Seizures, sensorineural deafness, ataxia, mental retardation, and electrolyte imbalance (SeSAME syndrome) caused by mutations in KCNJ10

Abstract

We describe members of 4 kindreds with a previously unrecognized syndrome characterized by seizures, sensorineural deafness, ataxia, mental retardation, and electrolyte imbalance (hypokalemia, metabolic alkalosis, and hypomagnesemia). By analysis of linkage we localize the putative causative gene to a 2.5-Mb segment of chromosome 1q23.2-23.3. Direct DNA sequencing of KCNJ10, which encodes an inwardly rectifying K(+) channel, identifies previously unidentified missense or nonsense mutations on both alleles in all affected subjects. These mutations alter highly conserved amino acids and are absent among control chromosomes. Many of these mutations have been shown to cause loss of function in related K(+) channels. These findings demonstrate that loss-of-function mutations in KCNJ10 cause this syndrome, which we name SeSAME. KCNJ10 is expressed in glia in the brain and spinal cord, where it is believed to take up K(+) released by neuronal repolarization, in cochlea, where it is involved in the generation of endolymph, and on the basolateral membrane in the distal nephron. We propose that KCNJ10 is required in the kidney for normal salt reabsorption in the distal convoluted tubule because of the need for K(+) recycling across the basolateral membrane to enable normal activity of the Na(+)-K(+)-ATPase; loss of this function accounts for the observed electrolyte defects. Mice deficient for KCNJ10 show a related phenotype with seizures, ataxia, and hearing loss, further supporting KCNJ10's role in this syndrome. These findings define a unique human syndrome, and establish the essential role of basolateral K(+) channels in renal electrolyte homeostasis.

Fig. 1.

Mapping the disease locus. (A) An ideogram of chromosome 1 is shown, with homozygous regions in patient 441–1 and 404–1 indicated by light gray boxes, and segments that are identical by descent (IBD) in the siblings 632–1 and 632–2 marked by dark gray boxes. The overlap of these intervals is marked, and represents the maximum likelihood location of the disease locus on chromosome 1q23.2-q23.3, a 4-cM interval covering 2.5 Mb from 158.1 M to 160.6 M base pairs. (B) The candidate interval contains KCNJ10 on chromosome 1q23.2. Neighbouring genes are represented by arrows in their corresponding transcriptional orientations. KCNJ10 comprises 2 exons indicated by boxes, with the coding sequence indicated in black.

Fig. 2.

Mutations in KCNJ10 in affected patients. In each panel the DNA sequences of the sense strand of wild-type subjects (Left) and affected subjects (Right) are shown. The sequence of the encoded peptide is indicated in single letter code. A ClustalW alignment of the Homo sapiens (H.s.) protein sequence with orthologs and paralogs from Mus musculus (M.m.), Gallus gallus (G.g.), Xenopus tropicalis (X.t.), Danio rerio (D.r.), and Drosophila melanogaster (D.m.) is shown next to each mutation. The human sequence and residues conserved in orthologs and paralogs are marked in yellow, and the mutant residue is indicated. (A) Patient 327–1 is compound heterozygous for a missense and a nonsense mutation in KCNJ10. (B) Patient 404–1 is homozygous for a missense mutation, changing codon TGT (C140) to CGT (R140). (C) A homozygous missense mutation was found in kindred 441, resulting in change of codon ACC (T164) to ATC (I164). (D) In kindred 632, both affected siblings are compound heterozygous for missense mutations: A167V and R297C.

Fig. 3.

Location of KCNJ10 mutations in patients with SeSAME syndrome. A schematic view of the protein is shown, with intracellular N- and C-termini, 2 transmembrane helices (plasma membrane shown in shaded gray), and 1 pore. This structure is characteristic of the inward rectifier family. Locations of mutations are indicated by black circles, and the respective amino acid change is noted.

Fig. 4.

A model of impaired ion transport in the distal convoluted tubule caused by mutations in the Kir4.1 inwardly rectifying potassium channel. (A) Kir4.1/5.1 heteromultimers in the basolateral membrane of the distal convoluted tubule (DCT) recycle potassium entering the cell via the Na⁺-K⁺-ATPase back into the interstitial space and contribute to the negative membrane potential that promotes basolateral chloride exit. On the luminal surface, sodium and potassium enter the cell via the thiazide sensitive cotransporter NCCT, and Mg²⁺ enters via TrpM6, using the favorable electrical gradient. (B) Disruption of Kir4.1 function inhibits the function of the Na⁺-K⁺-ATPase via loss of potassium recycling, reduces basolateral chloride reabsorption by rendering the membrane potential (E_m) less negative, and thereby inhibits both apical Na⁺ and Cl⁻ reabsorption by NCCT and Mg²⁺ reabsorption because of a less negative membrane potential. The resulting renal salt loss activates the renin-angiotensin-aldosterone system. Increased amounts of Na⁺ and Cl⁻ are delivered to the cortical collecting duct, where aldosterone dependent Na⁺ reabsoption via ENaC is coupled to K⁺ and H⁺ secretion (see Discussion), thus accounting for the hypokalemic alkalosis observed.