Transgene insertion in proximity to the c-myb gene disrupts erythroid-megakaryocytic lineage bifurcation

Gene therapy and gene editing strategies in inherited blood disorders

Gene therapy has shown significant potential in treating various diseases, particularly inherited blood disorders such as hemophilia, sickle cell disease, and thalassemia. Advances in understanding the regulatory network of disease-associated genes have led to the identification of additional therapeutic targets for treatment, especially for β-hemoglobinopathies. Erythroid regulatory factor BCL11A offers the most promising therapeutic target for β-hemoglobinopathies and reduction of its expression using the commercialized gene therapy product Casgevy was approved for use in the UK and USA in 2023. Notably, the emergence of innovative gene editing technologies has further broadened the gene therapy landscape, presenting new possibilities for treatment. Intensive studies indicate that base editing and prime editing, built upon CRISPR technology, enable precise single-base modification in hematopoietic stem cells for addressing inherited blood disorders ex vivo and in vivo. In this review, we present an overview of the current landscape of gene therapies, focusing on clinical research and gene therapy products for inherited blood disorders, evaluation of potential gene targets, and the gene editing tools employed in current gene therapy practices, which provides an insight for the establishment of safer and more effective gene therapy methods for a wider range of diseases in the future.

Myb overexpression synergizes with the loss of Pten and is a dependency factor and therapeutic target in T-cell lymphoblastic leukemia

T-lineage acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy that accounts for 10%-15% of pediatric and 25% of adult ALL cases. Although the prognosis of T-ALL has improved over time, the outcome of T-ALL patients with primary resistant or relapsed leukemia remains poor. Therefore, further progress in the treatment of T-ALL requires a better understanding of its biology and the development of more effective precision oncologic therapies. The proto-oncogene MYB is highly expressed in diverse hematologic malignancies, including T-ALLs with genomic aberrations that further potentiate its expression and activity. Previous studies have associated MYB with a malignant role in the pathogenesis of several cancers. However, its role in the induction and maintenance of T-ALL remains relatively poorly understood. In this study, we found that an increased copy number of MYB is associated with higher MYB expression levels, and might be associated with inferior event-free survival of pediatric T-ALL patients. Using our previously described conditional Myb overexpression mice, we generated two distinct MYB-driven T-ALL mouse models. We demonstrated that the overexpression of Myb synergizes with Pten deletion but not with the overexpression of Lmo2 to accelerate the development of T-cell lymphoblastic leukemias. We also showed that MYB is a dependency factor in T-ALL since RNA interference of Myb blocked cell cycle progression and induced apoptosis in both human and murine T-ALL cell lines. Finally, we provide preclinical evidence that targeting the transcriptional activity of MYB can be a useful therapeutic strategy for the treatment of T-ALL.

Multiphoton In Vivo Microscopy of Embryonic Thrombopoiesis Reveals the Generation of Platelets through Budding

Platelets are generated by specialized cells called megakaryocytes (MKs). However, MK's origin and platelet release mode have remained incompletely understood. Here, we established direct visualization of embryonic thrombopoiesis in vivo by combining multiphoton intravital microscopy (MP-IVM) with a fluorescence switch reporter mouse model under control of the platelet factor 4 promoter (Pf4CreRosa26mTmG). Using this microscopy tool, we discovered that fetal liver MKs provide higher thrombopoietic activity than yolk sac MKs. Mechanistically, fetal platelets were released from MKs either by membrane buds or the formation of proplatelets, with the former constituting the key process. In E14.5 c-Myb-deficient embryos that lack definitive hematopoiesis, MK and platelet numbers were similar to wild-type embryos, indicating the independence of embryonic thrombopoiesis from definitive hematopoiesis at this stage of development. In summary, our novel MP-IVM protocol allows the characterization of thrombopoiesis with high spatio-temporal resolution in the mouse embryo and has identified membrane budding as the main mechanism of fetal platelet production.

ASIS-Seq: Transgene Insertion Site Mapping by Nanopore Adaptive Sampling

Generation of transgenic mice by direct microinjection of foreign DNA into fertilized ova has become a routine technique in biomedical research. It remains an essential tool for studying gene expression, developmental biology, genetic disease models, and their therapies. However, the random integration of foreign DNA into the host genome that is inherent to this technology can lead to confounding effects associated with insertional mutagenesis and transgene silencing. Locations of most transgenic lines remain unknown because the methods are often burdensome (Nicholls et al., G3: Genes Genomes Genetics 9:1481-1486, 2019) or have limitations (Goodwin et al., Genome Research 29:494-505, 2019). Here, we present a method that we call Adaptive Sampling Insertion Site Sequencing (ASIS-Seq) to locate transgene integration sites using targeted sequencing on Oxford Nanopore Technologies' (ONT) sequencers. ASIS-Seq requires only about 3 ug of genomic DNA, 3 hours of hands-on sample preparation time, and 3 days of sequencing time to locate transgenes in a host genome.

3R measures in facilities for the production of genetically modified rodents

Sociocultural changes in the human-animal relationship have led to increasing demands for animal welfare in biomedical research. The 3R concept is the basis for bringing this demand into practice: Replace animal experiments with alternatives where possible, Reduce the number of animals used to a scientifically justified minimum and Refine the procedure to minimize animal harm. The generation of gene-modified sentient animals such as mice and rats involves many steps that include various forms of manipulation. So far, no coherent analysis of the application of the 3Rs to gene manipulation has been performed. Here we provide guidelines from the Committee on Genetics and Breeding of Laboratory Animals of the German Society for Laboratory Animal Science to implement the 3Rs in every step during the generation of genetically modified animals. We provide recommendations for applying the 3Rs as well as success/intervention parameters for each step of the process, from experiment planning to choice of technology, harm-benefit analysis, husbandry conditions, management of genetically modified lines and actual procedures. We also discuss future challenges for animal welfare in the context of developing technologies. Taken together, we expect that our comprehensive analysis and our recommendations for the appropriate implementation of the 3Rs to technologies for genetic modifications of rodents will benefit scientists from a wide range of disciplines and will help to improve the welfare of a large number of laboratory animals worldwide.

Detection of novel fusion genes by next-generation sequencing-based targeted RNA sequencing analysis in adenoid cystic carcinoma of head and neck

OBJECTIVE

Adenoid cystic carcinoma (AdCC) is a rare, indolent salivary gland tumor that is reported to be driven by fusion genes. However, MYB/MYBL1-NFIB fusions have been detected in <60% of all AdCC cases and the oncogenic driver mutations in approximately 40% of AdCC remain unknown. Our aim was to identify novel gene fusions in AdCC.

STUDY DESIGN

We investigated 20 AdCC cases using a targeted RNA sequencing panel to identify gene fusions and performed quantitative real-time reverse transcription polymerase chain reaction to assess MYB, MYBL1, and NFIB expression levels.

RESULTS

A total of 36 fusion transcripts in 15 cases were detected and validated by Sanger sequencing. The MYB-NFIB and MYBL1-NFIB fusion genes were detected in 9 and 3 cases, respectively, in a mutually exclusive manner. Furthermore, novel gene fusions, namely, NFIB-EPB41L2, MAP7-NFIB, NFIB-MCMDC2, MYBL1-C8orf34, C8orf34-NFIB, and NFIB-CASC20, were identified. Among them, NFIB-EPB41L2 and NFIB-MCMDC2 are thought to activate MYB and MYBL1 expression, respectively, through the insertion of a genomic segment in proximity to MYB and MYBL1 genes, respectively.

