Analyzing and reconciling colocalization and transcriptome-wide association studies from the perspective of inferential reproducibility.

Transcriptome-wide association studies and colocalization analysis are popular computational approaches for integrating genetic-association data from molecular and complex traits. They show the unique ability to go beyond variant-level genetic-association evidence and implicate critical functional units, e.g., genes, in disease etiology. However, in practice, when the two approaches are applied to the same molecular and complex-trait data, the inference results can be markedly different. This paper systematically investigates the inferential reproducibility between the two approaches through theoretical derivation, numerical experiments, and analyses of four complex trait GWAS and GTEx eQTL data. We identify two classes of inconsistent inference results. We find that the first class of inconsistent results (i.e., genes with strong colocalization but weak transcriptome-wide association study [TWAS] signals) might suggest an interesting biological phenomenon, i.e., horizontal pleiotropy; thus, the two approaches are truly complementary. The inconsistency in the second class (i.e., genes with weak colocalization but strong TWAS signals) can be understood and effectively reconciled. To this end, we propose a computational approach for locus-level colocalization analysis. We demonstrate that the joint TWAS and locus-level colocalization analysis improves specificity and sensitivity for implicating biologically relevant genes.

Quality assessment and refinement of chromatin accessibility data using a sequence-based predictive model.

Chromatin accessibility assays are central to the genome-wide identification of gene regulatory elements associated with transcriptional regulation. However, the data have highly variable quality arising from several biological and technical factors. To surmount this problem, we developed a sequence-based machine learning method to evaluate and refine chromatin accessibility data. Our framework, gapped k-mer SVM quality check (gkmQC), provides the quality metrics for a sample based on the prediction accuracy of the trained models. We tested 886 DNase-seq samples from the ENCODE/Roadmap projects to demonstrate that gkmQC can effectively identify "high-quality" (HQ) samples with low conventional quality scores owing to marginal read depths. Peaks identified in HQ samples are more accurately aligned at functional regulatory elements, show greater enrichment of regulatory elements harboring functional variants, and explain greater heritability of phenotypes from their relevant tissues. Moreover, gkmQC can optimize the peak-calling threshold to identify additional peaks, especially for rare cell types in single-cell chromatin accessibility data.

ADAR regulates APOL1 via A-to-I RNA editing by inhibition of MDA5 activation in a paradoxical biological circuit.

APOL1 risk variants are associated with increased risk of kidney disease in patients of African ancestry, but not all individuals with the APOL1 high-risk genotype develop kidney disease. As APOL1 gene expression correlates closely with the degree of kidney cell injury in both cell and animal models, the mechanisms regulating APOL1 expression may be critical determinants of risk allele penetrance. The APOL1 messenger RNA includes Alu elements at the 3' untranslated region that can form a double-stranded RNA structure (Alu-dsRNA) susceptible to posttranscriptional adenosine deaminase acting on RNA (ADAR)-mediated adenosine-to-inosine (A-to-I) editing, potentially impacting gene expression. We studied the effects of ADAR expression and A-to-I editing on APOL1 levels in podocytes, human kidney tissue, and a transgenic APOL1 mouse model. In interferon-γ (IFN-γ)-stimulated human podocytes, ADAR down-regulates APOL1 by preventing melanoma differentiation-associated protein 5 (MDA5) recognition of dsRNA and the subsequent type I interferon (IFN-I) response. Knockdown experiments showed that recognition of APOL1 messenger RNA itself is an important contributor to the MDA5-driven IFN-I response. Mathematical modeling suggests that the IFN-ADAR-APOL1 network functions as an incoherent feed-forward loop, a biological circuit capable of generating fast, transient responses to stimuli. Glomeruli from human kidney biopsies exhibited widespread editing of APOL1 Alu-dsRNA, while the transgenic mouse model closely replicated the edited sites in humans. APOL1 expression in mice was inversely correlated with Adar1 expression under IFN-γ stimuli, supporting the idea that ADAR regulates APOL1 levels in vivo. ADAR-mediated A-to-I editing is an important regulator of APOL1 expression that could impact both penetrance and severity of APOL1-associated kidney disease.

