WT1--more than a transcription factor?

RNA-binding proteins of COSMIC importance in cancer

Herculean efforts by the Wellcome Sanger Institute, the National Cancer Institute, and the National Human Genome Research Institute to sequence thousands of tumors representing all major cancer types have yielded more than 700 genes that contribute to neoplastic growth when mutated, amplified, or deleted. While some of these genes (now included in the COSMIC Cancer Gene Census) encode proteins previously identified in hypothesis-driven experiments (oncogenic transcription factors, protein kinases, etc.), additional classes of cancer drivers have emerged, perhaps none more surprisingly than RNA-binding proteins (RBPs). Over 40 RBPs responsible for virtually all aspects of RNA metabolism, from synthesis to degradation, are recurrently mutated in cancer, and just over a dozen are considered major cancer drivers. This Review investigates whether and how their RNA-binding activities pertain to their oncogenic functions. Focusing on several well-characterized steps in RNA metabolism, we demonstrate that for virtually all cancer-driving RBPs, RNA processing activities are either abolished (the loss-of-function phenotype) or carried out with low fidelity (the LoFi phenotype). Conceptually, this suggests that in normal cells, RBPs act as gatekeepers maintaining proper RNA metabolism and the "balanced" proteome. From the practical standpoint, at least some LoFi phenotypes create therapeutic vulnerabilities, which are beginning to be exploited in the clinic.

Classification of a frameshift/extended and a stop mutation in WT1 as gain-of-function mutations that activate cell cycle genes and promote Wilms tumour cell proliferation

The WT1 gene encodes a zinc finger transcription factor important for normal kidney development. WT1 is a suppressor for Wilms tumour development and an oncogene for diverse malignant tumours. We recently established cell lines from primary Wilms tumours with different WT1 mutations. To investigate the function of mutant WT1 proteins, we performed WT1 knockdown experiments in cell lines with a frameshift/extension (p.V432fsX87 = Wilms3) and a stop mutation (p.P362X = Wilms2) of WT1, followed by genome-wide gene expression analysis. We also expressed wild-type and mutant WT1 proteins in human mesenchymal stem cells and established gene expression profiles. A detailed analysis of gene expression data enabled us to classify the WT1 mutations as gain-of-function mutations. The mutant WT1^Wilms2 and WT1^Wilms3 proteins acquired an ability to modulate the expression of a highly significant number of genes from the G2/M phase of the cell cycle, and WT1 knockdown experiments showed that they are required for Wilms tumour cell proliferation. p53 negatively regulates the activity of a large number of these genes that are also part of a core proliferation cluster in diverse human cancers. Our data strongly suggest that mutant WT1 proteins facilitate expression of these cell cycle genes by antagonizing transcriptional repression mediated by p53. We show that mutant WT1 can physically interact with p53. Together the findings show for the first time that mutant WT1 proteins have a gain-of-function and act as oncogenes for Wilms tumour development by regulating Wilms tumour cell proliferation.

A novel cell-penetrating peptide derived from WT1 enhances p53 activity, induces cell senescence and displays antimelanoma activity in xeno- and syngeneic systems

The Wilms tumor protein 1 (WT1) transcription factor has been associated in malignant melanoma with cell survival and metastasis, thus emerging as a candidate for targeted therapy. A lysine-arginine rich peptide, WT1-pTj, derived from the ZF domain of WT1 was evaluated as an antitumor agent against A2058 human melanoma cells and B16F10-Nex2 syngeneic murine melanoma. Peptide WT1-pTj quickly penetrated human melanoma cells and induced senescence, recognized by increased SA-β-galactosidase activity, enhanced transcriptional activity of p53, and induction of the cell cycle inhibitors p21 and p27. Moreover, the peptide bound to p53 and competed with WT1 protein for binding to p53. WT1-pTj treatment led to sustained cell growth suppression, abrogation of clonogenicity and G2/M cell cycle arrest. Notably, in vivo studies showed that WT1-pTj inhibited both the metastases and subcutaneous growth of murine melanoma in syngeneic mice, and prolonged the survival of nude mice challenged with human melanoma cells. The 27-amino acid cell-penetrating WT1-derived peptide, depends on C(3) and H(16) for effective antimelanoma activity, inhibits proliferation of WT1-expressing human tumor cell lines, and may have an effective role in the treatment of WT1-expressing malignancies.

Expression of inhibin-alpha is regulated synergistically by Wilms' tumor gene 1 (Wt1) and steroidogenic factor-1 (Sf1) in sertoli cells

Wt1 encodes a zinc finger nuclear transcriptional factor, which is specifically expressed in testicular Sertoli cells and knockdown of Wt1 in Sertoli cells causes male mice subfertility. However, the underlying mechanism is still unclear. In this study, we found that expression of inhibin-α is significantly reduced in Wt1-deficient Sertoli cells. Luciferase assays using the inhibin-α promoter indicated that the inhibin-α promoter is transactivated by the Wt1 A, and B isoforms (−KTS), but not the C, and D isoforms (+KTS). Analysis of the Wt1 responsive element of the inhibin-α promoter region using site-directed mutagenesis showed that the nucleotides between −58 and −49 are essential for Wt1-dependent transactivation of the inhibin-α promoter. ChIP assays indicated that Wt1 directly interacts with the inhibin-α promoter. In addition, the inhibin-α promoter is activated synergistically by Wt1 and Sf1. Mutation of the ligand binding domain (LBD) of Sf1 (residues 235–238) completely abolished the synergistic action between Wt1 and Sf1, but did not affect the physical interaction between these two proteins, suggesting that other factor(s) may also be involved in the regulation of inhibin-α in Sertoli cells. Further studies demonstrated that β-catenin enhances the synergistic activation of Wt1 and Sf1 on the inhibin-α promoter. Given the fact that inhibin-α, a subunit of inhibin, is known to be involved in the regulation of spermatogenesis and testicular steroidogenesis, this study reveals a new regulatory mechanism of inhibin-α in Sertoli cells and also sheds light on the physiological functions of Wt1 in gonad development and spermatogenesis.