CONCLUSION

Six novel gene fusions other than MYB/MYBL1-NFIB were identified. The detection of novel fusion genes and investigation of the molecular mechanism will contribute to the development of novel molecular targeted therapies for this disease.

The role of bone marrow microRNA (miR) in erythropoietic dysfunction after severe trauma

BACKGROUND

Previous data has shown that severe traumatic injury is associated with bone marrow dysfunction, which manifests as persistent injury-associated anemia. This study sought to identify whether the expression of erythropoiesis-related microRNAs were altered in the bone marrow of trauma patients to determine if these microRNAs play a role in persistent injury-associated anemia.

METHODS

Bone marrow was collected from severely injured trauma patients who underwent fracture fixation as well as patients who underwent elective hip replacement. There were 27 trauma patients and 10 controls analyzed. Total RNA and microRNA were isolated from CD34-positive cells using the RNeasy Plus Mini kit, and genome-wide microRNA expression patterns were assayed. Genes with significant expression differences were found using BRB-ArrayTools with a significance of P < .01.

RESULTS

There were marked differences in expression of 108 microRNAs in the trauma group when compared with hip replacement patients. Four of these microRNAs play a role in regulating erythropoiesis: microRNA-150, microRNA-223, microRNA15a, and microRNA-24. These microRNAs were all upregulated significantly, with trauma/hip replacement fold changes of 1.7, 1.8, 1.2, and 1.2 respectively, and all act to suppress or regulate erythropoiesis.

CONCLUSION

Assessment of the bone marrow microRNA profile in trauma patients compared to those undergoing elective hip replacement revealed the differential expression of microRNA-150, microRNA-223, microRNA-15a, and microRNA-24. These microRNAs all play a role in decreased erythroid progenitor cell growth and provide important insight to the erythropoietic dysfunction seen after trauma.

Regulation of MYB by distal enhancer elements in human myeloid leukemia

MYB plays vital roles in regulating proliferation and differentiation of hematopoietic progenitor cells, dysregulation of MYB has been implicated in the pathogenesis of leukemia. Although the transcription of MYB has been well studied, its detailed underlying regulatory mechanisms still remain elusive. Here, we detected the long-range interaction between the upstream regions, −34k and −88k, and the MYB promoter in K562, U937, and HL-60 cells using circularized chromosome conformation capture (4C) assay, which declined when MYB was downregulated during chemical-induced differentiation. The enrichment of enhancer markers, H3K4me1 and H3K27ac, and enhancer activity at the −34k and −88k regions were confirmed by ChIP-qPCR and luciferase assay respectively. ChIP-qPCR showed the dynamic binding of GATA1, TAL1, and CCAAT/enhancer-binding protein (C/EBPβ) at −34k and −88k during differentiation of K562 cells. Epigenome editing by a CRISPR-Cas9-based method showed that H3K27ac at −34k enhanced TF binding and MYB expression, while DNA methylation inhibited MYB expression. Taken together, our data revealed that enhancer elements at −34k are required for MYB expression, TF binding, and epigenetic modification are closely involved in this process in human myeloid leukemia cells.

Regulation of Hemogenic Endothelial Cell Development and Function

Embryonic definitive hematopoiesis generates hematopoietic stem and progenitor cells (HSPCs) essential for establishment and maintenance of the adult blood system. This process requires the specification of a subset of vascular endothelial cells to become blood-forming, or hemogenic, and the subsequent endothelial-to-hematopoietic transition to generate HSPCs therefrom. The mechanisms that regulate these processes are under intensive investigation, as their recapitulation in vitro from human pluripotent stem cells has the potential to generate autologous HSPCs for clinical applications. In this review, we provide an overview of hemogenic endothelial cell development and highlight the molecular events that govern hemogenic specification of vascular endothelial cells and the generation of multilineage HSPCs from hemogenic endothelium. We also discuss the impact of hemogenic endothelial cell development on adult hematopoiesis.

Arsenic Metabolism in Mice Carrying a BORCS7/AS3MT Locus Humanized by Syntenic Replacement

BACKGROUND

Chronic exposure to inorganic arsenic (iAs) is a significant public health problem. Methylation of iAs by arsenic methyltransferase (AS3MT) controls iAs detoxification and modifies risks of iAs-induced diseases. Mechanisms underlying these diseases have been extensively studied using animal models. However, substantive differences between humans and laboratory animals in efficiency of iAs methylation have hindered the translational potential of the laboratory studies.

OBJECTIVES

The goal of this study was to determine whether humanization of the As3mt gene confers a human-like pattern of iAs metabolism in mice.

METHODS

We generated a mouse strain in which the As3mt gene along with the adjacent Borcs7 gene was humanized by syntenic replacement. We compared expression of the mouse As3mt and the human AS3MT and the rate and pattern of iAs metabolism in the wild-type and humanized mice.

RESULTS

AS3MT expression in mouse tissues closely modeled that of human and differed substantially from expression of As3mt. Detoxification of iAs was much less efficient in the humanized mice than in wild-type mice. Profiles for iAs and its methylated metabolites in tissues and excreta of the humanized mice were consistent with those reported in humans. Notably, the humanized mice expressed both the full-length AS3MT that catalyzes iAs methylation and the human-specific AS3MTd2d3 splicing variant that has been linked to schizophrenia.

CONCLUSIONS

These results suggest that AS3MT is the primary genetic locus responsible for the unique pattern of iAs metabolism in humans. Thus, the humanized mouse strain can be used to study the role of iAs methylation in the pathogenesis of iAs-induced diseases, as well as to evaluate the role of AS3MTd2d3 in schizophrenia. https://doi.org/10.1289/EHP6943.

Transcriptional activities of human elongation factor-1α and cytomegalovirus promoter in transgenic dogs generated by somatic cell nuclear transfer

Recent advances in somatic cell nuclear transfer (SCNT) in canines facilitate the production of canine transgenic models. Owing to the importance of stable and strong promoter activity in transgenic animals, we tested human elongation factor 1α (hEF1α) and cytomegalovirus (CMV) promoter sequences in SCNT transgenic dogs. After transfection, transgenic donor fibroblasts with the hEF1α-enhanced green fluorescence protein (EGFP) transgene were successfully isolated using fluorescence-activated cell sorting (FACS). We obtained four puppies, after SCNT, and identified three puppies as being transgenic using PCR analysis. Unexpectedly, EGFP regulated by hEF1α promoter was not observed at the organismal and cellular levels in these transgenic dogs. EGFP expression was rescued by the inhibition of DNA methyltransferases, implying that the hEF1α promoter is silenced by DNA methylation. Next, donor cells with CMV-EGFP transgene were successfully established and SCNT was performed. Three puppies of six born puppies were confirmed to be transgenic. Unlike hEF1α-regulated EGFP, CMV-regulated EGFP was strongly detectable at both the organismal and cellular levels in all transgenic dogs, even after 19 months. In conclusion, our study suggests that the CMV promoter is more suitable, than the hEF1α promoter, for stable transgene expression in SCNT-derived transgenic canine model.

