Loss of p53 Accelerates the Complications of Myelodysplastic Syndromes (MDS) In the NUP98-HOXD13 Transgenic Mouse Model

Abstract 2804

The myelodysplastic syndromes (MDS) are clonal stem cell disorders characterized by ineffective hematopoiesis leading to blood cytopenias and a high rate of progression to acute myelogenous leukemia (AML) (Nimer, 2008). Recent studies implicated an important role for p53 in 5q−MDS, a subtype where haploinsufficiency of the RPS14 ribosomal protein drives the anemia that accompanies this disease (Ebert et al., 2008; Barlow et al., 2010 and Dutt et al., 2011). The elevated level of p53 activity apparently triggers the excessive apoptosis and the dysplastic morphology seen in the erythroid cells of the “5q-mice” that lack one copy of the chromosomal region syntenic to the human 5q region that contains the RPS14 gene (Barlow et al., 2010). Thus, the cytopenias in some patients with MDS may be the result of defective ribosomal biosynthesis, leading to activation of p53 and excessive apoptosis. The nucleoporin gene NUP98 is fused to a number of different genes including HOXD13 by chromosomal translocations that are found in patients with MDS, AML and CML, blast crisis. Genetically engineered mice that express a NUP98-HOXD13 (NHD13) transgene (Tg) display the phenotypic features of MDS, including cytopenias, bone marrow dysplasia, and transformation to acute leukemia (Lin et al., 2005). We obtained these mice and analyzed the hematopoietic stem cells (HSC) and the erythroid compartment and found significantly decreased rps14 mRNA expression in the NHD13⁺ CD71⁺Ter119⁺ cells compared to the WT controls. Furthermore, flow cytometry analysis revealed increased intracellular p53 levels in the NHD13⁺ Lin⁻Sca-1⁺ c-Kit⁺ (LSK) and CD71⁺Ter119⁺ cells. These data suggest that defective rps14 ribosomal protein production and an increased p53 level may contribute to the anemic phenotype in NHD13⁺ Tg mice. To examine whether inhibition of p53 function can improve the cytopenias of NHD13⁺ Tg mice, mice were injected with Pifithrin-α (a reversible inhibitor of p53-mediated apoptosis and p53-dependent gene transcription, 2 mg/kg body weight) daily for five weeks. We observed partial rescue of the myeloid and lymphoid lineage differentiation defects, with no improvement in the hemoglobin level. To further investigate whether the presence or absence of p53 affects the MDS or AML phase of NHD13 driven disease, we generated NHD13⁺p53^+/− and NHD13⁺p53^−/− mice. Deletion of one allele of p53 rescues the myeloid progenitor cell compartment, while deletion of both alleles reversed the deficit in both the LSK and the MPP populations that was observed in the NHD13⁺ Tg mice. The clinically healthy NHD13⁺p53^−/− mice (aged 3 to 5 months) and the NHD13⁺p53^+/− mice (aged from 3 to 7 months) both had more severe leukopenia and anemia than the NHD13⁺ Tg mice or the WT controls (aged 4 to 7 months). 60% of the NHD13⁺p53^−/− mice and the NHD13⁺p53^+/− mice developed MDS with a median survival of 138 d and 190 d respectively; in contrast 30% of the NHD13⁺ Tg mice had MDS with a median survival of 236 d. These data indicate that the relative or absolute lack of p53 hastens the development of MDS, and shortens the median survival. Lack of one or two p53 alleles significantly accelerated the development of AML, which in NHD13⁺p53^+/− mice resulting in a median survival of 278 d. However, AML in the NHD13⁺p53^−/− mice is more undifferentiated with a median survival of 133 d, and the NHD13⁺ Tg mice showed AML with a median survival of 324 d. Taken together, these data demonstrated that the chronic loss one allele or two alleles of p53 does not rescue the MDS phenotype induced by NHD13 fusion gene. Rather it accelerates the development of NHD13 driven MDS and leukemia. Our studies suggest that targeting p53 transiently may temporally improve hematopoiesis in MDS, but over the long term has detrimental effects on hematopoiesis.