Bruton Agammaglobulinemia - X-Linked Agammaglobulinemia

Diagnosis

Indications for Testing

  • Recurrent infections in males during infancy or early childhood

Laboratory Testing

  • Nonspecific testing – CBC to rule out congenital neutropenic disorders
    • May observe severe neutropenia in 10-20% of patients if performed when patient has an infection
  • Immunoglobulin testing – quantitative
    • IgG typically <200 mg/dL; most between 100-200 mg/dL; 10% ≥200 mg/dL
    • IgM and IgA typically <20 mg/dL
    • Response to vaccination (polyvalent pneumococcal or diphtheria toxoid) – typically no response
  • Lymphocyte cell surface markers – significantly decreased CD19+ cells (B lymphocytes); <1% in circulation
  • B-cell immunodeficiency profile measures surface immunoglobulin on B cells – absent in Bruton agammaglobulinemia
  • Bruton tyrosine kinase (BTK) protein expression by flow cytometry – if protein expression is absent or reduced, suggests X-linked agammaglobulinemia (XLA) in males
  • Gene mutation analysis
    • Presence of BTK gene mutation is confirmatory
    • Other gene mutations causative for agammaglobulinemia are typically autosomal recessive, but autosomal dominant mutations may be difficult to differentiate from X-linked disease (eg, with LRRC8A mutations)

Differential Diagnosis

Clinical Background

Bruton agammaglobulinemia, or X-linked agammaglobulinemia (XLA), is a primary immunodeficiency characterized by recurrent bacterial infections in affected males.

Epidemiology

  • Incidence – estimated 1/250,000-700,000 male births per year
  • Age  
    • 50% diagnosed by 2 years
    • 80% diagnosed by school age
  • Sex – >99% male
  • Ethnicity – most commonly diagnosed in Caucasians

Risk Factors

  • Genetics – BTK gene with X-linked inheritance

Pathophysiology

  • BTK gene mutation
    • Leads to deficient development of B-cell lymphocytes and marked reduction in all classes of immunoglobulins
      • B-cell development in bone marrow is blocked at pro-B-cell stage to pre-B-cell stage
    • Hypogammaglobulinemia results in predisposition to life-threatening infections caused by encapsulated bacteria and enteroviruses

Clinical Presentation

  • Infants are usually asymptomatic during first 3 months of life due to passive transfer of immunoglobulins by their mothers
  • Most common clinical presentation of disease is respiratory infections with encapsulated organisms
  • Other common infections
    • Conjunctivitis
    • Chronic, recurrent diarrhea
    • Skin infections
  • Life-threatening infections uncommon

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
CBC with Platelet Count and Automated Differential 0040003
Method: Automated Cell Count/Differential

Initial screening for immunodeficiency; rule out congenital neutropenic disorders

   
Immunoglobulins (IgA, IgG, IgM), Quantitative 0050630
Method: Quantitative Nephelometry

Initial test in the evaluation of immunoglobulin disorders

Test components include serum quantitative IgA, IgG, IgE, and IgM

  If low IgG, IgM, or IgA, consider ordering B-cell memory and naive panel
B-Cell Memory and Naive Panel 2008901
Method: Flow Cytometry

Assess B-cell subsets in immunodeficiencies (eg, common variable immunodeficiency, B-cell reconstitution after bone marrow, or hematopoietic stem cell transplantation)

Measures B cells (CD19+), total memory B cells (CD19+/CD27+), class-switched memory B cells (CD19+/CD27+/IgD-), nonswitched/marginal-zone memory B cells (CD19+/CD27+/IgD+), and naïve B cells (CD19+/CD27-/IgD+)

   
Lymphocyte Subset Panel 7 - Congenital Immunodeficiencies 0095899
Method: Quantitative Flow Cytometry

Acceptable lymphocyte subset panel for the investigation of primary immunodeficiency disorders

Test includes percentage and absolute counts for CD2, CD3 (total T cells), HLA-DR, CD4 (helper T cells), CD45RA (naive helper T cells), CD45RO (memory helper T cells), CD8 (cytotoxic T cells), CD19 (B cells ), NK cells, and  CD4:CD8 ratio

