Cellular origin and extent of clonal involvement in multiple myeloma: genetic and phenotypic studies

Abstract

The cellular origin and extent of clonal involvement in multiple myeloma (MM) are controversial. The third-complementarity-determining region (CDR3) of the immunoglobulin heavy chain gene is the target region of V_H replacements and somatic mutations. We analysed the CDR3 sequences of myeloma cells from eight newly diagnosed and three relapsed patients in order to elucidate the target cell of malignant transformation in MM. We also examined the extent of clonal involvement in MM using a CDR3 clone-specific nucleic acid probe. The peripheral lymphocytes from the five MM patients were separated into fractions such as CD34⁺, CD20⁺CD10⁺, CD20⁺CD21⁺, CD20⁺CD19⁻ and CD2⁺ cells. Amplified CDR3 DNAs from these subpopulations were hybridized with the probe specific to each patient's tumour cells. We found no evidence of ongoing V_H replacements or somatic mutations in CDR3 in MM. However, frequent nucleotide mutations in D and J_H segments were observed. Circulating malignant cells were detected in the CD34⁺ and all of the CD20⁺ subpopulations, but not in the CD2⁺ fraction. MM is a neoplasm originating from a B-lineage cell which has already undergone antigendependent selection. Nevertheless, the tumour cells are composed of heterogeneous subpopulations at various stages of differentiation, similar to normal B-lineage cells. Conversely, T cells were not involved in MM. These results imply that there is an analogous developmental pathway between the normal B-lineage cells and the tumour cells of MM.