This web page was produced as an assignment for an undergraduate course at Davidson College.
Table of Contents:
Overview: Wiskott-Aldrich Syndrome (WAS) was independently characterized in the 1940s by the doctors that share the disease name. It wasn't until the 1960's, however, that WAS made the list of primary immunodeficiency diseases (IDF Patient Family Handbook, Chapter 7). Active research on the disease has uncovered a mechanistic understanding of the disease etiology and technological advancements have made WAS a less daunting disease. I hope to shed some light on the past, current, and future research concerning WAS by synthesizing a number of studies reported in scientific journals.
WAS is a rare X-linked recessive disease that affects 4 out of 1 million live male births. The WAS phenotype is uncommon in females, although females are carriers of the disease allele. In rare instances, non-random inactivation of the normal X chromosome in female lymphocytes can lead to expression of the disease. Median survival for patients with the disease continues to increase. While the median life expectancy is 11 years of age, the survival range falls between 1-35 and it is not uncommon for patients to live into their 2nd and 3rd decades of life. If a successful HLA-matched bone marrow transplant is peformed, there is an 80% chance of complete restoration of wild-type function (IDF Patient Family Handbook, Chapter 7). Through gene mapping studies, WAS has been found to result from mutations in the Wiskott-Aldrich Syndrome protein (WASP) that is transcribed from the short arm of the X chromosome (www.emedicine.com/MED/topic1162.htm). The gene has 12 exons that code for 502 amino acids and over 140 unique mutations have been implicated to cause the disease. There have not been any connections made between ethnicity or geographic origin and disease (Jin et al., 2004). As genetic testing becomes increasingly prevalent and less costly, informative markers on the X chromosome allows DNA analysis to detect female carriers and early prenatal diagnosis (Standen, 1991).
Condition Phenotype: A three year old male was brought to his primary care physician with persistent thrombocytpenia (low platelet count leading to excessive bleeding and bruising), recurrent infections, chronic eczema, small platelets, and decreased IgM antibody levels. The patient's immune system was challenged with T-cell dependent antigen and it failed to produce a cell-mediated immune response. Upon sequencing of the WASP gene, a 4 base pair deletion in exon 1 was determined to be the cause of a frameshift mutation leading to the introduction of a premature stop codon and a non-functional protein product. This patient presents a classical case of WAS (Ma et al., 2004).
The triad of symptoms seen with WAS include increased bleeding (thrombocytopenia), recurrent bacterial, viral, and fungal infection due to immunodeficiency, and eczema. WAS patients have an increased incidence of malignancies, especially lymphoma and leukemic cancers, which have been quantified as a 120-fold increase over the normal population (Standen, 1991). The malignant lymphomas associated with WAS are of monoclonal B cell origin. If the disease progression leads to malignancy, the prognosis is poor because of the toxicity of chemotherapy and the risk of fatal infection (Ma et al., 2004). Less common symptoms include asthma, autoimmune hemolytic anemia, and arthritis. The most common symptom (84% of patients) and the most easily diagnosed disease characteristic is a low platelet count and the presence of malformed, small platelets. The inefficient production of platelets due to defects in the lymphoproliferation of bone marrow cells and the increased removal of platelets by the spleen causes pinhead red spots on the skin called petechiae or excessive bruising. Other symptoms include bloody bowel movements, bleeding gums, or prolonged nose bleeds. Because WASP is exclusively expressed in hematopoietic cells, protein deficiency has a direct effect on B and T lymphocytes, the mechanism for which I will explain in the next section. This primary immunodeficiency makes WAS patients susceptible to opportunistic infection, especially by respiratory pathogens. Recurrent Herpes simplex virus and pneumonia are common. While eczema, a skin rash characterized by redness and dryness, may be absent in a number of WAS patients, this is a common symptom. Autoimmune symptoms persist in WAS patients and include anemia due to the destruction of red blood cells by self-antibodies, fever, swollen joints, and kidney inflammation. These symptoms usually last for a few days and come and go in waves (IDF Patient Family Handbook, Chapter 7).
