leukemia (AML), and solid tumours. Life expectancy is compromised by SBMF, malignancy, pulmonary and liver fibrosis, and GI bleeding. Objectives: Among patients with DC in Canada, aims were to: (1) characterize the genetic profile of DC in Canada, (2) define the spectrum of clinical features of DC, (3) determine the incidence and age when SBMF, MDS, AML or solid tumours develop, (4) identify factors that are associated with higher mortality risk, and (5) describe the causes of death. Methods: Data of patients enrolled in the Canadian Inherited Marrow Failure Registry (CIMFR) and meeting diagnostic criteria for DC between January 1, 2001 and March 1, 2018 were included. The CIMFR is a multicentre registry that captures data on patients with inherited marrow failure syndromes from pediatric tertiary referral centres across all Canadian provinces. We investigated several continuous (e.g. age at diagnosis of SBMF/MDS/AML) and categorical (e.g. mutated gene) variables that are associated with specific outcomes, namely overall survival and development of SBMF. Cox proportional hazard models were used to assess risk of death based on age at diagnosis and presence of SBMF. Kaplan-Meier curves were used to assess overall survival. Results: As of March 1st, 2018, 35 patients with DC were enrolled. The mean age of diagnosis was 10.94 years (0-39.9). The underlying genotypes were: DKC1 (7), TERT (6), TINF2 (5), RTEL1 (3), PARN (2), TERC (2) but remained undetermined in the others (10). Twenty-seven patients were classified as classical DC, 7 had Hoyeraal-Hreidarsson syndrome and 1 patient had Coats plus syndrome. Eight patients (23%) developed SBMF. The mean age of SBMF was 4.22 years (1-8.66). No statistical difference was found between genotypes and progression to SBMF (P=0.1). Modelling death as a function of time varying SBMF status using a cox proportional hazard regression model showed that the presence of SBMF in DC patients was predictive of higher mortality rate (P= 0.009, hazard ratio 5.7, CI 1.54-21.5). None of the patients developed malignancy during childhood (0-18 years). One adult patient developed skin cancer. Eleven patients (31%) received a hematopoietic stem cell transplant (HSCT). The mean age of HSCT was 9.5 years (0.5-37). Ten (29%) patients died, five of whom were recipients of HSCT. Mean age of death was 12.98 years (2-24.6). Extra-hematological complications included gastrointestinal bleeding (50%), pulmonary fibrosis (40%), overwhelming infection (40%), liver fibrosis (20%), cardiomyopathy (10%), hemolytic uremic syndrome (HUS) (10%) and thrombotic microangiopathy (TMA) (10%). Most patients had more than one organ dysfunction. Analysis of survival showed that all patients with TINF2 mutations have died (at median age of 10.8 years, range 2.5-23.25) whereas none died in the TERT group. Patients diagnosed at younger age had lower overall survival compared to patients diagnosed at older ages (P= 0.03, HR: 0.72, CI: 0.57-0.90). All deaths were due to organ dysfunction related to DC. Fifty percent of the patients had concurrent SBMF at the time of death. Conclusion: In this analysis, we characterised the genetic and phenotypic spectrum of DC patients registered in the CIMFR. We found a high mortality rate mainly related to organ dysfunction and SBMF, and described the impact of genotype, earlier age at diagnosis and presence of SBMF in predicting survival. We found that malignancy is an uncommon complication in the pediatric age group. Figure Disclosures Klaassen: Amgen Inc: Consultancy; TranQoL and KIT: Other: creater and owner of Kids ITP tool and TranQoL; Octapharma AG: Speakers Bureau; Baxalta: Speakers Bureau; Biogen Canada Limited: Speakers Bureau; Novo Nordisk Canada Inc: Consultancy; Hoffman-LaRoche Ltd: Consultancy; Agios Pharmaceuticals Inc: Consultancy; Shire Pharma Canada Inc: Consultancy. Pastore:Pfizer: Honoraria. Lipton:BMS: Consultancy, Research Funding; Takeda: Consultancy, Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria; Novartis: Consultancy, Research Funding; Ariad: Consultancy, Research Funding; Pfizer: Consultancy, Honoraria, Research Funding.

