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

Abstract

CXCR4^WHIM frameshift and nonsense mutations follow MYD88^L265P as the most common somatic variants in Waldenström 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 CXCR^WT cells (P < 0·05), though MAPK1 activation was more prolonged in CXCR4^S338X cells (P < 0·05). 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.