Wiskott–Aldrich syndrome (WAS) is a rare X-linked recessive disease characterized by eczema, thrombocytopenia (low platelet count), immune deficiency, and bloody diarrhea (secondary to the thrombocytopenia). It is also sometimes called the eczema-thrombocytopenia-immunodeficiency syndrome in keeping with Aldrich's original description in 1954. The WAS-related disorders of X-linked thrombocytopenia (XLT) and X-linked congenital neutropenia (XLN) may present with similar but less severe symptoms and are caused by mutations of the same gene.
Signs and symptoms WAS occurs most often in males due to its X-linked recessive pattern of inheritance, affecting between 1 and 10 males per million. The first signs are usually petechiae and bruising, resulting from a low platelet count (i.e. thrombocytopenia). Spontaneous nose bleeds and bloody diarrhea are also common and eczema typically develops within the first month of life. Recurrent bacterial infections typically develop by three months of age. The majority of children with WAS develop at least one autoimmune disorder, and cancers (mainly lymphoma and leukemia) develop in up to a third of patients. Immunoglobulin M (IgM) levels are reduced, IgA and IgE are elevated, and IgG levels can be normal, reduced, or elevated. In addition to thrombocytopenia, WAS patients have abnormally small platelets (i.e. microthrombocytes) and ~30% also have elevated eosinophil counts (i.e. eosinophilia).
Pathophysiology The microthrombocytes seen in WAS patients have only been observed in one other condition, ARPC1B deficiency. In both conditions the defective platelets are thought to be removed from circulation by the spleen and/or liver, leading to low platelet counts. WAS patients have increased susceptibility to infections, particularly of the ears and sinuses, and this immune deficiency has been linked to decreased antibody production and the inability of immune T cells to effectively combat infection.
Genetics
WAS is associated with mutations in a gene on the short arm of the X chromosome (Xp11.23) that was originally termed the Wiskott–Aldrich syndrome protein gene and is officially known as WAS (Gene ID: 7454). X-linked thrombocytopenia (XLT) is also linked to pathogenic variants in the WAS gene, although some variants tend to be more strongly associated with XLT versus others that are more associated with WAS. The rare disorder X-linked neutropenia has also been linked to a specific subset of WAS mutations. The protein product of WAS is known as WASp. It contains 502 amino acids and is mainly expressed in hematopoietic cells (the cells in the bone marrow that develop into blood cells). The main function of WASp is to activate actin polymerization by serving as a nucleation-promoting factor (NPF) for the Arp2/3 complex, which generates branched actin filaments. Several proteins can serve as NPFs, and it has been observed that in WAS platelets the Arp2/3 complex functions normally, indicating that WASp is not required for its activation in platelets. In T-cells, WASp is important because it is known to be activated via T-cell receptor signaling pathways to induce cortical actin cytoskeleton rearrangements that are responsible for forming the immunological synapse. The severity of the symptoms produced by pathogenic variants in the WAS gene generally correlates with their effects on WASp. Missense variants generally are associated with less severe disease than truncating variants that produce no protein due to nonsense-mediated decay. However, this correlation is not perfect, and sometimes the same variant can be seen both in XLT and in WAS (sometimes within two different members of the same family), a concept in genetics referred to as variable expressivity. Although autoimmune disease and malignancy may occur in both conditions, patients with loss of WASp are at higher risk. A defect in the CD43 molecule has also been found in WAS patients. CD43, a transmembrane sialoglycoprotein also known as a leukosialin, is part of a greater complex involved in T-cell activation and acts as a sensitive indicator of abnormal, malignant B cell populations. Defects in this molecule may be detrimental to WAS patients, who are at a much higher risk of autoimmune diseases that may be exacerbated in later-detected B-cell lymphomas.
Diagnosis The diagnosis can be made on the basis of clinical findings, the peripheral blood smear, and low immunoglobulin levels. Typically, IgM levels are low, IgA levels are elevated, and IgE levels may be elevated; paraproteins are occasionally observed. Skin immunologic testing (allergy testing) may reveal hyposensitivity. Individuals with Wiskott–Aldrich syndrome, however, are at higher risk for severe food allergies. Not all patients have a positive family history of the disorder; new mutations do occur. Often, leukemia may be suspected on the basis of low platelets and infections, and bone marrow biopsy may be performed. Decreased levels of WASp are typically observed. The current gold standard for diagnosis is DNA sequence analysis, which can detect WAS and the related disorders XLT and XLN in 95% of patients and carriers.
Classification Jin et al. (2004) employ a numerical grading of severity: This score, which ranges from 0 to 5, may have clinical utility for predicting disease severity. Those with higher WAS scores (e.g., 5) at younger ages (e.g., age less than 5 years old), are thought to be at highest risk for increased morbidity and mortality related to their condition. As individuals can develop more WAS-related symptoms (e.g. autoimmune disease, malignancy) with age, one's WAS score can increase over time. A lower WAS score may be more compatible with conservative management versus higher WAS scores that may favor intervention with treatments such as hematopoietic stem cell transplant.
Treatment
… excerpt ends here. Continue reading the full article.
