Defective nephrin trafficking caused by missense mutations in the NPHS1 gene: insight into the mechanisms of congenital nephrotic syndrome

Congenital nephrotic syndrome of the Finnish type (CNF or NPHS1) is an auto somal recessive kidney disorder resulting in severe proteinurea and renal d ysfunction. Although the disease occurs predominantly in the Finnish popula tion, many cases in other populations have also been reported. The disease gene (NPHS1) encodes nephrin, a podocyte transmembrane protein that is an e ssential component of the podocyte slit diaphragm, the renal ultrafilter. S ince the discovery of the gene, many mutations have been reported in the NP HS1 gene in patients with diverse ethnic background. A surprisingly large n umber of these mutations are missense mutations resulting in single amino a cid substitutions. In order to study the pathomechanism of these missense m utations, we have investigated the fate of 21 such mutations hitherto ident ified in NPHS1 patients. Immunostaining of stable transfected cells express ing the nephrin mutants demonstrated that most of the mutants showed only e ndoplasmic reticulum (ER) staining and no detectable cell surface localizat ion. Immunoelectron microscopy of cells expressing the wild-type and a muta nt nephrin further confirmed that the mutant nephrin could be abundantly fo und in the ER but not on the plasma membrane. Subcellular fractionation of wild-type and a mutant cell line clearly showed an altered subcellular dist ribution and molecular mobility of the mutant nephrin. In summary, our data indicate that a defective intracellular nephrin transport, most likely due to misfolding, is the most common consequence of missense mutations in NPH S1.