Novel Insights into the Pathogenesis of Monogenic Congenital Anomalies of the Kidney and Urinary Tract

Abstract

Congenital anomalies of the kidneys and urinary tract (CAKUT) comprise a large spectrum of congenital malformations ranging from severe manifestations, such as renal agenesis, to potentially milder conditions, such as vesicoureteral reflux. CAKUT causes approximately 40% of ESRD that manifests within the first three decades of life. Several lines of evidence indicate that CAKUT is often caused by recessive or dominant mutations in single (monogenic) genes. To date, approximately 40 monogenic genes are known to cause CAKUT if mutated, explaining 5%-20% of patients. However, hundreds of different monogenic CAKUT genes probably exist. The discovery of novel CAKUT-causing genes remains challenging because of this pronounced heterogeneity, variable expressivity, and incomplete penetrance. We here give an overview of known genetic causes for human CAKUT and shed light on distinct renal morphogenetic pathways that were identified as relevant for CAKUT in mice and humans.

Keywords: CAKUT; congenital anomalies of the kidneys and urinary tract; genetic kidney disease; monogenic disease.

Figure 1.

Development of the kidneys and urinary tract. (A) The bilateral nephric ducts (NDs; alternatively mesonephric ducts or Wolffian ducts) and the nephric cords (NCs) are the precursor structures of the adult urinary system. Both originate from the embryonic intermediate mesoderm. The cells of the ND undergo an early mesenchymal to epithelial transition and assemble into epithelial tube–like structures. The associated NC retains characteristics of mesenchymal tissue.^, (B) As the embryo develops, the ND elongates caudally. At approximately E9.5, the most caudal portion of the ND fuses with the cloacal epithelium (Cl). The cloaca is the embryonic precursor of the bladder, and it is derived from cloacal endoderm.^, (C) The NC reorganizes and forms a morphologically distinct domain: the MM. Renal morphogenesis is initiated and maintained by reciprocal interactions between the epithelial ND and the MM. In mice, at embryonic day approximately E10–E10.5, signals from the MM induce the formation of a circumscribed, broad swelling of the ND at the level of the MM.^, (D) At E10.5 in mice and around the fifth week of human gestation, the UB emerges from the swollen portion of the ND and grows dorsally toward the MM.^,,, The caudal part of the ND, which is located between the UB and the insertion into the Cl, is referred to as the common nephric duct (CND). (E) Stimulated by MM-derived signals, the UB begins to branch repeatedly (branching morphogenesis) at approximately E11.5. Through continuous reciprocal induction, the MM is important for promoting and maintaining branching events of the UB. The UB branching continues for approximately 9–13 cycles (mice) and then slows down after approximately E15.5. Via branching morphogenesis, the UB gives rise to the renal collecting system consisting of collecting ducts and renal pelvis as well as the ureter. Reciprocally, signals from the UB also support development of MM cells. The MM that is in closest proximity to the UB tips condenses and forms the so-called cap mesenchyme (CM). Stimulated by signals from the UB, the CM undergoes a mesenchymal to epithelial transition. The epithelial cell population subsequently gives rise to structures of the nephron (glomerulus, the proximal tubule, and the distal tubule). Modified from refs. and , with permission.

Figure 2.

