Mutations in GATA1 in both transient myeloproliferative disorder and acute megakaryoblastic leukemia of Down syndrome

Down syndrome and leukemia: An insight into the disease biology and current treatment options

Children with Down syndrome (DS) have a 10- to 20-fold greater predisposition to develop acute leukemia compared to the general population, with a skew towards myeloid leukemia (ML-DS). While ML-DS is known to be a subtype with good outcome, patients who relapse face a dismal prognosis. Acute lymphocytic leukemia in DS (DS-ALL) is considered to have poor prognosis. The relapse rate is high in DS-ALL compared to their non-DS counterparts. We have a better understanding about the mutational spectrum of DS leukemia. Studies using animal, embryonic stem cell- and induced pluripotent stem cell-based models have shed light on the mechanism by which these mutations contribute to disease initiation and progression. In this review, we list the currently available treatment strategies for DS-leukemias along with their outcome with emphasis on challenges with chemotherapy-related toxicities in children with DS. We focus on the mechanisms of initiation and progression of leukemia in children with DS and highlight the novel molecular targets with greater success in preclinical trials that have the potential to progress to the clinic.

Transient abnormal myelopoiesis with extramedullary involvement in a down syndrome preemie leading to an unresponsive course despite chemotherapy

INTRODUCTION

Transient abnormal myelopoiesis (TAM) is a transient, clonal myeloproliferative disorder unique to Down Syndrome (DS) babies. It is characterized by increased peripheral blasts and presence of GATA1 mutation. The clinical spectrum ranges from jaundice and hepatosplenomegaly to multi-organ failure and death. Here we present a case of a premature baby with DS diagnosed to have TAM with extramedullary involvement at birth who had a fatal outcome.

CASE REPORT

A 30.3-week-old female fetus with DS had leukocytosis (WBC: 187.82 K/uL) with neutrophilia (ANC 27.65 K/uL), macrocytic anemia (RBC: 2.41 m/uL, Hb 8.8 g/dL, MCV 108.3, MCH 36.5, MCHC 33.7) and thrombocytosis (platelet count 361 K/uL) at birth. Liver panels demonstrated normal bilirubin levels with elevated liver enzymes (AST = 239 U/L, ALT = 216 U/L).

RESULTS

Peripheral smear showed marked leukocytosis with increased blasts (70%), nucleated RBCs, giant platelets, and megakaryocytic elements. Flow cytometry demonstrated two populations of cells: 20% myeloblasts and 26% dim CD45 CD34- cells. GATA1 mutation was present. Based on these findings a diagnosis of TAM with extramedullary hematopoiesis was made. She received two cycles of cytarabine chemotherapy. Though her WBC levels reached a low of 18.93 K/uL, she developed multi-organ failure, eventually leading to death on day 45.

DISCUSSION

TAM is a transient condition resulting in disease resolution in around 80% of cases. Death is reported in 10% of cases. Risk factors associated with early death include prematurity, hyperleukocytosis, elevated bilirubin levels. Management of high-risk babies with chemotherapy is recommended to improve survival.

A Novel GATA1 Variant in the C-Terminal Zinc Finger Compared with the Platelet Phenotype of Patients with A Likely Pathogenic Variant in the N-Terminal Zinc Finger

The GATA1 transcription factor is essential for normal erythropoiesis and megakaryocytic differentiation. Germline GATA1 pathogenic variants in the N-terminal zinc finger (N-ZF) are typically associated with X-linked thrombocytopenia, platelet dysfunction, and dyserythropoietic anemia. A few variants in the C-terminal ZF (C-ZF) domain are described with normal platelet count but altered platelet function as the main characteristic. Independently performed molecular genetic analysis identified a novel hemizygous variant (c.865C>T, p.H289Y) in the C-ZF region of GATA1 in a German patient and in a Spanish patient. We characterized the bleeding and platelet phenotype of these patients and compared these findings with the parameters of two German siblings carrying the likely pathogenic variant p.D218N in the GATA1 N-ZF domain. The main difference was profound thrombocytopenia in the brothers carrying the p.D218N variant compared to a normal platelet count in patients carrying the p.H289Y variant; only the Spanish patient occasionally developed mild thrombocytopenia. A functional platelet defect affecting αIIbβ3 integrin activation and α-granule secretion was present in all patients. Additionally, mild anemia, anisocytosis, and poikilocytosis were observed in the patients with the C-ZF variant. Our data support the concept that GATA1 variants located in the different ZF regions can lead to clinically diverse manifestations.

GATA-1 Defects in Diamond-Blackfan Anemia: Phenotypic Characterization Points to a Specific Subset of Disease

Diamond−Blackfan anemia (DBA) is one of the inherited bone marrow failure syndromes marked by erythroid hypoplasia. Underlying variants in ribosomal protein (RP) genes account for 80% of cases, thereby classifying DBA as a ribosomopathy. In addition to RP genes, extremely rare variants in non-RP genes, including GATA1, the master transcription factor in erythropoiesis, have been reported in recent years in patients with a DBA-like phenotype. Subsequently, a pivotal role for GATA-1 in DBA pathophysiology was established by studies showing the impaired translation of GATA1 mRNA downstream of the RP haploinsufficiency. Here, we report on a patient from the Dutch DBA registry, in which we found a novel hemizygous variant in GATA1 (c.220+2T>C), and an Iranian patient with a previously reported variant in the initiation codon of GATA1 (c.2T>C). Although clinical features were concordant with DBA, the bone marrow morphology in both patients was not typical for DBA, showing moderate erythropoietic activity with signs of dyserythropoiesis and dysmegakaryopoiesis. This motivated us to re-evaluate the clinical characteristics of previously reported cases, which resulted in the comprehensive characterization of 18 patients with an inherited GATA-1 defect in exon 2 that is presented in this case-series. In addition, we re-investigated the bone marrow aspirate of one of the previously published cases. Altogether, our observations suggest that DBA caused by GATA1 defects is characterized by distinct phenotypic characteristics, including dyserythropoiesis and dysmegakaryopoiesis, and therefore represents a distinct phenotype within the DBA disease spectrum, which might need specific clinical management.

