ACTN1 mutations cause congenital macrothrombocytopenia

Dysregulation of Epstein-Barr Virus Infection in Hypomorphic ZAP70 Mutation

Background

Some patients with genetic defects develop Epstein-Barr virus (EBV)-associated lymphoproliferative disorder (LPD)/lymphoma as the main feature. Hypomophic mutations can cause different clinical and laboratory manifestations from null mutations in the same genes.

Methods

We sought to describe the clinical and immunologic phenotype of a 21-month-old boy with EBV-associated LPD who was in good health until then. A genetic and immunologic analysis was performed.

Results

Whole-exome sequencing identified a novel compound heterozygous mutation of ZAP70 c.703-1G>A and c.1674G>A. A small amount of the normal transcript was observed. Unlike ZAP70 deficiency, which has been previously described as severe combined immunodeficiency with nonfunctional CD4+ T cells and absent CD8+ T cells, the patient had slightly low numbers of CD8+ T cells and a small amount of functional T cells. EBV-specific CD8+ T cells and invariant natural killer T (iNKT) cells were absent. The T-cell receptor repertoire, determined using next generation sequencing, was significantly restricted.

Conclusions

Our patient showed that a hypomorphic mutation of ZAP70 can lead to EBV-associated LPD and that EBV-specific CD8+ T cells and iNKT cells are critically involved in immune response against EBV infection.

Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia

Actinin-1 mutations cause dominantly inherited congenital macrothrombocytopenia (CMTP), with mutations in the actin-binding domain increasing actinin's affinity for F-actin. In this study, we examined nine CMTP-causing mutations in the calmodulin-like and rod domains of actinin-1. These mutations increase, to varying degrees, actinin's ability to bundle actin filaments in vitro. Mutations within the calmodulin-like domain decrease its thermal stability slightly but do not dramatically affect calcium binding, with mutant proteins retaining calcium-dependent regulation of filament bundling in vitro. The G764S and E769K mutations increase cytoskeletal association of actinin in cells, and all mutant proteins colocalize with F-actin in cultured HeLa cells. Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP.

Inherited Thrombocytopenia: Update on Genes and Genetic Variants Which may be Associated With Bleeding

Inherited thrombocytopenia (IT) is comprised of a group of hereditary disorders characterized by a reduced platelet count as the main feature, and often with abnormal platelet function, which can subsequently lead to impaired haemostasis. Inherited thrombocytopenia results from genetic mutations in genes implicated in megakaryocyte differentiation and/or platelet formation and clearance. The identification of the underlying causative gene of IT is challenging given the high degree of heterogeneity, but important due to the presence of various clinical presentations and prognosis, where some defects can lead to hematological malignancies. Traditional platelet function tests, clinical manifestations, and hematological parameters allow for an initial diagnosis. However, employing Next-Generation Sequencing (NGS), such as Whole Genome and Whole Exome Sequencing (WES) can be an efficient method for discovering causal genetic variants in both known and novel genes not previously implicated in IT. To date, 40 genes and their mutations have been implicated to cause many different forms of inherited thrombocytopenia. Nevertheless, despite this advancement in the diagnosis of IT, the molecular mechanism underlying IT in some patients remains unexplained. In this review, we will discuss the genetics of thrombocytopenia summarizing the recent advancement in investigation and diagnosis of IT using phenotypic approaches, high-throughput sequencing, targeted gene panels, and bioinformatics tools.

Novel neuroblastoma amplified sequence (NBAS) mutations in a Japanese boy with fever-triggered recurrent acute liver failure

Biallelic mutations in the neuroblastoma amplified sequence (NBAS) gene have been reported to cause two different clinical spectra: short stature with optic nerve atrophy and Pelger-Huët anomaly (SOPH) syndrome and infantile liver failure syndrome 2 (ILFS2). Here, we describe a case of a 3-year-old Japanese boy who presented with fever-triggered recurrent acute liver failure (ALF). The clinical characteristics were considerable elevation of liver enzymes, severe coagulopathy, and acute renal failure. In addition to the liver phenotype, he had short stature and Pelger-Huët anomaly in the peripheral granulocytes. Whole-exome and Sanger sequencing of the patient and his parents revealed that he carried novel compound heterozygous missense mutations in NBAS, c.1018G>C (p.Gly340Arg) and c.2674 G>T (p.Val892Phe). Both mutations affect evolutionarily conserved amino acid residues and are predicted to be highly damaging. Immunoblot analysis of the patient’s skin fibroblasts showed a normal NBAS protein level but a reduced protein level of its interaction partner, p31, involved in Golgi-to-endoplasmic reticulum retrograde vesicular trafficking. We recommend NBAS gene analysis in children with unexplained fever-triggered recurrent ALF or liver dysfunction. Early antipyretic therapy may prevent further episodes of ALF.

Novel mutations in a gene called NBAS have been identified in a Japanese boy with recurrent acute liver failure. Researchers led by Junko Matsuda from Kawasaki Medical School, Okayama, Japan, searched for the genetic cause of a young boy’s recurrent episodes of fever-triggered liver dysfunction. They sequenced the entire protein-coding portion of his genome and that of his parents. They found that the boy had inherited two defective copies of the NBAS (neuroblastoma amplified sequence) gene, one from each parent. Laboratory experiments indicated that these mutations impaired the ability of the protein encoded by NBAS to function correctly. The authors recommend testing for NBAS mutations in any children with unexplained liver problems, and then treating with fever-reducing therapies to prevent future life-threatening episodes of liver failure.

Use of Targeted High-Throughput Sequencing for Genetic Classification of Patients with Bleeding Diathesis and Suspected Platelet Disorder

Inherited platelet disorders (IPD) form a rare and heterogeneous disease entity that is present in about 8% of patients with non-acquired bleeding diathesis. Identification of the defective cellular pathway is an important criterion for stratifying the patient's individual risk profile and for choosing personalized therapeutic options. While costs of high-throughput sequencing technologies have rapidly declined over the last decade, molecular genetic diagnosis of bleeding and platelet disorders is getting more and more suitable within the diagnostic algorithms. In this study, we developed, verified, and evaluated a targeted, panel-based next-generation sequencing approach comprising 59 genes associated with IPD for a cohort of 38 patients with a history of recurrent bleeding episodes and functionally suspected, but so far genetically undefined IPD. DNA samples from five patients with genetically defined IPD with disease-causing variants in WAS , RBM8A , FERMT3 , P2YR12 , and MYH9 served as controls during the validation process. In 40% of 35 patients analyzed, we were able to finally detect 15 variants, eight of which were novel, in 11 genes, ACTN1 , AP3B1 , GFI1B , HPS1 , HPS4 , HPS6 , MPL , MYH9 , TBXA2R , TPM4 , and TUBB1 , and classified them according to current guidelines. Apart from seven variants of uncertain significance in 11% of patients, nine variants were classified as likely pathogenic or pathogenic providing a molecular diagnosis for 26% of patients. This report also emphasizes on potentials and pitfalls of this tool and prospectively proposes its rational implementation within the diagnostic algorithms of IPD.

