Germline mutations in the transcription factor IKZF5 cause thrombocytopenia

Identification and Functional Analysis of a de novo IKZF3 Mutation in a Pediatric Patient with Combined Immunodeficiency

AIOLOS, a vital member of the IKAROS protein family, plays a significant role in lymphocyte development and function through DNA binding and protein-protein interactions. Mutations in the IKZF3 gene, which encodes AIOLOS, lead to a rare combined immunodeficiency often linked with infections and malignancy. In this study, we evaluated a 1-year-4-month-old female patient presenting with recurrent infections, diarrhea, and failure to thrive. Laboratory investigations revealed decreased T lymphocyte and immunoglobulin levels. Through whole-exome and Sanger sequencing, we discovered a de novo mutation in IKZF3 (NM_012481; exon 5 c.571G > C, p.Gly191Arg), corresponding to the third DNA-binding zinc finger region of the encoded protein AIOLOS. Notably, the patient with the AIOLOS G191R mutation showed reduced recent thymic emigrants in naïve CD4+T cells compared to healthy counterparts of the same age, while maintaining normal levels of Th1, Th2, Th17, Treg, and Tfh cells. This mutation also resulted in decreased switched memory B cells and lower CD23 and IgM expression. In vitro studies revealed that AIOLOS G191R does not impact the expression of AIOLOS but compromises its stability, DNA binding and pericentromeric targeting. Furthermore, AIOLOS G191R demonstrated a dominant-negative effect over the wild-type protein. This case represents the first reported instance of a mutation in the third DNA-binding zinc finger region of AIOLOS highlighting its pivotal role in immune cell functionality.

Evolution of the Ikaros family transcription factors: From a deuterostome ancestor to humans

Ikaros family proteins (Ikaros, Helios, Aiolos, Eos) are zinc finger transcription factors essential for the development and function of the adaptive immune system. They also control developmental events in neurons and other cell types, suggesting that they possess crucial functions across disparate cell types. These functions are likely shared among the organisms in which these factors exist, and it is thus important to obtain a view of their distribution and conservation across organisms. How this family evolved remains poorly understood. Here we mined protein, mRNA and DNA databases to identify proteins with DNA-binding domains homologous to that of Ikaros. We show that Ikaros-related proteins exist in organisms from all four deuterostome phyla (chordates, echinoderms, hemichordates, xenacoelomorpha), but not in more distant groups. While most non-vertebrates have a single family member, this family grew to six members in the acoel worm Hofstenia miamia, three in jawless and four in jawed vertebrates. Most residues involved in DNA contact from zinc fingers 2 to 4 were identical across the Ikaros family, suggesting conserved mechanisms for target sequence recognition. Further, we identified a novel KRKxxxPxK/R motif that inhibits DNA binding in vitro which was conserved across the deuterostome phyla. We also identified a EψψxxxψM(D/E)QAIxxAIxYLGA(D/E)xL motif conserved among human Ikaros, Aiolos, Helios and subsets of chordate proteins, and motifs that are specific to subsets of vertebrate family members. Some of these motifs are targets of mutations in human patients. Finally we show that the atypical family member Pegasus emerged only in vertebrates, which is consistent with its function in bone. Our data provide a novel evolutionary perspective for Ikaros family proteins and suggest that they have conserved regulatory functions across deuterostomes.

A review of the role of zinc finger proteins on hematopoiesis

The bone marrow is responsible for producing an incredible number of cells daily in order to maintain blood homeostasis through a process called hematopoiesis. Hematopoiesis is a greatly demanding process and one entirely dependent on complex interactions between the hematopoietic stem cell (HSC) and its surrounding microenvironment. Zinc (Zn2+) is considered an important trace element, playing diverse roles in different tissues and cell types, and zinc finger proteins (ZNF) are proteins that use Zn2+ as a structural cofactor. In this way, the ZNF structure is supported by a Zn2+ that coordinates many possible combinations of cysteine and histidine, with the most common ZNF being of the Cys2His2 (C2H2) type, which forms a family of transcriptional activators that play an important role in different cellular processes such as development, differentiation, and suppression, all of these being essential processes for an adequate hematopoiesis. This review aims to shed light on the relationship between ZNF and the regulation of the hematopoietic tissue. We include works with different designs, including both in vitro and in vivo studies, detailing how ZNF might regulate hematopoiesis.

