Paediatric mixed lineage leukaemia (MLL)-rearranged acute myeloid leukaemia (AML) patients face a poor prognosis with a 4-year survival rate of 20%. Previous studies have shown that beta-catenin signalling is required for the establishment of leukaemic stem cells (LSC) in MLL-rearranged AML, but is dispensable for the self-renewal of normal adult haematopoietic stem cells (HSC), hence representing an ideal therapeutic target for AML. Since beta-catenin remains difficult to target, this study aims to identify novel cell-surface effectors acting upstream of beta-catenin that can serve as therapeutic targets for MLL-rearranged AML.
In this study we have identified a novel beta-catenin regulator, GPR84, a member of the G protein-coupled receptor family that represents a highly tractable class of drug targets. High GPR84 expression levels were confirmed in human and mouse AML LSC compared to normal HSC. Furthermore, forced expression of GPR84 induced a significant upregulation of beta-catenin in pre-LSC (P=0.0034), while shRNA-mediated ablation of GPR84 resulted in significant downregulation of beta-catenin (P=0.0008). Suppression of GPR84 significantly reduced colony forming pre-LSC (P=0.0006) and inhibited cell growth by inducing a significant G1-phase cell cycle arrest in pre-LSC in vitro (P<0.0001). Importantly, GPR84 inhibition reduced LSC frequency and significantly impaired reconstitution of HSC-derived MLL-rearranged AML in vivo (P=0.0012), which represents a particularly aggressive and drug-resistant subtype of AML. The GPR84-deficient phenotype in established AML could be rescued by expression of constitutively active beta-catenin in vivo (P=0.0039). Furthermore, GPR84 conferred a significant growth advantage to pre-LSC in vitro (P=0.0018) and in vivo (P=0.0039). Microarray analysis demonstrated that GPR84 significantly upregulated a small set of MLL-fusion targets and beta-catenin effectors, and downregulated a hematopoietic cell cycle inhibitor. Altogether, our data reveal a previously unrecognized role of GPR84 in the maintenance of fully developed AML by sustaining aberrant beta-catenin signalling in LSC, and suggest that targeting the oncogenic GPR84/beta-catenin signalling axis may represent a novel therapeutic strategy for AML.