A Murine Model for the Transient Myeloproliferative Disorder of Down Syndrome: Phenotypic Influence of p53 on Reversible Megakaryoblastic Disorder
Authors: Elagib Kamaleldin E , Delehanty Lorrie L , Goldfarb Adam 
Published: October 2019

Abstract
Infants with Down syndrome (DS) display a high incidence of a reversible megakaryoblastic proliferation known as transient myeloproliferative disorder (DS-TMD). The clinical features of DS-TMD include marrow and liver infiltration by abnormal megakaryoctic precursors. These cells show a high propensity for spontaneous death, leading to liver damage and occasionally tumor lysis syndromes. Rapid disease onset is followed by gradual spontaneous remission over weeks to months. 10–20% of patients experience disease recurrence, which manifests as irreversible acute megakaryoblastic leukemia (DS-AMKL). DS-TMD pathogenesis requires trisomy 21 combined with acquired mutations of GATA-1, a transcription factor that programs megakaryocytic and erythroid maturation. The mutant sGATA-1 consists of an 84 amino acid amino-terminal truncation with diminished transcriptional activation. Evolution of DS-TMD to DS-AMKL may involve acquisition of p53 mutations, according to a recent clinical study in which such mutations occurred in 0/7 DS-TMD and in 2/3 DS-AMKL cases. Our lab has recently developed a murine model for DS-TMD based on GATA-1 cross-talk with the P-TEFb kinase complex promoting megakaryocytic maturation (see Elagib et al., Blood, prepublication 2008). In this model, Flavopiridol inhibition of P-TEFb in GATA-1Lo mice, which have megakaryocytic deficiency of GATA-1, induces a rapid onset megakaryoblastic proliferative disorder with many features of DS-TMD:
high rate of spontaneous cell death within megakaryoblasts, collateral damage to normal cells in involved tissues (marrow and spleen), defective megakaryoblastic polyploidization, aberrant coexpression of erythroid antigens on megakaryoblasts, reversibility of disease upon withdrawal of P-TEFb inhibitor, requirement for defective GATA-1 in megakaryocytes.
To determine the influence of p53 signaling on disease phenotype, the GATA-1Lo mutation was bred onto a TP53−/− background, followed by in vivo P-TEFb inhibition with low-dose Flavopiridol (5 mg/kg/day for 9 days). The resultant megakaryoblastic disorder in the GATA-1Lo::TP53−/− compound mutants showed several features distinct from findings in GATA-1Lo::TP53+/+ mice. In the peripheral blood, the GATA-1Lo::TP53−/− mice showed no significant decline in platelet counts: 2/5 mice had decreases, each <20%. By contrast, the majority (11/13) of GATA-1Lo::TP53+/+ mice showed marked declines (>50%) in platelet counts with P-TEFb inhibition. On necropsy, the GATA-1Lo::TP53−/− mice showed splenomegaly of ~2-fold, while GATA-1Lo::TP53+/+ mice showed splenic shrinkage. Light microscopy revealed extensive splenic infiltration by sheets of megakaryoblasts with minimal evidence of cell death in GATA-1Lo::TP53−/− mice. Marrows from these mice also showed infiltration by megakaryoblasts, but with relative preservation of tissue architecture and bystander cellular elements. Flow cytometry on these marrows confirmed the presence of an abnornal population of megakaryocytic cells with erythroid antigen coexpression and highlighted the lack of intramedullary cell death, distinct from the extensive cell death seen in the GATA-1Lo::TP53+/+ marrows. Another histologic feature unique to the GATA-1Lo::TP53−/− mice consisted of hepatic infiltration by megakaryoblasts, without evidence of hepatocellular damage. Withdrawal of Flavopiridol for 14 days lead to clearance of megakaryoblasts from all involved organs, as seen with GATA-1Lo::TP53+/+ mice. Thus, p53 clearly modulates the phenotype of the megakaryoblastic disease seen in GATA-1Lo mice undergoing P-TEFb inhibition. In the absence of p53 signaling, this disease shows more extensive proliferation, as indicated by the splenomegaly and liver infiltration, combined with markedly decreased cell death. This decrease in cell death is accompanied by a decrease in collateral damage of bystander cells/tissues and by an ability to maintain platelet counts at pre-treatment levels. These findings provide in vivo validation that P-TEFb inhibition can activate p53 (see Gomes et al., Genes Dev., 20:601, 2006) and suggest that the cell death, tissue damage, and spontaneous regression seen in human DS-TMD could be p53-driven. Loss of p53 function may promote transformation to irreversible leukemia, but P-TEFb under such circumstances retains the potential to induce disease regression.