Natural History and Clinicopathological Associations of TRPC6-Associated Podocytopathy.

BACKGROUND: Understanding the genetic basis of human diseases has become integral to drug development and precision medicine. Recent advancements have enabled the identification of molecular pathways driving diseases, leading to targeted treatment strategies. The increasing investment in rare diseases by the biotech industry underscores the importance of genetic evidence in drug discovery and approval processes. Here we studied a monogenic Mendelian kidney disease, TRPC6-associated podocytopathy (TRPC6-AP), to present its natural history, genetic spectrum, and clinicopathological associations in a large cohort of patients with causal variants in TRPC6, in order to help define the specific features of disease and further facilitate drug development and clinical trials design. METHODS: the study involved 64 individuals from 39 families with TRPC6 causal missense variants. Clinical data, including age of onset, laboratory results, response to treatment, kidney biopsy findings, and genetic information, were collected from multiple centers nationally and internationally. Exome or targeted sequencing was performed and variant classification was based on strict criteria. Structural and functional analyses of TRPC6 variants were conducted to understand their impact on protein function. In depth re-analysis of light and electron microscopy specimens for 9 available kidney biopsies was conducted to identify pathological features and correlates of TRPC6-AP. RESULTS: Large-scale sequencing data did not support causality for TRPC6 protein-truncating variants. We identified 21 unique TRPC6 missense variants, clustering in three distinct regions of the protein, and with different effects on TRPC6 3D protein structure. Kidney biopsy analysis revealed FSGS patterns of injury in most cases, along with distinctive podocyte features including diffuse foot process effacement and swollen cell bodies. The majority of patients presented in adolescence or early adulthood but with ample variation (average 22, SD ± 14 years), with frequent progression to kidney failure but with variability in time between presentation and ESKD. CONCLUSIONS: This study provides insights into the genetic spectrum, clinicopathological associations, and natural history of TRPC6-AP.

Idiopathic collapsing glomerulopathy is associated with APOL1 high-risk genotypes or Mendelian variants in most affected individuals in a highly admixed population.

Collapsing glomerulopathy (CG) is most often associated with fast progression to kidney failure with an incidence apparently higher in Brazil than in other countries. However, the reason for this occurrence is unknown. To better understand this, we performed an integrated analysis of clinical, histological, therapeutic, causative genetic and genetic ancestry data in a highly genetically admixed cohort of 70 children and adult patients with idiopathic CG (ICG). The disease onset occurred at 23 (interquartile range: 17-31) years and approximately half of patients progressed to chronic kidney disease requiring kidney replacement therapy (CKD-KRT) 36 months after diagnosis. Causative genetic bases, assessed by targeted-gene panel or whole-exome sequencing, were identified in 58.6% of patients. Among these cases, 80.5% harbored APOL1 high-risk genotypes (HRG) and 19.5% causative Mendelian variants (MV). Self-reported non-White patients more frequently had HRG. MV was an independent risk factor for progression to CKD-KRT by 36 months and the end of follow-up, while remission was an independent protective factor. All patients with HRG manifested CG at 9-44 years of age, whereas in those with APOL1 low-risk genotype, the disease arose throughout life. HRGs were associated with higher proportion of African genetic ancestry. Novel causative MVs were identified in COL4A5, COQ2 and PLCE1 and previously described causative MVs were identified in MYH9, TRPC6, COQ2, COL4A3 and TTC21B. Three patients displayed HRG combined with a variant of uncertain significance (ITGB4, LAMA5 or PTPRO). MVs were associated with worse kidney prognosis. Thus, our data reveal that the genetic status plays a major role in ICG pathogenesis, accounting for more than half of cases in a highly admixed Brazilian population.

Pediatric contributions and lessons learned from the NEPTUNE cohort study.

