Identification of a novel vertebrate homeobox gene expressed in haematopoietic cells

The transcription factor HHEX maintains glucocorticoid levels and protects adrenals from androgen-induced lipid depletion

Glucocorticoid-producing cells of the adrenal cortex (i.e. zona fasciculata, zF) constitute the critical effectors of the hypothalamic-pituitary-adrenal axis, mediating the mammalian stress response. With glucocorticoids being essential for life, it is not surprising that zF dysfunction perturbs multiple organs that participate in optimizing cardiometabolic fitness. The zF forms a dynamic and heterogenous cell population endowed with the capacity to remodel through the engagement of both proliferative and differentiation programs that enable the adrenal to adapt and respond to diverse stressors. However, the mechanisms that sustain such differential responsiveness remain poorly understood. In this study, we resolved the transcriptome of the steroidogenic lineage by scRNA-seq using Sf1-Cre; Rosa mT/mG reporter mice. We identified HHEX, a homeodomain protein, as the most enriched transcription factor in glucocorticoid-producing cells. We developed new genetic mouse models to demonstrate that HHEX deletion causes glucocorticoid insufficiency in male animals. Molecularly, we demonstrated that HHEX is an androgen receptor (AR) target gene, shaping the sexual dimorphism of the adrenal gland by repressing the female transcriptional program at puberty, while also maintaining zF cholesterol ester content by protecting lipid droplets from androgen-induced-lipophagy. Moreover, our study revealed that, in both sexes, HHEX is crucial for maintaining the identity of the innermost adrenocortical cell subpopulation. Specifically, loss of HHEX impairs the expression of Abcb1b (P-glycoprotein/MDR1), an efflux pump regulating steroid export and cellular levels of xenobiotics. Together, these data demonstrate that HHEX serves as a multi-functional regulator of post-natal adrenal maturation that is potentiated by androgens.

Chimeric Livers: Interspecies Blastocyst Complementation and Xenotransplantation for End-Stage Liver Disease

Orthotopic liver transplantation (OLT) currently serves as the sole definitive treatment for thousands of patients suffering from end-stage liver disease; and the existing supply of donor livers for OLT is drastically outpaced by the increasing demand. To alleviate this significant gap in treatment, several experimental approaches have been devised with the aim of either offering interim support to patients waiting on the transplant list or bioengineering complete livers for OLT by infusing them with fresh hepatic cells. Recently, interspecies blastocyst complementation has emerged as a promising method for generating complete organs in utero over a short timeframe. When coupled with gene editing technology, it has brought about a potentially revolutionary transformation in regenerative medicine. Blastocyst complementation harbors notable potential for generating complete human livers in large animals, which could be used for xenotransplantation in humans, addressing the scarcity of livers for OLT. Nevertheless, substantial experimental and ethical challenges still need to be overcome to produce human livers in larger domestic animals like pigs. This review compiles the current understanding of interspecies blastocyst complementation and outlines future possibilities for liver xenotransplantation in humans.

The Haematopoietically-expressed homeobox transcription factor: roles in development, physiology and disease

The Haematopoietically expressed homeobox transcription factor (Hhex) is a transcriptional repressor that is of fundamental importance across species, as evident by its evolutionary conservation spanning fish, amphibians, birds, mice and humans. Indeed, Hhex maintains its vital functions throughout the lifespan of the organism, beginning in the oocyte, through fundamental stages of embryogenesis in the foregut endoderm. The endodermal development driven by Hhex gives rise to endocrine organs such as the pancreas in a process which is likely linked to its role as a risk factor in diabetes and pancreatic disorders. Hhex is also required for the normal development of the bile duct and liver, the latter also importantly being the initial site of haematopoiesis. These haematopoietic origins are governed by Hhex, leading to its crucial later roles in definitive haematopoietic stem cell (HSC) self-renewal, lymphopoiesis and haematological malignancy. Hhex is also necessary for the developing forebrain and thyroid gland, with this reliance on Hhex evident in its role in endocrine disorders later in life including a potential role in Alzheimer's disease. Thus, the roles of Hhex in embryological development throughout evolution appear to be linked to its later roles in a variety of disease processes.

HOXA Amplification Defines a Genetically Distinct Subset of Angiosarcomas

Angiosarcoma is a rare, devastating malignancy with few curative options for disseminated disease. We analyzed a recently published genomic data set of 48 angiosarcomas and noticed recurrent amplifications of HOXA-cluster genes in 33% of patients. HOXA genes are master regulators of embryonic vascular development and adult neovascularization, which provides a molecular rationale to suspect that amplified HOXA genes act as oncogenes in angiosarcoma. HOXA amplifications typically affected multiple pro-angiogenic HOXA genes and co-occurred with amplifications of CD36 and KDR, whereas the overall mutation rate in these tumors was relatively low. HOXA amplifications were found most commonly in angiosarcomas located in the breast and were rare in angiosarcomas arising in sun-exposed areas on the head, neck, face and scalp. Our data suggest that HOXA-amplified angiosarcoma is a distinct molecular subgroup. Efforts to develop therapies targeting oncogenic HOX gene expression in AML and other sarcomas may have relevance for HOXA-amplified angiosarcoma.

Transcription factor Hhex negatively regulates osteoclast differentiation by controlling cyclin‐dependent kinase inhibitors

We investigated the role of hematopoietically expressed homeobox protein (Hhex) in osteoclast development. Trimethylation of lysine 27 of histone H3 at the cis‐regulatory element of Hhex was maintained and that of lysine 4 was reduced during receptor activator of nuclear factor κB ligand (RANKL)‐induced osteoclastogenesis, which was associated with a reduction of Hhex expression. Overexpression of Hhex in bone marrow–derived macrophages inhibited, whereas Hhex suppression promoted, RANKL‐induced osteoclastogenesis in vitro. Conditional deletion of Hhex in osteoclast‐lineage cells promoted osteoclastogenesis and reduced cancellous bone volume in mice, confirming the negative regulatory role of Hhex in osteoclast differentiation. Expression of cyclin‐dependent kinase inhibitors such as Cdkn2a and Cdkn1b in osteoclast precursors was negatively regulated by Hhex, and Hhex deletion increased the ratio of cells at the G1 phase of the cell cycle. In conclusion, Hhex is an inhibitor of osteoclast differentiation that is regulated in an epigenetic manner and regulates the cell cycle of osteoclast precursors and the skeletal homeostasis. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Role of NKL Homeobox Genes in T-Cell Malignancies

Homeobox genes encode transcription factors controlling basic developmental processes. The homeodomain is encoded by the homeobox and mediates sequence-specific DNA binding and interaction with cofactors, thus operating as a basic regulatory platform. Similarities in their homeobox sequences serve to arrange these genes in classes and subclasses, including NKL homeobox genes. In accordance with their normal functions, deregulated homeobox genes contribute to carcinogenesis along with hematopoietic malignancies. We have recently described the physiological expression of eleven NKL homeobox genes in the course of hematopoiesis and termed this gene expression pattern NKL-code. Due to the developmental impact of NKL homeobox genes these data suggest a key role for their activity in the normal regulation of hematopoietic cell differentiation including T-cells. On the other hand, aberrant overexpression of NKL-code members or ectopical activation of non-code members has been frequently reported in lymphoid and myeloid leukemia/lymphoma, demonstrating their oncogenic impact in the hematopoietic compartment. Here, we provide an overview of the NKL-code in normal hematopoiesis and discuss the oncogenic role of deregulated NKL homeobox genes in T-cell malignancies.