Interrenal organogenesis in the zebrafish model

In recent years, many genes that participate in the specification, differentiation and steroidogenesis of the interrenal organ, the teleostean homologue of the adrenal cortex, have been identified and characterized in zebrafish. In-depth studies of these genes have helped to delineate the morphogenetic steps of interrenal organ formation, as well as some of the molecular and cellular mechanisms that govern these processes. The co-development of interrenal tissue with the embryonic kidney (pronephros), surrounding endothelium and invading chromaffin cells has been analyzed, by virtue of the amenability of zebrafish embryos to a variety of genetic, developmental and histological approaches. Moreover, zebrafish embryos can be subject to molecular as well as biochemical assays for the unraveling of the transcriptional regulation program underlying interrenal development. To this end, the key mechanisms that control organogenesis and steroidogenesis of the zebrafish interrenal gland have been shown to resemble those in mammals, justifying the future utilization of zebrafish model for discovering novel genes associated with adrenal development and disease.

Psip1/Ledgf p52 binds methylated histone H3K36 and splicing factors and contributes to the regulation of alternative splicing

Increasing evidence suggests that chromatin modifications have important roles in modulating constitutive or alternative splicing. Here we demonstrate that the PWWP domain of the chromatin-associated protein Psip1/Ledgf can specifically recognize tri-methylated H3K36 and that, like this histone modification, the Psip1 short (p52) isoform is enriched at active genes. We show that the p52, but not the long (p75), isoform of Psip1 co-localizes and interacts with Srsf1 and other proteins involved in mRNA processing. The level of H3K36me3 associated Srsf1 is reduced in Psip1 mutant cells and alternative splicing of specific genes is affected. Moreover, we show altered Srsf1 distribution around the alternatively spliced exons of these genes in Psip1 null cells. We propose that Psip1/p52, through its binding to both chromatin and splicing factors, might act to modulate splicing.

Effective silencing of Sry gene with RNA interference in developing mouse embryos resulted in feminization of XY gonad

Delivering siRNA or shRNA into the developing embryos is still a main challenge to use of RNAi in mammalian systems. Here we analyze several factors influencing RNAi-mediated silencing of Sry gene, which is a tightly controlled spatiotemporal expressed gene and only shortly expressed in developing mouse embryo gonad. A Sry gene-specific shRNAs expression vector (pSilencer4.1/Sry565) was constructed. The shRNA constructs were mixed with polyethylenimines (PEIs) to form a complex and then injected into pregnant mice though tail vein. Our results showed that Sry gene was downregulated significantly in developing embryos. Further study revealed that knocking-down of Sry expression resulted in feminization of gonad development in mouse embryos and the expression level of Sox9 and Wt1 gene was also significantly changed by downregulation of Sry. The transfection efficiency is associated with the amount of plasmid DNA injection, injection time, injection speed, and volume. Our studies suggest that transplacental RNAi could be implemented by tail vein injection of plasmid vector into pregnant mice.

The vanishing testis: a histomorphologic and clinical assessment

Of patients with cryptorchidism, 5% have no palpable gonad. Physical examination or scrotal exploration demonstrates tissue nubbins or small nodules that constitute the vanishing testis syndrome. At the University of Chicago Hospitals (Chicago, IL; 2004-2008), 30 surgical pathology specimens from 29 patients with this clinical diagnosis underwent scrotal exploration. Histologic and immunohistochemical comparison was done with 7 fetal testes, 8 surgically removed nonneoplastic testes, and 2 cryptorchid testes. Routine histologic studies showed no seminiferous tubules in 18 cases (60%), fibrosis in all (100%), calcifications in 16 (53%), and hemosiderin deposits in 9 (30%). In 12 cases with seminiferous tubules (40%), there were Sertoli cells only. Scrotal exploration in such cases is clinically driven and results in the removal of any tissue present. Although published studies suggest the risk for future tumor development is low, possibly absent, the definitive removal of a testicle is established by an awareness of the histologic spectrum exhibited by testicular remnants.

A GATA4/WT1 cooperation regulates transcription of genes required for mammalian sex determination and differentiation

Background

In mammals, sex determination is genetically controlled. The SRY gene, located on Y chromosome, functions as the dominant genetic switch for testis development. The SRY gene is specifically expressed in a subpopulation of somatic cells (pre-Sertoli cells) of the developing urogenital ridge for a brief period during gonadal differentiation. Despite this tight spatiotemporal expression pattern, the molecular mechanisms that regulate SRY transcription remain poorly understood. Sry expression has been shown to be markedly reduced in transgenic mice harboring a mutant GATA4 protein (a member of the GATA family of transcription factors) disrupted in its ability to interact with its transcriptional partner FOG2, suggesting that GATA4 is involved in SRY gene transcription.

Results

Although our results show that GATA4 directly targets the pig SRY promoter, we did not observe similar action on the mouse and human SRY promoters. In the mouse, Wilms' tumor 1 (WT1) is an important regulator of both Sry and Müllerian inhibiting substance (Amh/Mis) expression and in humans, WT1 mutations are associated with abnormalities of sex differentiation. GATA4 transcriptionally cooperated with WT1 on the mouse, pig, and human SRY promoters. Maximal GATA4/WT1 synergism was dependent on WT1 but not GATA4 binding to their consensus regulatory elements in the SRY promoter and required both the zinc finger and C-terminal regions of the GATA4 protein. Although both isoforms of WT1 synergized with GATA4, synergism was stronger with the +KTS rather than the -KTS isoform. WT1/GATA4 synergism was also observed on the AMH promoter. In contrast to SRY, WT1/GATA4 action on the mouse Amh promoter was specific for the -KTS isoform and required both WT1 and GATA4 binding.

