Identification and characterization of cellular genes differentially expressed in a subset of tumors with non -functional WT1

Wilms tumor (WT) or nephroblastoma is a genetically heterogeneous pediatric renal tumor that accounts for 6–7% of all childhood cancers in the U.S. WT1, located at 11p13, is the sole WT gene cloned to date. Additional genomic regions containing genes that play a role in the development of Wilms tumor include 11p15, 7p, 16q, 1p, 17q and 19q. This heterogeneity has made it extremely difficult to develop an understanding of the pathways involved in the development of WT, even in the 5–20% of tumors that show mutations at the WT1 locus. My research addresses this gap in our current comprehension of the development of WT. ^ I have used two complementary approaches to extend the current understanding of molecular changes involved in the development of WT. In order to minimize complexities due to genetic heterogeneity, I confined my analysis to the WT1 pathway by assessing those genetically defined tumors that carry WT1 mutations. WT1 encodes a zinc finger transcription factor, and in vitro studies have identified many genes that are potentially regulated in vivo by WT1. However, there is very little in vivo data that suggests that they are transcriptionally regulated endogenously by WT1. In one approach I assessed the role of WT1 in the in vivo regulation of PDGFA and IGF2, two genes that are strong contenders for endogenous regulation by WT1. Using primary tissue samples, I found no correlation between the level of RNA expression of WT1 with either PDGFA or IGF2, suggesting that WT1 does not play a critical role in their expression in either normal kidney or WT. ^ In a parallel strategy, using differential display analysis I compared global gene expression in a subset of tumors with known homozygous inactivating WT1 mutations (WT1-tumors) to the gene expression in a panel of appropriate control tissues (fetal kidney, normal kidney, rhabdoid tumor and pediatric renal cell carcinoma). Transcripts that are aberrantly expressed in this subset of Wilms tumors are candidates for endogenous transcriptional regulation by WT1 as well as for potentially functioning in the development of WT. By this approach I identified several differentially expressed transcripts. I further characterized two of these transcripts, identifying a candidate WT gene in the process. I then performed a detailed analysis of this WT candidate gene, which maps to 7p. Future studies will shed more light on the role of these differentially expressed genes in WT. ^

Complete sequencing of the Fugu WAGR region from WT1 to PAX6: Dramatic compaction and conservation of synteny with human chromosome 11p13

The pufferfish Fugu rubripes has a genome ≈7.5 times smaller than that of mammals but with a similar number of genes. Although conserved synteny has been demonstrated between pufferfish and mammals across some regions of the genome, there is some controversy as to what extent Fugu will be a useful model for the human genome, e.g., [Gilley, J., Armes, N. & Fried, M. (1997) Nature (London) 385, 305–306]. We report extensive conservation of synteny between a 1.5-Mb region of human chromosome 11 and <100 kb of the Fugu genome in three overlapping cosmids. Our findings support the idea that the majority of DNA in the region of human chromosome 11p13 is intergenic. Comparative analysis of three unrelated genes with quite different roles, WT1, RCN1, and PAX6, has revealed differences in their structural evolution. Whereas the human WT1 gene can generate 16 protein isoforms via a combination of alternative splicing, RNA editing, and alternative start site usage, our data predict that Fugu WT1 is capable of generating only two isoforms. This raises the question of the extent to which the evolution of WT1 isoforms is related to the evolution of the mammalian genitourinary system. In addition, this region of the Fugu genome shows a much greater overall compaction than usual but with significant noncoding homology observed at the PAX6 locus, implying that comparative genomics has identified regulatory elements associated with this gene.

Modification of EWS/WT1 functional properties by phosphorylation

In many human cancers, tumor-specific chromosomal rearrangements are known to create chimeric products with the ability to transform cells. The EWS/WT1 protein is such a fusion product, resulting from a t(11;22) chromosomal translocation in desmoplastic small round cell tumors, where 265 aa from the EWS amino terminus are fused to the DNA binding domain of the WT1 tumor suppressor gene. Herein, we find that EWS/WT1 is phosphorylated in vivo on serine and tyrosine residues and that this affects DNA binding and homodimerization. We also show that EWS/WT1 can interact with, and is a substrate for, modification on tyrosine residues by c-Abl. Tyrosine phosphorylation of EWS/WT1 by c-Abl negatively regulates its DNA binding properties. These results indicate that the biological activity of EWS/WT1 is closely linked to its phosphorylation status.

Truncated WT1 mutants alter the subnuclear localization of the wild-type protein.

