Coordinated changes in gene expression kinetics underlie both mouse and human erythroid maturation.

BACKGROUND: Single-cell technologies are transforming biomedical research, including the recent demonstration that unspliced pre-mRNA present in single-cell RNA-Seq permits prediction of future expression states. Here we apply this RNA velocity concept to an extended timecourse dataset covering mouse gastrulation and early organogenesis. RESULTS: Intriguingly, RNA velocity correctly identifies epiblast cells as the starting point, but several trajectory predictions at later stages are inconsistent with both real-time ordering and existing knowledge. The most striking discrepancy concerns red blood cell maturation, with velocity-inferred trajectories opposing the true differentiation path. Investigating the underlying causes reveals a group of genes with a coordinated step-change in transcription, thus violating the assumptions behind current velocity analysis suites, which do not accommodate time-dependent changes in expression dynamics. Using scRNA-Seq analysis of chimeric mouse embryos lacking the major erythroid regulator Gata1, we show that genes with the step-changes in expression dynamics during erythroid differentiation fail to be upregulated in the mutant cells, thus underscoring the coordination of modulating transcription rate along a differentiation trajectory. In addition to the expected block in erythroid maturation, the Gata1-chimera dataset reveals induction of PU.1 and expansion of megakaryocyte progenitors. Finally, we show that erythropoiesis in human fetal liver is similarly characterized by a coordinated step-change in gene expression. CONCLUSIONS: By identifying a limitation of the current velocity framework coupled with in vivo analysis of mutant cells, we reveal a coordinated step-change in gene expression kinetics during erythropoiesis, with likely implications for many other differentiation processes.

Human erythroleukemia genetics and transcriptomes identify master transcription factors as functional disease drivers.

Acute erythroleukemia (AEL or acute myeloid leukemia [AML]-M6) is a rare but aggressive hematologic malignancy. Previous studies showed that AEL leukemic cells often carry complex karyotypes and mutations in known AML-associated oncogenes. To better define the underlying molecular mechanisms driving the erythroid phenotype, we studied a series of 33 AEL samples representing 3 genetic AEL subgroups including TP53-mutated, epigenetic regulator-mutated (eg, DNMT3A, TET2, or IDH2), and undefined cases with low mutational burden. We established an erythroid vs myeloid transcriptome-based space in which, independently of the molecular subgroup, the majority of the AEL samples exhibited a unique mapping different from both non-M6 AML and myelodysplastic syndrome samples. Notably, >25% of AEL patients, including in the genetically undefined subgroup, showed aberrant expression of key transcriptional regulators, including SKI, ERG, and ETO2. Ectopic expression of these factors in murine erythroid progenitors blocked in vitro erythroid differentiation and led to immortalization associated with decreased chromatin accessibility at GATA1-binding sites and functional interference with GATA1 activity. In vivo models showed development of lethal erythroid, mixed erythroid/myeloid, or other malignancies depending on the cell population in which AEL-associated alterations were expressed. Collectively, our data indicate that AEL is a molecularly heterogeneous disease with an erythroid identity that results in part from the aberrant activity of key erythroid transcription factors in hematopoietic stem or progenitor cells.

Captopril mitigates splenomegaly and myelofibrosis in the Gata1low murine model of myelofibrosis.

Allogeneic stem cell transplantation is currently the only curative therapy for primary myelofibrosis (MF), while the JAK2 inhibitor, ruxolitinib. Has been approved only for palliation. Other therapies are desperately needed to reverse life-threatening MF. However, the cell(s) and cytokine(s) that promote MF remain unclear. Several reports have demonstrated that captopril, an inhibitor of angiotensin-converting enzyme that blocks the production of angiotensin II (Ang II), mitigates fibrosis in heart, lung, skin and kidney. Here, we show that captopril can mitigate the development of MF in the Gata1low mouse model of primary MF. Gata1low mice were treated with 79 mg/kg/d captopril in the drinking water from 10 to 12 months of age. At 13 months of age, bone marrows were examined for fibrosis, megakaryocytosis and collagen expression; spleens were examined for megakaryocytosis, splenomegaly and collagen expression. Treatment of Gata1low mice with captopril in the drinking water was associated with normalization of the bone marrow cellularity; reduced reticulin fibres, splenomegaly and megakaryocytosis; and decreased collagen expression. Our findings suggest that treating with the ACE inhibitors captopril has a significant benefit in overcoming pathological changes associated with MF.

