Three Families with Autosomal Dominant Nephrogenic Diabetes Insipidus Caused by Aquaporin-2 Mutations in the C-Terminus

Abstract

The vasopressin-regulated water channel aquaporin-2 (AQP2) is known to tetramerize in the apical membrane of the renal tubular cells and contributes to urine concentration. We identified three novel mutations, each in a single allele of exon 4 of the AQP2 gene, in three families showing autosomal dominant nephrogenic diabetes insipidus (NDI). These mutations were found in the C-terminus of AQP2: a deletion of G at nucleotide 721 (721 delG), a deletion of 10 nucleotides starting at nucleotide 763 (763-772del), and a deletion of 7 nucleotides starting at nucleotide 812 (812-818del). The wild-type AQP2 is predicted to be a 271-amino acid protein, whereas these mutant genes are predicted to encode proteins that are 330-333 amino acids in length, because of the frameshift mutations. Interestingly, these three mutant AQP2s shared the same C-terminal tail of 61 amino acids. In Xenopus oocytes injected with mutant AQP2 cRNAs, the osmotic water permeability (Pf) was much smaller than that of oocytes with the AQP2 wild-type (14%-17%). Immunoblot analysis of the lysates of the oocytes expressing the mutant AQP2s detected a band at 34 kD, whereas the immunoblot of the plasma-membrane fractions of the oocytes and immunocytochemistry failed to show a significant surface expression, suggesting a defect in trafficking of these mutant proteins. Furthermore, coinjection of wild-type cRNAs with mutant cRNAs markedly decreased the oocyte Pf in parallel with the surface expression of the wild-type AQP2. Immunoprecipitation with antibodies against wild-type and mutant AQP2 indicated the formation of mixed oligomers composed of wild-type and mutant AQP2 monomers. Our results suggest that the trafficking of mutant AQP2 is impaired because of elongation of the C-terminal tail, and the dominant-negative effect is attributed to oligomerization of the wild-type and mutant AQP2s. Segregation of the mutations in the C-terminus of AQP2 with dominant-type NDI underlies the importance of this domain in the intracellular trafficking of AQP2.

Hereditary nephrogenic diabetes insipidus in Japanese patients: analysis of 78 families and report of 22 new mutations in AVPR2 and AQP2

“…The results from 3 of these families were previously reported [12]. These three families had monoallelic frame-shift deletion mutations (1-10 nucleotides) in the C-terminus of AQP2 (different mutations in each family), and showed an autosomal dominant inheritance with a slightly milder form of NDI [12].…”

“…These identified two deletion mutations cause frame shifts from Val251 and Leu259 and a new C-terminal tail ending at codon 334 (Table 4). We previously reported three small deletion mutations in the C-terminus that cause similar frame shifts and show dominant inheritance [12] (Table 4). These frame-shift mutations share the loss of the last tail of the AQP2 protein, the site where PDZ proteins and ubiquitines interact, and the presence of extended C-terminal tails that contain missorting signals.…”

“…Also, several cases have been reported separately before [12][13][14][15][16]. The AVPR2 and AQP2 genes are relatively small and all exons and intron-exon boundaries were sequenced with usual sequencing methods [12,17,18]. Usually, mutation analysis of AVPR2 was performed first.…”

Molecular Biology of Hereditary Diabetes Insipidus

“…In contrast, the dominant mutations reported to date are located in the region that codes for the carboxyl terminus of AQP2 (73)(74)(75). Dominant AQP2 mutants form heterotetramers with wt-AQP2 and are misrouted.…”

Novel mutations underlying nephrogenic diabetes insipidus in Arab families

“…11 The inheritance pattern is usually autosomal recessive, however dominant inheritance has been reported, albeit rarely. 12 Arab populations in which consanguinity and endogamy is common remain relatively culturally and geographically isolated 13 and largely understudied as evidenced by the finding of novel, generally population-specific mutations, even for common, well-studied disorders such as cystic fibrosis. 14 Despite 178 AVPR2 and 34 AQP2 mutations and being listed by the Human Gene Mutation Database (HGMD), 15 none and only 2, respectively, have been associated with Arabs to date, highlighting the need for this study.…”