The Role of Bone Marrow Mesenchymal Stromal Cell Senescence in Multiple Myeloma

Abstract

Multiple myeloma (MM) is a malignancy characterised by the uncontrolled clonal proliferation of neoplastic plasma cells (PCs) within the bone marrow (BM). MM PCs rely on mesenchymal stromal cells (MSCs) within the BM for their proliferation and survival. MM is universally preceded by an asymptomatic precancerous condition, monoclonal gammopathy of undetermined significance (MGUS). The risk of progression of MGUS-to-MM increases with advancing age, concurrent to the accumulation of senescent BM-MSCs. Moreover, MSC senescence has been implicated in promoting cancer growth, suggesting that it could play a role in MM progression. Here, we characterised BM-MSC senescence in ex vivo expanded BM-MSCs isolated from BM trephine biopsies from MGUS (median age: 67.5 [range: 42-84]) and MM (age: 70 [52- 84]) patients and aged (age: 88 [68-94]) and young (age: 21 [17-26]) healthy subjects by evaluating B-galactosidase activity, proliferation and cellular morphology. These studies confirmed that age-related increases in BM-MSC senescence are a characteristic feature of MGUS and MM. Additionally, there was no significant difference in the number of senescent BM-MSCs between aged healthy subjects, MGUS and MM patients, suggesting that this was not a function of disease stage. Strikingly, we found that the risk of progression to MM is significantly elevated in MGUS patients with increased BM-MSC senescence. Furthermore, we demonstrated that the induction of BM-MSC senescence promoted the proliferation of MM PCs relative to co-culture with non-senescent controls. Senescent MSCs display a senescence associated secretory phenotype (SASP) characterised by increased production of growth molecules. Notably, we identified that BM-MSC gene expression of the well-established SASP factor interleukin-6 (lL-6) and novel SASP factor Gremlin1 (GREMJ) at MGUS was predictive of progression to MM in our cohort. However, our studies showed that the MM growth-promoting effects of irradiated senescent BM-MSCs were not mediated by IL-6 and or Gremlin1 in vitro. Further RNA sequencing analysis of irradiated murine stroma identified 17 putative SASP factors, which were upregulated with irradiation and are expressed by MM patient BM-MSCs. These include, CXCL-12, GAS6 and IGF-1, which have been previously implicated in promoting MM PC growth. To investigate whether BM-MSC senescence plays a role in MM PC proliferation in vivo, we utilised a mouse model where cre-mediated knockout of enhancer of zeste 2 (EZH2) is driven by the mesenchymal Prrx1 promoter, which has previously been reported to induce senescence of mesenchymal lineage cells in vivo. Unexpectedly, our studies revealed that senescent cell burden within the bones and of BM-MSCs of EZH2⁺/⁻ mice was significantly reduced relative to that of WT controls. Furthermore, we did not detect a difference in Vk*Myc MM tumour growth within the BM following intravenous injection in these animals. Collectively, our findings suggest that the accumulation of senescent BM-MSCs with advancing biological age could constitute a growth-permissive niche at MGUS and facilitate the progression to MM. Excitingly, this highlights the potential utility of these cells as a novel therapeutic target and provides the impetus for future studies investigating the utility of senolytics and agents that inhibit the pro-tumorigenic effects of SASP to prevent MGUS-to- MM progression.