Loss-of-function mutations in PTPRJ cause a new form of inherited thrombocytopenia

Inherited thrombocytopenias (ITs) are a heterogeneous group of disorders characterized by low platelet count that may result in bleeding tendency. Despite progress being made in defining the genetic causes of ITs, nearly 50% of patients with familial thrombocytopenia are affected with forms of unknown origin. Here, through exome sequencing of 2 siblings with autosomal-recessive thrombocytopenia, we identified biallelic loss-of-function variants in PTPRJ . This gene encodes for a receptor-like PTP, PTPRJ (or CD148), which is expressed abundantly in platelets and megakaryocytes. Consistent with the predicted effects of the variants, both probands have an almost complete loss of PTPRJ at the messenger RNA and protein levels. To investigate the pathogenic role of PTPRJ deficiency in hematopoiesis in vivo, we carried out CRISPR/Cas9-mediated ablation of ptprja (the ortholog of human PTPRJ) in zebrafish, which induced a significantly decreased number of CD41⁺ thrombocytes in vivo. Moreover, megakaryocytes of our patients showed impaired maturation and profound defects in SDF1-driven migration and formation of proplatelets in vitro. Silencing of PTPRJ in a human megakaryocytic cell line reproduced the functional defects observed in patients' megakaryocytes. The disorder caused by PTPRJ mutations presented as a nonsyndromic thrombocytopenia characterized by spontaneous bleeding, small-sized platelets, and impaired platelet responses to the GPVI agonists collagen and convulxin. These platelet functional defects could be attributed to reduced activation of Src family kinases. Taken together, our data identify a new form of IT and highlight a hitherto unknown fundamental role for PTPRJ in platelet biogenesis.

Familial macrothrombocytopenia due to a double mutation in cis in the alpha-actinin 1 gene (ACTN1), previously considered to be chronic immune thrombocytopenic purpura

Congenital thrombocytopenia can easily be misdiagnosed as immune thrombocytopenic purpura, as is illustrated by this case of a woman and her two children. Doubts arose when steroid/IVIG therapy failed in the mother and the thrombocytopenia in the children persisted. By means of next-generation sequencing, two missense variants in cis in the ACTN1 gene of the affected family members were identified, both of unknown significance. We conclude, after further analysis of these mutations with, among others, in silico prediction tools, that the thrombocytopenia has a genetic cause, in particular the ACTN1 mutations, and is not immune mediated.

integRATE: a desirability-based data integration framework for the prioritization of candidate genes across heterogeneous omics and its application to preterm birth

BACKGROUND

The integration of high-quality, genome-wide analyses offers a robust approach to elucidating genetic factors involved in complex human diseases. Even though several methods exist to integrate heterogeneous omics data, most biologists still manually select candidate genes by examining the intersection of lists of candidates stemming from analyses of different types of omics data that have been generated by imposing hard (strict) thresholds on quantitative variables, such as P-values and fold changes, increasing the chance of missing potentially important candidates.

METHODS

To better facilitate the unbiased integration of heterogeneous omics data collected from diverse platforms and samples, we propose a desirability function framework for identifying candidate genes with strong evidence across data types as targets for follow-up functional analysis. Our approach is targeted towards disease systems with sparse, heterogeneous omics data, so we tested it on one such pathology: spontaneous preterm birth (sPTB).

RESULTS

We developed the software integRATE, which uses desirability functions to rank genes both within and across studies, identifying well-supported candidate genes according to the cumulative weight of biological evidence rather than based on imposition of hard thresholds of key variables. Integrating 10 sPTB omics studies identified both genes in pathways previously suspected to be involved in sPTB as well as novel genes never before linked to this syndrome. integRATE is available as an R package on GitHub ( https://github.com/haleyeidem/integRATE ).

CONCLUSIONS

Desirability-based data integration is a solution most applicable in biological research areas where omics data is especially heterogeneous and sparse, allowing for the prioritization of candidate genes that can be used to inform more targeted downstream functional analyses.

ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia

The inherited thrombocytopenias (IT) are a heterogeneous group of diseases resulting from mutations in more than 30 different genes. Among them, ACTN1-related thrombocytopenia (ACTN1-RT; Online Mendelian Inheritance in Man: 615193) is one of the most recently identified forms. It has been described as a mild autosomal dominant macrothrombocytopenia caused by mutations in ACTN1, a gene encoding for one of the two non-muscle isoforms of α-actinin. We recently identified seven new unrelated families with ACTN1-RT caused by different mutations. Two of them are novel missense variants (p.Trp128Cys and p.Pro233Leu), whose pathogenic role has been confirmed by in vitro studies. Together with the 10 families we have previously described, our cohort of ACTN1-RT now consists of 49 individuals carrying ACTN1 mutations. This is the largest case series ever collected and enabled a critical evaluation of the clinical aspects of the disease. We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases. The risk of bleeding, either spontaneous or upon haemostatic challenge, is negligible and there are no other associated defects, either congenital or acquired. Therefore, ACTN1-RT is a benign form of IT, whose diagnosis provides affected individuals and their families with a good prognosis.

Microtubule polyglutamylation and acetylation drive microtubule dynamics critical for platelet formation

Background

Upon maturation in the bone marrow, polyploid megakaryocytes elongate very long and thin cytoplasmic branches called proplatelets. Proplatelets enter the sinusoids blood vessels in which platelets are ultimately released. Microtubule dynamics, bundling, sliding, and coiling, drive these dramatic morphological changes whose regulation remains poorly understood. Microtubule properties are defined by tubulin isotype composition and post-translational modification patterns. It remains unknown whether microtubule post-translational modifications occur in proplatelets and if so, whether they contribute to platelet formation.

Results

Here, we show that in proplatelets from mouse megakaryocytes, microtubules are both acetylated and polyglutamylated. To bypass the difficulties of working with differentiating megakaryocytes, we used a cell model that allowed us to test the functions of these modifications. First, we show that α2bβ3integrin signaling in D723H cells is sufficient to induce β1tubulin expression and recapitulate the specific microtubule behaviors observed during proplatelet elongation and platelet release. Using this model, we found that microtubule acetylation and polyglutamylation occur with different spatio-temporal patterns. We demonstrate that microtubule acetylation, polyglutamylation, and β1tubulin expression are mandatory for proplatelet-like elongation, swelling formation, and cytoplast severing. We discuss the functional importance of polyglutamylation of β1tubulin-containing microtubules for their efficient bundling and coiling during platelet formation.

