Project description:Myelofibrosis (MF) is caused by genetic abnormalities involving the thrombopoietin (TPO)/MPL/JAK2 axis. Furthermore MF patients have elevated serum TPO levels. MF is also associated with reduced GATA1 content in MK suggesting that this abnormality represents a phenotypic modifier. In 2014, Dr. Crispino suggested that in MF abnormal TPO signaling induces a ribosomal deficiency hampering GATA1 mRNA translation in MK. Support for MK GATA1 deficiency as “phenotypic modifier” in MF was provided by the observation that mice carrying the Gata1low mutation reducing Gata1 transcription in MK develop myelofibrosis. Since reduced RBC half-life subject these mice to continuous “erythroid stress”, we investigated the TPO/Mpl axis in this model. In Gata1low and wild-type mice, TPO mRNA was expressed by bone marrow (BM), spleen and liver. The greatest expression (by 300-fold) was detected in liver. Gata1low livers expressed TPO mRNA levels 6-fold greater than wild-type livers. TPO protein was detected in BM, spleen, liver and peritoneum washes and plasma. The greatest levels where detected in plasma. Gata1low plasma contained TPO levels 2-fold lower than wild-type plasma, but 2-times greater than plasma from bleed wild-type mice and Mplnull mice with similar thrombocytopenia, suggesting that TPO is overproduced in Gata1low mice. JAK2 and STAT5 were easily detected in Gata1low BM bur barely detectable in wild-type BM, suggesting that in the former MPL is prompt to signaling activation. Furthermore, Gata1low LSK expressed levels of Mpl mRNA 3-times greater than wild-type cells but expressed cell-surface levels of MPL 2-times lower than wild-type cells and similar to those on LSK from TPO-treated wild-type mice, suggesting that MPL is down-modulated in Gata1low LSK. The Crispino’s hypothesis that in MF activation of TPO/MPL/JAK2 induces a ribosomal deficiency hampering GATA1 mRNA translation and the realization that this axis is activated in Gata1low mice made us question the original hypothesis that reduced content of GATA1 in Gata1low MK results from deletion of lineage-specific enhancers. Microarray analyses indeed identified that Gata1low BM express a discordant ribosome signature including reduced expression of RPS24 and RPS36A, two genes mutated in Diamond Blackfan Anemia, a disease characterized by inefficient GATA1 mRNA translation. Electron microscopy identified that the cytoplasm of Gata1low MK contained poorly developed endoplasmic reticulum with rare polysomes. In conclusion, these results validate the Gata1low model as a MF model by indicating that these mice express an activated TPO/MPL axis and an abnormal ribosomal signature which may reduce efficiency of Gata1 mRNA translation.

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Endocytosis of the thrombopoietin receptor Mpl regulates megakaryocyte and erythroid maturation in mice

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