Conclusion

Our data therefore provide new insights into the molecular mechanisms that contribute to the tissue-specific expression of the SRY and AMH genes in both normal development and certain syndromes of abnormal sex differentiation.

Wilms' tumor protein Wt1 is an activator of the anti-Müllerian hormone receptor gene Amhr2

The Wilms' tumor protein Wt1 plays an essential role in mammalian urogenital development. WT1 mutations in humans lead to a variety of disorders, including Wilms' tumor, a pediatric kidney cancer, as well as Frasier and Denys-Drash syndromes. Phenotypic anomalies in Denys-Drash syndrome include pseudohermaphroditism and sex reversal in extreme cases. We have used cDNA microarray analyses on Wt1 knockout mice to identify Wt1-dependent genes involved in sexual development. The gene most dramatically affected by Wt1 inactivation was Amhr2, encoding the anti-Müllerian hormone (Amh) receptor 2. Amhr2 is an essential factor for the regression of the Müllerian duct in males, and mutations in AMHR2 lead to the persistent Müllerian duct syndrome, a rare form of male pseudohermaphroditism. Here we show that Wt1 and Amhr2 are coexpressed during urogenital development and that the Wt1 protein binds to the promoter region of the Amhr2 gene. Inactivation and overexpression of Wt1 in cell lines was followed by immediate changes of Amhr2 expression. The identification of Amhr2 as a Wt1 target provides new insights into the role of Wt1 in sexual differentiation and indicates, in addition to its function in early gonad development and sex determination, a novel function for Wt1, namely, in Müllerian duct regression.

Sex determination and gonadal development in mammals

Arguably the most defining moment in our lives is fertilization, the point at which we inherit either an X or a Y chromosome from our father. The profoundly different journeys of male and female life are thus decided by a genetic coin toss. These differences begin to unfold during fetal development, when the Y-chromosomal Sry ("sex-determining region Y") gene is activated in males and acts as a switch that diverts the fate of the undifferentiated gonadal primordia, the genital ridges, towards testis development. This sex-determining event sets in train a cascade of morphological changes, gene regulation, and molecular interactions that directs the differentiation of male characteristics. If this does not occur, alternative molecular cascades and cellular events drive the genital ridges toward ovary development. Once testis or ovary differentiation has occurred, our sexual fate is further sealed through the action of sex-specific gonadal hormones. We review here the molecular and cellular events (differentiation, migration, proliferation, and communication) that distinguish testis and ovary during fetal development, and the changes in gene regulation that underpin these two alternate pathways. The growing body of knowledge relating to testis development, and the beginnings of a picture of ovary development, together illustrate the complex mechanisms by which these organ systems develop, inform the etiology, diagnosis, and management of disorders of sexual development, and help define what it is to be male or female.

Different isoforms of the Wilms' tumour protein WT1 have distinct patterns of distribution and trafficking within the nucleus

The Wilms' tumour suppressor gene WT1 encodes multiple isoforms of a transcription factor essential for correct mammalian urogenital development. Maintenance of the correct isoform ratio is critical. In humans, perturbation of this ratio causes Frasier syndrome, which is characterized by developmental defects of the kidney and urogenital tract. Different WT1 isoforms are thought to regulate transcription and participate in mRNA processing, functions reflected by a complex sub-nuclear distribution. However, the role of individual WT1 isoforms remains unclear and pathways leading to WT1 sub-nuclear localization are completely unknown. Here we use cells expressing green fluorescent protein-tagged WT1 to demonstrate that the two major WT1 isoforms occupy separate and dynamic intranuclear locations in which one isoform, WT1+KTS, preferentially associates with the nucleolus. The alternatively spliced zinc finger region is found to be critical for the initial sub-nuclear separation of WT1 isoforms, but interactions between different isoforms influence the sub-nuclear distribution of WT1. We illustrate how disruption of WT1 nuclear distribution might result in disease. This study contributes to the emerging picture of intranuclear protein trafficking.

Genetic and epigenetic alterations on the short arm of chromosome 11 are involved in a majority of sporadic Wilms' tumours

Wilms' tumour is one of the most common solid tumours of childhood. 11p13 (WT1 locus) and 11p15.5 (WT2 locus) are known to have genetic or epigenetic aberrations in these tumours. In Wilms' tumours, mutation of the Wilms tumour 1 (WT1) gene at the WT1 locus has been reported, and the WT2 locus, comprising the two independent imprinted domains IGF2/H19 and KIP2/LIT1, can undergo maternal deletion or alterations associated with imprinting. Although these alterations have been identified in many studies, it is still not clear how frequently combined genetic and epigenetic alterations of these loci are involved in Wilms' tumours or how these alterations occur. To answer both questions, we performed genetic and epigenetic analyses of these loci, together with an additional gene, CTNNB1, in 35 sporadic Wilms' tumours. Loss of heterozygosity of 11p15.5 and loss of imprinting of IGF2 were the most frequent genetic (29%) and epigenetic (40%) alterations in Wilms' tumours, respectively. In total, 83% of the tumours had at least one alteration at 11p15.5 and/or 11p13. One-third of the tumours had alterations at multiple loci. Our results suggest that chromosome 11p is not only genetically but also epigenetically critical for the majority of Wilms' tumours.

The makings of maleness: towards an integrated view of male sexual development

As the mammalian embryo develops, it must engage one of the two distinct programmes of gene activity, morphogenesis and organogenesis that characterize males and females. In males, sexual development hinges on testis determination and differentiation, but also involves many coordinated transcriptional, signalling and endocrine networks that underpin the masculinization of other organs and tissues, including the brain. Here we bring together current knowledge about these networks, identify gaps in the overall picture, and highlight the known defects that lead to disorders of male sexual development.

