Embryonic Kidney Development, Stem Cells and the Origin of Wilms Tumor
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G C A T T A C G G C A T genes Review Embryonic Kidney Development, Stem Cells and the Origin of Wilms Tumor Hao Li 1, Peter Hohenstein 2 and Satu Kuure 1,3,* 1 Stem Cells and Metabolism Research Program, Faculty of Medicine, University of Helsinki, FIN-00014 Helsinki, Finland; hao.li@helsinki.fi 2 Department of Human Genetics, Leiden University Medical Center, 2300 RC Leiden, The Netherlands; [email protected] 3 GM-Unit, Laboratory Animal Center, Helsinki Institute of Life Science, University of Helsinki, FIN-00014 Helsinki, Finland * Correspondence: satu.kuure@helsinki.fi; Tel.: +358-2941-59395 Abstract: 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. Keywords: kidney; organogenesis; pediatric cancer; stem cells; differentiation; self-renewal Citation: Li, H.; Hohenstein, P.; Kuure, S. Embryonic Kidney Development, Stem Cells and the Origin of Wilms Tumor. Genes 2021, 1. Introduction 12, 318. https://doi.org/10.3390/ genes12020318 Stem cells form a fundamental basis not only for normal development and tissue homeostasis but also for tumorigenesis. The adult mammalian kidney is mostly devoid of Academic Editor: Simona Chera stem cells and is thus considered a non-regenerative organ, especially when regeneration is defined by the ability to generate new nephrons and regain filtration capacity [1]. Contrary Received: 27 January 2021 to this, the embryonic kidney hosts at least three types of lineage-specific stem cells (referred Accepted: 18 February 2021 to here as progenitors) that give rise to the ureter and collecting duct system, nephrons, Published: 23 February 2021 and the connective tissue of the stroma [2,3]. These progenitor cells have raised substantial interest for use in regeneration-based renal medicine, but the safety of the undifferentiated Publisher’s Note: MDPI stays neutral cell maintenance in postnatal kidneys is a significantly less discussed topic. with regard to jurisdictional claims in An integral part of the safety assessment is a thorough understanding of tissue- published maps and institutional affil- residing progenitor regulation. The mechanisms guiding the maintenance and differen- iations. tiation of tissue-specific progenitors are aberrantly reactivated in many cancers [4]. This is especially obvious in embryonic-derived tumors such as Wilms tumor (WT; Figure1), medulloblastoma, and retinoblastoma. Nephrons—the functional filtration units of mammalian kidneys—and renal stroma Copyright: © 2021 by the authors. are derived from metanephric mesenchyme containing distinct progenitor pools for both Licensee MDPI, Basel, Switzerland. lineages [5,6]. Metanephric tissue is normally lost before birth in humans but remains part This article is an open access article of the undifferentiated nephrogenic rests in Wilms tumor patients [7,8]. Understanding distributed under the terms and the differences between normal tissue-residing progenitors and cancer-causing stem cells conditions of the Creative Commons is required to set the stage for better characterization of transformations that turn normal Attribution (CC BY) license (https:// kidney progenitors into Wilms tumor stem cells. This may essentially help with predict- creativecommons.org/licenses/by/ ing individualized disease prognosis and tumor chemosensitivity, thus facilitating better 4.0/). Genes 2021, 12, 318. https://doi.org/10.3390/genes12020318 https://www.mdpi.com/journal/genes Genes 2021, 12, 318 2 of 21 patient stratification and identification of the patients requiring aggressive treatment strate- gies [9]. This review gives an overview of kidney development followed by a summary of how collecting duct and nephron progenitors contribute to normal renal morphogenesis in Genes 2021, 12, x FOR PEER REVIEW 2 of 22 order to better facilitate the understanding of their fundamental similarities and differences with Wilms tumor biology. FigureFigure 1. Nephroblastomatosis 1. Nephroblastomatosis and and Wilms Wilms tumor. tumor. ( (AA)) An example ofof humanhuman postnatal postnatal kidney kidney with with nephrogenic nephrogenic rests rests (asterisks)(asterisks) of the of the nephroblastomatosis, nephroblastomatosis, which which areare seenseen as as tightly tightly packed packed darker darker blue blue cells cells in the in renalthe renal cortex. cortex. (B) An (B example) An exam- ple of human kidney kidney with with the the classical classical morphology morphology of Wilms of Wilms tumor. tumor. It resembles It resembles embryonic embryonic kidney by kidney exhibiting by exhibiti blastemalng (Bblaste-), malstromal (B), stromal (S), and (S) epithelial, and epithelial cells (arrows), cells (arrows), which, however, which, however fail to organize, fail to into organize typical into tissue typical structures. tissue structures. 