Conclusions

We characterized and validated a powerful cell model to address microtubule behavior in mature megakaryocytes, which allowed us to demonstrate the functional importance of microtubule acetylation and polyglutamylation for platelet release. Furthermore, we bring evidence of a link between the expression of a specific tubulin isotype, the occurrence of microtubule post-translational modifications, and the acquisition of specific microtubule behaviors. Thus, our findings could widen the current view of the regulation of microtubule behavior in cells such as osteoclasts, spermatozoa, and neurons, which express distinct tubulin isotypes and display specific microtubule activities during differentiation.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0584-6) contains supplementary material, which is available to authorized users.

Variants in exons 5 and 6 of ACTB cause syndromic thrombocytopenia

Germline mutations in the ubiquitously expressed ACTB, which encodes β-cytoplasmic actin (CYA), are almost exclusively associated with Baraitser-Winter Cerebrofrontofacial syndrome (BWCFF). Here, we report six patients with previously undescribed heterozygous variants clustered in the 3′-coding region of ACTB. Patients present with clinical features distinct from BWCFF, including mild developmental disability, microcephaly, and thrombocytopenia with platelet anisotropy. Using patient-derived fibroblasts, we demonstrate cohort specific changes to β-CYA filament populations, which include the enhanced recruitment of thrombocytopenia-associated actin binding proteins (ABPs). These perturbed interactions are supported by in silico modeling and are validated in disease-relevant thrombocytes. Co-examination of actin and microtubule cytoskeleton constituents in patient-derived megakaryocytes and thrombocytes indicates that these β-CYA mutations inhibit the final stages of platelet maturation by compromising microtubule organization. Our results define an ACTB-associated clinical syndrome with a distinct genotype-phenotype correlation and delineate molecular mechanisms underlying thrombocytopenia in this patient cohort.

Genetic variants in ACTB and ACTG1 have been associated with Baraitser-Winter Cerebrofrontofacial syndrome. Here, the authors report of a syndromic thrombocytopenia caused by variants in ACTB exons 5 or 6 that compromise the organization and coupling of the cytoskeleton, leading to impaired platelet maturation.

Genomics and transcriptomics of megakaryocytes and platelets: Implications for health and disease

The field of megakaryocyte and platelet biology has been transformed with the implementation of high throughput sequencing. The use of modern sequencing technologies has led to the discovery of causative mutations in congenital platelet disorders and has been a useful tool in uncovering many other mechanisms of altered platelet formation and function. Although the understanding of the presence of RNA in platelets is relatively novel, mRNA and miRNA expression profiles are being shown to play an increasingly important role in megakaryopoiesis and platelet function in normal physiology as well as in disease states. Understanding the genetic perturbations underlying platelet dysfunction provides insight into normal megakaryopoiesis and thrombopoiesis, as well as guiding the development of novel therapeutics.

High-throughput sequencing for diagnosing platelet disorders: lessons learned from exploring the causes of bleeding disorders

Inherited platelet disorders (IPDs) are a heterogeneous group of disorders caused by multiple genetic defects. Obtaining a molecular diagnosis for IPD patients using a phenotype- and laboratory-based approach is complex, expensive, time-consuming, and not always successful. High-throughput sequencing (HTS) methods offer a genotype-based approach to facilitate molecular diagnostics. Such approaches are expected to decrease time to diagnosis, increase the diagnostic rate, and they have provided novel insights into the genotype-phenotype correlation of IPDs. Some of these approaches have also focused on the discovery of novel genes and unexpected molecular pathways which modulate megakaryocyte and platelet biology were discovered. A growing number of genetic defects underlying IPDs have been identified and we will here provide an overview of the diverse molecular players. Screening of these genes will deliver a genetic diagnosis for about 40%-50% of the IPDs patients and we will compare different HTS applications that have been developed. A brief focus on gene variant interpretation and classification in a diagnostic setting will be given. Although it is true that successes in diagnostics and gene discovery have been reached, a large fraction of patients still remains without a conclusive diagnosis. In these patients, the sum of non-diagnostic variants in known genes or in potential novel genes might only be proven informative in future studies with larger patient cohorts and by data sharing among the diverse genome medicine initiatives. Finally, we still do not understand the role of the non-coding genome space for IPDs.

Structural basis of the filamin A actin-binding domain interaction with F-actin

Actin-cross-linking proteins assemble actin filaments into higher-order structures essential for orchestrating cell shape, adhesion, and motility. Missense mutations in the tandem calponin homology domains of their actin-binding domains (ABDs) underlie numerous genetic diseases, but a molecular understanding of these pathologies is hampered by the lack of high-resolution structures of any actin-cross-linking protein bound to F-actin. Here, taking advantage of a high-affinity, disease-associated mutant of the human filamin A (FLNa) ABD, we combine cryo-electron microscopy and functional studies to reveal at near-atomic resolution how the first calponin homology domain (CH1) and residues immediately N-terminal to it engage actin. We further show that reorientation of CH2 relative to CH1 is required to avoid clashes with actin and to expose F-actin-binding residues on CH1. Our data explain localization of disease-associated loss-of-function mutations to FLNaCH1 and gain-of-function mutations to the regulatory FLNaCH2. Sequence conservation argues that this provides a general model for ABD-F-actin binding.

De Novo Mutations Activating Germline TP53 in an Inherited Bone-Marrow-Failure Syndrome

Inherited bone-marrow-failure syndromes (IBMFSs) include heterogeneous genetic disorders characterized by bone-marrow failure, congenital anomalies, and an increased risk of malignancy. Many lines of evidence have suggested that p53 activation might be central to the pathogenesis of IBMFSs, including Diamond-Blackfan anemia (DBA) and dyskeratosis congenita (DC). However, the exact role of p53 activation in each clinical feature remains unknown. Here, we report unique de novo TP53 germline variants found in two individuals with an IBMFS accompanied by hypogammaglobulinemia, growth retardation, and microcephaly mimicking DBA and DC. TP53 is a tumor-suppressor gene most frequently mutated in human cancers, and occasional germline variants occur in Li-Fraumeni cancer-predisposition syndrome. Most of these mutations affect the core DNA-binding domain, leading to compromised transcriptional activities. In contrast, the variants found in the two individuals studied here caused the same truncation of the protein, resulting in the loss of 32 residues from the C-terminal domain (CTD). Unexpectedly, the p53 mutant had augmented transcriptional activities, an observation not previously described in humans. When we expressed this mutant in zebrafish and human-induced pluripotent stem cells, we observed impaired erythrocyte production. These findings together with close similarities to published knock-in mouse models of TP53 lacking the CTD demonstrate that the CTD-truncation mutations of TP53 cause IBMFS, providing important insights into the previously postulated connection between p53 and IBMFSs.

