Defective placental vasculogenesis causes embryonic lethality in VHL-deficient mice

Loss of pVHL is sufficient to cause HIF dysregulation in primary cells but does not promote tumor growth

Inactivation of the von Hippel-Lindau (VHL) gene is associated with the development of highly vascularized tumors. pVHL targets the alpha subunits of hypoxia inducible factor (HIF) for ubiquitin-mediated degradation in an oxygen-dependent manner. Although pVHL-deficient tumor cell lines demonstrate constitutive stabilization and activation of HIF, it has yet to be shown that loss of murine Vhl alone is sufficient to dysregulate HIF. We utilized a genetic approach to demonstrate that loss of Vhl is sufficient not only to stabilize HIF-alpha subunits under normoxia, but also fully activate HIF-mediated responses. These studies have implications for the hierarchy of signaling events leading to HIF stabilization, nuclear translocation, and target gene expression. We further demonstrate that loss of murine Vhl does not promote teratocarcinoma growth, indicating that other genetic changes must occur to facilitate Vhl-mediated tumorigenesis.

HIF and oxygen sensing; as important to life as the air we breathe?

Molecular oxygen (O2)is a basic requirement for cellular growth and viability and many aspects of anatomy and physiology are dedicated to achieving reliable distribution. Recent work has identified a specific sensing and response system, centred around a transcription complex called Hypoxia-inducible Factor 1 (HIF-1), which forms the focus of this review. The HIF-system operates in all cell types and modulates a very broad range of cellular pathways, consistent with the broad importance of oxygen. It is implicated in a rapidly expanding range of developmental, physiological and pathological settings, and is potentially relevant to almost all areas of clinical medicine. Excitingly, the pathway can be activated with low molecular weight compounds which should offer therapeutic benefit, especially in diseases where oxygen supply is compromised.

Highly penetrant, rapid tumorigenesis through conditional inversion of the tumor suppressor gene Snf5

Recent data suggest the SWI/SNF chromatin remodeling complex may also act as a tumor suppressor. Utilizing a reversibly inactivating conditional allele, we demonstrate that loss of Snf5/Ini1/Baf47/SmarcB1, a core subunit of SWI/SNF, results in highly penetrant cancer predisposition with 100% of mice developing mature CD8(+) T cell lymphoma or rare rhabdoid tumors with a median onset of only 11 weeks. Notably, while loss of Snf5 predisposes to aggressive cancers, it is also required for survival of virtually all nonmalignant cells in vivo. Reversible gene targeting demonstrates a critical and specific role for Snf5 in tumor suppression, provides a novel system in which to explore the genetic pathways involved in tumor suppression by Swi/Snf, and should be of wide use in evaluating other essential tumor suppressor genes.

Tumor suppressor genes on chromosome 3p involved in the pathogenesis of lung and other cancers

Loss of heterozygosity (LOH) involving several chromosome 3p regions accompanied by chromosome 3p deletions are detected in almost 100% of small (SCLCs) and more than 90% of non-small (NSCLCs) cell lung cancers. In addition, these changes appear early in the pathogenesis of lung cancer and are found as clonal lesions in the smoking damaged respiratory epithelium including histologically normal epithelium as well as in epithelium showing histologic changes of preneoplasia. These 3p genetic alterations lead to the conclusion that the short arm of human chromosome 3 contains several tumor suppressor gene(s) (TSG(s)). Although the first data suggesting that 3p alterations were involved in lung carcinogenesis were published more than 10 years ago, only recently has significant progress been achieved in identifying the candidate TSGs and beginning to demonstrate their functional role in tumor pathogenesis. Some of the striking results of these findings has been the discovery of multiple 3p TSGs and the importance of tumor acquired promoter DNA methylation as an epigenetic mechanism for inactivating the expression of these genes in lung cancer. This progress, combined with the well known role of smoking as an environmental causative risk factor in lung cancer pathogenesis, is leading to the development of new diagnostic and therapeutic strategies which can be translated into the clinic to combat and prevent the lung cancer epidemic. It is clear now that genetic and epigenetic abnormalities of several genes residing in chromosome region 3p are important for the development of lung cancers but it is still obscure how many of them exist and which of the numerous candidate TSGs are the key players in lung cancer pathogenesis. We review herein our current knowledge and describe the most credible candidate genes.

Genetic and functional analysis of the von Hippel-Lindau (VHL) tumour suppressor gene promoter

The VHL gatekeeper tumour suppressor gene is inactivated in the familial cancer syndrome von Hippel-Lindau disease and in most sporadic clear cell renal cell carcinomas. Recently the VHL gene product has been identified as a specific component of a SCF-like complex, which regulates proteolytic degradation of the hypoxia inducible transcription factors HIF-1 and HIF-2. pVHL is critical for normal development and mRNA expression studies suggest a role in nephrogenesis. Despite the importance of VHL in oncogenesis and development, little is known about the regulation of VHL expression. To investigate VHL promoter activity, we performed comparative sequence analysis of human, primate, and rodent 5' VHL sequences. We then proceeded to deletion analysis of regions showing significant evolutionary conservation between human and rat promoter sequences, and defined two positive and one negative regulatory regions. Analysis of specific putative transcription factor binding sites identified a functional Sp1 site, which was shown to be a regulatory element. Overlapping Sp1/AP2 sites were also identified and candidate E2F1 binding sites evaluated. Three binding sites for as yet unidentified transcription factors were mapped also. These investigations provide a basis for elucidating the regulation of VHL expression in development, the molecular pathology of epigenetic silencing of VHL in tumourigenesis, and suggest a possible link between Sp1, VHL, and nephrogenesis.

