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Myelodysplastic Syndrome in Man and Mouse
Progress in understanding and treating MDS has been hampered by a lack of suitable animal models for this disease, as current models either do not transform to acute leukemia or develop an incompletely penetrant, myeloproliferative disease (MPD) with dysplastic features. To generate a mouse model for MDS, we took advantage of the observation that a large number of chromosomal translocations involving the NUP98 gene have been associated with MDS. In particular, NUP98 is fused to at least seven different HOX genes in patients with MDS. To generate a mouse model, we cloned a NUP98-HOXD13 (NHD13) fusion gene from a pediatric patient with MDS and inserted this fusion gene, under the control of Vav regulatory elements, into the mouse germline. Vav regulatory elements were used because they are known to drive expression in all hematopoietic cells. Clinically healthy NHD13 transgenic mice developed MDS that resembled the human disease in terms of peripheral blood cytopenias, dysplasia, and increased apoptosis in the context of a hypercellular or normocellular bone marrow (Figure 1). Furthermore, the clinical course of the disease was similar to that of human MDS; some mice remained overtly healthy for an extended observation period, with mild-to-moderate cytopenias and dysplasia. Other mice died of severe anemia, and still other mice died following transformation of MDS to acute leukemia. Figure 1. Dysplasia and apoptosis of hematopoietic cells from NHD13 transgenic mice. A) hypersegmented neutrophil, B) multinucleated erythroblast, C) giant platelet, D) lane 1, bone marrow from NHD13 mouse; lane 2, bone marrow from normal littermate; note oligonucleosomal “ladder” in lane 1. Following leukemic transformation, the NHD13 transgenic mice displayed a wide variety of distinct leukemic subtypes, including myeloid, erythroid, megakaryocytic, undifferentiated, pre-T lymphoblastic, and pre-B lymphoblastic. The frequent transformation of MDS to pre-T lymphoblastic and pre-B lymphoblastic leukemia in these mice was unanticipated because human MDS only rarely transforms into a lymphoid malignancy. However, other NUP98 fusion genes, such as NUP98-RAP1GDS1 and NUP98-ADD3, have been found in patients with pre-T lymphoblastic leukemia, suggesting that expression of NUP98 fusion genes can lead to T-cell as well as myeloid malignancies. It seems likely that the NHD13 transgene exerts its oncogenic effect through the inhibition of normal hematopoietic differentiation. This assertion is supported by experiments demonstrating that overexpression of NHD13 can inhibit megakaryocytic differentiation, and experiments showing that NHD13 embryonic stem (ES) cells are severely impaired in their ability to differentiate in vitro. In addition, these findings are consistent with reports showing that overexpression of HOX or NUP98-HOX fusion genes can impair differentiation in other systems. This is the first animal model for MDS that accurately recapitulates all of the key features of the human disease, including ineffective hematopoiesis, peripheral blood cytopenias, dysplasia, and progression to acute leukemia. Predictably, generation of this model has led to more questions than it has answered. First, we presume that additional, collaborating mutations occur as the disease evolves from MDS to acute leukemia. What are these mutations? To answer this question, we have enlisted the support of expert collaborators both within and outside the CCR. The techniques we are using include retroviral tagging, comparative genomic hybridization, and restriction landmark genome scanning (RLGS) to identify collaborating mutations; the retroviral tagging and RLGS approaches have both yielded promising leads. A second fundamental question is whether MDS, which can be regarded as a premalignant condition, can be transplanted. Again, we have enlisted the support of CCR investigators to help us answer this question, through the transplantation of NHD13 bone marrow into non-transgenic, syngeneic mice. If the disease is transplantable, can we identify an “MDS stem cell” analogous to the leukemic stem cells that have been described? Finally, new chemotherapy agents are often screened for efficacy using a panel of cell lines. There are no MDS cell lines that can be used for this type of experiment, however, because MDS cells do not grow well in vitro. The “MDS” cell lines that have been established were derived from patients in whom MDS had transformed to acute leukemia, and are therefore actually leukemic cell lines rather than MDS cell lines. To determine whether the NHD13 mice are a valid preclinical model for MDS, we are treating the mice with agents known to be effective in human patients with MDS, as a proof-of-concept experiment. |
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yelodysplastic syndrome (MDS) is characterized by ineffective hematopoiesis, peripheral blood cytopenias, dysplasia, and transformation to acute leukemia. Patients with MDS may survive for an extended period of time before succumbing to complications of pancytopenia or leukemic transformation. A large number of chromosomal abnormalities, including deletions, amplifications, inversions, and translocations have been identified in the malignant cells of patients with MDS.