Primary glomerular diseases are rare entities. This has hampered efforts to better understand the underlying pathobiology and to develop novel safe and effective therapies. NEPTUNE is a rare disease network that is focused on patients of all ages with minimal change disease, focal segmental glomerulosclerosis, and membranous nephropathy. It is a longitudinal cohort study that collects detailed demographic, clinical, histopathologic, genomic, transcriptomic, and metabolomic data. The goal is to develop a molecular classification for these disorders that supersedes the traditional pathological features-based schema. Pediatric patients are important contributors to this ongoing project. In this review, we provide a snapshot of the children and adolescents enrolled in NEPTUNE and summarize some key observations that have been made based on the data accumulated during the study. In addition, we describe the development of NEPTUNE Match, a program that aims to leverage the multi-scalar information gathered for each individual patient to provide guidance about potential clinical trial participation based on the molecular characterization and non-invasive biomarker profile. This represents the first organized effort to apply principles of precision medicine to the treatment of patients with primary glomerular disease. NEPTUNE has proven to be an invaluable asset in the study of glomerular diseases in patients of all ages including children and adolescents.

Quantify and control reproducibility in high-throughput experiments.

Ensuring reproducibility of results in high-throughput experiments is crucial for biomedical research. Here, we propose a set of computational methods, INTRIGUE, to evaluate and control reproducibility in high-throughput settings. Our approaches are built on a new definition of reproducibility that emphasizes directional consistency when experimental units are assessed with signed effect size estimates. The proposed methods are designed to (1) assess the overall reproducible quality of multiple studies and (2) evaluate reproducibility at the individual experimental unit levels. We demonstrate the proposed methods in detecting unobserved batch effects via simulations. We further illustrate the versatility of the proposed methods in transcriptome-wide association studies: in addition to reproducible quality control, they are also suited to investigating genuine biological heterogeneity. Finally, we discuss the potential extensions of the proposed methods in other vital areas of reproducible research (for example, publication bias and conceptual replications).

The association of low birthweight and prematurity on outcomes in children and adults with nephrotic syndrome-a NEPTUNE cohort study.

BACKGROUND: In single-center studies, both preterm birth and low birth weight (LBW) are associated with worse outcomes in childhood nephrotic syndrome. Using the Nephrotic Syndrome Study Network (NEPTUNE) observational cohort, we tested the hypothesis that in patients with nephrotic syndrome, hypertension, proteinuria status, and disease progression would be more prevalent and more severe in subjects with LBW and prematurity singly or in combination (LBW/prematurity). METHODS: Three hundred fifty-nine adults and children with focal segmental glomerulosclerosis (FSGS) or minimal change disease (MCD) and available birth history were included. Estimated glomerular filtration rate (eGFR) decline and remission status were primary outcomes, and secondary outcomes were kidney histopathology, kidney gene expression, and urinary biomarkers. Logistic regression was used to identify associations with LBW/prematurity and these outcomes. RESULTS: We did not find an association between LBW/prematurity and remission of proteinuria. However, LBW/prematurity was associated with greater decline in eGFR. This decline in eGFR was partially explained by the association of LBW/prematurity with APOL1 high-risk alleles, but the association remained after adjustment. There were no differences in kidney histopathology or gene expression in the LBW/prematurity group compared to normal birth weight/term birth. CONCLUSION: LBW and premature babies who develop nephrotic syndrome have a more rapid decline in kidney function. We did not identify clinical or laboratory features that distinguished the groups. Additional studies in larger groups are needed to fully ascertain the effects of (LBW) and prematurity alone or in combination on kidney function in the setting of nephrotic syndrome.

Discovery of Autoantibodies Targeting Nephrin in Minimal Change Disease Supports a Novel Autoimmune Etiology.