Suppression of Cell Tumorigenicity by Non-neural Pro-differentiation Factors via Inhibition of Neural Property in Tumorigenic Cells

Our studies have demonstrated that cell tumorigenicity and pluripotent differentiation potential stem from neural stemness or a neural ground state, which is defined by a regulatory network of higher levels of machineries for basic cell physiological functions, including cell cycle, ribosome biogenesis, protein translation, spliceosome, epigenetic modification factors, reprogramming factors, etc., in addition to the neural stemness specific factors. These machineries and neural stemness factors mostly play cancer-promoting roles. It can be deduced that differentiation requires the repression of neural ground state and causes the reduction or loss of neural ground state and thus tumorigenicity in tumorigenic cells. Formerly, we showed that neuronal differentiation led to reduced tumorigenicity in tumorigenic cells. In the present study, we show that non-neural pro-differentiation factors, such as GATA3, HNF4A, HHEX, and FOXA3 that specify mesodermal or/and endodermal tissues during vertebrate embryogenesis, suppress tumorigenicity via repression of neural stemness and promotion of non-neural property in tumorigenic cells. Mechanistically, these transcription factors repress the transcription of neural enriched genes and meanwhile activate genes that specify non-neural properties via direct binding to the promoters of these genes. We also show that combined expression of HHEX and FOXA3 suppresses tumorigenesis effectively in the AOM/DSS model of colitis-associated cancer. We suggest that targeting the property of neural stemness could be an effective strategy for cancer therapy.

Transcription Control of Liver Development

During liver organogenesis, cellular transcriptional profiles are constantly reshaped by the action of hepatic transcriptional regulators, including FoxA1-3, GATA4/6, HNF1α/β, HNF4α, HNF6, OC-2, C/EBPα/β, Hex, and Prox1. These factors are crucial for the activation of hepatic genes that, in the context of compact chromatin, cannot access their targets. The initial opening of highly condensed chromatin is executed by a special class of transcription factors known as pioneer factors. They bind and destabilize highly condensed chromatin and facilitate access to other "non-pioneer" factors. The association of target genes with pioneer and non-pioneer transcription factors takes place long before gene activation. In this way, the underlying gene regulatory regions are marked for future activation. The process is called "bookmarking", which confers transcriptional competence on target genes. Developmental bookmarking is accompanied by a dynamic maturation process, which prepares the genomic loci for stable and efficient transcription. Stable hepatic expression profiles are maintained during development and adulthood by the constant availability of the main regulators. This is achieved by a self-sustaining regulatory network that is established by complex cross-regulatory interactions between the major regulators. This network gradually grows during liver development and provides an epigenetic memory mechanism for safeguarding the optimal expression of the regulators.

NKL-Code in Normal and Aberrant Hematopoiesis

Simple Summary

Gene codes represent expression patterns of closely related genes in particular tissues, organs or body parts. The NKL-code describes the activity of NKL homeobox genes in the hematopoietic system. NKL homeobox genes encode transcription factors controlling basic developmental processes. Therefore, aberrations of this code may contribute to deregulated hematopoiesis including leukemia and lymphoma. Normal and abnormal activities of NKL homeobox genes are described and mechanisms of (de)regulation, function, and diseases exemplified.

Abstract

We have recently described physiological expression patterns of NKL homeobox genes in early hematopoiesis and in subsequent lymphopoiesis and myelopoiesis, including terminally differentiated blood cells. We thereby systematized differential expression patterns of eleven such genes which form the so-called NKL-code. Due to the developmental impact of NKL homeobox genes, these data suggest a key role for their activity in normal hematopoietic differentiation processes. On the other hand, the aberrant overexpression of NKL-code-members or the ectopical activation of non-code members have been frequently reported in lymphoid and myeloid leukemia/lymphoma, revealing the oncogenic potential of these genes in the hematopoietic compartment. Here, I provide an overview of the NKL-code in normal hematopoiesis and instance mechanisms of deregulation and oncogenic functions of selected NKL genes in hematologic cancers. As well as published clinical studies, our conclusions are based on experimental work using hematopoietic cell lines which represent useful models to characterize the role of NKL homeobox genes in specific tumor types.

Transcription factors β-catenin and Hex in postnatal development of the rat adrenal cortex: implication in proliferation control

Transcriptional regulation of growth, maturation, and cell turnover in adrenal cortex during postnatal development has been significantly less studied than in embryonic period, while elucidation of factors mediating its normal postnatal morphogenesis could clarify mechanisms of tumorigenesis in adrenal cortex. Expression of transcription factors Hex, β-catenin, and Wnt signaling in the adrenal cortex of male pubertal and postpubertal Wistar rats were examined. Adrenal cortex morphology and hormone production during postnatal development were also studied. Adrenocortical zones demonstrated similar reduction of Ki-67-expressing cells, but different patterns of morphological and functional changes. Age-dependent decrease in percentage of cells with membrane localization of β-catenin and stable rate of cells with nuclear β-catenin, indicative of Wnt signaling activation, were revealed in each cortical zone. Nuclear β-catenin was not observed in immature areas of zona fasciculata. No association between Wnt signaling activation and rates of proliferation as well as changes in secretion of adrenocortical hormones was observed in postnatal development of rat adrenal cortex. Hex, known as antiproliferative factor, showed up-regulation of expression after puberty. Strong inverse correlations between ratio of Hex-positive cells and proliferating cells were found in zona glomerulosa and zona fasciculata. Zona reticularis demonstrated moderate correlation. Thus, these findings suggest a role for Hex in proliferation control during postnatal development of the rat adrenal cortex and possible implication of Hex down-regulation in adrenocortical dysplasia and neoplasia, which requires further study. Evaluation of Hex expression may also be considered a potent tool in assessment of cell proliferation in rat adrenal cortex.

Unraveling the Complex Interplay Between Transcription Factors and Signaling Molecules in Thyroid Differentiation and Function, From Embryos to Adults

Thyroid differentiation of progenitor cells occurs during embryonic development and in the adult thyroid gland, and the molecular bases of these complex and finely regulated processes are becoming ever more clear. In this Review, we describe the most recent advances in the study of transcription factors, signaling molecules and regulatory pathways controlling thyroid differentiation and development in the mammalian embryo. We also discuss the maintenance of the adult differentiated phenotype to ensure the biosynthesis of thyroid hormones. We will focus on endoderm-derived thyroid epithelial cells, which are responsible for the formation of the thyroid follicle, the functional unit of the thyroid gland. The use of animal models and pluripotent stem cells has greatly aided in providing clues to the complicated puzzle of thyroid development and function in adults. The so-called thyroid transcription factors - Nkx2-1, Foxe1, Pax8 and Hhex - were the first pieces of the puzzle identified in mice. Other transcription factors, either acting upstream of or directly with the thyroid transcription factors, were subsequently identified to, almost, complete the puzzle. Among them, the transcription factors Glis3, Sox9 and the cofactor of the Hippo pathway Taz, have emerged as important players in thyroid differentiation and development. The involvement of signaling molecules increases the complexity of the puzzle. In this context, the importance of Bmps, Fgfs and Shh signaling at the onset of development, and of TSH, IGF1 and TGFβ both at the end of terminal differentiation in embryos and in the adult thyroid, are well recognized. All of these aspects are covered herein. Thus, readers will be able to visualize the puzzle of thyroid differentiation with most - if not all - of the pieces in place.

Generating liver using blastocyst complementation: Opportunities and challenges

Orthotopic liver transplantation (OLT) is the only definitive treatment option for many patients with end-stage liver disease. Current supply of donor livers for OLT is not keeping up with the growing demand. To overcome this problem, a number of experimental strategies have been developed either to provide a bridge to transplant for patients on the waiting list or to bioengineer whole livers for OLT by replenishing them with fresh supplies of hepatic cells. In recent years, blastocyst complementation has emerged as the most promising approach for generating whole organs and, in combination with gene editing technology, it has revolutionized regenerative medicine. This methodology was successful in producing xenogeneic organs in animal hosts. Blastocyst complementation has the potential to produce whole livers in large animals that could be xenotransplanted in humans, thereby reducing the shortage of livers for OLT. However, significant experimental and ethical barriers remain for the production of human livers in domestic animals, such as the pig. This review summarizes the current knowledge and provides future perspectives for liver xenotransplantation in humans.

