Germ-line JAK2 mutations in the kinase domain are responsible for hereditary thrombocytosis and are resistant to JAK2 and HSP90 inhibitors

Abstract

Key Points• New germ-line mutations of JAK2 in the kinase domain were identified.• Specificity for MPL and resistance to JAK2 and HSP90 inhibitors was determined.The main molecular basis of essential thrombocythemia and hereditary thrombocytosis is acquired, and germ-line-activating mutations affect the thrombopoietin signaling axis. We have identified 2 families with hereditary thrombocytosis presenting novel heterozygous germ-line mutations of JAK2. One family carries the JAK2 R867Q mutation located in the kinase domain, whereas the other presents 2 JAK2 mutations, S755R/ R938Q, located in cis in both the pseudokinase and kinase domains. Expression of Janus kinase 2 (JAK2) R867Q and S755R/R938Q induced spontaneous growth of Ba/F3-thrombopoietin receptor (MPL) but not of Ba/F3-human receptor of erythropoietin cells. Interestingly, both Ba/F3-MPL cells expressing the mutants and platelets from patients displayed thrombopoietin-independent phosphorylation of signal transducer and activator of transcription 1. The JAK2 R867Q and S755R/R938Q proteins had significantly longer half-lives compared with JAK2 V617F. The longer half-lives correlated with increased binding to the heat shock protein 90 (HSP90) chaperone and with higher MPL cell-surface expression. Moreover, these mutants were less sensitive to JAK2 and HSP90 inhibitors than JAK2 V617F. Our results suggest that the mutations in the kinase domain of JAK2 may confer a weak activation of signaling specifically dependent on MPL while inducing a decreased sensitivity to clinically available JAK2 inhibitors. (Blood. 2014;123(9):1372-1383

Emerging EPO and EPO receptor regulators and signal transducers

“…80 In a Ba/F3 cell model, JAK2V617F's transforming potential is also promoted by EPOR (or thrombopoietin receptor [MPL]) expression. 81 For JAK2 R867Q or S755R/R938Q mutations (as associated primarily with thrombocytosis), however, transformation is supported by MPL and not the EPOR. These findings implicate selective EPOR (and MPL) interactions with mutated JAK2 alleles in a context of myeloproliferative disease.…”

How I treat essential thrombocythemia

“…23,24,65 Recent reports have described cases of hereditary thrombocytosis associated with noncanonical germ line mutations of JAK2. 23,[66][67][68][69] In familial ET, JAK2 (V617F) is always a somatically acquired event, as in the familial tree reported in Figure 2. 34,70,71 In our clinical practice, we interview all patients with thrombocytosis to find out if there is a family history of thrombocytosis or MPN.…”

Presence of atypical thrombopoietin receptor (MPL) mutations in triple-negative essential thrombocythemia patients

“…Cell proliferation was quantified 3 days later using a WST-1 assay (Takara [Ozyme]). 31 Experiments were done in triplicate.…”