Nephronophthisis (NPH), an autosomal-recessive tubulointerstitial nephritis, is the most common cause of hereditary end-stage renal disease in the first three decades of life. Since most NPH gene products (NPHP) function at the primary cilium, NPH is classified as a ciliopathy. We identified mutations in a candidate gene in eight individuals from five families presenting late-onset NPH with massive renal fibrosis. This gene encodes MAPKBP1, a poorly characterized scaffolding protein for JNK signaling. Immunofluorescence analyses showed that MAPKBP1 is not present at the primary cilium and that fibroblasts from affected individuals did not display ciliogenesis defects, indicating that MAPKBP1 may represent a new family of NPHP not involved in cilia-associated functions. Instead, MAPKBP1 is recruited to mitotic spindle poles (MSPs) during the early phases of mitosis where it colocalizes with its paralog WDR62, which plays a key role at MSP. Detected mutations compromise recruitment of MAPKBP1 to the MSP and/or its interaction with JNK2 or WDR62. Additionally, we show increased DNA damage response signaling in fibroblasts from affected individuals and upon knockdown of Mapkbp1 in murine cell lines, a phenotype previously associated with NPH. In conclusion, we identified mutations in MAPKBP1 as a genetic cause of juvenile or late-onset and cilia-independent NPH.
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We are grateful to the families and studied individuals for their contribution. We would like to thank Herv? Enslen, Sylvie Legrand-Poels, and Ami Aronheim for their kind gifts of JNK2, MAPKBP1, and WDR62 encoding plasmids; Christelle Arrondel for her precious help for immunohistochemistry; G?rard Pivert (Pathology Department, Necker Hospital) for kidney biopsies; the bioinformatic Plateform (Universit? Paris Descartes, Institut Imagine) as well as Solenn Pruvost and Mohammed Zarhrate for their support in exome sequencing; Morgan Gallazzini for his kind help on Phospho-JNK blots; Marie-Claire Gubler for her precious help with interpretation of kidney biopsies; Jorunn Skeie Bringsli for genotyping; Torbj?rn Leivestad at the Norwegian Renal Registry; and Joseph Szustakowski for his contribution in the early part of the project. The Mapkbp1 mutant mice were obtained from the Medical Research Council centre for mouse genetics (MRC-Harwell) which distributes these mice on behalf of the European Mouse Mutant Archive. The MRC-Harwell is also a member of the International Mouse Phenotyping Consortium (IMPC), which funded the generation of the Mapkbp1 mutant mice. This work was supported by grants from the Regional Health Authority Western Norway (grants no. 911688 and 911466 to?C.B.?and 911746 to E.R.), from the NIH (DK068306 to F.H.),?from the Fondation pour la Recherche M?dicale (FRM; DEQ20130326532 to S.S.), and the foundation GIS-Maladies Rares (FONDATION_HTS-RD; I201302013 to S.S.). The Imagine Institute is supported by an ANR grant ANR-A0-IAHU-01 and from Fondation ARC (EML20110602384) for the purchase of the LEICA SP8 confocal microscope.
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