Mapping the Chromosomal Insertion Site of the GFP Transgene of UBC-GFP Mice to the MHC Locus

GFP is frequently used as a marker for tracking donor cells adoptively transplanted into recipient animals. The human ubiquitin C promoter (UBC)-driven-GFP transgenic mouse is a commonly used source of donor cells for this purpose. This mouse was initially generated in the C57BL/6 inbred strain and has been backcrossed into the BALB/cBy strain for over 11 generations. Both the C57BL/6 inbred and BALB/cBy congenic UBC-GFP lines are commercially available and have been widely distributed. These UBC-GFP lines can be a convenient resource for tracking donor cells in both syngenic MHC-matched and in allogenic MHC-mismatched studies as C57BL/6 (H-2^b) and BALB/cBy (H-2^d) have disparate MHC haplotypes. In this report, we surprisingly discover that the UBC-GFP BALB/cBy congenic mice still retain the H-2^b MHC haplotype of their original C57BL/6 founder, suggesting that the UBC-GFP transgene integration site is closely linked to the MHC locus on chromosome 17. Using linear amplification-mediated PCR, we successfully map the UBC-GFP transgene to the MHC locus. This study highlights the importance and urgency of mapping the transgene integration site of transgenic mouse strains used in biomedical research. Furthermore, this study raises the possibility of alternative interpretations of previous studies using congenic UBC-GFP mice and focuses attention on the necessity for rigor and reproducibility in scientific research.

MicroRNAs as regulators and effectors of hematopoietic transcription factors

Hematopoiesis is a highly-regulated development process orchestrated by lineage-specific transcription factors that direct the generation of all mature blood cells types, including red blood cells, megakaryocytes, granulocytes, monocytes, and lymphocytes. Under homeostatic conditions, the hematopoietic system of the typical adult generates over 10¹¹ blood cells daily throughout life. In addition, hematopoiesis must be responsive to acute challenges due to blood loss or infection. MicroRNAs (miRs) cooperate with transcription factors to regulate all aspects of hematopoiesis, including stem cell maintenance, lineage selection, cell expansion, and terminal differentiation. Distinct miR expression patterns are associated with specific hematopoietic lineages and stages of differentiation and functional analyses have elucidated essential roles for miRs in regulating cell transitions, lineage selection, maturation, and function. MiRs function as downstream effectors of hematopoietic transcription factors and as upstream regulators to control transcription factor levels. Multiple miRs have been shown to play essential roles. Regulatory networks comprised of differentially expressed lineage-specific miRs and hematopoietic transcription factors are involved in controlling the quiescence and self-renewal of hematopoietic stem cells as well as proliferation and differentiation of lineage-specific progenitor cells during erythropoiesis, myelopoiesis, and lymphopoiesis. This review focuses on hematopoietic miRs that function as upstream regulators of central hematopoietic transcription factors required for normal hematopoiesis. This article is categorized under: RNA in Disease and Development > RNA in Development Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs.

Episomal minicircles persist in periods of transcriptional inactivity and can be transmitted through somatic cell nuclear transfer into bovine embryos

Episomal plasmids based on a scaffold/matrix attachment region (S/MAR) are extrachromosomal DNA entities that replicate once per cell cycle and are stably maintained in cells or tissue. We generated minicircles, episomal plasmids devoid of bacterial sequences, and show that they are stably transmitted in clonal primary bovine fibroblasts without selection pressure over more than two months. Total DNA, plasmid extraction and fluorescence in situ hybridization (FISH) analyses suggest that the minicircles remained episomal and were not integrated into the genome. Minicircles survived extended periods in serum-starved cells, which indicates that ongoing transcription in non-proliferating cells is not necessary for the maintenance of S/MAR-episomes. To test whether minicircles endure the process of somatic cell nuclear transfer (SCNT), we used cell-cycle synchronized, serum-starved, minicircle-containing cells. Analysis of cells outgrown from SCNT-derived blastocysts shows that the minicircles are maintained through SCNT and early embryonic development, which raises the prospect of using cell lines with episomal minicircles for the generation of transgenic animals.

Mapping Transgene Insertion Sites Reveals Complex Interactions Between Mouse Transgenes and Neighboring Endogenous Genes

Transgenic mouse lines are routinely employed to label and manipulate distinct cell types. The transgene generally comprises cell-type specific regulatory elements linked to a cDNA encoding a reporter or other protein. However, off-target expression seemingly unrelated to the regulatory elements in the transgene is often observed, it is sometimes suspected to reflect influences related to the site of transgene integration in the genome. To test this hypothesis, we used a proximity ligation-based method, Targeted Locus Amplification (TLA), to map the insertion sites of three well-characterized transgenes that appeared to exhibit insertion site-dependent expression in retina. The nearest endogenous genes to transgenes HB9-GFP, Mito-P, and TYW3 are Cdh6, Fat4 and Khdrbs2, respectively. For two lines, we demonstrate that expression reflects that of the closest endogenous gene (Fat4 and Cdh6), even though the distance between transgene and endogenous gene is 550 and 680 kb, respectively. In all three lines, the transgenes decrease expression of the neighboring endogenous genes. In each case, the affected endogenous gene was expressed in at least some of the cell types that the transgenic line has been used to mark and study. These results provide insights into the effects of transgenes and endogenous genes on each other's expression, demonstrate that mapping insertion site is valuable for interpreting results obtained with transgenic lines, and indicate that TLA is a reliable method for integration site discovery.

The ovarian cancer oncobiome

Humans and other mammals are colonized by microbial agents across the kingdom which can represent a unique microbiome pattern. Dysbiosis of the microbiome has been associated with pathology including cancer. We have identified a microbiome signature unique to ovarian cancers, one of the most lethal malignancies of the female reproductive system, primarily because of its asymptomatic nature during the early stages in development. We screened ovarian cancer samples along with matched, and non-matched control samples using our pan-pathogen array (PathoChip), combined with capture-next generation sequencing. The results show a distinct group of viral, bacterial, fungal and parasitic signatures of high significance in ovarian cases. Further analysis shows specific viral integration sites within the host genome of tumor samples, which may contribute to the carcinogenic process. The ovarian cancer microbiome signature provides insights for the development of targeted therapeutics against ovarian cancers.

Efficient mapping of transgene integration sites and local structural changes in Cre transgenic mice using targeted locus amplification

Cre/LoxP technology is widely used in the field of mouse genetics for spatial and/or temporal regulation of gene function. For Cre lines generated via pronuclear microinjection of a Cre transgene construct, the integration site is random and in most cases not known. Integration of a transgene can disrupt an endogenous gene, potentially interfering with interpretation of the phenotype. In addition, knowledge of where the transgene is integrated is important for planning of crosses between animals carrying a conditional allele and a given Cre allele in case the alleles are on the same chromosome. We have used targeted locus amplification (TLA) to efficiently map the transgene location in seven previously published Cre and CreERT2 transgenic lines. In all lines, transgene insertion was associated with structural changes of variable complexity, illustrating the importance of testing for rearrangements around the integration site. In all seven lines the exact integration site and breakpoint sequences were identified. Our methods, data and genotyping assays can be used as a resource for the mouse community and our results illustrate the power of the TLA method to not only efficiently map the integration site of any transgene, but also provide additional information regarding the transgene integration events.

