Analysis of the κ light chain variable region in multiple myeloma

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Abstract
The study of immunoglobulin heavy chain gene rearrangements in multiple myeloma has revealed extensive divergence from the germline sequences, but no intraclonal diversity with disease evolution. Our study investigated the state of the rearranged κ light chain variable region (Vκ) gene segments, as well as abortive Vκ family gene usage in cases of multiple myeloma expressing λ light chain. We studied 11 cases of κ and five cases of λ light chain‐expressing multiple myeloma. Total cellular RNA was extracted from the bone marrow of patients with overt disease and subjected to reverse transcription‐polymerase chain reaction (RT‐PCR) analysis to amplify clonally rearranged variable region sequences. Direct nucleotide sequencing by the dideoxy‐chain termination method was performed on the RT‐PCR products. We did not observe preferential usage of certain Vκ gene families. Mutation frequencies of the Vκ segments varied in number. In the majority of cases, extensive somatic mutations occurred within the complementarity determining regions (CDRs) of Vκ, whereas only a limited degree of divergence from the germline was observed in others. In all cases studied, replacement mutations tended to cluster in the CDRs, a finding compatible with an antigen‐driven somatic hypermutation process. In 3/5 cases of λ light‐chain expressing multiple myeloma, abortively rearranged Vκ gene segments were amplified from genomic DNA; in two cases a non‐templated nucleotide insertion rendering the Vκ sequences out‐of‐frame was observed, and in the third a stop codon was identified in the open reading frame of the Vκ sequence. Somatic mutations were observed in all cases of abortive Vκ genes studied; however, their distribution does not suggest selection by antigen.We conclude that somatic mutations observed in the Vκ regions of myeloma cells are of variable extent and suggest operation of the antigen selection process. Lack of or minimal somatic hypermutation in a few cases may be in some way implicated in the biological heterogeneity of the disease.

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