Abstract PR012: Pre-malignant plasma cells exhibit a senescence-like phenotype and accumulation of transposable elements
Authors: Borges Gabriel Alvares , Guilatco Angelo Jose , Hachfeld Christine M. , Ruan Ming , Royzenblat Sonya , Xu Ming , Edwards Claire M. , Diaz-delCastillo Marta , Andersen Thomas L. , Kourelis Taxiarchis , Tchkonia Tamar , Kirkland James L. , Drake Matthew T. , Weivoda Megan 
Published: January 2023

Abstract
Multiple myeloma (MM) is a clonal plasma cell (PC) cancer that is preceded by the benign conditions monoclonal gammopathy of undetermined significance (MGUS) and smoldering MM (SMM). While MGUS/SMM PCs are not proliferative, many of the oncogenes and chromosomal abnormalities in MM PCs are also present in MGUS/SMM. Since oncogenic stress is known to induce cellular senescence, we hypothesized that MGUS/SMM PCs may be in a senescent-like state. Our analysis of a published human dataset (GSE5900) revealed that compared to healthy PCs, MGUS/SMM PCs had significantly increased expression of senescence markers CDKN1A and GADD45A (FDR&lt;0.05). Gene Set Enrichment Analysis (GSEA) showed significant enrichment in MGUS/SMM PCs of our customized senescence phenotyping gene sets (q&lt;0.25). We next evaluated the KaLwRij MGUS mouse model. 10-mo-old female KaLwRij mice were treated with placebo or senolytics (dasatinib+quercetin, D+Q). D+Q treatment significantly reduced PCs compared to placebo, while restoring B cell number and functional gene expression. PCs isolated from D+Q-treated KaLwRij mice exhibited significant reductions in Myc and Il1b expression (p&lt;0.05) versus placebo and a trend towards reduced Trp53 expression (p=0.13), supporting that senescent PCs are targeted by D+Q. Single-cell RNA-seq and single sample (ss)GSEA of 24-mo-old KaLwRij PCs revealed a PC subset enriched for the ‘Plasma Cell Senescence’ gene set, which included genes from our customized gene sets that were differentially expressed in the human PCs dataset GSE5900. Notably, PCs in this subset showed enrichment for ‘Inflammatory SASP’ (median: 15.26%) and ‘Interferon (IFN) SASP’ (39.36%) gene sets. The IFN-SASP is characteristic of late senescence, driven by the accumulation of transposable elements. In the GSE5900 dataset, we found significant enrichment of ‘IFN-SASP’ in SMM (q&lt;0.25) but not MGUS PCs. Further, whole transcriptome RNA-sequencing showed that SMM PCs had increased expression of LINE1 retrotransposon L1HS in relation to normal PCs (FDR&lt;0.1), but not MGUS (FDR=0.83) or MM (FDR=0.5) PCs. Immunostaining confirmed increased cytosolic DNA:RNA hybrids in SMM (median frequency of cells exceeding the intensity threshold = 33%) versus MGUS (16.4%), MM (4.4%), and healthy PCs (1.5%), which is consistent with cytosolic DNA-mediated activation of the IFN SASP in SMM PCs. When these same patients were redistributed according to disease progression, L1HS was higher in patients with progressing SMM and newly diagnosed MM (NDMM) (FDR&lt;0.1) versus patients with stable MGUS (FDR=0.56) or advanced MM (FDR=0.38). Increased DNA:RNA staining was observed in progressing SMM PCs (median frequency = 41%), but the results for NDMM patients (4.4%) and advanced MM (3%) were comparable to healthy or stable patients (3.5%). These data demonstrate that MGUS and SMM PCs exhibit senescence features, suggest mechanisms that may contribute to MM tumorigenesis, and show that pharmacological ablation of senescent cells may prevent progression from MGUS/SMM to MM.
Citation Format: Gabriel Alvares Borges, Angelo Jose Guilatco, Christine M. Hachfeld, Ming Ruan, Sonya Royzenblat, Ming Xu, Claire M. Edwards, Marta Diaz-delCastillo, Thomas L. Andersen, Taxiarchis Kourelis, Tamar Tchkonia, James L. Kirkland, Matthew T. Drake, Megan Weivoda. Pre-malignant plasma cells exhibit a senescence-like phenotype and accumulation of transposable elements [abstract]. In: Proceedings of the AACR Special Conference: Aging and Cancer; 2022 Nov 17-20; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2022;83(2 Suppl_1):Abstract nr PR012.