BACKGROUND: Failure of the glomerular filtration barrier, primarily by loss of slit diaphragm architecture, underlies nephrotic syndrome in minimal change disease. The etiology remains unknown. The efficacy of B cell-targeted therapies in some patients, together with the known proteinuric effect of anti-nephrin antibodies in rodent models, prompted us to hypothesize that nephrin autoantibodies may be present in patients with minimal change disease. METHODS: We evaluated sera from patients with minimal change disease, enrolled in the Nephrotic Syndrome Study Network (NEPTUNE) cohort and from our own institutions, for circulating nephrin autoantibodies by indirect ELISA and by immunoprecipitation of full-length nephrin from human glomerular extract or a recombinant purified extracellular domain of human nephrin. We also evaluated renal biopsies from our institutions for podocyte-associated punctate IgG colocalizing with nephrin by immunofluorescence. RESULTS: In two independent patient cohorts, we identified circulating nephrin autoantibodies during active disease that were significantly reduced or absent during treatment response in a subset of patients with minimal change disease. We correlated the presence of these autoantibodies with podocyte-associated punctate IgG in renal biopsies from our institutions. We also identified a patient with steroid-dependent childhood minimal change disease that progressed to end stage kidney disease; she developed a massive post-transplant recurrence of proteinuria that was associated with high pretransplant circulating nephrin autoantibodies. CONCLUSIONS: Our discovery of nephrin autoantibodies in a subset of adults and children with minimal change disease aligns with published animal studies and provides further support for an autoimmune etiology. We propose a new molecular classification of nephrin autoantibody minimal change disease to serve as a framework for instigation of precision therapeutics for these patients.

A glomerular transcriptomic landscape of apolipoprotein L1 in Black patients with focal segmental glomerulosclerosis.

Apolipoprotein L1 (APOL1)-associated focal segmental glomerulosclerosis (FSGS) is the dominant form of FSGS in Black individuals. There are no targeted therapies for this condition, in part because the molecular mechanisms underlying APOL1's pathogenic contribution to FSGS are incompletely understood. Studying the transcriptomic landscape of APOL1 FSGS in patient kidneys is an important way to discover genes and molecular behaviors that are unique or most relevant to the human disease. With the hypothesis that the pathology driven by the high-risk APOL1 genotype is reflected in alteration of gene expression across the glomerular transcriptome, we compared expression and co-expression profiles of 15,703 genes in 16 Black patients with FSGS at high-risk vs 14 Black patients with a low-risk APOL1 genotype. Expression data from APOL1-inducible HEK293 cells and normal human glomeruli were used to pursue genes and molecular pathways uncovered in these studies. We discovered increased expression of APOL1 and nine other significant differentially expressed genes in high-risk patients. This included stanniocalcin, which has a role in mitochondrial and calcium-related processes along with differential correlations between high- and low-risk APOL1 and metabolism pathway genes. There were similar correlations with extracellular matrix- and immune-related genes, but significant loss of co-expression of mitochondrial genes in high-risk FSGS, and an NF-κB-down regulating gene, NKIRAS1, as the most significant hub gene with strong differential correlations with NDUF family (mitochondrial respiratory genes) and immune-related (JAK-STAT) genes. Thus, differences in mitochondrial gene regulation appear to underlie many differences observed between high- and low-risk Black patients with FSGS.

Genomic Mismatch at LIMS1 Locus and Kidney Allograft Rejection.

