(OA-42) Clonotypic B Cells are Passengers During Malignant Transformation and not Drivers of Disease Progression in Plasma Cell (PC) Neoplasms

Whether multiple myeloma (MM) and light-chain amyloidosis (AL) stem from terminally differentiated PC or earlier clonotypic B cells remains under debate. Addressing this issue would improve accurate diagnosis and treatment monitoring. Therefore, our aim was to identify the cell of origin and tumor reservoirs containing driver genetic alterations within the B cell lineage of MM and AL patients.

Methods:

18 healthy adults (HA), 22 MM and 17 AL patients (pts) were studied. Single cell RNA and B cell receptor sequencing (scRNA/BCRseq) was performed in sorted B cell precursors, mature B cells and PC. Exome sequencing was performed in sorted CD34 progenitors, B-cell precursors, mature B cells, normal and clonal PC, and T cells as control.

Results:

scRNA/BCRseq yielded paired transcriptomes and Ig gene rearrangements in 61,289 cells, classified into B cell precursors (n=3,808), mature B cells (n=28,589) and PC (n=28,892). Surprisingly, tumor BCR were detected in 0.3% B-cell precursors from MM pts, and in 0.4% and 0.1% mature B cells from MM and AL pts. Phenotypic hallmarks of the B cell differentiation (eg, CD10, CD19, CD20 or CD138) were similarly expressed in non-tumor vs tumor-related B cell subsets. However, the latter showed on average 329 differentially expressed genes when compared to normal counterparts in HA. These results confirm the immature phenotype of clonotypic cells and uncover altered transcriptomes possibly due to the disturbed tumor microenvironment.

Exome sequencing unveiled an average of ~30% somatic mutations shared between CD34 progenitors and normal PC from HA and pts. These data indicates a continuum of mutated cells throughout the normal and malignant B-cell lymphopoiesis. In AL, there were 30%, 34%, 31% and 28% shared mutations between clonal PC and CD34 progenitors, B cell precursors, mature B cells and normal PC. In MM, the respective percentages were 20%, 17%, 11% and 17%. Similar results were observed before and after transplant. Interestingly, driver mutations as well as copy number alterations were generally private in clonal PC, and absent in normal cell types from MM and AL pts.

We hypothesized that clonotypic B cells with a normal phenotype could result in sporadic cases of NGS+/ NGF- MRD. Such pts should have PFS similar to double negative MRD patients because clonotypic B cells lacked key genetic alterations and therefore cannot drive relapse. Thus, we studied 103 MM pts enrolled in the GEM2012MENOS65 trial who had simultaneous MRD assessment by NGF and NGS. Of the 103 pts, 7 were NGF-/NGS+ despite MRD levels above the limit of detection of NGF. After a median f/up of 80 months, these patients had a median PFS not reached near to those who were NGF-/NGS-. Similar discordances were observed in AL pts.

Conclusions:

Our results define clonotypic B cells as passengers during the malignant transformation of PC neoplasms and not drivers of disease progression. These findings shed light on the pathogenesis and inform on how to monitor MM and AL pts.