Distal regulation of c-myb expression during IL-6-induced differentiation in murine myeloid progenitor M1 cells

The c-Myb transcription factor is a major regulator that controls differentiation and proliferation of hematopoietic progenitor cells, which is frequently deregulated in hematological diseases, such as lymphoma and leukemia. Understanding of the mechanisms regulating the transcription of c-myb gene is challenging as it lacks a typical promoter and multiple factors are involved. Our previous studies identified some distal regulatory elements in the upstream regions of c-myb gene in murine myeloid progenitor M1 cells, but the detailed mechanisms still remain unclear. In the present study, we found that a cell differentiation-related DNase1 hypersensitive site is located at a -28k region upstream of c-myb gene and that transcription factors Hoxa9, Meis1 and PU.1 bind to the -28k region. Circular chromosome conformation capture (4C) assay confirmed the interaction between the -28k region and the c-myb promoter, which is supported by the enrichment of CTCF and Cohesin. Our analysis also points to a critical role for Hoxa9 and PU.1 in distal regulation of c-myb expression in murine myeloid cells and cell differentiation. Overexpression of Hoxa9 disrupted the IL-6-induced differentiation of M1 cells and upregulated c-myb expression through binding of the -28k region. Taken together, our results provide an evidence for critical role of the -28k region in distal regulatory mechanism for c-myb gene expression during differentiation of myeloid progenitor M1 cells.

miR-638 regulates differentiation and proliferation in leukemic cells by targeting cyclin-dependent kinase 2

MicroRNAs have been extensively studied as regulators of hematopoiesis and leukemogenesis. We identified miR-638 as a novel regulator in myeloid differentiation and proliferation of leukemic cells. We found that miR-638 was developmentally up-regulated in cells of myeloid but not lymphoid lineage. Furthermore, significant miR-638 down-regulation was observed in primary acute myeloid leukemia (AML) blasts, whereas miR-638 expression was dramatically up-regulated in primary AML blasts and leukemic cell lines undergoing forced myeloid differentiation. These observations suggest that miR-638 might play a role in myeloid differentiation, and its dysregulation may contribute to leukemogenesis. Indeed, ectopic expression of miR-638 promoted phorbol 12-myristate 13-acetate- or all-trans-retinoic acid-induced differentiation of leukemic cell lines and primary AML blasts, whereas miR-638 inhibition caused an opposite phenotype. Consistently, miR-638 overexpression induced G1 cell cycle arrest and reduced colony formation in soft agar. Cyclin-dependent kinase 2 (CDK2) was found to be a target gene of miR-638. CDK2 inhibition phenotypically mimicked the overexpression of miR-638. Moreover, forced expression of CDK2 restored the proliferation and the colony-forming ability inhibited by miR-638. Our data suggest that miR-638 regulates proliferation and myeloid differentiation by targeting CDK2 and may serve as a novel target for leukemia therapy or marker for AML diagnosis and prognosis.

Thrombopoiesis

The production of platelets is a complex process that involves hematopoietic stem cells (HSCs), their differentiated progeny, the marrow microenvironment and hematopoietic cytokines. Much has been learned in the 110 years since James Homer Wright postulated that marrow megakaryocytes were responsible for blood platelet production, at a time when platelets were termed the "dust of the blood". In the 1980s a number of in vitro culture systems were developed that could produce megakaryocytes, followed by the identification of several cytokines that could stimulate the process in vitro. However, none of these cytokines produced a substantial thrombocytosis when injected into animals or people, nor were blood levels inversely related to platelet count, the sine qua non of a physiological regulator. A major milestone in our understanding of thrombopoiesis occurred in 1994 when thrombopoietin, the primary regulator of platelet production was cloned and initially characterized. Since that time many of the molecular mechanisms of thrombopoiesis have been identified, including the effects of thrombopoietin on the survival, proliferation, and differentiation of hematopoietic stem and progenitor cells, the development of polyploidy and proplatelet formation, the final fragmentation of megakaryocyte cytoplasm to yield blood platelets, and the regulation of this process. While much progress has been made, several outstanding questions remain, such as the nature of the signals for final platelet formation, the molecular nature of the regulation of marrow stromal thrombopoietin production, and the role of these physiological processes in malignant hematopoiesis.

Fetal globin gene repressors as drug targets for molecular therapies to treat the β-globinopathies

The human β-globin locus is comprised of embryonic, fetal, and adult globin genes that are expressed in a developmental stage-specific manner. Mutations in the globin locus give rise to the β-globinopathies, β-thalassemia and sickle cell disease, which begin to manifest symptoms around the time of birth. Although the fetal globin genes are autonomously silenced in adult-stage erythroid cells, mutations lying both within and outside the locus lead to natural variations in the level of fetal globin gene expression, and some of these significantly ameliorate the clinical symptoms of the β-globinopathies. Multiple reports have now identified several transcription factors that are involved in fetal globin gene repression in definitive (adult)-stage erythroid cells (the TR2/TR4 heterodimer, MYB, KLFs, BCL11A, and SOX6). To carry out their repression functions, chromatin-modifying enzymes (such as DNA methyltransferase, histone deacetylases, and lysine-specific histone demethylase 1) are additionally involved as a consequence of forming large macromolecular complexes with the DNA-binding subunits of these cellular machines. This review focuses on the molecular mechanisms underlying fetal globin gene silencing and possible near-future molecularly targeted therapies for treating the β-globinopathies.

Analysis of MYB oncogene in transformed adenoid cystic carcinomas reveals distinct pathways of tumor progression

Adenoid cystic carcinomas can occasionally undergo dedifferentiation, a phenomenon also referred to as high-grade transformation. However, cases of adenoid cystic carcinomas have been described showing transformation to adenocarcinomas that are not poorly differentiated, indicating that high-grade transformation may not necessarily reflect a more advanced stage of tumor progression, but rather a transformation to another histological form, which may encompass a wide spectrum of carcinomas in terms of aggressiveness. The aim of this study was to gain more insight in the biology of this pathological phenomenon by means of genetic profiling of both histological components. Using microarray comparative genomic hybridization, we compared the genome-wide DNA copy-number changes of the conventional and transformed area of eight adenoid cystic carcinomas with high-grade transformation, comprising four with transformation into moderately differentiated adenocarcinomas and four into poorly differentiated carcinomas. In general, the poorly differentiated carcinoma cases showed a higher total number of copy-number changes than the moderately differentiated adenocarcinoma cases, and this correlated with a worse clinical course. Special attention was given to chromosomal translocation and protein expression of MYB, recently being considered to be an early and major oncogenic event in adenoid cystic carcinomas. Our data showed that the process of high-grade transformation is not always accompanied by an accumulation of genetic alterations; both conventional and transformed components harbored unique genetic alterations, which indicate a parallel progression. Our data further demonstrated that the MYB/NFIB translocation is not necessarily an early event or fundamental for the progression to adenoid cystic carcinoma with high-grade transformation.

HBS1L-MYB intergenic variants modulate fetal hemoglobin via long-range MYB enhancers

Genetic studies have identified common variants within the intergenic region (HBS1L-MYB) between GTP-binding elongation factor HBS1L and myeloblastosis oncogene MYB on chromosome 6q that are associated with elevated fetal hemoglobin (HbF) levels and alterations of other clinically important human erythroid traits. It is unclear how these noncoding sequence variants affect multiple erythrocyte characteristics. Here, we determined that several HBS1L-MYB intergenic variants affect regulatory elements that are occupied by key erythroid transcription factors within this region. These elements interact with MYB, a critical regulator of erythroid development and HbF levels. We found that several HBS1L-MYB intergenic variants reduce transcription factor binding, affecting long-range interactions with MYB and MYB expression levels. These data provide a functional explanation for the genetic association of HBS1L-MYB intergenic polymorphisms with human erythroid traits and HbF levels. Our results further designate MYB as a target for therapeutic induction of HbF to ameliorate sickle cell and β-thalassemia disease severity.

