Myoclonus epilepsy and ataxia due to KCNC1 mutation: Analysis of 20 cases and K+ channel properties

Potassium channel-related epilepsy: Pathogenesis and clinical features

Variants in potassium channel-related genes are one of the most important mechanisms underlying abnormal neuronal excitation and disturbances in the cellular resting membrane potential. These variants can cause different forms of epilepsy, which can seriously affect the physical and mental health of patients, especially those with refractory epilepsy or status epilepticus, which are common among pediatric patients and are potentially life-threatening. Variants in potassium ion channel-related genes have been reported in few studies; however, to our knowledge, no systematic review has been published. This study aimed to summarize the epilepsy phenotypes, functional studies, and pharmacological advances associated with different potassium channel gene variants to assist clinical practitioners and drug development teams to develop evidence-based medicine and guide research strategies. PubMed and Google Scholar were searched for relevant literature on potassium channel-related epilepsy reported in the past 5-10 years. Various common potassium ion channel gene variants can lead to heterogeneous epilepsy phenotypes, and functional effects can result from gene deletions and compound effects. Administration of select anti-seizure medications is the primary treatment for this type of epilepsy. Most patients are refractory to anti-seizure medications, and some novel anti-seizure medications have been found to improve seizures. Use of targeted drugs to correct aberrant channel function based on the type of potassium channel gene variant can be used as an evidence-based pathway to achieve precise and individualized treatment for children with epilepsy. PLAIN LANGUAGE SUMMARY: In this article, the pathogenesis and clinical characteristics of epilepsy caused by different types of potassium channel gene variants are reviewed in the light of the latest research literature at home and abroad, with the expectation of providing a certain theoretical basis for the diagnosis and treatment of children with this type of disease.

Myoclonus improvement after seizures in progressive myoclonic epilepsy type 7: a case report

BACKGROUND

Progressive Myoclonic Epilepsy (PME) is a group of rare diseases that are difficult to differentiate from one another based on phenotypical characteristics.

CASE REPORT

We report a case of PME type 7 due to a pathogenic variant in KCNC1 with myoclonus improvement after epileptic seizures.

DISCUSSION

Myoclonus improvement after seizures may be a clue to the diagnosis of Progressive Myoclonic Epilepsy type 7.

Potential Benefit of Channel Activators in Loss-of-Function Primary Potassium Channelopathies Causing Heredoataxia

Potassium channels (KCN) are transmembrane complexes that regulate the resting membrane potential and the duration of action potentials in cells. The opening of KCN brings about an efflux of K+ ions that induces cell repolarization after depolarization, returns the transmembrane potential to its resting state, and enables for continuous spiking ability. The aim of this work was to assess the role of KCN dysfunction in the pathogenesis of hereditary ataxias and the mechanisms of action of KCN opening agents (KCO). In consequence, a review of the ad hoc medical literature was performed. Among hereditary KCN diseases causing ataxia, mutated Kv3.3, Kv4.3, and Kv1.1 channels provoke spinocerebellar ataxia (SCA) type 13, SCA19/22, and episodic ataxia type 1 (EA1), respectively. The K+ efflux was found to be reduced in experimental models of these diseases, resulting in abnormally prolonged depolarization and incomplete repolarization, thereby interfering with repetitive discharges in the cells. Hence, substances able to promote normal spiking activity in the cerebellum could provide symptomatic benefit. Although drugs used in clinical practice do not activate Kv3.3 or Kv4.3 directly, available KCO probably could ameliorate ataxic symptoms in SCA13 and SCA19/22, as verified with acetazolamide in EA1, and retigabine in a mouse model of hypokalemic periodic paralysis. To summarize, ataxia could possibly be improved by non-specific KCO in SCA13 and SCA19/22. The identification of new specific KCO agents will undoubtedly constitute a promising therapeutic strategy for these diseases.

Targeted therapy improves cellular dysfunction, ataxia, and seizure susceptibility in a model of a progressive myoclonus epilepsy

The recurrent variant KCNC1-p.Arg320His causes progressive myoclonus epilepsy (EPM) type 7, defined by progressive myoclonus, epilepsy, and ataxia, and is without effective treatment. KCNC1 encodes the voltage-gated potassium channel subunit Kv3.1, specifically expressed in high-frequency-firing neurons. Variant subunits act via loss of function; hence, EPM7 pathogenesis may involve impaired excitability of Kv3.1-expressing neurons, while enhancing Kv3 activity could represent a viable therapeutic strategy. We generate a mouse model, Kcnc1-p.Arg320His/+, which recapitulates the core features of EPM7, including progressive ataxia and seizure susceptibility. Kv3.1-expressing cerebellar granule cells and neocortical parvalbumin-positive GABAergic interneurons exhibit abnormalities consistent with Kv3 channel dysfunction. A Kv3-specific positive modulator (AUT00206) selectively enhances the firing frequency of Kv3.1-expressing neurons and improves motor function and seizure susceptibility in Kcnc1-Arg320His/+ mice. This work identifies a cellular and circuit basis of dysfunction in EPM7 and demonstrates that Kv3 positive modulators such as AUT00206 have therapeutic potential for the treatment of EPM7.

Opening the KV3.1 gates: A therapeutic strategy for progressive myoclonus epilepsy type 7?

Progressive myoclonus epilepsy type 7, a debilitating neurological disorder, is caused by a loss-of-function mutation in the KV3.1 channel. Exciting work by Feng et al.1 utilizes a new knockin mouse model to identify a potential therapeutic intervention.

A structurally precise mechanism links an epilepsy-associated KCNC2 potassium channel mutation to interneuron dysfunction

De novo heterozygous variants in KCNC2 encoding the voltage-gated potassium (K+) channel subunit Kv3.2 are a recently described cause of developmental and epileptic encephalopathy (DEE). A de novo variant in KCNC2 c.374G > A (p.Cys125Tyr) was identified via exome sequencing in a patient with DEE. Relative to wild-type Kv3.2, Kv3.2-p.Cys125Tyr induces K+ currents exhibiting a large hyperpolarizing shift in the voltage dependence of activation, accelerated activation, and delayed deactivation consistent with a relative stabilization of the open conformation, along with increased current density. Leveraging the cryogenic electron microscopy (cryo-EM) structure of Kv3.1, molecular dynamic simulations suggest that a strong π-π stacking interaction between the variant Tyr125 and Tyr156 in the α-6 helix of the T1 domain promotes a relative stabilization of the open conformation of the channel, which underlies the observed gain of function. A multicompartment computational model of a Kv3-expressing parvalbumin-positive cerebral cortex fast-spiking γ-aminobutyric acidergic (GABAergic) interneuron (PV-IN) demonstrates how the Kv3.2-Cys125Tyr variant impairs neuronal excitability and dysregulates inhibition in cerebral cortex circuits to explain the resulting epilepsy.