GATA1 insufficiencies in primary myelofibrosis and other hematopoietic disorders: consequences for therapy.

INTRODUCTION: GATA1, the founding member of a family of transcription factors, plays important roles in the development of hematopoietic cells of several lineages. Although loss of GATA1 has been known to impair hematopoiesis in animal models for nearly 25 years, the link between GATA1 defects and human blood diseases has only recently been realized. Areas covered: Here the current understanding of the functions of GATA1 in normal hematopoiesis and how it is altered in disease is reviewed. GATA1 is indispensable mainly for erythroid and megakaryocyte differentiation. In erythroid cells, GATA1 regulates early stages of differentiation, and its deficiency results in apoptosis. In megakaryocytes, GATA1 controls terminal maturation and its deficiency induces proliferation. GATA1 alterations are often found in diseases involving these two lineages, such as congenital erythroid and/or megakaryocyte deficiencies, including Diamond Blackfan Anemia (DBA), and acquired neoplasms, such as acute megakaryocytic leukemia (AMKL) and the myeloproliferative neoplasms (MPNs). Expert commentary: Since the first discovery of GATA1 mutations in AMKL, the number of diseases that are associated with impaired GATA1 function has increased to include DBA and MPNs. With respect to the latter, we are only just now appreciating the link between enhanced JAK/STAT signaling, GATA1 deficiency and disease pathogenesis.

The transcription factor GATA1 regulates NBEAL2 expression through a long-distance enhancer.

Gray platelet syndrome is named after the gray appearance of platelets due to the absence of α-granules. It is caused by recessive mutations in NBEAL2, resulting in macrothrombocytopenia and myelofibrosis. Though using the term gray platelets for GATA1 deficiency has been debated, a reduced number of α-granules has been described for macrothrombocytopenia due to GATA1 mutations. We compared platelet size and number of α-granules for two NBEAL2 and two GATA1-deficient patients and found reduced numbers of α-granules for all, with the defect being more pronounced for NBEAL2 deficiency. We further hypothesized that the granule defect for GATA1 is due to a defective control of NBEAL2 expression. Remarkably, platelets from two patients, and Gata1-deficient mice, expressed almost no NBEAL2. The differentiation of GATA1 patient-derived CD34+ stem cells to megakaryocytes showed defective proplatelet and α-granule formation with strongly reduced NBEAL2 protein and ribonucleic acid expression. Chromatin immunoprecipitation sequencing revealed 5 GATA binding sites in a regulatory region 31 kb upstream of NBEAL2 covered by a H3K4Me1 mark indicative of an enhancer locus. Luciferase reporter constructs containing this region confirmed its enhancer activity in K562 cells, and mutagenesis of the GATA1 binding sites resulted in significantly reduced enhancer activity. Moreover, DNA binding studies showed that GATA1 and GATA2 physically interact with this enhancer region. GATA1 depletion using small interfering ribonucleic acid in K562 cells also resulted in reduced NBEAL2 expression. In conclusion, we herein show a long-distance regulatory region with GATA1 binding sites as being a strong enhancer for NBEAL2 expression.

GATA1-Deficient Dendritic Cells Display Impaired CCL21-Dependent Migration toward Lymph Nodes Due to Reduced Levels of Polysialic Acid.

