The Role of Gremlin1 in Multiple Myeloma

Abstract

Multiple myeloma (MM) is a fatal haematological malignancy characterised by the clonal proliferation of malignant plasma cells (PC) within the bone marrow (BM). In most instances, MM PCs are reliant on factors made by cells of the BM stroma for their survival and growth. To date, the nature and cellular composition of the BM tumour microenvironment and the critical factors which drive tumour progression remain imprecisely defined. The studies presented here show that Gremlin1 (Grem1), a highly conserved protein, which is abundantly secreted by a subset of BM mesenchymal stromal cells, plays a critical role in MM disease development. We describe, for the first time, a novel positive feedback loop between MM PCs and BM stroma, and that inhibiting this vicious cycle with a neutralising antibody directed against Grem1 dramatically reduced tumour burden in a preclinical mouse model of MM. Analysis of human BM stromal samples by quantitative PCR showed that GREM1 expression was significantly higher in MM patient-derived BM stromal cells compared to stromal cells from healthy, age-matched individuals (p<0.01 t-test). Additionally, a positive correlation between MM tumour burden and stromal-cell associated Grem1 expression was observed in 5TGM1 MM PC tumour-bearing mice (p<0.05, R=0.64, Pearson Correlation). Furthermore, BM-stromal cells cultured with 5TGM1 MM PCs expressed significantly higher levels of Grem1, compared to stromal cells alone (p<0.01, t-test), suggesting that MM PCs promote increased Grem1 expression in stromal cells. MM PC induction of stromal-Grem1 serves to drive a positive feedback loop, as proliferation of the murine MM cell line, 5TGM1, was found to be significantly increased when co-cultured with Grem1-overexpressing stromal cells (p<0.01, t-test). To further examine the role of Grem1 in MM tumour establishment and growth in vivo, we utilised the 5TGM1/KaLwRij mouse model of MM. Following 5TGM1 tumour cell inoculation, mice were randomly assigned to either a neutralising Grem1 antibody (anti-Grem1 antibody, UCB-Pharma) or IgG control treatment arm. Our studies showed that compared to Ig control-treated mice, anti-Grem1-treated mice showed a 54% decrease in tumour burden. This effect was even more pronounced when mice received treatment with a Grem1 neutralising antibody prior to 5TGM1 tumour cell inoculation, resulting in an 80% reduction in tumour burden. Moreover, MM tumour burden was reduced in a Vk*MYC/Grem1-genetic knockout mouse model, although this did not reach statistical significance. Collectively, the data presented in this thesis suggests that Grem1 is a key stromal-derived PC mitogen that promotes MM disease initiation and progression, and that antibody-mediated targeting of Grem1 significantly reduces disease burden. With few effective therapies directed against the critical relationship between MM PCs and the BM, the findings presented here, represent a novel treatment strategy to limit MM disease burden.