Role of Potassium Ion Channels in Epilepsy: Focus on Current Therapeutic Strategies

BACKGROUND

Epilepsy is one of the prevalent neurological disorders characterized by disrupted synchronization between inhibitory and excitatory neurons. Disturbed membrane potential due to abnormal regulation of neurotransmitters and ion transport across the neural cell membrane significantly contributes to the pathophysiology of epilepsy. Potassium ion channels (KCN) regulate the resting membrane potential and are involved in neuronal excitability. Genetic alterations in the potassium ion channels (KCN) have been reported to result in the enhancement of the release of neurotransmitters, the excitability of neurons, and abnormal rapid firing rate, which lead to epileptic phenotypes, making these ion channels a potential therapeutic target for epilepsy. The aim of this study is to explore the variations reported in different classes of potassium ion channels (KCN) in epilepsy patients, their functional evaluation, and therapeutic strategies to treat epilepsy targeting KCN.

METHODOLOGY

A review of all the relevant literature was carried out to compile this article.

RESULTS

A large number of variations have been reported in different genes encoding various classes of KCN. These genetic alterations in KCN have been shown to be responsible for disrupted firing properties of neurons. Antiepileptic drugs (AEDs) are the main therapeutic strategy to treat epilepsy. Some patients do not respond favorably to the AEDs treatment, resulting in pharmacoresistant epilepsy.

CONCLUSION

Further to address the challenges faced in treating epilepsy, recent approaches like optogenetics, chemogenetics, and genome editing, such as clustered regularly interspaced short palindromic repeats (CRISPR), are emerging as target-specific therapeutic strategies.

Identification, structural, and biophysical characterization of a positive modulator of human Kv3.1 channels

Voltage-gated potassium channels (Kv) are tetrameric membrane proteins that provide a highly selective pathway for potassium ions (K+) to diffuse across a hydrophobic cell membrane. These unique voltage-gated cation channels detect changes in membrane potential and, upon activation, help to return the depolarized cell to a resting state during the repolarization stage of each action potential. The Kv3 family of potassium channels is characterized by a high activation potential and rapid kinetics, which play a crucial role for the fast-spiking neuronal phenotype. Mutations in the Kv3.1 channel have been shown to have implications in various neurological diseases like epilepsy and Alzheimer's disease. Moreover, disruptions in neuronal circuitry involving Kv3.1 have been correlated with negative symptoms of schizophrenia. Here, we report the discovery of a novel positive modulator of Kv3.1, investigate its biophysical properties, and determine the cryo-EM structure of the compound in complex with Kv3.1. Structural analysis reveals the molecular determinants of positive modulation in Kv3.1 channels by this class of compounds and provides additional opportunities for rational drug design for the treatment of associated neurological disorders.

ILAE Genetics Literacy series: Progressive myoclonus epilepsies

Progressive Myoclonus Epilepsy (PME) is a rare epilepsy syndrome characterized by the development of progressively worsening myoclonus, ataxia, and seizures. A molecular diagnosis can now be established in approximately 80% of individuals with PME. Almost fifty genetic causes of PME have now been established, although some remain extremely rare. Herein, we provide a review of clinical phenotypes and genotypes of the more commonly encountered PMEs. Using an illustrative case example, we describe appropriate clinical investigation and therapeutic strategies to guide the management of this often relentlessly progressive and devastating epilepsy syndrome. This manuscript in the Genetic Literacy series maps to Learning Objective 1.2 of the ILAE Curriculum for Epileptology (Epileptic Disord. 2019;21:129).

A cytoskeleton-membrane interaction conserved in fast-spiking neurons controls movement, emotion, and memory

The pathogenesis of schizophrenia is believed to involve combined dysfunctions of many proteins including microtubule-associated protein 6 (MAP6) and Kv3.1 voltage-gated K+ (Kv) channel, but their relationship and functions in behavioral regulation are often not known. Here we report that MAP6 stabilizes Kv3.1 channels in parvalbumin-positive (PV+ ) fast-spiking GABAergic interneurons, regulating behavior. MAP6-/- and Kv3.1-/- mice display similar hyperactivity and avoidance reduction. Their proteins colocalize in PV+ interneurons and MAP6 deletion markedly reduces Kv3.1 protein level. We further show that two microtubule-binding modules of MAP6 bind the Kv3.1 tetramerization domain with high affinity, maintaining the channel level in both neuronal soma and axons. MAP6 knockdown by AAV-shRNA in the amygdala or the hippocampus reduces avoidance or causes hyperactivity and recognition memory deficit, respectively, through elevating projection neuron activity. Finally, knocking down Kv3.1 or disrupting the MAP6-Kv3.1 binding in these brain regions causes avoidance reduction and hyperactivity, consistent with the effects of MAP6 knockdown. Thus, disrupting this conserved cytoskeleton-membrane interaction in fast-spiking neurons causes different degrees of functional vulnerability in various neural circuits.

Potassium channels in behavioral brain disorders. Molecular mechanisms and therapeutic potential: A narrative review

Potassium channels (K+-channels) selectively control the passive flow of potassium ions across biological membranes and thereby also regulate membrane excitability. Genetic variants affecting many of the human K+-channels are well known causes of Mendelian disorders within cardiology, neurology, and endocrinology. K+-channels are also primary targets of many natural toxins from poisonous organisms and drugs used within cardiology and metabolism. As genetic tools are improving and larger clinical samples are being investigated, the spectrum of clinical phenotypes implicated in K+-channels dysfunction is rapidly expanding, notably within immunology, neurosciences, and metabolism. K+-channels that previously were considered to be expressed in only a few organs and to have discrete physiological functions, have recently been found in multiple tissues and with new, unexpected functions. The pleiotropic functions and patterns of expression of K+-channels may provide additional therapeutic opportunities, along with new emerging challenges from off-target effects. Here we review the functions and therapeutic potential of K+-channels, with an emphasis on the nervous system, roles in neuropsychiatric disorders and their involvement in other organ systems and diseases.

Differential diagnosis of familial adult myoclonic epilepsy

OBJECTIVE

Familial adult myoclonic epilepsy (FAME) is an under-recognized disorder characterized by cortical myoclonus, generalized tonic-clonic seizures, and additional clinical symptoms, which vary depending on the FAME subtype. FAME is caused by pentanucleotide repeat expansions of intronic TTTCA/TTTTA in different genes. FAME should be distinguished from a range of differential diagnoses.

METHODS

The differential diagnoses and frequent presentations leading to misdiagnosis of FAME were investigated from the available literature and reported based on an expert opinion survey.