Two methods for improved purification of full-length mammalian proteins that have poor expression and/or solubility using standard Escherichia coli procedures

Many mammalian proteins are multifunctional proteins with biological activities whose characterization often requires in vitro studies. However, these studies depend on generation of sufficient quantities of recombinant protein and many mammalian proteins cannot be easily expressed and purified as full-length products. One example is the Wilm's tumor gene product, WT1, which has proven difficult to express as a full-length purified recombinant protein using standard approaches. To facilitate expression of full-length WT1 we have developed approaches that optimized its expression and purification in Escherichia coli and mammalian cells. First, using a bicistronic vector system, we successfully expressed and purified WT1 containing a C-terminal tandem affinity tag in 293T cells. Second, using a specific strain of E. coli transformed with a modified GST vector, we successfully expressed and purified N-terminal GST tagged and C-terminal 2x FLAG tagged full-length human WT1. The benefits of these approaches include: (1) two-step affinity purification to allow high quality of protein purification, (2) large soluble tags that can be used for a first affinity purification step, but then conveniently removed with the highly site-specific TEV protease, and (3) the use of non-denaturing purification and elution conditions that are predicted to preserve native protein conformation and function.

Identification and comparative expression analysis of a second wt1 gene in zebrafish

The Wilms' tumor suppressor gene wt1 encodes a zinc-finger transcription factor that plays an important role in the development of the mammalian genitourinary system. Mutations in WT1 in humans lead to anomalies of kidney and gonad development and cause Wilms' tumor, a pediatric kidney cancer. The inactivation of both wt1 alleles in mice gives rise to multiple organ defects, among them agenesis of kidney, spleen, and gonads. In zebrafish, an ortholog of wt1 has been described that is expressed in the pronephric field and is later restricted to the podocytes. Here, we report the existence of a second wt1 gene in zebrafish, which we have named wt1b (we named the initial gene wt1a). The overall sequence identity of the two Wt1 proteins is 70% and 92% between the zinc-finger regions, respectively. In contrast to wt1a, wt1b is expressed from the earliest stages of development onward, albeit at low levels. Both wt1a and wt1b are expressed in the intermediate mesoderm, with wt1b being restricted to a smaller area lying at the caudal end of the wt1a expression domain. In adult fish, high expression levels for both genes can be found in gonads, kidney, heart, spleen, and muscle.

Two cases of isolated diffuse mesangial sclerosis with WT1 mutations

Here we report two cases of isolated diffuse mesangial sclerosis (IDMS) with early onset end-stage renal failure. These female patients did not show abnormalities of the gonads or external genitalia. Direct sequencing of WT1 PCR products from genomic DNA identified WT1 mutations in exons 8 (366 Arg>His) and 9 (396 Asp>Tyr). These mutations have been reported previously in association with Denys-Drash syndrome (DDS) with early onset renal failure. Therefore we suggest that, at least in part, IDMS is a variant of DDS and that investigations for the WT1 mutations should be performed in IDMS patients. In cases with identified WT1 mutations, the same attention to tumor development should be required as in DDS patients, and karyotyping and serial abdominal ultrasonograms to evaluate the gonads and kidney are warranted.

The cell-type-specificity of inherited predispositions to tumours: review and hypothesis

Most hereditary predispositions to tumours affect only one particular cell type of the body but the genes bearing the relevant germ-line mutation are not cell-type-specific. Some predisposition syndromes include increased risks of lesions (developmental or tumourous) of unrelated cell types, in any individual predisposed to the main lesion (e.g. osteosarcoma in patients predisposed to retinoblastoma). Other predispositions to additional lesions occur only in members of some families with the predisposition to the basic lesion (e.g. Gardner's syndrome in some families suffering familial adenomatous polyposis). In yet other predisposition syndromes, different mutations of the same gene are associated with markedly differing family-specific clinical syndromes. In particular, identical germline mutations (e.g. in APC, RET and PTEN genes), have been found associated with differing clinical syndromes in different families. This paper reviews previously suggested mechanisms of the cell-type specificity of inherited predispositions to tumour. Models of tumour formation in predisposition syndromes are discussed, especially those involving a germline mutation (the first 'hit') of a tumour suppressor gene (TSG) and a second (somatic) hit on the second allele of the same TSG. A modified model is suggested, such that the second hit is a co-mutation of the second allele of the TSG and a regulator which is specific for growth and/or differentiation of the cell type which is susceptible to the tumour predisposition. In some cases of tumour, the second hit may be large enough to be associated with a cytogenetically-demonstrable abnormality of the part of the chromosome carrying the TSG, but in other cases, the co-mutation may be of 'sub-cytogenetic' size (i.e. 10(2)-10(5) bases). For the latter, mutational mechanisms of frameshift and impaired fidelity of replication of DNA by DNA polyerases may sometimes be involved. Candidate cell-type-specific regulators may include microRNAs and perhaps transcription factors. It is suggested that searching the introns within 10(5)-10(6) bases either side of known of exonic mutations of TSGs associated with inherited tumour predisposition might reveal microRNA cell-type-specific regulators. Additional investigations may involve fluorescent in situ hybridisations on interphase tumour nuclei.

Transcriptional regulation by the Wilms' tumour suppressor protein WT1

Wilms' tumour is a paediatric malignancy of the kidneys and is the most common solid tumour found in children. The Wilms' tumour suppressor protein WT1 is mutated in approx. 15% of Wilms' tumours, and is aberrantly expressed in many others. WT1 can manifest both tumour suppressor and oncogenic activities, but the reasons for this are not yet clear. The Wilms' tumour suppressor protein WT1 is a transcriptional activator, the function of which is under cell-context-specific control. We have previously described a small region at the N-terminus of WT1 (suppression domain) that inhibits the transcriptional activation domain by contacting a co-suppressor protein. We recently identified BASP1 as one of the components of the co-suppressor. Here, we analyse the mechanism of action of the WT1 suppression domain, and discuss its function in the context of the role of WT1 as a regulator of development.