Disruption of the Hbs1l-Myb locus causes hereditary persistence of fetal hemoglobin in a mouse model

The human β-globin locus is comprised of embryonic, fetal, and adult globin genes, each of which is expressed at distinct stages of pre- and postnatal development. Functional defects in globin proteins or expression results in mild to severe anemia, such as in sickle-cell disease or β-thalassemia, but the clinical symptoms of both disorders are ameliorated by persistent expression of the fetal globin genes. Recent genome-wide association studies (GWAS) identified the intergenic region between the HBS1L and MYB loci as a candidate modifier of fetal hemoglobin expression in adults. However, it remains to be clarified whether the enhancer activity within the HBS1L-MYB regulatory domain contributes to the production of fetal hemoglobin in adults. Here we report a new mouse model of hereditary persistence of fetal hemoglobin (HPFH) in which a transgene was randomly inserted into the orthologous murine Hbs1l-Myb locus. This mutant mouse exhibited typically elevated expression of embryonic globins and hematopoietic parameters similar to those observed in human HPFH. These results support the contention that mutation of the HBS1L-MYB genomic domain is responsible for elevated expression of the fetal globin genes, and this model serves as an important means for the analysis of networks that regulate fetal globin gene expression.

Quantitative analysis of murine terminal erythroid differentiation in vivo: novel method to study normal and disordered erythropoiesis

Terminal erythroid differentiation is the process during which proerythroblasts differentiate to produce enucleated reticulocytes. Although it is well established that during murine erythropoiesis in vivo, 1 proerythroblast undergoes 3 mitosis to generate sequentially 2 basophilic, 4 polychromatic, and 8 orthochromatic erythroblasts, currently there is no method to quantitatively monitor this highly regulated process. Here we outline a method that distinguishes each distinct stage of erythroid differentiation in cells from mouse bone marrow and spleen based on expression levels of TER119, CD44, and cell size. Quantitative analysis revealed that the ratio of proerythroblasts:basophilic:polychromatic:orthromatic erythroblasts follows the expected 1:2:4:8 ratio, reflecting the physiologic progression of terminal erythroid differentiation in normal mice. Moreover, in 2 stress erythropoiesis mouse models, phlebotomy-induced acute anemia and chronic hemolytic anemia because of 4.1R deficiency, the ratio of these erythroblast populations remains the same as that of wild-type bone marrow. In contrast, in anemic β-thalassemia intermedia mice, there is altered progression which is restored to normal by transferrin treatment which was previously shown to ameliorate the anemic phenotype. The means to quantitate in vivo murine erythropoiesis using our approach will probably have broad application in the study of altered erythropoiesis in various red cell disorders.

The switch from fetal to adult hemoglobin

The fetal-to-adult hemoglobin switch and silencing of fetal hemoglobin (HbF) have been areas of long-standing interest among hematologists, given the fact that clinical induction of HbF production holds tremendous promise to ameliorate the clinical symptoms of sickle cell disease (SCD) and β-thalassemia. In this article, we discuss historic attempts to induce HbF that have resulted in some therapeutic approaches to manage SCD and β-thalassemia. We then go on to discuss how more recent molecular studies that have identified regulators, including BCL11A, MYB, and KLF1, hold great promise to develop targeted and more effective approaches for HbF induction. We go on to discuss strategies by which such approaches may be developed. Older studies in this field can provide important lessons for future studies aimed at developing more effective strategies for HbF induction, and we therefore chronologically cover the work accomplished as this field has evolved over the course of the past four decades.

PDCD2 knockdown inhibits erythroid but not megakaryocytic lineage differentiation of human hematopoietic stem/progenitor cells

Programmed cell death-2 (PDCD2) protein is enriched in embryonic, hematopoietic, and neural stem cells, however, its function in stem/progenitor cell differentiation is unclear. We investigated the effects of PDCD2 knockdown on the development and differentiation of hematopoietic progenitor cells (HPC). CD34(+) cells derived from normal human bone marrow and K562 leukemic cells were effectively transduced with short-hairpin RNA to knockdown PDCD2. Colony-forming assays were used to investigate the effects of PDCD2 loss on HPC clonogenic potential and on 12-O-tetradecanoyl-phorbol-13-acetate-and arabinofuranosylcytosine-induced terminal differentiation. In CD34(+) clonogenic progenitors, PDCD2 knockdown decreased the total number of colony-forming units, increased the number of colony-forming units-granulocyte-erythroid-macrophage-megakaryocyte and burst-forming unit-erythroid primitive colonies, and decreased the number of burst-forming unit-erythroid mature colonies. Similar results were observed in K562 cells, suggesting that PDCD2 is important for HPC differentiation and/or survival, and for erythroid lineage commitment. Furthermore, 12-O-tetradecanoyl-phorbol-13-acetate-induced megakaryocytic differentiation and proliferation of K562 cells was not affected by PDCD2 knockdown. In contrast, arabinofuranosylcytosine-induced erythroid differentiation of K562 cells was significantly reduced with PDCD2 knockdown, with no effect on cell proliferation. The effects of PDCD2 knockdown were attributed to a cell cycle arrest at G(0)/G(1), along with increased messenger RNA expression of early progenitor factors c-MYB and GATA-2, and decreased expression of erythroid factors GATA-1, EpoR, and γ-globin. We conclude that PDCD2 loss of function(s) impedes erythroid differentiation by inducing cell cycle arrest and increasing expression of early hematopoietic progenitor factors. These findings suggest that PDCD2 has a novel regulatory role in human hematopoiesis and is essential for erythroid development.

Three murine leukemia virus integration regions within 100 kilobases upstream of c-myb are proximal to the 5' regulatory region of the gene through DNA looping

Retroviruses integrated into genomic DNA participate in long-range gene activation from as far away as several hundred kilobases. Hypotheses have been put forth to account for these phenomena, but data have not been provided to support a physical mechanism that explains long-range activation. In murine leukemia virus-induced myeloid leukemia in mice, integrated proviruses have been found upstream of c-myb in three regions, named Mml1, Mml2, and Mml3 (25, 50, and 70 kb upstream, respectively). The transcription factor c-Myb is an oncogene whose dysregulation and/or mutation can lead to human leukemia. We hypothesized that the murine c-myb upstream region contains regulatory elements accessed by the retrovirus. To identify regulatory sites in the murine c-myb upstream region, we looked by chromatin immunoprecipitation with microarray technology (ChIP-on-chip) for histone modifications implicating gene activation in normal cells. H3K4me3, H3K4me1, and H3K9/14ac were enriched at Mml1 and/or Mml2 in the myeloblastic cell line M1, which expresses c-myb. The enrichment of all of these histone marks decreased with differentiation-induced downregulation of the gene in M1 cells but increased and spread in tumor cells containing integrated provirus. Importantly, using chromosome conformation capture (3C)-quantitative PCR assays, interactions between the 5' region, including the promoter and all Mml sites (Mml1, Mml2, and Mml3), were detected due to DNA looping in M1 cells and tumor cells with provirus in Mml1, Mml2, or Mml3. Therefore, our study provides a new mechanism of retrovirus insertional mutagenesis whereby spatial chromatin organization allows distally located provirus, with its own enhancer elements, to access the 5' regulatory region of the gene.

Dynamic long-range chromatin interactions control Myb proto-oncogene transcription during erythroid development

The key haematopoietic regulator Myb is essential for coordinating proliferation and differentiation. ChIP-Sequencing and Chromosome Conformation Capture (3C)-Sequencing were used to characterize the structural and protein-binding dynamics of the Myb locus during erythroid differentiation. In proliferating cells expressing Myb, enhancers within the Myb-Hbs1l intergenic region were shown to form an active chromatin hub (ACH) containing the Myb promoter and first intron. This first intron was found to harbour the transition site from transcription initiation to elongation, which takes place around a conserved CTCF site. Upon erythroid differentiation, Myb expression is downregulated and the ACH destabilized. We propose a model for Myb activation by distal enhancers dynamically bound by KLF1 and the GATA1/TAL1/LDB1 complex, which primarily function as a transcription elongation element through chromatin looping.

