Single-Cell Pathway Enrichment and Regulatory Profiling of Multiple Myeloma across Disease Stages
Authors: Wang Tianjiao , Sun Hua , Cui Zhou Daniel , Liu Ruiyang , Yao Lijun , Fiala Mark A. , Kohnen Daniel R. , O'Neal Julie , Rettig Michael P. , Fowles Jared S , Oh Stephen T. , Jayasinghe Reyka G , Vij Ravi , DiPersio John F. , Ding Li 
Published: February 2020

Multiple myeloma (MM) is a hematological malignancy, defined by aberrant monoclonal proliferation of plasma cells in the bone marrow, that to date remains an incurable disease despite advances in treatment. Key genetic and epigenetic alterations that drive MM pathogenesis have been identified, but a comprehensive profile of affected cellular pathways has yet to be fully characterized. In this study, we integrate whole-genome and whole-exome sequencing data with single-cell RNA sequencing (scRNA-seq) data from 13 patients across multiple treatment stages to 1) assess differential pathway enrichment between tumor subpopulations, 2) trace the clonal evolution of dominant disease mechanisms, and 3) investigate signaling interactions between surrounding cell types. We also analyzed bulk genomic and transcriptomic data from 662 additional Multiple Myeloma Research Foundation (MMRF) tumor samples as a large reference cohort for highly prevalent pathway disturbances.
To assess whether tumor subpopulations rely on different oncogenic programs for proliferation, we analyzed the differential expression of key genes (FDR-adjusted p-value &lt;0.05) in 12 canonical oncogenic pathways. Cell cycle, Hippo, RTK/RAS, and NFkB pathways contain the highest numbers of differentially expressed genes, with certain subclusters upregulating as many as 25% of annotated cell cycle genes and over 90% of annotated Hippo genes, whereas p53, Notch, Nrf2, and DNA repair genes tend to be uniformly expressed across subpopulations.
Next, we evaluated changes in pathway enrichment across different disease timepoints, with the goal of capturing the reorganization of functional profiles through successive treatment and relapse cycles. We assessed statistical enrichment of pathways containing differentially expressed genes (DEGs) unique to Smoldering Multiple Myeloma (SMM), primary, and relapse stages using the KEGG pathway database (n = 2, 17, and 7 pathways, respectively; FDR-adjusted p-value of enrichment &lt; 0.05). SMM is the only stage where hematopoietic differentiation and the PI3K-Akt pathways are variably expressed between plasma cell subpopulations, suggesting that these pathways may play a role in initiating events. Only primary tumor samples show significant intra-tumor variability of p53 regulation, which is lost in the relapsed tumor and may reflect selection due to treatment. Relative to SMM, primary and relapse samples are enriched for changes in the MAPK, NFkB, RAP1, and cell cycle pathways, indicating potential sources of tumor resistance.
We then analyzed pathway enrichment within the tumor microenvironment to enhance our understanding of tumor development in the context of surrounding tissues. We see frequent changes in many immune cell types in TLR signaling as the disease progresses, driven by differential expression of NFkB1A, JUN, and FOS, all of which are key upstream regulators of the AP-1 pathway and responders to the MAPK and PI3K signaling cascades. These microenvironment changes may be complementary to the PI3K and MAPK dysregulation observed in tumor plasma cells. Proteasome and ubiquitin genes, which affect secretion, autophagy, and apoptosis pathways that may be relevant to MM pathogenesis are also frequently differentially expressed in immune cells between disease stages.
Finally, we integrate bulk whole-exome and whole-genome sequencing analysis (from both the 13-patient cohort and MMRF) to obtain a more complete understanding of how pathways become dysregulated in MM. Our findings advance the understanding of how MM tumor subpopulations differentially regulate cellular pathways and interact within the tumor microenvironment.

Disclosures
O'Neal: Wugen: Patents & Royalties: Patent Pending; WashU: Patents & Royalties: Patent Pending. Rettig:WashU: Patents & Royalties: Patent Application 16/401,950. Oh:Incyte: Membership on an entity's Board of Directors or advisory committees; Blueprint Medicines: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy. Vij:Bristol-Myers Squibb: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; Genentech: Honoraria; Janssen: Honoraria; Karyopharm: Honoraria; Sanofi: Honoraria; Takeda: Honoraria, Research Funding. DiPersio:Amphivena Therapeutics: Consultancy, Research Funding; Magenta Therapeutics: Equity Ownership; Karyopharm Therapeutics: Consultancy; Incyte: Consultancy, Research Funding; RiverVest Venture Partners Arch Oncology: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy; Bioline Rx: Research Funding, Speakers Bureau; Macrogenics: Research Funding, Speakers Bureau; WUGEN: Equity Ownership, Patents & Royalties, Research Funding; NeoImmune Tech: Research Funding; Cellworks Group, Inc.: Membership on an entity's Board of Directors or advisory committees.