BASP1 is a transcriptional cosuppressor for the Wilms' tumor suppressor protein WT1

The Wilms' tumor suppressor protein WT1 is a transcriptional regulator that plays a key role in the development of the kidneys. The transcriptional activation domain of WT1 is subject to regulation by a suppression region within the N terminus of WT1. Using a functional assay, we provide direct evidence that this requires a transcriptional cosuppressor, which we identify as brain acid soluble protein 1 (BASP1). WT1 and BASP1 associate within the nuclei of cells that naturally express both proteins. BASP1 can confer WT1 cosuppressor activity in transfection assays, and elimination of endogenous BASP1 expression augments transcriptional activation by WT1. BASP1 is present in the developing nephron structures of the embryonic kidney and, coincident with that of WT1, its expression is restricted to the highly specialized podocyte cells of the adult kidney. Taken together, our results show that BASP1 is a WT1-associated factor that can regulate WT1 transcriptional activity.

Expression of Wilms' tumor suppressor in the liver with cirrhosis: relation to hepatocyte nuclear factor 4 and hepatocellular function

The Wilms' tumor suppressor WT1 is a transcriptional regulator present in the fetal but not in the mature liver. Its expression and functional role in liver diseases remains unexplored. In this study, we analyzed WT1 expression by reverse-transcription polymerase chain reaction (RT-PCR) and by immunohistochemistry in normal and diseased livers. In addition, we performed in vitro studies in isolated rat hepatocytes to investigate WT1 regulation and function. We detected WT1 messenger RNA (mRNA) in 18% of normal livers, 17% of chronic hepatitis with minimal fibrosis, 49% of chronic hepatitis with bridging fibrosis, and 71% of cirrhotic livers. In cirrhosis, WT1 immunoreactivity was localized to the nucleus of hepatocytes. WT1 mRNA abundance correlated inversely with prothrombin time (P =.04) and directly with serum bilirubin (P =.002) and with the MELD score (P =.001) of disease severity. In rats, WT1 expression was present in fetal hepatocytes and in the cirrhotic liver but not in normal hepatic tissue. In vitro studies showed that isolated primary hepatocytes express WT1 when stimulated with transforming growth factor beta (TGF-beta) or when the cells undergo dedifferentiation in culture. Moreover, we found that WT1 down-regulates hepatocyte nuclear factor 4 (HNF-4), a factor that is essential to maintain liver function and metabolic regulation in the mature organ. Hepatic expression of HNF-4 was impaired in advanced human cirrhosis and negatively correlated with WT1 mRNA levels (P =.001). In conclusion, we show that WT1 is induced by TGF-beta and down-regulates HNF-4 in liver cells. WT1 is reexpressed in the cirrhotic liver in relation to disease progression and may play a role in the development of hepatic insufficiency in cirrhosis.

Faithful expression of a tagged Fugu WT1 protein from a genomic transgene in zebrafish: efficient splicing of pufferfish genes in zebrafish but not mice

The teleost fish are widely used as model organisms in vertebrate biology. The compact genome of the pufferfish, Fugu rubripes, has proven a valuable tool in comparative genome analyses, aiding the annotation of mammalian genomes and the identification of conserved regulatory elements, whilst the zebrafish is particularly suited to genetic and developmental studies. We demonstrate that a pufferfish WT1 transgene can be expressed and spliced appropriately in transgenic zebrafish, contrasting with the situation in transgenic mice. By creating both transgenic mice and transgenic zebrafish with the same construct, we show that Fugu RNA is processed correctly in zebrafish but not in mice. Furthermore, we show for the first time that a Fugu genomic construct can produce protein in transgenic zebrafish: a full-length Fugu WT1 transgene with a C-terminal beta-galactosidase fusion is spliced and translated correctly in zebrafish, mimicking the expression of the endogenous WT1 gene. These data demonstrate that the zebrafish:Fugu system is a powerful and convenient tool for dissecting both vertebrate gene regulation and gene function in vivo.

Mice lacking the 68-amino-acid, mammal-specific N-terminal extension of WT1 develop normally and are fertile

Mutations in the Wilms' tumor 1 gene, WT1, cause pediatric nephroblastoma and the severe genitourinary disorders of Frasier and Denys-Drash syndromes. High levels of WT1 expression are found in the developing kidney, uterus, and testis--consistent with this finding, the WT1 knockout mouse demonstrates that WT1 is essential for normal genitourinary development. The WT1 gene encodes multiple isoforms of a zinc finger-containing protein by a combination of alternative splicing and alternative translation initiation. The use of an upstream, alternative CUG translation initiation codon specific to mammals results in the production of WT1 protein isoforms with a 68-amino-acid N-terminal extension. To determine the function in vivo of mammal-specific WT1 isoforms containing this extension, gene targeting was employed to introduce a subtle mutation into the WT1 gene. Homozygous mutant mice show a specific absence of the CUG-initiated WT1 isoforms yet develop normally to adulthood and are fertile. Detailed histological analysis revealed normal development of the genitourinary system.

