CCR1 as a therapeutic target in multiple myeloma: preventing tumour dissemination and therapeutic resistance

Abstract

Multiple myeloma (MM) is a haematological malignancy characterised by the clonal proliferation of plasma cells (PC) within the bone marrow (BM). While the introduction of novel therapies, such as the proteasome inhibitor bortezomib, has greatly improved patient outcomes, a subset of patients still does very poorly, due to intrinsic resistance to therapy, rapid disease relapse and increased tumour dissemination. However, the mechanisms which underpin MM PC dissemination and intrinsic resistance remain poorly understood. Our laboratory has previously shown that MM PC expression of the chemokine receptor CCR1 is associated with poor prognosis in newly diagnosed patients. In this thesis, the mechanisms underlying the prognostic disadvantage of elevated CCR1 expression are explored. In addition, the prognostic significance of MM PC expression of other chemokine receptors is also investigated. MM disease development, progression and relapse is dependent on MM PC haematogenous dissemination from one bone site to another. Currently, there is little understanding of which factors regulate spontaneous dissemination. Our previous studies suggested that elevated MM PC CCR1 expression is associated with increased numbers of circulating MM PC in newly diagnosed patients. Here, is was found that knockout of CCR1 in the MM cell line OPM2 resulted in almost no circulating tumour cells and prevented the formation of disseminated tumours following intratibial injection, suggesting that CCR1 is critical in egress from the BM to the circulation. In support of this, constitutive expression of CCR1 in 5TGM1 MM cells increased the incidence of disseminated tumours in C57BL/KaLwRij mice. Furthermore, pharmacological inhibition of CCR1 slowed the formation of disseminated tumours in intratibial RPMI-8226 or OPM2 tumour models. These studies support the further investigation of CCR1 inhibition as a therapeutic modality to inhibit MM tumour dissemination. The poor prognosis associated with CCR1 expression in MM patients suggests that CCR1 may also play a role in therapeutic resistance. In order to investigate this, the effect of CCR1 overexpression and knockdown in MM cell lines on response to bortezomib treatment was assessed. CCR1 knockout increased bortezomib sensitivity in vitro and in vivo, while constitutive expression of CCR1 decreased bortezomib sensitivity in vitro. Furthermore, RNA sequencing analysis revealed that elevated MM PC CCR1 expression at diagnosis or at relapse was associated with poorer overall survival in patients receiving bortezomib treatment. Future studies are warranted to assess the ability of CCR1 inhibitors to increase response to bortezomib. In order to investigate the potential role of other chemokine receptors in MM pathogenesis, we conducted an in silico analysis to determine the association between chemokine receptor expression on MM PCs and overall survival in MM patients. These studies identified, for the first time, that elevated CCR10 expression is associated with poor prognosis. Notably, elevated MM PC CCR10 mRNA or protein expression was not associated with other known prognostic factors in MM patients. Taken together, these studies suggest that CCR1 is a potential target to enhance response to therapy and prevent dissemination of MM PCs. In addition, they suggest that future studies are warranted to investigate the role of CCR10 in MM pathogenesis