WAS and the Immune System: There are two faces of WAS. First, WAS is a blood-clotting disorder due to low platelet production and excessive bleeding after tissue damage. Second, WAS is a primary immunodeficiency disorder that affects T cell signaling, cytoskeleton rearrangement of lymphocytes, B cell migration and attachment, and antigen presentation. Both humoral and cell-mediated immunity are affected by WAS. WAS patients challenged with T-cell dependent pathogens show absent or delayed hypersensitivity reactions and impaired lymphocyte responses to mitogens (peptides that induce mitosis in lymphocytes) and allogeneic antigens. It is believed that the lack of a functional WASP causes shifts in T cell subsets, specifically a reduction in CD4+ T cells. As a result, the activation of the correct isotype of B cells fails during pathogenic infection in WAS patients. Therefore, a common disease phenotype includes a skewed ratio of antibody isotypes including increased levels of IgA and IgE and decreased levels of circulating IgM. This humoral defect has been observed in patients injected with a T-cell independent cross-linking polysaccharide antigen. Failure to produce a correct response to such antigens causes a high susceptibility of WAS patients to bacterial pathogens. From this data, WAS has been characterized as a primary T cell immune defect (Standen, 1991). Also, WASP has been identified as the link between cellular signaling in lymphocytes and movement of actin filaments in the cytoskeleton. When this pathway is disrupted, the proper development of hematopoietic cells is halted and platelet and immune defects accumulate. The actin cytoskeleton is important in all cells. The cytoskeleton mediates cellular growth, endocytosis, exocytosis, and cytokinesis after cellular division. WASP is responsible for actin filament growth through rapid monomer addition, nucleation, and polymerization. WASP binds Rho family GTPases including Cdc42 and Rac, which help regulate actin polymerizaton and is essential for cell-cell interaction and normal lymphocyte migration and motility (www.emedicine.com/MED/topic1162.htm).
On a molecular level, WASP is an important component of the cytokine secretory pathway in CD4+ cells, while the secretory pathway for chemokine production and release is unaffected by WASP deficits. WASP deficient mice show an inability to polarize cytokines toward a specific target and, thus, the release of effector cytokine release into the immunological synapse (the space between adjacent cells) during cellular communication is disrupted. Because cellular communication is a key component of lymphocyte proliferation and development, such a defect causes signaling and cytoskeleton abnormalities (Morales-Tirado et al., 2004). Specifically, a failure in actin polymerization inhibits TCR signaling, organization of the immunological synapse, and IL-2 production (Cannon and Burkhardt, 2004). Because of this, WASP is the all-important link between cytoskeletal dynamics and T cell activation. IL-2 is a key cytokine that is most central to the development of an adaptive immune response. It is hypothesized that the absence of functional WASP causes defects in the activation of the NFAT transcription factor, which is responsible for the production of IL-2 during lymphocyte differentiation and proliferation. Based on this research, it is believed that WASP does not simply function as a regulator of actin-dependent immunological synapse formation (Cannon and Burkhardt, 2004). The roles of WASP continue to be elucidated.
Another hematopoietic cell subset affected by WAS are the NK cells that help clear viral pathogens that alter MHC class I presentation. A key pathway in NK cells is the activation of the transcription factors NFAT2 and NFkB. WASP deficiency has been shown to decrease the ability for calcineurin to modulate constituents of this gene transcription pathway. In WASP mice knockouts, a decrease in cellular proliferation, calcium influx, and IL-2 secretion was observed. Such defects severely inhibit the innate and adaptive immune response and NK cell differentiation. This altered phenotype is believed to be independent of the disorganized actin skeleton phenotype, however, the exact mechanism is unclear (Huang et al., 2005).
Upon antigen challenge, a delayed and reduced humoral immune response is observed in WAS patients. In wild type individuals, the normal immune response includes the activation of B cells by T cells in secondary lymphoid organs. The T cells secrete cytokines or express co-stimulatory molecules, thus telling the B cells to divide and form germinal centers. In WASP deficient mice, the formation of germinal centers is reduced and B cell areas of the spleen are significantly reduced. Also, B cells fail to migrate and adhere properly, ultimately leading to a decreased production of antibodies. While the exact mechanism is unknown, it is believed that the impairment of B cell polarization and aggregation is due to distorted cell surface microvilli. This hypothesis is in agreement with the data concerning the role of WASP in cytoskeleton arrangement (Westerberg et al., 2005).