A novel function for human factor C1 (HCF-1), a host protein required for herpes simplex virus infection, in pre-mRNA splicing

Human factor C1 (HCF-1) is needed for the expression of herpes simplex virus 1 (HSV-1) immediate-early genes in infected mammalian cells. Here, we provide evidence that HCF-1 is required for spliceosome assembly and splicing in mammalian nuclear extracts. HCF-1 interacts with complexes containing splicing snRNPs in uninfected mammalian cells and is a stable component of the spliceosome complex. We show that a missense mutation in HCF-1 in the BHK21 hamster cell line tsBN67, at the non-permissive temperature, inhibits the protein's interaction with U1 and U5 splicing snRNPs, causes inefficient spliceosome assembly and inhibits splicing. Transient expression of wild-type HCF-1 in tsBN67 cells restores splicing at the non-permissive temperature. The inhibition of splicing in tsBN67 cells correlates with the temperature-sensitive cell cycle arrest phenotype, suggesting that HCF-1-dependent splicing events may be required for cell cycle progression.

Human telomerase and its regulation

The telomere is a special functional complex at the end of linear eukaryotic chromosomes, consisting of tandem repeat DNA sequences and associated proteins. It is essential for maintaining the integrity and stability of linear eukaryotic genomes. Telomere length regulation and maintenance contribute to normal human cellular aging and human diseases. The synthesis of telomeres is mainly achieved by the cellular reverse transcriptase telomerase, an RNA-dependent DNA polymerase that adds telomeric DNA to telomeres. Expression of telomerase is usually required for cell immortalization and long-term tumor growth. In humans, telomerase activity is tightly regulated during development and oncogenesis. The modulation of telomerase activity may therefore have important implications in antiaging and anticancer therapy. This review describes the currently known components of the telomerase complex and attempts to provide an update on the molecular mechanisms of human telomerase regulation.

The Wilms tumor suppressor WT1 regulates early gonad development by activation of Sf1

In mammals, several genes including the Wilms tumor suppressor gene Wt1, the Lim homeobox gene Lhx9, and the gene encoding steroidogenic factor 1 (Sf1) have been implicated in the development of the indifferent gonad prior to sexual differentiation. Interactions among these genes have not yet been elucidated. Using biochemical and genetic experiments, we demonstrate here that WT1 and LHX9 function as direct activators of the Sf1 gene. Interestingly, only the -KTS form of WT1 is able to bind to and transactivate the Sf1 promoter. This observation is consistent with differential roles for the -KTS and +KTS variants of WT1 which have been postulated on the basis of human disorders such as the Frasier syndrome. Our data suggest a pathway in which the products of the Wt1 and Lhx9 genes activate expression of Sf1 and thus mediate early gonadogenesis.

Activity, function, and gene regulation of the catalytic subunit of telomerase (hTERT)

Recent interest in the regulation of telomerase, the enzyme that maintains chromosomal termini, has lead to the discovery and characterization of the catalytic subunit of telomerase, hTERT. Many studies have suggested that the transcription of hTERT represents the rate-limiting step in telomerase expression and key roles for hTERT have been implied in cellular aging, immortalization, and transformation. Before the characterization of the promoter of hTERT in 1999, regulatory mechanisms suggested for this gene were limited to speculation. The successful cloning and characterization of the hTERT 5' gene regulatory region has enabled its formal investigation and analysis of potential mechanisms controlling hTERT expression. Although these studies have provided important information about hTERT gene regulation, there has been some confusion regarding the nucleotide boundaries of this region, the location, number, and importance of various transcription factor binding motifs, and the results of promoter activity assays. We feel that this uncertainty, combined with the sheer volume of recent publications on hTERT regulation, calls for consolidation and review. In this analysis we examine recent advances in the regulation of the hTERT gene and attempt to resolve discrepancies resulting from the nearly simultaneous nature of publications in this fast-moving area. Additionally, we aim to summarize the extant knowledge of hTERT gene regulation and its role in important biological processes such as cancer and aging.

The fission yeast ortholog of the coregulator SKIP interacts with the small subunit of U2AF

The mode of action of transcriptional coregulators may involve the recruitment of spliceosome components. Using the two-hybrid screen, we examined the interaction partners of spSNW1, the S. pombe ortholog of the human coregulator SNW1/SKIP/NCoA-62, and found it to interact with the small subunit of the splicing factor U2AF (spU2AF23). The interaction involves the C-terminal parts of spU2AF23 and spSNW1. Tagged variants of both proteins were expressed in S. pombe and the interaction was proved by coprecipitation in nuclear extracts. This interaction would explain the finding of SKIP in nuclear speckles (Mintz, P. J., et al., EMBO J. 18, 4308-4320, 1999) and in reconstituted spliceosomes (Neubauer, G., et al., Nat. Genet. 20, 46-50, 1998). We deleted the spSNW1 gene in the diploid strain and demonstrated that spSNW1 is an essential gene in S. pombe.

Update in podocyte biology

Knowledge of podocyte biology is growing rapidly. Podocytes are crucially involved in most hereditary diseases affecting the glomerulus, which all exhibit podocyte-specific defects, that is, foot process effacement and protein leakage. Efforts to understand molecular mechanisms causing these derangements are increasingly successful and will allow a better targeting of interventions to halt the progression of chronic renal disease.

The speckling domain of the Wilms tumor suppressor WT1 overlaps with the transcriptional repression domain

The Wilms tumor suppressor gene WT1 encodes a zinc finger protein, expressed as different splicing variants, that has all the hallmarks of a transcription factor. The -KTS form of WT1 displays a homogeneous localization within the nucleus and has been shown to activate or repress the activity of various target genes. In contrast, the WT1(+KTS) variant demonstrates a speckled pattern of expression within the nucleus. This and its association with factors of the splicing machinery has led to the hypothesis that WT1(+KTS) might play a role in post-transcriptional processes. By the generation of a series of deletion constructs and subsequent immunofluorescence analysis, we have identified and characterized the domain which is responsible for the localization of WT1 variants in nuclear speckles. The speckling domain comprises amino acids 76-120 within the N-terminus of WT1 and is sufficient to target other proteins into distinct nuclear domains. Interestingly the WT1 speckling domain does not overlap with the domain required for interaction with the splicing factor U2AF65 but overlaps with the transcriptional repression domain. Thus our data challenge the view that association of WT1 with spliceosomes is responsible for the speckling phenotype.