Dendritic cells (DCs) play a pivotal role in the regulation of the immune response. DC development and activation is finely orchestrated through transcriptional programs. GATA1 transcription factor is required for murine DC development, and data suggest that it might be involved in the fine-tuning of the life span and function of activated DCs. We generated DC-specific Gata1 knockout mice (Gata1-KODC), which presented a 20% reduction of splenic DCs, partially explained by enhanced apoptosis. RNA sequencing analysis revealed a number of deregulated genes involved in cell survival, migration, and function. DC migration toward peripheral lymph nodes was impaired in Gata1-KODC mice. Migration assays performed in vitro showed that this defect was selective for CCL21, but not CCL19. Interestingly, we show that Gata1-KODC DCs have reduced polysialic acid levels on their surface, which is a known determinant for the proper migration of DCs toward CCL21.

Clinical and genomic heterogeneity of Diamond Blackfan anemia in the Russian Federation.

BACKGROUND: Diamond Blackfan anemia (DBA) is a genetically and clinically heterogeneous ribosomopathy and inherited bone marrow failure syndrome characterized by anemia, reticulocytopenia, and decreased erythroid precursors in the bone marrow with an increased risk of malignancy and, in approximately 50%, physical abnormalities. METHODS: We retrospectively analyzed clinical data from 77 patients with DBA born in the Russian Federation from 1993 to 2014. In 74 families there was one clinically affected individual; in only three instances a multiplex family was identified. Genomic DNA from 57 DBA patients and their first-degree relatives was sequenced for mutations in RPS19, RPS10, RPS24, RPS26, RPS7, RPS17, RPL5, RPL11, RPL35a, and GATA1. RESULTS: Severe anemia presented before 8 months of age in all 77 patients; before 2 months in 61 (78.2%); before 4 months in 71 (92.2%). Corticosteroid therapy was initiated after 1 year of age in the majority of patients. Most responded initially to steroids, while 5 responses were transient. Mutations in RP genes were detected in 35 of 57 patients studied: 15 in RPS19, 6 in RPL5, 3 in RPS7, 3 each in RPS10, RPS26, and RPL11 and 1 each in RPS24 and RPL35a; 24 of these mutations have not been previously reported. One patient had a balanced chromosomal translocation involving RPS19. No mutations in GATA1 were found. CONCLUSION: In our cohort from an ethnically diverse population the distribution of mutations among RP genes was approximately the same as was reported by others, although within genotypes most of the mutations had not been previously reported.

Mouse prenatal platelet-forming lineages share a core transcriptional program but divergent dependence on MPL.

The thrombopoietic environment of the neonate is established during prenatal life; therefore, a comprehensive understanding of platelet-forming cell development during embryogenesis is critical to understanding the etiology of early-onset thrombocytopenia. The recent discovery that the first platelet-forming cells of the conceptus are not megakaryocytes (MKs) but diploid platelet-forming cells (DPFCs) revealed a previously unappreciated complexity in thrombopoiesis. This raises important questions, including the following. When do conventional MKs appear? Do pathogenic genetic lesions of adult MKs affect DPFCs? What role does myeloproliferative leukemia virus (MPL), a key regulator of adult megakaryopoiesis, play in prenatal platelet-forming lineages? We performed a comprehensive study to determine the spatial and temporal appearance of prenatal platelet-forming lineages. We demonstrate that DPFCs originate in the yolk sac and then rapidly migrate to other extra- and intraembryonic tissues. Using gene disruption models of Gata1 and Nfe2, we demonstrate that perturbing essential adult MK genes causes an analogous phenotype in the early embryo before the onset of hematopoietic stem/progenitor cell-driven (definitive) hematopoiesis. Finally, we present the surprising finding that DPFC and MK commitment from their respective precursors is MPL independent in vivo but that completion of MK differentiation and establishment of the prenatal platelet mass is dependent on MPL expression.

Eosinophils Contribute to Early Clearance of Pneumocystis murina Infection.