Gastrointestinal Hemorrhage: A Manifestation of the Telomere Biology Disorders

To describe the clinical features, therapeutic interventions, and patient outcomes of gastrointestinal (GI) hemorrhage in individuals with a telomere biology disorder, including dyskeratosis congenita, Hoyeraal-Hreidarsson syndrome, Revesz syndrome, and Coats plus. Clinical Care Consortium for Telomere Associated Ailments members were invited to contribute data on individuals with telomere biology disorders at their institutions who experienced GI bleeding. Patient demographic, laboratory, imaging, procedural, and treatment information and outcomes were extracted from the medical record. Sixteen patients who experienced GI hemorrhage were identified at 11 centers. Among 14 patients who underwent genetic testing, 8 had mutations in TINF2, 4 had mutations in CTC1 or STN1, and 1 patient each had a mutation in TERC and RTEL1. Ten patients had a history of hematopoietic cell transplantation. The patients with Coats plus and those without Coats plus had similar clinical features and courses. Angiodysplasia of the stomach and/or small bowel was described in 8 of the 12 patients who underwent endoscopy; only 4 had esophageal varices. Various medical interventions were trialed. No single intervention was uniformly associated with cessation of bleeding, although 1 patient had a sustained response to treatment with bevacizumab. Recurrence was common, and the overall long-term outcome for affected patients was poor. GI bleeding in patients with telomere biology disorders is associated with significant morbidity and with vascular ectasias rather than varices.

An RTEL1 Mutation Links to Infantile-Onset Ulcerative Colitis and Severe Immunodeficiency.

More than 50 different monogenic disorders causing inflammatory bowel disease (IBD) have been identified. Our goal was to characterize the clinical phenotype, genetic workup, and immunologic alterations in an Ashkenazi Jewish patient that presented during infancy with ulcerative colitis and unique clinical manifestations. Immune workup and whole-exome sequencing were performed, along with Sanger sequencing for confirmation. Next-generation sequencing of the TCRB and IgH was conducted for immune repertoire analysis. Telomere length was evaluated by in-gel hybridization assay. Mass cytometry was performed on patient's peripheral blood mononuclear cells, and compared with control subjects and patients with UC. The patient presented in infancy with failure to thrive and dysmorphic features, consistent with a diagnosis of dyskeratosis congenita and Hoyeraal-Hreidarsson syndrome. Severe ulcerativecolitis manifested in the first year of life and proceeded to the development of a primary immunodeficiency, presenting as Pneumocystis jiroveci pneumonia and hypogammaglobulinemia. Genetic studies identified a deleterious homozygous C.3791G>A missense mutation in the helicase regulator of telomere elongation 1 (RTEL1), leading to short telomeres in the index patient. Immune repertoire studies showed polyclonal T and B cell receptor distribution, while mass cytometry analysis demonstrated marked immunological alterations, including a predominance of naïve T cells, paucity of B cells, and a decrease in various innate immune subsets. RTEL1 mutations are associated with significant alterations in immune landscape and can manifest with infantile-onset IBD. A high index of suspicion is required in Ashkenazi Jewish families where the carriage rate of the C.3791G>A variant is high.

Full length RTEL1 is required for the elongation of the single-stranded telomeric overhang by telomerase.

Telomeres cap the ends of eukaryotic chromosomes and distinguish them from broken DNA ends to suppress DNA damage response, cell cycle arrest and genomic instability. Telomeres are elongated by telomerase to compensate for incomplete replication and nuclease degradation and to extend the proliferation potential of germ and stem cells and most cancers. However, telomeres in somatic cells gradually shorten with age, ultimately leading to cellular senescence. Hoyeraal-Hreidarsson syndrome (HHS) is characterized by accelerated telomere shortening and diverse symptoms including bone marrow failure, immunodeficiency, and neurodevelopmental defects. HHS is caused by germline mutations in telomerase subunits, factors essential for its biogenesis and recruitment to telomeres, and in the helicase RTEL1. While diverse phenotypes were associated with RTEL1 deficiency, the telomeric role of RTEL1 affected in HHS is yet unknown. Inducible ectopic expression of wild-type RTEL1 in patient fibroblasts rescued the cells, enabled telomerase-dependent telomere elongation and suppressed the abnormal cellular phenotypes, while silencing its expression resulted in gradual telomere shortening. Our observations reveal an essential role of the RTEL1 C-terminus in facilitating telomerase action at the telomeric 3′ overhang. Thus, the common etiology for HHS is the compromised telomerase action, resulting in telomere shortening and reduced lifespan of telomerase positive cells.

Naturally occurring modified ribonucleosides.