Pathogenic human variant that dislocates GATA2 zinc fingers disrupts hematopoietic gene expression and signaling networks

Although certain human genetic variants are conspicuously loss of function, decoding the impact of many variants is challenging. Previously, we described a patient with leukemia predisposition syndrome (GATA2 deficiency) with a germline GATA2 variant that inserts 9 amino acids between the 2 zinc fingers (9aa-Ins). Here, we conducted mechanistic analyses using genomic technologies and a genetic rescue system with Gata2 enhancer-mutant hematopoietic progenitor cells to compare how GATA2 and 9aa-Ins function genome-wide. Despite nuclear localization, 9aa-Ins was severely defective in occupying and remodeling chromatin and regulating transcription. Variation of the inter-zinc finger spacer length revealed that insertions were more deleterious to activation than repression. GATA2 deficiency generated a lineage-diverting gene expression program and a hematopoiesis-disrupting signaling network in progenitors with reduced granulocyte-macrophage colony-stimulating factor (GM-CSF) and elevated IL-6 signaling. As insufficient GM-CSF signaling caused pulmonary alveolar proteinosis and excessive IL-6 signaling promoted bone marrow failure and GATA2 deficiency patient phenotypes, these results provide insight into mechanisms underlying GATA2-linked pathologies.

The Analysis of the Human Megakaryocyte and Platelet Coding Transcriptome in Healthy and Diseased Subjects

Platelets are generated and released into the bloodstream from their precursor cells, megakaryocytes that reside in the bone marrow. Though platelets have no nucleus or DNA, they contain a full transcriptome that, during platelet formation, is transported from the megakaryocyte to the platelet. It has been described that transcripts in platelets can be translated into proteins that influence platelet response. The platelet transcriptome is highly dynamic and has been extensively studied using microarrays and, more recently, RNA sequencing (RNA-seq) in relation to diverse conditions (inflammation, obesity, cancer, pathogens and others). In this review, we focus on bulk and single-cell RNA-seq studies that have aimed to characterize the coding transcriptome of healthy megakaryocytes and platelets in humans. It has been noted that bulk RNA-seq has limitations when studying in vitro-generated megakaryocyte cultures that are highly heterogeneous, while single-cell RNA-seq has not yet been applied to platelets due to their very limited RNA content. Next, we illustrate how these methods can be applied in the field of inherited platelet disorders for gene discovery and for unraveling novel disease mechanisms using RNA from platelets and megakaryocytes and rare disease bioinformatics. Next, future perspectives are discussed on how this field of coding transcriptomics can be integrated with other next-generation technologies to decipher unexplained inherited platelet disorders in a multiomics approach.

Genetics of inherited thrombocytopenias

The inherited thrombocytopenia syndromes are a group of disorders characterized primarily by quantitative defects in platelet number, though with a variety demonstrating qualitative defects and/or extrahematopoietic findings. Through collaborative international efforts applying next-generation sequencing approaches, the list of genetic syndromes that cause thrombocytopenia has expanded significantly in recent years, now with over 40 genes implicated. In this review, we focus on what is known about the genetic etiology of inherited thrombocytopenia syndromes and how the field has worked to validate new genetic discoveries. We highlight the important role for the clinician in identifying a germline genetic diagnosis and strategies for identifying novel causes through research-based endeavors.

AIOLOS Variants Causing Immunodeficiency in Human and Mice

AIOLOS is encoded by IKZF3 and is a member of the IKAROS zinc finger transcription factor family. Heterozygous missense variants in the second zinc finger of AIOLOS have recently been reported to be found in the families of patients with inborn errors of immunity. The AIOLOS^G159R variant was identified in patients with B-lymphopenia and familial Epstein–Barr virus-associated lymphoma. Early B-cell progenitors were significantly reduced in the bone marrow of patients with AIOLOS^G159R. Another variant, AIOLOS^N160S was identified in the patients presented with hypogammaglobulinemia, susceptibility to Pneumocystis jirovecii pneumonia, and chronic lymphocytic leukemia. Patients with AIOLOS^N160S had mostly normal B cell counts but showed increased levels of CD21^lo B cells, decreased CD23 expression, and abrogated CD40 response. Both variants were determined to be loss-of-function. Mouse models harboring the corresponding patient’s variants recapitulated the phenotypes of the patients. AIOLOS is therefore a novel disease-causing gene in human adaptive immune deficiency.

Screening and diagnosis of inherited platelet disorders

Inherited platelet disorders are important conditions that often manifest with bleeding. These disorders have heterogeneous underlying pathologies. Some are syndromic disorders with non-blood phenotypic features, and others are associated with an increased predisposition to developing myelodysplasia and leukemia. Platelet disorders can present with thrombocytopenia, defects in platelet function, or both. As the underlying pathogenesis of inherited thrombocytopenias and platelet function disorders are quite diverse, their evaluation requires a thorough clinical assessment and specialized diagnostic tests, that often challenge diagnostic laboratories. At present, many of the commonly encountered, non-syndromic platelet disorders do not have a defined molecular cause. Nonetheless, significant progress has been made over the past few decades to improve the diagnostic evaluation of inherited platelet disorders, from the assessment of the bleeding history to improved standardization of light transmission aggregometry, which remains a "gold standard" test of platelet function. Some platelet disorder test findings are highly predictive of a bleeding disorder and some show association to symptoms of prolonged bleeding, surgical bleeding, and wound healing problems. Multiple assays can be required to diagnose common and rare platelet disorders, each requiring control of preanalytical, analytical, and post-analytical variables. The laboratory investigations of platelet disorders include evaluations of platelet counts, size, and morphology by light microscopy; assessments for aggregation defects; tests for dense granule deficiency; analyses of granule constituents and their release; platelet protein analysis by immunofluorescent staining or flow cytometry; tests of platelet procoagulant function; evaluations of platelet ultrastructure; high-throughput sequencing and other molecular diagnostic tests. The focus of this article is to review current methods for the diagnostic assessment of platelet function, with a focus on contemporary, best diagnostic laboratory practices, and relationships between clinical and laboratory findings.

Ikaros Proteins in Tumor: Current Perspectives and New Developments

Ikaros is a zinc finger transcription factor (TF) of the Krüppel family member, which significantly regulates normal lymphopoiesis and tumorigenesis. Ikaros can directly initiate or suppress tumor suppressors or oncogenes, consequently regulating the survival and proliferation of cancer cells. Over recent decades, a series of studies have been devoted to exploring and clarifying the relationship between Ikaros and associated tumors. Therapeutic strategies targeting Ikaros have shown promising therapeutic effects in both pre-clinical and clinical trials. Nevertheless, the increasingly prominent problem of drug resistance targeted to Ikaros and its analog is gradually appearing in our field of vision. This article reviews the role of Ikaros in tumorigenesis, the mechanism of drug resistance, the progress of targeting Ikaros in both pre-clinical and clinical trials, and the potential use of associated therapy in cancer therapy.

Hemostatic phenotypes and genetic disorders

This review is focused on genetic regulators of bleeding and thrombosis with a focus on next-generation sequencing (NGS) technologies for diagnosis and research of patients with inherited disorders. The molecular diagnosis of hemostatic phenotypes relies on the detection of genetic variants in the 99 curated disease-causing genes implicated for bleeding, platelet, and thrombotic disorders through the use of multigene panel tests. In this review, we will provide an overview of the advantages and disadvantages of using such multigene panel tests for diagnostics. During the past decade, NGS technologies have also been used for the gene discovery of 32 novel genes involved in inherited hemostatic phenotypes. We will provide a brief overview of these genes and discuss what information (eg, linkage, consanguinity, multiple index cases with similar phenotypes, mouse models, and more) was used to support the gene discovery process. Next, we provide examples on how RNA sequencing is useful to explore disease mechanisms of novel and often unexpected genes. This review will summarize the important findings concerning NGS technologies for diagnostics and gene discovery that were presented at the ISTH 2021 conference. Finally, future perspectives in our field mainly deal with finding the needle in the haystack for some still unexplained patients and the need for exploring the noncoding gene space and rapid disease validation models.

Megakaryopoiesis and Platelet Biology: Roles of Transcription Factors and Emerging Clinical Implications

Platelets play a critical role in hemostasis and thrombus formation. Platelets are small, anucleate, and short-lived blood cells that are produced by the large, polyploid, and hematopoietic stem cell (HSC)-derived megakaryocytes in bone marrow. Approximately 3000 platelets are released from one megakaryocyte, and thus, it is important to understand the physiologically relevant mechanism of development of mature megakaryocytes. Many genes, including several key transcription factors, have been shown to be crucial for platelet biogenesis. Mutations in these genes can perturb megakaryopoiesis or thrombopoiesis, resulting in thrombocytopenia. Metabolic changes owing to inflammation, ageing, or diseases such as cancer, in which platelets play crucial roles in disease development, can also affect platelet biogenesis. In this review, I describe the characteristics of platelets and megakaryocytes in terms of their differentiation processes. The role of several critical transcription factors have been discussed to better understand the changes in platelet biogenesis that occur during disease or ageing.

Inborn errors of IKAROS and AIOLOS

IKAROS is a pioneer protein of the IKZF family of transcription factors that plays an essential role in lymphocyte development. Recently, inborn errors of IKAROS have been identified in patients with B cell deficiency and hypogammaglobulinemia, and these patients often present with recurrent sinopulmonary infection. Autoimmunity and hematologic malignancies are other characteristic complications seen in the patients with IKAROS deficiency. Missense mutation involving asparagine at the 159th position results in combined immunodeficiency, often presenting with Pneumocystis jirovecii pneumonia. Inborn errors of AIOLOS, HELIOS, and PEGASUS have also been reported in patients with B cell deficiency, Evans syndrome, and hereditary thrombocytopenia, respectively. Here, we briefly review the phenotype and genotype of IKZF mutations, especially IKAROS.

Inherited Platelet Disorders: An Updated Overview

Platelets play a major role in hemostasis as ppwell as in many other physiological and pathological processes. Accordingly, production of about 10¹¹ platelet per day as well as appropriate survival and functions are life essential events. Inherited platelet disorders (IPDs), affecting either platelet count or platelet functions, comprise a heterogenous group of about sixty rare diseases caused by molecular anomalies in many culprit genes. Their clinical relevance is highly variable according to the specific disease and even within the same type, ranging from almost negligible to life-threatening. Mucocutaneous bleeding diathesis (epistaxis, gum bleeding, purpura, menorrhagia), but also multisystemic disorders and/or malignancy comprise the clinical spectrum of IPDs. The early and accurate diagnosis of IPDs and a close patient medical follow-up is of great importance. A genotype-phenotype relationship in many IPDs makes a molecular diagnosis especially relevant to proper clinical management. Genetic diagnosis of IPDs has been greatly facilitated by the introduction of high throughput sequencing (HTS) techniques into mainstream investigation practice in these diseases. However, there are still unsolved ethical concerns on general genetic investigations. Patients should be informed and comprehend the potential implications of their genetic analysis. Unlike the progress in diagnosis, there have been no major advances in the clinical management of IPDs. Educational and preventive measures, few hemostatic drugs, platelet transfusions, thrombopoietin receptor agonists, and in life-threatening IPDs, allogeneic hematopoietic stem cell transplantation are therapeutic possibilities. Gene therapy may be a future option. Regular follow-up by a specialized hematology service with multidisciplinary support especially for syndromic IPDs is mandatory.

Learning the Ropes of Platelet Count Regulation: Inherited Thrombocytopenias

Inherited thrombocytopenias (IT) are a group of hereditary disorders characterized by a reduced platelet count sometimes associated with abnormal platelet function, which can lead to bleeding but also to syndromic manifestations and predispositions to other disorders. Currently at least 41 disorders caused by mutations in 42 different genes have been described. The pathogenic mechanisms of many forms of IT have been identified as well as the gene variants implicated in megakaryocyte maturation or platelet formation and clearance, while for several of them the pathogenic mechanism is still unknown. A range of therapeutic approaches are now available to improve survival and quality of life of patients with IT; it is thus important to recognize an IT and establish a precise diagnosis. ITs may be difficult to diagnose and an initial accurate clinical evaluation is mandatory. A combination of clinical and traditional laboratory approaches together with advanced sequencing techniques provide the highest rate of diagnostic success. Despite advancement in the diagnosis of IT, around 50% of patients still do not receive a diagnosis, therefore further research in the field of ITs is warranted to further improve patient care.

Advances in germline predisposition to acute leukaemias and myeloid neoplasms

Although much work has focused on the elucidation of somatic alterations that drive the development of acute leukaemias and other haematopoietic diseases, it has become increasingly recognized that germline mutations are common in many of these neoplasms. In this Review, we highlight the different genetic pathways impacted by germline mutations that can ultimately lead to the development of familial and sporadic haematological malignancies, including acute lymphoblastic leukaemia, acute myeloid leukaemia (AML) and myelodysplastic syndrome (MDS). Many of the genes disrupted by somatic mutations in these diseases (for example, TP53, RUNX1, IKZF1 and ETV6) are the same as those that harbour germline mutations in children and adolescents who develop these malignancies. Moreover, the presumption that familial leukaemias only present in childhood is no longer true, in large part due to the numerous studies demonstrating germline DDX41 mutations in adults with MDS and AML. Lastly, we highlight how different cooperating events can influence the ultimate phenotype in these different familial leukaemia syndromes.

Inherited thrombocytopenias: an updated guide for clinicians

The great advances in the knowledge of inherited thrombocytopenias (ITs) made since the turn of the century have significantly changed our view of these conditions. To date, ITs encompass 45 disorders with different degrees of complexity of the clinical picture and very wide variability in the prognosis. They include forms characterized by thrombocytopenia alone, forms that present with other congenital defects, and conditions that predispose to acquire additional diseases over the course of life. In this review, we recapitulate the clinical features of ITs with emphasis on the forms predisposing to additional diseases. We then discuss the key issues for a rational approach to the diagnosis of ITs in clinical practice. Finally, we aim to provide an updated and comprehensive guide to the treatment of ITs, including the management of hemostatic challenges, the treatment of severe forms, and the approach to the manifestations that add to thrombocytopenia.

Novel manifestations of immune dysregulation and granule defects in gray platelet syndrome

Gray platelet syndrome (GPS) is a rare recessive disorder caused by biallelic variants in NBEAL2 and characterized by bleeding symptoms, the absence of platelet α-granules, splenomegaly, and bone marrow (BM) fibrosis. Due to the rarity of GPS, it has been difficult to fully understand the pathogenic processes that lead to these clinical sequelae. To discern the spectrum of pathologic features, we performed a detailed clinical genotypic and phenotypic study of 47 patients with GPS and identified 32 new etiologic variants in NBEAL2. The GPS patient cohort exhibited known phenotypes, including macrothrombocytopenia, BM fibrosis, megakaryocyte emperipolesis of neutrophils, splenomegaly, and elevated serum vitamin B12 levels. Novel clinical phenotypes were also observed, including reduced leukocyte counts and increased presence of autoimmune disease and positive autoantibodies. There were widespread differences in the transcriptome and proteome of GPS platelets, neutrophils, monocytes, and CD4 lymphocytes. Proteins less abundant in these cells were enriched for constituents of granules, supporting a role for Nbeal2 in the function of these organelles across a wide range of blood cells. Proteomic analysis of GPS plasma showed increased levels of proteins associated with inflammation and immune response. One-quarter of plasma proteins increased in GPS are known to be synthesized outside of hematopoietic cells, predominantly in the liver. In summary, our data show that, in addition to the well-described platelet defects in GPS, there are immune defects. The abnormal immune cells may be the drivers of systemic abnormalities such as autoimmune disease.

Updates in diagnosis of the inherited platelet disorders

PURPOSE OF REVIEW

To provide a comprehensive update on the current available methodologies and techniques for diagnosis of inherited platelet disorders (IPD).

RECENT FINDINGS

The contributions of many groups have resulted in the significant progress in the molecular diagnosis of IPD including the identification of many genes responsible for the various phenotypes. The widespread use and availability of next-generation sequencing has brought to the forefront ethical challenges associated with nontargeted sequencing as well as provided us with novel variants to functionally validate. These requirements have driven the development of novel tools for functional assessment of platelets, although none of the novel techniques beyond sequencing have yet taken clinical hold.

SUMMARY

Much work is ongoing on functional and molecular assessment of platelet disorders and the incorporation of combined assessments is likely to yield the highest diagnostic results.

Bleeding risks for uncharacterized platelet function disorders

BACKGROUND

The bleeding risks for nonsyndromic platelet function disorders (PFDs) that impair aggregation responses and/or cause dense granule deficiency (DGD) are uncertain.

OBJECTIVES

Our goal was to quantify bleeding risks for a cohort of consecutive cases with uncharacterized PFD.

METHODS

Sequential cases with uncharacterized PFDs that had reduced maximal aggregation (MA) with multiple agonists and/or nonsyndromic DGD were invited to participate along with additional family members to reduce bias. Index cases were further evaluated by exome sequencing, with analysis of RUNX1-dependent genes for cases with RUNX1 sequence variants. Bleeding assessment tools were used to estimate bleeding scores, with bleeding risks estimated as odds ratios (ORs) relative to general population controls. Relationships between symptoms and laboratory findings were also explored.

RESULTS

Participants with uncharacterized PFD (n = 37; 23 index cases) had impaired aggregation function (70%), nonsyndromic DGD (19%) or both (11%), unlike unaffected relatives. Probable pathogenic RUNX1 variants were found in 2 (9%) index cases/families, whereas others had PFD of unknown cause. Participants with PFD had increased bleeding scores compared to unaffected family members and general population controls, and increased risks for mucocutaneous (OR, 4-207) and challenge-related bleeding (OR, 12-43), and for receiving transfusions for bleeding (OR, 100). Reduced MA with collagen was associated with wound healing problems and bruising, and more severe DGD was associated with surgical bleeding (P < .04).

CONCLUSIONS

PFDs that impair MA and/or cause nonsyndromic DGD have significantly increased bleeding risks, and some symptoms are more common in those with more severe DGD or impaired collagen aggregation.

Whole-genome sequencing of patients with rare diseases in a national health system

Most patients with rare diseases do not receive a molecular diagnosis and the aetiological variants and causative genes for more than half such disorders remain to be discovered1. Here we used whole-genome sequencing (WGS) in a national health system to streamline diagnosis and to discover unknown aetiological variants in the coding and non-coding regions of the genome. We generated WGS data for 13,037 participants, of whom 9,802 had a rare disease, and provided a genetic diagnosis to 1,138 of the 7,065 extensively phenotyped participants. We identified 95 Mendelian associations between genes and rare diseases, of which 11 have been discovered since 2015 and at least 79 are confirmed to be aetiological. By generating WGS data of UK Biobank participants2, we found that rare alleles can explain the presence of some individuals in the tails of a quantitative trait for red blood cells. Finally, we identified four novel non-coding variants that cause disease through the disruption of transcription of ARPC1B, GATA1, LRBA and MPL. Our study demonstrates a synergy by using WGS for diagnosis and aetiological discovery in routine healthcare.

Inherited thrombocytopenias: history, advances and perspectives

Over the last 100 years the role of platelets in hemostatic events and their production by megakaryocytes have gradually been defined. Progressively, thrombocytopenia was recognized as a cause of bleeding, first through an acquired immune disorder; then, since 1948, when Bernard-Soulier syndrome was first described, inherited thrombocytopenia became a fascinating example of Mendelian disease. The platelet count is often severely decreased and platelet size variable; associated platelet function defects frequently aggravate bleeding. Macrothrombocytopenia with variable proportions of enlarged platelets is common. The number of circulating platelets will depend on platelet production, consumption and lifespan. The bulk of macrothrombocytopenias arise from defects in megakaryopoiesis with causal variants in transcription factor genes giving rise to altered stem cell differentiation and changes in early megakaryocyte development and maturation. Genes encoding surface receptors, cytoskeletal and signaling proteins also feature prominently and Sanger sequencing associated with careful phenotyping has allowed their early classification. It quickly became apparent that many inherited thrombocytopenias are syndromic while others are linked to an increased risk of hematologic malignancies. In the last decade, the application of next-generation sequencing, including whole exome sequencing, and the use of gene platforms for rapid testing have greatly accelerated the discovery of causal genes and extended the list of variants in more common disorders. Genes linked to an increased platelet turnover and apoptosis have also been identified. The current challenges are now to use next-generation sequencing in first-step screening and to define bleeding risk and treatment better.

Highly impaired platelet ultrastructure in two families with novel IKZF5 variants

Heterozygous variants in the IKZF5 gene, encoding transcription factor Pegasus, were recently discovered to be causal of inherited thrombocytopenia (IT). We screened 90 patients suspected of inherited thrombocytopenia for variants in 101 genes associated with inherited bleeding disorders and report the clinical presentation of two Danish families with novel variants in IKZF5. Platelet ultrastructure and cytoskeleton were evaluated by immunofluorescent microscopy (IF) and found to be highly abnormal, demonstrating severe disturbances of distribution and expression of non-muscular myosin, filamin, β-tubulin and α tubulin. Number of alpha granules were reduced, and platelets elongated when evaluated by TEM. In both families a child carrying a rare IKZF5 variant was affected by developmental delay. The proband of family A presented with recurrent infections and was examined for an immunodeficiency. The concentration of naive B-cells was found moderately reduced by leucocyte subpopulation examination, indicating an impaired cellular immunity. T-cells were marginally low with reduced share and concentration of CD45RApos, CD31pos, CD4pos recent thymic immigrants as signs of reduced thymic output. The novel IKZF5 variants co-segregated with thrombocytopenia in both families and both probands had significant bleeding tendency. Through comprehensive characterizations of the platelet morphology and function linked to the specific phenotypes we add novel insight to IKZF5-associated thrombocytopenia, which may help to identify and classify more cases with IKZF5 associated IT.

Diagnosis of Inherited Platelet Disorders on a Blood Smear

Inherited platelet disorders (IPDs) are rare diseases featured by low platelet count and defective platelet function. Patients have variable bleeding diathesis and sometimes additional features that can be congenital or acquired. Identification of an IPD is desirable to avoid misdiagnosis of immune thrombocytopenia and the use of improper treatments. Diagnostic tools include platelet function studies and genetic testing. The latter can be challenging as the correlation of its outcomes with phenotype is not easy. The immune-morphological evaluation of blood smears (by light- and immunofluorescence microscopy) represents a reliable method to phenotype subjects with suspected IPD. It is relatively cheap, not excessively time-consuming and applicable to shipped samples. In some forms, it can provide a diagnosis by itself, as for MYH9-RD, or in addition to other first-line tests as aggregometry or flow cytometry. In regard to genetic testing, it can guide specific sequencing. Since only minimal amounts of blood are needed for the preparation of blood smears, it can be used to characterize thrombocytopenia in pediatric patients and even newborns further. In principle, it is based on visualizing alterations in the distribution of proteins, which result from specific genetic mutations by using monoclonal antibodies. It can be applied to identify deficiencies in membrane proteins, disturbed distribution of cytoskeletal proteins, and alpha as well as delta granules. On the other hand, mutations associated with impaired signal transduction are difficult to identify by immunofluorescence of blood smears. This review summarizes technical aspects and the main diagnostic patterns achievable by this method.