Hhex Directly Represses BIM-Dependent Apoptosis to Promote NK Cell Development and Maintenance

Hhex encodes a homeobox transcriptional regulator important for embryonic development and hematopoiesis. Hhex is highly expressed in NK cells, and its germline deletion results in significant defects in lymphoid development, including NK cells. To determine if Hhex is intrinsically required throughout NK cell development or for NK cell function, we generate mice that specifically lack Hhex in NK cells. NK cell frequency is dramatically reduced, while NK cell differentiation, IL-15 responsiveness, and function at the cellular level remain largely normal in the absence of Hhex. Increased IL-15 availability fails to fully reverse NK lymphopenia following conditional Hhex deletion, suggesting that Hhex regulates developmental pathways extrinsic to those dependent on IL-15. Gene expression and functional genetic approaches reveal that Hhex regulates NK cell survival by directly binding Bcl2l11 (Bim) and repressing expression of this key apoptotic mediator. These data implicate Hhex as a transcriptional regulator of NK cell homeostasis and immunity.

The NKL-code for innate lymphoid cells reveals deregulated expression of NKL homeobox genes HHEX and HLX in anaplastic large cell lymphoma (ALCL)

NKL homeobox genes encode developmental transcription factors and display an NKL-code according to their physiological expression pattern in hematopoiesis. Here, we analyzed public transcriptome data from primary innate lymphoid cells (ILCs) for NKL homeobox gene activities and found that ILC3 expressed exclusively HHEX while in ILC1 and ILC2 these genes were silenced. Deregulation of the NKL-code promotes hematopoietic malignancies, including anaplastic large cell lymphoma (ALCL) which reportedly may derive from ILC3. Accordingly, we analyzed NKL homeobox gene activities in ALCL cell lines and investigated their role in this malignancy. Transcriptome analyses demonstrated low expression levels of HHEX but powerfully activated HLX. Forced expression of HHEX in ALCL cell lines induced genes involved in apoptosis and ILC3 differentiation, indicating tumor suppressor activity. ALCL associated NPM1-ALK and JAK-STAT3-signalling drove enhanced expression of HLX while discounting HHEX. Genomic profiling revealed copy number gains at the loci of HLX and STAT3 in addition to genes encoding both STAT3 regulators (AURKA, BCL3, JAK3, KPNB1, NAMPT, NFAT5, PIM3, ROCK1, SIX1, TPX2, WWOX) and targets (BATF3, IRF4, miR135b, miR21, RORC). Transcriptome data of ALCL cell lines showed absence of STAT3 mutations while MGA was mutated and downregulated, encoding a novel potential STAT3 repressor. Furthermore, enhanced IL17F-signalling activated HLX while TGFbeta-signalling inhibited HHEX expression. Taken together, our data extend the scope of the NKL-code for ILCs and spotlight aberrant expression of NKL homeobox gene HLX in ALCL. HLX represents a direct target of ALCL hallmark factor STAT3 and deregulates cell survival and differentiation in this malignancy.

Zebrafish hhex-null mutant develops an intrahepatic intestinal tube due to de-repression of cdx1b and pdx1

The hepatopancreatic duct (HPD) system links the liver and pancreas to the intestinal tube and is composed of the extrahepatic biliary duct, gallbladder, and pancreatic duct. Haematopoietically expressed-homeobox (Hhex) protein plays an essential role in the establishment of HPD; however, the molecular mechanism remains elusive. Here, we show that zebrafish hhex-null mutants fail to develop the HPD system characterized by lacking the biliary marker Annexin A4 and the HPD marker sox9b. The hepatobiliary duct part of the mutant HPD system is replaced by an intrahepatic intestinal tube characterized by expressing the intestinal marker fatty acid-binding protein 2a (fabp2a). Cell lineage analysis showed that this intrahepatic intestinal tube is not originated from hepatocytes or cholangiocytes. Further analysis revealed that cdx1b and pdx1 are expressed ectopically in the intrahepatic intestinal tube and knockdown of cdx1b and pdx1 could restore the expression of sox9b in the mutant. Chromatin-immunoprecipitation analysis showed that Hhex binds to the promoters of pdx1 and cdx1b genes to repress their expression. We therefore propose that Hhex, Cdx1b, Pdx1, and Sox9b form a genetic network governing the patterning and morphogenesis of the HPD and digestive tract systems in zebrafish.

Blood platelets stimulate cancer extravasation through TGFβ-mediated downregulation of PRH/HHEX

Cancer cells go through a process known as epithelial–mesenchymal transition (EMT) during which they acquire the ability to migrate and invade extracellular matrix. Some cells also acquire the ability to move across a layer of endothelial cells to enter and exit the bloodstream; intra- and extravasation, respectively. The transcription factor PRH/HHEX (proline-rich homeodomain/haematopoietically expressed homeobox) controls cell proliferation and cell migration/invasion in a range of cell types. Our previous work showed that PRH activity is downregulated in prostate cancer cells owing to increased inhibitory PRH phosphorylation and that this increases cell proliferation and invasion. PRH inhibits migration and invasion by prostate and breast epithelial cells in part by activating the transcription of Endoglin, a transforming growth factor β (TGFβ) co-receptor. Here we show that depletion of PRH in immortalised prostate epithelial cells results in increased extravasation in vitro. We show that blood platelets stimulate extravasation of cells with depleted PRH and that inhibition of TGFβ signalling blocks the effects of platelets on these cells. Moreover, TGFβ induces changes characteristic of EMT including decreased E-Cadherin expression and increased Snail expression. We show that in prostate cells PRH regulates multiple genes involved in EMT and TGFβ signalling. However, both platelets and TGFβ increase PRH phosphorylation. In addition, TGFβ increases binding of its effector pSMAD3 to the PRH/HHEX promoter and downregulates PRH protein and mRNA levels. Thus, TGFβ signalling downregulates PRH activity by multiple mechanisms and induces an EMT that facilitates extravasation and sensitises cells to TGFβ.

A Runaway PRH/HHEX-Notch3-Positive Feedback Loop Drives Cholangiocarcinoma and Determines Response to CDK4/6 Inhibition

Aberrant Notch and Wnt signaling are known drivers of cholangiocarcinoma (CCA), but the underlying factors that initiate and maintain these pathways are not known. Here, we show that the proline-rich homeodomain protein/hematopoietically expressed homeobox (PRH/HHEX) transcription factor forms a positive transcriptional feedback loop with Notch3 that is critical in CCA. PRH/HHEX expression is elevated in CCA, and depletion of PRH reduces CCA tumor growth in a xenograft model. Overexpression of PRH in primary human biliary epithelial cells is sufficient to increase cell proliferation and produce an invasive phenotype. Interrogation of the gene networks regulated by PRH and Notch3 reveals that unlike Notch3, PRH directly activates canonical Wnt signaling. These data indicate that hyperactivation of Notch and Wnt signaling is independent of the underlying mutational landscape and has a common origin in dysregulation of PRH. Moreover, they suggest new therapeutic options based on the dependence of specific Wnt, Notch, and CDK4/6 inhibitors on PRH activity. SIGNIFICANCE: The PRH/HHEX transcription factor is an oncogenic driver in cholangiocarcinoma that confers sensitivity to CDK4/6 inhibitors.

