Development of a Mouse Model of DS Megakaryoblastic Leukemia and Identification of the Hsa21 Gene DYRK1A as a Prominent Contributor to DS-AMKL

Abstract 469

Children with Down syndrome (DS) have a 500-fold increased risk of Acute Megakaryoblastic Leukemia (DS-AMKL). In addition to trisomy 21, DS-AMKL blasts harbor somatic mutations in the gene encoding GATA-1, an essential transcriptional regulator of erythroid and megakaryocytic differentiation. In every case, the GATA1 mutations lead to a block in expression of the full-length protein, but allow for the translation of a shortened isoform named GATA-1s, which lacks the N-terminal transcriptional activation domain. Recent studies have shown that mutations in of JAK2, JAK3 and MPL are another feature of AMKL. Whereas the impact on hematopoiesis of individual GATA-1s, JAK2 V617F and MPL W515L mutants is relatively well known, the specific effects of trisomy 21, the initiating event in DS leukemogenesis, remains unclear. Here, we used two different experimental models to identify specific genes on human chromosome 21 (Hsa21) that participate in leukemogenesis: human DS-AMKL cell lines and the Ts1Rhr mouse strain, which is trisomic for only 33 orthologs of Hsa21 genes. Comprehensive analysis of hematopoiesis in progressively more complex mouse models of DS disorders, including Ts1Rhr mice, Ts1Rhr/KI-Gata-1s compound mutant mice, and recipients of Ts1Rhr/KI-Gata-1s hematopoietic progenitors expressing MPL W515L, revealed that the presence of trisomy 21 impacts the phenotype of each genetic background. Ts1Rhr mice develop a progressive myeloproliferative disease with megakaryocytic features while Ts1Rhr/KI-Gata1s double transgenic mice develop thrombocytosis at an earlier age compared to Gata1s mutant or Ts1Rhr mice alone. However neither the Ts1Rhr nor the compound mutant mice develop AMKL. In stark contrast, expression of MPL W515L in Ts1Rhr/KI-Gata1s hematopoietic progenitors led to a rapid lethal megakaryoblastic disorder characterized by profound marrow and splenic fibrosis and the presence of immature megakaryoblasts in lethally irradiated recipients. Of note, transplantation of MPL W515L expressing singly mutant Ts1Rhr or KI-Gata-1s progenitors failed to cause a similar disease. These findings show that three oncogenic events are necessary and sufficient to induce a megakaryoblastic leukemia in recipient mice. In parallel, to identify specific genes on Hsa21 that contribute to AMKL, we performed shRNA screening in DS-AMKL cell lines by knocking down the human orthologs of the 33 genes trisomic in the Ts1Rhr mice. We then evaluated the consequence of knockdown on megakaryocyte proliferation, survival and/or differentiation. This study revealed that altered expression of ERG, DYRK1A, CHAF1B and HLCS affected megakaryocyte growth. Analysis of their expression in human samples suggest that these four genes likely play a role in etiology of DS-AMKL. Moreover expression evaluation of those genes in our in vivo models of 1, 2 or 3 genetic events revealed that ERG, a known megakaryoblastic oncogene, and DYRK1A are the most prominent genes implicated in these megakaryocytic disorders. DYRK1A (dual specificity tyrosine-regulated kinase 1A) has been functionally implicated in many developmental disorders associated with DS and shown to be constitutively phosphorylated/activated in human AMKL cell lines. We found that overexpression of DYRK1A cooperates with Gata1s to lead to a increase in megakaryocyte growth in liquid culture and in colony assays in vitro. In summary, we show that partially trisomic Ts1Rhr mice display progressive defects in the megakaryocytic compartment, that mutagenesis of Gata1 in the Ts1Rhr background exacerbates the Ts1Rhr phenotype, and that addition of an activating mutation of MPL leads to a fulminant megakaryocytic leukemia with myelofibrosis. Moreover, among the 33 genes present in 3 copies in the Ts1Rhr background, we identify DYRK1A as a likely megakaryoblastic oncogene in DS-AMKL.