The thrombopoietin/MPL axis is activated in the Gata1low mouse 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 Gata1(low) mutation, which reduces the levels of Gata1 mRNA in megakaryocytes, develop MF, we investigated whether the TPO axis is hyperactive in this model. Gata1(low) mice contained two times more Tpo mRNA in liver and TPO in plasma than wild-type littermates. Furthermore, Gata1(low) LSKs 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. Gata1(low) marrow and spleen contained more JAK2/STAT5 than wild-type tissues, an indication that these organs were reach of TPO-responsive cells. Moreover, treatment of Gata1(low) mice with the JAK inhibitor ruxolitinib reduced their splenomegaly. Also in Gata1(low) mice 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 Gata1(low) megakaryocytes contained poorly developed endoplasmic reticulum with rare polysomes. In summary, Gata1(low) mice are a bona fide model of MF, which recapitulates the hyperactivation of the TPO/MPL/JAK2 axis observed in megakaryocytes from myelofibrotic patients.