The thrombocytopenia of WAS: a familial form of ITP?

Next generation sequencing (NGS)-based approach to diagnosing Algerian patients with suspected inborn errors of immunity (IEIs)

The advent of next-generation sequencing (NGS) technologies has greatly expanded our understanding of both the clinical spectra and genetic landscape of inborn errors of immunity (IEIs). Endogamous populations may be enriched for unique, ancestry-specific disease-causing variants, a consideration that significantly impacts molecular testing and analysis strategies. Herein, we report on the application of a 2-step NGS-based testing approach beginning with targeted gene panels (TGPs) tailored to specific IEI subtypes and reflexing to whole exome sequencing (WES) if negative for Northwest Algerian patients with suspected IEIs. Our overall diagnostic yield of 57% is comparable to others broadly applying short-read NGS to IEI detection, but data from our localized cohort show some similarities and differences from NGS studies performed on larger regional IEI cohorts. This suggests the importance of tailoring diagnostic strategies to local demographics and needs, but also highlights ongoing concerns inherent to the application of genomics for clinical IEI diagnostics.

A Review on Secondary Immune Thrombocytopenia in Malaysia

Immune thrombocytopenia (ITP) is an acquired autoimmune disease that occurs in adults and children. In Malaysia, the clinical practice guideline (CPG) for the management of ITP was issued in 2006, which focused almost exclusively on primary ITP (pITP), and only a few secondary ITP (sITP) forms were addressed. All published (twenty-three) sITP articles among children and adults in Malaysia, identified on the academic databases were retrieved. The articles were published between 1981 and 2019, at a rate of 0.62 publications per year. The publications were considered low and mainly focused on rare presentation and followed-up of secondary diseases. This review revealed that sITP in Malaysia is commonly associated with autoimmune diseases (Evan’s syndrome, SLE and WAS), malignancy (Kaposi’s sarcoma and breast cancer) and infection (dengue haemorrhagic fever, Helicobacter pylori and hepatitis C virus). The relationship between ITP and autoimmune diseases, malignancy and infections raise the question concerning the mechanism involved in these associations. Further studies should be conducted to bridge the current knowledge gap, and the further information is required to update the existing CPG of management of ITP in Malaysia.

When WAS Gene Diagnosis Is Needed: Seeking Clues Through Comparison Between Patients With Wiskott-Aldrich Syndrome and Idiopathic Thrombocytopenic Purpura

Background: Wiskott-Aldrich syndrome (WAS) is a rare and severe X-linked disorder with variable clinical phenotypes correlating with the type of mutations in the WAS gene. The syndrome is difficult to differentiate from idiopathic thrombocytopenic purpura (ITP) before genetic diagnosis. We retrospectively reviewed patients suspected to have WAS who were referred to our hospital from 2004 to 2016 and compared the clinical features and laboratory examination of genetically confirmed WAS patients and of patients diagnosed with ITP in order to seek some clues to distinguish WAS and ITP before genetic diagnosis.

Methods: Seventy-eight children suspected to have WAS from 78 unrelated families were enrolled in this study. The clinical data and laboratory examination of children were reviewed in the present study. The distribution of lymphocyte subsets from peripheral blood was examined by how cytometry. WASP mutations were identified by direct sequencing of PCR-amplified genomic DNA.

Results: Forty-two patients were finally diagnosed with WAS genetically. The median onset age of these patients was 1 month (range: 1 day−10 months). The median diagnosis lag was 4.6 months (range: 0 months−9.42 years). Fifteen patients (35.71%) had positive family histories. More than half of the patients (n = 23, 54.76%) had diarrhea. Twenty-three (54.76%) had pneumonia, 7 with severe symptoms. Major bleeding events included skin spots or petechiae (n = 27, 64.29%), per-rectal bleeding (n = 21, 50.00%), epistaxis (n = 7, 16.67%) and intracranial bleeding (n = 2, 4.76%). Twenty-nine patients (69.05%) had eczema, and one patient had a drug allergy. Three patients had autoimmune diseases, among whom 2 had autoimmune hemolytic anemia and one had autoimmune hemolytic anemia and IgA nephropathy. A total of 42 mutations in WASP were identified, including 19 novel mutations. Eight patients received hematopoietic stem cell transplantation (HSCT) and all survived. Compared with the 30 patients diagnosed with ITP, the WAS patients had higher EOS counts and elevated IgE level, increased NK cell numbers but fewer CD8⁺T lymphocytes.

Conclusion: The WAS gene diagnosis should be considered in all males with ITP-like features, especially for patients with a very early onset age, decreased MPV (<6.5 fl), higher EOS counts and elevated IgE level, increased NK cell number, diminished CD8⁺T lymphocyte count.