Modeling Down Syndrome Myeloid Leukemia by Sequential Introduction of GATA1 and STAG2 Mutations in Induced Pluripotent Stem Cells with Trisomy 21

Children with Down syndrome (DS) have a high risk for acute myeloid leukemia (DS-ML). Genomic characterization of DS-ML blasts showed the presence of unique mutations in GATA1, an essential hematopoietic transcription factor, leading to the production of a truncated from of GATA1 (GATA1s). GATA1s, together with trisomy 21, is sufficient to develop a pre-leukemic condition called transient abnormal myelopoiesis (TAM). Approximately 30% of these cases progress into DS-ML by acquisition of additional somatic mutations in a stepwise manner. We previously developed a model for TAM by introducing disease-specific GATA1 mutation in trisomy 21-induced pluripotent stem cells (iPSCs), leading to the production of N-terminally truncated short form of GATA1 (GATA1s). In this model, we used CRISPR/Cas9 to introduce a co-operating mutation in STAG2, a member of the cohesin complex recurrently mutated in DS-ML but not in TAM. Hematopoietic differentiation of GATA1 STAG2 double-mutant iPSC lines confirmed GATA1s expression and the loss of functional STAG2 protein, leading to enhanced production of immature megakaryocytic population compared to GATA1 mutant alone. Megakaryocyte-specific lineage expansion of the double-mutant HSPCs exhibited close resemblance to the DS-ML immunophenotype. Transcriptome analysis showed that GATA1 mutation resulted in downregulation of megakaryocytic and erythrocytic differentiation pathways and interferon α/β signaling, along with an upregulation of pathways promoting myeloid differentiation such as toll-like receptor cascade. The co-occurrence of STAG2 knockout partially reverted the expression of genes involved in myeloid differentiation, likely leading to enhanced self-renewal and promoting leukemogenesis. In conclusion, we developed a DS-ML model via hematopoietic differentiation of gene-targeted iPSCs bearing trisomy 21.

Molecular Mechanisms of the Genetic Predisposition to Acute Megakaryoblastic Leukemia in Infants With Down Syndrome

Individuals with Down syndrome are genetically predisposed to developing acute megakaryoblastic leukemia. This myeloid leukemia associated with Down syndrome (ML–DS) demonstrates a model of step-wise leukemogenesis with perturbed hematopoiesis already presenting in utero, facilitating the acquisition of additional driver mutations such as truncating GATA1 variants, which are pathognomonic to the disease. Consequently, the affected individuals suffer from a transient abnormal myelopoiesis (TAM)—a pre-leukemic state preceding the progression to ML–DS. In our review, we focus on the molecular mechanisms of the different steps of clonal evolution in Down syndrome leukemogenesis, and aim to provide a comprehensive view on the complex interplay between gene dosage imbalances, GATA1 mutations and somatic mutations affecting JAK-STAT signaling, the cohesin complex and epigenetic regulators.

Modeling Transient Abnormal Myelopoiesis Using Induced Pluripotent Stem Cells and CRISPR/Cas9 Technology

Approximately 1%–2% of children with Down syndrome (DS) develop acute myeloid leukemia (AML) prior to age 5 years. AML in DS children (ML-DS) is characterized by the pathognomonic mutation in the gene encoding the essential hematopoietic transcription factor GATA1, resulting in N-terminally truncated short form of GATA1 (GATA1s). Trisomy 21 and GATA1s together are sufficient to induce transient abnormal myelopoiesis (TAM) exhibiting pre-leukemic characteristics. Approximately 30% of these cases progress into ML-DS by acquisition of additional somatic mutations. We employed disease modeling in vitro by the use of customizable induced pluripotent stem cells (iPSCs) to generate a TAM model. Isogenic iPSC lines derived from the fibroblasts of DS individuals with trisomy 21 and with disomy 21 were used. The CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)/Cas9 system was used to introduce GATA1 mutation in disomic and trisomic iPSC lines. The hematopoietic stem and progenitor cells (HSPCs) derived from GATA1 mutant iPSC lines expressed GATA1s. The expression of GATA1s concomitant with loss of full-length GATA1 reduced the erythroid population, whereas it augmented megakaryoid and myeloid populations, characteristic of TAM. In conclusion, we have developed a model system representing TAM, which can be used for modeling ML-DS by stepwise introduction of additional mutations.

The histone H3K4 demethylase JARID1A directly interacts with haematopoietic transcription factor GATA1 in erythroid cells through its second PHD domain

Chromatin remodelling and transcription factors play important roles in lineage commitment and development through control of gene expression. Activation of selected lineage-specific genes and repression of alternative lineage-affiliated genes result in tightly regulated cell differentiation transcriptional programmes. However, the complex functional and physical interplay between transcription factors and chromatin-modifying enzymes remains elusive. Recent evidence has implicated histone demethylases in normal haematopoietic differentiation as well as in malignant haematopoiesis. Here, we report an interaction between H3K4 demethylase JARID1A and the haematopoietic-specific master transcription proteins SCL and GATA1 in red blood cells. Specifically, we observe a direct physical contact between GATA1 and the second PHD domain of JARID1A. This interaction has potential implications for normal and malignant haematopoiesis.

