The role of podocyte genetics in childhood nephrotic syndrome

The nephrotic syndrome (NS) is characterized by massive proteinuria, edema and hypertriglyceridemia. It can appear as a primary or a secondary disease, idiopathic or familial, monogenic or complex, responsive to medication or progressing inevitably towards end-stage renal disease. As the manifestations of the disease are varied so is the etiologies behind it and still much remains to be discovered. However, structural compromise can be observed in the glomeruli of the NS patients, especially in podocytes, specialized epithelial cells of the glomerular filtration barrier. When causative genetic variants are found they are predominantly in the genes coding proteins involved in the structure and function of the podocytes. To date, over thirty podocyte protein-coding genes have been implicated. In this study we looked into genetic and cellular mechanisms in the podocyte underlying different forms of NS: congenital nephrotic syndrome of the Finnish type (CNF), steroid sensitive nephrotic syndrome (SSNS) and steroid resistant nephrotic syndrome (SRNS). Specifically, we used CNF kidney samples to study how the different compartments of the podocyte are affected by the lack of nephrin, characteristic structural component of the unique cellular junction of the podocytes (slit diaphragm (SDs)). We also looked into genetic variation in the podocyte protein coding genes in the Finnish SRNS patients to map out the variant spectrum and to see if this population shows difference to other populations. In addition, we aimed to find genetic and clinical markers predicting the course and severity of SSNS, which would have significant clinical value. The analyses were carried out using an array of molecular biological methods. Protein expression was studied using immunohistochemistry and light microscopy, genetic variants with PCR and sequence analysis. In addition, immunoprecipitation and western blot analyses were carried out to study the functionality of a particular variant. Genome wide sequencing analysis was carried out to gain wider perspective on the variant spectrum in NS patients. Our results showed that the lack of nephrin leads to widespread effects in the podocyte on protein level, especially on the expression of other proteins of the SD. In other compartments of the cell (basal membrane, actin cytoskeleton, apical membrane) the effect was clear but considering the profound structural damage seen in CNF the orders of magnitude of the observed changes were modest. We also showed that the analyzed clinical features or variants in these key genes coding podocyte proteins cannot reliably predict severity of SSNS. Children with difficult disease (multiple relapses, dependence on steroids) are more likely to suffer from NS in adulthood but other correlations between early patient features and prognosis could not be made. Single nucleotide polymorphisms (SNP) analysis of children with idiopathic NS revealed that variants in some genes (e.g. MDR1) may be useful in predicting responsiveness to steroids but the correlation is not sufficiently strong to warrant routine clinical testing. We found few causative variants in patients with idiopathic SRNS but did uncover a de novo NPHS2 variant co-segregating with familial dominant SRNS and unusual course of the disease. Healthy function of the podocyte relies on interaction and communication between multitudes of protein components in the different compartments of the cell. If any of these components are faulty it may cascade into widespread podocyte damage and proteinuria. The precise nature of the defect may play a significant role in the disease phenotype and not only the function of the damaged gene but also the nature of the variant ought to be considered when carrying out genetic analysis. Not in all cases causative variants or precise faulty structures can be identified. The genetic factors behind complex traits remain elusive, and even though some pieces of the puzzle have been found, in small part by this study, much is still to be discovered