Homeobox protein Hhex negatively regulates Treg cells by inhibiting Foxp3 expression and function

Regulatory T (Treg) cells play an essential role in maintaining immune homeostasis, but the suppressive function of Treg cells can be an obstacle in the treatment of cancer and chronic infectious diseases. Here, we identified the homeobox protein Hhex as a negative regulator of Treg cells. The expression of Hhex was lower in Treg cells than in conventional T (Tconv) cells. Hhex expression was repressed in Treg cells by TGF-β/Smad3 signaling. Retroviral overexpression of Hhex inhibited the differentiation of induced Treg (iTreg) cells and the stability of thymic Treg (tTreg) cells by significantly reducing Foxp3 expression. Moreover, Hhex-overexpressing Treg cells lost their immunosuppressive activity and failed to prevent colitis in a mouse model of inflammatory bowel disease (IBD). Hhex expression was increased; however, Foxp3 expression was decreased in Treg cells in a delayed-type hypersensitivity (DTH) reaction, a type I immune reaction. Hhex directly bound to the promoters of Foxp3 and other Treg signature genes, including Il2ra and Ctla4, and repressed their transactivation. The homeodomain and N-terminal repression domain of Hhex were critical for inhibiting Foxp3 and other Treg signature genes. Thus, Hhex plays an essential role in inhibiting Treg cell differentiation and function via inhibition of Foxp3.

Deregulated NKL Homeobox Genes in B-Cell Lymphoma

Recently, we have described physiological expression patterns of NKL homeobox genes in early hematopoiesis and in subsequent lymphopoiesis. We identified nine genes which constitute the so-called NKL-code. Aberrant overexpression of code-members or ectopically activated non-code NKL homeobox genes are described in T-cell leukemia and in T- and B-cell lymphoma, highlighting their oncogenic role in lymphoid malignancies. Here, we introduce the NKL-code in normal hematopoiesis and focus on deregulated NKL homeobox genes in B-cell lymphoma, including HLX, MSX1 and NKX2-2 in Hodgkin lymphoma; HLX, NKX2-1 and NKX6-3 in diffuse large B-cell lymphoma; and NKX2-3 in splenic marginal zone lymphoma. Thus, the roles of various members of the NKL homeobox gene subclass are considered in normal and pathological hematopoiesis in detail.

Systematic alanine scanning of PAX8 paired domain reveals functional importance of the N-subdomain

Thyroid-specific transcription factor PAX8 has an indispensable role in the thyroid gland development, which is evidenced by the facts that PAX8/Pax8 mutations cause congenital hypothyroidism in humans and mice. More than 90% of known PAX8 mutations were located in the paired domain, suggesting the central role of the domain in exerting the molecular function. Structure-function relationships of PAX8, as well as other PAX family transcription factors, have never been investigated in a systematic manner. Here, we conducted the first alanine scanning mutagenesis study, in which 132 alanine variants located in the paired domain of PAX8 were created and systematically evaluated in vitro. We found that 76 alanine variants (55%) were loss of function (LOF) variants (defined by <30% activity as compared with wild type PAX8). Importantly, the distribution of LOF variants were skewed, with more frequently observed in the N-subdomain (65% of the alanine variants in the N-subdomain) than in the C-subdomain (45%). Twelve out of 13 alanine variants in residues that have been affected in patients with congenital hypothyroidism were actually LOF, suggesting that the alanine scanning data can be used to evaluate the functional importance of mutated residues. Using our in vitro data, we tested the accuracy of seven computational algorithms for pathogenicity prediction, showing that they are sensitive but not specific to evaluate on the paired domain alanine variants. Collectively, our experiment-based data would help better understand the structure-function relationships of the paired domain, and would provide a unique resource for pathogenicity prediction of future PAX8 variants.

Hhex induces promyelocyte self-renewal and cooperates with growth factor independence to cause myeloid leukemia in mice

The hematopoietically expressed homeobox (Hhex) transcription factor is overexpressed in human myeloid leukemias. Conditional knockout models of murine acute myeloid leukemia indicate that Hhex maintains leukemia stem cell self-renewal by enabling Polycomb-mediated epigenetic repression of the Cdkn2a tumor suppressor locus, encoding p16^Ink4a and p19^Arf However, whether Hhex overexpression also affects hematopoietic differentiation is unknown. To study this, we retrovirally overexpressed Hhex in hematopoietic progenitors. This enabled serial replating of myeloid progenitors, leading to the rapid establishment of interleukin-3 (IL-3)-dependent promyelocytic cell lines. Use of a Hhex-ERT2 fusion protein demonstrated that continuous nuclear Hhex is required for transformation, and structure function analysis demonstrated a requirement of the DNA-binding and N-terminal-repressive domains of Hhex for promyelocytic transformation. This included the N-terminal promyelocytic leukemia protein (Pml) interaction domain, although deletion of Pml failed to prevent Hhex-induced promyelocyte transformation, implying other critical partners. Furthermore, deletion of p16^Ink4a or p19^Arf did not promote promyelocyte transformation, indicating that repression of distinct Hhex target genes is required for this process. Indeed, transcriptome analysis showed that Hhex overexpression resulted in repression of several myeloid developmental genes. To test the potential for Hhex overexpression to contribute to leukemic transformation, Hhex-transformed promyelocyte lines were rendered growth factor-independent using a constitutively active IL-3 receptor common β subunit (βcV449E). The resultant cell lines resulted in a rapid promyelocytic leukemia in vivo. Thus, Hhex overexpression can contribute to myeloid leukemia via multiple mechanisms including differentiation blockade and enabling epigenetic repression of the Cdkn2a locus.

Micropattern differentiation of mouse pluripotent stem cells recapitulates embryo regionalized cell fate patterning

During gastrulation epiblast cells exit pluripotency as they specify and spatially arrange the three germ layers of the embryo. Similarly, human pluripotent stem cells (PSCs) undergo spatially organized fate specification on micropatterned surfaces. Since in vivo validation is not possible for the human, we developed a mouse PSC micropattern system and, with direct comparisons to mouse embryos, reveal the robust specification of distinct regional identities. BMP, WNT, ACTIVIN and FGF directed mouse epiblast-like cells to undergo an epithelial-to-mesenchymal transition and radially pattern posterior mesoderm fates. Conversely, WNT, ACTIVIN and FGF patterned anterior identities, including definitive endoderm. By contrast, epiblast stem cells, a developmentally advanced state, only specified anterior identities, but without patterning. The mouse micropattern system offers a robust scalable method to generate regionalized cell types present in vivo, resolve how signals promote distinct identities and generate patterns, and compare mechanisms operating in vivo and in vitro and across species.

Hhex Regulates Hematopoietic Stem Cell Self-Renewal and Stress Hematopoiesis via Repression of Cdkn2a

The hematopoietically expressed homeobox transcription factor (Hhex) is important for the maturation of definitive hematopoietic progenitors and B-cells during development. We have recently shown that in adult hematopoiesis, Hhex is dispensable for maintenance of hematopoietic stem cells (HSCs) and myeloid lineages but essential for the commitment of common lymphoid progenitors (CLPs) to lymphoid lineages. Here, we show that during serial bone marrow transplantation, Hhex-deleted HSCs are progressively lost, revealing an intrinsic defect in HSC self-renewal. Moreover, Hhex-deleted mice show markedly impaired hematopoietic recovery following myeloablation, due to a failure of progenitor expansion. In vitro, Hhex-null blast colonies were incapable of replating, implying a specific requirement for Hhex in immature progenitors. Transcriptome analysis of Hhex-null Lin^- Sca⁺ Kit⁺ cells showed that Hhex deletion leads to derepression of polycomb repressive complex 2 (PRC2) and PRC1 target genes, including the Cdkn2a locus encoding the tumor suppressors p16^Ink 4^a and p19^Arf . Indeed, loss of Cdkn2a restored the capacity of Hhex-null blast colonies to generate myeloid progenitors in vitro, as well as hematopoietic reconstitution following myeloablation in vivo. Thus, HSCs require Hhex to promote PRC2-mediated Cdkn2a repression to enable continued self-renewal and response to hematopoietic stress. Stem Cells 2017;35:1948-1957.