Somatic point mutation of the wild-type allele detected in tumors of patients with VHL germline deletion

The majority of patients with Von Hippel-Lindau (VHL) disease are affected by a VHL germline mutation involving one copy of the VHL gene. Loss of heterozygosity of the second VHL allele can be consistently demonstrated in tumor tissue from these patients, suggesting that allelic deletion is a very early or even initiating event for tumorigenesis. Approximately 20% of VHL disease patients, however, exhibit germline deletion of one entire copy or at least a substantial part of the VHL gene. To investigate the nature of the "second genetic hit" in this patient population, we analysed two renal cell carcinomas and one CNS hemangioblastoma from three unrelated patients for genetic changes of the second copy of the VHL gene. All three tumors showed retention of one VHL allele by FISH. Single-strand conformation polymorphism and mutation analysis of microdissected tumor DNA revealed somatic point mutations of the wild-type VHL copies in each of the three tumors. The results indicate that the "two hit model" is equally applicable to patients with VHL germline mutation and VHL germline deletion. In contrast to tumors from patients with VHL germline mutation, however, point mutations of the wild-type allele can be detected in tumors from patients with VHL germline deletion.

Oxygen sensors and angiogenesis

Local oxygen tension has a profound effect on the vasculature, which compensates vascular insufficiency through the induction of angiogenesis. An important mediator in this process is the hypoxia-inducible factor (HIF) complex, which is activated in hypoxic cells and increases transcription of a broad range of genes including angiogenic growth factors such as VEGF. HIF is primarily regulated through oxygen-dependent proteasomal destruction of the regulatory subunit, HIF-1 alpha or HIF-2 alpha. Regulation is through the modification of specific prolines in HIF- alpha chains which are hydroxylated by a recently identified family of enzymes which require molecular oxygen and 2-oxoglutarate as cosubstrates, and iron as a cofactor. Following modification HIF- alpha chains are captured by a ubiquitin ligase E3 complex containing the von Hippel-Lindau (VHL) tumour suppressor protein. The HIF prolyl hydroxylases (PHD enzymes) act as oxygen sensors regulating HIF, and hence angiogenesis. The PHD-HIF-VHL system provides a range of opportunities for therapeutic manipulation.

Animal models of tumor-suppressor genes

The development of cancer requires multiple genetic alterations perturbing distinct cellular pathways. In human cancers, these alterations often arise owing to mutations in tumor-suppressor genes whose normal function is to either inhibit the proliferation, apoptosis, or differentiation of cells, or maintain their genomic integrity. Mouse models for tumor suppressors frequently provide definitive evidence for the antitumorigenic functions of these genes. In addition, animal models permit the identification of previously unsuspected roles of these genes in development and differentiation. The availability of null and tissue-specific mouse mutants for tumor-suppressor genes has greatly facilitated our understanding of the mechanisms leading to cancer. In this review, we describe mouse models for tumor-suppressor genes.

Hypoxia-inducible factor and the development of stem cells of the cardiovascular system

Decreased oxygen (O2) levels activate hypoxia-inducible factor (HIF-1) to induce genes involved in glycolysis, glucose transport, erythropoiesis, and angiogenesis. Mutations in various HIF-1 subunits have contributed to our understanding of the role hypoxia plays during early embryonic development in general and the cardiovascular system in particular. We propose that HIF-1 is important for the generation, proliferation, maintenance, and differentiation of the early cardiovascular system. Understanding aberrations in these hypoxic responses is important since they contribute to serious human disease such as ischemia and tumorigenesis. In this review we will focus on the critical role of O2 in regulating cardiovascular events during early embryonic development.

Renal cancer: molecular mechanisms and newer therapeutic options

Renal cell carcinomas account for 80-85% of all primary renal neoplasms. Recent identification of VHL, c-met and TSC as candidate genes mutated in various types of renal carcinomas has greatly enhanced our understanding of the pathogenesis of renal carcinomas and has provided novel therapeutic options for patients with renal cancer. Furthermore, developments in angiogenesis and in tumor immunology have given us additional treatment modalities for cancer patients, especially those with renal cancer. This review highlights the genetic abnormalities seen in renal cell carcinomas and reviews current and future therapeutic options.

Playing Tag with HIF: The VHL Story

Inactivation of the von Hippel-Lindau (VHL) tumour suppressor gene product pVHL is the cause of inherited VHL disease and is associated with sporadic kidney cancer. pVHL is found in a multiprotein complex with elongins B/C, Cul2, and Rbx1 forming an E3 ubiquitin ligase complex called VEC. This modular enzyme targets the alpha subunits of hypoxia-inducible factor (HIF) for ubiquitin-mediated destruction. Consequently, tumour cells lacking functional pVHL overproduce the products of HIF-target genes such as vascular endothelial growth factor (VEGF), which promotes angiogenesis. This likely accounts for the hypervascular nature of VHL-associated neoplasms. Although pVHL has been linked to the cell-cycle, differentiation, and the regulation of extracellular matrix assembly, microenvironment pH, and tissue invasiveness, this review will focus on the recent insights into the molecular mechanisms governing the E3 ubiquitin ligase function of VEC.