RESULTS

The phenotypic features of FAME, including generalized tonic-clonic and myoclonic seizures, are also seen in other epilepsy syndromes, such as juvenile myoclonic epilepsy, with a resultant risk of misdiagnosis and lack of identification of the underlying cause. Cortical myoclonus may mimic essential tremor or drug-induced tremor. In younger individuals, the differential diagnosis includes progressive myoclonus epilepsies (PMEs), such as Unverricht-Lundborg disease, whereas, in adulthood, late-onset variants of PMEs, such as sialidoses, myoclonus epilepsy, and ataxia due to potassium channel pathogenic variants should be considered. PMEs may also be suggested by cognitive impairment, cerebellar signs, or psychiatric disorders. Electroencephalography (EEG) may show similarities to other idiopathic generalized epilepsies or PMEs, with generalized spike-wave activity. Signs of cortical hyperexcitability may be seen, such as an increased amplitude of somatosensory evoked potentials or enhanced cortical reflex to sensory stimuli, together with the neurophysiological pattern of the movement disorder.

SIGNIFICANCE

Recognition of FAME will inform prognostic and genetic counseling and diagnosis of the insidious progression, which may occur in older individuals who show mild cognitive deterioration. Distinguishing FAME from other disorders in individuals or families with this constellation of symptoms is essential to allow the identification of underlying etiology.

A KCNC1-related neurological disorder due to gain of Kv3.1 function

OBJECTIVE

To further clarify genotype:phenotype correlations associated with variants in KCNC1 encoding the voltage-gated potassium (K+) channel subunit Kv3.1 and which are an emerging cause of a spectrum of neurological disease including intellectual disability, isolated myoclonus, progressive myoclonus epilepsy, and developmental and epileptic encephalopathy.

METHODS

We describe the clinical and genetic characteristics of a series of three patients with de novo heterozygous missense variants in KCNC1 associated with nonspecific developmental delay/intellectual disability and central hypotonia without epilepsy or ataxia. All three variants lead to amino acids alterations with mild predicted differences in physicochemical properties yet are localized to the S6 pore region of the Kv3.1 protein between the selectivity filter and PXP motif important for K+ channel gating. We performed whole-cell voltage clamp electrophysiological recording of wild-type versus variants in a heterologous mammalian expression system.

RESULTS

We demonstrate a prominent leftward (hyperpolarized) shift in the voltage dependence of activation and slowed deactivation of all variants in the clinically defined series.

INTERPRETATION

Electrophysiological recordings are consistent with a gain of K+ channel function that is predicted to exert a loss of function on the excitability of Kv3-expressing high frequency- firing neurons based on the unique electrophysiological properties of Kv3 channels. These results define a clinical-genetic syndrome within the spectrum of KCNC1-related neurological disorders.

Potassium channels and epilepsy

With the development and application of next-generation sequencing technology, the aetiological diagnosis of genetic epilepsy is rapidly becoming easier and less expensive. Additionally, there is a growing body of research into precision therapy based on genetic diagnosis. The numerous genes in the potassium ion channel family constitute the largest family of ion channels: this family is divided into different subtypes. Potassium ion channels play a crucial role in the electrical activity of neurons and are directly involved in the mechanism of epileptic seizures. In China, scientific research on genetic diagnosis and studies of precision therapy for genetic epilepsy are progressing rapidly. Many cases of epilepsy caused by mutation of potassium channel genes have been identified, and several potassium channel gene targets and drug candidates have been discovered. The purpose of this review is to briefly summarize the progress of research on the precise diagnosis and treatment of potassium ion channel-related genetic epilepsy, especially the research conducted in China. Here in, we review several large cohort studies on the genetic diagnosis of epilepsy in China in recent years, summarized the proportion of potassium channel genes. We focus on the progress of precison therapy on some hot epilepsy related potassium channel genes: KCNA1, KCNA2, KCNB1, KCNC1, KCND2, KCNQ2, KCNQ3, KCNMA1, and KCNT1.

Cryo-EM structure of the human Kv3.1 channel reveals gating control by the cytoplasmic T1 domain

Kv3 channels have distinctive gating kinetics tailored for rapid repolarization in fast-spiking neurons. Malfunction of this process due to genetic variants in the KCNC1 gene causes severe epileptic disorders, yet the structural determinants for the unusual gating properties remain elusive. Here, we present cryo-electron microscopy structures of the human Kv3.1a channel, revealing a unique arrangement of the cytoplasmic tetramerization domain T1 which facilitates interactions with C-terminal axonal targeting motif and key components of the gating machinery. Additional interactions between S1/S2 linker and turret domain strengthen the interface between voltage sensor and pore domain. Supported by molecular dynamics simulations, electrophysiological and mutational analyses, we identify several residues in the S4/S5 linker which influence the gating kinetics and an electrostatic interaction between acidic residues in α6 of T1 and R449 in the pore-flanking S6T helices. These findings provide insights into gating control and disease mechanisms and may guide strategies for the design of pharmaceutical drugs targeting Kv3 channels.

Defining causal variants in rare epilepsies: an essential team effort between biomedical scientists, geneticists and epileptologists

In the last few years, with the advent of next generation sequencing (NGS), our knowledge of genes associated with monogenic epilepsies has significantly improved. NGS is also a powerful diagnostic tool for patients with epilepsy, through gene panels, exomes and genomes. This has improved diagnostic yield, reducing the time between the first seizure and a definitive molecular diagnosis. However, these developments have also increased the complexity of data interpretation, due to the large number of variants identified in a given patient and due to the phenotypic variability associated with many of the epilepsy-related genes. In this paper, we present examples of variant classification in "real life" clinic situations. We emphasize the importance of accurate phenotyping of the epilepsies including recognising variable/milder phenotypes and expansion of previously described phenotypes. There are some important issues specific to rare epilepsies - mosaicism and reduced penetrance - which affect genetic counselling. These challenges may be overcome through multidisciplinary meetings including epileptologists, pediatric neurologists, and clinical and molecular geneticists, in which every specialist learns from the others in a process which leads to for rapid and accurate diagnosis. This is an important milestone to achieve as targeted therapiesbased on the functional effects of pathogenic variants become available.

Apo and ligand-bound high resolution Cryo-EM structures of the human Kv3.1 channel reveal a novel binding site for positive modulators

Kv3 ion-channels constitute a class of functionally distinct voltage-gated ion channels characterized by their ability to fire at a high frequency. Several disease relevant mutants, together with biological data, suggest the importance of this class of ion channels as drug targets for CNS disorders, and several drug discovery efforts have been reported. Despite the increasing interest for this class of ion channels, no structure of a Kv3 channel has been reported yet. We have determined the cryo-EM structure of Kv3.1 at 2.6 Å resolution using full-length wild type protein. When compared to known structures for potassium channels from other classes, a novel domain organization is observed with the cytoplasmic T1 domain, containing a well-resolved Zinc site and displaying a rotation by 35°. This suggests a distinct cytoplasmic regulation mechanism for the Kv3.1 channel. A high resolution structure was obtained for Kv3.1 in complex with a novel positive modulator Lu AG00563. The structure reveals a novel ligand binding site for the Kv class of ion channels located between the voltage sensory domain and the channel pore, a region which constitutes a hotspot for disease causing mutations. The discovery of a novel binding site for a positive modulator of a voltage-gated potassium channel could shed light on the mechanism of action for these small molecule potentiators. This finding could enable structure-based drug design on these targets with high therapeutic potential for the treatment of multiple CNS disorders.