On the Way to in vitro Platelet Production

The severely decreased platelet counts (10–30. 10³ platelets/μL) frequently observed in patients undergoing chemotherapy, radiation treatment, or organ transplantation are associated with life-threatening increased bleeding risks. To circumvent these risks, platelet transfusion remains the treatment of choice, despite some limitations which include a limited shelf-life, storage-related deterioration, the development of alloantibodies in recipients and the transmission of infectious diseases. A sustained demand has evolved in recent years for controlled blood products, free of infectious, inflammatory, and immune risks. As a consequence, the challenge for blood centers in the near future will be to ensure an adequate supply of blood platelets, which calls for a reassessment of our transfusion models. To meet this challenge, many laboratories are now turning their research efforts toward the in vitro and customized production of blood platelets. In recent years, there has been a major enthusiasm for the cultured platelet production, as illustrated by the number of reviews that have appeared in recent years. The focus of the present review is to critically asses the arguments put forward in support of the culture of platelets for transfusion purposes. In light of this, we will recapitulate the main advances in this quickly evolving field, while noting the technical limitations to overcome to make cultured platelet a transfusional alternative.

On the way to in vitro platelet production

The severely decreased platelet counts (10-30.10³ platelets/μL) frequently observed in patients undergoing chemotherapy, radiation treatment or organ transplantation are associated with life-threatening increased bleeding risks. To circumvent these risks, platelet transfusion remains the treatment of choice, despite some limitations which include a limited shelf-life, storage-related deterioration, the development of alloantibodies in recipients and the transmission of infectious diseases. A sustained demand has evolved in recent years for controlled blood products, free of infectious, inflammatory and immune risks. As a consequence, the challenge for blood centers in the near future will be to ensure an adequate supply of blood platelets, which calls for a reassessment of our transfusion models. To meet this challenge, many laboratories are now turning their research efforts towards the in vitro and customized production of blood platelets.

Inherited Macrothrombocytopenia: Correlating Morphology, Epidemiology, Molecular Pathology and Clinical Features

Inherited macrothrombocytopenia is increasingly being recognized as a relatively common condition. This descriptive review aims at focusing on the different areas of advancement that have taken place with this condition with particular reference to India. A pubmed search of articles between January 1990 and October 2017 with the key words-macrothrombocytopenia, asymptomatic macrothrombocytopenia, macrothrombocytopenia India, syndromic macrothrombocytopenia, molecular pathology, megakaryopoiesis and platelet formation were searched. The shortlisted articles were then read. Review articles provided additional references and the articles thus obtained were also read. Special interest and research conducted by the authors provided further sources of information. A total of 487 articles were found of which 68 articles were related to our subject of review. Review articles were read and additional articles from the reference quoted. Forty-four percent of nonsyndromic Inherited macrothrombocytopenia showed mutations of MYH9, GP1BB, GP1Ba, GPIX, ABCG5 and 8, ACTN, FLI, TUBB and RUNX1 frequently in heterozygous state. All types of inheritance pattern namely autosomal dominant, recessive and sex linked patterns have been described. Syndromic causes of this phenomenon are well known and have been described. Many asymptomatic patients do have mild or moderate bleeding history. Clinical algorithms to differentiate chronic ITP associated macrothrombocytopenia from inherited variety have been explored. Inherited macrothrombocytopenia is an emerging area of interest in platelet biology with its implication in diagnosis, prognosis, genetic counseling, management and in transfusion medicine.

Whole-exome analysis to detect congenital hemolytic anemia mimicking congenital dyserythropoietic anemia

Congenital dyserythropoietic anemia (CDA) is a heterogeneous group of rare congenital disorders characterized by ineffective erythropoiesis and dysplastic changes in erythroblasts. Diagnosis of CDA is based primarily on the morphology of bone marrow erythroblasts; however, genetic tests have recently become more important. Here, we performed genetic analysis of 10 Japanese patients who had been diagnosed with CDA based on laboratory findings and morphological characteristics. We examined 10 CDA patients via central review of bone marrow morphology and genetic analysis for congenital bone marrow failure syndromes. Sanger sequencing for CDAN1, SEC23B, and KLF1 was performed for all patients. We performed whole-exome sequencing in patients without mutation in these genes. Three patients carried pathogenic CDAN1 mutations, whereas no SEC23B mutations were identified in our cohort. WES unexpectedly identified gene mutations known to cause congenital hemolytic anemia in two patients: canonical G6PD p.Val394Leu mutation and SPTA1 p.Arg28His mutation. Comprehensive genetic analysis is warranted for more effective diagnosis of patients with suspected CDA.

Congenital Disorders of Platelet Function and Number

Mucocutaneous bleeding symptoms and/or persistent thrombocytopenia occur in individuals with congenital disorders of platelet function and number. Apart from bleeding, these disorders are often associated with additional hematologic and clinical manifestations, including auditory, immunologic, and oncologic disease. Autosomal recessive, dominant, and X-linked inheritance patterns have been demonstrated. Precise delineation of the molecular cause of the platelet disorder can aid the pediatrician in the detection and prevention of specific disorder-associated manifestations and guide appropriate treatment and anticipatory care for the patient and family.

Sarcomeric and nonmuscle α-actinin isoforms exhibit differential dynamics at skeletal muscle Z-lines

The α-actinin proteins are a highly conserved family of actin crosslinkers that mediate interactions between several cytoskeletal and sarcomeric proteins. Nonsarcomeric α-actinin-1 and α-actinin-4 crosslink actin filaments in the cytoskeleton, while sarcomeric α-actinin-2 and α-actinin-3 serve a crucial role in anchoring actin filaments to the muscle Z-line. To assess the difference in turnover dynamics and structure/function properties between the α-actinin isoforms at the sarcomeric Z-line, we used Fluorescence Recovery After Photobleaching (FRAP) in primary myofiber cultures. We found that the recovery kinetics of these proteins followed three distinct patterns: α-actinin-2/α-actinin-3 had the slowest turn over, α-actinin-1 recovered to an intermediate degree, and α-actinin-4 had the fastest recovery. Interestingly, the isoforms' patterns of recovery were reversed at adhesion plaques in fibroblasts. This disparity suggests that the different α-actinin isoforms have unique association kinetics in myofibers and that nonmuscle isoform interactions are more dynamic at the sarcomeric Z-line. Protein domain-specific investigations using α-actinin-2/4 chimeric proteins showed that differential dynamics between sarcomeric and nonmuscle isoforms are regulated by cooperative interactions between the N-terminal actin-binding domain, the spectrin-like linker region and the C-terminal calmodulin-like EF hand domain. Together, these findings demonstrate that α-actinin isoforms are unique in binding dynamics at the Z-line and suggest differentially evolved interactive and Z-line association capabilities of each functional domain.

