HCK is a survival determinant transactivated by mutated MYD88, and a direct target of ibrutinib

Activating mutations in MYD88 are present in approximately 95% of patients with Waldenstrom Macroglobulinemia (WM), as well as other B-cell malignancies including ABC DLBCL. In WM, mutated MYD88 triggers activation of BTK. Ibrutinib, a pleiotropic kinase inhibitor that targets BTK, is highly active in patients with mutated MYD88. We observed that mutated MYD88 WM and ABC DLBCL cell lines, as well as primary WM cells show enhanced HCK transcription and activation, and that HCK is activated by IL6. Over-expression of mutated MYD88 triggers HCK and IL6 transcription, while knockdown of HCK reduced survival and attenuated BTK, PI3K/AKT, and MAPK/ERK signaling in mutated MYD88 WM and/or ABC DLBCL cells. Ibrutinib and the more potent HCK inhibitor A419259 blocked HCK activation and induced apoptosis in mutated MYD88 WM and ABC DLBCL cells. Docking and pull-down studies confirmed that HCK was a target of ibrutinib. Ibrutinib and A419259 also blocked ATP binding to HCK, while transduction of mutated MYD88 expressing WM cells with a mutated HCK gatekeeper greatly increased the EC50 for ibrutinib and A419259. The findings support that HCK expression and activation is triggered by mutated MYD88, supports the growth and survival of mutated MYD88 WM and ABC DLBCL cells, and is a direct target of ibrutinib. HCK represents a novel target for therapeutic development in MYD88 mutated WM and ABC DLBCL, and possibly other diseases driven by mutated MYD88.