Comparative developmental anatomy of the murine and human definitive placentae

The placenta of eutherian mammals is a remarkable biological structure. It is composed of both zygote-derived and maternal cells, and mediates the complex interactions between the mother and the fetus that are necessary for fetal growth and survival. While the genetic basis of human placental development and function is largely unknown, its understanding is of immense clinical importance because placentopathies of unknown genetic aetiology are thought to be the cause of many types of pregnancy complications including unexplained miscarriage and intrauterine growth retardation. The mouse is the best-studied mammalian experimental genetic model system and research is not restricted by the inherent ethical and practical limitations associated with the human. As a result, knowledge about the genetic control of mouse placental development has expanded greatly in recent years. In order for this to be of benefit to medical practice, extrapolations from murine to human placentation have to be made. However, comprehensive comparisons of the placentae of these two species are rare. This review therefore compares the developmental anatomy of the placenta between humans and mice with emphasis on structures and cell types that might be analogous between the two species. This could be of particular benefit to mouse developmental geneticists who study placental development and have an interest in the possible clinical implications of their work.

Hypoxia: the tumor's gateway to progression along the angiogenic pathway

Decreased aerobic (hypoxic) conditions in tumors induce the release of cytokines that promote vascularization and thereby enhance tumor growth and metastasis. Recent major advances have provided insight into the role hypoxia plays in cancer biology. The domain structure of the hypoxia-inducible factor 1alpha (HIF-1alpha) has been elucidated, as has the mechanism by which stabilization of HIF-1alpha leads to initiation of the transcription of target genes involved in growth of blood vessels.

Use of transgenic mice model for understanding the placentation: towards clinical applications in human obstetrical pathologies?

The mammalian embryo and fetus are unable to develop without a well-established, functional placenta. This transitory yet indispensable structure attaches the conceptus to the uterus and establishes the vascular connections necessary for nutrient and gaseous exchange between maternal and fetal compartments. Genetic targeting strategy allows the generation of mice lacking a specific gene. Such approaches reveal: (i) the high incidence of mutant embryonic or fetal death in utero, and (ii) the extraembryonic (placental) causes of these deaths. Due to the similarities presented between mouse and human placenta, we propose to use the potential of mouse targeting experiments as a model in order to understand human obstetrical pathologies. In this paper, we first review genes that have been demonstrated to be required in mice for implantation, choriovitelline and chorioallantoic placentation. Using examples (integrins, homeoboxs, hepatocyte growth factor or epidermal growth factor receptor...) we demonstrate the reality and efficiency of such an approach. Other candidate genes (receptor of leukemia inhibitory factor, Wnt2 or retinoic acid receptor alpha...) in order to understand, prevent and treat human obstetrical pathologies.

Vascular abnormalities and deregulation of VEGF in Lkb1-deficient mice

The LKB1 tumor suppressor gene, mutated in Peutz-Jeghers syndrome, encodes a serine/threonine kinase of unknown function. Here we show that mice with a targeted disruption of Lkb1 die at midgestation, with the embryos showing neural tube defects, mesenchymal cell death, and vascular abnormalities. Extraembryonic development was also severely affected; the mutant placentas exhibited defective labyrinth layer development and the fetal vessels failed to invade the placenta. These phenotypes were associated with tissue-specific deregulation of vascular endothelial growth factor (VEGF) expression, including a marked increase in the amount of VEGF messenger RNA. Moreover, VEGF production in cultured Lkb1(-/-) fibroblasts was elevated in both normoxic and hypoxic conditions. These findings place Lkb1 in the VEGF signaling pathway and suggest that the vascular defects accompanying Lkb1 loss are mediated at least in part by VEGF.

Placental development: lessons from mouse mutants

The placenta is the first organ to form during mammalian embryogenesis. Problems in its formation and function underlie many aspects of early pregnancy loss and pregnancy complications in humans. Because the placenta is critical for survival, it is very sensitive to genetic disruption, as reflected by the ever-increasing list of targeted mouse mutations that cause placental defects. Recent studies of mouse mutants with disrupted placental development indicate that signalling interactions between the placental trophoblast and embryonic cells have a key role in placental morphogenesis. Furthering our understanding of mouse trophoblast development should provide novel insights into human placental function.

Treatment prospects for autosomal-dominant polycystic kidney disease

An increased understanding of the molecular genetic and cellular pathophysiologic mechanisms responsible for the development of autosomal-dominant polycystic kidney disease (ADPKD), made possible by the advances in molecular biology and genetics of the last three decades, has laid the foundation for the development of effective therapies. As the concept that a polycystic kidney is a neoplasm in disguise is becoming increasingly accepted, the development of therapies for ADPKD may benefit greatly from the expanding body of information on cancer chemoprevention and chemosuppression. This review summarizes the observations that already have been made and discusses therapies for PKD that deserve investigation.

