Bruton's tyrosine kinase--an integral protein of B cell development that also has an essential role in the innate immune system

Btk is the protein affected in XLA, a disease identified as a B cell differentiation defect. Btk is crucial for B cell differentiation and activation, but its role in other cells is not fully understood. This review focuses on the function of Btk in monocytes, neutrophils, and platelets and the receptors and signaling cascades in such cells with which Btk is associated.

Atypical X-linked agammaglobulinaemia caused by a novel BTK mutation in a selective immunoglobulin M deficiency patient

Background

X-linked agammaglobulinaemia (XLA) is the most common inherited humoural immunodeficiency disorder. Mutations in the gene coding for Bruton’s tyrosine kinase (BTK) have been identified as the cause of XLA. Most affected patients exhibit a marked reduction of serum immunoglobulins, mature B cells, and an increased susceptibility to recurrent bacterial infections. However, the diagnosis of XLA can be a challenge in certain patients who have near-normal levels of serum immunoglobulin. Furthermore, reports on XLA with renal involvement are scant.

Case presentation

We report an atypical XLA patient who presented with selective immunoglobulin M (IgM) immunodeficiency and nephropathy. He was diagnosed with selective IgM immunodeficiency, based on his normal serum immunoglobulin G (IgG) and immunoglobulin A (IgA) levels but undetectable serum IgM level. Intravenous immunoglobulin was initiated due to increased infections and persistent proteinuria but no improvement in proteinuria was found. A lupus-like nephritis was detected in his kidney biopsy and the proteinuria subsided after receiving a mycophenolate mofetil regimen. Although he had a history of recurrent bacterial infections since childhood, XLA was not diagnosed until B-lymphocyte surface antigen studies and a genetic analysis were conducted.

Conclusions

We suggest that B-lymphocyte surface antigen studies and a BTK mutation analysis should be performed in familial patients with selective IgM deficiency to rule out atypical XLA.