Two coding variants in the apolipoprotein L1 (APOL1) gene (termed G1 and G2) are strongly associated with increased risk of nondiabetic kidney disease in people of recent African ancestry. The mechanisms by which the risk variants cause kidney damage, although not well-understood, are believed to involve injury to glomerular podocytes. The intracellular localization and function of APOL1 in podocytes remain unclear, with recent studies suggesting possible roles in the endoplasmic reticulum (ER), mitochondria, endosomes, lysosomes, and autophagosomes. Here, we demonstrate that APOL1 also localizes to intracellular lipid droplets (LDs). While a large fraction of risk variant APOL1 (G1 and G2) localizes to the ER, a significant proportion of wild-type APOL1 (G0) localizes to LDs. APOL1 transiently interacts with numerous organelles, including the ER, mitochondria, and endosomes. Treatment of cells that promote LD formation with oleic acid shifted the localization of G1 and G2 from the ER to LDs, with accompanying reduction of autophagic flux and cytotoxicity. Coexpression of G0 APOL1 with risk variant APOL1 enabled recruitment of G1 and G2 from the ER to LDs, accompanied by reduced cell death. The ability of G0 APOL1 to recruit risk variant APOL1 to LDs may help explain the recessive pattern of kidney disease inheritance. These studies establish APOL1 as a bona fide LD-associated protein, and reveal that recruitment of risk variant APOL1 to LDs reduces cell toxicity, autophagic flux, and cell death. Thus, interventions that divert APOL1 risk variants to LDs may serve as a novel therapeutic strategy to alleviate their cytotoxic effects.

BackgroundOver the past two decades, the importance of genetic factors in the development of FSGS has become increasingly clear. However, despite many known monogenic causes of FSGS, single gene defects explain only 30% of cases.MethodsTo investigate mutations underlying FSGS, we sequenced 662 whole exomes from individuals with sporadic or familial FSGS. After quality control, we analyzed the exome data from 363 unrelated family units with sporadic or familial FSGS and compared this to data from 363 ancestry-matched controls. We used rare variant burden tests to evaluate known disease-associated genes and potential new genes.ResultsWe validated several FSGS-associated genes that show a marked enrichment of deleterious rare variants among the cases. However, for some genes previously reported as FSGS related, we identified rare variants at similar or higher frequencies in controls. After excluding such genes, 122 of 363 cases (33.6%) had rare variants in known disease-associated genes, but 30 of 363 controls (8.3%) also harbored rare variants that would be classified as “causal” if detected in cases; applying American College of Medical Genetics filtering guidelines (to reduce the rate of false-positive claims that a variant is disease related) yielded rates of 24.2% in cases and 5.5% in controls. Highly ranked new genes include SCAF1, SETD2, and LY9. Network analysis showed that top-ranked new genes were located closer than a random set of genes to known FSGS genes.ConclusionsAlthough our analysis validated many known FSGS-causing genes, we detected a nontrivial number of purported “disease-causing” variants in controls, implying that filtering is inadequate to allow clinical diagnosis and decision making. Genetic diagnosis in patients with FSGS is complicated by the nontrivial rate of variants in known FSGS genes among people without kidney disease.

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Introduction: Focal segmental glomerulosclerosis (FSGS) is a histopathologically defined kidney lesion. FSGS can be observed with various underlying causes, including highly penetrant monogenic renal disease. We recently identified pathogenic variants of UMOD, a gene encoding the tubular protein uromodulin, in 8 families with suspected glomerular disease.Methods: To validate pathogenic variants of UMOD, we reviewed the clinical and pathology reports of members of 8 families identified to have variants of UMOD. Clinical, laboratory, and pathologic data were collected, and genetic confirmation for UMOD was performed by Sanger sequencing.Results: Biopsy-proven cases of FSGS were verified in 21% (7 of 34) of patients with UMOD variants. The UMOD variants seen in 7 families were mutations previously reported in autosomal dominant tubulointerstitial kidney disease-uromodulin (ADTKD-UMOD). For one family with 3 generations affected, we identified p.R79G in a noncanonical transcript variant of UMOD co-segregating with disease. Consistent with ADTKD, most patients in our study presented with autosomal dominant inheritance, subnephrotic range proteinuria, minimal hematuria, and renal impairment. Kidney biopsies showed histologic features of glomerular injury consistent with secondary FSGS, including focal sclerosis and partial podocyte foot process effacement. Conclusion:Our study demonstrates that with the use of standard clinical testing and kidney biopsy, clinicians were unable to make the diagnosis of ADTKD-UMOD; patients were often labeled with a clinical diagnosis of FSGS. We show that genetic testing can establish the diagnosis of ADTKD-UMOD with secondary FSGS. Genetic testing in individuals with FSGS histology should not be limited to genes that directly impair podocyte function.