Human cytotrophoblast expression of the von Hippel-Lindau protein is downregulated during uterine invasion in situ and upregulated by hypoxia in vitro

The von Hippel-Lindau tumor-suppressor protein (pVHL) regulates the stability of HIF1 alpha and HIF2 alpha and thus is pivotal in cellular responses to changes in oxygen tension. Paradoxically, human cytotrophoblasts proliferate under hypoxic conditions comparable to those measured in the early gestation placenta (2% O(2)), but differentiate into tumorlike invasive cells under well-oxygenated conditions such as those found in the uterus. We sought to explain this phenomenon in terms of pVHL expression. In situ, pVHL immunolocalized to villous cytotrophoblast stem cells, and expression was enhanced at sites of cell column initiation; in both of these relatively hypoxic locations, cytoplasmic staining for HIF2 alpha was also detected. As cytotrophoblasts attached to and invaded the uterus, which results in their increased exposure to oxygen, pVHL staining was abruptly downregulated concordant with localization of HIF2 alpha to the nucleus. In vitro, hypoxia (2% O(2)) upregulated cytotrophoblast pVHL expression together with HIF2 alpha, which localized to the cytoplasm; culture under well-oxygenated conditions greatly reduced levels of both molecules. These results, together with the placental defects previously observed in VHL(-/-) mice, suggest that pVHL is a component of the mechanism that transduces local differences in oxygen tension at the maternal-fetal interface to changes in the biological behavior of cytotrophoblasts. Furthermore, these data provide the first example of oxygen-dependent changes in pVHL abundance.

Genes governing placental development

The placenta is essential for fetal growth because it promotes the delivery of nutrients and oxygen from the maternal circulation. In mice, many gene mutations disrupt formation of the placenta, with specific effects at different times and on different components. Studies of these mutations are beginning to provide insights into both the molecular pathways required for formation of different placental substructures and the nature of intercellular interactions, between trophoblast, mesenchymal and vascular components, that regulate placental development. Conserved gene expression patterns in humans should enable the elucidation of the molecular basis of human placental dysfunction.

VHL tumor suppressor regulates Cl-/HCO3- exchange and Na+/H+ exchange activities in renal carcinoma cells

Mutations in the von Hippel-Lindau (VHL) tumor suppressor gene are thought to play a critical role in the pathogenesis of both sporadic and VHL disease-associated clear-cell renal carcinomas (RCC). Differential display-PCR identified the AE2 anion exchanger as a candidate VHL target gene. AE2 mRNA and polypeptide levels were approximately threefold higher in 786-O VHL cells than in 786-O Neo cells. In contrast, Cl(-)/HCO(3)(-) exchange activity in 786-O VHL cells was 50% lower than in 786-O Neo cells. Since resting intracellular pH (pH(i)) values were indistinguishable, we postulated that Na(+)/H(+) exchange activity (NHE) might be similarly reduced in 786-O VHL cells. NHE-mediated pH(i) recovery from acid load was less than 50% that in 786-O Neo cells, whereas hypertonicity-stimulated, amiloride-sensitive NHE was indistinguishable in the two cell lines. The NHE3 mRNA level was higher in 786-O VHL than 786-O Neo cells, but NHE1 mRNA levels did not differ. AE2 and NHE3 are the first transcripts reported to be upregulated by pVHL. Elucidation of mechanisms responsible for downregulation of both ion exchange activities will require further investigation.

The von Hippel-Lindau tumor suppressor gene

Germline mutations of the von Hippel-Lindau tumor suppressor gene (VHL) in humans causes a hereditary cancer syndrome characterized by the development of retinal and central nervous system hemangioblastomas. Other tumors associated with von Hippel-Lindau disease include clear cell renal carcinomas and pheochromocytomas. Tumor development in this setting is due to functional loss of the remaining wild-type VHL allele. Biallelic VHL inactivation is also common in nonhereditary hemangioblastomas and clear cell renal carcinomas, in keeping with Knudson's 2-Hit Model of carcinogenesis. The VHL gene product, pVHL, is a component of an E3 ubiquitin ligase that targets the alpha subunits of the HIF (hypoxia-inducible factor) transcription factor for destruction in the presence of oxygen. Consequently, tumor cells lacking pVHL overproduce the products of HIF target genes such as vascular endothelial growth factor and transforming growth factor alpha. pVHL has been implicated in a variety of processes that are central to carcinogenesis including cell-cycle control, differentiation, extracellular matrix formation and turnover, and angiogenesis.

