Phenotypic quantification of Nphs1-deficient mice.

Background: Steroid-resistant nephrotic syndrome (SRNS) is the second most frequent cause of chronic kidney disease in children and young adults. The most severe form of steroid-resistant nephrotic syndrome is congenital nephrotic syndrome Finnish type (CNSF), caused by biallelic loss-of-function variants in NPHS1, encoding nephrin. Since each of the 68 monogenic causes of steroid-resistant nephrotic syndrome represents a rare cause of the disease, tailoring therapeutic interventions to multiple molecular targets remains challenging, suggesting gene replacement therapy (GRT) as a viable alternative. To set the ground for a gene replacement study in vivo, we established rigorous, quantifiable, and reproducible phenotypic assessment of a conditional Nphs1 knockout mouse model.

Methods: By breeding a floxed Nphs1 mouse (Nphs1/J) previously studied for pancreatic β-cell survival with a podocin promoter-driven Cre recombinase mouse model (Tg(NPHS2-Cre)/J), we generated mice with podocyte-specific nephrin deficiency (Nphs1 NPHS2-Cre +).

Results: We observed a median survival to postnatal day P5 in nephrin-deficient mice, whereas heterozygous control mice and wild type (WT) control group showed 90% and 100% survival, respectively (at P50 days). Light microscopy analysis showed a significantly higher number of renal-tubular microcysts per kidney section in nephrin-deficient mice compared to the control groups (P < 0.0022). Transmission electron microscopy demonstrated reduced foot process (FP) density in nephrin-deficient mice compared to controls (P < 0.0001). Additionally, proteinuria quantitation using urine albumin-to-creatinine ratio (UACR) was significantly higher in nephrin-deficient mice compared to controls.

Conclusions: This study represents the first comprehensive description of the kidney phenotype in a nephrin-deficient mouse model, laying the foundation for future gene replacement therapy endeavors.