P805: extramedullary multiple myeloma: molecular pathogenesis and novel therapeutic targets

Topic: 13. Myeloma and other monoclonal gammopathies - Biology & Translational Research Background: Extramedullary disease (EMD) is an aggressive manifestation of multiple myeloma (MM), where MM plasma cells become independent of the bone marrow (BM) microenvironment and infiltrate other tissues and organs. The incidence of EMD is increasing and is associated with worse prognosis and drug resistance. Importantly, the molecular processes underpinning the pathogenesis and resistance of extramedullary form of MM are poorly understood. Therefore, we performed the most comprehensive next generation sequencing (NGS) study of EMD cells to date, combining the whole exome sequencing (WES) and RNA sequencing (RNA-seq) data from 12 patients with EMD. Aims: To identify key molecular features of EMD. To evaluate expression of therapeutic targets in EMD plasma cells. To suggest new treatment strategies steaming from EMD biology. Methods: We sorted plasma cells from EMD and BM at the time of diagnosis. We analyzed whole exome (WES) and transcriptomic (RNA-seq) data from 12 and 11 patients with EMD, respectively. Of those, we were able to obtain 7 WES and 6 RNA-seq from paired BM samples from the time of diagnosis. To match the number of samples in EMD and BM cohort for differential expression analysis, we sequenced RNA also for 5 unrelated newly diagnosed MM (NDMM) samples Results: We detected 1q21 gain or amplification in 11/12 patients (92%; 2 gains and 9 amplifications). Del(13q) and del(17p) were detected in 6 (50%) and 5 (42%) EMD cases, respectively. Next, we identified overall higher mutational load in EMD and at least one mutated gene in MAPK signalling in virtually all EMD samples except one (11/12, 92%). All mutations in EMD tumors were detected to be clonal (CCF > 0.8) which suggest that all preceded the extramedullary tumor development. To infer the biological features of EMD we used RNA-seq to compare 11 EMD samples with 11 NDMM samples from which 6 samples were paired. In total, we identified 906 significantly deregulated genes. Pathway enrichment analysis revealed up-regulation of pathways connected to cell cycle and cell division and down-regulation of pathways connected to immune response. Interestingly, CXCR4 gene, important for a homing of plasma cells in bone marrow, was significantly down-regulated. Importantly, we observed production of interleukin 6 (IL6), a potent growth factor for PCs, in majority of EMD samples. Immunoglobulin production decreased in EMD compared to paired NDMM samples whilst the ratio of heavy/light chains was shifted towards light chains in EMD in almost all samples. Importantly, our data also revealed a decrease in the expression of several immunotherapeutic targets(e.g. CD38, SLAMF7, BCMA) on EMD cells. In addition we also observed lower expression of MHC-1 molecules on EMD cells, that could be responsible for immune evasion and thus could explain lower efficacy of treatment T cell engagers in patients with EMD. Intriguingly, our data show EMD elevated expression of EZH2 and CD70 that are used in lymphomas and solid tumors and might represent promising targets also in EMD. Summary/Conclusion: We identified 1q21 amp and mutated MAPK pathway as crucial mutational events for EMD development, respectively. Further, EMD cells display up-regulated pathways connected to cell division, that is probably regulated in autocrine stimulation by IL6 and down-regulation of molecules connected to plasma cell function e.g. IGs or CXCR4. Finally, we showed lower expression of key MM molecular targets and MHC-1 molecules on EMD cells and propose CD70 and EZH2 as potentially new promising targets for EMD treatment. Keywords: Multiple myeloma