Vascular tumors in livers with targeted inactivation of the von Hippel-Lindau tumor suppressor

von Hippel-Lindau (VHL) disease is a pleomorphic familial tumor syndrome that is characterized by the development of highly vascularized tumors. Homozygous disruption of the VHL gene in mice results in embryonic lethality. To investigate VHL function in the adult we have generated a conditional VHL null allele (2-lox allele) and null allele (1-lox allele) by Cre-mediated recombination in embryonic stem cells. We show here that mice heterozygous for the 1-lox allele develop cavernous hemangiomas of the liver, a rare manifestation of the human disease. Histologically these tumors were associated with hepatocellular steatosis and focal proliferations of small vessels. To study the cellular origin of these lesions we inactivated VHL tissue-specifically in hepatocytes. Deletion of VHL in the liver resulted in severe steatosis, many blood-filled vascular cavities, and foci of increased vascularization within the hepatic parenchyma. These histopathological changes were similar to those seen in livers from mice heterozygous for the 1-lox allele. Hypoxia-inducible mRNAs encoding vascular endothelial growth factor, glucose transporter 1, and erythropoietin were up-regulated. We thus provide evidence that targeted inactivation of mouse VHL can model clinical features of the human disease and underline the importance of the VHL gene product in the regulation of hypoxia-responsive genes in vivo.

The von Hippel-Lindau tumor suppressor protein

The von Hippel-Lindau tumor suppressor protein (pVHL) has been shown to bind directly to the alpha subunits of the heterodimeric transcription factor HIF (hypoxia inducible factor). pVHL directs the polyubiquitination and, hence, destruction of HIF in the presence of oxygen. Loss of pVHL function leads to deregulation of HIF target genes, which play critical roles in angiogenesis.

A mouse model of multiple endocrine neoplasia, type 1, develops multiple endocrine tumors

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant cancer syndrome, characterized primarily by multiple tumors in the parathyroid glands, endocrine pancreas, and anterior pituitary. Other tumors, including gastrinoma, carcinoid, adrenal cortical tumors, angiofibroma, collagenoma, and lipoma, also occur in some patients. Individuals with MEN1 almost always have loss-of-function mutations in the MEN1 gene on chromosome 11, and endocrine tumors arising in these patients usually show somatic loss of the remaining wild-type allele. To examine the role of MEN1 in tumor formation, a mouse model was generated through homologous recombination of the mouse homolog Men1. Homozygous mice die in utero at embryonic days 11.5-12.5, whereas heterozygous mice develop features remarkably similar to those of the human disorder. As early as 9 months, pancreatic islets show a range of lesions from hyperplasia to insulin-producing islet cell tumors, and parathyroid adenomas are also frequently observed. Larger, more numerous tumors involving pancreatic islets, parathyroids, thyroid, adrenal cortex, and pituitary are seen by 16 months. All of the tumors tested to date show loss of the wild-type Men1 allele, further supporting its role as a tumor suppressor gene.

Genes regulating embryonic and fetal survival

Embryonic mortality in both farm animals and humans occurs most frequently during the first few weeks after conception. It can be attributed to abnormalities in the earliest developmental processes during embryogenesis that include implantation, maternal recognition of pregnancy, and formation of the placenta and cardiovascular system. The molecular mechanisms that are essential for all of these early processes are being elucidated at a rapid pace using transgenic and gene knockout approaches in mice. Two important general conclusions have emerged from this work. First, placental defects can occur by a number of different molecular mechanisms and can result from defects in the development or function of its trophoblast, mesenchymal or vascular components. Second, placental and cardiovascular functions are intimately linked. Cells of the placenta, for example, produce hormones that have profound effects on maternal and fetal cardiac and vascular function. In addition, development of the two is linked mechanistically through the use of some genes that are essential for development of both. Understanding the molecular basis of these processes should help to address the major limits to the success of embryo transfer, IVF and embryo cloning practices in livestock species.

Essential role for p38alpha mitogen-activated protein kinase in placental angiogenesis

The p38 family of mitogen-activated protein kinases (MAPKs) mediates signaling in response to environmental stresses and inflammatory cytokines, but the requirements for the p38 MAPK pathway in normal mammalian development have not been elucidated. Here, we show that targeted disruption of the p38alpha MAPK gene results in homozygous embryonic lethality because of severe defects in placental development. Although chorioallantoic placentation is initiated appropriately in p38alpha null homozygotes, placental defects are manifest at 10.5 days postcoitum as nearly complete loss of the labyrinth layer and significant reduction of the spongiotrophoblast. In particular, p38alpha mutant placentas display lack of vascularization of the labyrinth layer as well as increased rates of apoptosis, consistent with a defect in placental angiogenesis. Furthermore, p38alpha mutants display abnormal angiogenesis in the embryo proper as well as in the visceral yolk sac. Thus, our results indicate a requirement for p38alpha MAPK in diploid trophoblast development and placental vascularization and suggest a more general role for p38 MAPK signaling in embryonic angiogenesis.

Inherited epithelial tumors of the kidney: old and new diseases

This review summarizes information on inherited epithelial tumors of the kidney. Emphasis is placed on identifying clinically distinct inherited forms of renal cancer because each distinct clinical syndrome defines a different renal cancer susceptibility gene. So far, two genes that predispose to epithelial cancers of the kidney have been identified, VHL and the MET proto-oncogene. Available evidence suggests that several renal cancer genes remain to be identified.

