Test Catalog

Test ID: BTKK    
Bruton Tyrosine Kinase (BTK) Genotype, Known Mutation, Blood

Useful For Suggests clinical disorders or settings where the test may be helpful

As a follow-up confirmatory genetic test for relatives of X-linked agammaglobulinemia (XLA) patients with a previously identified Bruton tyrosine kinase gene (BTK) variant, after abnormal Btk protein expression has been previously demonstrated (eg, BTK / Bruton Tyrosine Kinase [Btk], Protein Expression, Flow Cytometry, Blood)

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

X-linked agammaglobulinemia (XLA) is a humoral primary immunodeficiency affecting males in approximately 1 in 200,000 live births. XLA is caused by variants 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). Variants causing XLA have been found in all domains of the BTK gene, as well as noncoding regions of the gene. Over 800 unique variants in BTK have been detected by full gene sequencing and are listed in BTKbase, a database for BTK variants (http://structure.bmc.lu.se/idbase/BTKbase/).(5)  Missense variants account for approximately 33% of unique variants, nonsense variants 13%, frameshift 25%, in-frame deletions and insertions 4%, large deletions 3% to 5%, and intronic and complex variants make up the remainder. Patients with a large deletion spanning the BTK gene may also impact the adjacent TIMM8A gene (also known as DDP) resulting in both XLA and deafness-dystonia-optic neuropathy syndrome (DDS or Mohr-Tranebjaerg syndrome). Genotype-phenotype correlations have not been completely defined for BTK, but it is clear that nonsense and frameshift variants are overrepresented 4-fold compared with substitutions, which indicates that the latter may be tolerated without causing a phenotype or with a milder phenotype or later age at presentation. Some individuals present 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 variant is present. While the disease is considered fully penetrant, the clinical phenotype can vary considerably depending on the nature of the specific BTK variant.(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). Conversely, some BTK variants 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 variant. 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 variant with normal protein expression but aberrant protein function. The diagnosis is confirmed only in those individuals with appropriate clinical history who have a variant identified within BTK by gene sequencing or who have other male family members with hypogammaglobulinemia with absent or low B cells.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

A patient-specific interpretive report is provided.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Patients who have received a heterologous blood transfusion within the preceding 6 weeks, or who have received an allogeneic hematopoietic stem cell transplant, can have inaccurate genetic test results due to presence of donor DNA.

 

This method will not detect variants that occur in intronic (other than exon-intron boundaries) and regulatory regions of the gene or large rearrangement type variants. This assay is not designed to detect large deletions.

 

Btk protein and genetic tests are not meant for patients with hematological neoplasias on kinase inhibitor therapy, including but not restricted to the selective Btk inhibitor, Ibrutonib. This test is only meant for the assessment of patients with a suspected monogenic primary immunodeficiency, X-linked agammaglobulinemia, caused by germline variants in the Bruton tyrosine kinase gene.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Tsukada S, Saffran DC, Rawlings DJ, et al: Deficient expression of a B cell cytoplasmic tyrosine kinase in human X- linked agammaglobulinemia. Cell 1993 Jan 29;72(2):279-290

2. Noordzij JG, de Bruin-Versteeg S, Comans-Bitter WM, et al: Composition of precursor B-cell compartment in bone marrow from patients with X-linked agammaglobulinemia compared with healthy children. Pediatr Res 2002 Feb;51(2):159-168

3. Conley ME, Broides A, Hernandez-Trujillo V, et al: Genetic analysis of patients with defects in early B-cell development. Immunol Rev 2005 Feb;203:216-234

4. Lindvall JM, Blomberg KE, Valiaho J, et al: Bruton’s tyrosine kinase: cell biology, sequence conservation, mutation spectrum, siRNA modifications, and expression profiling. Immunol Rev 2005 Feb;203:200-215

5. Valiaho J, Smith CI, Vihinen M: BTKbase: the mutation database for X-linked agammaglobulinemia. Hum Mutat 2006 Dec;27(12):1209-1217

6. Takada H, Kanegane H, Nomura A, et al: Female agammaglobulinemia due to the Bruton tyrosine kinase deficiency caused by extremely skewed X-chromosome inactivation. Blood 2004 Jan 1;103(1):185-187

Special Instructions Library of PDFs including pertinent information and forms related to the test