Ania Koziell

Mutations of the novel renal glomerular genes NPHS1 and NPHS2 encoding nephrin and podocin cause two types of severe nephrotic syndrome presenting in early life, Finnish type congenital nephrotic syndrome (CNF) and a form of autosomal recessive familial focal segmental glomerulosclerosis (SRN1), respectively. To investigate the mechanisms by which mutations might cause glomerular protein leak, we analysed NPHS1/NPHS2 genotype/phenotype relationships in 41 non-Finnish CNF patients, four patients with congenital (onset 0 to 3 months) focal segmental glomerulosclerosis and five patients with possible SRN1 (onset 6 months to 2 years). We clarify the range of NPHS1 mutations in CNF, detecting mutation 'hot-spots' within the NPHS1 coding sequence. In addition, we describe a novel discordant CNF phenotype characterized by variable clinical severity, apparently influenced by gender. Moreover, we provide evidence that CNF may be genetically heterogeneous by detection of NPHS2 mutations in some CNF patients in whom NPHS1 mutations were not found. We confirm an overlap in the NPHS1/NPHS2 mutation spectrum with the characterization of a unique di-genic inheritance of NPHS1 and NPHS2 mutations, which results in a 'tri-allelic' hit and appears to modify the phenotype from CNF to one of congenital focal segmental glomerulosclerosis (FSGS). This may result from an epistatic gene interaction, and provides a rare example of multiple allelic hits being able to modify an autosomal recessive disease phenotype in humans. Our findings provide the first evidence for a functional inter-relationship between NPHS1 and NPHS2 in human nephrotic disease, thus underscoring their critical role in the regulation of glomerular filtration.

Microalbuminuria is significant both as the earliest stage of diabetic nephropathy and as an independent cardiovascular risk factor in nondiabetic subjects, in whom it is associated with insulin resistance. The link between disorders of cellular insulin metabolism and albuminuria has been elusive. Here, we report using novel conditionally immortalized human podocytes in vitro and human glomeruli ex vivo that the podocyte, the principal cell responsible for prevention of urinary protein loss, is insulin responsive and able to approximately double its glucose uptake within 15 min of insulin stimulation. Conditionally immortalized human glomerular endothelial cells do not respond to insulin, suggesting that insulin has a specific effect on the podocyte in the glomerular filtration barrier. The insulin response of the podocyte occurs via the facilitative glucose transporters GLUT1 and GLUT4, and this process is dependent on the filamentous actin cytoskeleton. Insulin responsiveness in this key structural component of the glomerular filtration barrier may have central relevance for understanding of diabetic nephropathy and for the association of albuminuria with states of insulin resistance.

BACKGROUND AND OBJECTIVES: Up to 95% of children presenting with steroid-resistant nephrotic syndrome in early life will have a pathogenic single-gene mutation in 1 of 24 genes currently associated with this disease. Others may be affected by polymorphic variants. There is currently no accepted diagnostic algorithm for clinical genetic testing. The hypothesis was that the increasing reliability of next generation sequencing allows comprehensive one-step genetic investigation of this group and similar patient groups. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: This study used next generation sequencing to screen 446 genes, including the 24 genes known to be associated with hereditary steroid-resistant nephrotic syndrome. The first 36 pediatric patients collected through a national United Kingdom Renal Registry were chosen with comprehensive phenotypic detail. Significant variants detected by next generation sequencing were confirmed by conventional Sanger sequencing. RESULTS: Analysis revealed known and novel disease-associated variations in expected genes such as NPHS1, NPHS2, and PLCe1 in 19% of patients. Phenotypically unexpected mutations were also detected in COQ2 and COL4A4 in two patients with isolated nephropathy and associated sensorineural deafness, respectively. The presence of an additional heterozygous polymorphism in WT1 in a patient with NPHS1 mutation was associated with earlier-onset disease, supporting modification of phenotype through genetic epistasis. CONCLUSIONS: This study shows that next generation sequencing analysis of pediatric steroid-resistant nephrotic syndrome patients is accurate and revealing. This analysis should be considered part of the routine genetic workup of diseases such as childhood steroid-resistant nephrotic syndrome, where the chance of genetic mutation is high but requires sequencing of multiple genes.