Pneumocystis pneumonia remains a common opportunistic infection in the diverse immunosuppressed population. One clear risk factor for susceptibility to Pneumocystis is a declining CD4(+) T cell count in the setting of HIV/AIDS or primary immunodeficiency. Non-HIV-infected individuals taking immunosuppressive drug regimens targeting T cell activation are also susceptible. Given the crucial role of CD4(+) T cells in host defense against Pneumocystis, we used RNA sequencing of whole lung early in infection in wild-type and CD4-depleted animals as an unbiased approach to examine mechanisms of fungal clearance. In wild-type mice, a strong eosinophil signature was observed at day 14 post Pneumocystis challenge, and eosinophils were increased in the bronchoalveolar lavage fluid of wild-type mice. Furthermore, eosinophilopoiesis-deficient Gata1(tm6Sho)/J mice were more susceptible to Pneumocystis infection when compared with BALB/c controls, and bone marrow-derived eosinophils had in vitro Pneumocystis killing activity. To drive eosinophilia in vivo, Rag1(-/-) mice were treated with a plasmid expressing IL-5 (pIL5) or an empty plasmid control via hydrodynamic injection. The pIL5-treated mice had increased serum IL-5 and eosinophilia in the lung, as well as reduced Pneumocystis burden, compared with mice treated with control plasmid. In addition, pIL5 treatment could induce eosinophilia and reduce Pneumocystis burden in CD4-depleted C57BL/6 and BALB/c mice, but not eosinophilopoiesis-deficient Gata1(tm6Sho)/J mice. Taken together, these results demonstrate that an early role of CD4(+) T cells is to recruit eosinophils to the lung and that eosinophils are a novel candidate for future therapeutic development in the treatment of Pneumocystis pneumonia in the immunosuppressed population.

GATA-1 deficiency rescues trabecular but not cortical bone in OPG deficient mice.

GATA-1(low/low) mice have an increase in megakaryocytes (MKs) and trabecular bone. The latter is thought to result from MKs directly stimulating osteoblastic bone formation while simultaneously inhibiting osteoclastogenesis. Osteoprotegerin (OPG) is known to inhibit osteoclastogenesis and OPG(-/-) mice have reduced trabecular and cortical bone due to increased osteoclastogenesis. Interestingly, GATA-1(low/low) mice have increased OPG levels. Here, we sought to determine whether GATA-1 knockdown in OPG(-/-) mice could rescue the observed osteoporotic bone phenotype. GATA-1(low/low) mice were bred with OPG(-/-) mice and bone phenotype assessed. GATA-1(low/low) × OPG(-/-) mice have increased cortical bone porosity, similar to OPG(-/-) mice. Both OPG(-/-) and GATA-1(low/low) × OPG(-/-) mice, were found to have increased osteoclasts localized to cortical bone, possibly producing the observed elevated porosity. Biomechanical assessment indicates that OPG(-/-) and GATA-1(low/low) × OPG(-/-) femurs are weaker and less stiff than C57BL/6 or GATA-1(low/low) femurs. Notably, GATA-1(low/low) × OPG(-/-) mice had trabecular bone parameters that were not different from C57BL/6 values, suggesting that GATA-1 deficiency can partially rescue the trabecular bone loss observed with OPG deficiency. The fact that GATA-1 deficiency appears to be able to partially rescue the trabecular, but not the cortical bone phenotype suggests that MKs can locally enhance trabecular bone volume, but that MK secreted factors cannot access cortical bone sufficiently to inhibit osteoclastogenesis or that OPG itself is required to inhibit osteoclastogenesis in cortical bone.

PSTPIP2 dysregulation contributes to aberrant terminal differentiation in GATA-1-deficient megakaryocytes by activating LYN.