Transient leukemia of Down syndrome

Childhood leukemia is mostly a "developmental accident" during fetal hematopoiesis and may require multiple prenatal and postnatal "hits". The World Health Organization defines transient leukemia of Down syndrome (DS) as increased peripheral blood blasts in neonates with DS and classifies this type of leukemia as a separate entity. Although it was shown that DS predisposes children to myeloid leukemia, neither the nature of the predisposition nor the associated genetic lesions have been defined. Acute myeloid leukemia of DS is a unique disease characterized by a long pre-leukemic, myelodysplastic phase, unusual chromosomal findings and a high cure rate. In the present manuscript, we present a comprehensive review of the literature about clinical and biological findings of transient leukemia of DS (TL-DS) and link them with the genetic discoveries in the field. We address the manuscript to the pediatric generalist and especially to the next generation of pediatric hematologists.

Recent advances in the understanding of transient abnormal myelopoiesis in Down syndrome

Neonates with Down syndrome (DS) have a propensity to develop the unique myeloproliferative disorder, transient abnormal myelopoiesis (TAM). TAM usually resolves spontaneously in ≤3 months, but approximately 10% of patients with TAM die from hepatic or multi-organ failure. After remission, 20% of patients with TAM develop acute myeloid leukemia associated with Down syndrome (ML-DS). Blasts in both TAM and ML-DS have trisomy 21 and GATA binding protein 1 (GATA1) mutations. Recent studies have shown that infants with DS and no clinical signs of TAM or increases in peripheral blood blasts can have minor clones carrying GATA1 mutations, referred to as silent TAM. Low-dose cytarabine can improve the outcomes of patients with TAM and high white blood cell count. A number of studies using fetal liver cells, mouse models, or induced pluripotent stem cells have elucidated the roles of trisomy 21 and GATA1 mutations in the development of TAM. Next-generation sequencing of TAM and ML-DS patient samples identified additional mutations in genes involved in epigenetic regulation. Xenograft models of TAM demonstrate the genetic heterogeneity of TAM blasts and mimic the process of clonal selection and expansion of TAM clones that leads to ML-DS. DNA methylation analysis suggests that epigenetic dysregulation may be involved in the progression from TAM to ML-DS. Unraveling the mechanisms underlying leukemogenesis and identification of factors that predict progression to leukemia could assist in development of strategies to prevent progression to ML-DS. Investigation of TAM, a unique pre-leukemic condition, will continue to strongly influence basic and clinical research into the development of hematological malignancies.

A novel role of CKIP-1 in promoting megakaryocytic differentiation

Casein kinase 2-interacting protein-1 (CKIP-1) is a known regulator of cardiomyocytes and macrophage proliferation. In this study, we showed that CKIP-1 was involved in the process of megakaryocytic differentiation. During megakaryocytic differentiation of K562 cells, CKIP-1 was dramatically upregulated and this upregulation induced by PMA was mediated through downregulation of transcription factor GATA-1. By transient transfection, oligonucleotide-directed mutagenesis and chromatin immunoprecipitation assays, we identified the transcriptional regulation of CKIP-1 by GATA-1. Overexpression of CKIP-1 initiated events of spontaneous megakaryocytic differentiation in K562 cells. Conversely, knockdown of CKIP-1 in cell lines suppressed megakaryocytic differentiation. Mechanistically, overexpression of CKIP-1 changed the expression levels of transcription factors that have been shown to be critical in erythro-megakaryocytic differentiation such as Fli-1, c-Myb and c-Myc. In vivo analysis confirmed that CKIP-1^−/− mice had decreased number of CD41⁺ cells harvested from bone marrow, and lower platelet levels when compared to wild-type littermates. This is the first direct evidence suggesting that CKIP-1 is a novel regulator of megakaryocytic differentiation.

CRISPR/Cas9-Directed Reassignment of the GATA1 Initiation Codon in K562 Cells to Recapitulate AML in Down Syndrome

Using a CRISPR/Cas9 system, we have reengineered a translational start site in the GATA1 gene in K562 cells. This mutation accounts largely for the onset of myeloid leukemia in Down syndrome (ML-DS). For this reengineering, we utilized CRISPR/Cas9 to generate mammalian cell lines that express truncated versions of the Gata1s protein similar to that seen in ML-DS, as determined by analyzing specific genetic alterations resulting from CRISPR/Cas9 cleavage. During this work, 73 cell lines were clonally expanded, with allelic variance analyzed. Using Tracking of Indels by DEcomposition (TIDE) and Sanger sequencing, we defined the DNA sequence and variations within each allele. We found significant heterogeneity between alleles in the same clonally expanded cell, as well as among alleles from other clonal expansions. Our data demonstrate and highlight the importance of the randomness of resection promoted by non-homologous end joining after CRISPR/Cas9 cleavage in cells undergoing genetic reengineering. Such heterogeneity must be fully characterized to predict altered functionality inside target tissues and to accurately interpret the associated phenotype. Our data suggest that in cases where the objective is to rearrange specific nucleotides to redirect gene expression in human cells, it is imperative to analyze genetic composition at the individual allelic level.