Tracheal development and the von Hippel-Lindau tumor suppressor homolog in Drosophila

von Hippel-Lindau disease is a hereditary cancer syndrome. Mutations in the VHL tumor suppressor gene predispose individuals to highly vascularized tumors. However, VHL-deficient mice die in utero due to a lack of vascularization in the placenta. To resolve the contradiction, we cloned the Drosophila VHL homologue (d-VHL) and studied its function. It showed an overall 50% similarity to the human counterpart and 76% similarity in the crucial functional domain: the elongin C binding site. The putative d-VHL protein can bind Drosophila elongin C in vitro. During embryogenesis, d-VHL is expressed in the developing tracheal regions where tube outgrowth no longer occurs. Reduced d-VHL activity (using RNA interference methodology) caused breakage of the main vasculature accompanied by excessive looping of smaller branches, whereas over-expression caused a general lack of vasculature. Importantly, human VHL can induce the same gain-of-function phenotypes. VHL is likely involved in halting cell migration at the end of vascular tube outgrowth. Loss of VHL activity can therefore lead to disruption of major vasculature (as in the mouse embryo), which requires precise cell movement and tube fusion, or ectopic outgrowth from existing secondary vascular branches (as in the adult tumors). Oncogene (2000) 19, 2803 - 2811

Molecular basis of angiogenesis. Role of VEGF and VE-cadherin

The formation of new blood vessels (angiogenesis) is essential for embryonic development and contributes to the pathogenesis of numerous disorders. In contrast, insufficient angiogenesis may lead to tissue ischemia and failure. The recent discovery of novel angiogenic molecules has initiated efforts to improve tissue perfusion via therapeutic angiogenesis. However, rational design of such treatment strategies mandates a better understanding of the molecular mechanisms of angiogenesis. In this brief review, the role of a prime angiogenic candidate, namely vascular endothelial growth factor (VEGF) and its homologues, in physiological and pathological angiogenesis will be discussed with particular attention to myocardial ischemia and heart failure. In addition, a novel interaction between the junctional protein vascular endothelial-cadherin (VE-cadherin) and VEGF, essential for the endothelial survival function of VEGF, will be reviewed.

Genetic insights into trophoblast differentiation and placental morphogenesis

The placenta is comprised of an inner vascular network covered by an outer epithelium, called trophoblast, all designed to promote the delivery of nutrients to the fetus. Several specialized trophoblast cell subtypes arise during development to promote this function, including cells that invade the uterus to promote maternal blood flow to the implantation site, and other cells that fuse into a syncytium, expand and fold to increase the surface area for efficient transport. Mutation of many genes in mice results in embryonic mortality or fetal growth restriction due to defects in placental development. Several important principles about placental development have emerged from these studies. First, distinct molecular pathways regulate the differentiation of the various trophoblast cell subtypes. Second, trophoblast proliferation, differentiation and morphogenesis are highly regulated by interactions with adjacent cell types. Finally, the specific classes of mutant phenotypes observed in the placenta of knockout mice resemble those seen in humans that are associated with preeclampsia and intrauterine growth restriction.

Placental failure in mice lacking the mammalian homolog of glial cells missing, GCMa

The GCM family of transcription factors consists of Drosophila melanogaster GCM, an important regulator of gliogenesis in the fly, and its two mammalian homologs, GCMa and GCMb. To clarify the function of these mammalian homologs, we deleted GCMa in mice. Genetic ablation of murine GCMa (mGCMa) is embryonic lethal, with mice dying between 9.5 and 10 days postcoitum. At the time of death, no abnormalities were apparent in the embryo proper. Nervous system development, in particular, was not impaired, as might have been expected in analogy to Drosophila GCM. Instead, placental failure was the cause of death. In agreement with the selective expression of mGCMa in labyrinthine trophoblasts, mutant placentas did not develop a functional labyrinth layer, which is necessary for nutrient and gas exchange between maternal and fetal blood. Only a few fetal blood vessels entered the placenta, and these failed to thrive and branch normally. Labyrinthine trophoblasts did not differentiate. All other layers of the placenta, including spongiotrophoblast and giant cell layer, formed normally. Our results indicate that mGCMa plays a critical role in trophoblast differentiation and the signal transduction processes required for normal vascularization of the placenta.

Hereditary cancer and developmental abnormalities

About 1% of all cancers are hereditary, caused by germ-line mutations in specific cancer-related genes. More than 25 different hereditary cancer syndromes are known, most of them involving mutations in tumor suppressor genes. These genes, which are related to cellular proliferation, might also be involved in differentiation. Hence, the phenotype of hereditary cancer syndromes might include developmental abnormalities, in addition to cancer predisposition. The information summarized here indicates that developmental phenotypes appear in both human patients and mouse models of the various hereditary cancer syndromes. These developmental abnormalities, which involve a variety of tissues and organs, usually lead to embryonic malformation that prevents the birth of viable homozygous offspring, but can also be detected in heterozygotes. In some of the syndromes a correlation exists between tumor types and developmentally affected tissues. Comparison of mice and human phenotypes from both the cancer and the developmental aspects indicates that many of the mouse models mimic the human syndromes. Our analysis indicates that most tumor suppressor genes participate not only in the regulation of cell proliferation, but also in differentiation and embryogenesis.

