Genetic and clinical spectrum of steroid-resistant nephrotic syndrome with nuclear pore gene mutation

Abstract

Background: Steroid-resistant nephrotic syndrome (SRNS) is insensitive to steroid therapy and overwhelmingly progresses to kidney failure (KF), the known pathogenic genes of which include key subunits of the nuclear pore complex (NPC), a less-recognized contributor to glomerular podocyte injury.

Methods: After analyzing their clinical characterizations and obtaining parental consent, whole-exome sequencing (WES) was performed on patients with SRNS. Several nucleoporin (NUP) biallelic pathogenic variants were identified and further analyzed by cDNA-PCR sequencing from white cells of peripheral blood, minigene assay, immunohistochemical (IHC) staining, and electron microscopy (EM) ultrastructure observation of kidney biopsy, as well as multiple in silico prediction tools, including 3D protein modeling.

Results: Here, in six families with SRNS, we identified pathogenic mutations in NUP85/93/107/160 genes. Specifically, the patient with NUP93 mutation developed KF six months after diagnosis at 1 year 2 months. Two missense mutations, c.1655A > G and c.1604A > C, disrupted the protein stability of NUP93 by IHC staining of kidney biopsy. Ultrastructurally, the above mutations led to severe vacuolization and deformed nucleus in podocytes, torn and dissolved glomerular basement membrane, and diffuse foot process effacement. The patient with NUP85 mutation reached chronic kidney disease (CKD) stage 3 after 4 years follow-up, with exons 2-5 in-frame loss and a missense variant at c.511C > T, not affecting NUP85 expression but possibly weakened interaction with Seh1. Additionally, an extended endoplasmic reticulum (ER) tubule was readily observed under EM. Meanwhile, dilated ER was also found in two children with NUP160 mutations (c.3330 delA and c.2407 G > A; c.2241 + 1 (IVS17) G > T and c.3656 T > G), one of which has undergone kidney transplantation. Compound heterozygous variants in NUP107, c.1695 G > C and c.1360 C > T, were found in a 14-year-old girl initially diagnosed with CKD stage 5, with the former variant causing exon 19 skipping and early translation termination. c.1311 + 1(IVS15) G > A and c.1790 C > T were identified in the second affected girl, with the former causing exon 15 skipping and an in-frame loss of aa417-438, which disrupted the stability of NUP107 and interaction with NUP133.

Conclusions: Our findings expand the spectrum of phenotypes and genotypes of NUPs-associated SRNS and suggest its possible pathogenic mechanism in nuclear and ER homeostasis.

Keywords: Endoplasmic reticulum stress; Gene variants; Nuclear deformity; Nucleoporin; Podocyte; Steroid-resistant nephrotic syndrome.