Megakaryocyte ontogeny: Clinical and molecular significance

Fetal megakaryocytes (Mks) differ from adult Mks in key parameters that affect their capacity for platelet production. However, despite being smaller, more proliferative, and less polyploid, fetal Mks generally mature in the same manner as adult Mks. The phenotypic features unique to fetal Mks predispose patients to several disease conditions, including infantile thrombocytopenia, infantile megakaryoblastic leukemias, and poor platelet recovery after umbilical cord blood stem cell transplantations. Ontogenic Mk differences also affect new strategies being developed to address global shortages of platelet transfusion units. These donor-independent, ex vivo production platforms are hampered by the limited proliferative capacity of adult-type Mks and the inferior platelet production by fetal-type Mks. Understanding the molecular programs that distinguish fetal versus adult megakaryopoiesis will help in improving approaches to these clinical problems. This review summarizes the phenotypic differences between fetal and adult Mks, the disease states associated with fetal megakaryopoiesis, and recent advances in the understanding of mechanisms that determine ontogenic Mk transitions.

Genetic heterogeneity of uncharacterized childhood autoimmune diseases with lymphoproliferation

Autoimmune diseases in children are rare and can be difficult to diagnose.  Single causative genes have been identified for some pediatric autoimmune diseases. Such orphan diseases may not be diagnosed properly due to the variability of patients' phenotypes. Guidelines for the diagnostic process need to be developed. Fifteen patients with uncharacterized childhood autoimmune diseases with lymphoproliferation that had negative testing for autoimmune lymphoproliferative syndrome were subjected to whole-exome sequencing to identify genes associated with these conditions. Five causative genes, CTLA4, STAT3, TNFAIP3, IKZF1, and PSTPIP1, were identified. These genes should be considered as candidates for uncharacterized childhood autoimmune diseases with lymphoproliferation.

A novel heterozygous ITGB3 p.T720del inducing spontaneous activation of integrin αIIbβ3 in autosomal dominant macrothrombocytopenia with aggregation dysfunction

We identified a novel heterozygous ITGB3 p.T720del mutation in a pedigree with macrothrombocytopenia exhibiting aggregation dysfunction. Platelet aggregation induced by ADP and collagen was significantly reduced, while ristocetin aggregation was normal. Integrin αIIbβ3 was partially activated in a resting status, but platelet expression of αIIbβ3 was downregulated. Functional analysis using a cell line showed spontaneous phosphorylation of FAK in αIIb/β3 (p.T720del)-transfected 293T cells in suspension conditions. Abnormal cytoplasmic protrusions, membrane ruffling, and cytoplasmic localization of αIIbβ3 were observed in αIIb/β3 (p.T720del)-transfected CHO cells. Such morphological changes were reversed by treatment with an FAK inhibitor. These findings imply spontaneous, but partial, activation of αIIbβ3 followed by phosphorylation of FAK as the initial mechanism of abnormal thrombopoiesis. Internalization and decreased surface expression of αIIbβ3 would contribute to aggregation dysfunction. We reviewed the literature of congenital macrothrombocytopenia associated with heterozygous ITGA2B or ITGB3 mutations. Reported mutations were highly clustered at the membrane proximal region of αIIbβ3, which affected the critical interaction between αIIb R995 and β3 D723, resulting in a constitutionally active form of the αIIbβ3 complex. Macrothrombocytopenia caused by a heterozygous activating mutation of ITGA2B or ITGB3 at the membrane proximal region forms a distinct entity of rare congenital thrombocytopenia.

Hereditary thrombocytopenias: a growing list of disorders

The introduction of high throughput sequencing (HTS) techniques greatly improved the knowledge of inherited thrombocytopenias (ITs) over the last few years. A total of 33 different forms caused by molecular defects affecting at least 32 genes have been identified; along with the discovery of new disease-causing genes, pathogenetic mechanisms of thrombocytopenia have been better elucidated. Although the clinical picture of ITs is heterogeneous, bleeding has been long considered the major clinical problem for patients with IT. Conversely, the current scenario indicates that patients with some of the most common ITs are at risk of developing additional disorders more dangerous than thrombocytopenia itself during life. In particular, MYH9 mutations result in congenital macrothrombocytopenia and predispose to kidney failure, hearing loss, and cataracts, MPL and MECOM mutations cause congenital thrombocytopenia evolving into bone marrow failure, whereas thrombocytopenias caused by RUNX1, ANKRD26, and ETV6 mutations are characterized by predisposition to hematological malignancies. Making a definite diagnosis of these forms is crucial to provide patients with the most appropriate treatment, follow-up, and counseling. In this review, the ITs known to date are discussed, with specific attention focused on clinical presentations and diagnostic criteria for ITs predisposing to additional illnesses. The currently available therapeutic options for the different forms of IT are illustrated.

Genetics of Venous Thrombosis: update in 2015

Venous thrombosis (VT) is a common multifactorial disease with a genetic component that was first suspected nearly 60 years ago. In this review, we document the genetic determinants of the disease, and update recent findings delivered by the application of high-throughput genotyping and sequencing technologies. To date, 17 genes have been robustly demonstrated to harbour genetic variations associated with VT risk: ABO, F2, F5, F9, F11, FGG, GP6, KNG1, PROC, PROCR, PROS1, SERPINC1, SLC44A2, STXBP5, THBD, TSPAN15 and VWF. The common polymorphisms are estimated to account only for a modest part (~5 %) of the VT heritability. Much remains to be done to fully disentangle the exact genetic (and epigenetic) architecture of the disease. A large suite of powerful tools and research strategies can be deployed on the large collections of patients that have already been assembled (and additional are ongoing).

ACTN1-related Macrothrombocytopenia: A Novel Entity in the Progressing Field of Pediatric Thrombocytopenia

The most common cause of thrombocytopenia in children is immune thrombocytopenia. Nevertheless, some atypical cases should evoke the hypothesis of genetic thrombocytopenia. Indeed, in the past years, 30 new genes had been described in the field of inherited thrombocytopenia. We report a series of 11 cases of a newly diagnosed entity: ACTN1-related macrothrombocytopenia. Mutations in the gene ACTN1 cause mild macrothrombocytopenia characterized by elevated mean platelet volume and elevated immature platelet fraction, and low bleeding tendency. Its transmission is autosomal dominant. Molecular diagnosis is made by sequencing the ACTN1 gene. Its potential role in hematological malignancy predisposition remains unclear and should be clarified.

CONCLUSION

We identified 11 patients with ACTN1-related macrothrombocytopenia diagnosed through pediatric probands. The aim was to underline the specificities of this entity, especially in children, and bring it to the knowledge of pediatricians.