GATA1 mutations are tightly associated with transient myeloproliferative disorder (TMD) and acute megakaryoblstic leukemia (AMKL) in children with Down syndrome. Numerous genes are altered in GATA-1-deficient megakaryocytes, which may contribute to the hyperproliferation and abnormal terminal differentiation of these malignant cells. In this study, we demonstrate that Pstpip2 is a GATA-1-repressed gene that controls megakaryopoiesis. Ectopic expression of PSTPIP2 impaired megakaryocytic differentiation as evidenced by a decrease of CD41 expression and reduced DNA content in K562 cells. PSTPIP2 overexpression also caused enhanced activation of Src family kinases and subsequently reduced ERK phosphorylation. Consistently, PSTPIP2 knockdown showed the opposite effect on differentiation and signaling. Moreover, the W232A mutant of PSTPIP2, defective in its interaction with PEST family phosphatases that recruit c-Src terminal kinase (CSK) to suppress Src family kinases, failed to inhibit differentiation and lost its ability to enhance Src family kinases or reduce ERK phosphorylation. In fact, the W232A mutant of PSTPIP2 promoted megakaryocyte differentiation. These observations suggest that PSTPIP2 recruiting PEST phosphatases somehow blocked CSK activity and led to enhanced activation of Src family kinases and reduced ERK phosphorylation, which ultimately repressed megakaryocyte differentiation. Supporting this idea, PSTPIP2 interacted with LYN and the expression of a dominant negative LYN (LYN DN) overwhelmed the inhibitory effect of PSTPIP2 on differentiation and ERK signaling. In addition, a constitutively active LYN (LYN CA) normalized the enhanced megakaryocyte differentiation and repressed ERK signaling in PSTPIP2 knockdown cells. Finally, we found that PSTPIP2 repressed ERK signaling, differentiation, and proliferation and verified that PSTPIP2 upregulation repressed megakaryocyte development in primary mouse bone marrow cells. Our study thus reveals a novel mechanism by which dysregulation of PSTPIP2 due to GATA-1 deficiency may contribute to abnormal megakaryocyte proliferation and differentiation in pathogenesis of related diseases.

[Connecting isolated congenital asplenia to the ribosome]. / De l'asplénie congénitale isolée au ribosome⋆⋆.

Isolated congenital asplenia is characterized by the absence of a spleen at birth without any other developmental defect. Isolated congenital asplenia is a rare and life-threatening disease that predisposes patients to severe bacterial infections. The first and main genetic etiology of isolated congenital asplenia was discovered in 2013. Mutations in the gene RPSA, which encodes ribosomal protein SA, cause more than half of the cases of isolated congenital asplenia. These disease-causing mutations lead to haploinsufficiency of RPSA. Haploinsufficiency of genes encoding other ribosomal proteins have been reported to cause other developmental defects in humans, and in model organisms like the fly or the mouse. About half of the patients with Diamond-Blackfan anemia, which is a well-characterized ribosomopathy, present developmental defects such as craniofacial defects, cardiac defects or thumb abnormalities. The mechanism of pathogenesis linking mutations in ribosomal proteins, which are highly and ubiquitously expressed, to specific developmental defects remains to be elucidated. One hypothesis is that the ribosome, and ribosomal proteins in particular, regulate the expression of specific genes during development.

The effects of GATA-1 and NF-E2 deficiency on bone biomechanical, biochemical, and mineral properties.

Mice deficient in GATA-1 or NF-E2, transcription factors required for normal megakaryocyte (MK) development, have increased numbers of MKs, reduced numbers of platelets, and a striking high bone mass phenotype. Here, we show the bone geometry, microarchitecture, biomechanical, biochemical, and mineral properties from these mutant mice. We found that the outer geometry of the mutant bones was similar to controls, but that both mutants had a striking increase in total bone area (up to a 35% increase) and trabecular bone area (up to a 19% increase). Interestingly, only the NF-E2 deficient mice had a significant increase in cortical bone area (21%) and cortical thickness (27%), which is consistent with the increase in bone mineral density (BMD) seen only in the NF-E2 deficient femurs. Both mutant femurs exhibited significant increases in several biomechanical properties including peak load (up to a 32% increase) and stiffness (up to a 13% increase). Importantly, the data also demonstrate differences between the two mutant mice. GATA-1 deficient femurs break in a ductile manner, whereas NF-E2 deficient femurs are brittle in nature. To better understand these differences, we examined the mineral properties of these bones. Although none of the parameters measured were different between the NF-E2 deficient and control mice, an increase in calcium (21%) and an increase in the mineral/matrix ratio (32%) was observed in GATA-1 deficient mice. These findings appear to contradict biomechanical findings, suggesting the need for further research into the mechanisms by which GATA-1 and NF-E2 deficiency alter the material properties of bone.