BACKGROUND: In the context of kidney transplantation, genomic incompatibilities between donor and recipient may lead to allosensitization against new antigens. We hypothesized that recessive inheritance of gene-disrupting variants may represent a risk factor for allograft rejection. METHODS: We performed a two-stage genetic association study of kidney allograft rejection. In the first stage, we performed a recessive association screen of 50 common gene-intersecting deletion polymorphisms in a cohort of kidney transplant recipients. In the second stage, we replicated our findings in three independent cohorts of donor-recipient pairs. We defined genomic collision as a specific donor-recipient genotype combination in which a recipient who was homozygous for a gene-intersecting deletion received a transplant from a nonhomozygous donor. Identification of alloantibodies was performed with the use of protein arrays, enzyme-linked immunosorbent assays, and Western blot analyses. RESULTS: In the discovery cohort, which included 705 recipients, we found a significant association with allograft rejection at the LIMS1 locus represented by rs893403 (hazard ratio with the risk genotype vs. nonrisk genotypes, 1.84; 95% confidence interval [CI], 1.35 to 2.50; P = 9.8×10-5). This effect was replicated under the genomic-collision model in three independent cohorts involving a total of 2004 donor-recipient pairs (hazard ratio, 1.55; 95% CI, 1.25 to 1.93; P = 6.5×10-5). In the combined analysis (discovery cohort plus replication cohorts), the risk genotype was associated with a higher risk of rejection than the nonrisk genotype (hazard ratio, 1.63; 95% CI, 1.37 to 1.95; P = 4.7×10-8). We identified a specific antibody response against LIMS1, a kidney-expressed protein encoded within the collision locus. The response involved predominantly IgG2 and IgG3 antibody subclasses. CONCLUSIONS: We found that the LIMS1 locus appeared to encode a minor histocompatibility antigen. Genomic collision at this locus was associated with rejection of the kidney allograft and with production of anti-LIMS1 IgG2 and IgG3. (Funded by the Columbia University Transplant Center and others.).

A Rare Autosomal Dominant Variant in Regulator of Calcineurin Type 1 (RCAN1) Gene Confers Enhanced Calcineurin Activity and May Cause FSGS.

BACKGROUND: Podocyte dysfunction is the main pathologic mechanism driving the development of FSGS and other morphologic types of steroid-resistant nephrotic syndrome (SRNS). Despite significant progress, the genetic causes of most cases of SRNS have yet to be identified. METHODS: Whole-genome sequencing was performed on 320 individuals from 201 families with familial and sporadic NS/FSGS with no pathogenic mutations in any known NS/FSGS genes. RESULTS: Two variants in the gene encoding regulator of calcineurin type 1 (RCAN1) segregate with disease in two families with autosomal dominant FSGS/SRNS. In vitro, loss of RCAN1 reduced human podocyte viability due to increased calcineurin activity. Cells expressing mutant RCAN1 displayed increased calcineurin activity and NFAT activation that resulted in increased susceptibility to apoptosis compared with wild-type RCAN1. Treatment with GSK-3 inhibitors ameliorated this elevated calcineurin activity, suggesting the mutation alters the balance of RCAN1 regulation by GSK-3ß, resulting in dysregulated calcineurin activity and apoptosis. CONCLUSIONS: These data suggest mutations in RCAN1 can cause autosomal dominant FSGS. Despite the widespread use of calcineurin inhibitors in the treatment of NS, genetic mutations in a direct regulator of calcineurin have not been implicated in the etiology of NS/FSGS before this report. The findings highlight the therapeutic potential of targeting RCAN1 regulatory molecules, such as GSK-3ß, in the treatment of FSGS.

Copy Number Variant Analysis and Genome-wide Association Study Identify Loci with Large Effect for Vesicoureteral Reflux.

BACKGROUND: Vesicoureteral reflux (VUR) is a common, familial genitourinary disorder, and a major cause of pediatric urinary tract infection (UTI) and kidney failure. The genetic basis of VUR is not well understood. METHODS: A diagnostic analysis sought rare, pathogenic copy number variant (CNV) disorders among 1737 patients with VUR. A GWAS was performed in 1395 patients and 5366 controls, of European ancestry. RESULTS: Altogether, 3% of VUR patients harbored an undiagnosed rare CNV disorder, such as the 1q21.1, 16p11.2, 22q11.21, and triple X syndromes ((OR, 3.12; 95% CI, 2.10 to 4.54; P=6.35×10-8) The GWAS identified three study-wide significant and five suggestive loci with large effects (ORs, 1.41-6.9), containing canonical developmental genes expressed in the developing urinary tract (WDPCP, OTX1, BMP5, VANGL1, and WNT5A). In particular, 3.3% of VUR patients were homozygous for an intronic variant in WDPCP (rs13013890; OR, 3.65; 95% CI, 2.39 to 5.56; P=1.86×10-9). This locus was associated with multiple genitourinary phenotypes in the UK Biobank and eMERGE studies. Analysis of Wnt5a mutant mice confirmed the role of Wnt5a signaling in bladder and ureteric morphogenesis. CONCLUSIONS: These data demonstrate the genetic heterogeneity of VUR. Altogether, 6% of patients with VUR harbored a rare CNV or a common variant genotype conferring an OR >3. Identification of these genetic risk factors has multiple implications for clinical care and for analysis of outcomes in VUR.

