CJASN ePress. Published on April 23, 2020 as doi: 10.2215/CJN.11440919 Genetic Disorders of the Glomerular Filtration Barrier Anna S. Li,1,2 Jack F. Ingham,1 and Rachel Lennon 1,3 Abstract The glomerular filtration barrier is a highly specialized capillary wall comprising fenestrated endothelial cells, podocytes, and an intervening basement membrane. In glomerular disease, this barrier loses functional integrity, allowing the passage of macromolecules and cells, and there are associated changes in both cell morphology and the extracellular matrix. Over the past three decades there has been a transformation in our understanding about glomerular disease, fueled by genetic discovery, and this is leading to exciting advances in our knowledge about 1Division of Cell- glomerular biology and pathophysiology. In current clinical practice, a genetic diagnosis already has important Matrix Biology and implications for management ranging from estimating the risk of disease recurrence post-transplant to the life- Regenerative changing advances in the treatment of atypical hemolytic uremic syndrome. Improving our understanding about Medicine, Wellcome Centre for Cell-Matrix the mechanistic basis of glomerular disease is required for more effective and personalized therapy options. In this Research, School of review we describe genotype and phenotype correlations for genetic disorders of the glomerular filtration barrier, Biological Sciences, with a particular emphasis on how these gene defects cluster by both their ontology and patterns of glomerular Faculty of Biology, pathology. Medicine and Health, CJASN ccc–ccc Manchester Academic 15: , 2020. doi: https://doi.org/10.2215/CJN.11440919 Health Science Centre, The University of Manchester, Introduction fenestrae that are 70–100 nm in diameter and covered Manchester, United . Kingdom; Defects in 80 genes have been found to cause by an endothelial surface layer consisting largely of 2 fi Department of disorders of the glomerular ltration barrier, with a glycocalyx that contributes to the charge-selective Nephrology, rapid rise in gene discovery over the last 10 years barrier (10). The GBM is a unique basement mem- Manchester University (Figure 1). This list continues to expand with the use brane formed by secreted products from both endo- Hospitals National Health Service of next-generation sequencing in large cohorts of pa- thelial cells and podocytes during glomerulogenesis – Foundation Trust, tients who have been deeply phenotyped (1 4). Whole- (11). It is a complex gel-like structure consisting of Manchester Academic exome and whole-genome sequencing allows rapid core basement membrane components, including Health Science screening for both pathogenic and novel variants in laminin isoforms, type IV collagen, nidogen, and Centre, Manchester, affected individuals and their families who lack a United Kingdom; and heparan sulfate proteoglycans, in addition to many 3 genetic diagnosis (5). Furthermore, advances in cell- Department of fi more less abundant structural and regulatory compo- Paediatric speci c genetic engineering techniques such as Cre-lox nents (12). Podocytes are specialized epithelial cells Nephrology, Royal recombination (6) and more recent genome-editing covering the outer aspect of the glomerular capillary. Manchester Children’s approaches with CRISPR-Cas9 (7) have enabled fo- Hospital, Manchester These cells are basally anchored to the GBM through cused in vivo and in vitro modeling of gene function. University Hospitals transmembrane receptors such as integrins and National Health These approaches can elicit mechanistic details about dystroglycans (10) and specialized cell-cell junctions Service Foundation genetic variants and cellular physiology to establish a known as slit diaphragms interconnect podocytes Trust, Manchester causal link from gene to clinical phenotype (8). Using Academic Health this pipeline from gene discovery to experimental laterally (10). Podocyte foot processes have an actin- Science Centre, modeling, our understanding of the glomerular filtra- based cytoskeleton connecting basal, lateral, and Manchester, United via Kingdom tion barrier in health and disease has been trans- apical domains linker proteins to both cell-cell formed. This review focuses on known human genetic and cell-matrix junctions and thereby convey cues from the extracellular environment (10). Correspondence: disorders of the filtration barrier by considering the Prof. Rachel Lennon, key component parts: podocytes, endothelial cells, and Pathogenic variants in a wide range of genes with Division of Cell- fi the glomerular basement membrane. functional relevance in glomerular ltration have Matrix Biology, been identified to date. As a high-level starting point, Wellcome Centre