WASP deficient cells have abnormal dendritic cell migration. It has been reported in experimental mouse strains that correct actin organization is important for dendritic cell attachment and detachment. The activation of Langerhans cells and their subsequent maturation into dendritic cells is impaired in these WASP knockouts. The ability of dendritic cells to act as sentinels of injury and infection in WASP patients fails because the dendritic cells do not home to the correct T cell areas of secondary lymphoid organs. Therefore, the actin skeleton must be important in regulating and directing dendritic cell motility based on their response to chemokines and their ability to extravasate through tissues (Noronha et al., 2005).
Diagnosis and Treatment: Identifying the mutation in the WASP gene on the X chromosome is the only definitive diagnosis for WAS. However, there are some common symptoms, which if seen in combination, can lead to a correct disease identification. The simplest and most useful diagnostic test is to obtain a platelet count through a blood test and to carefully examine the platelet morphology. Diseased platelets will be about half the size of normal platelets. Also, the levels of certain antibody isotypes can be determined by looking at quantitative serum Ig levels. The common disease phenotype includes low IgM levels, but elevated IgA and IgE concentrations. Also, functional testing of humoral and cellular immunity can quickly determine the extent of immunodeficiency in WAS patients. This is done by challenging the immune system with standard polysaccharide and protein antigens and measuring the proliferation of lymphocytes. By using T cell dependent and independent antigens, the exact arm of the immune system that is affected can be determined. Also, genetic testing for carrier status is a good option for women who have a history of WAS in their family pedigree (www.emedicine.com/MED/topic1162.htm).
The only permanent cure for WAS is a successful bone marrow transplant or umbilical cord stem cell transplantation. An 80-90% cure rate can be achieved if the donor and recipient bone marrow are HLA-identical, meaning that all the HLA alleles at the MHC locus are syngeneic. The use of parental bone marrow is not as successful because the offspring only share half the MHC alleles of either parent. If the HLA alleles are unmatched, the cure rate falls to only 23%. Also, the surgical removal of the patient's spleen, known as a splenectomy, fixes the low platelet count in 90% of the cases. This is an important treatment due to the fact that 23% of the deaths related to WAS occur from excessive blood loss. However, removing the spleen also increases the patient's susceptibility to bacterial infection and 44% of WAS patients succumb to chronic infections (Ma et al., 2004). Besides surgical intervention, it is very important to treat the symptoms of the disease in order to maintain the highest level of comfort and health for WAS patients. Moisterizure should be used to treat eczema, steroid cream can be used to decrease inflammation, and platelet transfusions are important to prevent excessive bleeding. The most commonly prescribed corticosteroids include prednisone, methylprednisolone, and fluocinolone. Also, immunoglobulin transfusions can be given to patients in order to restore proper levels of antibodies to patients who can not respond to bacterial antigens (IDF Patient Family Handbook, Chapter 7).
Summary: WAS is a difficult disease for any family to deal with, but the future prospects are surely promising. With increased technology, medical advancements, and an understanding of the underlying mechanisms of the disease comes an increase in the average life expectancy of WAS patients and a high chance of a complete cure if a matched bone marrow recipient is found. The fact that the study of WAS is an active area of research should be reassuring for anyone who is affected by the disease. I have complete confidence that an even better prognosis for WAS patients awaits in the near future.
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Huang, Winifred, et al. "The Wiskott-Aldrich Syndrome Protein Regulates Nucler Translocation of NFAT2 and NFkB Independently of Its Role in Filamentous Actin Polymerization and Actin Cytoskeletal Rearrangement." The Journal of Immunology 174 (2005): 2602-2611.
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"Wiskott-Aldrich Syndrome." www.emedicine.com/MED/topic1162.htm. Last Updated June 10, 2005.
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