2. WilmsNephrons Tumor—the functional filtration units of mammalian kidneys—and renal stroma are derivedWilms from tumors metanephric (Figure1) are mesenchyme one of the most containing common soliddistinct tumors progenitor in children pool withs for an both lineagesincidence [5,6] of. 1:10,000,Metanephric usually tissue appearing is normally before lost the before age of five.birth Although in humans good but treatmentremains part ofoptions the undifferentiated exist for some histologicalnephrogenic tumor rests typesin Wilms and theirtumor survival patients rates [7,8] are. Understanding good, other thetypes, differences like anaplastic between Wilms normal tumors, tissue still-residing have a progenitors 5-year survival and of cancer only 50%.-causing Therefore, stem cells is thererequired remains to set a clear the stage clinical for need better for thecharacterization improvement of of Wilms transformations tumor therapeutic that turn options; normal kidneyto achieve progenitors this, a better into basicWilms understanding tumor stem cells of these. This tumors may essentially is essential. help with predicting individualizedAs reviewed disease extensively prognosis elsewhere and tumor [7], Wilms chemosensitivity tumors have been, thus intriguing facilitating clinicians, better pa- tientpathologists, stratification and and cancer identification geneticists forof th ae long patients time. requiring The tumors aggressive are the direct treatmen resultt strate- of giesproblems [9]. This during review the gives embryonic an overview development of kidney of the development kidney, and followed it was one by of a the summary cancer of types based on which Alfred Knudson developed his two-hit model for tumor suppressor how collecting duct and nephron progenitors contribute to normal renal morphogenesis genes [10]. It is widely believed that nephrogenic rests—embryonic tissues that are retained in order to better facilitate the understanding of their fundamental similarities and differ- in the postnatal kidney—are the precursor lesions of Wilms tumors, and indeed, the tumors encesthemselves with Wilms contain tumor cell typesbiology normally. found only in the embryonic kidney. A subset of cases, the ones caused by the loss of the Wilms Tumor1 (WT1) tumor suppressor gene, 2. additionallyWilms Tumor present with the development of ectopic, non-renal tissues, especially striated muscle. All this points to the disruption of normal development as the primary cause of WilmsWilms tumors. tumors We therefore(Figure 1) need are toone understand of the most normal common kidney solid development tumors in inchildren order to with anreally incidence understand of 1:10 Wilms,000, tumorsusually and appearing exploit this before understanding the age of to five. develop Although better and good more treat- mentspecific options therapies. exist Atfor the some same histological time, the primary tumor defect types leading and their to Wilms survival tumors rates could are begood, otherconsidered types, like a naturally anaplastic occurring Wilms model tumors, to understand still have a normal 5-year kidneysurvival development. of only 50%. There- fore, there remains a clear clinical need for the improvement of Wilms tumor therapeutic options3. Embryonic; to achieve Kidney this, a better basic understanding of these tumors is essential. AsKidney reviewed morphogenesis extensively is elsewhere a classic example [7], Wilms of balanced tumors have reciprocal been tissue intriguing interac- clini- cians,tions pathologists [11–13]. Much, and of ourcancer basic genet understandingicists for a oflong how time. the kidneyThe tumors develops are derivesthe direct from result the classical in vitro tissue recombination/induction experiments in different model organ- of problems during the embryonic development of the kidney, and it was one of the cancer isms that are complemented with in vivo gene inactivation studies