for these genes can be classified according to their Cell-Matrix Research, fi School of Biological The Glomerular Filtration Barrier location within the three-layered ltration barrier Sciences, Faculty of The filtration barrier comprises three layers: (Figure 2). An alternative representation is demon- Biology Medicine and podocytes and endothelial cells with an intervening strated with enrichment analysis of Gene Ontology Health, University of cellular compartment annotations for known genes Manchester, Oxford glomerular basement membrane (GBM). Together, this Road, Manchester specialized capillary wall allows selective ultrafiltration (Figure 3). This analysis demonstrates the importance M13 9PT, United while retaining circulating cells and plasma proteins. of the cellular cytoskeleton, cell-cell junctions, and Kingdom. Email: The barrier is both size selective and charge selective, extracellular matrix in genetic disorders of the filtra- Rachel.Lennon@ manchester.ac.uk as demonstrated by early studies of transferrin tion barrier. In the following sections, we demonstrate accumulation (9). Glomerular endothelial cells have how genetic discovery has highlighted particular www.cjasn.org Vol 15 December, 2020 Copyright © 2020 by the American Society of Nephrology 1 2 CJASN role of these genes in normal glomerular function. They also reaffirm the essential role of the podocyte in the function of the filtration barrier with the majority of genes implicated in steroid-resistant nephrotic syndrome localizing to the podocyte. Slit Diaphragm–Related Genes NPHS1 encodes nephrin, a transmembrane protein and a core component of the slit diaphragm. Between adjacent foot processes, nephrin molecules form homo- and hetero- dimers with its homolog Neph1 forming the slit diaphragm in a zipper-like pattern. Intracellularly, nephrin phosphoryla- tion by the tyrosine kinase Fyn leads to enhanced PI3K/Akt/ Rac pathway activation, promoting dynamic modeling of actin cytoskeleton. Nephrin interacts with podocin (encoded by NPHS2), an integral membrane protein, and CD2-associated protein (encoded by CD2AP), an adapter protein connecting nephrin to the actin cytoskeleton as well as interacting with the PI3K/Akt pathway. Nephrin also interacts with IQ motif–containing GTPase-activating protein 1 (IQGAP1), which in turn interacts with phospho- lipase C«1 (encoded by PLCE1), a phospholipase generating downstream messengers such as such as inositol 1,4,5- NPHS1 fi trisphosphate (IP3) and diacylglycerol. was identi ed as the pathogenic gene in congenital nephrotic syndrome of the Finnish type (13). Disease-causing homozygous NPHS1 variants are common in Finland, affecting approximately Figure 1. | The rapid rise in gene discovery. Thenumberofnewgene 1 in 10,000 children. Two truncating variants (Fin-major discoveries is shown together with the cumulative total. There is rapid rise and Fin-minor) account for .94% of Finnish cases, in discoveries in the last 10 years, from 29 genes in 2009 to 87 in 2019. demonstrating a founder effect. Outside Finland, classic NPHS1 variants are rare. Missense variants are associated cellular components and biologic pathways and helped to with a milder FSGS phenotype (14). Homozygous variants NPHS2 PLCE1 build understanding about glomerular biology and patho- in and causing steroid-resistant nephrotic physiology. syndrome display genotype-phenotype correlation. Differ- ent NPHS2 variants can cause steroid-resistant nephrotic syndrome with variant-specific disease severity and onset Podocyte Genetics from early childhood to early adulthood. Truncating var- Inherited podocyte dysfunction typically manifests as iants in PLCE1 cause early onset of proteinuria and pro- nephrotic syndrome, characterized by the clinical triad of gression to kidney failure, whereas missense variants are massive proteinuria, albumin ,25 g/L, and edema. Many found in later-onset nephrotic syndrome and cause slower genetic causes of nephrotic syndrome are resistant to progression. Some individuals with PLCE1 variants were treatment with glucocorticoids, leading to the classifica- responsive to corticosteroids and immunosuppressive ther- tion of steroid-resistant nephrotic syndrome. The prog- apy. Recently, defects in proteins encoded by KIRREL1 and nosis for patients with steroid-resistant nephrotic KIRREL2, members of the NEPH family that interact with syndrome is poor and they invariably progress to kidney podocin, have also been found in children with steroid- failure. Although kidney biopsy remains an important resistant nephrotic syndrome (15,16). clinical tool for the histologic diagnosis of nephrotic Enhanced calcium influx appears to be a key factor for syndrome, the availability of genetic testing has enabled a mediating podocyte injury