Repercussion of Megakaryocyte-Specific Gata1 Loss on Megakaryopoiesis and the Hematopoietic Precursor Compartment

During hematopoiesis, transcriptional programs are essential for the commitment and differentiation of progenitors into the different blood lineages. GATA1 is a transcription factor expressed in several hematopoietic lineages and essential for proper erythropoiesis and megakaryopoiesis. Megakaryocyte-specific genes, such as GP1BA, are known to be directly regulated by GATA1. Mutations in GATA1 can lead to dyserythropoietic anemia and pseudo gray-platelet syndrome. Selective loss of Gata1 expression in adult mice results in macrothrombocytopenia with platelet dysfunction, characterized by an excess of immature megakaryocytes. To specifically analyze the impact of Gata1 loss in mature committed megakaryocytes, we generated Gata1-Lox|Pf4-Cre mice (Gata1cKO^MK). Consistent with previous findings, Gata1cKO^MK mice are macrothrombocytopenic with platelet dysfunction. Supporting this notion we demonstrate that Gata1 regulates directly the transcription of Syk, a tyrosine kinase that functions downstream of Clec2 and GPVI receptors in megakaryocytes and platelets. Furthermore, we show that Gata1cKO^MK mice display an additional aberrant megakaryocyte differentiation stage. Interestingly, these mice present a misbalance of the multipotent progenitor compartment and the erythroid lineage, which translates into compensatory stress erythropoiesis and splenomegaly. Despite the severe thrombocytopenia, Gata1cKO^MK mice display a mild reduction of TPO plasma levels, and Gata1cKO^MK megakaryocytes show a mild increase in Pf4 mRNA levels; such a misbalance might be behind the general hematopoietic defects observed, affecting locally normal TPO and Pf4 levels at hematopoietic stem cell niches.

GATA family transcriptional factors: emerging suspects in hematologic disorders

GATA transcription factors are zinc finger DNA binding proteins that regulate transcription during development and cell differentiation. The three important GATA transcription factors GATA1, GATA2 and GATA3 play essential roles in the development and maintenance of hematopoietic systems. GATA1 is required for the erythroid and megakaryocytic commitment during hematopoiesis. GATA2 is crucial for the proliferation and survival of early hematopoietic cells, and is also involved in lineage specific transcriptional regulation as the dynamic partner of GATA1. GATA3 plays an essential role in T lymphoid cell development and immune regulation. As a result, mutations in genes encoding the GATA transcription factors or alteration in the protein expression level or their function have been linked to a variety of human hematologic disorders. In this review, we summarized the current knowledge regarding the disrupted biologic function of GATA in various hematologic disorders.

Shotgun metabolomic profiles in maternal urine identify potential mass spectral markers of abnormal fetal biochemistry - dihydrouracil and progesterone in the metabolism of Down syndrome

In Down syndrome (DS) in particular, the precise cellular mechanisms linking genotype to phenotype is not straightforward despite a clear mapping of the genetic cause. Metabolomic profiling might be more revealing in understanding molecular-cellular mechanisms of inborn errors of metabolism/syndromes than genomics alone and also result in new prenatal screening approaches. The urinary metabolome of 122 maternal urine from women with and without an aneuploid pregnancy (predominantly Down syndrome) were compared by both zwitterionic hydrophilic interaction chromatography (ZIC-HILIC) and reversed-phase liquid chromatography (RPLC) coupled to hybrid ion trap time of flight mass spectral analysis. ZIC-HILIC mass spectrometry resolved 10-fold more unique molecular ions than RPLC mass spectrometry, of which molecules corresponding to ions of m/z 114.07 and m/z 314.20 showed maternal urinary level changes that significantly coincided with the presence of a DS fetus. The ion of m/z 314.20 was identified as progesterone and m/z 114.07 as dihydrouracil. A metabolomics profiling-based maternal urinary screening test modelled from this separation data would detect approximately 87 and 60.87% (using HILIC-MS and RPLC-MS, respectively) of all DS pregnancies between 9 and 23 weeks of gestation with no false positives.

Transient Abnormal Myelopoiesis in Neonates: GATA Get the Diagnosis

Transient abnormal myelopoiesis occurs exclusively in patients with Down syndrome (constitutional trisomy 21), manifests in the neonatal period, and is characterized by circulating megakaryoblasts with varied degrees of multisystem organ involvement. In most cases, this process resolves spontaneously by 3 to 6 months of age, but for some, the disease can be fatal. Affected patients are particularly prone to develop acute megakaryoblastic leukemia in early childhood. Somatic GATA1 mutations are believed to be pivotal in the development of transient abnormal myelopoiesis and have proven to be a marker of clonal identity in its evolution to megakaryoblastic leukemia. We describe a study case of transient abnormal myelopoiesis and review the clinical manifestations, laboratory features, natural history, molecular genetics, and postulated disease pathogenesis of this disorder.

The genome-wide molecular signature of transcription factors in leukemia

Transcription factors control expression of genes essential for the normal functioning of the hematopoietic system and regulate development of distinct blood cell types. During leukemogenesis, aberrant regulation of transcription factors such as RUNX1, CBFβ, MLL, C/EBPα, SPI1, GATA, and TAL1 is central to the disease. Here, we will discuss the mechanisms of transcription factor deregulation in leukemia and how in recent years next-generation sequencing approaches have helped to elucidate the molecular role of many of these aberrantly expressed transcription factors. We will focus on the complexes in which these factors reside, the role of posttranslational modification of these factors, their involvement in setting up higher order chromatin structures, and their influence on the local epigenetic environment. We suggest that only comprehensive knowledge on all these aspects will increase our understanding of aberrant gene expression in leukemia as well as open new entry points for therapeutic intervention.