Novel chromosomal rearrangements and break points at the t(6;9) in salivary adenoid cystic carcinoma: association with MYB-NFIB chimeric fusion, MYB expression, and clinical outcome

OBJECTIVE

To investigate the molecular genetic heterogeneity associated with the t(6:9) in adenoid cystic carcinoma (ACC) and correlate the findings with patient clinical outcome.

EXPERIMENTAL DESIGN

Multimolecular and genetic techniques complemented with massive pair-ended sequencing and single-nucleotide polymorphism array analyses were used on tumor specimens from 30 new and 52 previously analyzed fusion transcript-negative ACCs by reverse transcriptase PCR (RT-PCR). MYB mRNA expression level was determined by quantitative RT-PCR. The results of 102 tumors (30 new and 72 previously reported cases) were correlated with the clinicopathologic factors and patients' survival.

RESULTS

The FISH analysis showed 34 of 82 (41.5%) fusion-positive tumors and molecular techniques identified fusion transcripts in 21 of the 82 (25.6%) tumors. Detailed FISH analysis of 11 out the 15 tumors with gene fusion without transcript formation showed translocation of NFIB sequences to proximal or distal sites of the MYB gene. Massive pair-end sequencing of a subset of tumors confirmed the proximal translocation to an NFIB sequence and led to the identification of a new fusion gene (NFIB-AIG1) in one of the tumors. Overall, MYB-NFIB gene fusion rate by FISH was in 52.9% whereas fusion transcript forming incidence was 38.2%. Significant statistical association between the 5' MYB transcript expression and patient survival was found.

CONCLUSIONS

We conclude that: (i) t(6;9) results in complex genetic and molecular alterations in ACC, (ii) MYB-NFIB gene fusion may not always be associated with chimeric transcript formation, (iii) noncanonical MYB-NFIB gene fusions occur in a subset of tumors, (iv) high MYB expression correlates with worse patient survival.

FOG-1 and GATA-1 act sequentially to specify definitive megakaryocytic and erythroid progenitors

The transcription factors that control lineage specification of haematopoietic stem cells (HSCs) have been well described for the myeloid and lymphoid lineages, whereas transcriptional control of erythroid (E) and megakaryocytic (Mk) fate is less understood. We here use conditional removal of the GATA-1 and FOG-1 transcription factors to identify FOG-1 as required for the formation of all committed Mk- and E-lineage progenitors, whereas GATA-1 was observed to be specifically required for E-lineage commitment. FOG-1-deficient HSCs and preMegEs, the latter normally bipotent for the Mk and E lineages, underwent myeloid transcriptional reprogramming, and formed myeloid, but not erythroid and megakaryocytic cells in vitro. These results identify FOG-1 and GATA-1 as required for formation of bipotent Mk/E progenitors and their E-lineage commitment, respectively, and show that FOG-1 mediates transcriptional Mk/E programming of HSCs as well as their subsequent Mk/E-lineage commitment. Finally, C/EBPs and FOG-1 exhibited transcriptional cross-regulation in early myelo-erythroid progenitors making their functional antagonism a potential mechanism for separation of the myeloid and Mk/E lineages.

cMyb regulates hematopoietic stem/progenitor cell mobilization during zebrafish hematopoiesis

The establishment of the HSC pool in vertebrates depends not only on the formation and the propagation of these stem cells but also on their proper trafficking among the defined hematopoietic organs. However, the physiologic mechanisms that regulate HSC mobilization remain elusive. Through analysis of the zebrafish cmyb mutant cmyb(hkz3), we show that the suppression of cMyb function abrogates larval and adult hematopoiesis, with concomitant accumulation of hematopoietic stem/progenitor cells (HSPCs) in their birthplace, the ventral wall of the dorsal aorta (VDA). Cell tracking and time-lapse recording reveal that the accumulation of HSPCs in cmyb(hkz3) mutants is caused by the impairment of HSPC egression from the VDA. Further analysis demonstrates that the HSPC migratory defects in cmyb(hkz3) mutants are at least partly because of adversely elevated levels of chemokine stromal cell-derived factor 1a (Sdf1a). Our study reveals that cMyb plays a hitherto unidentified role in dictating physiologic HSPC migration by modulating Sdf1a signaling.

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.

The hemangioblast: from concept to authentication

The hemangioblast hypothesis has been hotly debated for over a century. Hemangioblasts are defined as multipotent cells that can give rise to both hematopoietic cells and endothelial cells. The existence of hemangioblasts has now been confirmed and many important molecules and several signaling pathways are involved in their generation and differentiation. Fibroblast growth factor, renin-angiotensin system and runt-related transcription factor 1 (Runx1) direct the formation of hemangioblasts through highly selective gene expression patterns. On the other hand, the hemogenic endothelium theory and a newly discovered pattern of hematopoietic/endothelial differentiation make the genesis of hemangioblasts more complicated. But how hemangioblasts are formed and how hematopoietic cells or endothelial cells are derived from remains largely unknown. Here we summarize the current knowledge of the signaling pathways and molecules involved in hemangioblast development and suggest some future clinical applications.

Geminin deletion from hematopoietic cells causes anemia and thrombocytosis in mice

HSCs maintain the circulating blood cell population. Defects in the orderly pattern of hematopoietic cell division and differentiation can lead to leukemia, myeloproliferative disorders, or marrow failure; however, the factors that control this pattern are incompletely understood. Geminin is an unstable regulatory protein that regulates the extent of DNA replication and is thought to coordinate cell division with cell differentiation. Here, we set out to determine the function of Geminin in hematopoiesis by deleting the Geminin gene (Gmnn) from mouse bone marrow cells. This severely perturbed the pattern of blood cell production in all 3 hematopoietic lineages (erythrocyte, megakaryocyte, and leukocyte). Red cell production was virtually abolished, while megakaryocyte production was greatly enhanced. Leukocyte production transiently decreased and then recovered. Stem and progenitor cell numbers were preserved, and Gmnn(–/–) HSCs successfully reconstituted hematopoiesis in irradiated mice. CD34(+) Gmnn(–/–) leukocyte precursors displayed DNA overreplication and formed extremely small granulocyte and monocyte colonies in methylcellulose. While cultured Gmnn(–/–) mega-karyocyte-erythrocyte precursors did not form erythroid colonies, they did form greater than normal numbers of megakaryocyte colonies. Gmnn(–/–) megakaryocytes and erythroblasts had normal DNA content. These data led us to postulate that Geminin regulates the relative production of erythrocytes and megakaryocytes from megakaryocyte-erythrocyte precursors by a replication-independent mechanism.

Regulation of hematopoietic stem cells by their mature progeny

Thrombopoietin (TPO), acting through its receptor Mpl, has two major physiological roles: ensuring production of sufficient platelets via stimulation of megakaryocyte production and maintaining hematopoietic stem cell (HSC) quiescence. Mpl also controls circulating TPO concentration via receptor-mediated internalization and degradation. Here, we demonstrate that the megakaryocytosis and increased platelet mass in mice with mutations in the Myb or p300 genes causes reduced circulating TPO concentration and TPO starvation of the stem-cell compartment, which is exacerbated because these cells additionally exhibit impaired responsiveness to TPO. HSCs from Myb(Plt4/Plt4) mice show altered expression of TPO-responsive genes and, like HSCs from Tpo and Mpl mutant mice, exhibit increased cycling and a decline in the number of HSCs with age. These studies suggest that disorders of platelet number can have profound effects on the HSC compartment via effects on the feedback regulation of circulating TPO concentration.