Epilepsy Combined With Multiple Gene Heterozygous Mutation

The fast pace of gene discovery has resulted in groundbreaking advances in the field of epilepsy genetics. Clinical testing using comprehensive gene panels, exomes, or genomes is now increasingly available and has significantly increased the diagnostic yield for early-onset epilepsies and enabled precision medicine approaches. In this paper, we report a case of epilepsy in a pedigree. The proband had heterozygous mutations in KCNC1 (NM_001112741.1:c.959G>A, p. Arg320His), CAPN3 (NM_000070.2:c.526G>A, p. Val176Met), and NEFH (NM_021076.3:c. 2595 delC, p. Lys866Argfs^*51). Sanger sequencing verification was consistent with the results of whole-exome sequencing. The KCNC1 mutation was a de novo mutation, and the CAPN3 and NEFH mutations were inherited from their father and mother, respectively. Based on the American College of Medical Genetics and Genomics (ACMG) guidelines, a heterozygous mutation was found for APOB (NM_000384.2: c.10579C > T, p. Arg3527Trp). The heterozygous mutation at this site was inherent in the pedigree. Coexpression analysis indicated that heterozygous mutations of KCNC1, CAPN3, NEFH, and APOB were closely related to the clinical phenotypes of the patient, and the clinical phenotypic heterogeneity of the disease may be the result of the interaction of multiple genes.

Progressive Myoclonus Epilepsies

Background and Objectives

To assess the current diagnostic yield of genetic testing for the progressive myoclonus epilepsies (PMEs) of an Italian series described in 2014 where Unverricht-Lundborg and Lafora diseases accounted for ∼50% of the cohort.

Methods

Of 47/165 unrelated patients with PME of indeterminate genetic origin, 38 underwent new molecular evaluations. Various next-generation sequencing (NGS) techniques were applied including gene panel analysis (n = 7) and/or whole-exome sequencing (WES) (WES singleton n = 29, WES trio n = 7, and WES sibling n = 4). In 1 family, homozygosity mapping was followed by targeted NGS. Clinically, the patients were grouped in 4 phenotypic categories: “Unverricht-Lundborg disease-like PME,” “late-onset PME,” “PME plus developmental delay,” and “PME plus dementia.”

Results

Sixteen of 38 (42%) unrelated patients reached a positive diagnosis, increasing the overall proportion of solved families in the total series from 72% to 82%. Likely pathogenic variants were identified in NEU1 (2 families), CERS1 (1 family), and in 13 nonfamilial patients in KCNC1 (3), DHDDS (3), SACS, CACNA2D2, STUB1, AFG3L2, CLN6, NAXE, and CHD2. Across the different phenotypic categories, the diagnostic rate was similar, and the same gene could be found in different phenotypic categories.

Discussion

The application of NGS technology to unsolved patients with PME has revealed a collection of very rare genetic causes. Pathogenic variants were detected in both established PME genes and in genes not previously associated with PME, but with progressive ataxia or with developmental encephalopathies. With a diagnostic yield >80%, PME is one of the best genetically defined epilepsy syndromes.

Kv3.1 channelopathy: a novel loss-of-function variant and the mechanistic basis of its clinical phenotypes

Background

KCNC1 encodes Kv3.1, a subunit of the Kv3 voltage-gated potassium channels. It is predominantly expressed in inhibitory GABAergic interneurons and cerebellar neurons. Kv3.1 channelopathy has been linked to a variety of human diseases including epilepsy, developmental delay, and ataxia. Characterization of structural and functional disturbances of this channel, and its relationship to a heterogenous group of clinical phenotypes, is a current topic of research. We herein characterize the clinical phenotype as well as the functional and structural consequences of the novel KCNC1 p.R317S variant. We further set out to explore the mechanistic basis for the spectrum of KCNC1 related channelopathies.

Methods

Variant was identified via whole-exome sequencing and its functional impact was determined using two-electrode voltage clamp recordings in Xenopus laevis oocytes. Homolog modeling and in silico structural analysis were performed on the p.R317S variant and other KCNC1 related variants.

Results

We identified a novel loss-of-function KCNC1 variant c.949C>A (p.R317S) presenting with symptoms similar to myoclonic epilepsy and ataxia due to potassium channel (MEAK), but with distinct radiological features. Functional analysis in the Xenopus laevis oocyte’s expression system revealed that the current amplitudes were significantly decreased in the p.R317S variant compared to the wild type, indicating a dominant-negative effect. Atomic structural analysis of the KCNC1 related variants provided a possible mechanistic explanation for the heterogeneity in the clinical spectrum.

Conclusions

We have identified the p.R317S loss-of-function variant in the KCNC1 gene, expanded the spectrum of potassium channelopathy and provided mechanistic insights into KCNC1 related disorders.

Gene Editing and Modulation: the Holy Grail for the Genetic Epilepsies?

Epilepsy is a complex neurological disorder for which there are a large number of monogenic subtypes. Monogenic epilepsies are often severe and disabling, featuring drug-resistant seizures and significant developmental comorbidities. These disorders are potentially amenable to a precision medicine approach, of which genome editing using CRISPR/Cas represents the holy grail. Here we consider mutations in some of the most 'common' rare epilepsy genes and discuss the different CRISPR/Cas approaches that could be taken to cure these disorders. We consider scenarios where CRISPR-mediated gene modulation could serve as an effective therapeutic strategy and discuss whether a single gene corrective approach could hold therapeutic potential in the context of homeostatic compensation in the developing, highly dynamic brain. Despite an incomplete understanding of the mechanisms of the genetic epilepsies and current limitations of gene editing tools, CRISPR-mediated approaches have game-changing potential in the treatment of genetic epilepsy over the next decade.

Progressive myoclonus epilepsy KCNC1 variant causes a developmental dendritopathy

OBJECTIVE

Mutations in KCNC1 can cause severe neurological dysfunction, including intellectual disability, epilepsy, and ataxia. The Arg320His variant, which occurs in the voltage-sensing domain of the channel, causes a highly penetrant and specific form of progressive myoclonus epilepsy with severe ataxia, designated myoclonus epilepsy and ataxia due to potassium channel mutation (MEAK). KCNC1 encodes the voltage-gated potassium channel KV 3.1, a channel that is important for enabling high-frequency firing in interneurons, raising the possibility that MEAK is associated with reduced interneuronal function.