Direct protein interactions are responsible for Ikaros-GATA and Ikaros-Cdk9 cooperativeness in hematopoietic cells

Ikaros (Ik) is a critical regulator of hematopoietic gene expression. Here, we established that the Ik interactions with GATA transcription factors and cyclin-dependent kinase 9 (Cdk9), a component of the positive transcription elongation factor b (P-TEFb), are required for transcriptional activation of Ik target genes. A detailed dissection of Ik-GATA and Ik-Cdk9 protein interactions indicated that the C-terminal zinc finger domain of Ik interacts directly with the C-terminal zinc fingers of GATA1, GATA2, and GATA3, whereas the N-terminal zinc finger domain of Ik is required for interaction with the kinase and T-loop domains of Cdk9. The relevance of these interactions was demonstrated in vivo in COS-7 and primary hematopoietic cells, in which Ik facilitated Cdk9 and GATA protein recruitment to gene promoters and transcriptional activation. Moreover, the oncogenic isoform Ik6 did not efficiently interact with Cdk9 or GATA proteins in vivo and perturbed Cdk9/P-TEFb recruitment to Ik target genes, thereby affecting transcription elongation. Finally, characterization of a novel nuclear Ik isoform revealed that Ik exon 6 is dispensable for interactions with Mi2 and GATA proteins but is essential for the Cdk9 interaction. Thus, Ik is central to the Ik-GATA-Cdk9 regulatory network, which is broadly utilized for gene regulation in hematopoietic cells.

Spontaneous remission of childhood acute marrow fibrosis and megakaryoblastic leukemia

Spontaneous remission in 2 children with myelofibrosis, one with megakaryocytic acute myeloblastic leukemia and t(1;22) (with recurrence later) and one with Down syndrome and GATA1 mutation (permanent), are described. One had sepsis and was treated with antibiotics and blood products, whereas the other received only blood products. Remission was spontaneous, without chemotherapy treatment. Possible explanations for these outcomes include immunologic response to sepsis by a leukemia-specific T-cell response or the release of various cytokines, such as tumor necrosis factor and interleukin-2, during infections. Natural killer and cytotoxic T cells transfused with blood products might have also triggered an immune response.

The GATA1s isoform is normally down-regulated during terminal haematopoietic differentiation and over-expression leads to failure to repress MYB, CCND2 and SKI during erythroid differentiation of K562 cells

Background

Although GATA1 is one of the most extensively studied haematopoietic transcription factors little is currently known about the physiological functions of its naturally occurring isoforms GATA1s and GATA1FL in humans—particularly whether the isoforms have distinct roles in different lineages and whether they have non-redundant roles in haematopoietic differentiation. As well as being of general interest to understanding of haematopoiesis, GATA1 isoform biology is important for children with Down syndrome associated acute megakaryoblastic leukaemia (DS-AMKL) where GATA1FL mutations are an essential driver for disease pathogenesis.

Methods

Human primary cells and cell lines were analyzed using GATA1 isoform specific PCR. K562 cells expressing GATA1s or GATA1FL transgenes were used to model the effects of the two isoforms on in vitro haematopoietic differentiation.

Results

We found no evidence for lineage specific use of GATA1 isoforms; however GATA1s transcripts, but not GATA1FL transcripts, are down-regulated during in vitro induction of terminal megakaryocytic and erythroid differentiation in the cell line K562. In addition, transgenic K562-GATA1s and K562-GATA1FL cells have distinct gene expression profiles both in steady state and during terminal erythroid differentiation, with GATA1s expression characterised by lack of repression of MYB, CCND2 and SKI.

Conclusions

These findings support the theory that the GATA1s isoform plays a role in the maintenance of proliferative multipotent megakaryocyte-erythroid precursor cells and must be down-regulated prior to terminal differentiation. In addition our data suggest that SKI may be a potential therapeutic target for the treatment of children with DS-AMKL.

Myeloid leukemia in Down syndrome

Although adults with Down syndrome (DS) show a decreased incidence of cancer compared to individuals without DS, children with DS are at an increased risk of leukemia. Nearly half of these childhood leukemias are classified as acute megakaryoblastic leukemia (AMKL), a relatively rare subtype of acute myeloid leukemia (AML). Here, we summarize the clinical features of myeloid leukemia in DS, review recent research on the mechanisms of leukemogenesis, including the roles of GATA1 mutations and trisomy 21, and discuss treatment strategies. Given that trisomy 21 is a relatively common event in hematologic malignancies, greater knowledge of how the genes on chromosome 21 contribute to DS-AMKL will increase our understanding of a broader class of patients with leukemia.

Transient myeloproliferative disorder and GATA1 mutation in neonates with and without Down syndrome

OBJECTIVE

To report clinical experiences and cytogenetic findings of transient myeloproliferative disorder (TMD) in neonates with and without Down syndrome (DS).

METHODS

GATA1 gene was screened in DNA samples from neonates presenting with TMD during their leukemic and remission status.

RESULTS

Six neonates (2 phenotypically normal and 4 DS) born in the past 6 years had presented with TMD; all had trisomy 21 during leukemic status. Two DS infants died during early infancy, one of hepatic failure and one of cardiac complication. One non-DS infant evolved into myelodysplastic syndrome (MDS) and acute leukemia since 14 months old. Three other patients have not developed true leukemia after follow-up of 8, 9, and 70 months, respectively. The authors detected mutations within exon 2 of GATA1 gene in 3 DS and 2 non-DS infants. All these mutations disappeared after remission of TMD, but an identical mutation was detected in one non-DS patient when evolving into MDS. Trisomy 21 was confined to leukemic clone in non-DS patients.

CONCLUSIONS

TMD should be considered in case of congenital leukemia with megakaryoblastic features and accompanied by trisomy 21 and GATA1 mutation. Both DS and non-DS patients will possibly develop true leukemia within few years.

