We report a five-month-old male infant of Lebanese descent who presented with infantile spasms, axial hypotonia, and a distinctive facial gestalt. Whole exome sequencing (WES) identified a heterozygous pathogenic variant (c.460G>A; p.Gly154Ser) in the KCNJ6 gene, confirming a diagnosis of Keppen-Lubinsky Syndrome (KPLBS). This case underscores the utility of WES in diagnosing rare dysmorphic syndromes, discusses the therapeutic challenges of channelopathy-related epilepsy, and outlines the guarded prognosis associated with KPLBS.

Keppen-Lubinsky syndrome is caused by pathogenic variants in KCNJ6, which encodes the inwardly rectifying channel subfamily J6. The four confirmed cases reported to date were characterized by severe intellectual disability, global developmental delay, feeding difficulties, and dysmorphic features. All but one of the cases also had a severe form of lipodystrophy, resulting in tightly adherent facial skin and appearance of premature aging. Here, we describe a 36-year-old female with a de novo pathogenic variant in KCNJ6 (NM_002240.5: c.460G>T; p.(Gly154Cys)) presenting with mild intellectual disability, subtle dysmorphic features, obsessive-compulsive disorder, and an exaggerated startle response. This case indicates that KCNJ6-related disorders should be considered in patients with less pronounced dysmorphic features and milder cognitive impairment, as well as in patients with startle disorders.

To investigate the clinical characteristics and genetic basis for a child with Keppen-Lubinsky syndrome (KPLBS). Trio-whole exome sequencing (Trio-WES) was carried out for the proband and her parents. Candidate variant was verified by Sanger sequencing and bioinformatic analysis. The child has featured peculiar facies including large eyes, alar hypoplasia, microretrognathia, premature aging appearance in addition with growth delay and mental retardation. Trio-WES has identified that she has carried a de novo variant of the KCNJ6 gene, namely c.460G>C (p.Gly154Arg). The variant has not been recorded in the database. Prediction of protein structure indicated that the variant may affect the potassium ion selective filtration structure channel in the transmembrane region of KCNJ6 protein, which may result in up regulation of the function of the channel. The de novo c.460G>C (p.Gly154Arg) variant of the KCNJ6 gene probably underlay the KPLBS in this child. Above finding has enriched the genotypic and phenotype spectrum of this syndrome.

Potassium channels are a heterogeneous group of membrane-bound proteins, whose functions support a diverse range of biological processes. Genetic disorders arising from mutations in potassium channels are classically recognized by symptoms arising from acute channel dysfunction, such as periodic paralysis, ataxia, seizures, or cardiac conduction abnormalities, often in a patient with otherwise normal examination findings. In this chapter, we review a distinct subgroup of rare potassium channelopathies whose presentations are instead suggestive of a developmental disorder, with features including intellectual disability, craniofacial dysmorphism or other physical anomalies. Known conditions within this subgroup are: Andersen-Tawil syndrome, Birk-Barel syndrome, Cantú syndrome, Keppen-Lubinsky syndrome, Temple-Baraitser syndrome, Zimmerman-Laband syndrome and a very similar disorder called Bauer-Tartaglia or FHEIG syndrome. Ion channelopathies are unlikely to be routinely considered in the differential diagnosis of children presenting with developmental concerns, and so detailed description and photographs of the clinical phenotype are provided to aid recognition. For several of these disorders, functional characterization of the genetic mutations responsible has led to identification of candidate therapies, including drugs already commonly used for other indications, which adds further impetus to their prompt recognition. Together, these cases illustrate the potential for mechanistic insights gained from genetic diagnosis to drive translational work toward targeted, disease-modifying therapies for rare disorders.

Here, we describe a fourth case of a human with a de novo KCNJ6 (GIRK2) mutation, who presented with clinical findings of severe hyperkinetic movement disorder and developmental delay, similar to the Keppen-Lubinsky syndrome but without lipodystrophy. Whole-exome sequencing of the patient's DNA revealed a heterozygous de novo variant in the KCNJ6 (c.512T>G, p.Leu171Arg). We conducted in vitro functional studies to determine if this Leu-to-Arg mutation alters the function of GIRK2 channels. Heterologous expression of the mutant GIRK2 channel alone produced an aberrant basal inward current that lacked G protein activation, lost K+ selectivity and gained Ca2+ permeability. Notably, the inward current was inhibited by the Na+ channel blocker QX-314, similar to the previously reported weaver mutation in murine GIRK2. Expression of a tandem dimer containing GIRK1 and GIRK2(p.Leu171Arg) did not lead to any currents, suggesting heterotetramers are not functional. In neurons expressing p.Leu171Arg GIRK2 channels, these changes in channel properties would be expected to generate a sustained depolarization, instead of the normal G protein-gated inhibitory response, which could be mitigated by expression of other GIRK subunits. The identification of the p.Leu171Arg GIRK2 mutation potentially expands the Keppen-Lubinsky syndrome phenotype to include severe dystonia and ballismus. Our study suggests screening for dominant KCNJ6 mutations in the evaluation of patients with severe movement disorders, which could provide evidence to support a causal role of KCNJ6 in neurological channelopathies.