Single Cell RNA-Seq Reveals Clonal Consistency and Differential Malignancy in Extramedullary Plasmacytoma of Multiple Myeloma

Extramedullary Plasmacytoma (EMP) is a minor yet devastating metastatic form of Multiple Myeloma (MM), shortening patients' survival from 10 years to 6 months on average. Genetic cause of EMP in MM is yet to be defined. Transcriptome difference between EMP+ patients and EMP- patients is studied here on single cell level by RNA Sequencing (RNA-Seq). We sorted CD38+CD138+ malignant plasma cells from bone marrow and peripheral blood samples by flow cytometry, then picked up single malignant plasma cell and performed single cell RNA-Seq with SmartSeq2 protocol followed by Tn5-based library preparation from bone marrow, peripheral blood and extramedullary tissue of EMP patients. From the single cell RNA-Seq results, in bone marrow we found differential gene expression between EMP+ and EMP- samples, such as CTAG2, STMN1 and RRM2. By comparing circulating malignant plasma cells in PBMC and malignant plasma cell from the sample EMP+ patient, we observed metastatic clone in blood with the same VDJ immunoglobulin heavy chain as in bone marrow. Several genes' expression of these metastatic cells are down-regulated than in bone marrow, such as PAGE2, GTSF1, DICER1. These genes may correlate with egress capability of MM cells into peripheral to become circulating plasma cells (cPCs), and EMP eventually.