Deducing the stage of origin of Wilms' tumours from a developmental series of Wt1-mutant mice

Hallmark discoveries in the biology of Wilms tumour

The modern study of Wilms tumour was prompted nearly 50 years ago, when Alfred Knudson proposed the 'two-hit' model of tumour development. Since then, the efforts of researchers worldwide have substantially expanded our knowledge of Wilms tumour biology, including major advances in genetics - from cloning the first Wilms tumour gene to high-throughput studies that have revealed the genetic landscape of this tumour. These discoveries improve understanding of the embryonal origin of Wilms tumour, familial occurrences and associated syndromic conditions. Many efforts have been made to find and clinically apply prognostic biomarkers to Wilms tumour, for which outcomes are generally favourable, but treatment of some affected individuals remains challenging. Challenges are also posed by the intratumoural heterogeneity of biomarkers. Furthermore, preclinical models of Wilms tumour, from cell lines to organoid cultures, have evolved. Despite these many achievements, much still remains to be discovered: further molecular understanding of relapse in Wilms tumour and of the multiple origins of bilateral Wilms tumour are two examples of areas under active investigation. International collaboration, especially when large tumour series are required to obtain robust data, will help to answer some of the remaining unresolved questions.

Wilms' tumor gene 1: lessons from the interface between kidney development and cancer

In 1990, mutations of the Wilms' tumor-1 gene (WT1), encoding a transcription factor in the embryonic kidney, were found in 10-15% of Wilms' tumors; germline WT1 mutations were associated with hereditary syndromes involving glomerular and reproductive tract dysplasia. For more than three decades, these discoveries prompted investigators to explore the embryonic role of WT1 and the mechanisms by which loss of WT1 leads to malignant transformation. Here, we discuss how alternative splicing of WT1 generates isoforms that act in a context-specific manner to activate or repress target gene transcription. WT1 also regulates posttranscriptional regulation, alters the epigenetic landscape, and activates miRNA expression. WT1 functions at multiple stages of kidney development, including the transition from resting stem cells to committed nephron progenitor, which it primes to respond to WNT9b signals from the ureteric bud. WT1 then drives nephrogenesis by activating WNT4 expression and directing the development of glomerular podocytes. We review the WT1 mutations that account for Denys-Drash syndrome, Frasier syndrome, and WAGR syndrome. Although the WT1 story began with Wilms' tumors, an understanding of the pathways that link aberrant kidney development to malignant transformation still has some important gaps. Loss of WT1 in nephrogenic rests may leave these premalignant clones with inadequate DNA repair enzymes and may disturb the epigenetic landscape. Yet none of these observations provide a complete picture of Wilms' tumor pathogenesis. It appears that the WT1 odyssey is unfinished and still holds a great deal of untilled ground to be explored.

The tumor suppressor WT1 drives progenitor cell progression and epithelialization to prevent Wilms tumorigenesis in human kidney organoids

Wilms tumor is the most widespread kidney cancer in children and frequently associated with homozygous loss of the tumor suppressor WT1. Pediatric tumorigenesis is largely inaccessible in humans. Here, we develop a human kidney organoid model for Wilms tumor formation and show that deletion of WT1 during organoid development induces overgrowth of kidney progenitor cells at the expense of differentiating glomeruli and tubules. Functional and gene expression analyses demonstrate that absence of WT1 halts progenitor cell progression at a pre-epithelialized cell state and recapitulates the transcriptional changes detected in a subgroup of Wilms tumor patients with ectopic myogenesis. By "transplanting" WT1 mutant cells into wild-type kidney organoids, we find that their propagation requires an untransformed microenvironment. This work defines the role of WT1 in kidney progenitor cell progression and tumor suppression, and establishes human kidney organoids as a phenotypic model for pediatric tumorigenesis.

In vitro Modeling of Embryonal Tumors

A subset of pediatric tumors affects very young children and are thought to arise during fetal life. A common theme is that these embryonal tumors hijack developmental programs, causing a block in differentiation and, as a consequence, unrestricted proliferation. Embryonal tumors, therefore typically maintain an embryonic gene signature not found in their differentiated progeny. Still, the processes underpinning malignant transformation remain largely unknown, which is hampering therapeutic innovation. To gain more insight into these processes, in vitro and in vivo research models are indispensable. However, embryonic development is an extremely dynamic process with continuously changing cellular identities, making it challenging to define cells-of-origin. This is crucial for the development of representative models, as targeting the wrong cell or targeting a cell within an incorrect developmental time window can result in completely different phenotypes. Recent innovations in in vitro cell models may provide more versatile platforms to study embryonal tumors in a scalable manner. In this review, we outline different in vitro models that can be explored to study embryonal tumorigenesis and for therapy development.

