ANALYSIS OF THE KAPPA-LIGHT-CHAIN VARIABLE REGION IN MULTIPLE-MYELOMA

The study of immunoglobulin heavy chain gene rearrangements in multipl e myeloma has revealed extensive divergence from the germline sequence s, but no intraclonal diversity with disease evolution. Our study inve stigated the state of the rearranged kappa light chain variable region (V kappa) gene segments, as well as abortive V kappa family gene usag e in cases of multiple myeloma expressing lambda light chain. We studi ed 11 cases of kappa and five cases of lambda light chain-expressing m ultiple 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 rearr anged variable region sequences. Direct nucleotide sequencing by the d ideoxy-chain termination method was performed on the RT-PCR products. We did not observe preferential usage of certain V kappa gene families . Mutation frequencies of the V kappa segments varied in number. In th e majority of cases, extensive somatic mutations occurred within the c omplementarity determining regions (CDRs) of V kappa, whereas only a l imited degree of divergence from the germline was observed in others. In all cases studied, replacement mutations tended to cluster in the C DRs, a finding compatible with an antigen-driven somatic hypermutation process, In 3/5 cases of lambda light-chain expressing multiple myelo ma, abortively rearranged V kappa gene segments were amplified from ge nomic DNA: in two cases a non-templated nucleotide insertion rendering the V kappa sequences out-of-frame was observed, and in the third a s top codon was identified in the open reading frame of the V kappa sequ ence. Somatic mutations were observed in all cases of abortive V kappa genes studied; however, their distribution does not suggest selection by antigen. We conclude that somatic mutations observed in the V kapp a regions of myeloma cells are of variable extent and suggest operatio n of the antigen selection process. Lack of or minimal somatic hypermu tation in a few cases may be in some way implicated in the biological heterogeneity of the disease.