Molecular Interrogation of Biclonal Multiple Myeloma for Clonal Relatedness.

Abstract 2928 Multiple myeloma (MM) is caused by the clonal expansion of malignant plasma cells (PCs) producing high levels of monoclonal immunoglobulin (Ig) or M-protein. However, ∼2% of all patients with monoclonal gammopathies (MG) have more than 1 M-protein, a condition termed biclonal gammopathy or double monoclonal gammopathy (DMG). These patients can express M-proteins with different heavy chain (HC) isotypes and different light chains (LCs) or different HCs that are LC-matched. Reports suggest that ∼87% of DMGs with different HCs have matched LCs, but without molecular analysis at the nucleotide level, clonal relationships are unknown. Thus, DMG patients may have either two independent malignant PC clones that coincidentally express the same type of LC or the two clones are related and reflect previous or ongoing class switch recombination (CSR). This latter variant would offer novel insight into the origin and differentiation of the malignant clone. Of note, prior reports of HC-dissimilar/LC-matched DMGs are limited to the analysis of single patients and with few exceptions; do not include molecular analysis of the variable regions in Ig HC or LC genes at the nucleotide level. In this study, we interrogate a cohort of MM patients with known HC-dissimilar/LC-matched DMGs for the frequency of clonal relatedness. The cohort was identified by searching our MG database for patients with: 1) HC-dissimilar/LC-matched Igs as determined by immunofixation; 2) >10% bone marrow (BM) PCs; and 3) no transplant prior to BM collection. Eight patients (7 MM and 1 smoldering MM/primary amyloid) were identified for further molecular evaluation. All 8 cases expressed IgG and IgA HCs. Three patients expressed kappa LC (K-LC) and 5 expressed lambda LC (L-LC). To determine if the IgG and IgA M-proteins were clonally related in each patient, we performed Ig HC variable gene (IGHV) PCR with sense primers to all IGHV genes and an antisense primer specific to IgG or IgA. In all 8 patients, a single productive IGHV gene was determined with IgG and IgA. In 2/8 patients, the IgG and IgA HC variable regions were clearly distinct and therefore genuinely biclonal. Of note, PCs from 6/8 patients used the same IGH V-D-J genes in both IgG and IgA transcripts. In 5 of these 6 patients, the entire IgG and IgA variable region and HCDR3 were identical at nucleotide and amino acid (AA) levels. Of interest, one patient had common, as well as distinguishing IGHV region somatic mutations (SHM) between the IgG and IgA transcripts. The HCDR3 regions (including N nucleotides) were highly homologous but differed by 7 nucleotides which resulted in 3 AA differences between the 2 clonally related sequences. While these are compelling data, it remained possible that the malignant PCs in these patients simultaneously expressed IgG and IgA as a result of long nuclear transcripts. To test this possibility, we used immunofluorescence (IF) and verified the presence of both IgG and IgA expressing PCs in all cases. Parallel analysis of both K-LC and L-LC variable gene by PCR was done on the 6 patients with the same IGHV gene in both IgG and IgA transcripts. In 3/6 patients, there was a single somatically mutated LC identified corresponding to the LC detected by immunofixation. These results were also confirmed by IF. The remaining 3 patients appeared to express more than 1 LC rearrangement. Taken together these results confirm that HC-dissimilar/LC-matched DMG can present with various phenotypes at the molecular level. We describe the largest cohort to date documenting the frequency of DMG patients that can be classified as expressing 2 identical HCs with either a single LC or >1 LC based on molecular analysis. The former case may reflect an original progenitor cell that divided with one daughter cell subsequently experiencing further CSR yet the LC remained untouched. In the case where the HCs were slightly different, there may have been CSR with additional SHM of a daughter cell, or the molecular differences may reflect random somatic mutations occurring independent of germinal center reactions. These results suggest that HC-dissimilar/LC-matched DMG may occur more often than thought. Larger studies tracking M-protein levels over time accompanied by cytogenetic analyses are needed to determine if the two clonally-related class switch variants in DMG patients exhibit similar biological properties. This analysis would offer new insight into the genesis of this unique subtype of MM. Disclosures: No relevant conflicts of interest to declare.