The chemical identity of RNA molecules beyond the four standard ribonucleosides has fascinated scientists since pseudouridine was characterized as the “fifth” ribonucleotide in 1951. Since then, the ever‐increasing number and complexity of modified ribonucleosides have been found in viruses and throughout all three domains of life. Such modifications can be as simple as methylations, hydroxylations, or thiolations, complex as ring closures, glycosylations, acylations, or aminoacylations, or unusual as the incorporation of selenium. While initially found in transfer and ribosomal RNAs, modifications also exist in messenger RNAs and noncoding RNAs. Modifications have profound cellular outcomes at various levels, such as altering RNA structure or being essential for cell survival or organism viability. The aberrant presence or absence of RNA modifications can lead to human disease, ranging from cancer to various metabolic and developmental illnesses such as Hoyeraal–Hreidarsson syndrome, Bowen–Conradi syndrome, or Williams–Beuren syndrome. In this review article, we summarize the characterization of all 143 currently known modified ribonucleosides by describing their taxonomic distributions, the enzymes that generate the modifications, and any implications in cellular processes, RNA structure, and disease. We also highlight areas of active research, such as specific RNAs that contain a particular type of modification as well as methodologies used to identify novel RNA modifications.This article is categorized under:RNA Processing > RNA Editing and Modification

Characterizing Dyskeratosis Congenita Caused By Parn Mutations in the Zebrafish

Inherited bone marrow failure syndromes (IBMFs) are a group of rare genetic disorders characterized by deficient hematopoiesis and extra-hematologic traits. Most known entities are related to a specific gene or group of genes, but others still remain unclassified. In many cases, the involved proteins are required for critical processes involved in cell survival, such as ribosome biogenesis, maintenance of telomere length, and DNA repair. Importantly, patients with IBMFs have higher risks of developing a variety of cancers from leukemia to solid tumours of the head and neck. In 2015, we published four patients from three different families with mutations in the Poly(A)-specific ribonuclease (PARN) gene. This gene encodes a ribonuclease which is involved in degradation of the poly(A) tails, which regulate mRNA turnover, and thus gene expression. Three of the patients presented with several degrees of mental illness and/or developmental delay. The fourth patient, harbored both a monoallelic deletion and a point mutation at the catalytic domain of the protein, and presented with bone marrow failure and hypomyelination, similar to a severe form of dyskeratosis congenital (DC) known as Hoyeraal-Hreidarsson syndrome. In the last two decades, zebrafish (Danio rerio) has emerged as an excellent animal model for human disease, and is especially relevant in hematology, since many of the transcription factors and cell types are highly conserved. Zebrafish have a single PARN ortholog,with 64% sequence identity to the human gene. Using CRISPR-Cas9 genome editing and a combination of six sgRNAs, we generated a 1.2 kb deletion in the zebrafish parn ortholog extending from exon 5 to 13, causing a premature stop codon. Homozygous fish were generated by incrossing to replicate the complete loss-of-function observed in the patient with the DC-like phenotype. Using whole-mount in situ hybridization (WISH) at 48 hours post-fertilization (hpf), we observed a decrease in the number of several mature myeloid cell lineages including neutrophils (labeled with mpx; p&lt;0.0001), macrophages (lcp1; p=0.0005) and mast cells (cpa5; p=0.0005). We also observed a decrease in the amount of hemoglobin (o-dianisidine staining; p&lt;0.0001). However, the number of hematopoietic stem cells (HSCs) was unchanged in parn mutants. This data parallels similar findings using parn directed splice site and translation start site morpholinos. PARN is described as a protein involved in RNA processing, but has also been associated with telomere maintenance. This latter process is crucial for cell senescence and genome integrity, and is a known cause of several IBMFSs. The telomerase ribonucleoprotein complex is highly active in hematopoietic stem and progenitor cells (HSPCs) and plays a role in cell differentiation. This complex is composed of a reverse transcriptase (TERT), RNA template (TERC), and the dyskerin protein complex (DKC1), mutations of which represent a common cause of DC. qPCR analysis in zebrafish parn mutants revealed a 2.98-fold reduction in tert expression compared with the wild type fish. Combined, these findings suggest that PARN plays an important role in HSC differentiation into myeloid and erythroid lineages, resulting in a bone marrow failure phenotype. Our model provides a unique in vivo platform to study the role of PARN in hematopoiesis and for identifying compounds that restore normal blood cell ratios, which may have the potential to prevent future leukemic transformation. Disclosures No relevant conflicts of interest to declare.

CNS manifestations in patients with telomere biology disorders.