WT1 encodes a zinc-finger protein, expressed as distinct isoforms, that is inactivated in a subset of Wilms tumors. Both constitutional and somatic mutations disrupting the DNA-binding domain of WT1 result in a potentially dominant-negative phenotype. In generating inducible cell lines expressing wild-type isoforms of WT1 and WT1 mutants, we observed dramatic differences in the subnuclear localization of the induced proteins. The WT1 isoform that binds with high affinity to a defined DNA target, WT1(-KTS), was diffusely localized throughout the nucleus. In contrast, expression of an alternative splicing variant with reduced DNA binding affinity, WT1 (+KTS), or WT1 mutants with a disrupted zinc-finger domain resulted in a speckled pattern of expression within the nucleus. Although similar in appearance, the localization of WT1 variants to subnuclear clusters was clearly distinct from that of the essential splicing factor SC35, suggesting that WT1 is not directly involved in pre-mRNA splicing. Localization to subnuclear clusters required the N terminus of WT1, and coexpression of a truncated WT1 mutant and wild-type WT1(-KTS) resulted in their physical association, the redistribution of WT1(-KTS) from a diffuse to a speckled pattern, and the inhibition of its transactivational activity. These observations suggest that different WT1 isoforms and WT1 mutants have distinct subnuclear compartments. Dominant-negative WT1 proteins physically associate with wild-type WT1 in vivo and may result in its sequestration within subnuclear structures.

Antagonism of WT1 activity by protein self-association.

Germline loss-of-function mutations at the Wilms tumor (WT) suppressor locus WT1 are associated with a predisposition to WTs and mild genital system anomalies. In contrast, germ-line missense mutations within the WT1 gene encoding the DNA-binding domain often yield a more severe phenotype consisting of WT, sexual ambiguity, and renal nephropathy. In this report, we demonstrate that the products of mutant alleles that impair DNA recognition can antagonize WT1-mediated transcriptional repression. We demonstrate that WT1 can self-associate in vitro and in vivo and that the responsible domain maps to the amino-terminal region of the protein. Oligomers of full-length protein form less efficiently or produce less stable complexes than oligomers between truncated polypeptides and full-length protein. Our data suggest a molecular mechanism to explain how WT1 mutations may act in deregulating cellular proliferation and differentiation.

G1 phase arrest induced by Wilms tumor protein WT1 is abrogated by cyclin/CDK complexes.

WT1, the Wilms tumor-suppressor gene, maps to the human chromosomal region 11p13 and encodes a transcriptional repressor, WT1, implicated in controlling normal urogenital development. Microinjection of the WT1 cDNA into quiescent cells or cells in early to mid G1 phase blocked serum-induced cell cycle progression into S phase. The activity of WT1 varied significantly depending on the presence or absence of an alternatively spliced region located upstream of the zinc finger domain. The inhibitory activity of WT1 was abrogated by the overexpression of cyclin E/CDK2 as well as cyclin D1/CDK4. Furthermore, both CDK4- and CDK2-associated kinase activities were downregulated in cells overexpressing WT1, whereas the levels of CDK4, CDK2, and cyclin D1 expression were unchanged. These findings suggest that inhibition of the activity of cyclin/CDK complexes may be involved in mediating the WT1-induced cell cycle block.

Anlaysis of complementary expression profiles following WT1 induction versus repression reveals the cholesterol/fatty acid synthetic pathways as a possible major target of WT1

The Wilms' tumour suppressor gene, WT1, encodes a zinc-finger protein that is mutated in Wilms' tumours and other malignancies. WT1 is one of the earliest genes expressed during kidney development. WT1 proteins can activate and repress putative target genes in vitro, although the in vivo relevance of such target genes often remains unverified. To better understand the role of WT1 in tumorigenesis and kidney development, we need to identify downstream target genes. In this study, we have expression pro. led human embryonic kidney 293 cells stably transfected to allow inducible WT1 expression and mouse mesonephric M15 cells transfected with a WT1 antisense construct to abolish endogenous expression of all WT1 isoforms to identify WT1-responsive genes. The complementary overlap between the two cell lines revealed a pronounced repression of genes involved in cholesterol biosynthesis by WT1. This pathway is transcriptionally regulated by the sterol responsive element-binding proteins (SREBPs). Here, we provide evidence that the C-terminal end of the WT1 protein can directly interact with SREBP, suggesting that WT1 may modify the transcriptional function of SREBPs via a direct protein-protein interaction. Therefore, the tumour suppressor activities of WT1 may be achieved by repressing the mevalonate pathway, thereby controlling cellular proliferation and promoting terminal differentiation.