APOL1 at 10 years: progress and next steps.

APOL1 kidney risk variants (RVs) were identified in 2010 as major drivers of glomerular, tubulointerstitial, and renal microvascular disease in individuals with sub-Saharan African ancestry. In December 2020, the "APOL1 at Ten" conference summarized the first decade of progress and discussed controversies and uncertainties that remain to be addressed. Topics included trypanosome infection and its role in the evolution of APOL1 kidney RVs, clinical phenotypes in APOL1-associated nephropathy, relationships between APOL1 RVs and background haplotypes on cell injury and molecular mechanisms initiating disease, the role of clinical APOL1 genotyping, and development of novel therapies for kidney disease. Future goals were defined, including improved characterization of various APOL1 RV phenotypes in patients and experimental preclinical models; further dissection of APOL1-mediated pathways to cellular injury and dysfunction in kidney (and other) cells; clarification of gene-gene and gene-environment interactions; and evaluation of the role for existing and novel therapies.

An eQTL Landscape of Kidney Tissue in Human Nephrotic Syndrome.

Expression quantitative trait loci (eQTL) studies illuminate the genetics of gene expression and, in disease research, can be particularly illuminating when using the tissues directly impacted by the condition. In nephrology, there is a paucity of eQTL studies of human kidney. Here, we used whole-genome sequencing (WGS) and microdissected glomerular (GLOM) and tubulointerstitial (TI) transcriptomes from 187 individuals with nephrotic syndrome (NS) to describe the eQTL landscape in these functionally distinct kidney structures. Using MatrixEQTL, we performed cis-eQTL analysis on GLOM (n = 136) and TI (n = 166). We used the Bayesian "Deterministic Approximation of Posteriors" (DAP) to fine-map these signals, eQTLBMA to discover GLOM- or TI-specific eQTLs, and single-cell RNA-seq data of control kidney tissue to identify the cell type specificity of significant eQTLs. We integrated eQTL data with an IgA Nephropathy (IgAN) GWAS to perform a transcriptome-wide association study (TWAS). We discovered 894 GLOM eQTLs and 1,767 TI eQTLs at FDR < 0.05. 14% and 19% of GLOM and TI eQTLs, respectively, had >1 independent signal associated with its expression. 12% and 26% of eQTLs were GLOM specific and TI specific, respectively. GLOM eQTLs were most significantly enriched in podocyte transcripts and TI eQTLs in proximal tubules. The IgAN TWAS identified significant GLOM and TI genes, primarily at the HLA region. In this study, we discovered GLOM and TI eQTLs, identified those that were tissue specific, deconvoluted them into cell-specific signals, and used them to characterize known GWAS alleles. These data are available for browsing and download via our eQTL browser, "nephQTL."

APOL1 in an ethnically diverse pediatric population with nephrotic syndrome: implications in focal segmental glomerulosclerosis and other diagnoses.