The Sos1 and Sos2 Ras-specific exchange factors: differences in placental expression and signaling properties

Targeted disruption of both alleles of mouse sos1, which encodes a Ras-specific exchange factor, conferred mid-gestational embryonic lethality that was secondary to impaired placental development and was associated with very low placental ERK activity. The trophoblastic layers of sos1(-/-) embryos were poorly developed, correlating with high sos1 expression in wild-type trophoblasts. A sos1(-/-) cell line, which expressed readily detectable levels of the closely related Sos2 protein, formed complexes between Sos2, epidermal growth factor receptor (EGFR) and Shc efficiently, gave normal Ras.GTP and ERK responses when treated with EGF for < or =10 min and was transformed readily by activated Ras. However, the sos1(-/-) cells were resistant to transformation by v-Src or by overexpressed EGFR and continuous EGF treatment, unlike sos1(+/-) or wild-type cells. This correlated with Sos2 binding less efficiently than Sos1 to EGFR and Shc in cells treated with EGF for > or =90 min or to v-Src and Shc in v-Src-expressing cells, and with less ERK activity. We conclude that Sos1 participates in both short- and long-term signaling, while Sos2-dependent signals are predominantly short-term.

Transgenic mouse models in angiogenesis and cardiovascular disease

Novel gene technologies have allowed us to manipulate the genetic balance of candidate molecules in mice in a controllable manner. Homologous or site-specific recombination in embryonic stem cells allows us to study the consequences of deficiencies, mutations, and conditional or tissue-specific expression of gene products in transgenic mice. These technological breakthroughs have significantly advanced biomedical research and broadened our understanding of the pathophysiological role of candidate disease genes. In addition, gene transfer allows us to test the possible therapeutic use of gene products for gene therapy. A variety of assays have been miniaturized, allowing analysis of cardiovascular physiology in the mouse. With the advent of genome sequencing programmes, these gene technologies provide means of studying gene function in a conclusive manner. Furthermore, disease models can be generated which can be used as test models for (gene) therapy or for the discovery of novel genes using differential gene profiling techniques. The present review will focus on the molecular basis of how blood vessels form (angiogenesis and arteriogenesis) and how they become diseased. A selected number of molecules that have been studied in the authors' laboratory will be reviewed in more detail.

Mice lacking HSP90beta fail to develop a placental labyrinth

The 90 kDa heat-shock proteins (HSP90s) play important roles during stress situations as general chaperones and under physiological conditions in the conformational activation of specific protein substrates. Vertebrates express two cytosolic HSP90s (HSP90alpha and HSP90beta) ubiquitously. We have mutated the Hsp90beta gene in murine embryonic stem cells and generated Hsp90beta mutant mice. Heterozygous animals were phenotypically normal. Interestingly, homozygous embryos developed normally until embryonic day 9.0/9.5. Then, although Hsp90beta is expressed ubiquitously, they exhibited phenotypic abnormalities restricted to the placenta. The mutant concepti failed to form a fetal placental labyrinth and died a day later. Fusion between the allantois and the chorionic plate occurred, allantoic blood vessels invaded the chorion, but then did not expand. Mutant trophoblast cells failed to differentiate into trilaminar labyrinthine trophoblast. Despite conspicuous similarities between HSP90alpha and HSP90beta at the molecular level, our data suggest that HSP90beta has a key role in placenta development that cannot be performed by the endogenous HSP90alpha alone. Analysis of chimeric concepti consisting of mutant embryos and tetraploid embryos or ES cells revealed that wild-type allantois was able to induce mutant trophoblast to differentiate. In contrast, trophoblast wild type at the Hsp90beta locus was unable to differentiate when in contact with mutant allantois. Therefore, the primary defect caused by the Hsp90beta mutation resided in the allantois. The allantois mesoderm is thought to induce trophoblast differentiation. Our results show that Hsp90beta is a necessary component of this induction process.

Tumor suppressor genes in ophthalmology

Tumor suppressor genes have a diversity of functions, but they have in common the property of inhibiting neoplastic transformation. When they become inactivated, a constraint is removed that allows cells to grow inappropriately. Mutations in these genes are now thought to be the initiating events in most cancers. The first tumor suppressor gene was discovered through its role in retinoblastoma, and many other tumor suppressor genes also have important ophthalmic manifestations. The first group of tumor suppressor genes to be discussed are those involved in retinoblastoma and uveal melanoma. These are among the most frequently mutated genes in human cancer and are key regulators of growth and homeostasis. The second group of genes is associated with specific hereditary tumor syndromes with ophthalmic manifestations. These genes function in a variety of molecular pathways and are associated with neoplastic and non-neoplastic abnormalities in restricted tissue distributions. Research on tumor suppressor genes continues to shed light on the molecular pathophysiology of ophthalmic tumors and will increasingly yield diagnostic and therapeutic applications.

