The thrombopoietin/MPL axis is activated in the Gata1mouse model of myelofibrosis and is associated with a defective RPS14 signature

Myelofibrosis (MF) is characterized by hyperactivation of thrombopoietin (TPO) signaling, which induces a RPS14 deficiency that de-regulates GATA1 in megakaryocytes by hampering its mRNA translation. As mice carrying the hypomorphic Gata1lowmutation, which reduces the levels of Gata1 mRNA in megakaryocytes, develop MF, we investigated whether the TPO axis is hyperactive in this model. Gata1lowmice contained two times more Tpo mRNA in liver and TPO in plasma than wild-type littermates. Furthermore, Gata1lowLSKs expressed levels of Mpl mRNA (five times greater than normal) and protein (two times lower than normal) similar to those expressed by LSKs from TPO-treated wild-type mice. Gata1lowmarrow and spleen contained more JAK2/STAT5 than wild-type tissues, an indication that these organs were reach of TPO-responsive cells. Moreover, treatment of Gata1lowmice with the JAK inhibitor ruxolitinib reduced their splenomegaly. Also in Gata1lowmice activation of the TPO/MPL axis was associated with a RSP14 deficiency and a discordant microarray ribosome signature (reduced RPS24, RPS26 and SBDS expression). Finally, electron microscopy revealed that Gata1lowmegakaryocytes contained poorly developed endoplasmic reticulum with rare polysomes. In summary, Gata1lowmice are a bona fide model of MF, which recapitulates the hyperactivation of the TPO/MPL/JAK2 axis observed in megakaryocytes from myelofibrotic patients.