Down syndrome and GATA1 mutations in transient abnormal myeloproliferative disorder: mutation classes correlate with progression to myeloid leukemia

Twenty percent to 30% of transient abnormal myelopoiesis (TAM) observed in newborns with Down syndrome (DS) develop myeloid leukemia of DS (ML-DS). Most cases of TAM carry somatic GATA1 mutations resulting in the exclusive expression of a truncated protein (GATA1s). However, there are no reports on the expression levels of GATA1s in TAM blasts, and the risk factors for the progression to ML-DS are unidentified. To test whether the spectrum of transcripts derived from the mutant GATA1 genes affects the expression levels, we classified the mutations according to the types of transcripts, and investigated the modalities of expression by in vitro transfection experiments using GATA1 expression constructs harboring mutations. We show here that the mutations affected the amount of mutant protein. Based on our estimates of GATA1s protein expression, the mutations were classified into GATA1s high and low groups. Phenotypic analyses of 66 TAM patients with GATA1 mutations revealed that GATA1s low mutations were significantly associated with a risk of progression to ML-DS (P < .001) and lower white blood cell counts (P = .004). Our study indicates that quantitative differences in mutant protein levels have significant effects on the phenotype of TAM and warrants further investigation in a prospective study.

Loss of the Gata1 gene IE exon leads to variant transcript expression and the production of a GATA1 protein lacking the N-terminal domain

GATA1 is essential for the differentiation of erythroid cells and megakaryocytes. The Gata1 gene is composed of multiple untranslated first exons and five common coding exons. The erythroid first exon (IE exon) is important for Gata1 gene expression in hematopoietic lineages. Because previous IE exon knockdown analyses resulted in embryonic lethality, less is understood about the contribution of the IE exon to adult hematopoiesis. Here, we achieved specific deletion of the floxed IE exon in adulthood using an inducible Cre expression system. In this conditional knock-out mouse line, the Gata1 mRNA level was significantly down-regulated in the megakaryocyte lineage, resulting in thrombocytopenia with a marked proliferation of megakaryocytes. By contrast, in the erythroid lineage, Gata1 mRNA was expressed abundantly utilizing alternative first exons. Especially, the IEb/c and newly identified IEd exons were transcribed at a level comparable with that of the IE exon in control mice. Surprisingly, in the IE-null mouse, these transcripts failed to produce full-length GATA1 protein, but instead yielded GATA1 lacking the N-terminal domain inefficiently. With low level expression of the short form of GATA1, IE-null mice showed severe anemia with skewed erythroid maturation. Notably, the hematological phenotypes of adult IE-null mice substantially differ from those observed in mice harboring conditional ablation of the entire Gata1 gene. The present study demonstrates that the IE exon is instrumental to adult erythropoiesis by regulating the proper level of transcription and selecting the correct transcription start site of the Gata1 gene.

Induction of hyperproliferative fetal megakaryopoiesis by an N-terminally truncated GATA1 mutant

Two GATA1-related leukemias have been described: one is an erythroleukemia that develops in mice as a consequence of diminished expression of wild-type GATA1, whereas the other is an acute megakaryoblastic leukemia (AMKL) that arises in Down syndrome children as a consequence of somatic N-terminal truncation (DeltaNT) of GATA1. We discovered that mice expressing the shortened GATA1 protein (DeltaNTR mice) phenocopies the human transient myeloproliferative disorder (TMD) that precedes AMKL in Down syndrome children. In perinatal livers of the DeltaNTR mutant mice, immature megakaryocytes accumulate massively, and this fraction contains cells that form hyperproliferative megakaryocytic colonies. Furthermore, showing good agreement with the clinical course of TMD in humans, DeltaNTR mutant mice undergo spontaneous resolution from the massive megakaryocyte accumulation concomitant with the switch of hematopoietic microenvironment from liver to bone marrow/spleen. These results thus demonstrate that expression of the GATA1/Gata1 N-terminal deletion mutant per se induces hyperproliferative fetal megakaryopoiesis. This mouse model serves as an important means to clarify how impaired GATA1 function contributes to the multi-step leukemogenesis.

The 2008 revision of the World Health Organization (WHO) classification of myeloid neoplasms and acute leukemia: rationale and important changes

Recently the World Health Organization (WHO), in collaboration with the European Association for Haematopathology and the Society for Hematopathology, published a revised and updated edition of the WHO Classification of Tumors of the Hematopoietic and Lymphoid Tissues. The 4th edition of the WHO classification incorporates new information that has emerged from scientific and clinical studies in the interval since the publication of the 3rd edition in 2001, and includes new criteria for the recognition of some previously described neoplasms as well as clarification and refinement of the defining criteria for others. It also adds entities-some defined principally by genetic features-that have only recently been characterized. In this paper, the classification of myeloid neoplasms and acute leukemia is highlighted with the aim of familiarizing hematologists, clinical scientists, and hematopathologists not only with the major changes in the classification but also with the rationale for those changes.

Hematopoietic defects in the Ts1Cje mouse model of Down syndrome

Down syndrome (DS) persons are born with various hematopoietic abnormalities, ranging from relatively benign, such as neutrophilia and macrocytosis, to a more severe transient myeloproliferative disorder (TMD). In most cases, these abnormalities resolve in the first few months to years of life. However, sometimes the TMD represents a premalignant disease that develops into acute megakaryocytic leukemia (AMKL), usually in association with acquired GATA1 mutations. To gain insight into the mechanisms responsible for these abnormalities, we analyzed the hematopoietic development of the Ts1Cje mouse model of DS. Our analyses identified defects in mature blood cells, including macrocytosis and anemia, as well as abnormalities in fetal liver and bone marrow stem and progenitor cell function. Despite these defects, the Ts1Cje mice do not develop disease resembling either TMD or AMKL, and this was not altered by a loss of function allele of Gata1. Thus, loss of Gata1 and partial trisomy of chromosome 21 orthologs, when combined, do not appear to be sufficient to induce TMD or AMKL-like phenotypes in mice.

