Misleading findings of homozygosity mapping resulting from three novel mutations in NPHS1 encoding nephrin in a highly inbred community

Purpose: Congenital nephrotic syndrome of the Finnish type (CNF, NPHS1) is a rare autosomal recessive disease caused by mutations in the NPHS1 gene encoding nephrin. We diagnosed congenital nephrotic syndrome in 12 children living in a village near Jerusalem. Most of the inhabitants are descendants of one Muslim family and have maintained their isolation by preference of consanguineous marriages. The aim of this study was to confirm that the NPHS1 gene is responsible for congenital nephrotic syndrome in our population, applying homozygosity mapping. Methods: DNA samples were genotyped by four microsatellite markers that were in linkage disequilibrium with the NPHS1 gene on chromosome 19q13.1. Immunoperoxidase staining was used to study the expression of nephrin, and mutations were subsequently identified by direct sequencing of the entire coding region of the NPHS1 gene. Results: Haplotype analysis revealed several different haplotypes, leading us to assume erroneously that there was genetic heterogeneity of congenital nephrotic syndrome. Because nephrin was completely absent in kidney tissue of one patient, direct sequencing of all DNA samples was performed, yielding three novel mutations: c.1138C > T (p.Gln380X), c.2160_2161insC (p.Cys721fs), and c.1707C > G (p.Ser569Arg). Patients were either homozygous for one of these mutations or compound heterozygotes, and they differed in their phenotype. Conclusion: We report the potential pitfalls of performing homozygosity mapping in a highly consanguineous population and discuss the phenomenon of multiple mutations in a given gene within an isolate.