Mining for JAK-STAT mutations in cancer.

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway mediates signaling by cytokines, which control survival, proliferation and differentiation of several cell types. Constitutive JAK activation leads to persistent activation of STAT transcription factors, and several cancers exhibit constitutive STAT activation, in the absence of JAK or STAT activating mutations. Recently, a unique somatic mutation in JAK2 was identified in a majority of patients with myeloproliferative neoplasms. This mutation, encoding a V617F substitution, promotes JAK2 catalytic activation and cytokine-independent signaling. JAK2 and JAK3 mutations have also been identified in a minority of polycythemia vera and acute megakaryoblastic leukemia patients, and it is predicted that further JAK-STAT mutations will be identified in different cancers. Recent discoveries also suggest that mutated JAK proteins will be potent targets for anti-cancer therapy

miR-28 is a thrombopoietin receptor targeting microRNA detected in a fraction of myeloproliferative neoplasm patient platelets

BCR-ABL negative myeloproliferative neoplasms (MPNs; polycythemia vera, essential thrombocythemia, primary myelofibrosis) are malignant diseases arising from a multipotent hematopoietic progenitor, frequently altered by JAK2 V617F or other JAK/STAT activating mutations. The thrombopoietin receptor (TpoR, MPL) is one of the major dimeric cytokine receptors that use JAK2 in the myeloid lineage, and was found to be down-modulated in certain MPN patients. We searched for negative regulators of MPL expression. Here we report that miR-28 targets the 3' untranslated (3'UTR) region of MPL, inhibiting its translation, as well as other proteins potentially involved in megakaryocyte differentiation, such as E2F6. Expression of miR-28 in CD34-derived megakaryocytes inhibited terminal differentiation. miR-28 was found to be overexpressed in platelets of a fraction of MPN patients, while it was expressed at constant low levels in platelets from healthy subjects. Constitutive activation of STAT5 leading to autonomous growth of hematopoietic cell lines was associated with increased miR-28 expression. We discuss how down-modulating MPL and other targets of miR-28, and of related miR-708 and miR-151, could contribute to MPN pathogenicity. (Blood. 2010; 116(3): 437-445