METHODS

To determine how this variant triggers MEAK, we expressed KV 3.1bR320H in cortical interneurons in vitro and investigated the effects on neuronal function and morphology. We also performed electrophysiological recordings of oocytes expressing KV 3.1b to determine whether the mutation introduces gating pore currents.

RESULTS

Expression of the KV 3.1bR320H variant profoundly reduced excitability of mature cortical interneurons, and cells expressing these channels were unable to support high-frequency firing. The mutant channel also had an unexpected effect on morphology, severely impairing neurite development and interneuron viability, an effect that could not be rescued by blocking KV 3 channels. Oocyte recordings confirmed that in the adult KV 3.1b isoform, R320H confers a dominant negative loss-of-function effect by slowing channel activation, but does not introduce potentially toxic gating pore currents.

SIGNIFICANCE

Overall, our data suggest that, in addition to the regulation of high-frequency firing, KV 3.1 channels play a hitherto unrecognized role in neuronal development. MEAK may be described as a developmental dendritopathy.

Potassium channels as prominent targets and tools for the treatment of epilepsy

INTRODUCTION

K⁺ channels are of great interest to epilepsy research as mutations in their genes are found in humans with inherited epilepsy. At the level of cellular physiology, K+ channels control neuronal intrinsic excitability and are the main contributors to membrane repolarization of active neurons. Recently, a genetically modified voltage-dependent K⁺ channel has been patented as a remedy for epileptic seizures.

AREAS COVERED

We review the role of potassium channels in excitability, clinical and experimental evidence for the association of potassium channelopathies with epilepsy, the targeting of K+ channels by drugs, and perspectives of gene therapy in epilepsy with the expression of extra K+ channels in the brain.

EXPERT OPINION

Control over K+ conductance is of great potential benefit for the treatment of epilepsy. Nowadays, gene therapy affecting K+ channels is one of the most promising approaches to treat pharmacoresistant focal epilepsy.

Progressive myoclonus epilepsies-Residual unsolved cases have marked genetic heterogeneity including dolichol-dependent protein glycosylation pathway genes

Progressive myoclonus epilepsies (PMEs) comprise a group of clinically and genetically heterogeneous rare diseases. Over 70% of PME cases can now be molecularly solved. Known PME genes encode a variety of proteins, many involved in lysosomal and endosomal function. We performed whole-exome sequencing (WES) in 84 (78 unrelated) unsolved PME-affected individuals, with or without additional family members, to discover novel causes. We identified likely disease-causing variants in 24 out of 78 (31%) unrelated individuals, despite previous genetic analyses. The diagnostic yield was significantly higher for individuals studied as trios or families (14/28) versus singletons (10/50) (OR = 3.9, p value = 0.01, Fisher's exact test). The 24 likely solved cases of PME involved 18 genes. First, we found and functionally validated five heterozygous variants in NUS1 and DHDDS and a homozygous variant in ALG10, with no previous disease associations. All three genes are involved in dolichol-dependent protein glycosylation, a pathway not previously implicated in PME. Second, we independently validate SEMA6B as a dominant PME gene in two unrelated individuals. Third, in five families, we identified variants in established PME genes; three with intronic or copy-number changes (CLN6, GBA, NEU1) and two very rare causes (ASAH1, CERS1). Fourth, we found a group of genes usually associated with developmental and epileptic encephalopathies, but here, remarkably, presenting as PME, with or without prior developmental delay. Our systematic analysis of these cases suggests that the small residuum of unsolved cases will most likely be a collection of very rare, genetically heterogeneous etiologies.

Cortico-muscular and cortico-cortical coherence changes resulting from Perampanel treatment in patients with cortical myoclonus

OBJECTIVE

To investigate the mechanisms by which Perampanel (PER) reduces the severity of action myoclonus, we studied on MEG signals the changes occurring in cortico-muscular coherence (CMC) and cortico-cortical connectivity in patients with progressive myoclonus epilepsies.

METHODS

The subjects performed an isometric extension of the hand; CMC and cortico-cortical connectivity were assessed using autoregressive models and generalized partial-directed coherence. The contralateral (Co) sensors showing average CMC values >0.7 of the maximum (set to 1) were grouped as central (C) regions of interest (ROI), while adjacent sensors showing CMC values >0.3 were grouped as Surrounding (Sr) ROIs.

RESULTS

Under PER treatment, CMC decreased on Co C and Sr ROIs, but also on homologous ipsilateral (Ip) ROIs; out-degrees and betweenness centrality increased in Co ROIs and decreased in Ip ROIs. The flow from Ip to Co ROIs and from activated muscles to Ip C ROI decreased.

CONCLUSION

The improvement of myoclonus corresponded to decreased CMC and recovered leadership of the cortical regions directly involved in the motor task, with a reduced interference of ipsilateral areas.

SIGNIFICANCE

Our study highlights on mechanisms suitable to treating myoclonus and suggests the role of a reduced local synchronization together a better control of distant synaptic effects.

A survey of the European Reference Network EpiCARE on clinical practice for selected rare epilepsies

Objective

Clinical care of rare and complex epilepsies is challenging, because evidence-based treatment guidelines are scarce, the experience of many physicians is limited, and interdisciplinary treatment of comorbidities is required. The pathomechanisms of rare epilepsies are, however, increasingly understood, which potentially fosters novel targeted therapies. The objectives of our survey were to obtain an overview of the clinical practice in European tertiary epilepsy centers treating patients with 5 arbitrarily selected rare epilepsies and to get an estimate of potentially available patients for future studies.

Methods

Members of the European Reference Network for rare and complex epilepsies (EpiCARE) were invited to participate in a web-based survey on clinical practice of patients with Dravet syndrome, tuberous sclerosis complex (TSC), autoimmune encephalitis, and progressive myoclonic epilepsies including Unverricht Lundborg and Unverricht-like diseases. A consensus-based questionnaire was generated for each disease.

Results

Twenty-six of 30 invited epilepsy centers participated. Cohorts were present in most responding centers for TSC (87%), Dravet syndrome (85%), and autoimmune encephalitis (71%). Patients with TSC and Dravet syndrome represented the largest cohorts in these centers. The antiseizure drug treatments were rather consistent across the centers especially with regard to Dravet syndrome, infantile spasms in TSC, and Unverricht Lundborg / Unverricht-like disease. Available, widely used targeted therapies included everolimus in TSC and immunosuppressive therapies in autoimmune encephalitis. Screening for comorbidities was routinely done, but specific treatment protocols were lacking in most centers.

