CXCR4 WHIM-like frameshift and nonsense mutations promote ibrutinib resistance but do not supplant MYD88L265P-directed survival signalling in Waldenstrom macroglobulinaemia cells

CXCR4(WHIM) frameshift and nonsense mutations follow MYD88(L265P) as the most common somatic variants in Waldenstrom Macroglobulinaemia (WM), and impact clinical presentation and ibrutinib response. While the nonsense (CXCR4(S338X)) mutation has been investigated, little is known about CXCR4 frameshift (CXCR4(FS)) mutations. We engineered WM cells to express CXCR4(FS) mutations present in patients, and compared their CXCL12 (SDF-1a) induced signalling and ibrutinib sensitivity to CXCR4(wild-type (WT)) and CXCR4(S338X) cells. Following CXCL12 stimulation, CXCR4(FS) and CXCR4(S338X) WM cells showed impaired CXCR4 receptor internalization, and enhanced AKT1 (also termed AKT) and MAPK1 (also termed ERK) activation versus CXCRWT cells (P<005), though MAPK1 activation was more prolonged in CXCR4(S338X) cells (P<005). CXCR4(FS) and CXCR4(S338X) cells, but not CXCR4(WT) cells, were rescued from ibrutinib-triggered apoptosis by CXCL12 that was reversed by AKT1, MAPK1 or CXCR4 antagonists. Treatment with an inhibitor that blocks MYD88(L265P) signalling triggered similar levels of apoptosis that was not abrogated by CXCL12 treatment in CXCR4(WT) and CXCR4(WHIM) cells. These studies show a functional role for CXCR4(FS) mutations in WM, and provide a framework for the investigation of CXCR4 antagonists with ibrutinib in CXCR4(WHIM)-mutated WM patients. Direct inhibition of MYD88(L265P) signalling overcomes CXCL12 triggered survival effects in CXCR4(WHIM)-mutated cells supporting a primary role for this survival pathway in WM.