Par4 is a coactivator for a splice isoform-specific transcriptional activation domain in WT1

The Wilms' tumor suppressor protein WT1 is a transcriptional regulator involved in differentiation and the regulation of cell growth. WT1 is subject to alternative splicing, one isoform including a 17-amino acid region that is specific to mammals. The function of this 17-amino acid insertion is not clear, however. Here, we describe a transcriptional activation domain in WT1 that is specific to the WT1 splice isoform that contains the 17-amino acid insertion. We show that the function of this domain in transcriptional activation is dependent on a specific interaction with the prostate apoptosis response factor par4. A mutation in WT1 found in Wilms' tumor disturbs the interaction with par4 and disrupts the function of the activation domain. Analysis of WT1 derivatives in cells treated to induce par4 expression showed a strong correlation between the transcription function of the WT1 17-amino acid insertion and the ability of WT1 to regulate cell survival and proliferation. Our results provide a molecular mechanism by which alternative splicing of WT1 can regulate cell growth in development and disease.

Identification of connective tissue growth factor as a target of WT1 transcriptional regulation

The Wilms tumor suppressor WT1 has transcription-activating and -suppressing capabilities. WT1-responsive promoters have been described; however, in large part, it remains unclear which potential downstream genes are physiologically relevant and mediate the function of WT1 in tumorigenesis and development. To identify genes regulated by WT1 in vivo, we used a dominant-negative version of WT1 to modulate WT1 activity in a Wilms tumor cell line. Screening oligonucleotide arrays with RNA from these cells uncovered a number of genes whose expression was altered by abrogation of WT1 function. Several of the genes encode members of the CCN family of growth regulators. The promoter of one of these genes, connective tissue growth factor (CTGF), is suppressed by WT1 both in its endogenous location and in reporter constructs. WT1 regulation of CTGF expression is not mediated by previously identified WT1 recognition elements and may therefore involve a novel mechanism. Our results indicate that CTGF is a bona fide target of WT1 transcriptional suppression and likely plays a role in Wilms tumorigenesis and associated disease syndromes.

Physical and functional interaction between two pluripotent proteins, the Y-box DNA/RNA-binding factor, YB-1, and the multivalent zinc finger factor, CTCF

CTCF is a unique, highly conserved, and ubiquitously expressed 11 zinc finger (ZF) transcriptional factor with multiple DNA site specificities. It is able to bind to varying target sequences to perform different regulatory roles, including promoter activation or repression, creating hormone-responsive gene silencing elements, and functional block of enhancer-promoter interactions. Because different sets of ZFs are utilized to recognize different CTCF target DNA sites, each of the diverse DNA.CTCF complexes might engage different essential protein partners to define distinct functional readouts. To identify such proteins, we developed an affinity chromatography method based on matrix-immobilized purified recombinant CTCF. This approach resulted in isolation of several CTCF protein partners. One of these was identified as the multifunctional Y-box DNA/RNA-binding factor, YB-1, known to be involved in transcription, replication, and RNA processing. We examined CTCF/YB-1 interaction by reciprocal immunoprecipitation experiments with anti-CTCF and anti-YB-1 antibodies, and found that CTCF and YB-1 form complexes in vivo. We show that the bacterially expressed ZF domain of CTCF is fully sufficient to retain YB-1 in vitro. To assess possible functional significance of CTCF/YB-1 binding, we employed the very first identified by us, negatively regulated, target for CTCF (c-myc oncogene promoter) as a model in co-transfection assays with both CTCF and YB-1 expression vectors. Although expression of YB-1 alone had no effect, co-expression with CTCF resulted in a marked enhancement of CTCF-driven c-myc transcriptional repression. Thus our findings demonstrate, for the first time, the biological relevance of the CTCF/YB-1 interaction.

Direct coupling of transcription and mRNA processing through the thermogenic coactivator PGC-1

Transcription and mRNA processing are coupled events in vivo, but the mechanisms that coordinate these processes are largely unknown. PGC-1 is a transcriptional coactivator that plays a major role in the regulation of adaptive thermogenesis. PGC-1 also has certain motifs characteristic of splicing factors. We demonstrate here that mutations in the serine- and arginine-rich domain and RNA recognition motif of PGC-1 interfere with the ability of PGC-1 to induce mRNAs of target genes. These mutations also disrupt the ability of PGC-1 to co-localize and associate with RNA processing factors. PGC-1 can alter the processing of an mRNA, but only when it is loaded onto the promoter of the gene. These data demonstrate the coordinated regulation of RNA transcription and processing through PGC-1.

E-cadherin is a WT1 target gene

The WT1 tumor suppressor gene encodes a transcription factor that can activate and repress gene expression. Transcriptional targets relevant for the growth suppression functions of WT1 are poorly understood. We found that mesenchymal NIH 3T3 fibroblasts stably expressing WT1 exhibit growth suppression and features of epithelial differentiation including up-regulation of E-cadherin mRNA. Acute expression of WT1 in NIH 3T3 fibroblasts after retroviral infection induced murine E-cadherin expression. In transient transfection experiments, the human and murine E-cadherin promoters were activated by co-expression of WT1. E-cadherin promoter activity was increased in cells overexpressing WT1 and was blocked by a dominant negative form of WT1. WT1 activated the murine E-cadherin promoter through a conserved GC-rich sequence similar to an EGR-1 binding site as well as through a CAAT box sequence. WT1 produced in vitro or derived from nuclear extracts bound to the WT1-response element within the murine E-cadherin promoter, but not the CAAT box. E-cadherin, a gene important in epithelial differentiation and neoplastic transformation, represents a downstream target gene that links the roles of the WT1 in differentiation and growth control.