Transcriptional regulation of Hhex in hematopoiesis and hematopoietic stem cell ontogeny

Hematopoietic stem cells (HSCs) emerge during development via an endothelial-to-hematopoietic transition from hemogenic endothelium of the dorsal aorta (DA). Using in situ hybridization and analysis of a knock-in RedStar reporter, we show that the transcriptional regulator Hhex is expressed in endothelium of the dorsal aorta (DA) and in clusters of putative HSCs as they are specified during murine development. We exploited this observation, using the Hhex locus to define cis regulatory elements, enhancers and interacting transcription factors that are both necessary and sufficient to support gene expression in the emerging HSC. We identify an evolutionarily conserved non-coding region (ECR) in the Hhex locus with the capacity to bind the hematopoietic-affiliated transcriptional regulators Gata2, SCL, Fli1, Pu.1 and Ets1/2. This region is sufficient to drive the expression of a transgenic GFP reporter in the DA endothelium and intra-aortic hematopoietic clusters. GFP-positive AGM cells co-expressed HSC-associated markers c-Kit, CD34, VE-Cadherin, and CD45, and were capable of multipotential differentiation and long term engraftment when transplanted into myelo-ablated recipients. The Hhex ECR was also sufficient to drive expression at additional blood sites including the yolk sac blood islands, fetal liver, vitelline and umbilical arteries and the adult bone marrow, suggesting a common mechanism for Hhex regulation throughout ontogenesis of the blood system. To explore the physiological requirement for the Hhex ECR region during hematoendothelial development, we deleted the ECR element from the endogenous locus in the context of a targeted Hhex-RedStar reporter allele. Results indicate a specific requirement for the ECR in blood-associated Hhex expression during development and further demonstrate a requirement for this region in the adult HSC compartment. Taken together, our results identified the ECR region as an enhancer both necessary and sufficient for gene expression in HSC development and homeostasis. The Hhex ECR thus appears to be a core node for the convergence of the transcription factor network that governs the emergence of HSCs.

Selection of reliable reference genes for normalization of quantitative RT-PCR from different developmental stages and tissues in amphioxus

Amphioxus is a closest living proxy to the ancestor of cephalochordates with vertebrates, and key animal for novel understanding in the evolutionary origin of vertebrate body plan, genome, tissues and immune system. Reliable analyses using quantitative real-time PCR (qRT-PCR) for answering these scientific questions is heavily dependent on reliable reference genes (RGs). In this study, we evaluated stability of thirteen candidate RGs in qRT-PCR for different developmental stages and tissues of amphioxus by four independent (geNorm, NormFinder, BestKeeper and deltaCt) and one comparative algorithms (RefFinder). The results showed that the top two stable RGs were the following: (1) S20 and 18 S in thirteen developmental stages, (2) EF1A and ACT in seven normal tissues, (3) S20 and L13 in both intestine and hepatic caecum challenged with lipopolysaccharide (LPS), and (4) S20 and EF1A in gill challenged with LPS. The expression profiles of two target genes (EYA and HHEX) in thirteen developmental stages were used to confirm the reliability of chosen RGs. This study identified optimal RGs that can be used to accurately measure gene expression under these conditions, which will benefit evolutionary and functional genomics studies in amphioxus.

Acute myeloid leukemia requires Hhex to enable PRC2-mediated epigenetic repression of Cdkn2a

Here, Shields et al. demonstrate that the hematopoietically expressed homeobox gene Hhex is overexpressed in acute myeloid leukemia (AML) and is essential for the initiation and propagation of MLL-ENL-induced AML but dispensable for normal myelopoiesis, indicating a specific requirement for Hhex for leukemic growth. The findings in this study describe for the first time a nonclustered homeobox transcription factor that is essential for AML initiation and maintenance and provide mechanistic insight into these processes.

Unlike clustered HOX genes, the role of nonclustered homeobox gene family members in hematopoiesis and leukemogenesis has not been extensively studied. Here we found that the hematopoietically expressed homeobox gene Hhex is overexpressed in acute myeloid leukemia (AML) and is essential for the initiation and propagation of MLL-ENL-induced AML but dispensable for normal myelopoiesis, indicating a specific requirement for Hhex for leukemic growth. Loss of Hhex leads to expression of the Cdkn2a-encoded tumor suppressors p16^INK4a and p19^ARF, which are required for growth arrest and myeloid differentiation following Hhex deletion. Mechanistically, we show that Hhex binds to the Cdkn2a locus and directly interacts with the Polycomb-repressive complex 2 (PRC2) to enable H3K27me3-mediated epigenetic repression. Thus, Hhex is a potential therapeutic target that is specifically required for AML stem cells to repress tumor suppressor pathways and enable continued self-renewal.

Screening of HHEX Mutations in Chinese Children with Thyroid Dysgenesis

OBJECTIVE

Congenital hypothyroidism (CH) is a frequent neonatal endocrine disease with an incidence of about 1:2500 worldwide. Although thyroid dysgenesis (TD) is the most frequent cause of CH cases, its pathogenesis remains unclear. The aim of this study was to screen the hematopoietically-expressedhomeobox gene (HHEX) mutations in Chinese children with TD.

METHODS

Genomic deoxyribonucleic acid was extracted from peripheral blood leukocytes in 234 TD patients from Shandong Province. Mutations in all exons and nearby introns of HHEX were analyzed by direct sequencing after polymerase chain reaction amplification.

RESULTS

Sequencing analysis of HHEX indicated that no causative mutations were present in the coding regionof the TD patients. However, a genetic variant (IVS2+ 127 G/T, 10.26%) was observed in the intron 2 in HHEX.

CONCLUSION

Our results indicate that the frequency of HHEX mutation is very low and may not be the main causative factor in Chinese TD patients. However, these results need to be replicated using larger datasets collected from different populations.

Misregulation of the proline rich homeodomain (PRH/HHEX) protein in cancer cells and its consequences for tumour growth and invasion

The proline rich homeodomain protein (PRH), also known as haematopoietically expressed homeobox (HHEX), is an essential transcription factor in embryonic development and in the adult. The PRH protein forms oligomeric complexes that bind to tandemly repeated PRH recognition sequences within or at a distance from PRH-target genes and recruit a variety of PRH-interacting proteins. PRH can also bind to other transcription factors and co-regulate specific target genes either directly through DNA binding, or indirectly through effects on the activity of its partner proteins. In addition, like some other homeodomain proteins, PRH can regulate the translation of specific mRNAs. Altered PRH expression and altered PRH intracellular localisation, are associated with breast cancer, liver cancer and thyroid cancer and some subtypes of leukaemia. This is consistent with the involvement of multiple PRH-interacting proteins, including the oncoprotein c-Myc, translation initiation factor 4E (eIF4E), and the promyelocytic leukaemia protein (PML), in the control of cell proliferation and cell survival. Similarly, multiple PRH target genes, including the genes encoding vascular endothelial growth factor (VEGF), VEGF receptors, Endoglin, and Goosecoid, are known to be important in the control of cell proliferation and cell survival and/or the regulation of cell migration and invasion. In this review, we summarise the evidence that implicates PRH in tumourigenesis and we review the data that suggests PRH levels could be useful in cancer prognosis and in the choice of treatment options.

