Abstract 641: Single-cell RNA sequencing of rare circulating tumor cells in precursor myeloma patients reveals molecular underpinnings of tumor cell circulation

Abstract Background: Multiple Myeloma (MM) is a hematological malignancy characterized by abnormal proliferation of terminally differentiated plasma cells (PCs) in the bone marrow (BM). MM is almost always preceded by the precursor stage smoldering multiple myeloma (SMM). BM biopsies are useful to monitor disease progression, but they are invasive and not routinely collected from patients for disease monitoring during precursor stages. Profiling circulating tumor cells (CTCs) from peripheral blood (PB) could aid early detection, disease monitoring, and biomarker identification to predict patients at high risk of progression that may benefit from early therapeutic intervention. Methods: Paired PB and BM aspirates were collected from 40 SMM patients enrolled in the PCROWD study (IRB #14-174) at Dana-Farber Cancer Institute. Malignant PCs were enriched by magnetic bead-based methods and underwent 5’ single-cell RNA sequencing (scRNA-seq) and single-cell B-cell receptor sequencing (scBCR-seq) (10x Genomics). Results: We analyzed 105,246 BM PCs and 33,234 PB PCs from 15 patients. To differentiate malignant from normal PCs, we used clonal V(D)J rearrangements, assessed by concurrent scBCR-seq. A total of 86,986 BM tumor cells and 8,718 CTCs were captured. A median of 5, 26, and 47 CTCs were present per mL of blood from low, intermediate, and high-risk SMM patients as defined by the International Myeloma Working Group (IMWG) “20/2/20” criteria, suggesting sequencing-based CTC enumeration corresponds to prognosis. High levels of driver genes commonly upregulated in patients with specific translocations, including CCND1 and MAF, were detected in both BM tumor and CTC clusters in 3 patients with t(11;14) and t(14;16) confirmed by fluorescence in situ hybridization (FISH) clinical testing, and 2 additional patients with inconclusive FISH results (Wilcoxon, q <10-3), supporting the idea of CTC-based prognostication. Differential expression (DE) analysis revealed 8 genes that were significantly upregulated and 3 genes that were significantly downregulated in CTCs compared to BM tumor cells robustly across 15 paired samples. Gene set enrichment analysis (GSEA) revealed genes DE in CTCs are associated with TNF-α and NF-κB signaling, which are commonly induced by extrinsic factors in the bone marrow milieu, providing insight into the biology of tumor cell circulation. Conclusions: This study highlights the utility of scRNA-seq for molecular profiling of CTCs, even in asymptomatic low tumor burden disease. Additional analyses are ongoing in the expanded cohort of 40 patients with paired samples to help gain further insight into CTC heterogeneity. Overall, this study will help enable the design of new molecular liquid biopsy-based approaches to diagnosis, disease monitoring, and biological insights to improve treatment strategies for precursor myeloma patients. Citation Format: Elizabeth D. Lightbody, Danielle T. Firer, Romanos Sklavenitis-Pistofidis, Michael Agius, Ankit K. Dutta, Michelle Aranha, Jean-Baptiste Alberge, Laura Hevenor, Nang Kham Su, Cody Boehner, Erica Horowitz, Jacqueline Perry, Anna Cowan, Hadley Barr, Anna Justis, Daniel Auclair, Catherine R. Marinac, Gad Getz, Irene Ghobrial. Single-cell RNA sequencing of rare circulating tumor cells in precursor myeloma patients reveals molecular underpinnings of tumor cell circulation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 641.