Molecular basis of inherited thrombocytopenias: an update

PURPOSE OF REVIEW

Inherited thrombocytopenias are a heterogeneous group of diseases caused by mutations in many genes. They account for approximately only 50% of cases, suggesting that novel genes have yet to be identified for a comprehensive understanding of platelet biogenesis defects. This review provides an update of the last year of discoveries on inherited thrombocytopenias focusing on the molecular basis and potential pathogenic mechanisms affecting megakaryopoiesis and platelet production.

RECENT FINDINGS

Most of the novel discoveries are related to identification of mutations in novel inherited thrombocytopenia genes using a next-generation sequencing approach. They include MECOM, DIAPH1, TRPM7, SRC, FYB, and SLFN14, playing different roles in megakaryopoiesis and platelet production. Moreover, it is worth mentioning data on hypomorphic mutations of FLI1 and the association of single nucleotide polymorphisms, such as that identified in ACTN1, with thrombocytopenia.

SUMMARY

Thanks to the application of next-generation sequencing, the number of inherited thrombocytopenia genes is going to increase rapidly. Considering the wide genetic heterogeneity (more than 30 genes), these technologies can also be used for diagnostic purpose. Whatever is the aim, extreme caution should be taken in interpreting data, as inherited thrombocytopenias are mainly autosomal dominant diseases caused by variants of apparent unknown significance.

Twinfilin 2a regulates platelet reactivity and turnover in mice

Regulated reorganization of the actin cytoskeleton is a prerequisite for proper platelet production and function. Consequently, defects in proteins controlling actin dynamics have been associated with platelet disorders in humans and mice. Twinfilin 2a (Twf2a) is a small actin-binding protein that inhibits actin filament assembly by sequestering actin monomers and capping filament barbed ends. Moreover, Twf2a binds heterodimeric capping proteins, but the role of this interaction in cytoskeletal dynamics has remained elusive. Even though Twf2a has pronounced effects on actin dynamics in vitro, only little is known about its function in vivo. Here, we report that constitutive Twf2a-deficient mice (Twf2a^-/-) display mild macrothrombocytopenia due to a markedly accelerated platelet clearance in the spleen. Twf2a^-/- platelets showed enhanced integrin activation and α-granule release in response to stimulation of (hem) immunoreceptor tyrosine-based activation motif (ITAM) and G-protein-coupled receptors, increased adhesion and aggregate formation on collagen I under flow, and accelerated clot retraction and spreading on fibrinogen. In vivo, Twf2a deficiency resulted in shortened tail bleeding times and faster occlusive arterial thrombus formation. The hyperreactivity of Twf2a^-/- platelets was attributed to enhanced actin dynamics, characterized by an increased activity of n-cofilin and profilin 1, leading to a thickened cortical cytoskeleton and hence sustained integrin activation by limiting calpain-mediated integrin inactivation. In summary, our results reveal the first in vivo functions of mammalian Twf2a and demonstrate that Twf2a-controlled actin rearrangements dampen platelet activation responses in a n-cofilin- and profilin 1-dependent manner, thereby indirectly regulating platelet reactivity and half-life in mice.

A Fast Association Test for Identifying Pathogenic Variants Involved in Rare Diseases

We present a rapid and powerful inference procedure for identifying loci associated with rare hereditary disorders using Bayesian model comparison. Under a baseline model, disease risk is fixed across all individuals in a study. Under an association model, disease risk depends on a latent bipartition of rare variants into pathogenic and non-pathogenic variants, the number of pathogenic alleles that each individual carries, and the mode of inheritance. A parameter indicating presence of an association and the parameters representing the pathogenicity of each variant and the mode of inheritance can be inferred in a Bayesian framework. Variant-specific prior information derived from allele frequency databases, consequence prediction algorithms, or genomic datasets can be integrated into the inference. Association models can be fitted to different subsets of variants in a locus and compared using a model selection procedure. This procedure can improve inference if only a particular class of variants confers disease risk and can suggest particular disease etiologies related to that class. We show that our method, called BeviMed, is more powerful and informative than existing rare variant association methods in the context of dominant and recessive disorders. The high computational efficiency of our algorithm makes it feasible to test for associations in the large non-coding fraction of the genome. We have applied BeviMed to whole-genome sequencing data from 6,586 individuals with diverse rare diseases. We show that it can identify multiple loci involved in rare diseases, while correctly inferring the modes of inheritance, the likely pathogenic variants, and the variant classes responsible.

Diagnosis of inherited platelet disorders on a blood smear: a tool to facilitate worldwide diagnosis of platelet disorders

Essentials There are many hereditary platelet disorders (HPD) but diagnosing these is challenging. We provide a method to diagnose several HPDs using standard blood smears requiring < 100 µL blood. By this approach, the underlying cause of HPD was characterized in ~25-30% of referred individuals. The method facilitates diagnosis of HPD for patients of all ages around the world.

SUMMARY

Background Many hereditary thrombocytopenias and/or platelet function disorders have been identified, but diagnosis of these conditions remains challenging. Diagnostic laboratory techniques are available only in a few specialized centers and, using fresh blood, often require the patient to travel long distances. For the same reasons, patients living in developing countries usually have limited access to diagnosis. Further, the required amount of blood is often prohibitive for pediatric patients. Objectives By a collaborative international approach of four centers, we aimed to overcome these limitations by developing a method using blood smears prepared from less than 100 μL blood, for a systematic diagnostic approach to characterize the platelet phenotype. Methods We applied immunofluorescence labelling (performed centrally) to standard air-dried peripheral blood smears (prepared locally, shipped by regular mail), using antibodies specific for proteins known to be affected in specific hereditary platelet disorders. Results By immunofluorescence labelling of blood smears we characterized the underlying cause in 877/3217 (27%) patients with suspected hereditary platelet disorders (HPD). Currently about 50 genetic causes for HPD are identified. Among those, the blood smear method was especially helpful to identify MYH9 disorders/MYH9-related disease, biallelic Bernard-Soulier syndrome, Glanzmann thrombasthenia and gray platelet syndrome. Diagnosis could be established for GATA1 macrothrombocytopenia, GFI1B macrothrombocytopenia, ß1-tubulin macrothrombocytopenia, filamin A-related thrombocytopenia and Wiskott-Aldrich syndrome. Conclusion Combining basic and widely available preanalytical methods with the immunomorphological techniques presented here, allows detailed characterization of the platelet phenotype. This supports genetic testing and facilitates diagnosis of hereditary platelet disorders for patients of all ages around the world.

