Attenuated Essential Thrombocytemia Induced in Mice by the Expression of AML1/MDS1/EVI1 (AME) or an AME Mutant: Comparison between Two New Models of Myeloproliferative Disorders.

Essential thrombocytemia (ET) is a myeloproliferative disorder characterized by a primary increase in platelets count often associated with splenomegaly and/or hepatomegaly often developing/evolving in thrombosis. The t(3;21)(q26;q22) resulting in the AML1/MDS1/EVI1 (AME) fusion gene is associated with several hematological disorders often characterized by severe dysmegakaryopoiesis, suggesting that AME alters the megakaryocytic program.

By BM infection and transplantation we generated two groups of mice that express either wild type AME or a point/internal-deletion mutant of AME (AME-D-C). This mutant lacks the eighth zinc finger motif and is unable to interact with the CtBP corepressor. Both groups of mice developed a fatal disease with features of human ET, however Kaplan-Meyer analysis showed a significant difference (p < 0.0032) in survival between the two groups with the AME-D-C-mice living considerably longer. The morphological analyses of the peripheral blood smears did not show significant differences between the AME- and AME-D-C-mice. However while the AME-mice were anemic, the AME-D-C-mice had normal counts. In both groups the platelet number was increased and atypical features like macro-platelets, agranular platelets and small aggregates were observed. Both groups of mice developed thrombocytosis with median platelet values consistently above the normal whereas the neutrophil counts were normal. It is possible that death resulted from bleeding complications maybe due to an intrinsic platelet dysfunction more evident in the AME population. Peripheral blood smears showed evidence of megakaryocytic fragments, large cytoplasm pieces containing pro-platelet, and nucleus scraps in both groups of mice. Seven out of 12 AME-mice and 8 out of 12 AME-D-C-mice showed slight neuthrophil dysplastic aspects such as hypersegmentation and open nuclei. The red blood cells were normocromic and normocytic in all mice however minimal anisocytosis was observed during the course of disease, with a variation in degree of polycromatophilia at the time of death.

The bone core biopsies were normocellular in eight AME mice, three were hypocellular and only one was hypercellular. In contrast the cellularity in all the AME-D-C samples was normal. The most prominent aspects of the AME-mice were an increased number of BM megakaryocytes often grouped in clusters of four to six elements located in proximity of sinuses or in paratrabeculae and rare megakaryocytes that contained intact white cells in their cytoplasm. These two features previously described as endosteal dislocation and emperipolesis, often seen in patients with MPD or ET, were absent in AME-D-C-mice, in which only a minimum increase of megakaryocyte without atypical aspects was observed. No significant differences were found in spleen size, weight and histological features aside from a slight increase in fibrosis in AME-D-C-mice. These results suggest that a mutation in the eighth zinc finger motif and the abrogation of CtBP binding could attenuate the myeloproliferative phenotype caused by AME particularly in terms of histological features and overall survival.