In a Japanese brother and sister with a mild variant of Finnish congenital nephrosis (NPHS1; 256300), Kitamura et al. (2007) identified compound heterozygosity for a 793T-C transition in the NPHS1 gene, resulting in a cys265-to-arg (C265R) substitution in Ig domain 3, and a 2464G-A transition in the NPHS1 gene, resulting in a val822-to-met (V822M; 602716.0009) substitution in the spacer region between Ig domains 7 and 8. The parents, who had no urinary abnormalities, were each heterozygous for 1 of the missense mutations, respectively, which were not identified in 74 control chromosomes. Expression studies in COS-7 cells revealed that the C265R mutation, which disrupts formation of a disulfide bond within the third Ig domain, resulted in mutant protein that was predominantly trapped within the endoplasmic reticulum (ER). In contrast, the V822M variant protein reached the plasma membrane, although partial ER retention was observed. Antibody crosslinking experiments revealed a fine granular pattern of puncta on the plasma membrane with the V822M variant compared to the distinct large punctate staining seen with wildtype nephrin, suggesting impairment of lateral oligomerization of the V822M-mutant protein.
Shono et al. (2009) reported the biochemical effects of compound heterozygosity for the nephrin variants C265R and V822M. When heterologously expressed, these variants exhibited normal metabolic half-life and raft binding. Clustering of V822M-mutant protein failed to evoke a maximum tyrosine-phosphorylation and actin reorganization, suggesting an inability to assemble into functioning membrane microdomains. Shono et al. (2009) suggested that C265R and V822M mutants may compose a dysfunctional slit diaphragm complex due to their mixed defects comprising reduced cell surface targeting and ineffective assembly of signaling microdomains. The defective slit diaphragm may confer a susceptibility to immunogenic stimuli and predisposition to a relapsing phenotype.