A novel mutation in the GATA1 gene associated with acute megakaryoblastic leukemia in a Korean Down syndrome patient

Although acquired mutations in the GATA1 gene have been reported for Down syndrome-related acute megakaryoblastic leukemia (DS-AMKL) in Caucasians, this is the first report of a Korean Down syndrome patient with AMKL carrying a novel mutation of the GATA1 gene. A 3-yr-old Korean girl with Down syndrome was admitted to our hospital complaining of pallor and fever. The findings of a peripheral blood smear and bone marrow study were compatible with the presence of AMKL. A chromosome study showed 48,XX,-7,+21c,+21,+r[3]/47,XX,+21c[17]. Following GATA1 gene mutation analysis, a novel mutation, c.145dupG (p.Ala49GlyfsX18), was identified in the N-terminal activation domain of the GATA1 gene. This mutation caused a premature termination at codon 67 and expression of an abnormal GATA-1 protein with a defective N-terminal activation domain, and the absence of full-length GATA-1 protein. This case demonstrates that a leukemogenic mechanism for DS-AMKL is contributed by a unique collaboration between overexpressed genes from trisomy 21 and an acquired GATA1 mutation previously seen in Caucasians and now in a Korean patient.

GATA1-related leukaemias

GATA1 is a prototypical lineage-restricted transcription factor that is central to the correct differentiation, proliferation and apoptosis of erythroid and megakaryocytic cells. Mutations in GATA1 can generate a truncated protein, which contributes to the genesis of transient myeloproliferative disorder (TMD) and acute megakaryoblastic leukaemia (AMKL) in infants with Down syndrome. Similarly, Gata1 knockdown to 5% of its wild-type level causes high incidence of erythroid leukaemia in mice. The GATA1-related leukaemias in both human and mouse could provide important insights into the mechanism of multi-step leukaemogenesis. Efforts are afoot to produce mouse models that are reflective of TMD and AMKL.

Transcription factors GATA-1 and Fli-1 regulate human HOXA10 expression in megakaryocytic cells

HOXA10 is a member of the HOX family of regulatory genes that are involved in hematopoiesis. Its role in megakaryopoiesis has been suggested by its expression in immature megakaryocytes and by the proliferation of megakaryocyte-primitive blast colonies upon HOXA10 overexpression. We sought to understand the role of HOXA10 in megakaryopoiesis better, by investigating its transcriptional regulation. Analysis of the 5' untranslated region and transfection of promoter/plasmids into human tissue culture cell lines identified transcriptionally active sequences that contain GATA-1 and Ets-1 sites and a putative binding site for its neighboring gene, HOXA11. Gel shift assays confirmed protein-DNA interactions at these sites. Mutation of the GATA-1 and the Ets-1 motifs amplified the expression of HOXA10 in HEL and K562 cells, confirming the importance of these cis-acting elements in regulating HOXA10 expression in megakaryocytic cells. Chromatin immunoprecipitation (ChIP) and chloramphenicol acetyl transferase (CAT) assays confirm that HOXA11 binds to the putative binding site, resulting in repression of HOXA10 expression. These data taken together give insight into the regulation of HOXA10 expression in megakaryocytic differentiation.

Mechanisms of HDAC inhibitor-induced thrombocytopenia

Histone deacetylase inhibitors (HDAC inhibitors) are an emerging class of anticancer agents. To elucidate the mechanism of HDAC inhibitor-induced thrombocytopenia, we focused on the effects of HDAC inhibitors on megakaryocyte differentiation and performed Affymetrix GeneChip analysis of human megakaryocytic HEL cells treated with or without HDAC inhibitors. Here, we report that GATA-1 and 10 haematopoietic factors (SCL, NF-E2, EKLF, Pleckstrin, Thrombin-R, LMO2, PU.1, Fli-1, AML1, and TCF11) are transcriptionally repressed by HDAC inhibitors in a similar pattern (R>0.98), and putative GATA-1-binding sites are found in almost all promoters of these genes. In addition, luciferase reporter assays reveal that mutations of GATA-1-binding sites in the GATA-1 promoter abolish its sensitivity to HDAC inhibitor-mediated down-regulation in HEL cells. Further, this report also asserts that HDAC inhibitor increases megakaryocyte counts and inhibits GATA-1 gene expression in rat spleen. Together, these results suggest that HDAC inhibitors inhibit GATA-1 gene expression by decreasing the transactivation function of GATA-1 itself, and that this may in turn lead to a delay in megakaryocyte maturation and finally cause thrombocytopenia. Our findings may help our understanding of the molecular mechanism of HDAC inhibitor-mediated GATA-1 transcriptional repression and to reduce the risk of HDAC inhibitor-induced thrombocytopenia.

Pharmacogenetics of alkylator-associated acute myeloid leukemia

Therapy-related acute myeloid leukemia (t-AML) is a lethal late complication of alkylator chemotherapy. The genetic basis of susceptibility to t-AML is poorly understood. Both t-AML and de novo AML are complex genetic diseases, requiring cooperating mutations in interacting pathways for disease initiation and progression. Germline variants of these ‘leukemia pathway’ genes may cooperate with somatic mutations to induce both de novo and therapy-related AML. Several cancer susceptibility syndromes have been identified that cause an inherited predisposition to de novo and t-AML. The genes responsible for these syndromes are also somatically mutated in sporadic AML. We reason that germline polymorphism in any gene somatically mutated in AML could contribute to t-AML risk in the general population. Identification of these susceptibility alleles should help clinicians develop tailored therapies that reduce the relative risk of t-AML.