Significance

The survey summarizes the current clinical practice for selected rare epilepsies in tertiary European epilepsy centers and demonstrates consistency as well as heterogeneity in the treatment, underscoring the need for controlled trials and recommendations. The survey also provides estimates for potential participants of clinical trials recruited via EpiCARE, emphasizing the great potential of Reference Networks for future studies to evaluate new targeted therapies and to identify novel biomarkers.

Computer modeling of whole-cell voltage-clamp analyses to delineate guidelines for good practice of manual and automated patch-clamp

The patch-clamp technique and more recently the high throughput patch-clamp technique have contributed to major advances in the characterization of ion channels. However, the whole-cell voltage-clamp technique presents certain limits that need to be considered for robust data generation. One major caveat is that increasing current amplitude profoundly impacts the accuracy of the biophysical analyses of macroscopic ion currents under study. Using mathematical kinetic models of a cardiac voltage-gated sodium channel and a cardiac voltage-gated potassium channel, we demonstrated how large current amplitude and series resistance artefacts induce an undetected alteration in the actual membrane potential and affect the characterization of voltage-dependent activation and inactivation processes. We also computed how dose-response curves are hindered by high current amplitudes. This is of high interest since stable cell lines frequently demonstrating high current amplitudes are used for safety pharmacology using the high throughput patch-clamp technique. It is therefore critical to set experimental limits for current amplitude recordings to prevent inaccuracy in the characterization of channel properties or drug activity, such limits being different from one channel type to another. Based on the predictions generated by the kinetic models, we draw simple guidelines for good practice of whole-cell voltage-clamp recordings.

Utilising Automated Electrophysiological Platforms in Epilepsy Research

Genetic mutations have long been implicated in epilepsy, particularly in genes that encode ion channels and neurotransmitter receptors. Among some of those identified are voltage-gated sodium, potassium and calcium channels, and ligand-gated gamma-aminobutyric acid (GABA), neuronal nicotinic acetylcholine (CHRN), and glutamate receptors, making them key therapeutic targets. In this chapter we discuss the use of automated electrophysiological technologies to examine the impact of gene defects in two potassium channels associated with different epilepsy syndromes. The hKCNC1 gene encodes the voltage-gated potassium channel hK_V3.1, and mutations in this gene cause progressive myoclonus epilepsy (PME) and ataxia due to a potassium channel mutation (MEAK). The hKCNT1 gene encodes the weakly voltage-dependent sodium-activated potassium channel hKCNT1, and mutations in this gene cause a wide spectrum of seizure disorders, including severe autosomal dominant sleep-related hypermotor epilepsy (ADSHE) and epilepsy of infancy with migrating focal seizures (EIMFS), both conditions associated with drug-resistance. Importantly, both of these potassium channels play vital roles in regulating neuronal excitability. Since its discovery in the late nineteen seventies, the patch-clamp technique has been regarded as the bench-mark technology for exploring ion channel characteristics. In more recent times, innovations in automated patch-clamp technologies, of which there are many, are enabling the study of ion channels with much greater productivity that manual systems are capable of. Here we describe aspects of Nanion NPC-16 Patchliner, examining the effects of temperature on stably and transiently transfected mammalian cells, the latter of which for most automated systems on the market is quite challenging. Remarkable breakthroughs in the development of other automated electrophysiological technologies, such as multielectrode arrays that support extracellular signal recordings, provide additional features to examine network activity in the area of ion channel research, particularly epilepsy. Both of these automated technologies enable the acquisition of consistent, robust, and reproducible data. Numerous systems have been developed with very similar capabilities, however, not all the systems on the market are adapted to work with primary cells, particularly neurons that can be problematic. This chapter also showcases methods that demonstrate the versatility of Nanion NPC-16 Patchliner and the Multi Channel Systems (MCS) multielectrode array (MEA) assay for acutely dissociated murine primary cortical neurons, enabling the study of potassium channel mutations implicated in severe refractory epilepsies.

Myoclonus-Ataxia Syndromes: A Diagnostic Approach

Background

A myriad of disorders combine myoclonus and ataxia. Most causes are genetic and an increasing number of genes are being associated with myoclonus-ataxia syndromes (MAS), due to recent advances in genetic techniques. A proper etiologic diagnosis of MAS is clinically relevant, given the consequences for genetic counseling, treatment, and prognosis.

Objectives

To review the causes of MAS and to propose a diagnostic algorithm.

Methods

A comprehensive and structured literature search following PRISMA criteria was conducted to identify those disorders that may combine myoclonus with ataxia.

Results

A total of 135 causes of combined myoclonus and ataxia were identified, of which 30 were charted as the main causes of MAS. These include four acquired entities: opsoclonus-myoclonus-ataxia syndrome, celiac disease, multiple system atrophy, and sporadic prion diseases. The distinction between progressive myoclonus epilepsy and progressive myoclonus ataxia poses one of the main diagnostic dilemmas.

Conclusions

Diagnostic algorithms for pediatric and adult patients, based on clinical manifestations including epilepsy, are proposed to guide the differential diagnosis and corresponding work-up of the most important and frequent causes of MAS. A list of genes associated with MAS to guide genetic testing strategies is provided. Priority should be given to diagnose or exclude acquired or treatable disorders.

Clinical phenotype features and genetic etiologies of 38 children with progressive myoclonic epilepsy

Background

Progressive myoclonic epilepsy (PME) is a group of neurodegenerative diseases with genetic heterogeneity and phenotypic similarities, and many cases remain unknown of the genetic causes. This study is aim to summarize the clinical features and study the genetic causes of PME patients.

Methods

Sanger sequencing of the target gene, Next Generation Sequencing (NGS) panels of epilepsy, trio-based Whole Exome Sequencing (WES) and detection of cytosine-adenine-guanine (CAG) repeat number were used to investigate the genetic causes of PME patients.

Results

Thirty-eight children with PME whose seizure onset age ranged from 3 months to 12 years were collected from February 2012 to November 2019 in three hospitals in Beijing, China. The seizure types included myoclonic seizures (n = 38), focal seizures (n = 19), generalized tonic-clonie seizure (GTCS) (n = 13), absence seizures (n = 4), atonic seizures (n = 3), epileptic spasms (n = 2) and tonic seizures (n = 1). Twenty-seven cases were sporadic and 11 had family members affected. Established PME-related genes were identified in 30 out of 38 (78.9%) patients who had either recessively inherited or de novo heterozygous mutations. Among these 30 cases, there were 12 cases (31.6%) of neuronal ceroid lipofuscinoses (the causing gene contains TPP1, PPT1, CLN5, CLN6 and MFSD8), two cases of sialidosis (the causing gene is NEU1), two cases of neuronopathic Gaucher disease (the causing gene is GBA), one case of spinal muscular atrophy-progressive myoclonic epilepsy (the causing gene is ASAH1), four cases of KCNC1 mutation-related PME, four cases of KCTD7 mutation-related PME, two cases of TBC1D24 mutation-related PME, one case of GOSR2 related PME, and two of dentatorubral-pallidoluysian atrophy (the causing gene is ATN1). In total, 13 PME genes were identified in our cohort. The etiology was not clear in eight patients.