The genetics of platelet count and volume in humans

The last decade has witnessed an explosion in the depth, variety, and amount of human genetic data that can be generated. This revolution in technical and analytical capacities has enabled the genetic investigation of human traits and disease in thousands to now millions of participants. Investigators have taken advantage of these advancements to gain insight into platelet biology and the platelet's role in human disease. To do so, large human genetics studies have examined the association of genetic variation with two quantitative traits measured in many population and patient based cohorts: platelet count (PLT) and mean platelet volume (MPV). This article will review the many human genetic strategies-ranging from genome-wide association study (GWAS), Exomechip, whole exome sequencing (WES), to whole genome sequencing (WGS)-employed to identify genes and variants that contribute to platelet traits. Additionally, we will discuss how these investigations have examined and interpreted the functional implications of these newly identified genetic factors and whether they also impart risk to human disease. The depth and size of genetic, phenotypic, and other -omic data are primed to continue their growth in the coming years and provide unprecedented opportunities to gain critical insights into platelet biology and how platelets contribute to disease.

Mutations in tropomyosin 4 underlie a rare form of human macrothrombocytopenia

Platelets are anuclear cells that are essential for blood clotting. They are produced by large polyploid precursor cells called megakaryocytes. Previous genome-wide association studies in nearly 70,000 individuals indicated that single nucleotide variants (SNVs) in the gene encoding the actin cytoskeletal regulator tropomyosin 4 (TPM4) exert an effect on the count and volume of platelets. Platelet number and volume are independent risk factors for heart attack and stroke. Here, we have identified 2 unrelated families in the BRIDGE Bleeding and Platelet Disorders (BPD) collection who carry a TPM4 variant that causes truncation of the TPM4 protein and segregates with macrothrombocytopenia, a disorder characterized by low platelet count. N-Ethyl-N-nitrosourea–induced (ENU-induced) missense mutations in Tpm4 or targeted inactivation of the Tpm4 locus led to gene dosage–dependent macrothrombocytopenia in mice. All other blood cell counts in Tpm4-deficient mice were normal. Insufficient TPM4 expression in human and mouse megakaryocytes resulted in a defect in the terminal stages of platelet production and had a mild effect on platelet function. Together, our findings demonstrate a nonredundant role for TPM4 in platelet biogenesis in humans and mice and reveal that truncating variants in TPM4 cause a previously undescribed dominant Mendelian platelet disorder.

Rare variants in GP1BB are responsible for autosomal dominant macrothrombocytopenia

The von Willebrand receptor complex, which is composed of the glycoproteins Ibα, Ibβ, GPV, and GPIX, plays an essential role in the earliest steps in hemostasis. During the last 4 decades, it has become apparent that loss of function of any 1 of 3 of the genes encoding these glycoproteins (namely, GP1BA, GP1BB, and GP9) leads to autosomal recessive macrothrombocytopenia complicated by bleeding. A small number of variants in GP1BA have been reported to cause a milder and dominant form of macrothrombocytopenia, but only 2 tentative reports exist of such a variant in GP1BB By analyzing data from a collection of more than 1000 genome-sequenced patients with a rare bleeding and/or platelet disorder, we have identified a significant association between rare monoallelic variants in GP1BB and macrothrombocytopenia. To strengthen our findings, we sought further cases in 2 additional collections in the United Kingdom and Japan. Across 18 families exhibiting phenotypes consistent with autosomal dominant inheritance of macrothrombocytopenia, we report on 27 affected cases carrying 1 of 9 rare variants in GP1BB.

Clinical utility of next-generation sequencing for inherited bone marrow failure syndromes

PURPOSE

Precise genetic diagnosis of inherited bone marrow failure syndromes (IBMFS), a heterogeneous group of genetic disorders, is challenging but essential for precise clinical decision making.

METHODS

We analyzed 121 IBMFS patients using a targeted sequencing covering 184 associated genes and 250 IBMFS patients using whole-exome sequencing (WES).

RESULTS

We achieved successful genetic diagnoses for 53 of 121 patients (44%) using targeted sequencing and for 68 of 250 patients (27%) using WES. In the majority of cases (targeted sequencing: 45/53, 85%; WES: 63/68, 93%), the detected variants were concordant with, and therefore supported, the clinical diagnoses. However, in the remaining 13 cases (8 patients by target sequencing and 5 patients by WES), the clinical diagnoses were incompatible with the detected variants.

CONCLUSION

Our approach utilizing targeted sequencing and WES achieved satisfactory diagnostic rates and supported the efficacy of massive parallel sequencing as a diagnostic tool for IBMFS.Genet Med advance online publication 19 January 2017.

Effect of amino acid mutations on intra-dimer tubulin–tubulin binding strength of microtubules

Binding strength inside αβ-tubulin dimers of a microtubule (MT) with atomic resolutions are of importance in determining the structural stability of the MT as well as designing self-assembled functional structures from it. Through simulations, this study proposes a new strategy to tune the binding strength inside microtubules through point mutations of amino acids on the intra-dimer interface.

Haploinsufficiency of TNFAIP3 (A20) by germline mutation is involved in autoimmune lymphoproliferative syndrome

BACKGROUND

Autoimmune diseases in children are rare and can be difficult to diagnose. Autoimmune lymphoproliferative syndrome (ALPS) is a well-characterized pediatric autoimmune disease caused by mutations in genes associated with the FAS-dependent apoptosis pathway. In addition, various genetic alterations are associated with the ALPS-like phenotype.

OBJECTIVE

The aim of the present study was to elucidate the genetic cause of the ALPS-like phenotype.

METHODS

Candidate genes associated with the ALPS-like phenotype were screened by using whole-exome sequencing. The functional effect of the identified mutations was examined by analyzing the activity of related signaling pathways.

RESULTS

A de novo heterozygous frameshift mutation of TNF-α-induced protein 3 (TNFAIP3, A20), a negative regulator of the nuclear factor κB pathway, was identified in one of the patients exhibiting the ALPS-like phenotype. Increased activity of the nuclear factor κB pathway was associated with haploinsufficiency of TNFAIP3 (A20).

CONCLUSION

Haploinsufficiency of TNFAIP3 (A20) by a germline heterozygous mutation leads to the ALPS phenotype.

Inherited platelet function disorders. Diagnostic approach and management

Inherited platelet function disorders (IPFDs) make up a significant proportion of congenital bleeding diatheses, but they remain poorly understood and often difficult to diagnose. Therefore, a rational diagnostic approach, based on a standardized sequence of laboratory tests, with consecutive steps of increasing level of complexity, plays a crucial role in the diagnosis of most IPFDs. In this review we discuss a diagnostic approach through platelet phenotyping and genotyping and we give an overview of the options for the management of bleeding in these disorders and an account of the few systematic studies on the bleeding risk associated with invasive procedures and its treatment.