Megakaryocyte biology and related disorders

Platelets, derived from megakaryocytes, have an essential role in thrombosis and hemostasis. Over the past 10 years, a great deal of new information has been obtained concerning the various aspects of hematopoiesis necessary to maintain a steady-state platelet level to support physiologic hemostasis. Here we discuss the differentiation of HSCs into megakaryocytes, with emphasis on the key cytokine signaling pathways and hematopoietic transcription factors. Recent insight into these processes elucidates the molecular bases of numerous acquired and inherited hematologic disorders. It is anticipated that the growing knowledge in these areas may be exploited for new therapeutic strategies to modulate both platelet numbers and their thrombogenicity.

GATA1 as a new target to detect minimal residual disease in both transient leukemia and megakaryoblastic leukemia of Down syndrome

Acquired mutations in exon 2 of the GATA1 gene are detected in most Down syndrome (DS) patients with transient leukemia (TL) and acute megakaryoblastic leukemia (AMKL). We sought to determine if GATA1 mutations can be utilized as markers for minimal residual disease (MRD). GATA1 mutations were screened by SSCP analysis and sequenced. Using GATA1 mutation-specific primers, follow-up bone marrow samples from four patients were assayed by quantitative PCR. We show that molecular monitoring of GATA1 mutations is possible in Down syndrome patients with TL and AMKL, and GATA1 could be a stable marker for MRD monitoring.

GATA Transcription Factors in Hematologic Disease

GATA family transcription factors play essential roles in broad developmental settings. GATA-1, one of the hematopoietically expressed members, is required for normal erythroid and megakaryocytic differentiation. Over the past few years, mutations in the gene encoding GATA-1 have been linked to several human hematologic disorders, including X-linked dyserythropoietic anemia and thrombocytopenia, X-linked thrombocytopenia and beta-thalassemia, and Down syndrome acute megakaryoblastic leukemia. This review summarizes the role of GATA-1 during normal hematopoiesis and discusses how disease-associated mutations may affect its function.

Leukemogenesis caused by incapacitated GATA-1 function

GATA-1 is essential for the development of erythroid and megakaryocytic lineages. We found that GATA-1 gene knockdown female (GATA-1.05/X) mice frequently develop a hematopoietic disorder resembling myelodysplastic syndrome that is characterized by the accumulation of progenitors expressing low levels of GATA-1. In this study, we demonstrate that GATA-1.05/X mice suffer from two distinct types of acute leukemia, an early-onset c-Kit-positive nonlymphoid leukemia and a late-onset B-lymphocytic leukemia. Since GATA-1 is an X chromosome gene, two types of hematopoietic cells reside within heterozygous GATA-1 knockdown mice, bearing either an active wild-type GATA-1 allele or an active mutant GATA-1.05 allele. In the hematopoietic progenitors with the latter allele, low-level GATA-1 expression is sufficient to support survival and proliferation but not differentiation, leading to the accumulation of progenitors that are easily targeted by oncogenic stimuli. Since such leukemia has not been observed in GATA-1-null/X mutant mice, we conclude that the residual GATA-1 activity in the knockdown mice contributes to the development of the malignancy. This de novo model recapitulates the acute crisis found in preleukemic conditions in humans.

Possibilities for tailored and targeted therapy in paediatric acute myeloid leukaemia

The clinical outcome of acute myeloid leukaemia (AML) in children has improved considerably using intensive chemotherapy and/or stem cell transplantation. This leads to cure in 50-70% of patients, and also results in significant morbidity and mortality. Hence, we need other ways to improve the cure rate. This review discusses possibilities for tailored therapy, reviewing in vitro cellular drug sensitivity data. The results provide suggestions regarding the adaptation of clinical protocols in certain AML subgroups, although further clinical studies will show whether this is effective. Secondly, we review type 1 genetic abnormalities (such as receptor tyrosine kinase mutations) that result in enhanced survival and proliferation of leukaemic cells, which can be detected in approximately 50% of paediatric AML samples, and are non-randomly associated with French-American-British type and cytogenetic subgroups. FLT3 internal tandem duplication is associated with poor clinical outcome, and may be used for risk-group stratification. The first results with small molecule inhibitors in adult AML do not suggest their use in children as yet. International collaboration is needed to further improve outcome by developing treatment protocols for subgroups of paediatric AML.

A molecular dissection of the interaction between the transcription factor Gata-1 zinc finger and DNA

The production of circulating blood cells from bone marrow stem cells during hematopoiesis is accompanied by overall changes in gene expression which cause production of required functional proteins, such as hemoglobin in erythroid cells, as well as control of cell growth, preventing apoptosis of differentiating cells. Hematopoietic gene regulation is controlled by several specific transcription factors, including the factor Gata-1, which is required for erythrocyte maturation. Based on contacts observed in the NMR structure of the cGata-1 binding domain in complex with DNA, the protein's key DNA interface is interesting in being quite hydrophobic in nature, due to the presence of three leucine side chains protruding toward the DNA. Given the T-rich composition of the GATA DNA binding site, it is possible that thymine's unique 5-methyl group may mediate some of these hydrophobic contacts to increase the stability of binding. The hypothesis that thymine methyl groups are important to the free energy of binding between Gata and DNA is tested by measuring binding of an oligonucleotide substrate in which individual thymine bases are substituted with uracil. To test for any important base-pair specific interactions which may be hydrogen-bonded in character, we have also assayed Gata binding to oligonucleotides with base analogs which cannot make hydrogen bonds. We report that out of the binding site's five thymine methyl groups, only one appeared to make a notable contribution to binding affinity, with removal causing a loss of less than 1kcal/mol of binding free energy. On the other hand, perturbing the potential hydrogen-bonding surface of the DNAs major groove was found to cause a larger decrease in binding affinity than removal of any of the thymine methyl groups, with a loss of 2-3kcal/mol of binding free energy.