Clonal Analysis of IgM Cells in Multiple Myeloma Patients Suggests the Co-Existence of Normal and Malignant Arms of the Myeloma Clone.
Authors: Taylor Brian J. , Ye Ming , Strachan Erin R. , Tiffinger Tara M. , Belch Andrew R. , Pilarski Linda M. 
Published: April 2020

Abstract
Analysis of immunoglobulin V genes, which undergo stepwise changes during B cell differentiation such as VDJ rearrangement, somatic hypermutation, and class switch recombination, provides insight into the point of transformation of B cell tumors. In Multiple Myeloma (MM), clonotypic VDJ sequences of malignant plasma cells are mutated, homogeneous, and associated with post-switch constant regions (either IgG or IgA, called the clinical isotype), suggesting the malignant arm of the MM clone arises from transformation events in the late stages of the germinal centre reaction. By contrast, the existence of clonotypic VDJ associated with pre-switch IgM is well established, and we have shown persistent clonotypic IgM is associated with advanced disease at diagnosis and poor survival in MM. Whether clonotypic IgM cells represent a malignant progenitor or a non-malignant population that parallels disease severity is unclear. To address these possibilities, we focused our analysis of clonotypic VDJ mutation profiles on IgM+ cells sorted by immunomagnetic separation from MM patient peripheral blood cells (PBMC). IgM clonotypic transcripts were amplified by hemi-nested RT-PCR targeting the CDR2-C mu constant region in IgM+ cells from 4/7 patients. These products were cloned, and 122, 28, 27, and 25 IgM clonotypic colonies were identified by specific CDR2/CDR3 PCR for patients 1–4 respectively. Each of these clones was sequenced, and mutations were identified by comparison with the closest germline V gene and tumor derived plasma cell VDJ sequences. An average mutation frequency of 0.005, significantly greater than the Taq error rate, was obtained for the 250–280 bp fragment downstream of CDR2, including the D-J-C mu region. Typically, MM clones were observed with 1–2 mutations in this region, many localizing to the D-J-C mu region. Small deletions that preserve reading frame were also observed in the D region of single clones of patients 1 and 4 respectively. The detection of intraclonal heterogeneity amongst clonotypic IgM cells may reflect a normal arm of the myeloma clone that co-exists with the post-switch malignant arm. In previous work examining bulk PBMC populations we had detected diversified clonotypic cells in the non-clinical isotype compartment of one patient, but, in accordance with studies performed by several other groups, were unable to detect diversified pre-switch counterparts. In this work we have focused on IgM+ MM B cells, a compartment of the MM clone that may remain driven by antigenic selection and undergo persistent clonal expansion. Our analysis gives insight into the nature of this proposed normal arm of the myeloma clone, revealing two coexisting subsets of pre-switch clonotypic IgM cells: a major set exhibiting homogeneity, identity with post-switch tumor VDJ, and questionable transformation status, and a minor clonally heterogeneous set which may represent the pre-malignant clone from which myeloma arose.