Embryonic Kidney Development, Stem Cells and the Origin of Wilms Tumor

The adult mammalian kidney is a poorly regenerating organ that lacks the stem cells that could replenish functional homeostasis similarly to, e.g., skin or the hematopoietic system. Unlike a mature kidney, the embryonic kidney hosts at least three types of lineage-specific stem cells that give rise to (a) a ureter and collecting duct system, (b) nephrons, and (c) mesangial cells together with connective tissue of the stroma. Extensive interest has been raised towards these embryonic progenitor cells, which are normally lost before birth in humans but remain part of the undifferentiated nephrogenic rests in the pediatric renal cancer Wilms tumor. Here, we discuss the current understanding of kidney-specific embryonic progenitor regulation in the innate environment of the developing kidney and the types of disruptions in their balanced regulation that lead to the formation of Wilms tumor.

Expansion of the renal capsular stroma, ureteric bud branching defects and cryptorchidism in mice with Wilms tumor 1 gene deletion in the stromal compartment of the developing kidney

Development of the mammalian kidney is orchestrated by reciprocal interactions of stromal and nephrogenic mesenchymal cells with the ureteric bud epithelium. Previous work showed that the transcription factor Wilms tumor 1 (WT1) acts in the nephrogenic lineage to maintain precursor cells, to drive the epithelial transition of aggregating precursors into a renal vesicle and to specify and maintain the podocyte fate. However, WT1 is expressed not only in the nephrogenic lineage but also transiently in stromal progenitors in the renal cortex. Here we report that specific deletion of Wt1 in the stromal lineage using the Foxd1^cre driver line results at birth in cryptorchidism and hypoplastic kidneys that harbour fewer and enlarged ureteric bud tips and display an expansion of capsular stroma into the cortical region. In vivo and ex vivo analysis at earlier stages revealed that stromal loss of Wt1 reduces stromal proliferation, and delays and alters branching morphogenesis, resulting in a variant architecture of the collecting duct tree with an increase of single at the expense of bifurcated ureteric bud tips. Molecular analysis identified a transient reduction of Aldh1a2 expression and of retinoic acid signalling activity in stromal progenitors, and of Ret in ureteric bud tips. Administration of retinoic acid partly rescued the branching defects of mutant kidneys in culture. We propose that WT1 maintains retinoic acid signalling in the cortical stroma, which, in turn, assures proper levels and dynamics of Ret expression in the ureteric bud tips, and thus normal ramification of the ureteric tree. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

In Vivo Developmental Trajectories of Human Podocyte Inform In Vitro Differentiation of Pluripotent Stem Cell-Derived Podocytes

The renal corpuscle of the kidney comprises a glomerular vasculature embraced by podocytes and supported by mesangial myofibroblasts, which ensure plasma filtration at the podocyte-generated slit diaphragm. With a spectrum of podocyte-expressed gene mutations causing chronic disease, an enhanced understanding of podocyte development and function to create relevant in vitro podocyte models is a clinical imperative. To characterize podocyte development, scRNA-seq was performed on human fetal kidneys, identifying distinct transcriptional signatures accompanying the differentiation of functional podocytes from progenitors. Interestingly, organoid-generated podocytes exhibited highly similar, progressive transcriptional profiles despite an absence of the vasculature, although abnormal gene expression was pinpointed in late podocytes. On transplantation into mice, organoid-derived podocytes recruited the host vasculature and partially corrected transcriptional profiles. Thus, human podocyte development is mostly intrinsically regulated and vascular interactions refine maturation. These studies support the application of organoid-derived podocytes to model disease and to restore or replace normal kidney functions.

Mouse Models of Congenital Kidney Anomalies

Congenital anomalies of the kidney and urinary tract (CAKUT) are common birth defects, which cause the majority of chronic kidney diseases in children. CAKUT covers a wide range of malformations that derive from deficiencies in embryonic kidney and lower urinary tract development, including renal aplasia, hypodysplasia, hypoplasia, ectopia, and different forms of ureter abnormalities. The majority of the genetic causes of CAKUT remain unknown. Research on mutant mice has identified multiple genes that critically regulate renal differentiation. The data generated from this research have served as an excellent resource to identify the genetic bases of human kidney defects and have led to significantly improved diagnostics. Furthermore, genetic data from human CAKUT studies have also revealed novel genes regulating kidney differentiation.