A unique role of GATA1s in Down syndrome acute megakaryocytic leukemia biology and therapy.

BACKGROUND: Acute megakaryocytic leukemia (AMkL) in Down syndrome (DS) children is uniformly associated with somatic GATA1 mutations, which result in the synthesis of a shorter protein (GATA1s) with altered transactivation activity compared to the wild-type GATA1. It is not fully established whether leukemogenesis and therapeutic responses in DS AMkL patients are due to loss of the wild-type GATA1 or due to a unique function of GATA1s. METHODOLOGY: Stable clones of CMK cells with decreased GATA1s or Bcl-2 levels were generated by using GATA1- or BCL-2-specific lentivirus shRNAs. In vitro ara-C, daunorubicin, and VP-16 cytotoxicities of the shRNA stable clones were determined by using the Cell Titer-blue reagent. Apoptosis and cell cycle distribution were determined by flow cytometry analysis. Changes in gene transcript levels were determined by gene expression microarray and/or real-time RT-PCR. Changes in protein levels were measured by Western blotting. In vivo binding of GATA1s to IL1A promoter was determined by chromatin immunoprecipitation assays. RESULTS: Lentivirus shRNA knockdown of the GATA1 gene in the DS AMkL cell line, CMK (harbors a mutated GATA1 gene and only expresses GATA1s), resulting in lower GATA1s protein levels, promoted cell differentiation towards the megakaryocytic lineage and repressed cell proliferation. Increased basal apoptosis and sensitivities to ara-C, daunorubicin, and VP-16 accompanied by down-regulated Bcl-2 were also detected in the CMK GATA1 shRNA knockdown clones. Essentially the same results were obtained when Bcl-2 was knocked down with lentivirus shRNA in CMK cells. Besides Bcl-2, down-regulation of GATA1s also resulted in altered expression of genes (e.g., IL1A, PF4, and TUBB1) related to cell death, proliferation, and differentiation. CONCLUSION: Our results suggest that GATA1s may facilitate leukemogenesis and potentially impact therapeutic responses in DS AMkL by promoting proliferation and survival, and by repressing megakaryocytic lineage differentiation, potentially by regulating expression of Bcl-2 protein and other relevant genes.

c-Myb and GATA-1 alternate dominant roles during megakaryocyte differentiation.

BACKGROUND: Transcription factors are essential for blood cell formation. Mice expressing low levels of c-Myb (c-Myb(low)) have an increased number of bone marrow megakaryocytes (MKs) and corresponding thrombocytosis. In contrast, mice engineered to express low levels of GATA-1 (GATA-1(low)) in the megakaryocytic lineage exhibit aberrant megakaryocytopoiesis with hyperproliferation of progenitors and defective terminal differentiation leading to thrombocytopenia. These seemingly opposite roles may affect platelet turnover and thus be of clinical relevance. OBJECTIVE: To determine how these two transcription factors act together to control megakaryocytopoiesis and platelet formation. METHODS: We used a combination of cellular and molecular in vitro assays to examine the ability of bone marrow cells from mice expressing low levels of both c-Myb and GATA-1 (referred to as double(low)) to produce MKs and platelets. RESULTS: Double(low) cells, or those with low GATA-1 levels in which c-Myb is conditionally deleted, lack the hyperproliferative capacity of GATA-1(low) cells, allowing the cells to proceed towards more committed MKs that are, however, impaired in their capacity to produce fully differentiated cells, as confirmed by the abundance of morphologically aberrant cells that lack the ability to form proplatelets. CONCLUSION: c-Myb and GATA-1 act in concert to achieve correct megakaryocytic differentiation. GATA-1 regulates both the proliferation of megakaryocytic progenitors and their terminal maturation. c-Myb also acts at the level of the progenitor by influencing its commitment to differentiation, but in contrast to GATA-1 it does not have any effect on the process of terminal differentiation.

Down-regulation of GATA1 uncouples STAT5-induced erythroid differentiation from stem/progenitor cell proliferation.

