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Unraveling of the Polymorphic C2-C3 Amplification and the Ke⁺Oz^- Polymorphism in the Human Ig Locus

Department of Immunology, Erasmus University Rotterdam/University Hospital Rotterdam, Rotterdam, The Netherlands

Two polymorphisms of the human Ig (IGL) locus have been described. The first polymorphism concerns a single, 2- or 3-fold amplification of 5.4 kb of DNA in the C2-C3 region. The second polymorphism is the Mcg^-Ke⁺Oz^- isotype, which has only been defined via serological analyses in Bence-Jones proteins of multiple myeloma patients and was assumed to be encoded by a polymorphic C2 segment because of its high homology with the Mcg^-Ke^-Oz^- C2 isotype. It has been speculated that the Mcg^-Ke⁺Oz^- isotype might be encoded by a C gene segment of the amplified C2-C3 region. We now unraveled both IGL gene polymorphisms. The amplification polymorphism appeared to result from a duplication, triplication, or quadruplication of a functional J-C2 region and is likely to have originated from unequal crossing over of the J-C2 and J-C3 region via a 2.2-kb homologous repeat. The amplification polymorphism was found to result in the presence of one to five extra functional J-C2 per genome regions, leading to decreased Ig:Ig ratios on normal peripheral blood B cells. Via sequence analysis, we demonstrated that the Mcg^-Ke⁺Oz^- isotype is encoded by a polymorphic C2 segment that differs from the normal C2 gene segment at a single nucleotide position. This polymorphism was identified in only 1.5% (2 of 134) of individuals without J-C2 amplification polymorphism and was not found in the J-C2 amplification polymorphism of 44 individuals, indicating that the two IGL gene polymorphisms are not linked.

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