EVIDENCE THAT THE CLONOGENIC CELL IN MULTIPLE-MYELOMA ORIGINATES FROMA PRE-SWITCHED BUT SOMATICALLY MUTATED B-CELL

There is still much controversy about the precursor cell type in multi ple myeloma (MM). Some authors claim that it is a pre-B cell, others s tate that it is a memory B cell or plasmablast. We have recently shown that the VDJ region of the MM immunoglobulin heavy chain gene is soma tically hypermutated and antigen selected, without intraclonal variati on or evolution in time. By using a patient-specific PCR approach we h ave now obtained evidence that the pre-myeloma cell can be situated in the pre-switched B-cell compartment and that heavy chain switching ca n occur without further somatic mutation. Based on the MM immunoglobul in sequences derived from the bone marrow, patient-specific CDR2 and C DR3 oligonucleotides were designed. B lymphocytes were separated from plasma cells based on the expression of CD19 and HLA class II or surfa ce bound IgM using immunomagnetic beads. The expressed Ig sequences we re amplified by RT-PCR using patient specific CDR2 primers and isotype specific primers (C mu, C gamma and C alpha). Myeloma-specific Ig seq uences were detected by a myeloma-specific CDR3 probe and sequenced. I n one out of five cases we found in the peripheral blood clonally rela ted IgM and IgA sequences with the same somatic mutations as the MM-Ig G sequence. In another case of an IgG MM we found in the bone marrow c lonally related IgA sequences with the same somatic mutations. These f indings, together with the fact that myeloma-Ig genes contain somatic mutations without intraclonal variation, suggest that the clonogenic c ell in multiple myeloma can originate from a pre-switched but somatica lly mutated B cell.