Steroid-resistant nephrotic syndrome in infants caused by a novel compound heterozygous mutation of the NUP93: A CARE case report

Nucleoporin-associated steroid-resistant nephrotic syndrome

Nucleoporins (Nups) are a class of proteins that assemble to form nuclear pore complexes, which are related to nucleocytoplasmic transport, gene expression, and the cell cycle. Pathogenic variants in six genes encoding Nups, NUP85, NUP93, NUP107, NUP133, NUP160, and NUP205, cause monogenic steroid-resistant nephrotic syndrome (SRNS), referred to as nucleoporin-associated SRNS. In this paper, we review the epidemiology, structure and function of Nups, pathogenesis, phenotypes and genotypes, and management of nucleoporin-associated SRNS as well as implications for genetic counseling. Affected individuals exhibit autosomal recessive isolated and syndromic SRNS, whose extrarenal manifestations include neurological disorders, growth and development disorders, cardiovascular disorders, and congenital malformations. The median ages at onset of NUP85-, NUP93-, NUP107-, NUP133-, NUP160-, and NUP205-associated SRNS are 7, 3, 4.1, 9, 7, and 2 years, respectively. Kidney biopsies reveal focal segmental glomerulosclerosis in 89% of patients. Most affected individuals are resistant to immunosuppressants. For the six subtypes of nucleoporin-associated SRNS, patients show progression to kidney failure at median ages of 8.5, 3.7, 6.9, 13, 15, and 7 years, respectively. Only two patients with NUP93-associated SRNS with nephrotic syndrome relapse post-transplant have been reported, and the recurrence rate is 12.5%. Next-generation sequencing using a targeted gene panel is recommended in cases of suspected nucleoporin-associated SRNS for genetic diagnosis. Renin-angiotensin-aldosterone system inhibitors are recommended for patients with nucleoporin-associated SRNS. Once genetic diagnosis is confirmed, immunosuppressant discontinuation should be considered, and kidney transplant is preferred when patients progress to kidney failure. Genetic counselling should be provided for asymptomatic siblings and future siblings of an affected individual. Further studies on the pathogenesis of nucleoporin-associated SRNS are needed to seek new therapeutic interventions.

Mutations in the NUP93, NUP107 and NUP160 genes cause steroid-resistant nephrotic syndrome in Chinese children

BACKGROUND

The variants of nucleoporins are extremely rare in hereditary steroid-resistant nephrotic syndrome (SRNS). Most of the patients carrying such variants progress to end stage kidney disease (ESKD) in their childhood. More clinical and genetic data from these patients are needed to characterize their genotype-phenotype relationships and elucidate the role of nucleoporins in SRNS.

METHODS

Four patients of SRNS carrying biallelic variants in the NUP93, NUP107 and NUP160 genes were presented. The clinical and molecular genetic characteristics of these patients were summarized, and relevant literature was reviewed.

RESULTS

All four patients in this study were female and initially presented with SRNS. The median age at the onset of the disease was 5.08 years, ranging from 1 to 10.5 years. Among the four patients, three progressed to ESKD at a median age of 7 years, ranging from 1.5 to 10.5 years, while one patient reached stage 3 chronic kidney disease (CKD3). Kidney biopsies revealed focal segmental glomerulosclerosis in three patients. Biallelic variants were detected in NUP93 in one patient, NUP107 in two patients, as well as NUP160 in one patient respectively. Among these variants, five yielded single amino acid substitutions, one led to nonsense mutation causing premature termination of NUP107 translation, one caused a single nucleotide deletion resulting in frameshift and truncation of NUP107. Furthermore, one splicing donor mutation was observed in NUP160. None of these variants had been reported previously.

CONCLUSION

This report indicates that biallelic variants in NUP93, NUP107 and NUP160 can cause severe early-onset SRNS, which rapidly progresses to ESKD. Moreover, these findings expand the spectrum of phenotypes and genotypes and highlight the importance of next-generation sequencing in elucidating the molecular basis of SRNS and allowing rational treatment for affected individuals.