Notch/Delta4 signaling inhibits human megakaryocytic terminal differentiation

The effects of Notch signaling on human megakaryocytic and erythroid differentiation were investigated by exposing human CD34(+) progenitor cells to an immobilized chimeric form of the Notch ligand, Delta-like4 (Dll4Fc). Exposure of human cord blood CD34(+) cells to Dll4Fc induced a modest enhancement of erythroid cell production. Conversely, under megakaryocytic culture conditions, Dll4Fc strongly impaired platelet production by reducing the generation of mature CD41a(+)CD42b(+) megakaryocytes (MKs) and platelet-forming cells. The inhibitory activity of Dll4 on terminal MK differentiation was confirmed by culturing CD34(+) cells onto Dll-4-expressing stroma cells (engineered to express the membrane-anchored form of Dll4). The reduced production of mature CD41a(+)CD42(+) cells was rescued by inhibiting Notch signaling either with the N-N-(3,5-difluorophenacetyl-L-alanyl)-S-phenylglycine t-butyl ester γ-secretase inhibitor or the dominant-negative version of Mastermind. Dll4 impaired the generation of mature CD41a(+)CD42b(+) cells and proplatelet formation without affecting earlier steps of MK differentiation, such as production of megakaryocytic/erythroid progenitors and colony-forming units-MKs. This blockade was accompanied by a modulation of the transcriptional program of megakaryocytic differentiation. All these results indicate that Dll4/Notch signaling inhibits human terminal MK differentiation.

Inducible Fli-1 gene deletion in adult mice modifies several myeloid lineage commitment decisions and accelerates proliferation arrest and terminal erythrocytic differentiation

This study investigated the role of the ETS transcription factor Fli-1 in adult myelopoiesis using new transgenic mice allowing inducible Fli-1 gene deletion. Fli-1 deletion in adult induced mild thrombocytopenia associated with a drastic decrease in large mature megakaryocytes number. Bone marrow bipotent megakaryocytic-erythrocytic progenitors (MEPs) increased by 50% without increase in erythrocytic and megakaryocytic common myeloid progenitor progeny, suggesting increased production from upstream stem cells. These MEPs were almost unable to generate pure colonies containing large mature megakaryocytes, but generated the same total number of colonies mainly identifiable as erythroid colonies containing a reduced number of more differentiated cells. Cytological and fluorescence-activated cell sorting analyses of MEP progeny in semisolid and liquid cultures confirmed the drastic decrease in large mature megakaryocytes but revealed a surprisingly modest (50%) reduction of CD41-positive cells indicating the persistence of a megakaryocytic commitment potential. Symmetrical increase and decrease of monocytic and granulocytic progenitors were also observed in the progeny of purified granulocytic-monocytic progenitors and common myeloid progenitors. In summary, this study indicates that Fli-1 controls several lineages commitment decisions at the stem cell, MEP, and granulocytic-monocytic progenitor levels, stimulates the proliferation of committed erythrocytic progenitors at the expense of their differentiation, and is a major regulator of late stages of megakaryocytic differentiation.

Genetic analysis of hierarchical regulation for Gata1 and NF-E2 p45 gene expression in megakaryopoiesis

GATA1 and NF-E2 p45 are two important regulators of megakaryopoiesis. Whereas GATA1 is known to regulate the p45 gene, details of the GATA1 contribution to the spatiotemporal expression of the p45 gene remain to be elucidated. To clarify the relationship between GATA1 and p45, we performed genetic complementation rescue analysis of p45 function in megakaryocytes utilizing the hematopoietic regulatory domain of the Gata1 gene (G1HRD). We established transgenic mouse lines expressing p45 under G1HRD regulation and crossed the mice with p45-null mice. Compound mutant mice displayed normal platelet counts and no sign of hemorrhage, indicating that G1HRD has the ability to express p45 in a spatiotemporally correct manner. However, deletion of 38 amino acids from the N-terminal region of p45 abrogated the p45 rescue function, suggesting the presence of an essential transactivation activity in the region. We then crossed the G1HRD-p45 transgenic mice with megakaryocyte-specific Gata1 gene knockdown (Gata1(Delta)(neo)(Delta)(HS)) mice. The G1HRD-p45 transgene was insufficient for complete rescue of the Gata1(Delta)(neo)(Delta)(HS) megakaryocytes, suggesting that GATA1 or other factors regulated by GATA1 are required to cooperate with p45 for normal megakaryopoiesis. This study thus provides a unique in vivo validation of the hierarchical relationship between GATA1 and p45 in megakaryocytes.

NF-E2 domination over Nrf2 promotes ROS accumulation and megakaryocytic maturation

In megakaryocytes, the maturation process and oxidative stress response appear to be closely related. It has been suggested that increased oxygen tension and reactive oxygen species (ROS) promote megakaryopoiesis and that the expression of stress-responsive genes responsible for ROS elimination declines during megakaryocytic maturation. NF-E2 p45 is an essential regulator of megakaryopoiesis, whereas Nrf2 is a key activator of stress-responsive genes. Because p45 and Nrf2 have similar DNA-binding specificities, we hypothesized that p45 competes with Nrf2 to repress stress-responsive genes and achieves favorable intracellular conditions to allow ROS to be efficiently used as signaling molecules. We conducted comprehensive gene expression profiling with wild-type and p45-null megakaryocytes and examined the functional relationship between p45 and Nrf2. We found that 2 characteristic gene clusters are defined within p45 target genes: platelet genes and cytoprotective genes. The former are unique targets activated by p45, whereas the latter are common targets of p45 and Nrf2. Further analysis suggested that, as a less efficacious activator, p45 maintains moderate expression of cytoprotective genes through competing with Nrf2 and promotes ROS accumulation. Increased ROS enhanced platelet gene expression. These results suggest that p45 dominates over Nrf2 to enhance megakaryocytic maturation by promoting ROS accumulation.

miR-34a contributes to megakaryocytic differentiation of K562 cells independently of p53

The role of miRNAs in regulating megakaryocyte differentiation was examined using bipotent K562 human leukemia cells. miR-34a is strongly up-regulated during phorbol ester–induced megakaryocyte differentiation, but not during hemin-induced erythrocyte differentiation. Enforced expression of miR-34a in K562 cells inhibits cell proliferation, induces cell-cycle arrest in G1 phase, and promotes megakaryocyte differentiation as measured by CD41 induction. miR-34a expression is also up-regulated during thrombopoietin-induced differentiation of CD34+ hematopoietic precursors, and its enforced expression in these cells significantly increases the number of megakaryocyte colonies. miR-34a directly regulates expression of MYB, facilitating megakaryocyte differentiation, and of CDK4 and CDK6, to inhibit the G1/S transition. However, these miR-34a target genes are down-regulated rapidly after inducing megakaryocyte differentiation before miR-34a is induced. This suggests that miR-34a is not responsible for the initial down-regulation but may contribute to maintaining their suppression later on. Previous studies have implicated miR-34a as a tumor suppressor gene whose transcription is activated by p53. However, in p53-null K562 cells, phorbol esters induce miR-34a expression independently of p53 by activating an alternative phorbol ester-responsive promoter to produce a longer pri-miR-34a transcript.

