A human myeloma IgA with a hybrid heavy chain resulting from putative somatic gene conversion

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Abstract
A human monoclonal IgA1‐IgA2λ. hybrid molecule was detected in a myeloma patient homozygous for the A2m(1) allotype during a systematic study of monoclonal IgA with subclass‐specific monoclonal antibodies (mAb) and the lectin jacalin. This monoclonal immunoglobulin (GAU) reacted with both, although not with all, anti‐α1 and anti‐α2 mAb. Its heavy (H) chain contained an al hinge region as shown by jacalin reactivity, the presence of disulfide‐linked H and light chains in spite of its belonging to the IgA2m(1) allotype and amino acid composition of the isolated hinge region. The complete sequence of the H chain was deduced from that of complementary DNA clones from bone marrow cells. The CHI domain, hinge region and beginning of the CH2 domain and the membrane peptide were encoded by the al gene, with an insertion of an α2m(1) gene sequence accounting for the end of the CH2 and part or all of the CH3 domain (sequence identity between the two normal genes precludes a precise definition of breakpoints). The region of the 5′ recombination site included a repeat of a six base pair sequence which might play a role in the genetic exchange. The GAU hybrid a gene was undetectable in the patient's genomic DNA from polymorphonuclear cells. The genetic event which occurred at the level of the proliferating plasma cell clone is most likely to be a gene conversion.