Phosphatase and tensin homolog (PTEN) mutation can cause activated phosphatidylinositol 3-kinase δ syndrome-like immunodeficiency

BACKGROUND

Activated phosphatidylinositol 3-kinase δ syndrome (APDS) is a recently discovered primary immunodeficiency disease (PID). Excess phosphatidylinositol 3-kinase (PI3K) activity linked to mutations in 2 PI3K genes, PIK3CD and PIK3R1, causes APDS through hyperphosphorylation of AKT, mammalian target of rapamycin (mTOR), and S6.

OBJECTIVE

This study aimed to identify novel genes responsible for APDS.

METHODS

Whole-exome sequencing was performed in Japanese patients with PIDs. Immunophenotype was assessed through flow cytometry. Hyperphosphorylation of AKT, mTOR, and S6 in lymphocytes was examined through immunoblotting, flow cytometry, and multiplex assays.

RESULTS

We identified heterozygous mutations of phosphatase and tensin homolog (PTEN) in patients with PIDs. Immunoblotting and quantitative PCR analyses indicated that PTEN expression was decreased in these patients. Patients with PTEN mutations and those with PIK3CD mutations, including a novel E525A mutation, were further analyzed. The clinical symptoms and immunologic defects of patients with PTEN mutations, including lymphocytic AKT, mTOR, and S6 hyperphosphorylation, resemble those of patients with APDS. Because PTEN is known to suppress the PI3K pathway, it is likely that defective PTEN results in activation of the PI3K pathway.

CONCLUSION

PTEN loss-of-function mutations can cause APDS-like immunodeficiency because of aberrant PI3K pathway activation in lymphocytes.

Should any genetic defect affecting α-granules in platelets be classified as gray platelet syndrome?

There is much current interest in the role of the platelet storage pool of α-granule proteins both in hemostasis and non-hemostatic events. As well as in the arrest of bleeding, the secreted proteins participate in wound healing, inflammation, and innate immunity while in pathology they may be actors in arterial thrombosis and atherosclerosis as well as cancer and metastasis. For a long time, gray platelet syndrome (GPS) has been regarded as the classic inherited platelet disorder caused by an absence of α-granules and their contents. While NBEAL2 is the major source of mutations in GPS, other gene variants may give rise to significant α-granule deficiencies in platelets. These include GATA1, VPS33B, or VIPAS39 in the arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome and now GFI1B. Nevertheless, many phenotypic differences are associated with mutations in these genes. This critical review was aimed to assess genotype/phenotype variability in disorders of platelet α-granule biogenesis and to urge caution in grouping all genetic defects of α-granules as GPS. Am. J. Hematol. 91:714-718, 2016. © 2016 Wiley Periodicals, Inc.

Functional characterization of a novel GFI1B mutation causing congenital macrothrombocytopenia

Essentials Two groups recently reported GFI1B as a novel causative gene for congenital macrothrombocytopenia. We performed functional analysis of a novel GFI1B mutation and previous mutations. An immunofluorescence analysis of the platelet CD34 expression can be useful as a screening test. Mutant-transduced megakaryocytes produced enlarged proplatelet tips which were reduced in number.

SUMMARY

Background GFI1B is an essential transcription factor for megakaryocyte and erythrocyte development. Two groups have recently identified GFI1B as a novel causative gene for congenital macrothrombocytopenia associated with α-granule deficiency. Methods We performed whole exome sequencing and identified a novel GFI1B p.G272fsX274 mutation in a family with macrothrombocytopenia, and a decreased number of platelet α-granules and abnormally shaped red blood cells. p.G272fsX274 and the previous two mutations all predicted disruption of an essential DNA-binding domain in GFI1B. We therefore performed functional studies to characterize the biochemical and biological effects of these three patient-derived mutations. Results An immunofluorescence analysis revealed decreased thrombospondin-1 and increased CD34 expression in platelets from our patient. Consistent with the previous studies, the three patient-derived mutants were unable to repress the expression of the reporter gene and had a dominant-negative effect over wild-type GFI1B. In addition, the three mutations abolished recognition of a consensus-binding site in gel shift assays. Furthermore, transduction of mouse fetal liver-derived megakaryocytes with the three GFI1B mutants resulted in the production of abnormally large proplatelet tips, which were reduced in number. Conclusions Our study provides further proof of concept that GFI1B is an essential protein for the normal development of the megakaryocyte lineage.

Inherited platelet disorders: Insight from platelet genomics using next-generation sequencing

Inherited platelet disorders (IPDs) are a heterogeneous group of disorders associated with normal or reduced platelet counts and bleeding diatheses of varying severities. The identification of the underlying cause of IPDs is clinically challenging due to the absence of a gold-standard platelet test, and is often based on a clinical presentation and normal values in other hematology assays. As a consequence, a DNA-based approach has a potentially important role in the investigation of these patients. Next-generation sequencing (NGS) technologies are allowing the rapid analysis of genes that have been previously implicated in IPDs or that are known to have a key role in platelet regulation, as well as novel genes that have not been previously implicated in platelet dysfunction. The potential limitations of NGS arise with the interpretation of the sheer volume of genetic information obtained from whole exome sequencing (WES) or whole genome sequencing (WGS) in order to identify function-disrupting variants. Following on from bioinformatic analysis, a number of candidate genetic variants usually remain, therefore adding to the difficulty of phenotype–genotype segregation verification. Linking genetic changes to an underlying bleeding disorder is an ongoing challenge and may not always be feasible due to the multifactorial nature of IPDs. Nevertheless, NGS will play a key role in our understanding of the mechanisms of platelet function and the genetics involved.

Whole exome sequencing identifies genetic variants in inherited thrombocytopenia with secondary qualitative function defects

Inherited thrombocytopenias are a heterogeneous group of disorders characterized by abnormally low platelet counts which can be associated with abnormal bleeding. Next-generation sequencing has previously been employed in these disorders for the confirmation of suspected genetic abnormalities, and more recently in the discovery of novel disease-causing genes. However its full potential has not yet been exploited. Over the past 6 years we have sequenced the exomes from 55 patients, including 37 index cases and 18 additional family members, all of whom were recruited to the UK Genotyping and Phenotyping of Platelets study. All patients had inherited or sustained thrombocytopenia of unknown etiology with platelet counts varying from 11×10⁹/L to 186×10⁹/L. Of the 51 patients phenotypically tested, 37 (73%), had an additional secondary qualitative platelet defect. Using whole exome sequencing analysis we have identified "pathogenic" or "likely pathogenic" variants in 46% (17/37) of our index patients with thrombocytopenia. In addition, we report variants of uncertain significance in 12 index cases, including novel candidate genetic variants in previously unreported genes in four index cases. These results demonstrate that whole exome sequencing is an efficient method for elucidating potential pathogenic genetic variants in inherited thrombocytopenia. Whole exome sequencing also has the added benefit of discovering potentially pathogenic genetic variants for further study in novel genes not previously implicated in inherited thrombocytopenia.