Conclusion

PME is a group of clinically and genetically heterogeneous diseases. Genetic diagnosis was clear in 78.9% of PME patients. Various of genetic testing methods could increase the rate of genetic diagnosis. Neuronal ceroid lipofuscinoses (NCL) is the most common etiology of PME in children. Nearly one third PME children were diagnosed with NCL. GOSR2 related PME was in our cohort in Asia for the first time.

Deep Transcriptomic Analysis Reveals the Dynamic Developmental Progression during Early Development of Channel Catfish (Ictalurus punctatus)

The transition from fertilized egg to larva in fish is accompanied with various biological processes. We selected seven early developmental stages in channel catfish, Ictalurus punctatus, for transcriptome analysis, and covered 22,635 genes with 590 million high-quality RNA-sequencing (seq) reads. Differential expression analysis between neighboring developmental timepoints revealed significantly enriched biological categories associated with growth, development and morphogenesis, which was most evident at 2 vs. 5 days post fertilization (dpf) and 5 vs. 6 dpf. A gene co-expression network was constructed using the Weighted Gene Co-expression Network Analysis (WGCNA) approach and four critical modules were identified. Among candidate hub genes, GDF10, FOXA2, HCEA and SYCE3 were involved in head formation, egg development and the transverse central element of synaptonemal complexes. CK1, OAZ2, DARS1 and UBE2V2 were mainly associated with regulation of cell cycle, growth, brain development, differentiation and proliferation of enterocytes. IFI44L and ZIP10 were critical for the regulation of immune activity and ion transport. Additionally, TCK1 and TGFB1 were related to phosphate transport and regulating cell proliferation. All these genes play vital roles in embryogenesis and regulation of early development. These results serve as a rich dataset for functional genomic studies. Our work reveals new insights of the underlying mechanisms in channel catfish early development.

The expanding spectrum of movement disorders in genetic epilepsies

An ever-increasing number of neurogenetic conditions presenting with both epilepsy and atypical movements are now recognized. These disorders within the 'genetic epilepsy-dyskinesia' spectrum are clinically and genetically heterogeneous. Increased clinical awareness is therefore necessary for a rational diagnostic approach. Furthermore, careful interpretation of genetic results is key to establishing the correct diagnosis and initiating disease-specific management strategies in a timely fashion. In this review we describe the spectrum of movement disorders associated with genetically determined epilepsies. We also propose diagnostic strategies and putative pathogenic mechanisms causing these complex syndromes associated with both seizures and atypical motor control. WHAT THIS PAPER ADDS: Implicated genes encode proteins with very diverse functions. Pathophysiological mechanisms by which epilepsy and movement disorder phenotypes manifest are often not clear. Early diagnosis of treatable disorders is essential and next generation sequencing may be required.

Genetic potassium channel-associated epilepsies: Clinical review of the Kv family

Next-generation sequencing has enhanced discovery of many disease-associated genes in previously unexplained epilepsies, mainly in developmental and epileptic encephalopathies and familial epilepsies. We now classify these disorders according to the underlying molecular pathways, which encompass a diverse array of cellular and sub-cellular compartments/signalling processes including voltage-gated ion-channel defects. With the aim to develop and increase the use of precision medicine therapies, understanding the pathogenic mechanisms and consequences of disease-causing variants has gained major relevance in clinical care. The super-family of voltage-gated potassium channels is the largest and most diverse family among the ion channels, encompassing approximately 80 genes. Key potassium channelopathies include those affecting the K_V, K_Ca and K_ir families, a significant proportion of which have been implicated in neurological disease. As for other ion channel disorders, different pathogenic variants within any individual voltage-gated potassium channel gene tend to affect channel protein function differently, causing heterogeneous clinical phenotypes. The focus of this review is to summarise recent clinical developments regarding the key voltage-gated potassium (K_V) family-related epilepsies, which now encompasses approximately 12 established disease-associated genes, from the KCNA-, KCNB-, KCNC-, KCND-, KCNV-, KCNQ- and KCNH-subfamilies.

North Sea Progressive Myoclonus Epilepsy is Exacerbated by Heat, A Phenotype Primarily Associated with Affected Glia

Progressive myoclonic epilepsies (PMEs) comprise a group of rare disorders of different genetic aetiologies, leading to childhood-onset myoclonus, myoclonic seizures and subsequent neurological decline. One of the genetic causes for PME, a mutation in the gene coding for Golgi SNAP receptor 2 (GOSR2), gives rise to a PME-subtype prevalent in Northern Europe and hence referred to as North Sea Progressive Myoclonic Epilepsy (NS-PME). Treatment for NS-PME, as for all PME subtypes, is symptomatic; the pathophysiology of NS-PME is currently unknown, precluding targeted therapy. Here, we investigated the pathophysiology of NS-PME. By means of chart review in combination with interviews with patients (n = 14), we found heat to be an exacerbating factor for a majority of NS-PME patients (86%). To substantiate these findings, we designed a NS-PME Drosophila melanogaster model. Downregulation of the Drosophila GOSR2-orthologue Membrin leads to heat-induced seizure-like behaviour. Specific downregulation of GOSR2/Membrin in glia but not in neuronal cells resulted in a similar phenotype, which was progressive as the flies aged and was partially responsive to treatment with sodium barbital. Our data suggest a role for GOSR2 in glia in the pathophysiology of NS-PME.

Kv2.1 voltage-gated potassium channels in developmental perspective

Kv2.1 voltage-gated potassium channels consist of two types of α-subunits: (a) electrically-active Kcnb1 α-subunits and (b) silent or modulatory α-subunits plus β-subunits that, similar to silent α-subunits, also regulate electrically-active subunits. Voltage-gated potassium channels were traditionally viewed, mainly by electrophysiologists, as regulators of the electrical activity of the plasma membrane in excitable cells, a role that is performed by transmembrane protein domains of α-subunits that form the electric pore. Genetic studies revealed a role for this region of α-subunits of voltage-gated potassium channels in human neurodevelopmental disorders, such as epileptic encephalopathy. The N- and C-terminal domains of α-subunits interact to form the cytoplasmic subunit of heterotetrameric potassium channels that regulate electric pores. Subsequent animal studies revealed the developmental functions of Kcnb1-containing voltage-gated potassium channels and illustrated their role during brain development and reproduction. These functions of potassium channels are discussed in this review in the context of regulatory interactions between electrically-active and regulatory subunits.

