Detailed immunophenotypic and genetic characterization of bone marrow clonal plasma cells and B-cells in early-stage monoclonal gammopathies.

Monoclonal gammopathy (MG) of undetermined significance (MGUS) is the earliest and most prevalent stage of MG that may progress to symptomatic multiple myeloma (MM) or Waldenström's macroglobulinemia (WM) through intermediate smoldering forms of both diseases. We used next-generation flow cytometry (NGF) to identify and purify bone marrow (BM) clonal plasma cells (cPC) and B-cells/lymphoplasmacytic cells (cB-cells) from 25 asymptomatic individuals with MG diagnosed within the population-based iStopMM screening program for further molecular analysis by next-generation sequencing (NGS). IgM-MGUS cases consistently exhibited WM-like phenotypes by NGF, and MYD88 p.L265P and/or CXCR4 mutations in the coexisting cPC and cB-cells. Conversely, cPC from non-IgM MGUS and smoldering MM (SMM) displayed a MM-like phenotype associated with genetic drivers -i.e., t(11;14)-, frequently coexisting with genetically unrelated cB-cells. Interestingly, recurrent RUNX1 single nucleotide variants were detected in cPC, but not coexisting cB-cells, from WM-like cases, suggesting a role for RUNX1 in PC differentiation. In summary, NGF enabled detailed immunophenotypic characterization of BM cPC and cB-cell populations in all cases analyzed, subdivided in IgM vs non-IgM cases with WM-like or MM-like phenotypic and genetic profiles, respectively. These findings underscore the potential contribution of NGF and NGS for accurate classification or early-stage disease, beyond conventional criteria.