CXCR 4 WHIM ‐like frameshift and nonsense mutations promote ibrutinib resistance but do not supplant MYD 88 L265P ‐directed survival signalling in W aldenström macroglobulinaemia cells

Summary CXCR 4 WHIM frameshift and nonsense mutations follow MYD 88 L265P as the most common somatic variants in W aldenström M acroglobulinaemia ( WM ), and impact clinical presentation and ibrutinib response. While the nonsense ( CXCR 4 S338X ) mutation has been investigated, little is known about CXCR 4 frameshift ( CXCR 4 FS ) mutations. We engineered WM cells to express CXCR 4 FS mutations present in patients, and compared their CXCL 12 ( SDF ‐1a) induced signalling and ibrutinib sensitivity to CXCR 4 wild‐type (WT) and CXCR 4 S338X cells. Following CXCL 12 stimulation, CXCR 4 FS and CXCR 4 S338X WM cells showed impaired CXCR 4 receptor internalization, and enhanced AKT 1 (also termed AKT ) and MAPK 1 (also termed ERK ) activation versus CXCR WT cells ( P  < 0·05), though MAPK 1 activation was more prolonged in CXCR 4 S338X cells ( P  < 0·05). CXCR 4 FS and CXCR 4 S338X cells, but not CXCR 4 WT cells, were rescued from ibrutinib‐triggered apoptosis by CXCL 12 that was reversed by AKT 1, MAPK 1 or CXCR 4 antagonists. Treatment with an inhibitor that blocks MYD 88 L265P signalling triggered similar levels of apoptosis that was not abrogated by CXCL 12 treatment in CXCR 4 WT and CXCR 4 WHIM cells. These studies show a functional role for CXCR 4 FS mutations in WM , and provide a framework for the investigation of CXCR 4 antagonists with ibrutinib in CXCR 4 WHIM ‐mutated WM patients. Direct inhibition of MYD 88 L265P signalling overcomes CXCL 12 triggered survival effects in CXCR 4 WHIM ‐mutated cells supporting a primary role for this survival pathway in WM .