Hhex Is Necessary for the Hepatic Differentiation of Mouse ES Cells and Acts via Vegf Signaling

Elucidating the molecular mechanisms involved in the differentiation of stem cells to hepatic cells is critical for both understanding normal developmental processes as well as for optimizing the generation of functional hepatic cells for therapy. We performed in vitro differentiation of mouse embryonic stem cells (mESCs) with a null mutation in the homeobox gene Hhex and show that Hhex^-/- mESCs fail to differentiate from definitive endoderm (Sox17⁺/Foxa2⁺) to hepatic endoderm (Alb⁺/Dlk⁺). In addition, hepatic culture elicited a >7-fold increase in Vegfa mRNA expression in Hhex^-/- cells compared to Hhex^+/+ cells. Furthermore, we identified VEGFR2⁺/ALB⁺/CD34^- in early Hhex^+/+ hepatic cultures. These cells were absent in Hhex^-/- cultures. Finally, through manipulation of Hhex and Vegfa expression, gain and loss of expression experiments revealed that Hhex shares an inverse relationship with the activity of the Vegf signaling pathway in supporting hepatic differentiation. In summary, our results suggest that Hhex represses Vegf signaling during hepatic differentiation of mouse ESCs allowing for cell-type autonomous regulation of Vegfr2 activity independent of endothelial cells.

Spontaneous Pancreatitis Caused by Tissue-Specific Gene Ablation of Hhex in Mice

BACKGROUND & AIMS

Perturbations in pancreatic ductal bicarbonate secretion cause chronic pancreatitis. The physiologic mechanism of ductal secretion is known, but its transcriptional control is not. We determine the role of the transcription factor hematopoietically expressed homeobox protein (Hhex) in ductal secretion and pancreatitis.

METHODS

We derived mice with pancreas-specific, Cremediated Hhex gene ablation to determine the requirement of Hhex in the pancreatic duct in early life and in adult stages. Histologic and immunostaining analyses were used to detect the presence of pathology. Pancreatic primary ductal cells were isolated to discover differentially expressed transcripts upon acute Hhex ablation on a cell autonomous level.

RESULTS

Hhex protein was detected throughout the embryonic and adult ductal trees. Ablation of Hhex in pancreatic progenitors resulted in postnatal ductal ectasia associated with acinar-to-ductal metaplasia, a progressive phenotype that ultimately resulted in chronic pancreatitis. Hhex ablation in adult mice, however, did not cause any detectable pathology. Ductal ectasia in young mice did not result from perturbation of expression of Hnf6, Hnf1β, or the primary cilia genes. RNA-seq analysis of Hhex-ablated pancreatic primary ductal cells showed mRNA levels of the G-protein coupled receptor natriuretic peptide receptor 3 (Npr3), implicated in paracrine signaling, up-regulated by 4.70-fold.

CONCLUSIONS

Although Hhex is dispensable for ductal cell function in the adult, ablation of Hhex in pancreatic progenitors results in pancreatitis. Our data highlight the critical role of Hhex in maintaining ductal homeostasis in early life and support ductal hypersecretion as a novel etiology of pediatric chronic pancreatitis.

Thyroid transcription factors in development, differentiation and disease

Identification of the thyroid transcription factors (TTFs), NKX2-1, FOXE1, PAX8 and HHEX, has considerably advanced our understanding of thyroid development, congenital thyroid disorders and thyroid cancer. The TTFs are fundamental to proper formation of the thyroid gland and for maintaining the functional differentiated state of the adult thyroid; however, they are not individually required for precursor cell commitment to a thyroid fate. Although knowledge of the mechanisms involved in thyroid development has increased, the full complement of genes involved in thyroid gland specification and the signals that trigger expression of the genes that encode the TTFs remain unknown. The mechanisms involved in thyroid organogenesis and differentiation have provided clues to identifying the genes that are involved in human congenital thyroid disorders and thyroid cancer. Mutations in the genes that encode the TTFs, as well as polymorphisms and epigenetic modifications, have been associated with thyroid pathologies. Here, we summarize the roles of the TTFs in thyroid development and the mechanisms by which they regulate expression of the genes involved in thyroid differentiation. We also address the implications of mutations in TTFs in thyroid diseases and in diseases not related to the thyroid gland.

A crucial role for the homeodomain transcription factor Hhex in lymphopoiesis

The hematopoietically expressed homeobox gene, Hhex, is a transcription factor that is important for development of definitive hematopoietic stem cells (HSCs) and B cells, and that causes T-cell leukemia when overexpressed. Here, we have used an Hhex inducible knockout mouse model to study the role of Hhex in adult hematopoiesis. We found that loss of Hhex was tolerated in HSCs and myeloid lineages, but resulted in a progressive loss of B lymphocytes in the circulation. This was accompanied by a complete loss of B-cell progenitors in the bone marrow and of transitional B-cell subsets in the spleen. In addition, transplantation and in vitro culture experiments demonstrated an almost complete failure of Hhex-null HSCs to contribute to lymphoid lineages beyond the common lymphoid precursor stage, including T cells, B cells, NK cells, and dendritic cells. Gene expression analysis of Hhex-deleted progenitors demonstrated deregulated expression of a number of cell cycle regulators. Overexpression of one of these, cyclin D1, could rescue the B-cell developmental potential of Hhex-null lymphoid precursors. Thus, Hhex is a key regulator of early lymphoid development, functioning, at least in part, via regulation of the cell cycle.

Hhex regulates Kit to promote radioresistance of self-renewing thymocytes in Lmo2-transgenic mice

Lmo2 is an oncogenic transcription factor that is frequently overexpressed in T-cell acute leukemias, in particular poor prognosis early T-cell precursor-like (ETP-) acute lymphoblastic leukemia (ALL). The primary effect of Lmo2 is to cause self-renewal of developing CD4(-)CD8(-) (double negative, DN) T cells in the thymus, leading to serially transplantable thymocytes that eventually give rise to leukemia. These self-renewing thymocytes are intrinsically radioresistant implying that they may be a source of leukemia relapse after therapy. The homeobox transcription factor, Hhex, is highly upregulated in Lmo2-transgenic thymocytes and can phenocopy Lmo2 in inducing thymocyte self-renewal, implying that Hhex may be a key component of the Lmo2-induced self-renewal program. To test this, we conditionally deleted Hhex in the thymi of Lmo2-transgenic mice. Surprisingly, this did not prevent accumulation of DN thymocytes, nor alter the rate of overt leukemia development. However, deletion of Hhex abolished the transplantation capacity of Lmo2-transgenic thymocytes and overcame their radioresistance. We found that Hhex regulates Kit expression in Lmo2-transgenic thymocytes and that abrogation of Kit signaling phenocopied loss of Hhex in abolishing the transplantation capacity and radioresistance of these cells. Thus, targeting the Kit signaling pathway may facilitate the eradication of leukemia-initiating cells in immature T-cell leukemias in which it is expressed.

HHEX promotes hepatic-lineage specification through the negative regulation of eomesodermin

Human embryonic stem cells (hESCs) could provide a major window into human developmental biology, because the differentiation methods from hESCs mimic human embryogenesis. We previously reported that the overexpression of hematopoietically expressed homeobox (HHEX) in the hESC-derived definitive endoderm (DE) cells markedly promotes hepatic specification. However, it remains unclear how HHEX functions in this process. To reveal the molecular mechanisms of hepatic specification by HHEX, we tried to identify the genes directly targeted by HHEX. We found that HHEX knockdown considerably enhanced the expression level of eomesodermin (EOMES). In addition, HHEX bound to the HHEX response element located in the first intron of EOMES. Loss-of-function assays of EOMES showed that the gene expression levels of hepatoblast markers were significantly upregulated, suggesting that EOMES has a negative role in hepatic specification from the DE cells. Furthermore, EOMES exerts its effects downstream of HHEX in hepatic specification from the DE cells. In conclusion, the present results suggest that HHEX promotes hepatic specification by repressing EOMES expression.

The HHEX gene is not related to congenital heart disease in 296 Chinese patients

BACKGROUND

The hematopoietically expressed homeobox (HHEX) gene is an important determinant of mammalian heart development. This study aimed to identify the potential mutations of the gene in Chinese patients with congenital heart disease (CHD).