BACKGROUND: APOL1 high-risk genotypes (HRG) are associated with increased risk of kidney disease in individuals of African ancestry. We analyzed the effects of APOL1 risk variants on an ethnically diverse Brazilian pediatric nephrotic syndrome (NS) cohort. METHODS: Multicenter study including 318 NS patients, categorized as progressors to advanced CKD [estimated glomerular filtration rate (eGFR)] < 30 mL/min/1.73 m2] and slow/non-progressors (eGFR > 30 mL/min/1.73 m2 through the study). We employed Cox regression with progression time as the outcome and APOL1 genotype as the independent variable. We tested this association in the entire cohort and three subgroups; (1) focal segmental glomerulosclerosis (FSGS), (2) steroid-resistant NS (SRNS), and (3) those who underwent kidney biopsy. RESULTS: Nineteen patients (6%) had an HRG. Of these, 47% were self-reported White. Patients with HRG manifested NS at older ages and presented higher frequencies of FSGS and SRNS. HRG patients progressed to advanced CKD more often than low-risk-genotype (LRG) children in the whole NS cohort (p = 0.001) and the three subgroups. In SRNS and biopsied patients, a single risk variant was associated with trends of higher CKD progression risk. CONCLUSIONS: Novel discoveries include a substantial prevalence of HRG among patients self-reported White, worse kidney outcomes in HRG versus LRG children in the FSGS subgroup, and a trend of higher CKD progression risk associated with a single risk variant in the SRNS cohort. These findings suggest APOL1-associated NS extends beyond patients self-reported non-White, the HRG effect is independent of FSGS, and a single risk variant may have a detrimental impact in children with NS.

APOL1 genotype-associated morphologic changes among patients with focal segmental glomerulosclerosis.

BACKGROUND: The G1 and G2 alleles of apolipoprotein L1 (APOL1) are common in the Black population and associated with increased risk of focal segmental glomerulosclerosis (FSGS). The molecular mechanisms linking APOL1 risk variants with FSGS are not clearly understood, and APOL1's natural absence in laboratory animals makes studying its pathobiology challenging. METHODS: In a cohort of 90 Black patients with either FSGS or minimal change disease (MCD) enrolled in the Nephrotic Syndrome Study Network (58% pediatric onset), we used kidney biopsy traits as an intermediate outcome to help illuminate tissue-based consequences of APOL1 risk variants and expression. We tested associations between APOL1 risk alleles or glomerular APOL1 mRNA expression and 83 light- or electron-microscopy traits measuring structural and cellular kidney changes. RESULTS: Under both recessive and dominant models in the FSGS patient subgroup (61%), APOL1 risk variants were significantly correlated (defined as FDR <0.1) with decreased global mesangial hypercellularity, decreased condensation of cytoskeleton, and increased tubular microcysts. No significant correlations were detected in MCD cohort. Independent of risk alleles, glomerular APOL1 expression in FSGS patients was not correlated with morphologic features. CONCLUSIONS: While APOL1-associated FSGS is associated with two risk alleles, both one and two risk alleles are associated with cellular/tissue changes in this study of FSGS patients. Our lack of discovery of a large group of tissue differences in FSGS and no significant difference in MCD may be due to the lack of power but also supports investigating whether machine learning methods may more sensitively detect APOL1-associated changes.

UBD modifies APOL1-induced kidney disease risk.

People of recent African ancestry develop kidney disease at much higher rates than most other groups. Two specific coding variants in the Apolipoprotein-L1 gene APOL1 termed G1 and G2 are the causal drivers of much of this difference in risk, following a recessive pattern of inheritance. However, most individuals with a high-risk APOL1 genotype do not develop overt kidney disease, prompting interest in identifying those factors that interact with APOL1 We performed an admixture mapping study to identify genetic modifiers of APOL1-associated kidney disease. Individuals with two APOL1 risk alleles and focal segmental glomerulosclerosis (FSGS) have significantly increased African ancestry at the UBD (also known as FAT10) locus. UBD is a ubiquitin-like protein modifier that targets proteins for proteasomal degradation. African ancestry at the UBD locus correlates with lower levels of UBD expression. In cell-based experiments, the disease-associated APOL1 alleles (known as G1 and G2) lead to increased abundance of UBD mRNA but to decreased levels of UBD protein. UBD gene expression inversely correlates with G1 and G2 APOL1-mediated cell toxicity, as well as with levels of G1 and G2 APOL1 protein in cells. These studies support a model whereby inflammatory stimuli up-regulate both UBD and APOL1, which interact in a functionally important manner. UBD appears to mitigate APOL1-mediated toxicity by targeting it for destruction. Thus, genetically encoded differences in UBD and UBD expression appear to modify the APOL1-associated kidney phenotype.