The HBS1L-MYB intergenic interval associated with elevated HbF levels shows characteristics of a distal regulatory region in erythroid cells

HBS1L-MYB intergenic polymorphism (HMIP) on chromosome 6q23 is associated with elevated fetal hemoglobin levels and has pleiotropic effects on several hematologic parameters. To investigate potential regulatory activity in the region, we have measured sensitivity of the sequences to DNase I cleavage that identified 3 tissue-specific DNase I hypersensitive sites in the core intergenic interval. Chromatin immunoprecipitation with microarray (ChIP-chip) analysis showed strong histone acetylation in a defined interval of 65 kb corresponding to the core HBS1L-MYB intergenic region in primary human erythroid cells but not in non–MYB-expressing HeLa cells. ChIP-chip analysis also identified several potential cis-regulatory elements as strong GATA-1 signals that coincided with the DNase I hypersensitive sites present in MYB-expressing erythroid cells. We suggest that HMIP contains regulatory sequences that could be important in hematopoiesis by controlling MYB expression. This study provides the functional link between genetic association of HMIP with control of fetal hemoglobin and other hematologic parameters. We also present a large-scale analysis of histone acetylation as well as RNA polymerase II and GATA-1 interactions on chromosome 6q, and α and β globin gene loci. The data suggest that GATA-1 regulates numerous genes of various functions on chromosome 6q.

MicroRNAs in normal and malignant myelopoiesis

MicroRNAs (miRNAs) are a class of non-coding protein, single-stranded RNA of 18-22 nucleotides, that exert their actions at post-transcriptional level, mostly through base pairing with the 3'-untranslated region of the target mRNA, thus leading to its translational repression and/or degradation. Recent studies have shown that miRNAs play a crucial role in normal hematopoiesis through the control of the expression of key regulators of hematopoiesis (i.e., transcription factors, growth factor receptors, chemokine receptors), involving regulatory loops that selectively operate in the various hematopoietic lineages. Extensive miRNA deregulation has been observed in leukemia and functional studies support a role for miRNAs in the pathogenesis of these disorders.

Reduced c-Myb activity compromises HSCs and leads to a myeloproliferation with a novel stem cell basis

Murine haematopoietic stem cells (HSCs) are contained in the Kit+Sca1+Lin(-) (KSL) population of bone marrow and are able to repopulate lethally irradiated mice. Myeloproliferative disorders (MPDs) are thought to be clonogenic diseases arising at the level of the HSC. Here, we show that mice expressing low levels of the transcription factor c-Myb, as the result of genetic knockdown, develop a transplantable myeloproliferative phenotype that closely resembles the human disease essential thrombocythaemia (ET). Unlike wild-type cells, the KSL population in c-myb knockdown bone marrow cannot repopulate irradiated mice and does not transfer the disease. Instead, cells positive for Kit and expressing low to medium levels of CD11b acquire self-renewing stem cell properties and are responsible for the perpetuation of the myeloproliferative phenotype.

Mutational inhibition of c-Myb or p300 ameliorates treatment-induced thrombocytopenia

The transcription factor c-Myb and coregulator p300 have a key role in maintaining production of controlled numbers of megakaryocytes and platelets. In mice, mutations in c-Myb or p300 cause thrombocytosis in otherwise wild-type animals and can ameliorate the thrombocytopenia in mice lacking the thrombopoietin receptor, c-Mpl, a model for human congenital amegakaryocytic thrombocytopenia. To examine whether inhibition of c-Myb/p300 is effective in other models of thrombocytopenia, the effect of the c-Myb(Plt4) mutation on thrombocytopenia associated with reduced platelet life span in Bcl-X(Plt20/Plt20) mice was assessed, as were responses in c-Myb(Plt4) and/or p300(Plt6) mutant mice to thrombocytopenia associated with antiplatelet antibodies, chemotherapy, or bone marrow transplantation. Homozygosity of the c-Myb(Plt4) allele ameliorated thrombocytopenia associated with reduced platelet life span, and c-Myb(Plt4/+) mice exhibited more rapid than normal recovery from thrombocytopenia caused by antiplatelet serum or bone marrow transplantation. Recovery to pretreatment platelet levels was unaltered in 5-fluorouracil-treated c-Myb(Plt4/+) mice relative to wild-type controls, but enhanced platelet production during subsequent thrombocytosis was evident. More modest enhancement of platelet recovery after 5-fluorouracil or bone marrow transplantation was also evident in p300(Plt6/+) animals. The data suggest potential utility of c-Myb/p300 as a target for therapeutic intervention in thrombocytopenia of diverse origins.

The c-myb proto-oncogene and microRNA-15a comprise an active autoregulatory feedback loop in human hematopoietic cells

The c-myb proto-oncogene encodes an obligate hematopoietic cell transcription factor important for lineage commitment, proliferation, and differentiation. Given its critical functions, c-Myb regulatory factors are of great interest but remain incompletely defined. Herein we show that c-Myb expression is subject to posttranscriptional regulation by microRNA (miRNA)-15a. Using a luciferase reporter assay, we found that miR-15a directly binds the 3'-UTR of c-myb mRNA. By transfecting K562 myeloid leukemia cells with a miR-15a mimic, functionality of binding was shown. The mimic decreased c-Myb expression, and blocked the cells in the G(1) phase of cell cycle. Exogenous expression of c-myb mRNA lacking the 3'-UTR partially rescued the miR-15a induced cell-cycle block. Of interest, the miR-15a promoter contained several potential c-Myb protein binding sites. Occupancy of one canonical c-Myb binding site was demonstrated by chromatin immunoprecipitation analysis and shown to be required for miR-15a expression in K562 cells. Finally, in studies using normal human CD34(+) cells, we showed that c-Myb and miR-15a expression were inversely correlated in cells undergoing erythroid differentiation, and that overexpression of miR-15a blocked both erythroid and myeloid colony formation in vitro. In aggregate, these findings suggest the presence of a c-Myb-miR-15a autoregulatory feedback loop of potential importance in human hematopoiesis.

Thrombopoietin regulates c-Myb expression by modulating micro RNA 150 expression

OBJECTIVE

Mice harboring c-Myb hypomorphic mutations display enhanced thrombopoiesis because of increased numbers of megakaryocytes and their progenitors. Thrombopoietin induces these same effects, which lead us to hypothesize that the hormone acts through modulation of c-Myb expression, as c-Myb levels falls during thrombopoietin-induced megakaryocyte (MK) maturation. Micro RNAs (miRs) downregulate gene expression by binding to the 3' untranslated region (UTR) of specific messenger RNAs (mRNAs); we noted that the 3'UTR of c-Myb contains four miR-150 binding sites.

MATERIALS AND METHODS

We used quantitative reverse transcriptase polymerase chain reaction, Western blotting, and reporter gene analyses to assess the response of c-Myb to thrombopoietin stimulation and to gain of and loss of miR-150 expression.

RESULTS

We found that thrombopoietin reduced c-Myb mRNA and protein levels within 7 hours in megakaryocytes and UT7/thrombopoietin (TPO) cells. Using a reporter gene containing the c-Myb 3'UTR region, including its four miR150 binding sites, we found that expression of miR150 reduced luciferase expression to 50% of baseline at 24 hours and to 25% at 48 hours in UT7/TPO cells. Quantitative polymerase chain reaction and Western blotting also revealed that miR-150 reduced endogenous c-Myb mRNA and protein to 50% in UT7/TPO cells, and to 65% in mature megakaryocytes. Converse experiments utilizing anti-miR150 increased luciferase activity twofold over control anti-miR. Finally, TPO increased miR150 expression 1.8-fold within 24 hours and 3.4-fold within 48 hours.

CONCLUSIONS

These findings establish that miR150 downmodulates c-Myb levels, and because TPO affects miR150 expression, our results indicate that, in addition to affecting MK progenitor cell growth, TPO downmodulates c-Myb expression through induction of miR-150.