Unusual gamma heavy chain disease protein in a patient with splenic marginal-zone lymphoma

We conducted an electrophoretic analysis of monoclonal gamma-globulin found in the serum of a patient with splenic marginal-zone lymphoma. This monoclonal protein showed electrophoretic mobility to the gamma region and reacted with anti-immunoglobulin (IgG) antiserum but not with anti-kappa or anti-lambda light chain antisera. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting of the monoclonal protein-rich gamma-globulin fraction extracted from the sliced gel revealed the presence of two types of abnormal IgG molecule, low- and high-molecular-weight IgG, neither of which reacted with anti-kappa or anti-lambda light chain antisera. Additionally, an abnormal high-molecular-weight gamma heavy chain was identified by reducing SDS-PAGE. These findings suggest that this monoclonal protein is composed only of gamma heavy chains of normal and larger size. The presence of abnormal serum immunoglobulin composed of only gamma heavy chain has been known as a fundamental feature of gamma heavy chain disease (HCD). However, the unique composition of monoclonal gamma-globulin makes our case distinct from typical gammaHCD, which is characterized by an abnormal truncated low-molecular-weight gamma heavy chain. Thus, the unusual monoclonal protein may have been produced by a somatic mutation of IgH gene associated with splenic marginal-zone lymphoma.