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


CONCLUSIONS
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.
Frishberg
Yaacov
Y
Division of Pediatric Nephrology, Shaare Zedek Medical Center, Jerusalem, Israel. yaacov@md.huji.ac.il
Ben-Neriah
Ziva
Z
Suvanto
Maija
M
Rinat
Choni
C
Männikkö
Minna
M
Feinstein
Sofia
S
Becker-Cohen
Rachel
R
Jalanko
Hannu
H
Zlotogora
Joel
J
Kestilä
Marjo
M
D002888:Chromosomes, Human, Pair 19 / Q000235:genetics; D003241:Consanguinity; D005260:Female; D030342:Genetic Diseases, Inborn / Q000235:genetics*; D006801:Humans; D015810:Linkage Disequilibrium; D008297:Male; D008565:Membrane Proteins / Q000235:genetics*; D009404:Nephrotic Syndrome / Q000151:congenital / Q000235:genetics*; D010375:Pedigree; D017354:Point Mutation
D008565:Membrane Proteins; C113267:nephrin