ECM proteins cause murine and human CAKUT. Schematic of the interface between the UB and the MM in renal development. Proteins encoded by genes that, if mutated, cause monogenic CAKUT in humans are highlighted in yellow. Red frames indicate proteins encoded by genes mutated in CAKUT in mice. (Left panel) FRAS1 and FREM2 localize in epithelial cells of the UB as transmembrane proteins with intracytoplasmic tail regions.^,, The interaction with PDZ domains of the intracellular protein GRIP1 is essential for targeting to the basal surface of the UB cells and shedding of FRAS1 from the membrane.^, FREM1 is produced by the MM and secreted into the extracellular space.^, FRAS1, FREM2, and FREM1 assemble at the epithelial-mesenchymal interface to form the FC. Mutations in the genes encoding these proteins cause CAKUT (Supplemental Figure 2). Also, mutations in GRIP1 result in human and murine CAKUT. Nephronectin (NPNT) functions as an adaptor to interconnect the ternary FC with the ITGA8/integrin-β1 heterodimer on the surface of MM cells. ITGA8/integrin-β1 signaling leads to an increased expression of GDNF by the MM, thereby promoting renal morphogenesis. Mutations in ITGA8 in humans and mice cause CAKUT. Loss of FC integrity results in a significant decrease in GDNF expression in the MM, thereby hampering the interaction between the UB and the MM and consequentially, impeding renal morphogenesis. (Right panel) Laminins are cross-shaped ECM molecules that consist of three distinct chains (α, β, and γ). Laminins simultaneously interact with cell surface receptors (integrins), HSPG, collagen, and nidogen 1 (see box). Laminins that contain γ1-chains form high-affinity interactions with nidogen 1. Fourteen distinct laminin isoforms have been identified to date, of which ten possess the laminin-γ1 chain.^, A homozygous ablation of the nidogen binding site in laminin-γ1 has been shown to result in severe CAKUT phenotypes (including bilateral renal agenesis) in mice. Despite its important role in ECM assembly, targeted inactivation of nidogen 1 did not result in a CAKUT phenotype. Both nidogen 1 and Laminin are known interactors of HSPG. HPSE is one of the few enzymes with the ability to break down HSPG. Knockdown of neither HSPG nor HPSE has been reported to cause CAKUT phenotypes in mice.^,,, However, recessive mutations in HPSE2 have been identified in human patients with urofacial syndrome,^, with a similar phenotype in mice. Although HPSE2 has no detectable enzymatic activity itself, it has been shown to function as an endogenous inhibitor of HPSE (Supplemental Figure 3). Modified from refs. and , with permission.

Figure 3.

RA signaling plays a central role for the development of the kidneys and urinary tract. Genes encoding the proteins highlighted by red frames have been implicated in murine cases of syndromic CAKUT (Supplemental Table 1). Genes highlighted in yellow, if mutated, constitute causes of human CAKUT. Mutations in genes encoding multiple proteins involved in intracellular RA processing and signaling have previously been identified to cause syndromic CAKUT in mice (Retinol Dehydrogenase 10 [Rdh10], Aldehyde Dehydrogenase 1 Family Member A2 [Aldh1a2], and Cytochrome P450 Family 26 [Cyp26], Rxr, Rarα, and Rarβ).^,– RDH10 catalyzes the reversible reaction from retinol to retinal. Retinal is then converted into the active metabolite all-trans RA via an irreversible reaction catalyzed by the enzyme ALD1A2. RA then either enters the nucleus or is metabolized via enzymes of the CYP26 family (e.g., CYP26A1)^, in the endoplasmic reticulum. In the nucleus, RA derivatives can serve as a ligand for two receptors: RXR (9-cis-RA and rexinoids) and RAR (all-trans-RA).^, Both receptor proteins have at least three subtypes and isoforms each. RXR and RAR heterodimers bind to retinoic acid–responsive elements (RAREs) of the nuclear DNA.^, RAREs are predominantly located in promoter regions of target genes. The presence of RA recruits coactivators and leads to enhanced binding to RAREs and expression of target genes.^, The transcriptional control of downstream genes is further modified by the binding of additional coregulators (coactivators and corepressors, including NRIP1) to RXR and RAR.^,, We recently identified mutations in the corepressor NRIP1 (highlighted in yellow) as causing CAKUT in humans. NRIP1 may interact with both RXRs and RARs.

Figure 4.

Members of the BMP signaling cascade with a role in CAKUT. Proteins encoded by genes that, if mutated, cause murine CAKUT are outlined in red, and proteins highlighted in yellow constitute causes of isolated and/or syndromic manifestations of human CAKUT (Supplemental Table 4). BMPs act as ligands for two classes of transmembrane serine-threonine-kinase receptors on the surface of the corresponding effector cells (e.g., bone morphogenic protein receptor type 1A [BMPR1A] and BMPR2). After activated, these receptors initiate intracellular signaling cascades, including canonical SMAD signaling and noncanonical signaling (e.g., via mitogen-activated protein kinase [MAPK]), which eventually result in increased expression of distinct target genes in the cell nucleus.^, The cellular consequences of BMP signaling depend on the location and local concentration of the BMP ligand. The availability of ligands for receptor binding is regulated by diverse intra- and extracellular antagonists and agonists of BMP and includes, for example, Gremlin 1 (GREM1), Follistatin (FST), and bone morphogenic protein binding to the endothelial regulator (BMPER).^– Please note that, BMPR2, although depicted in the figure, has not been implicated in the pathogenesis of CAKUT to date. CRIM1, cysteine-rich transmembrane bone morphogenic protein regulator 1; CTDNEP1, CTD nuclear envelope phosphatase 1; GPC3, Glypican 3.