Wiskott–Aldrich syndrome

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 similar but less severe symptoms and are caused by mutations of the same gene.

Signs and symptoms
Due to its mode of inheritance, the overwhelming majority are male. The first signs of WAS are usually petechiae and bruising, resulting from thrombocytopenia (low platelet counts). Spontaneous nose bleeds and bloody diarrhea are common. Eczema develops within the first month of life. Recurrent bacterial infections develop by three months. Splenomegaly is not an uncommon finding. The majority of WAS children develop at least one autoimmune disorders, and malignancies (mainly lymphoma and leukemia) develop in up to a third of patients.

IgM levels are reduced, IgA and IgE are elevated, and IgG levels can be reduced or elevated.

Diagnosis
The diagnosis is made on the basis of clinical parameters, the blood film and low immunoglobulin levels. Typically, immunoglobulin M (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. It must be remembered that not all patients will have a family history, since they may be the first to harbor the gene mutation. Often, leukemia may initially be suspected on the basis of the low platelets and the infections, and bone marrow biopsy may be performed. Decreased levels of Wiskott-Aldrich syndrome protein and/or confirmation of a causative mutation provides the most definitive diagnosis.

Sequence analysis can detect the WAS-related disorders of Wiskott–Aldrich syndrome (WAS), X-linked thrombocytopenia (XLT), and X-linked congenital neutropenia (XLN). Sequence analysis of the WASp gene can detect about 98% of mutations in males and 97% of mutations in female carriers. Because XLT and XLN symptoms may be less severe than full WAS and because female carriers are usually asymptomatic, clinical diagnosis can be elusive. In these cases, genetic testing can be instrumental in diagnosis of WAS-related disorders.

Classification
Jin et al. (2004) employ a numerical grading of severity:
 * 0.5: intermittent thrombocytopenia
 * 1.0: thrombocytopenia and small platelets (microthrombocytopenia)
 * 2.0: microthrombocytopenia plus normally responsive eczema or occasional upper respiratory tract infections
 * 2.5: microthrombocytopenia plus therapy-responsive but severe eczema or airway infections requiring antibiotics
 * 3.0: microthrombocytopenia plus both eczema and airway infections requiring antibiotics
 * 4.0: microthrombocytopenia plus eczema continuously requiring therapy and/or severe or life threatening infections
 * 5.0: microthrombocytopenia plus autoimmune disease or malignancy

Pathophysiology
In Wiskott–Aldrich syndrome, the platelets are small and do not function properly. They are removed by the spleen, which leads to low platelet counts.

Wiskott–Aldrich syndrome was linked in 1994 to mutations in a gene on the short arm of the X chromosome, which was termed Wiskott-Aldrich syndrome protein (WASp). It was later discovered that the disease X-linked thrombocytopenia (XLT) was also due to WASp mutations, but different ones from those that cause full-blown Wiskott–Aldrich syndrome. Furthermore, the rare disorder X-linked neutropenia has been linked to particular mutations of the WASp gene.

The WASp gene codes for the protein by the same name, which is 502 amino acids long and is mainly expressed in hematopoietic cells (the cells in the bone marrow that develop into blood cells). Its exact function is being investigated, but signal transduction and cytoskeleton maintenance have been suggested.

The immune deficiency is caused by decreased antibody production, although T cells are also affected (making it a combined immunodeficiency). This leads to increased susceptibility to infections, particularly of the ears and sinuses. T cells are unable to reorganize their actin cytoskeleton. The type of mutation to the WASp gene correlates significantly with the degree of severity: those that led to the production of a truncated protein caused significantly more symptoms than those with a missense mutation but a normal-length WASp. Although autoimmune disease and malignancy occur in both types of mutation, those patients with truncated WASp carry a higher risk.

A defect in CD43 molecule has been found to be associated in patients with Wiskott–Aldrich syndrome.

Epidemiology
The combined incidence of WAS and XLT is about 4-10 in 1 million live births. There is no geographical factor.

Treatment
Treatment of Wiskott–Aldrich syndrome is currently based on correcting symptoms. Aspirin and other non-steroidal anti-inflammatory drugs should be avoided, since these may interfere with platelet function. A protective helmet can protect children from bleeding into the brain which could result from head injuries. For severely low platelet counts, patients may require platelet transfusions or a splenectomy. For patients with frequent infections, intravenous immunoglobulins (IVIG) can be given to boost the immune system. Anemia from bleeding may require iron supplementation or blood transfusion.

As Wiskott–Aldrich syndrome is primarily a disorder of the blood-forming tissues, a hematopoietic stem cell transplant, accomplished through a cord blood or bone marrow transplant offers the only current hope of cure. This may be recommended for patients with HLA-identical donors, matched sibling donors, or even in cases of incomplete matches if the patient is age 5 or under.

Studies of correcting Wiskott–Aldrich syndrome with gene therapy using a lentivirus have begun. Proof-of-principle for successful hematopoietic stem cell gene therapy has been provided for patients with Wiskott–Aldrich syndrome. Currently, many investigators continue to develop optimized gene therapy vectors.

History
The syndrome is named after Dr Robert Anderson Aldrich (1917–1998), an American pediatrician who described the disease in a family of Dutch-Americans in 1954, and Dr Alfred Wiskott (1898–1978), a German pediatrician who first noticed the syndrome in 1937. Wiskott described three brothers with a similar disease, whose sisters were unaffected. In 2006 a German research group analysed family members of Wiskott's three cases, and surmised that they probably shared a novel frameshift mutation of the first exon of the WAS gene.