A new case of γ-heavy chain disease (LIA protein) with deletion of the hinge region

An IgG1K monoclonal component with abnormal covalent H and L chains structure (LIA protein) was identified during a systematic screening of myeloma proteins by means of nonreducing/reducing SDS-polyacrylamide gel electrophoresis. Using immunochemical and immunogenetic analysis the mutation was characterized as a hinge region deletion, with loss of L-H and H-H disulphide bridges and direct L-L bonds. Moreover, non-expression of the Glm(z) allotype suggested that the deletion might start at residue 216, a preferential site previously observed in other HCD proteins. This feature is in agreement with the discontinuous structure of immunoglobulin C_H genes and suggests that an abnormal switch mechanism is responsible for the deletion.