From the sideline: Tissue-specific nucleoporin function in health and disease, an update

The subcellular compartmentalisation of eukaryotic cells requires selective exchange between the cytoplasm and the nucleus. Intact nucleocytoplasmic transport is vital for normal cell function and mutations in the executing machinery have been causally linked to human disease. Central players in nucleocytoplasmic exchange are nuclear pore complexes (NPCs), which are built from ~30 distinct proteins collectively termed nucleoporins. Aberrant nucleoporin expression was detected in human cancers and autoimmune diseases since quite some time, while it was through the increasing use of next generation sequencing that mutations in nucleoporin genes associated with mainly rare hereditary diseases were revealed. The number of newly identified mutations is steadily increasing, as is the number of diseases. Mutational hotspots have emerged: mutations in the scaffold nucleoporins seemingly affect primarily inner organs, such as heart, kidney, and ovaries, whereas genetic alterations in peripheral, cytoplasmic nucleoporins affect primarily the central nervous system and development. In this review, we summarise latest insights on altered nucleoporin function in the context of human hereditary disorders, with a focus on those where mechanistic insights are beginning to emerge.

Steroid-Resistant Nephrotic Syndrome Caused by NUP93 Pathogenic Variants

BACKGROUND

Although steroid therapy is a standard of care for nephrotic syndrome treatment, 15-20% of patients do not respond to it. Finding the genetic background is possible in >10% of steroid-resistant nephrotic syndrome (SRNS) cases. Variants in genes encoding nuclear pore complex proteins are a novel cause of paediatric steroid-resistant nephrotic syndrome (SRNS). Recent studies suggest NUP93 variants to be a significant cause of paediatric onset SRNS. The clinical data on certain variants and disease history are still very limited.

METHODS AND RESULTS

We report the SRNS case of a 12-year-old boy with two detected NUP93 variants, which are pathogenic and possibly pathogenic. The onset of the disease was early and severe. The patient was admitted to the paediatric nephrology department due to nephrotic-range proteinuria and hypoalbuminemia with a long medical history of steroid and non-steroid immunosuppressive treatment. The genetic panel targeting 50 genes, clinically relevant for nephrotic syndrome, was performed. The only gene which was found to be affected by mutations, namely c.2326C>T and c.1162C>T, respectively, was NUP93. Conclusions: NUP93 variants are rarely identified as causes of SRNS. Clinical data are of utmost importance to establish the standard of care for SRNS patients suffering from this genetic disfunction. This is the first case of a heterozygous patient with the c.2326C>T and c.1162C>T variants and confirmed clinical history of the SRNS described so far. Our data suggest the clinical relevance of the c.1162C>T variant.

A rare cause of steroid-resistant nephrotic syndrome - a case report

Steroid resistance is a common condition occurring in children with nephrotic syndrome. Until now, over 50 genes involved in steroid-resistant nephrotic syndrome (SRNS) pathogenesis have been identified, among which the most prevalent are NPHS1, NPHS2, CD2AP, and PTPRO. The patterns of inheritance of SRNS are autosomal recessive, autosomal dominant, or mitochondrial, and tissues of those patients show focal segmental glomerulosclerosis (FSGS) signs in histopathological image analysis. We present a case of a 6-year-old girl who was admitted to the pediatric nephrology department due to nephrotic range proteinuria and edema of the lower leg. We started therapy with prednisone at a dose of 45 mg (60 mg/m2), enalapril as a nephroprotection, and antihistamines as an additional treatment. During in-patient treatment, we detected increased blood pressure. Due to persistent proteinuria in spite of 6-week treatment with steroids at the maximal dose, we confirmed disease resistance to steroids. Additionally, FSGS signs were confirmed in kidney biopsy samples. After genetic screening for SRNS and detection of the rare gene mutation NUP93 we reduced prednisone but maintained nephroprotective treatment and administered cyclosporin A. The girl remains currently under the care of nephrologists with normal arterial blood pressure, trace proteinuria in follow-up examination, and normal kidney function. NUP93 mutation is extremely rare; therefore few cases have been described to date. The onset of the symptoms in all pediatric patients appeared before the age of 8 and they developed end stage kidney disease (ESKD). They might manifest symptoms from the other systems.

