Research ArticleGain-of-function KCNJ6 Mutation in a Severe Hyperkinetic Movement Disorder Phenotype☆
Introduction
Genetic neurological channelopathies are a group of heterogeneous disorders, usually with dominant inheritance, and causing some form of paroxysmal neurological disturbances in function, which may become permanent in time (Spillane et al., 2016). For example, ion channelopathies have been implicated in diseases including epileptic encephalopathies, ataxia, paroxysmal dyskinesias, migraine, pain syndromes, skeletal muscle disorders, and periodic paralysis (Spillane et al., 2016). Under normal conditions, ion channels serve an important role of permitting rapid and selective movement of ions across plasma membranes, influencing the excitability of neurons and subsequent signaling in the brain. Channelopathies result from defects in the ion channel function or from changes in trafficking to the plasma membrane. A large number of channelopathies involve potassium channels, which can be divided into three families: voltage-gated K+ channels (KV1–18), two-pore K+ channels (K2P1–K2P18), and inwardly rectifying K+ channels (KIR1–KIR7) (Serratrice et al., 2010). KV channels contribute to the repolarization of the action potential. Defects in this family of channels commonly lead to some form of epilepsy. For example, mutations in KCNQ2 (KV7.2) and KCNA1 (KV1.2) cause benign familial neonatal seizures and myoclonic epilepsy, respectively (Villa and Combi, 2016). Thus, loss of these channels leads to dramatic increase in neuronal excitability. K2P channelopathies have been linked to familial migraine with aura, and have been attributed to have a role in pain sensation (Serratrice et al., 2010).
Inwardly rectifying potassium (KIR) channels play a key role in the maintenance of the resting potential and regulation of cell excitability (Hibino et al., 2010, Lüscher and Slesinger, 2010). These channels lack the voltage-sensitivity of their Kv counterparts and preferentially limit the outward flow of K+ to a voltage range near the resting membrane potential. KIR channels can have high basal activity (constitutively open), such as KIR2 channels, or low basal activity that can be enhanced by ligands such as G proteins and alcohol, such as KIR3 channels. There are seven different KIR families (KIR1–KIR7) that form either homotetramers or heterotetramers. However, KIR channels typically assemble with only members from the same subfamily, for example, KIR3.1 does not coassemble with KIR2.1 but it does co-assemble with KIR3.2 (Hibino et al., 2010, Lüscher and Slesinger, 2010). The constitutively active KIR2 (KCNJ2) has been associated with the Andersen syndrome, a human disease that is characterized by periodic paralysis, cardiac arrhythmia, and dysmorphism (Plaster et al., 2001). Point mutations in KIR6.2 (KCNJ11) have been described for the human disease developmental delay, epilepsy and neonatal diabetes mellitus (DEND) syndrome, a treatable channelopathy of the brain and pancreas (Hibino et al., 2010, Lüscher and Slesinger, 2010). The G protein-gated inwardly-rectifying K+ (GIRK) channel belongs to the KIR3 subfamily and is a regulator of cardiac and neuronal excitability (Hibino et al., 2010, Lüscher and Slesinger, 2010). There are four GIRK channel subunits in humans, referred to as KIR3.1 (KCNJ3), KIR3.2 (KCNJ6, Girk2), KIR3.3 (KCNJ9), and KIR3.4 (KCNJ5) (for review, see Hibino et al., 2010, Lüscher and Slesinger, 2010). Mice lacking GIRK channels display altered responses to addictive drugs and in the case of the Girk2 knockout, result in epileptic seizures (for review, see Rifkin et al., 2017). Heretofore, human neurological diseases caused by mutations in GIRK channels have been uncommon.
Recently, dominant mutations in KCNJ6 (GIRK2) have been linked to the Keppen–Lubinsky Syndrome (MIM# 614098) (Gorlin and Hennekam, 2001), a disorder characterized by lipodystrophy, severe developmental delay, intellectual disability, hypertonia, hyperreflexia and growth retardation (Gorlin and Hennekam, 2001, Masotti et al., 2015). Three patients were reported to have KCNJ6 mutations near the ion selective pore; one patient with a mutation (p. Gly154Ser) corresponding to the developmentally impaired weaver mouse (Rakic and Sidman, 1973, Rakic and Sidman, 1973, Goldowitz and Mullen, 1982, Hatten et al., 1986, Goldowitz, 1989, Patil et al., 1995, Hess, 1996) and two patients with deletions in the pore (p.Thr152del). These three patients all show lipodystrophy, which ranges from a generalized loss of adipose tissue in one case, to a restricted loss of facial adipose tissue in two cases (Masotti et al., 2015). (De Brasi et al., 2003, Basel-Vanagaite et al., 2009). Thus, lipodystrophy is a hallmark of the Keppen–Lubinsky Syndrome.