Autoimmunity in primary T-cell immunodeficiencies

Primary immunodeficiency diseases (PID) are a genetically heterogeneous group of more than 270 disorders that affect distinct components of both humoral and cellular arms of the immune system. Primary T cell immunodeficiencies affect subjects at the early age of life. In most cases, T-cell PIDs become apparent as combined T- and B-cell deficiencies. Patients with T-cell PID are prone to life-threatening infections. On the other hand, non-infectious complications such as lymphoproliferative diseases, cancers and autoimmunity seem to be associated with the primary T-cell immunodeficiencies. Autoimmune disorders of all kinds (organ specific or systemic ones) could be subjected to this class of PIDs; however, the most frequent autoimmune disorders are immune thrombocytopenic purpura (ITP) and autoimmune hemolytic anemia (AIHA). In this review, we discuss the proposed mechanisms of autoimmunity and review the literature reported on autoimmune disorder in each type of primary T-cell immunodeficiencies.

Absence of WASp Enhances Hematopoietic and Megakaryocytic Differentiation in a Human Embryonic Stem Cell Model

The Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency caused by mutations in the WAS gene and characterized by severe thrombocytopenia. Although the role of WASp in terminally differentiated lymphocytes and myeloid cells is well characterized, its role in early hematopoietic differentiation and in platelets (Plts) biology is poorly understood. In the present manuscript, we have used zinc finger nucleases targeted to the WAS locus for the development of two isogenic WAS knockout (WASKO) human embryonic stem cell lines (hESCs). Upon hematopoietic differentiation, hESCs-WASKO generated increased ratios of CD34⁺CD45⁺ progenitors with altered responses to stem cell factor compared to hESCs-WT. When differentiated toward the megakaryocytic linage, hESCs-WASKO produced increased numbers of CD34⁺CD41⁺ progenitors, megakaryocytes (MKs), and Plts. hESCs-WASKO-derived MKs and Plts showed altered phenotype as well as defective responses to agonist, mimicking WAS patients MKs and Plts defects. Interestingly, the defects were more evident in WASp-deficient MKs than in WASp-deficient Plts. Importantly, ectopic WAS expression using lentiviral vectors restored normal Plts development and MKs responses. These data validate the AND-1_WASKO cell lines as a human cellular model for basic research and for preclinical studies for WAS.

Clinical spectrum, pathophysiology and treatment of the Wiskott-Aldrich syndrome

PURPOSE OF REVIEW

The Wiskott-Aldrich syndrome (WAS), caused by mutations in the WAS gene, is a complex and diverse disorder with X-linked inheritance. This review focuses on recent developments in the understanding of its basic pathophysiology, diverse clinical phenotypes and optimal patient management including novel therapies.

RECENT FINDINGS

The protein encoded by the WAS gene is a multifunctional signaling element expressed in immune and hematopoietic cells that plays a critical role in cytoskeletal reorganization, immune synapse formation and intracellular signaling. The type of specific mutation, its location within the gene and its effect on protein expression play a major role in determining an individual patient's clinical phenotype. Recent clinical observations and molecular studies have created a sophisticated picture of the disease spectrum. The improved outcome of stem cell transplantation from related and unrelated matched donors and promising early results from the first clinical gene therapy trial have added new therapeutic options for these patients.

SUMMARY

Classic WAS, X-linked thrombocytopenia and X-linked neutropenia are caused by WAS gene mutations, each having a distinct pattern of clinical symptoms and disease severity. New developments in the understanding of these syndromes and novel therapeutic options will have a major impact on the treatment of individuals with WAS mutations.

Loss of the F-BAR protein CIP4 reduces platelet production by impairing membrane-cytoskeleton remodeling

Megakaryocytes generate platelets through extensive reorganization of the cytoskeleton and plasma membrane. Cdc42 interacting protein 4 (CIP4) is an F-BAR protein that localizes to membrane phospholipids through its BAR domain and interacts with Wiskott-Aldrich Syndrome Protein (WASP) via its SRC homology 3 domain. F-BAR proteins promote actin polymerization and membrane tubulation. To study its function, we generated CIP4-null mice that displayed thrombocytopenia similar to that of WAS(-) mice. The number of megakaryocytes and their progenitors was not affected. However, the number of proplatelet protrusions was reduced in CIP4-null, but not WAS(-), megakaryocytes. Electron micrographs of CIP4-null megakaryocytes showed an altered demarcation membrane system. Silencing of CIP4, not WASP, expression resulted in fewer proplatelet-like extensions. Fluorescence anisotropy studies showed that loss of CIP4 resulted in a more rigid membrane. Micropipette aspiration demonstrated decreased cortical actin tension in megakaryocytic cells with reduced CIP4 or WASP protein. These studies support a new biophysical mechanism for platelet biogenesis whereby CIP4 enhances the complex, dynamic reorganization of the plasma membrane (WASP independent) and actin cortex network (as known for WASP and cortical actin) to reduce the work required for generating proplatelets. CIP4 is a new component in the highly coordinated system of megakaryocytic membrane and cytoskeletal remodeling affecting platelet production.

Biochemical and functional significance of F-BAR domain proteins interaction with WASP/N-WASP

The Bin-Amphiphysin-Rvs (BAR) domain family of proteins includes groups which promote positive (classical BAR, N-BAR, and F-BAR) and negative (I-BAR) membrane deformation. Of these groups, the F-BAR subfamily is the most diverse in its biochemical properties. F-BAR domain proteins dimerize to form a tight scaffold about the membrane. The F-BAR domain provides a banana-shaped, alpha-helical structure that senses membrane curvature. Different types of F-BAR domain proteins contain tyrosine kinase or GTPase activities; some interact with phosphatases and RhoGTPases. Most possess an SH3 domain that facilitates the recruitment and activation of WASP/N-WASP. Thus, F-BAR domain proteins affect remodeling of both membrane and the actin cytoskeleton. The purpose of this review is to highlight the role of F-BAR proteins in coupling WASP/N-WASP to cytoskeletal remodeling. A role for F-BAR/WASP interaction in human diseases affecting nervous, blood, and neoplastic tissues is discussed.

Use of zinc-finger nucleases to knock out the WAS gene in K562 cells: a human cellular model for Wiskott-Aldrich syndrome

Mutations in the WAS gene cause Wiskott-Aldrich syndrome (WAS), which is characterized by eczema, immunodeficiency and microthrombocytopenia. Although the role of WASP in lymphocytes and myeloid cells is well characterized, its role on megakaryocyte (MK) development is poorly understood. In order to develop a human cellular model that mimics the megakaryocytic-derived defects observed in WAS patients we used K562 cells, a well-known model for study of megakaryocytic development. We knocked out the WAS gene in K562 cells using a zinc-finger nuclease (ZFN) pair targeting the WAS intron 1 and a homologous donor DNA that disrupted WASP expression. Knockout of WASP on K562 cells (K562WASKO cells) resulted in several megakaryocytic-related defects such as morphological alterations, lower expression of CD41ɑ, lower increments in F-actin polymerization upon stimulation, reduced CD43 expression and increased phosphatidylserine exposure. All these defects have been previously described either in WAS-knockout mice or in WAS patients, validating K562WASKO as a cell model for WAS. However, K562WASPKO cells showed also increased basal F-actin and adhesion, increased expression of CD61 and reduced expression of TGFβ and Factor VIII, defects that have never been described before for WAS-deficient cells. Interestingly, these phenotypic alterations correlate with different roles for WASP in megakaryocytic differentiation. All phenotypic alterations observed in K562WASKO cells were alleviated upon expression of WAS following lentiviral transduction, confirming the role of WASP in these phenotypes. In summary, in this work we have validated a human cellular model, K562WASPKO, that mimics the megakaryocytic-related defects found in WAS-knockout mice and have found evidences for a role of WASP as regulator of megakaryocytic differentiation. We propose the use of K562WASPKO cells as a tool to study the molecular mechanisms involved in the megakaryocytic-related defects observed in WAS patients and as a cellular model to study new therapeutic strategies.

Mice lacking the ITIM-containing receptor G6b-B exhibit macrothrombocytopenia and aberrant platelet function

Platelets are highly reactive cell fragments that adhere to exposed extracellular matrix (ECM) and prevent excessive blood loss by forming clots. Paradoxically, megakaryocytes, which produce platelets in the bone marrow, remain relatively refractory to the ECM-rich environment of the bone marrow despite having the same repertoire of receptors as platelets. These include the ITAM (immunoreceptor tyrosine-based activation motif)-containing collagen receptor complex, which consists of glycoprotein VI (GPVI) and the Fc receptor γ-chain, and the ITIM (immunoreceptor tyrosine-based inhibition motif)-containing receptor G6b-B. We showed that mice lacking G6b-B exhibited macrothrombocytopenia (reduced platelet numbers and the presence of enlarged platelets) and a susceptibility to bleeding as a result of aberrant platelet production and function. Platelet numbers were markedly reduced in G6b-B-deficient mice compared to those in wild-type mice because of increased platelet turnover. Furthermore, megakaryocytes in G6b-B-deficient mice showed enhanced metalloproteinase production, which led to increased shedding of cell-surface receptors, including GPVI and GPIbα. In addition, G6b-B-deficient megakaryocytes exhibited reduced integrin-mediated functions and defective formation of proplatelets, the long filamentous projections from which platelets bud off. Together, these findings establish G6b-B as a major inhibitory receptor regulating megakaryocyte activation, function, and platelet production.

