Shared N-Terminal Sequences in Monoclonal IgMκ and IgGκ Proteins from a Patient with a Complex Multiple Paraprotein Disorder

Abstract

Abstract The N-terminal sequence analyses were performed on the heavy (H) and light (L) chains of the idiotypically identical IgMκ and IgGκ paraproteins isolated from the serum of patient, Cam. The N-terminal 39 residues of the κ chains of the IgM and IgG were identical and belonged to the human VκIII subgroup. This sequenced stretch included the first L chain hypervariable region. The N-terminal 27 residues of the variable regions (VH) of the respective µ and γ heavy chains were also identical and belonged to the human VHIII subgroup. These identical VH sequences were unique with lysine residues at positions 13 and 19. This dual lysine substitution has not been seen in 37 other human VHIII sequences reported in the literature. This N-terminal sequence homology in the V-regions of Cam IgMκ and IgGκ paraproteins and the shared idiotypy expressed by Cam IgM, IgG, and IgA proteins strongly suggest the existence of complete structural homology in the variable regions of the H and L chains of these Ig molecules of three separate Ig classes. At the cellular and genetic level, these results point toward a common clonal origin for the idiotypically related Ig molecules and suggest that identical V-region (VH and VL) genes were utilized by the Cam lymphoid clone in the biosynthesis of the respective IgM, IgG, and IgA proteins.