OA-33: Aberrant B cell differentiation induce plasma cell malignancy

Introduction: Multiple myeloma (MM) is the second most common hematological malignancy. It is generally believed that MM cells develop from either B cells or plasma cells differentiated from hematopoietic stem cells (HSCs) in the bone marrow (BM). However, the nature of the earliest precursors of MM cells is still poorly defined.

Methods: Screening the cellular origin of important genetic variation in MM patients and the key factor in cell subsets using scRNA-seq and scATAC-seq combined plasma cell differentiation. By using FISH and scPCR technology to verify the important genetic variation and differential genes of MM cell origin. Gene editing technology and adoptive B cell transplantation mouse model to verify the malignant transformation effect of screening key factor.

Results: Firstly,we sorted HSCs, B cells and plasma cell of new diagnosis MM for scRNA-seq and scATAC-seq sequencing. Principal component analysis revealed the number of B cells declined and abnormal proportion. Further time pseudotime analysis found that MM patients with abnormal differentiation compared with the healthy donor, and “memory B” like subgroups had the potential to differentiate into plasma cells.
By using inferCNV analysis, we found 1q amplification (1qAmp) could be detected in plasma cell and B cells but no HSCs. Furthermore we found that 1qAmp+ B cells have a CD24-FCRL5+phenotype. The use of FISH to validate clinical specimens further confirms this result. Finally, we sorted CD24-FCRL5+subgroups induction in vitro found that they had stronger differentiation potential of plasma cell. Suggesting that the CD24-FCRL5+ B cell subset is the cellular origin of 1qAmp .
In vitro induced differentiation results display interference with FCRL5 expression inhibited B cell proliferation and plasma cell differentiation by used cas9 tag mice. The results were also confirmed by MM patients. The adoptive B cell transplantation by Vκmyc mice confirmed that over expression of FCRL5 promotes B cell malignant transformation and bone destruction. Further omics data analysis found that the open region of FCRL5 chromatin had SPI1 binding site, and FCRL5 upregulated the expression of proliferation related gene MCL1 and B cell differentiation gene IRF4. CHIP qPCR verified that SPI1 directly bound to the promoter region of MCL1 and IRF4. It is suggested that FCRL5 can promote B cell malignancy by recruiting SPI1 and regulating MCL1 and IRF4.

Conclusions: Evidence from our previous study has demonstrated that the 1q amplification, which is one of the most common cytogenetic abnormalities of MM, is detected in CD24-FCRL5+B cell sub-population by single-cell omics. Interestingly, FCRL5 drives MM malignant transformation through the promotive process of B cell proliferation and plasma cell differentiation.Our research will not only provide novel insights for the origin and mechanism of MM with different genetic events, but also provide a new intervention strategy for refractory and relapsed from the perspective of cell origin of MM.