Children with Down syndrome (DS) display macrocytosis, thrombocytosis, and a 500-fold increased risk of developing megakaryocytic leukemia; however, the specific effects of trisomy 21 on hematopoiesis remain poorly defined. To study this question, we analyzed blood cell development in the Ts65Dn mouse model of DS. Ts65Dn mice are trisomic for 104 orthologs of Hsa21 genes and are the most widely used mouse model for DS. We discovered that Ts65Dn mice display persistent macrocytosis and develop a myeloproliferative disease (MPD) characterized by profound thrombocytosis, megakaryocyte hyperplasia, dysplastic megakaryocyte morphology, and myelofibrosis. In addition, these animals bear distorted hematopoietic stem and myeloid progenitor cell compartments compared with euploid control littermates. Of the 104 trisomic genes in Ts65Dn mice, Aml1/Runx1 attracts considerable attention as a candidate oncogene in DS-acute megakaryoblastic leukemia (DS-AMKL). To determine whether trisomy for Aml1/Runx1 is essential for MPD, we restored disomy at the Aml1/Runx1 locus in the Ts65Dn strain. Surprisingly, trisomy for Aml1/Runx1 is not required for megakaryocyte hyperplasia and myelofibrosis, suggesting that trisomy for one or more of the remaining genes can promote this disease. Our studies demonstrate the potential of DS mouse models to improve our understanding of chromosome 21 gene dosage effects in human hematologic malignancies.

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Down syndrome-associated acute megakaryocytic leukemia (DS-AMKL) is a complex malignancy that evolves in hematopoietic progenitors with trisomy 21 that acquire a somatic mutation in the blood transcription factor GATA1. The mechanistic relationship between these two genetic factors that leads to leukemia is poorly understood. In order to study the interplay between trisomy 21 and GATA1 mutations, we are developing a mouse model of this malignancy. The Ts65Dn mouse, which contains a segmental trisomy for mouse chromosome 16, homologous to human 21, has been reported to display several of the cognitive and craniofacial phenotypes seen in humans with DS, but the hematopoietic system has not been assessed as a model for blood development in humans with DS. We have evaluated adult hematopoiesis in the Ts65Dn strain by comparing monthly complete blood counts (CBC) of peripheral blood from 14 trisomic and 20 disomic littermates. Similar to humans with DS, Ts65Dn trisomic mice display persistent erythrocyte macrocytosis, with values at the high end of the normal range. Trisomic mice also harbor decreased numbers of red blood cells, mildly elevated platelet counts, a higher percentage of monocytes and a lower hemoglobin concentration. Interestingly infants with DS frequently display thrombocytosis. In addition, we have characterized fetal liver hematopoiesis in the Ts65Dn strain by FACS analysis of hematopoietic precursors and by performing colony assays. In general, we did not detect any significant differences in erythroid, myeloid, or megakaryocytic colony formation between trisomic or disomic fetuses. Likewise flow cytometry for CD34, TER119, and CD41 demonstrated overall similar numbers of cells in these compartments for Ts65Dn mice and disomic littermates. However, one of seven trisomic embryos displayed a significant increase in the proportion of CD34+ cells with concomitant decrease in both Ter119+ and CD41+ populations. In addition, cells from this fetal liver gave rise to seven-fold and three-fold increases in BFU-E and CFU-Mk colonies respectively, with no change in the CFU-GM. Although the sample size is small, these findings suggest that a subset of Ts65Dn trisomic fetuses exhibit aberrant hematopoiesis. Taken together, our study indicates that the Ts65Dn trisomic mouse may be an excellent model to study human DS hematopoiesis.

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Trisomy 21 causes human Down syndrome (DS), a heterogeneous group of phenotypes including marked predisposition to leukemia. Children with Down syndrome are 500 times more likely to develop acute megakaryoblastic leukemia (AMKL) than other children. Furthermore, non-DS children with AMKL often have acquired trisomy 21 in their leukemic clones, suggesting that trisomy 21 in hematopoietic cells contributes to leukemic transformation. To better understand the impact of trisomy 21 on blood cell homeostasis and leukemia, we studied hematopoiesis in the Ts65Dn mouse model of Down syndrome. Ts65Dn mice harbor a segmental trisomy for mouse chromosome 16, homologous to human chromosome 21, and display many of the phenotypes associated with human DS, including craniofacial anomalies and learning deficits. To define the hematopoietic parameters for this strain, we performed monthly complete blood counts for a cohort of trisomic mice and their disomic littermates and discovered the development of progressive thrombocytosis and mild anemia in trisomic animals. Increased numbers of CD41+ megakaryocytes with lower modal ploidy were detected in the bone marrow and spleen of Ts65Dn mice as early as three months of age. Over time, expansion of the megakaryocyte population was accompanied by a decrease in TER119+ cells in the bone marrow, myelofibrosis, splenomegaly, and extramedullary hematopoiesis. Colony forming assays demonstrated increased colony forming ability in the spleens of trisomic mice along with variable decreased hematopoiesis in the bone marrow. Further, characterization of stem cells in the bone marrow indicated a hyperproliferative stem cell population. Importantly, the mice did not develop malignant leukemia by the age of 18 months and no mutations were found in the blood transcription factor GATA1, which is commonly affected in human AMKL. While Ts65Dn mice do not develop the AMKL seen in humans with DS, our results indicate that, trisomy 16 can cause hyperproliferation of the myeloid lineages, extramedullary hematopoiesis, and bone marrow fibrosis in mice. Additionally, these results suggest that trisomy 21 in humans may initiate a similar process in hematopoietic stem cells, which may contribute to leukemogenesis. Unexpectedly, this phenotype also bears significant resemblance to the human myeloproliferative disease chronic idiopathic myelofibrosis (CIMF). These findings may provide insight into the origins and progression of human myeloid diseases, including AMKL and CIMF.

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