Improvement of membranous nephropathy by inhibition of miR-193a to affect podocytosis via targeting WT1

The objective of this paper was to explore the role and molecular mechanism of miR-193a in membranous nephropathy (MN). Experimental rats and podocytes were randomly divided into four groups: control, MN, miR-NC, and miR-193a inhibitor groups. The relative mRNA level of miR-193a was determined. The mRNA level and protein expression of PODXL, NPHS1, and Notch1 were determined by real-time polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. The mRNA level and protein expression of WT1 in podocytes were also determined by RT-PCR and Western blot analysis. The relative mRNA level of miR-193a in the MN group was significantly higher than that in the control group, and inhibition of miR-193a inhibited the increase successfully. Inhibition of miR-193a inhibited renal injury, podocyte injury, and tissue cell apoptosis resulting from MN. The expression of PODXL, NPHS1, and Notch1 was decreased in the MN group, while the expression was increased in the miR-193a inhibitor group. WT1 was verified as a target gene of miR-193a and the expression of WT1 increased after inhibition of miR-193a. Inhibition of miR-193a by targeting WT1 could inhibit renal function injury, renal tissue cell apoptosis, and podocytosis.

Tamoxifen- and Mifepristone-Inducible Versions of CRISPR Effectors, Cas9 and Cpf1

Methods for making specific modifications to the genomes of living cells are powerful research tools. Two methods currently dominate, CRISPR and Cre recombinase. CRISPR has the advantage that it can act on unmodified target genes; Cre has the advantage of being available in drug-inducible versions, allowing temporal control, but it requires engineering ("floxing") of the target gene. Here, we have combined these advantages by constructing drug (tamoxifen/mifepristone)-inducible Cas9 and Cpf1 CRISPR effectors. We demonstrate their low background activity and robust activation with drugs, by using gRNAs to target them to TetR, in a cell carrying a Tet-repressed reporter gene. As well as being useful in their own right, the research tools generated here will pave the way to making further drug-inducible effector proteins.

Kidney Nephron Determination

The nephron is a multifunctional filtration device equipped with an array of sophisticated sensors. For appropriate physiological function in the human and mouse, nephrons must be stereotypically arrayed in large numbers, and this essential structural property that defines the kidney is determined during its fetal development. This review explores the process of nephron determination in the fetal kidney, providing an overview of the foundational literature in the field as well as exploring new developments in this dynamic research area. Mechanisms that ensure that a large number of nephrons can be formed from a small initial number of progenitor cells are central to this process, and the question of how the nephron progenitor cell population balances epithelial differentiation with renewal in the progenitor state is a subject of particular interest. Key growth factor signaling pathways and transcription factor networks are discussed.

Wilms' tumour 1 (WT1) in development, homeostasis and disease

The study of genes mutated in human disease often leads to new insights into biology as well as disease mechanisms. One such gene is Wilms' tumour 1 (WT1), which plays multiple roles in development, tissue homeostasis and disease. In this Primer, I summarise how this multifaceted gene functions in various mammalian tissues and organs, including the kidney, gonads, heart and nervous system. This is followed by a discussion of our current understanding of the molecular mechanisms by which WT1 and its two major isoforms regulate these processes at the transcriptional and post-transcriptional levels.

WT1 Is Necessary for the Proliferation and Migration of Cells of Renin Lineage Following Kidney Podocyte Depletion

Wilms' tumor suppressor 1 (WT1) plays an important role in cell proliferation and mesenchymal-epithelial balance in normal development and disease. Here, we show that following podocyte depletion in three experimental models, and in patients with focal segmental glomerulosclerosis (FSGS) and membranous nephropathy, WT1 increased significantly in cells of renin lineage (CoRL). In an animal model of FSGS in RenWt1^fl/fl reporter mice with inducible deletion of WT1 in CoRL, CoRL proliferation and migration to the glomerulus was reduced, and glomerular disease was worse compared with wild-type mice. To become podocytes, CoRL undergo mesenchymal-to-epithelial transformation (MET), typified by reduced staining for mesenchymal markers (MYH11, SM22, αSMA) and de novo expression of epithelial markers (E-cadherin and cytokeratin18). Evidence for changes in MET markers was barely detected in RenWt1^fl/fl mice. Our results show that following podocyte depletion, WT1 plays essential roles in CoRL proliferation and migration toward an adult podocyte fate.