Here we report a case study with a de novo KCNJ6 missense mutation [NM_002240: c.512T>G; NP_002231: p. Leu171Arg], without recognizable lipodystrophy, i.e., no obvious absence of facial adipose tissue. Instead, the patient presented with developmental delay, hypotonia, and a severe hyperkinetic movement disorder. Epilepsy was also not present. The p. Leu171Arg mutation was determined to be a gain-of-function, with dramatic changes in G protein activation and ion selectivity. Though, p. Leu171Arg resides in a different region of the protein, the electrophysiological phenotype resembles that described in the murine weaver (wv) mouse, which has extensive developmental defects and also carries a mutation in KCNJ6 (Patil et al., 1995, Hess, 1996).
Section snippets
Experimental procedures
Whole-exome sequencing. Patient and family were enrolled into the TIDEX gene discovery study (UBC IRB approval H12-00067) and provided informed and written consent for data and sample collection, WES, as well as publication of the current case report. WES was performed for the index and her unaffected parents using the Agilent SureSelect kit and Illumina HiSeq 2000 (Perkin-Elmer, Waltham, MA, USA). Data were analyzed using our semi-automated bioinformatics pipeline (Tarailo-Graovac et al., 2016
Clinical findings
The patient is a 4-year old girl from non-consanguineous parents, with an unremarkable pregnancy and term delivery. Her birth weight was 3210 g (30th percentile), and there were no neonatal complications. The first concerns arose when she did not achieve head control by the age of 6 months. Her mother also noticed posturing of her legs, abnormal movements of her arms, and arching of her back. The patient had frequent episodes in which her eyes rolled up. These movements at times were quite
Discussion
Here, we report a gain-of-function mutation in the KCNJ6 (Girk2) gene contributes to a human neurological disease that has many similarities to the Keppen–Lubinsky syndrome. Whole-exome sequencing of the patient and her parents revealed a de novo variant, p.Leu171Arg, located on chromosome 21 in KCNJ6 of the patient. Expression of the GIRK2 mouse homolog with the mutation, GIRK2(L173R), alone or in the presence of the GIRK1 subunit, revealed several significant changes in channel properties.
Video consent
The authors received a signed release form for the patient videotaped, authorizing the offline and/or online distribution of this video material.
Acknowledgments
We gratefully acknowledge the patient and family for their participation in this study; Dr. G. Sinclair and Dr. H. Vallance for interpretation of biochemical data; Ms. E. Aisenberg for creating the mouse L173R mutation in Girk2, Ms. X. Han for Sanger sequencing; Ms. A. Ghani for consenting and data management; Mrs. M. Higginson for DNA extraction, sample handling and technical data; Mr. D. Arenillas and Mr. M. Hatas for systems support, Ms E. Lomba and Mrs. D. Pak for research management
Author contributions
Gabriella Horvath worked on conception and design, acquisition of data and drafted a significant portion of the manuscript. Yulin Zhao collected all of the electrophysiology data, analyzed data and edited the manuscript. Maja Tarailo-Graovac worked on conception and design and edited a significant portion of the manuscript, worked on acquisition and analysis of the data, identifying the KCNJ6 variant. Cyrus Boelman worked on acquisition and analysis of the data and edited a significant portion
Conflict of interest
The authors have no conflict(s) of interest to declare.
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This work was supported by grants from the National Institutes of Health (DA037170 to PAS; AA018734 to PAS); the B.C. Children’s Hospital Foundation (“1st Collaborative Area of Innovation” www.tidebc.org); Genome British Columbia (SOF-195); the Canadian Institutes of Health Research (#301221); NeuroDevNet (Strategic Opportunity Grant); and informatics infrastructure supported by Genome British Columbia and Genome Canada (ABC4DE Project). Dr. van Karnebeek is a recipient of the Michael Smith Foundation for Health Research Scholar Award.
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Denotes equal contribution.