ObjectiveWe systematically evaluated CNS manifestations in patients with inherited telomere biology disorders (TBDs) to better understand the clinical and biological consequences of germline aberrations in telomere biology.MethodsForty-four participants with TBDs (31 dyskeratosis congenita, 12 Hoyeraal-Hreidarsson syndrome, and 1 Revesz syndrome) enrolled in an institutional review board–approved longitudinal cohort study underwent detailed clinical assessments, brain MRI, and genetic testing. Lymphocyte telomere length Z-scores were calculated to adjust for age.ResultsIn this cohort, 25/44 (57%) patients with a TBD had at least 1 structural brain abnormality or variant, most commonly cerebellar hypoplasia (39%). Twenty-one patients (48%) had neurodevelopmental disorder or psychomotor abnormality. Twelve had psychiatric diagnoses, including depression and/or anxiety disorders. Other findings such as hypomyelination, prominent cisterna magna, and cavum septum pellucidum were more frequent than in the general population (p < 0.001). Shorter lymphocyte telomere length was associated with an increased number of MRI findings (p = 0.02) and neurodevelopmental abnormalities (p < 0.001). Patients with autosomal recessive or X-linked TBDs had more neurologic findings than those with autosomal dominant disease.ConclusionsStructural brain abnormalities and variants are common in TBDs, as are neurologic and psychiatric symptoms. The connection between neurodevelopment and telomere biology warrants future study.

Generation of an Rtel1-CreERT2 knock-in mouse model for lineage tracing RTEL1+ stem cells during development.

Regulator of telomere length 1 (RTEL1) DNA helicase has been demonstrated to be essential for the maintenance of telomeres and genomic stability. This function of RTEL1 could be required for protecting stem cells from genomic mutations as suggested by its selective expression in stem cell-zones, as well as by RTEL1 mutations identified in Hoyeraal-Hreidarsson syndrome, a severe dyskeratosis congenita that targets primarily stem cell compartments. As a first step to establish a role of RTEL1 in stem cells, we generated an Rtel1CreERT2 mouse allele in which a tamoxifen-inducible Cre (CreERT2) cDNA was specifically knocked into the Rtel1 genomic locus and controlled by the endogenous Rtel1 regulatory elements. By crossing with a Cre-dependent LacZ reporter mouse strain (R26RLacZ), we further demonstrated that Cre activity in Rtel1CreERT2 mice could be specifically induced by tamoxifen, which allowed the fate of RTEL1+ cells to be traced at various stages of development. Using this tracing assay, we showed for the first time that RTEL1+ cells in the intestine and the testis can act as stem cells that have the capacity to self-renew and differentiate into progeny cells. Therefore, the Rtel1CreERT2 mice generated in this study will be a valuable transgenic tool to explore the function of RTEL1 in stem cells.

Retinopathy and bone marrow failure revealing Coats plus syndrome

The inherited bone marrow failure syndromes (IBMFS) are a group of clinically heterogeneous disorders in which at least one hematopoietic cell lineage is significantly reduced in number.1 Dyskeratosis congenita (DC),...

Human RTEL1 stabilizes long G-overhangs allowing telomerase-dependent over-extension.

Telomere maintenance protects the cell against genome instability and senescence. Accelerated telomere attrition is a characteristic of premature aging syndromes including Dyskeratosis congenita (DC). Mutations in hRTEL1 are associated with a severe form of DC called Hoyeraal-Hreidarsson syndrome (HHS). HHS patients carry short telomeres and HHS cells display telomere damage. Here we investigated how hRTEL1 contributes to telomere maintenance in human primary as well as tumor cells. Transient depletion of hRTEL1 resulted in rapid telomere shortening only in the context of telomerase-positive cells with very long telomeres and high levels of telomerase. The effect of hRTEL1 on telomere length is telomerase dependent without impacting telomerase biogenesis or targeting of the enzyme to telomeres. Instead, RTEL1 depletion led to a decrease in both G-overhang content and POT1 association with telomeres with limited telomere uncapping. Strikingly, overexpression of POT1 restored telomere length but not the overhang, demonstrating that G-overhang loss is the primary defect caused by RTEL1 depletion. We propose that hRTEL1 contributes to the maintenance of long telomeres by preserving long G-overhangs, thereby facilitating POT1 binding and elongation by telomerase.

When Telomerase Causes Telomere Loss

Telomerase counteracts telomere shortening, preventing cellular senescence. Telomerase deficiency causes telomere syndromes because of premature telomere exhaustion in highly proliferative cells. Paradoxically, in a recent issue of Cell, Margalef etal. (2018) demonstrate that telomerase causes telomere loss in cells lacking the RTEL1 helicase, which is defective in Hoyeraal-Hreidarsson syndrome (HHS).