Physical and functional interaction between the HCMV IE2 protein and the Wilms' tumor suppressor WT1

Human cytomegalovirus (HCMV) is a major renal pathogen in congenitally infected infants and renal allograft recipients. It has been shown that human kidney cells of glomerular, tubular, and vascular origin were all infected by HCMV in vitro. It has previously been demonstrated that the IE2 protein of HCMV directly associates with the zinc finger domain of Egr-1. The zinc finger region of WT1 is a sequence-specific DNA-binding domain which also recognizes the consensus DNA binding site (5'-CGCCCCCGC-3') of Egr-1, thus suggesting a possible interaction between WT1 and IE2. Here we demonstrate that HCMV IE2 binds to the C-terminal region of WT1 containing zinc finger domain in vivo as well as in vitro and that WT1 can inhibit IE2-driven transactivation of the responsive promoter. Our results suggest that WT1 may be able to regulate the functional activity of HCMV IE2. Furthermore, these data may provide new insights into the possible involvement of HCMV in WT1-related pathogeneses.

The Wilms' tumor 1 tumor suppressor gene represses transcription of the human telomerase reverse transcriptase gene

Regulation of the human telomerase reverse transcriptase (hTERT) gene is the primary determinant for telomerase enzyme activity, which is found in tumor cells but is largely absent from normal somatic cells. Recent studies have shown that Myc protein can transcriptionally activate the hTERT gene. However, little is known about the repression mechanism of the hTERT gene and telomerase enzyme. Here, we developed an expression cloning strategy to identify cDNAs whose products can repress hTERT promoter activity in telomerase-positive immortal cells. Using this screen, we isolated the Wilms' tumor 1 suppressor gene (WT1). WT1 can repress hTERT promoter activity in 293 kidney cells. The WT1 binding site on the hTERT promoter was identified by deletional analysis. Alteration of the WT1 binding site markedly derepresses transcription from an isolated hTERT promoter by inhibiting interaction of WT1 with DNA. These specific repression effects of WT1 were not observed in HeLa cells, which express no endogenous WT1. Furthermore, we show that WT1 can repress the endogenous hTERT promoter and telomerase enzyme activities. These results suggest that WT1 may be a transcriptional repressor of the hTERT gene, at least in some specific cells.

Regulation of the Wilms' tumour suppressor protein transcriptional activation domain

The Wilms' tumour suppressor protein WT1 contains a transcriptional regulatory domain that can either activate or repress transcription depending upon its cellular environment. The mechanistic basis for this dichotomy is unclear however. Here, we dissect the transcriptional regulatory domains of WT1. We find that a region within the domain of WT1 attributed to transcriptional repression is a potent suppressor of the activation domain at several promoters and in different cell types. In vitro transcription analysis suggests that the mechanism of suppression of the activation domain occurs at the level of transcription initiation. Furthermore we find that the WT1 suppression domain is able to inhibit a heterologous activation domain when fused in cis. Dissection of this domain resulted in the delineation of a 30 amino acid region that was sufficient to confer suppression of a transcriptional activation domain both in vivo and in vitro. Additionally, we find that the WT1 transcriptional activation domain interacts with the general transcription factor TFIIB and that this interaction is not affected by the suppression domain. Taken together, these studies suggest that the suppression domain of WT1 interacts with a cosuppressor protein to mediate inhibition of the WT1 transcriptional activation domain.

The cellular response to p53: the decision between life and death

The p53 tumor suppressor protein plays a crucial role in regulating cell growth following exposure to various stress stimuli. p53 induces either growth arrest, which prevents the replication of damaged DNA, or programmed cell death (apoptosis), which is important for eliminating defective cells. Whether the cell enters growth arrest or undergoes apoptosis, depends on the final integration of incoming signals with antagonistic effects on cell growth. Many factors affect the cellular response to activated p53. These include the cell type, the oncogenic status of the cell with emphasis on the Rb/E2F balance, the extracellular growth and survival stimuli, the intensity of the stress signals, the level of p53 expression and the interaction of p53 with specific proteins. p53 is regulated both at the levels of protein stability and biochemical activities. This complex regulation is mediated by a range of viral and cellular proteins. This review discusses this intriguing complexity which affects the cell response to p53 activation.

The Wilms tumor suppressor gene wt1 is required for development of the spleen

The Wilms tumor suppressor gene WT1 (wt1 in mouse) is unique among tumor suppressors because, in addition to its involvement in cancer [1] [2] and various other diseases [3] [4] [5] [6], it has an essential role in the development of certain organs. This is revealed by the phenotype of mice with inactivated wt1 alleles [7]. These animals exhibit a complete failure of kidney and gonad development as well as abnormalities of the heart and mesothelial structures. On a C57BL/6 genetic background, wt1(-/-) animals die between day 13.5 (E13.5) and 15.5 (E15.5) of embryonic development [7]. We report here that crossing of the wt1 mutation onto different mouse backgrounds delayed embryonic lethality until birth. In wt1(-/-) mice on these different genetic backgrounds, we observed a dramatic failure of spleen development, in addition to the well characterized phenotypic abnormalities. The spleen anlage formed at around E12 to E13 and involuted by the E15 stage, before the invasion of hematopoietic cells. The absence of proper spleen development in these wt1(-/-) embryos correlated with enhanced apoptosis in the primordial spleen cells. The expression of hox11, a gene that also controls development of the spleen [8] [9], was not altered by the inactivation of wt1. In situ hybridization revealed that the two genes are regulated independently. These findings demonstrate that the penetrance of the wt1(-/-) phenotype depends on the existence of one or more modifier gene(s) and that wt1 plays a pivotal role in the development of the spleen, thereby extending its role in organogenesis.