Analysis of the Immunoglobulin Heavy and Light Chain Gene Expression and Mutations in POEMS Patients.

Abstract 4986

The plasma cell disorder POEMS syndrome is defined by the following major features: peripheral neuropathy (P), organomegaly (O), endocrinopathy (E), monoclonal (immunoglobulin) protein expression (M), and skin changes (S). One of the most noticeable characteristics of this disease is that virtually all patients have lambda (γ) rather than the kappa (κ) restricted clonal plasma cells. We hypothesized that this restricted expression and possibly mutated heavy and light chain genes may be linked to the pathogenesis of POEMS. To this end, we analyzed light and heavy chain gene sequences from 21 POEMS patients at Mayo Clinic Rochester. We found that 16 of these patients had dominant light chain gene sequences, and that the expression of light chain was restricted to IGLV 1 subfamily and more specifically, to the IGLV1-40 or the IGLV 1-44 subtype. In contrast, no such stereotypy was observed at the immunoglobulin heavy chain locus (IGHV), though IGHV gene expression indicated a high degree of clonality. We aligned these sequences with their corresponding germline consensus sequences in IMGT database to assess similarity and divergence. IGLV1-40 and IGLV 1-44 sequences from POEMS patients were also aligned with the sequences in the light chain library on Boston University Amyloid Database (http://pulm.bumc.bu.edu/aldb), and POEMS IGHV sequences were compared to IGHV sequences reported in the literature for multiple myeloma, MGUS (monoclonal gammopathy of unknown significance) and CLL (chronic lymphocytic leukaemia). We found that: i) both the restricted light- and the non-restricted heavy chain genes in POEMS patients varied significantly (>2%) from their germline counterparts; ii) the incidence of somatic hypermutation in IGHV was higher in POEMS than that reported for other plasma cell disorders like multiple myeloma, MGUS and CLL, iii) in IGLV, mutations occurred at non-random, clustered hotspots located in specified areas of the gene (such as the structural elements that contribute the antigen binding CDR domain); iv) mutations occurring in the CDR regions tended to be conserved (the amino acid in POEMS was similar to that in germline in its physicochemical properties), and v) the histidine residue at position 40 (H40) in IGLV1-40 and the threonine at position 38 (T38) in IGLV1-44 are POEMS-specific. These observations suggest that an antigen driven somatic hypermutation mechanism may mark a post-germinal precursor plasma cell for selection, but further transformation of this selected cell into a malignant POEMS clone occurs via a different, hitherto unknown, mechanism. They also highlight the significance of H40 (in IGLV 1-40) and T38 (in IGLV 1-44) residues, which seem to be highly POEMS-specific (occurring seldom in other plasma cell disorders), and which may bias the selected plasma cell clone towards POEMS syndrome phenotype.