   
Bruton Tyrosine Kinase (BTK) Protein Expression by Flow Cytometry 2012002
Method: Qualitative Flow Cytometry

Preferred test for initial screening  for individuals with clinical suspicion of agammaglobulinemia/hypogammaglobulinemia    

Normal expression of BTK protein

  • Occurs in 20-30% of patients with XLA due to truncated or inactive BTK protein with abnormal function; genetic analysis is recommended
  • Does not exclude mutations or defective protein function
  • Does not rule out XLA in males
  • Does not rule out XLA carrier status in females
 
Agammaglobulinemia Panel, Sequencing (9 Genes) and Deletion/Duplication (6 Genes) 2011151
Method: Massive Parallel Sequencing/Exonic Oligonucleotide-based CGH Microarray

Preferred test for individuals with clinical phenotype of agammaglobulinemia

Clinical sensitivity/analytical sensitivity, specificity – ~90%; 99%

Genes sequenced – BLNK, BTK, CD79A, CD79B, IGHM, IGLL1, LRRC8A, PIK3R1, SH2D1A

Genes analyzed for deletions/duplications – BLNK, BTK, CD79A, CD79B, IGHM, IGLL1

Not determined or evaluated – mutations in genes not included on the panel; deep intronic and regulatory region mutations; breakpoints for large deletions/duplications; translocations

Deletions/duplications will not be detected in LRRC8A, PIK3R1, SH2D1A

Small deletions or insertions may not be detected

Diagnostic errors can occur due to rare sequence variations

Lack of detectable gene mutation does not exclude diagnosis of agammaglobulinemia

 
Primary Antibody Deficiency Panel, Sequencing (35 Genes) and Deletion/Duplication (26 Genes)  2011156
Method: Massive Parallel Sequencing/Exonic Oligonucleotide-based CGH Microarray

Preferred test for individuals with clinical phenotype of antibody deficiency (eg, agammaglobulinemia, common variable immunodeficiency)

Preferred genetic test for individual with clinical phenotype of primary antibody deficiency (eg, common variable immunodeficiency)

For hyper-IgM syndrome genetic testing, refer to hyper-IgM syndrome sequencing and deletion/duplication panel; for agammaglobulinemia genetic testing, refer to agammaglobulinemia sequencing and deletion/duplication panel

Genes sequenced –  ADA, AICDA, ATM, BLNK, BTK, CD19, CD40, CD40LG, CD79A, CD79B, CD81, CR2, ICOS, IGHM, IGLL1, IKBKG, LRBA, LRRC8A, MRE11A, MS4A1, NBN/NBS1, NFKB2, NFKBIA, PIK3CD, PIK3R1, PLCG2, PRKCD, PTPRC, RAG2, SH2D1A, TNFRSF13B, TNFRSF13C, UNG, VAV1, XIAP/BIRC4

Genes analyzed for deletions/duplications – ADA, AICDA, ATM, BLNK, BTK, CD19, CD40, CD40LG, CD79A, CD79B, CD81, CR2, ICOS, IGHM, IGLL1, MRE11A, MS4A1, NBN/NBS1, NFKB2, NFKBIA, PTPRC, RAG2, TNFRSF13B, TNFRSF13C, UNG, VAV1

Not determined or evaluated – mutations in genes not included on the panel; deep intronic and regulatory region mutations; breakpoints for large deletions/duplications; translocations

Deletions/duplications will not be detected in IKBKG, LRBA, LRRC8A, PIK3CD, PIK3R1, PLCG2, PRKCD, SH2D1A, or XIAP/BIRC4 gene

Small deletions or insertions may not be detected

Diagnostic errors can occur due to rare sequence variations

Lack of a detectable gene mutation does not exclude a diagnosis of primary antibody deficiency

 
Additional Tests Available
 
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Lymphocyte Subset Panel 6 - Total Lymphocyte Enumeration with CD45RA and CD45RO 0095862
Method: Quantitative Flow Cytometry

Useful for assessing primary T-cell immunodeficiency disorders

Measures percentage and absolute numbers of CD4  (helper T cells), CD45RA (naive helper T cells), CD45RO (memory helper T cells), CD8 (cytotoxic T cells), CD4: CD8 ratio, CD3 (total T cells), CD19 (B cells), NK cells