Mutation in XPO5 causes adult-onset autosomal dominant familial focal segmental glomerulosclerosis

Background

Focal and segmental glomerulosclerosis (FSGS) is a histological pathology that characterizes a wide spectrum of diseases. Many genes associated with FSGS have been studied previously, but there are still some FSGS families reported in the literature without the identification of known gene mutations. The aim of this study was to investigate the new genetic cause of adult-onset FSGS.

Methods

This study included 40 FSGS families, 77 sporadic FSGS cases, 157 non-FSGS chronic kidney disease (CKD) families and 195 healthy controls for analyses. Whole-exome sequencing (WES) and Sanger sequencing were performed on probands and family members of all recruited families and sporadic FSGS cases.

Results

Using WES, we have identified a novel heterozygous missense variant (c.T1655C:p.V552A) in exportin 5 gene (XPO5) in two families (FS-133 and CKD-05) affected with FSGS and CKD. Sanger sequencing has confirmed the co-segregation of this identified variant in an autosomal dominant pattern within two families, while this variant was absent in healthy controls. Furthermore, the identified mutation was absent in 195 ethnically matched healthy controls by Sanger sequencing. Subsequently, in silico analysis demonstrated that the identified variant was highly conservative in evolution and likely to be pathogenic.

Conclusions

Our study reports an adult-onset autosomal dominant inheritance of the XPO5 variant in familial FSGS for the first time. Our study expanded the understanding of the genotypic, phenotypic and ethnical spectrum of mutation in this gene.

Exploring the relevance of NUP93 variants in steroid-resistant nephrotic syndrome using next generation sequencing and a fly kidney model

BACKGROUND

Variants in genes encoding nuclear pore complex (NPC) proteins are a newly identified cause of paediatric steroid-resistant nephrotic syndrome (SRNS). Recent reports describing NUP93 variants suggest these could be a significant cause of paediatric onset SRNS. We report NUP93 cases in the UK and demonstrate in vivo functional effects of Nup93 depletion in a fly (Drosophila melanogaster) nephrocyte model.

METHODS

Three hundred thirty-seven paediatric SRNS patients from the National cohort of patients with Nephrotic Syndrome (NephroS) were whole exome and/or whole genome sequenced. Patients were screened for over 70 genes known to be associated with Nephrotic Syndrome (NS). D. melanogaster Nup93 knockdown was achieved by RNA interference using nephrocyte-restricted drivers.

RESULTS

Six novel homozygous and compound heterozygous NUP93 variants were detected in 3 sporadic and 2 familial paediatric onset SRNS characterised histologically by focal segmental glomerulosclerosis (FSGS) and progressing to kidney failure by 12 months from clinical diagnosis. Silencing of the two orthologs of human NUP93 expressed in D. melanogaster, Nup93-1, and Nup93-2 resulted in significant signal reduction of up to 82% in adult pericardial nephrocytes with concomitant disruption of NPC protein expression. Additionally, nephrocyte morphology was highly abnormal in Nup93-1 and Nup93-2 silenced flies surviving to adulthood.

CONCLUSION

We expand the spectrum of NUP93 variants detected in paediatric onset SRNS and demonstrate its incidence within a national cohort. Silencing of either D. melanogaster Nup93 ortholog caused a severe nephrocyte phenotype, signaling an important role for the nucleoporin complex in podocyte biology. A higher resolution version of the Graphical abstract is available as Supplementary information.