METHODS

We collected 296 CHD patients and 200 controls, and classified the cardiac deformities. Then we conducted sequence analyses of the HHEX gene in those patients.

RESULTS

In all the CHD patients, we did not find any causative mutations in the coding region of the HHEX gene.

CONCLUSION

To our knowledge, this is the first study to examine the HHEX gene in non-symptomatic CHD cases, and this has expanded our knowledge about its etiology.

Regulation of liver growth by glypican 3, CD81, hedgehog, and Hhex

Previous studies from our laboratory have found glypican 3 (GPC3) as a negative regulator of growth. CD81 was found to be a binding partner for GPC3, and its expression and co-localization with GPC3 increased at the end of hepatocyte proliferation. However, the mechanisms through which these two molecules might regulate liver regeneration are not known. We tested the hypothesis that GPC3 down-regulates the hedgehog (HH) signaling pathway by competing with patched-1 for HH binding. We found decreased GPC3-Indian HH binding at peak proliferation in mice followed by increase in glioblastoma 1 protein (effector of HH signaling). We performed a yeast two-hybrid assay and identified hematopoietically expressed homeobox (Hhex, a known transcriptional repressor) as a binding partner for CD81. We tested the hypothesis that Hhex binding to CD81 keeps it outside the nucleus. However, when GPC3 binds to CD81, CD81-Hhex binding decreases, resulting in nuclear translocation of Hhex and transcriptional repression. In support of this, we found decreased GPC3-CD81 binding at hepatocyte proliferation peak, increased CD81-Hhex binding, and decreased nuclear Hhex. GPC3 transgenic mice were used as an additional tool to test our hypothesis. Overall, our data suggest that GPC3 down-regulates cell proliferation by binding to HH and down-regulating the HH signaling pathway and binding with CD81, thus making it unavailable to bind to Hhex and causing its nuclear translocation.

Study of base pair mutations in proline-rich homeodomain (PRH)-DNA complexes using molecular dynamics

Proline-rich homeodomain (PRH) is a regulatory protein controlling transcription and gene expression processes by binding to the specific sequence of DNA, especially to the sequence 5'-TAATNN-3'. The impact of base pair mutations on the binding between the PRH protein and DNA is investigated using molecular dynamics and free energy simulations to identify DNA sequences that form stable complexes with PRH. Three 20-ns molecular dynamics simulations (PRH-TAATTG, PRH-TAATTA and PRH-TAATGG complexes) in explicit solvent water were performed to investigate three complexes structurally. Structural analysis shows that the native TAATTG sequence forms a complex that is more stable than complexes with base pair mutations. It is also observed that upon mutation, the number and occupancy of the direct and water-mediated hydrogen bonds decrease. Free energy calculations performed with the thermodynamic integration method predict relative binding free energies of 0.64 and 2 kcal/mol for GC to AT and TA to GC mutations, respectively, suggesting that among the three DNA sequences, the PRH-TAATTG complex is more stable than the two mutated complexes. In addition, it is demonstrated that the stability of the PRH-TAATTA complex is greater than that of the PRH-TAATGG complex.

The dual roles of homeobox genes in vascularization and wound healing

Homeobox genes represent a family of highly conserved transcription factors originally discovered to regulate organ patterning during development. More recently, several homeobox genes were shown to affect processes in adult tissue, including angiogenesis and wound healing. Whereas a subset of members of the Hox-family of homeobox genes activate growth and migration to promote angiogenesis or wound healing, other Hox genes function to restore or maintain quiescent, differentiated tissue function. Pathological tissue remodeling is linked to differential expression of activating or stabilizing Hox genes and dysregulation of Hox expression can contribute to disease progression. Studies aimed at understanding the role and regulation of Hox genes have provided insight into how these potent morphoregulatory genes can be applied to enhance tissue engineering or limit cancer progression.

Heterogeneity in white blood cells has potential to confound DNA methylation measurements

Epigenetic studies are commonly conducted on DNA from tissue samples. However, tissues are ensembles of cells that may each have their own epigenetic profile, and therefore inter-individual cellular heterogeneity may compromise these studies. Here, we explore the potential for such confounding on DNA methylation measurement outcomes when using DNA from whole blood. DNA methylation was measured using pyrosequencing-based methodology in whole blood (n = 50-179) and in two white blood cell fractions (n = 20), isolated using density gradient centrifugation, in four CGIs (CpG Islands) located in genes HHEX (10 CpG sites assayed), KCNJ11 (8 CpGs), KCNQ1 (4 CpGs) and PM20D1 (7 CpGs). Cellular heterogeneity (variation in proportional white blood cell counts of neutrophils, lymphocytes, monocytes, eosinophils and basophils, counted by an automated cell counter) explained up to 40% (p<0.0001) of the inter-individual variation in whole blood DNA methylation levels in the HHEX CGI, but not a significant proportion of the variation in the other three CGIs tested. DNA methylation levels in the two cell fractions, polymorphonuclear and mononuclear cells, differed significantly in the HHEX CGI; specifically the average absolute difference ranged between 3.4-15.7 percentage points per CpG site. In the other three CGIs tested, methylation levels in the two fractions did not differ significantly, and/or the difference was more moderate. In the examined CGIs, methylation levels were highly correlated between cell fractions. In summary, our analysis detects region-specific differential DNA methylation between white blood cell subtypes, which can confound the outcome of whole blood DNA methylation measurements. Finally, by demonstrating the high correlation between methylation levels in cell fractions, our results suggest a possibility to use a proportional number of a single white blood cell type to correct for this confounding effect in analyses.

Overexpression of hematopoietically expressed homeoprotein induces nonapoptotic cell death in mouse prechondrogenic ATDC5 cells

Physiological cell death is an essential event in normal development and maintenance of homeostasis. Recently, the morphological and pharmacological characteristics of programmed cell death, which are distinct from those of apoptosis under physiological and pathological conditions, have been reported. However, the molecular mechanism and executioner of this type of cell death are unknown. We show that overexpression of hematopoietically expressed homeoprotein (Hex), a homeoprotein of divergent type, and enhanced green fluorescent protein (EGFP) fusion protein (Hex-EGFP) induces cell death in mouse chondrogenic cell line ATDC5. The expression rate of Hex-EGFP decreased more rapidly than that of EGFP 96 h after transfection. The time-lapse image of living cells revealed the Hex-EGFP-positive cells rapidly died in a necrosis-like fashion. The nuclei of Hex-EGFP-expressing cells were rarely fragmented; however, these cells were negative for terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) staining. The expression rate of Hex-EGFP clearly increased by treatment with radical scavengers, propyl gallate and butylated hydroxyanisole, slightly increased with a caspase inhibitor, zVAD-fmk, and was not affected by N-acetyl cysteine in ATDC5 cells. A fluorescent probe indicated that reactive oxygen species (ROS) were localized near the nuclei in Hex-EGFP-positive cells. In differentiated ATDC5 cells, as hypertrophic chondrocyte-like cells, the expression rate of Hex-EGFP increased above that in uninduced ATDC5 cells. These results suggest that Hex induces nonapoptotic cell death through local accumulation of reactive oxygen species, and mature chondrocytes, which express Hex, might be able to escape cell death induced by Hex in cartilage.