IVS6+5G>A found in Wiskott-Aldrich syndrome and X-linked thrombocytopenia in a Korean family

Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT) are caused by a mutation in the WAS gene on Xp11.22. We report two patients with IVS6+5G>A of WAS in a Korean family. The proband presented with classic WAS, whereas his maternal cousin had symptoms limited to XLT. Their mothers were proved to be carriers. The IVS6+5G>A mutation was reported to result in incomplete splicing of the donor site and typically associated with mild form of disease, XLT. Our observation of the intrafamilial variability of clinical manifestations of WAS further expands the genotype-phenotype correlations and suggests the presence of modifying genetic factors.

A unique presentation of Wiskott-Aldrich syndrome in relation to platelet size

Wiskott-Aldrich Syndrome (WAS) is a triad of immunodeficiency, eczema, and thrombocytopenia. Despite the heterogeneity of genetic and clinical findings, a correlation with small platelet size is routinely observed. Herein we describe a case with a unique phenotype that links normal mean platelet volume with the classic characteristics of this disease. The diagnosis was verified by genetic analysis showing a novel and de novo mutation. Our case illustrates that a high index of suspicion of WAS is warranted even in the setting of normal sized platelets.

Hematopoietic cell transplantation for Wiskott-Aldrich syndrome: advances in biology and future directions for treatment

The Wiskott-Aldrich syndrome (WAS) is an X-linked disorder characterized by a triad of diagnostic clinical elements: immunodeficiency, eczema, and hemorrhage caused by thrombocytopenia with small-sized platelets. The formal proof that hematopoietic cell transplantation (HCT) could be used to cure WAS revealed a requirement for both immunosuppression and myelosuppression that still underlies the standard approach to curative therapy today. The current short- and long-term toxicities of HCT are the main stumbling block for the ability to cure every patient with WAS and X-linked thrombocytopenia, and much remains to be done.

WASP plays a novel role in regulating platelet responses dependent on alphaIIbbeta3 integrin outside-in signalling

The most consistent feature of Wiskott Aldrich syndrome (WAS) is profound thrombocytopenia with small platelets. The responsible gene encodes WAS protein (WASP), which functions in leucocytes as an actin filament nucleating agent -yet- actin filament nucleation proceeds normally in patient platelets regarding shape change, filopodia and lamellipodia generation. Because WASP localizes in the platelet membrane skeleton and is mobilized by alphaIIbbeta3 integrin outside-in signalling, we questioned whether its function might be linked to integrin. Agonist-induced alphaIIbbeta3 activation (PAC-1 binding) was normal for patient platelets, indicating normal integrin inside-out signalling. Inside-out signalling (fibrinogen, JON/A binding) was also normal for wasp-deficient murine platelets. However, adherence/spreading on immobilized fibrinogen was decreased for patient platelets and wasp-deficient murine platelets, indicating decreased integrin outside-in responses. Another integrin outside-in dependent response, fibrin clot retraction, involving contraction of the post-aggregation actin cytoskeleton, was also decreased for patient platelets and wasp-deficient murine platelets. Rebleeding from tail cuts was more frequent for wasp-deficient mice, suggesting decreased stabilisation of the primary platelet plug. In contrast, phosphatidylserine exposure, a pro-coagulant response, was enhanced for WASP-deficient patient and murine platelets. The collective results reveal a novel function for WASP in regulating pro-aggregatory and pro-coagulant responses downstream of integrin outside-in signalling.

Recent advances in understanding the pathophysiology of Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a severe X-linked immunodeficiency caused by mutations in the gene encoding for WASP, a key regulator of signaling and cytoskeletal reorganization in hematopoietic cells. Mutations in WASP result in a wide spectrum of clinical manifestations ranging from the relatively mild X-linked thrombocytopenia to the classic full-blown WAS phenotype characterized by thrombocytopenia, immunodeficiency, eczema, and high susceptibility to developing tumors and autoimmune manifestations. The life expectancy of patients affected by severe WAS is reduced, unless they are successfully cured by bone marrow transplantation from related identical or matched unrelated donors. Because many patients lack a compatible bone marrow donor, the administration of WAS gene-corrected autologous hematopoietic stem cells could represent an alternative therapeutic approach. In the present review, we focus on recent progress in understanding the molecular and cellular mechanisms contributing to the pathophysiology of WAS. Although molecular and cellular studies have extensively analyzed the mechanisms leading to defects in T, B, and dendritic cells, the basis of autoimmunity and thrombocytopenia still remains poorly understood. A full understanding of these mechanisms is still needed to further implement new therapeutic strategies for this peculiar immunodeficiency.