Maternally inherited Birk Barel mental retardation dysmorphism syndrome caused by a mutation in the genomically imprinted potassium channel KCNK9

Abstract

We describe a maternally transmitted genomic-imprinting syndrome of mental retardation, hypotonia, and unique dysmorphism with elongated face. We mapped the disease-associated locus to approximately 7.27 Mb on chromosome 8q24 and demonstrated that the disease is caused by a missense mutation in the maternal copy of KCNK9 within this locus. KCNK9 is maternally transmitted (imprinted with paternal silencing) and encodes K(2P)9.1, a member of the two pore-domain potassium channel (K(2P)) subfamily. The mutation fully abolishes the channel's currents--both when functioning as a homodimer or as a heterodimer with K(2P)3.1.

Figure 1

Clinical Features (A) Pedigree of the affected Israeli Arab family. The numbers denote individuals whose DNA samples were analyzed. It can be seen that the disease is maternally transmitted (paternally imprinted). (B and C) Dysmorphic features of affected siblings. (D) Prominent incisor teeth of young affected individual.

Figure 2

Mapping the KCNK9 Locus (A) Partial pedigree and segregation analysis of the affected family. The numbers denote individuals whose DNA samples were analyzed. Haplotypes are represented as columns of numbers; the disease-associated haplotype is shaded. Physical distances between the markers are shown. (B) Two-point LOD-score analysis for the polymorphic markers at the KCNK9 locus. The arrow indicates the gene location.

Figure 3

Analysis of the 770G→A Mutation in Exon 2 of KCNK9 Sequence analysis of a healthy obligatory-carrier mother and her affected and healthy offspring.

Figure 4

The K_2P9.1-G236R Mutant Does Not Produce Functional Channels Macroscopic currents measured by two-electrode voltage clamp. (A) Raw current traces for representative oocytes expressing WT-K_2P9.1 and K_2P9.1-G236R channels and bathed in 5 mM potassium solution. Oocyte membrane potential was held at −80 mV and pulsed from −155 to +40 mV in 15 mV voltage steps for 70 ms with 1 s interpulse intervals. (B) K_2P9.1 steady-state current-voltage relationships for WT-K_2P9.1 (full circles), K_2P9.1-G236R mutated channels (open circles), and coexpression of WT-K_2P9.1 + K_2P9.1-G236R (triangles) (mean ± SEM, n = 6-9 cells). (C) Currents at +40 mV of oocytes injected with equal cRNA amounts of the different channels, as indicated (mean ± standard error of the mean, n = 6–9 cells). (D) Predicted membrane topology of a K_2P9.1 subunit indicating the two P domains and four transmembrane segments (M1–4). The predicted position of G236 is highlighted in red. (E) Surface representation of two opposing KcsA subunits. G104 was replaced by an arginine residue (highlighted in red), as in the K_2P9.1-G236 mutant. Ribbon representation of the carbon backbone is in dark green, and potassium ions are in light green. Molecular graphics images were produced with the UCSF Chimera (an extensible molecular-modeling system) package from the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco.