Cryocrystalglobulinemia is characterized by a monoclonal immunoglobulin that is both cryoprecipitable and crystalforming (cryocrystalglobulin). It occours in humans as a rare complication of lymphoproliferative disorders. The monoclonal immunoglobulins may crystallize in serum and synovial fluid and are responsible for both microcrystalline synovial inflammation and occlusive vasculopathy in kidneys and skin. At present, there are no reports on sequence analysis and tertiary structure of human cryocrystalglobulins. In this study, we established the first complete light and heavy chain variable region sequences of a human cryocrystalglobulin (BEL) from a patient with severe microcrystalline arthropathy, cutaneous purpura and sacroiliitis. Serum cryoprecipitate was composed of unbound monoclonal IgGk at immunofixation (type I cryoglobulinemia) and revealed homogeneous crystals under light microscopy. Extra and intracellular crystals were also found in synovial fluid. Analysis of bone marrow aspirate showed 4% monoclonal plasma cell infiltration. Total RNA was extracted from Ficoll-separated bone marrow mononuclear cells and complete heavy (VH) and light chain (Vk) variable domain sequences of monoclonal Ig were obtained by means of an unbiased inverse-PCR strategy. Sequences were then matched in databases to identify rearranged germline V, D and J segments. The heavy chain sequence belonged to the g2 class and its variable region was most closely related (94% identity) to the VH3-30 germline gene (VH3 family) rearranged to D2-2 and JH4b segments, whereas the Vk region was derived (97% identity) from the germline VkL6 (VkIII) rearranged to Jk5. Partial protein sequence of serum cryocrystalprecipitate confirmed the correct identification of the monoclonal sequences. A previously reported murine model of cryocrystalglobulinemia suggested that acquisition of specific amino acids at critical positions within the VH domain (positions 6 and 23) could influence cryoprecipitation (

Rengers J-U et al,
). Analysis of BEL VH domain did not confirm these observations. Protein modelling and site-directed mutagenesis studies are undergoing to clarify the role of amino acid substitutions and interactions in immunoglobulin crystal-like aggregation and cryoprecipitation.

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