Homeoprotein Hex is expressed in mouse developing chondrocytes

Endochondral ossification is a complex process involving the formation of cartilage and the subsequent replacement by mineralized bone. Although the proliferation and differentiation of chondrocytes are strictly regulated, the molecular mechanisms involved are not completely understood. Here, we show that a divergent-type homeobox gene, hematopoietically expressed homeobox gene (HEX), is expressed in mouse chondrogenic cell line ATDC5. The expression of Hex protein drastically increased during differentiation. The chondrogenic differentiation-enhanced expression of Hex protein was also observed in chondrocytes in the tibia of embryonic day 15.5 (E15.5) mouse embryos. The localization of Hex protein in the chondrocytes of the tibia changed in association with maturation; namely, there was Hex protein in the cytoplasm near the endoplasmic reticulum (ER) in resting chondrocytes, which moved to the nucleus in prehypertrophic chondrocytes, and thereafter entered the ER in hypertrophic chondrocytes. These results suggest Hex expression and subcellular localization are associated with chondrocyte maturation.

DNA compaction by the higher-order assembly of PRH/Hex homeodomain protein oligomers

Protein self-organization is essential for the establishment and maintenance of nuclear architecture and for the regulation of gene expression. We have shown previously that the Proline-Rich Homeodomain protein (PRH/Hex) self-assembles to form oligomeric complexes that bind to arrays of PRH binding sites with high affinity and specificity. We have also shown that many PRH target genes contain suitably spaced arrays of PRH sites that allow this protein to bind and regulate transcription. Here, we use analytical ultracentrifugation and electron microscopy to further characterize PRH oligomers. We use the same techniques to show that PRH oligomers bound to long DNA fragments self-associate to form highly ordered assemblies. Electron microscopy and linear dichroism reveal that PRH oligomers can form protein-DNA fibres and that PRH is able to compact DNA in the absence of other proteins. Finally, we show that DNA compaction is not sufficient for the repression of PRH target genes in cells. We conclude that DNA compaction is a consequence of the binding of large PRH oligomers to arrays of binding sites and that PRH is functionally and structurally related to the Lrp/AsnC family of proteins from bacteria and archaea, a group of proteins formerly thought to be without eukaryotic equivalents.

Development of the endothelium: an emphasis on heterogeneity

The endothelium is composed of specialized epithelial cells that line the vasculature, the lymph vessels, and the heart. These endothelial cells are characterized by their stratification and are connected via intercellular junctions that confer specific permeability. Although all endothelium acts as a barrier, considerable heterogeneity exists among different organs and even within vessels. During development, the endothelial cells are specified before they migrate to their final destination, and then they commit to an arterial or venous fate. From the venous endothelial cell population, a subset of cells is further specified as lymphatic endothelium. The endothelium can be highly permeable, as in the lymph vessels, or impenetrable, as in the blood-brain barrier. These differences arise during development and are orchestrated through a series of signaling pathways. This review details how endothelial cells arise and are directed to their specific fate, specifically targeting what differentiates endothelial populations.

PRH/Hhex controls cell survival through coordinate transcriptional regulation of vascular endothelial growth factor signaling

The proline-rich homeodomain protein (PRH) plays multiple roles in the control of gene expression during embryonic development and in the adult. Vascular endothelial growth factor (VEGF) is a mitogen that stimulates cell proliferation and survival via cell surface receptors including VEGFR-1 and VEGFR-2. VEGF signaling is of critical importance in angiogenesis and hematopoiesis and is elevated in many tumors. Here we show that PRH binds directly to the promoter regions of the Vegf, Vegfr-1, and Vegfr-2 genes and that in each case PRH represses transcription. We demonstrate that overexpression or knockdown of PRH directly impinges on the survival of both leukemic and tumor cells and that the modulation of VEGF and VEGF receptor signaling by PRH mediates these effects. Our findings demonstrate that PRH is a key regulator of the VEGF signaling pathway and describe a mechanism whereby PRH plays an important role in tumorigenesis and leukemogenesis.

The homeobox gene Hex regulates hepatocyte differentiation from embryonic stem cell-derived endoderm

We investigated the role of the hematopoietically expressed homeobox (Hex) in the differentiation and development of hepatocytes within embryonic stem cell (ESC)-derived embryoid bodies (EBs). Analyses of hepatic endoderm derived from Hex(-/-) EBs revealed a dramatic reduction in the levels of albumin (Alb) and alpha-fetoprotein (Afp) expression. In contrast, stage-specific forced expression of Hex in EBs from wild-type ESCs led to the up-regulation of Alb and Afp expression and secretion of Alb and transferrin. These inductive effects were restricted to c-kit(+) endoderm-enriched EB-derived populations, suggesting that Hex functions at the level of hepatic specification of endoderm in this model. Microarray analysis revealed that Hex regulated the expression of a broad spectrum of hepatocyte-related genes, including fibrinogens, apolipoproteins, and cytochromes. When added to the endoderm-induced EBs, bone morphogenetic protein 4 acted synergistically with Hex in the induction of expression of Alb, Afp, carbamoyl phosphate synthetase, transcription factor 1, and CCAAT/enhancer binding protein alpha. These findings indicate that Hex plays a pivotal role during induction of liver development from endoderm in this in vitro model and suggest that this strategy may provide important insight into the generation of functional hepatocytes from ESCs.

Interaction between Hhex and SOX13 modulates Wnt/TCF activity

Fine-tuning of the Wnt/TCF pathway is crucial for multiple embryological processes, including liver development. Here we describe how the interaction between Hhex (hematopoietically expressed homeobox) and SOX13 (SRY-related high mobility group box transcription factor 13), modulates Wnt/TCF pathway activity. Hhex is a homeodomain factor expressed in multiple endoderm-derived tissues, like the liver, where it is essential for proper development. The pleiotropic expression of Hhex during embryonic development and its dual role as a transcriptional repressor and activator suggest the presence of different tissue-specific partners capable of modulating its activity and function. While searching for developmentally regulated Hhex partners, we set up a yeast two-hybrid screening using an E9.5-10.5 mouse embryo library and the N-terminal domain of Hhex as bait. Among the putative protein interactors, we selected SOX13 for further characterization. We found that SOX13 interacts directly with full-length Hhex, and we delineated the interaction domains within the two proteins. SOX13 is known to repress Wnt/TCF signaling by interacting with TCF1. We show that Hhex is able to block the SOX13-dependent repression of Wnt/TCF activity by displacing SOX13 from the SOX13 x TCF1 complex. Moreover, Hhex de-repressed the Wnt/TCF pathway in the ventral foregut endoderm of cultured mouse embryos electroporated with a SOX13-expressing plasmid. We conclude that the interaction between Hhex and SOX13 may contribute to control Wnt/TCF signaling in the early embryo.

A mechanism of nucleocytoplasmic trafficking for the homeodomain protein PRH

Proline-rich homeodomain (PRH)/hematopoietically expressed homeodomain (Hex) is a homeodomain protein that plays an important role in early embryonic patterning and hematopoiesis. PRH can act as either a tumor suppressor or an oncogene and its expression is dysregulated in certain types of lymphoid and myeloid leukemias. Aberrant exclusion of PRH from the nuclei has been associated with thyroid and breast cancers and a subset of myeloid leukemias. Accordingly, nuclear localization of PRH was found to be necessary for the inhibition of eIF4E-dependent transformation. Since PRH's nuclear-cytoplasmic localization has been associated with neoplastic transformation we sought to better understand how PRH is transported to the nuclear compartment. Here, we report an essential element that controls the mechanism of PRH nucleocytoplasmic trafficking, namely that it is imported into the nuclei by Karyopherin/Importin 7. Kap7 was identified as a binding partner for PRH in a GST-pull down from a HeLa cell protein lysate, followed by mass-spectrometry. The Kap7-PRH complex is dissociated in the presence of RanGTP, as expected for a nuclear import complex. Kap7 can bind directly to PRH in a GST-pull down assay with purified proteins, as well as mediates the transport of PRH to the nuclear compartment in a digitonin permeabilized cells assay. Finally, in vivo depletion of Kap7 dramatically reduces accumulation of PRH in the nucleus. Our data open the way for investigations of the mechanism of perturbed PRH localization in tumors and possible therapeutic interventions.