A novel activating, germline JAK2 mutation, JAK2R564Q, causes familial essential thrombocytosis

Along with the most common mutation, JAK2V(617)F, several other acquired JAK2 mutations have now been shown to contribute to the pathogenesis of myeloproliferative neoplasms (MPNs). However, here we describe for the first time a germline mutation that leads to familial thrombocytosis that involves a residue other than Val(617). The novel mutation JAK2R(564)Q, identified in a family with autosomal dominant essential thrombocythemia, increased cell growth resulting from suppression of apoptosis in Ba/F3-MPL cells. Although JAK2R(564)Q and JAK2V(617)F have similar levels of increased kinase activity, the growth-promoting effects of JAK2R(564)Q are much milder than those of JAK2V(617)F because of at least 2 counter regulatory mechanisms. Whereas JAK2V(617)F can escape regulation by the suppressor of cytokine signaling 3 and p27/Kip1, JAK2R(564)Q-expressing cells cannot. Moreover, JAK2R(564)Q-expressing cells are much more sensitive to the JAK inhibitor, ruxolitinib, than JAK2V(617)F-expressers, suggesting that lower doses of this drug may be effective in treating patients with MPNs associated with alternative JAK2 mutations, allowing many undesirable adverse effects to be avoided. This work provides a greater understanding of the cellular effects of a non-JAK2V(617)F, MPN-associated JAK2 mutation; provides insights into new treatment strategies for such patients; and describes the first case of familial thrombosis caused by a JAK2 residue other than Val(617).