JAK2^V617F-negative ET patients do not display constitutively active JAK/STAT signaling

Objective: Presence of the JAK2^V617F mutation in only 40% to 60% of patients with essential thrombocythemia (ET) underscores the heterogeneity of this myeloproliferative disorder (MPD). Several distinct mutations, either in JAK2 (exon 12) or in c-Mpl (W515L) have been described in subsets of other MPDs, polycythemia vera, and idiopathic myelofibrosis. Analogous to JAK2,^V617F these mutations cause constitutive JAK2 and signal transducer and activation of transcription (STAT) activation. It has therefore been proposed that constitutive activation of the JAK/STAT pathway underlies the molecular etiology of all MPDs. We investigated the alternative hypothesis that distinct alterations, separate from the JAK/STAT signal transduction pathway, underlie a subset of JAK2^V617F-negative ET. Methods: cDNA microarrays and quantitative reverse transcriptase polymerase chain reactions were used to compare gene expression in 40 ET patients with and without the JAK2^V617F mutation. Results: Unsupervised clustering of gene-expression patterns in ET patients revealed two distinct subclasses of patients. These subclasses differed in presence or absence of the JAK2^V617F mutation. Patients lacking the JAK2^V617F mutation displayed significantly lower expression of the JAK/STAT target genes Pim-1 and suppressor of cytokine signaling-2. In addition, JAK2^V617F-negative patients showed lower levels of STAT3 phosphorylation. Conclusions: These data demonstrate that a large proportion of JAK2^V617F-negative ET patients do not display constitutive JAK/STAT signaling. Hence, we propose that alterations in different signal transduction pathways can lead to the clinical phenotype of ET. Elucidation of novel ET-inducing changes will facilitate both a molecular classification of ET and development of rationally designed therapies.