Angioblast-mesenchyme induction of early kidney development is mediated by Wt1 and Vegfa

Most studies on kidney development have considered the interaction of the metanephric mesenchyme and the ureteric bud to be the major inductive event that maintains tubular differentiation and branching morphogenesis. The mesenchyme produces Gdnf, which stimulates branching, and the ureteric bud stimulates continued growth of the mesenchyme and differentiation of nephrons from the induced mesenchyme. Null mutation of the Wt1 gene eliminates outgrowth of the ureteric bud, but Gdnf has been identified as a target of Pax2, but not of Wt1. Using a novel system for microinjecting and electroporating plasmid expression constructs into murine organ cultures, it has been demonstrated that Vegfa expression in the mesenchyme is regulated by Wt1. Previous studies had identified a population of Flk1-expressing cells in the periphery of the induced mesenchyme, and adjacent to the stalk of the ureteric bud, and that Vegfa was able to stimulate growth of kidneys in organ culture. Here it is demonstrated that signaling through Flk1 is required to maintain expression of Pax2 in the mesenchyme of the early kidney, and for Pax2 to stimulate expression of Gdnf. However, once Gdnf stimulates branching of the ureteric bud, the Flk1-dependent angioblast signal is no longer required to maintain branching morphogenesis and induction of nephrons. Thus, this work demonstrates the presence of a second set of inductive events, involving the mesenchymal and angioblast populations, whereby Wt1-stimulated expression of Vegfa elicits an as-yet-unidentified signal from the angioblasts, which is required to stimulate the expression of Pax2 and Gdnf, which in turn elicits an inductive signal from the ureteric bud.

Multifunctional role of steroidogenic factor 1 and disorders of sex development

Disorders of sex development (DSD) involve several conditions that result from abnormalities during gonadal determination and differentiation. Some of these disorders may manifest at birth by ambiguous genitalia; others are diagnosed only at puberty, by the delayed onset of secondary sexual characteristics. Sex determination and differentiation in humans are processes that involve the interaction of several genes such as WT1, NR5A1, NR0B1, SOX9, among others, in the testicular pathway, and WNT4, DAX1, FOXL2 and RSPO1, in the ovarian pathway. One of the major proteins in mammalian gonadal differentiation is the steroidogenic nuclear receptor factor 1 (SF1). This review will cover some of the most recent data on SF1 functional roles and findings related to mutations in its coding gene, NR5A1.

Down-regulation of the candidate tumor suppressor gene PAR-4 is associated with poor prognosis in breast cancer

Substantial experimental evidence indicates that PAWR gene (PKC apoptosis WT1 regulator; also named PAR-4, prostate apoptosis response-4) is a central player in cancer cell survival and a potential target for cancer-selective targeted therapeutics. However, little is known about the role of PAR-4 in breast cancer. We investigated the possible role of PAR-4 expression in breast cancer. IHC results on tissue microarrays containing 1,161 primary breast tumor samples showed that 57% (571/995) of analyzable cases were negative for PAR-4 nuclear staining. Down-regulation of nuclear PAR-4 protein expression predicted a poor prognosis for breast cancer patients (OS; P=0.041, log-rank test). PAR-4 down-regulation also correlates with poor survival in the group of patients with luminal A subtype breast cancer (P=0.028). Additionally, in this large series of breast cancer patients, we show that ERBB2/HER2, EGFR and pAKT protein expression are significantly associated with shorter disease-free survival and overall survival, but the prognosis was even worse for HER2-positive, EGFR-positive or pAKT-positive breast cancer patients with tumors negative for nuclear PAR-4 expression. Furthermore, using three-dimensional (3D) cell culture we provide preliminary results showing that PAR-4 is highly expressed in the MCF10A cells inside the acini structure, suggesting that PAR-4 might have a role in the lumen acini formation. Taken together, our results provide, for the first time, evidence that PAR-4 may have a role in the process of the mammary eland morphogenesis and its functional inactivation is associated with tumor aggressive phenotype and might represent an additional prognostic and predictive marker for breast cancer.

Exogenous Slit2 does not affect ureteric branching or nephron formation during kidney development

In an attempt to elucidate the role of Slit2 invertebrate kidney development, the effect of adding exogenous human Slit2 protein (hSlit2) to developing murine metanephric kidney explants was examined. To confirm the activity of the recombinant Slit2 protein, neurons from 8 day old chick sympathetic nerve chain dorsal root ganglia were cultured with hSlit2 protein, which induced significant neurite branching and outgrowth. Using kidney explants as a model system, metanephric development in the presence of hSlit2 protein was examined. Addition of hSlit2 up to a final concentration of 1 mug/ml had no detectable effect on the formation of nephrons or on branching morphogenesis of the ureteric tree after 2 or 4 days in culture, as assessed via immunofluorescence for the markers WT1 and calbindin 28K respectively. Similarly, maturation of the nephrogenic mesenchyme occurred in a phenotypically normal fashion. In situ analysis of the Slit receptors, Robot and Robot, the vasculogenic markers VEGFA and Flk-1, and the stromal cell marker BF2 displayed no difference in comparison to controls.

