MYELOMA IG HEAVY-CHAIN-V REGION SEQUENCES REVEAL PRIOR ANTIGENIC SELECTION AND MARKED SOMATIC MUTATION BUT NO INTRACLONAL DIVERSITY

The Ig V-H region sequence in 48 patients with multiple myeloma (MM) w as analyzed to characterize the malignant cell of origin. The sequence s were obtained after amplification of bone marrow cDNA by using V-H f amily-specific and C-H primers, then compared with either directly seq uenced patient germ-line or published V, gene sequences to assay for s omatic mutation. Because somatic hypermutation of the V, gene occurs l ate in B cell development, its presence has been helpful in determinin g the cell of origin in other B cell malignancies. Overall, a median o f 8.2% of the nucleotides had evidence of substitution within each V-H gene sequence (range = 2.7% to 16.5%), which is more prevalent than i n any other reported tumor type. Strong evidence of prior antigenic se lection pressure was also evident. The ratio of nucleotide substitutio ns that resulted in amino acid replacement was significantly higher in the complementarity-determining region than in the framework region ( 3.25 vs 1.56, respectively; p < 0.00005). No V-H gene intraclonal dive rsity was noted, despite sequencing multiple clones (3-16) from each p atient, nor was there evidence of further VH gene somatic mutation ove r the course of three patients' disease. These findings strongly imply that the malignant clone in MM evolves from a cell late in B cell dev elopment.