Clinical Information

X-linked agammaglobulinemia (XLA) is a humoral primary immunodeficiency affecting males in approximately 1/200,000 live births. XLA is caused by mutations in the Bruton tyrosine kinase gene (BTK),(1) which results in a profound block in B-cell development within the bone marrow and a significant reduction, or complete absence, of mature B cells in peripheral blood.(2) Approximately 85% of male patients with defects in early B-cell development have XLA.(3) Due to the lack of mature B cells, XLA patients have markedly reduced levels of all major classes of immunoglobulins in the serum and are, therefore, susceptible to severe and recurrent bacterial infections. Pneumonia, otitis media, enteritis, and recurrent sinopulmonary infections are among the key diagnostic clinical characteristics of the disease. The spectrum of infectious complications also includes enteroviral meningitis, septic arthritis, cellulitis, and empyema, among others. The disease typically manifests in male children younger than 1 year.

BTK, the only gene associated with XLA, maps to the X chromosome at Xq21.3-Xq22 and consists of 19 exons spanning 37.5 kb genomic DNA.(4) BTK encodes a nonreceptor tyrosine kinase of the Btk/Tec family. The Btk protein consists of 5 structural domains (PH, TH, SH3, SH2, and TK). Mutations causing XLA have been found in all domains of the BTK gene, as well as noncoding regions of the gene. Missense mutations account for 40% of all mutations, while nonsense mutations account for 17%, deletions 20%, insertions 7%, and splice-site mutations 16%. Over 600 unique mutations in BTK have been detected by full gene sequencing and are listed in the BTKbase, a database for BTK mutations (http://bioinf.uta.fi/BTKbase).(5) Genotype-phenotype correlations have not been completely defined for BTK, but it is clear that nonsense mutations are overrepresented 4-fold compared with substitutions, which indicates that the latter may be tolerated without causing a phenotype. The type and location of the mutation in the gene clearly affects the severity of the clinical phenotype. Some mutations manifest within the first 2 years of life, enabling an early diagnosis. Others present with milder phenotypes, resulting in diagnosis later in childhood or in adulthood.(5) Delayed diagnoses can be partly explained by the variable severity of XLA, even within families in which the same mutation is present. While the disease is considered fully penetrant, the clinical phenotype can vary considerably depending on the nature of the specific BTK mutation.(5) Lyonization of this gene is not typical and only 1 case of XLA in a female has been reported so far due to skewed lyonization in a carrier female. Therefore, females with clinical features that are identical to XLA should be evaluated for autosomal recessive agammaglobulinemia when deemed clinically appropriate(6) and for XLA, if a male parent is affected with the disease.

A diagnosis of XLA should be suspected in males with 1) early-onset bacterial infections, 2) marked reduction in all classes of serum immunoglobulins, and 3) absent B cells (CD19+ cells). The decrease in numbers of peripheral B cells is a key feature, though this also can be seen in a small subset of patients with common variable immunodeficiency (CVID). As well, some BTK mutations can preserve small numbers of circulating B cells and, therefore, all 3 of the criteria mentioned above need to be evaluated.

The preferred approach for confirming a diagnosis of XLA in males and identifying carrier females requires testing for the Btk protein expression on B cells by flow cytometry and genetic testing for a BTK mutation. Patients can be screened for the presence of Btk protein by flow cytometry (BTK / Bruton Tyrosine Kinase [Btk], Protein Expression, Flow Cytometry, Blood); however, normal results by flow cytometry do not rule out the presence of a BTK mutation with normal protein expression but aberrant protein function. The diagnosis is confirmed only in those individuals with appropriate clinical history who have a mutation identified within BTK by gene sequencing or who have other male family members with hypogammaglobulinemia with absent or low B cells.