SOX8 is expressed during testis differentiation in mice and synergizes with SF1 to activate the Amh promoter in vitro

Sox8 is a member of the Sox family of developmental transcription factor genes and is closely related to Sox9, a key gene in the testis determination pathway in mammals. Like Sox9, Sox8 is expressed in the developing mouse testis around the time of sex determination, suggesting that it might play a role in regulating the expression of testis-specific genes. An early step in male sex differentiation is the expression of anti-Mullerian hormone (AMH) in Sertoli cells. Expression of the Amh gene during sex differentiation requires the interaction of several transcription factors, including SF1, SOX9, GATA4, WT1, and DAX1. Here we show that SOX8 may also be involved in regulating the expression of Amh. Expression of Sox8 begins just prior to that of Amh at 12 days post coitum (dpc) in mouse testes and continues beyond 16 dpc in Sertoli cells. In vitro assays showed that SOX8 binds specifically to SOX binding sites within the Amh minimal promoter and, like SOX9, acts synergistically with SF1 through direct protein-protein interaction to enhance Amh expression, albeit at lower levels compared with SOX9. SOX8 and SOX9 appear to have arisen from a common ancestral gene and may have retained some common functions during sexual development. Our data provide the first evidence that SOX8 may partially compensate for the reduced SOX9 activity in campomelic dysplasia and substitute for Sox9 where Sox9 is either not expressed or expressed too late to be involved in sex determination or regulation of Amh expression.

Estudo mutacional em tumor de Wilms por sequenciação de nova geração

O Tumor de Wilms (TW) é o tumor renal mais comum da infância, com uma incidência de 1 em ~10000 crianças. Esta patologia é de etiologia genética complexa e diversificada. No entanto, cerca de um terço dos doentes apresenta mutações somáticas associadas aos genes WT1, CTNNB1, TP53 e/ou AMER1. Assim, foi desenvolvido um painel de amplicões destes 4 genes para a identificação de mutações num grupo de doentes portugueses com TW, através de uma metodologia baseada na sequenciação de nova geração. As bibliotecas de DNA foram preparadas a partir de amostras de sangue periférico e tumor de 36 doentes com TW e sequenciadas no MiSeq. Foram identificadas alterações somáticas em 7 dos 36 (19,4%) doentes estudados. Conclui-se que a sequenciação de um painel de genes é um método rápido para a deteção de mutações somáticas quando desenhado com cuidado de forma a serem evitados problemas de perda de cobertura.

Cells derived from the coelomic epithelium contribute to multiple gastrointestinal tissues in mouse embryos.

Gut mesodermal tissues originate from the splanchnopleural mesenchyme. However, the embryonic gastrointestinal coelomic epithelium gives rise to mesenchymal cells, whose significance and fate are little known. Our aim was to investigate the contribution of coelomic epithelium-derived cells to the intestinal development. We have used the transgenic mouse model mWt1/IRES/GFP-Cre (Wt1(cre)) crossed with the Rosa26R-EYFP reporter mouse. In the gastrointestinal duct Wt1, the Wilms' tumor suppressor gene, is specific and dynamically expressed in the coelomic epithelium. In the embryos obtained from the crossbreeding, the Wt1-expressing cell lineage produces the yellow fluorescent protein (YFP) allowing for colocalization with differentiation markers through confocal microscopy and flow cytometry. Wt1(cre-YFP) cells were very abundant throughout the intestine during midgestation, declining in neonates. Wt1(cre-YFP) cells were also transiently observed within the mucosa, being apparently released into the intestinal lumen. YFP was detected in cells contributing to intestinal vascularization (endothelium, pericytes and smooth muscle), visceral musculature (circular, longitudinal and submucosal) as well as in Cajal and Cajal-like interstitial cells. Wt1(cre-YFP) mesenchymal cells expressed FGF9, a critical growth factor for intestinal development, as well as PDGFRα, mainly within developing villi. Thus, a cell population derived from the coelomic epithelium incorporates to the gut mesenchyme and contribute to a variety of intestinal tissues, probably playing also a signaling role. Our results support the origin of interstitial cells of Cajal and visceral circular muscle from a common progenitor expressing anoctamin-1 and SMCα-actin. Coelomic-derived cells contribute to the differentiation of at least a part of the interstitial cells of Cajal.