Background: Diseases of the kidney's glomerular filtration barrier are a leading cause of end stage renal failure. Despite a growing understanding of genes involved in glomerular disorders in children, the vast majority of adult patients lack a clear genetic diagnosis. The protein podocin p.R229Q, which results from the most common missense variant in NPHS2 , is enriched in cohorts of patients with FSGS. However, p.R229Q has been proposed to cause disease only when transassociated with specific additional genetic alterations, and population-based epidemiologic studies on its association with albuminuria yielded ambiguous results.
Methods: To test whether podocin p.R229Q may also predispose to the complex disease pathogenesis in adults, we introduced the exact genetic alteration in mice using CRISPR/Cas9-based genome editing ( Pod R231Q ). We assessed the phenotype using super-resolution microscopy and albuminuria measurements and evaluated the stability of the mutant protein in cell culture experiments.
Results: Heterozygous Pod R231Q/wild-type mice did not present any overt kidney disease or proteinuria. However, homozygous Pod R231Q/R231Q mice developed increased levels of albuminuria with age, and super-resolution microscopy revealed preceding ultrastructural morphologic alterations that were recently linked to disease predisposition. When injected with nephrotoxic serum to induce glomerular injury, heterozygous Pod R231Q/wild-type mice showed a more severe course of disease compared with Pod wild-type/wild-type mice. Podocin protein levels were decreased in Pod R231Q/wild-type and Pod R231Q/R231Q mice as well as in human cultured podocytes expressing the podocin R231Q variant. Our in vitro experiments indicate an underlying increased proteasomal degradation.
Conclusions: Our findings demonstrate that podocin R231Q exerts a pathogenic effect on its own, supporting the concept of podocin R229Q contributing to genetic predisposition in adult patients.
(Copyright © 2022 by the American Society of Nephrology.)