Characterisation of the Tumour Proteome in Primary Extramedullary Multiple Myeloma Identifies Key Proteins Associated with Transendothelial Migration

Introduction: Extramedullary multiple myeloma (EMM) refers to the spread of clonal plasma cells to tissues extending outside of the bone marrow microenvironment. EMM is present at the time of diagnosis in 6-10% of patients, however, this increases to 13-26% in patients with disease progression and relapse. Cancer cells are suspected to spread to new tissues and organs via the circulatory system as a result of molecular changes that allow malignant cells to escape the bone marrow (BM). For example, downregulation of CXCR4 (C-X-C Motif Chemokine Receptor 4), an important factor in cellular homing to the BM, has frequently been reported to be linked to the EMM phenotype. In addition, the majority of patients presenting with EMM have highly complex cytogenetic abnormalities and high-risk cytogenetic markers such as t(14;16). As EMM is an indicator of a more aggressive disease, more intensive treatment, including combination chemotherapy, is often recommended. Many of the underlying molecular mechanisms accompanying EMM are yet to be characterised. Our mass-spectrometry (MS)-based proteomic study provides insight into the unique molecular mechanisms associated with EMM, identifying key proteins linked to the progression of medullary multiple myeloma (MM) to EMM. Methods: Label-free liquid chromatography mass spectrometric analysis of age and gender matched medullary MM (n=8) and EMM (n=9) bone-marrow derived mononuclear cells (MNCs) was carried out using a Thermo Orbitrap Fusion Tribrid mass spectrometer (Thermo Fisher Scientific). Proteome Discoverer 2.2 using Sequest HT (Thermo Fisher Scientific) and a percolator were employed for the identification of peptides and proteins. For protein identification, the following search parameters were used: (i) 0.02 Da for MS/MS mass tolerance, (ii) 10 ppm for peptide mass tolerance, (iii) variable modification settings for methionine oxidation, (iv) fixed modification settings in relation to carbamido-methylation and (v) tolerance for up to two missed cleavages. Peptide probability was set to high confidence. Datasets were imported into Progenesis QI (version 2.0) software for further analysis. Data was filtered based on an ANOVA p-value of ≤0.05, fold change >1.5 between experimental groups, and proteins with ≥1 unique peptides contributing to the identification. Proteins with less than 70% valid values were removed from the analysis. G:profiler and STRING were utilized for functional enrichment and the characterisation of protein interaction patterns. Results: Our quantitative MS-based proteomic analysis identified a total of 492 proteins with significantly altered abundances between EMM and MM bone marrow MNC. Of these significant proteins, 275 were found to be increased in EMM compared to medullary MM and 217 were found to be decreased in EMM compared to medullary MM. Hierarchical clustering was performed to highlight the proteomic profile associated with extramedullary disease (Figure 1A). KEGG pathway analysis and gene ontology (GO) analysis of proteins found to be increased in EMM indicated an increase in proteins associated with cell adhesion, invasion, and migration pathways (Figure 1B). Interestingly, several proteins involved in leukocyte transendothelial migration were significantly increased in EMM indicating their potential involvement in the dissemination of MM cells from the bone marrow microenvironment to distal tissues (Figure 1C). Among the proteins found to be involved in this biological pathway was junctional adhesion molecule-A (F11R), a protein previously reported to play a role in EMD pathophysiology [1]. Other proteins involved in MM invasion and migration including Rho-associated protein kinase 2 (ROCK2), Ras-related C3 botulinum toxin substrate 1 (Rac1) and platelet endothelial cell adhesion molecule (PECAM-1) were significantly increased in EMM. Conclusion: Using high-resolution mass spectrometry to characterise the tumour proteome of MM patients with extramedullary disease, we have identified a significant increase in the abundance of proteins associated with leukocyte transendothelial invasion in primary EMM samples. Our study provides further insight into the molecular mechanisms within EMM and thus holds potential to enhance current efforts to provide a more personalised therapeutic approach for EMM patients. References: [1] A.G. Solimando et al., Blood 2018; 132 (Supplement 1): 4455. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.