Previously, we have shown that overexpression of an activated mutant of signal transducer and activator of transcription-5 (STAT5) induces erythropoiesis, impaired myelopoiesis, and an increase in long-term proliferation of human hematopoietic stem/progenitor cells. Because GATA1 is a key transcription factor involved in erythropoiesis, the involvement of GATA1 in STAT5-induced phenotypes was studied by shRNA-mediated knockdown of GATA1. CD34(+) cord blood cells were double transduced with a conditionally active STAT5 mutant and a lentiviral vector expressing a short hairpin against GATA1. Erythropoiesis was completely abolished in the absence of GATA1, indicating that STAT5-induced erythropoiesis is GATA1-dependent. Furthermore, the impaired myelopoiesis in STAT5-transduced cells was restored by GATA1 knockdown. Interestingly, early cobblestone formation was only modestly affected, and long-term growth of STAT5-positive cells was increased in the absence of GATA1, whereby high progenitor numbers were maintained. Thus, GATA1 down-regulation allowed the dissection of STAT5-induced differentiation phenotypes from the effects on long-term expansion of stem/progenitor cells. Gene expression profiling allowed the identification of GATA1-dependent and GATA1-independent STAT5 target genes, and these studies revealed that several proliferation-related genes were up-regulated by STAT5 independent of GATA1, whereas several erythroid differentiation-related genes were found to be GATA1 as well as STAT5 dependent.

Immature and mature megakaryocytes enhance osteoblast proliferation and inhibit osteoclast formation.

Recent data suggest that megakaryocytes (MKs) play a role in skeletal homeostasis. In vitro and in vivo data show that MKs stimulate osteoblast (OB) proliferation and inhibit osteoclast (OC) formation, thus favoring net bone deposition. There are several mouse models with dysregulated megakaryopoiesis and resultant high bone mass phenotypes. One such model that our group has extensively studied is GATA-1 deficient mice. GATA-1 is a transcription factor required for normal megakaryopoiesis, and mice deficient in GATA-1 have increases in immature MK number and a striking increase in bone mass. While the increased bone mass could simply be a result of increased MK number, here we take a more in depth look at the MKs of these mice to see if there is a unique factor inherent to GATA-1 deficient MKs that favors increased bone deposition. We show that increased MK number does correspond with increased OB proliferation and decreased OC formation that stage of maturation does not alter the effect of MKs on bone cell lineages beyond the megakaryoblast stage, and that GATA-1 deficient MKs survive longer than wild-type controls. So while increased MK number in GATA-1 deficient mice likely contributes to the high bone mass phenotype, we propose that the increased longevity of this lineage also plays a role. Since GATA-1 deficient MKs live longer they are able to exert both more proliferative influence on OBs and more inhibitory influence on OCs.

GATA-2 reinforces megakaryocyte development in the absence of GATA-1.

GATA-2 is an essential transcription factor that regulates multiple aspects of hematopoiesis. Dysregulation of GATA-2 is a hallmark of acute megakaryoblastic leukemia in children with Down syndrome, a malignancy that is defined by the combination of trisomy 21 and a GATA1 mutation. Here, we show that GATA-2 is required for normal megakaryocyte development as well as aberrant megakaryopoiesis in Gata1 mutant cells. Furthermore, we demonstrate that GATA-2 indirectly controls cell cycle progression in GATA-1-deficient megakaryocytes. Genome-wide microarray analysis and chromatin immunoprecipitation studies revealed that GATA-2 regulates a wide set of genes, including cell cycle regulators and megakaryocyte-specific genes. Surprisingly, GATA-2 also negatively regulates the expression of crucial myeloid transcription factors, such as Sfpi1 and Cebpa. In the absence of GATA-1, GATA-2 prevents induction of a latent myeloid gene expression program. Thus, GATA-2 contributes to cell cycle progression and the maintenance of megakaryocyte identity of GATA-1-deficient cells, including GATA-1s-expressing fetal megakaryocyte progenitors. Moreover, our data reveal that overexpression of GATA-2 facilitates aberrant megakaryopoiesis.