The human nephrin Y1139RSL motif is essential for podocyte foot process organization and slit diaphragm formation during glomerular development.

Nephrin is an immunoglobulin-type cell-adhesion molecule with a key role in the glomerular interpodocyte slit diaphragm. Mutations in the nephrin gene are associated with defects in the slit diaphragm, leading to early-onset nephrotic syndrome, typically resistant to treatment. Although the endocytic trafficking of nephrin is essential for the assembly of the slit diaphragm, nephrin's specific endocytic motifs remain unknown. To search for endocytic motifs, here we performed a multisequence alignment of nephrin and identified a canonical YXXØ-type motif, Y1139RSL, in the nephrin cytoplasmic tail, expressed only in primates. Using site-directed mutagenesis, various biochemical methods, single-plane illumination microscopy, a human podocyte line, and a human nephrin-expressing zebrafish model, we found that Y1139RSL is a novel endocytic motif and a structural element for clathrin-mediated nephrin endocytosis that functions as a phosphorylation-sensitive signal. We observed that Y1139RSL motif-mediated endocytosis helps to localize nephrin to specialized plasma membrane domains in podocytes and is essential for normal foot process organization into a functional slit diaphragm between neighboring foot processes in zebrafish. The importance of nephrin Y1139RSL for healthy podocyte development was supported by population-level analyses of genetic variations at this motif, revealing that such variations are very rare, suggesting that mutations in this motif have autosomal-recessive negative effects on kidney health. These findings expand our understanding of the mechanism underlying nephrin endocytosis and may lead to improved diagnostic tools or therapeutic strategies for managing early-onset, treatment-resistant nephrotic syndrome.

Disruption of the exocyst induces podocyte loss and dysfunction.

Although the slit diaphragm proteins in podocytes are uniquely organized to maintain glomerular filtration assembly and function, little is known about the underlying mechanisms that participate in trafficking these proteins to the correct location for development and homeostasis. Identifying these mechanisms will likely provide novel targets for therapeutic intervention to preserve podocyte function following glomerular injury. Analysis of structural variation in cases of human nephrotic syndrome identified rare heterozygous deletions of EXOC4 in two patients. This suggested that disruption of the highly-conserved eight-protein exocyst trafficking complex could have a role in podocyte dysfunction. Indeed, mRNA profiling of injured podocytes identified significant exocyst down-regulation. To test the hypothesis that the exocyst is centrally involved in podocyte development/function, we generated homozygous podocyte-specific Exoc5 (a central exocyst component that interacts with Exoc4) knockout mice that showed massive proteinuria and died within 4 weeks of birth. Histological and ultrastructural analysis of these mice showed severe glomerular defects with increased fibrosis, proteinaceous casts, effaced podocytes, and loss of the slit diaphragm. Immunofluorescence analysis revealed that Neph1 and Nephrin, major slit diaphragm constituents, were mislocalized and/or lost. mRNA profiling of Exoc5 knockdown podocytes showed that vesicular trafficking was the most affected cellular event. Mapping of signaling pathways and Western blot analysis revealed significant up-regulation of the mitogen-activated protein kinase and transforming growth factor-ß pathways in Exoc5 knockdown podocytes and in the glomeruli of podocyte-specific Exoc5 KO mice. Based on these data, we propose that exocyst-based mechanisms regulate Neph1 and Nephrin signaling and trafficking, and thus podocyte development and function.