Cantú syndrome resulting from activating mutation in the KCNJ8 gene

Abstract

ATP-sensitive potassium (KATP ) channels, composed of inward-rectifying potassium channel subunits (Kir6.1 and Kir6.2, encoded by KCNJ8 and KCNJ11, respectively) and regulatory sulfonylurea receptor (SUR1 and SUR2, encoded by ABCC8 and ABCC9, respectively), couple metabolism to excitability in multiple tissues. Mutations in ABCC9 cause Cantú syndrome (CS), a distinct multiorgan disease, potentially via enhanced KATP channel activity. We screened KCNJ8 in an ABCC9 mutation-negative patient who also exhibited clinical hallmarks of CS (hypertrichosis, macrosomia, macrocephaly, coarse facial appearance, cardiomegaly, and skeletal abnormalities). We identified a de novo missense mutation encoding Kir6.1[p.Cys176Ser] in the patient. Kir6.1[p.Cys176Ser] channels exhibited markedly higher activity than wild-type channels, as a result of reduced ATP sensitivity, whether coexpressed with SUR1 or SUR2A subunits. Our results identify a novel causal gene in CS, but also demonstrate that the cardinal features of the disease result from gain of KATP channel function, not from a Kir6-independent SUR2 function.

Keywords: Cantú syndrome; KATP; KCNJ8; Kir6.1; hypertrichosis.

Fig 1. Gain-of-function KCNJ8 mutation Kir6.1[p.Cys176Ser] underlies Cantu Syndrome

(A) Sequence chromatogram of patient and unaffected parents genomic DNA confirms de novo g.21,919,406A>T transition mutation in the patient, that is absent in the mother and father. (B) Clinical phenotype of the patient. Photographs of the patient at 13 years reveal extensive hypertrichosis, macrocephaly, coarse facial appearance, long arm and torso to height ratio, and gingival hyperplasia with thickened lips. (C) Ribbon diagram of two of the four Kir6.1 subunits that form the K⁺-selective pore in K_ATP, based on the crystal structures of KirBac1.1 (Kuo et al., 2003) and cytoplasmic domain of Kir3.1 (Nishida et al., 2002). Shown are Kir6.1 residues mutated in CS (p.Cys176Ser), and reported in association with the J-wave syndrome (p.Ser422Leu). (D) Rate constants for K_ATP-dependent ⁸⁶Rb⁺ efflux (k₂) in basal conditions relative to metabolic inhibition (MI) for WT and mutant K_ATP channels (mean ± s.e.m., from 4–6 experiments). *P < 0.01 compared to wild-type K_ATP by unpaired Student’s t test.

Fig. 2. Reduced ATP-sensitivity of reconstituted heteromeric K_ATP channels containing Kir6.1[p.Cys176Ser] subunits

(A) Rate constants for K_ATP-dependent ⁸⁶Rb⁺ efflux (k₂) in basal conditions relative to metabolic inhibition (MI) from COS cells expressing heteromeric Kir6.1/Kir6.2 or Kir6.1[p.Cys176Ser] (C176S)/Kir6.2 plus SUR2A K_ATP channels. (B) Representative currents recorded from inside-out membrane patches from COS cells expressing heteromeric Kir6.1/Kir6.2 or Kir6.1[p.Cys176Ser] (C176S)/Kir6.2 plus SUR2A K_ATP channels at −50 mV in K_int solution (see methods). Patches were exposed to differing [ATP] and baseline current was determined by exposure to ATP (5mM). (C) Steady-state dependence of membrane current on [ATP] (relative to current in zero ATP (I_rel)) for wild-type and p.Cys176Ser-containing channels. Data points represent the mean ± s.e.m. (n = 6–8 patches). The fitted lines correspond to least squares fits of a Hill equation (see methods). *P < 0.01 compared to wild-type K_ATP by unpaired Student’s t test.