Abnormalities of Cytoskeletal Linking Proteins – Wiskott-Aldrich Syndrome Protein
Definition and History
The Wiskott-Aldrich syndrome, which affects 4 of every 1 million males worldwide, is an X chromosome-linked inherited disorder characterized by small platelets, thrombocytopenia, recurrent infections, and eczema, although only a minority of patients have all of the classic manifestations.722–724 In addition, a variety of immunologic abnormalities affecting T-lymphocyte function, immunoglobulin levels, cellular immunity, and responsiveness to polysaccharide antigens are commonly present.725,726 The immune defects are probably responsible for an increase in autoimmune phenomena and lymphoreticular malignancies associated with the disorder. Death from infection, hemorrhage, or malignancy is common before adulthood.
The Wiskott-Aldrich syndrome protein (WASP) has been cloned and its amino acid sequence deduced from the complementary DNA. The protein contains a unique Wiskott homology domain, which is also present in a number of other genes that convey signals from the surface of cells to the actin cytoskeleton. A number of other domains interact with a large number of other proteins.725,727 WASP is found in all hematopoietic stem cell-derived lineages. It is likely that signals from G-protein-coupled receptors can initiate actin bundling via WASP. Most, but not all patients with Wiskott-Aldrich syndrome have mutations in the WASP protein723 and so other genes may be involved. Moreover, some patients with X-linked thrombocytopenia without the other associated features of Wiskott-Aldrich syndrome have been found to have mutations in WASP.728 Of note, an apparently X-linked severe congenital neutropenia was found in a family with a constitutively activating mutation in WASP.729 An international database of WASP mutations730 was compiled through 2004 and can be accessed at http://homepage.mac.com/kohsukeimai/wasp/WASPbase.html.
A defect in the surface glycoprotein sialophorin (CD43, gp115, leukosialin) has also been described in Wiskott-Aldrich syndrome,731 but its significance is uncertain. Deficiencies in platelet GPIb have also been described in some patients with the Wiskott-Aldrich syndrome,731,732 but this is not an invariant finding.733,734 Deficiencies in platelet integrin 2 have also been recorded in some, but not all, patients.731 Similarly, decreases in platelet IIb3 and GPIV have been reported based on flow cytometry studies even after normalizing for platelet size.734
The mechanism(s) underlying the thrombocytopenia and small platelet size in Wiskott-Aldrich syndrome remains uncertain despite extensive investigation. Inconsistent data have been reported on shortened platelet survival, and in those studies in which shortened survival has been observed, it is unclear whether the defect was selective for patient platelets compared to normal platelets.735–739 Marrow megakaryocyte numbers have been reported to be normal in most, but not all patients735,738,740,741; similarly, ex vivo defects in megakaryopoiesis and/or proplatelet formation have been described in some, but not all, studies.742–745 Moreover, both decreased and increased levels of immature platelets (thiazole orange-positive; “reticulated”) have been reported in Wiskott-Aldrich patients.734,746 Splenectomy consistently improves the platelet count,747,748 but recurrent severe thrombocytopenia and hemorrhage can occur, often on an autoimmune basis.725,741,749
A specific connection between Wiskott-Aldrich syndrome and immune thrombocytopenia has been proposed and an increase in other autoimmune diseases is found in Wiskott-Aldrich syndrome.750 Thus, immune mechanisms may contribute to the thrombocytopenia in some patients734,738,751–753 and the patients’ other abnormalities in platelet production and removal may make them particularly susceptible to even low-level immune-mediated premature removal.750 Moreover, based on data from the murine model, opsonized Wiskott-Aldrich platelets may be especially susceptible to clearance because of increased ex vivo phagocytosis.754 This observation has also been proposed to explain why intravenous immunoglobulin G usually fails to increase the platelet count in patients with Wiskott-Aldrich syndrome, as the abnormality(ies) leading to enhanced phagocytosis may limit the effectiveness of intravenous immunoglobulin G. It is also possible that enhanced phagocytosis of Wiskott-Aldrich platelets contributes to the development of immune thrombocytopenia.750
The cause of the small size of the platelets is unknown. Although it is likely related in part to an abnormality in the connection between the membrane and the cytoskeleton caused by the defect in the WASP, it is less clear whether this becomes manifest before and/or after platelets enter the circulation. Intrinsic cytoskeletal defects in Wiskott-Aldrich platelets may also result in a greater propensity to develop microparticles, which may contribute to both the small size and the documented increase in plasma microparticles.755
Variant forms of Wiskott-Aldrich syndrome characterized by thrombocytopenia and X chromosome-linked inheritance (XLT) have been reported,728,756 some of which are associated with mutations of WASP.728,740,757 The WASP mutations in XLT are primarily in the N-terminal region of the molecule, which is the region that links WASP to IIb3 via calcium and integrin binding protein.757 Platelets from patients with XLT have been reported to have reduced adhesion to fibrinogen and have a defect in IIb3 activation.757
Platelets from most patients with Wiskott-Aldrich syndrome have qualitative as well as quantitative abnormalities. Most common is a deficiency in the storage pool of adenine nucleotides, producing a reduced positive feedback mechanism during platelet activation and aggregation.735,737,756 Abnormalities in platelet energy metabolism have also been described.737,758
Platelet aggregation in Wiskott-Aldrich syndrome has been reported to be reduced, normal, or enhanced.755,757,759,760 These studies are confounded by the low platelet count. Despite the role of the WASP protein in cytoskeletal reorganization, shape change and actin polymerization are normal in Wiskott-Aldrich platelets.760–762 Similarly, although WASP undergoes tyrosine phosphorylation in response to collagen and collagen-related peptide, aggregation in response to the latter has been reported to be normal.760,763 Although it is not possible to reconcile all of the reported findings, the subcellular localization of WASP and its association with IIb3 may be functionally important. Thus, although in nucleated cells WASP is predominantly a cytosolic protein that becomes activated when it is recruited to the membrane, in platelets, approximately 25 percent of WASP in unactivated platelets is associated with the membrane skeleton. WASP undergoes transient phosphorylation, redistribution, and cleavage by calpain (or a similar enzyme) with platelet activation; all of these changes require IIb3 outside-in signaling. WASP has been shown to bind to calcium and integrin binding protein when platelets are activated, and the latter protein is known to bind the cytoplasmic domain of IIb. Inhibiting the interaction between WASP and calcium and integrin binding protein inhibits inside-out signaling of IIb3 and fibrinogen binding. Because cytoskeletal traction has been implicated in activation of IIb3,65 the loss of WASP binding to IIb via calcium and integrin binding protein may contribute to the reduced ability to activate IIb3.
Clinical Features & Sign ,Symptoms – Wiskott-Aldrich Syndrome Protein
Hemorrhage, recurrent infections, eczema, and lymphoreticular malignancies dominate the clinical picture. Autoimmune diseases, including arthritis, vasculitis, autoimmune hemolytic anemia, and immune thrombocytopenia, may complicate the disorders.749 There is enormous variability in disease severity, and this even extends to variability within individual kindreds.725 Correlations between WASP gene mutations and clinical manifestations are inexact, but patients whose cells express full-length protein may have better immunologic function.725,730
Laboratory test Features
The platelet count is variably reduced, with 44 percent of patients in a large study having platelet counts ≤20,000/L at the time of diagnosis, and the platelet volume is significantly reduced in nearly all patients.722,741 Lymphopenia and eosinophilia are present in a minority of patients. The bleeding time is usually prolonged to a greater extent than would be expected from the platelet count, but when the reduced platelet mass is considered, the bleeding time prolongation may not be inappropriate. Platelet aggregation and release of dense body contents are variably abnormal. Platelet ultrastructural abnormalities have been reported, but on balance it appears that platelet morphology is essentially normal.519
Results of immunologic evaluations vary significantly, but some patients have decreased numbers of CD8+ T cells.722 Serum levels of IgG are usually normal, whereas serum levels of IgM are usually depressed and serum levels of IgA and IgE are usually elevated.741 Variable deficiencies in immune response to antigenic challenge, especially polysaccharide antigens, are common.741
Flow cytometry may be used to assess quantitative abnormalities of WASP in blood mononuclear cells and may be useful in carrier detection under certain circumstances.764,765 X-inactivation in some carriers of Wiskott-Aldrich syndrome is nonrandom, however, and thus caution is required in interpreting such studies.766
Theatment , Course, and Prognosis of Wiskott-Aldrich Syndrome Protein
Splenectomy usually, but not invariably, improves the thrombocytopenia and usually partially corrects the defect in platelet size, at least temporarily.722,747,748 It may also improve platelet function. Thus, splenectomy should be considered in patients with excessive hemorrhage. Opportunistic infections present serious problems.725,741,747 There is an increased risk of overwhelming bacterial sepsis after splenectomy, but this can be reduced by the use of pneumococcal, meningococcal, and Haemophilus influenzae vaccines, as well as prophylactic antibiotics and intravenous immunoglobulin. Patients with Wiskott-Aldrich syndrome tend to have hypercatabolism of IgG and thus may require both a higher dose and more frequent dosing of IgG.741 If platelet transfusion is required to stop hemorrhage, the platelets should be irradiated to prevent transfusion-related graft-versus-host disease. It is preferable to obtain platelets from donors who are free of cytomegalovirus.
Bone marrow transplant 0r Stem cell transplant for Wiskott-Aldrich Syndrome Protein – Cure
Hematopoietic reconstitution with stem cells from blood, cord blood or marrow can cure the disorder.725,748 Because the prognosis is otherwise very poor, intervening before the onset of significant immunodeficiency has been recommended when a histocompatible donor is available.725,748 Transplantation from matched unrelated donors can be successful in young patients, but the success rate declines after 5 to 6 years of age.741