Neuronal Proteins as Antigenic Targets in Membranous Nephropathy

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

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.

Diagnostic Challenges and Emerging Pathogeneses of Selected Glomerulopathies

Recent progress in glomerular immune complex and complement-mediated diseases have refined diagnostic categories and informed mechanistic understanding of disease development in pediatric patients. Herein, we discuss selected advances in 3 categories. First, membranous nephropathy antigens are increasingly utilized to characterize disease in pediatric patients and include phospholipase A2 receptor (PLA2R), Semaphorin 3B (Sema3B), neural epidermal growth factor-like 1 (NELL1), and protocadherin FAT1, as well as the lupus membranous-associated antigens exostosin 1/2 (EXT1/2), neural cell adhesion molecule 1 (NCAM1), and transforming growth factor beta receptor 3 (TGFBR3). Second, we examine advances in techniques for paraffin and light chain immunofluorescence (IF), including the former's function as a salvage technique and their necessity for diagnosis in adolescent cases of membranous-like glomerulopathy with masked IgG kappa deposits (MGMID) and proliferative glomerulonephritis with monotypic Ig deposits (PGNMID), respectively. Finally, progress in understanding the roles of complement in pediatric glomerular disease is reviewed, with specific attention to overlapping clinical, histologic, and genetic or functional alternative complement pathway (AP) abnormalities among C3 glomerulopathy (C3G), infection-related and post-infectious GN, "atypical" post-infectious GN, immune complex mediated membranoproliferative glomerulonephritis (IC-MPGN), and atypical hemolytic uremic syndrome (aHUS).

Novel Antigens and Clinical Updates in Membranous Nephropathy

Membranous nephropathy (MN), an autoimmune kidney disease and leading cause of nephrotic syndrome, leads to kidney failure in up to one-third of affected individuals. Most MN cases are due to an autoimmune reaction against the phospholipase A2 receptor (PLA2R) located on kidney podocytes. Serum PLA2R antibody quantification is now part of routine clinical practice because antibody titers correlate with disease activity and treatment response. Recent advances in target antigen detection have led to the discovery of more than 20 other podocyte antigens, yet the clinical impact of additional antigen detection remains unknown and is under active investigation. Here we review recent findings and hypothesize how current research will inform future care of patients with MN.