Tissue factor is required for uterine hemostasis and maintenance of the placental labyrinth during gestation

We employed a novel mouse line that expresses low levels of human tissue factor (TF) in the absence of murine TF to analyze the role of TF in gestation. Low-TF female mice had a 14-18% incidence of fatal postpartum uterine hemorrhage, suggesting that TF plays an important role in uterine hemostasis. Low-TF female mice mated with low-TF male mice had a 42% incidence of fatal midgestational hemorrhage (n = 41), whereas no fatal midgestational hemorrhages were observed in low-TF female mice mated with wild-type male mice (n = 43). Placentas of low-TF embryos from both low-TF and normal (+/-) TF females were abnormal and contained numerous maternal blood pools in the labyrinth. Placentas of TF null embryos surviving beyond embryonic day 10.5 exhibited similar defects. The mouse maternal-embryonic placental barrier consists of four cellular layers (layers I, II, and III and endothelial cells), where layer I lines the maternal lacunae. Comparison of TF-deficient placentas with control placentas by immunohistochemical and ultrastructural analyses revealed thinning of layer I and a reduction in the number of cellular contacts of layer I trophoblasts spanning the maternal blood space between adjacent trabeculae. These structural changes in low-TF and TF null placentas result in enlarged maternal lacunae, as determined by morphometric analysis, and placental hemorrhage, which leads to midgestational death of low-TF female mice. This study demonstrated that TF is required for uterine hemostasis and revealed an unexpected role of TF in the maintenance of the placental labyrinth.

The von Hippel-Lindau tumour suppressor protein: new perspectives

von Hippel-Lindau (VHL) disease is a hereditary cancer syndrome caused by germline mutations of the VHL tumour suppressor gene. The VHL gene product, pVHL, forms multiprotein complexes that contain elongin B, elongin C and Cul-2, and negatively regulates hypoxia-inducible mRNAs. pVHL is suspected to play a role in ubiquitination given the similarity of elongin C and Cul-2 with Skp1 and Cdc53, respectively. pVHL can also interact with fibronectin and is required for the assembly of a fibronectin matrix. Finally, pVHL, at least indirectly, plays a role in the ability of cells to exit the cell cycle. Thus, pVHL is a tumour suppressor protein that regulates angiogenesis, extracellular matrix formation and the cell cycle.

Protective function of von Hippel-Lindau protein against impaired protein processing in renal carcinoma cells

The absence of functional von Hippel-Lindau (VHL) tumor suppressor gene leads to the development of neoplasias characteristic of VHL disease, including renal cell carcinoma (RCC). Here, we compared the sensitivity of RCC cells lacking VHL gene function with that of RCC cells expressing the wild-type VHL gene (wtVHL) after exposure to various stresses. While the response to most treatments was not affected by the VHL gene status, glucose deprivation was found to be much more cytotoxic for RCC cells lacking VHL gene function than for wtVHL-expressing cells. The heightened sensitivity of VHL-deficient cells was not attributed to dissimilar energy requirements or to differences in glucose uptake, but more likely reflects a lesser ability of VHL-deficient cells to handle abnormally processed proteins arising from impaired glycosylation. In support of this hypothesis, other treatments which act through different mechanisms to interfere with protein processing (i.e., tunicamycin, brefeldin A, and azetidine) were also found to be much more toxic for VHL-deficient cells. Furthermore, ubiquitination of cellular proteins was elevated in VHL-deficient cells, particularly after glucose deprivation, supporting a role for the VHL gene in ubiquitin-mediated proteolysis. Accordingly, the rate of elimination of abnormal proteins was lower in cells lacking a functional VHL gene than in wtVHL-expressing cells. Thus, pVHL appears to participate in the elimination of misprocessed proteins, such as those arising in the cell due to the unavailability of glucose or to other stresses.

Tumour suppressor gene mutations in humans and mice: parallels and contrasts

Tumour suppressor genes prevent cancer development. They can be identified by studying humans, but a full understanding of the mechanisms of their action requires the production of animal models. Mice with mutations in tumour suppressor genes can be produced by gene targeting. The phenotypic consequences of tumour suppressor gene mutations in mice and humans show parallels and contrasts, and both can contribute to the elucidation of disease processes.

The VHL tumour-suppressor gene paradigm

The VHL tumour-suppressor gene (TSG) has a critical 'gatekeeper' role in regulating growth and differentiation of human kidney cells, and inactivation of the VHL gene is the most frequent genetic event in human kidney cancer. There are many similarities between the genetics of the VHL and retinoblastoma TSGs, but the VHL tumourigenesis model is more complex. Here, we examine the current knowledge of the genetics and functional aspects of the VHL TSG, and emphasize how the VHL gene provides a paradigm that illustrates many aspects of TSG biology.

The von Hippel-Lindau tumor suppressor protein is required for proper assembly of an extracellular fibronectin matrix

Fibronectin coimmunoprecipitated with wild-type von Hippel-Lindau protein (pVHL) but not tumor-derived pVHL mutants. Immunofluorescence and biochemical fractionation experiments showed that fibronectin colocalized with a fraction of pVHL associated with the endoplasmic reticulum, and cold competition experiments suggested that complexes between fibronectin and pVHL exist in intact cells. Assembly of an extracellular fibronectin matrix by VHL-/- renal carcinoma cells, as determined by immunofluorescence and ELISA assays, was grossly defective compared with VHL+/+ renal carcinoma cells. Reintroduction of wildtype, but not mutant, pVHL into VHL-/- renal carcinoma cells partially corrected this defect. Finally, extracellular fibronectin matrix assembly by VHL-/- mouse embryos and mouse embryo fibroblasts (MEFs), unlike their VHL+/+ counterparts, was grossly impaired. These data support a direct role of pVHL in fibronectin matrix assembly.