A Kinetic Map of the Homomeric Voltage-Gated Potassium Channel (Kv) Family

The voltage-gated potassium (Kv) channels, encoded by 40 genes, repolarize all electrically excitable cells, including plant, cardiac, and neuronal cells. Although these genes were fully sequenced decades ago, a comprehensive kinetic characterization of all Kv channels is still missing, especially near physiological temperature. Here, we present a standardized kinetic map of the 40 homomeric Kv channels systematically characterized at 15, 25, and 35°C. Importantly, the Kv kinetics at 35°C differ significantly from commonly reported kinetics, usually performed at room temperature. We observed voltage-dependent Q₁₀ for all active Kv channels and inherent heterogeneity in kinetics for some of them. Kinetic properties are consistent across different host cell lines and conserved across mouse, rat, and human. All electrophysiology data from all Kv channels are made available through a public website (Channelpedia). This dataset provides a solid foundation for exploring kinetics of heteromeric channels, roles of auxiliary subunits, kinetic modulation, and for building accurate Kv models.

An Italian multicentre study of perampanel in progressive myoclonus epilepsies

Perampanel (PER) is a novel anti-seizure medication useful in different types of epilepsy. We intended to assess the effectiveness of PER on cortical myoclonus and seizure frequency in patients with progressive myoclonus epilepsy (PME), using quantitative validated scales. Forty-nine patients aged 36.6 ± 15.6 years with PME of various aetiology (18 EPM1, 12 EPM2, five with sialidosis, one with Kufs disease, one with EPM7, and 12 undetermined) were enrolled between January 2017 and June 2018. PER at the dose of 2-12 mg (5.3 ± 2.5) was added to existing therapy. Myoclonus severity was assessed using a minimal myoclonus scale (MMS) in all the patients before and after 4-6 months of steady PER dose, and by means of the Unified Myoclonus Rating Scale (UMRS) in 20 patients. Logistic regression analysis was used to identify the factors potentially predicting treatment efficacy. Four patients dropped out in the first two months due to psychiatric side effects. In the remaining patients, PER reduced myoclonus severity as assessed using MMS (Wilcoxon test: p < 0.001) and UMRS (p < 0.001), with the 'Action myoclonus' section of the UMRS showing the greatest improvement. The patients with EPM1 or EPM1-like phenotype were more likely to improve with PER (p = 0.011). Convulsive seizures which have recurred at least monthly in 17 patients were reduced by >50%. Side effects occurred in 22/49 (44.8%) patients, the most common being irritability followed by drowsiness. PER is effective in treating myoclonus and seizures in PME patients. The frequency of psychiatric side effects suggests the need for careful patient monitoring.

Encephalopathies with KCNC1 variants: genotype-phenotype-functional correlations

OBJECTIVE

To analyze clinical phenotypes associated with KCNC1 variants other than the Progressive Myoclonus Epilepsy-causing variant p.Arg320His, determine the electrophysiological functional impact of identified variants and explore genotype-phenotype-physiological correlations.

METHODS

Ten cases with putative pathogenic variants in KCNC1 were studied. Variants had been identified via whole-exome sequencing or gene panel testing. Clinical phenotypic data were analyzed. To determine functional impact of variants detected in the Kv 3.1 channel encoded by KCNC1, Xenopus laevis oocyte expression system and automated two-electrode voltage clamping were used.

RESULTS

Six unrelated patients had a Developmental and Epileptic Encephalopathy and a recurrent de novo variant p.Ala421Val (c.1262C > T). Functional analysis of p.Ala421Val revealed loss of function through a significant reduction in whole-cell current, but no dominant-negative effect. Three patients had a contrasting phenotype of Developmental Encephalopathy without seizures and different KCNC1 variants, all of which caused loss of function with reduced whole-cell currents. Evaluation of the variant p.Ala513Val (c.1538C > T) in the tenth case, suggested it was a variant of uncertain significance.

INTERPRETATION

These are the first reported cases of Developmental and Epileptic Encephalopathy due to KCNC1 mutation. The spectrum of phenotypes associated with KCNC1 is now broadened to include not only a Progressive Myoclonus Epilepsy, but an infantile onset Developmental and Epileptic Encephalopathy, as well as Developmental Encephalopathy without seizures. Loss of function is a key feature, but definitive electrophysiological separation of these phenotypes has not yet emerged.

KCNC1-related disorders: new de novo variants expand the phenotypic spectrum

A recurrent de novo missense variant in KCNC1, encoding a voltage-gated potassium channel expressed in inhibitory neurons, causes progressive myoclonus epilepsy and ataxia, and a nonsense variant is associated with intellectual disability. We identified three new de novo missense variants in KCNC1 in five unrelated individuals causing different phenotypes featuring either isolated nonprogressive myoclonus (p.Cys208Tyr), intellectual disability (p.Thr399Met), or epilepsy with myoclonic, absence and generalized tonic-clonic seizures, ataxia, and developmental delay (p.Ala421Val, three patients). Functional analyses demonstrated no measurable currents for all identified variants and dominant-negative effects for p.Thr399Met and p.Ala421Val predicting neuronal disinhibition as the underlying disease mechanism.

KCTD7 deficiency defines a distinct neurodegenerative disorder with a conserved autophagy-lysosome defect

OBJECTIVE

Several small case series identified KCTD7 mutations in patients with a rare autosomal recessive disorder designated progressive myoclonic epilepsy (EPM3) and neuronal ceroid lipofuscinosis (CLN14). Despite the name KCTD (potassium channel tetramerization domain), KCTD protein family members lack predicted channel domains. We sought to translate insight gained from yeast studies to uncover disease mechanisms associated with deficiencies in KCTD7 of unknown function.

METHODS

Novel KCTD7 variants in new and published patients were assessed for disease causality using genetic analyses, cell-based functional assays of patient fibroblasts and knockout yeast, and electron microscopy of patient samples.

RESULTS

Patients with KCTD7 mutations can exhibit movement disorders or developmental regression before seizure onset, and are distinguished from similar disorders by an earlier age of onset. Although most published KCTD7 patient variants were excluded from a genome sequence database of normal human variations, most newly identified patient variants are present in this database, potentially challenging disease causality. However, genetic analysis and impaired biochemical interactions with cullin 3 support a causal role for patient KCTD7 variants, suggesting deleterious alleles of KCTD7 and other rare disease variants may be underestimated. Both patient-derived fibroblasts and yeast lacking Whi2 with sequence similarity to KCTD7 have impaired autophagy consistent with brain pathology.

INTERPRETATION

Biallelic KCTD7 mutations define a neurodegenerative disorder with lipofuscin and lipid droplet accumulation but without defining features of neuronal ceroid lipofuscinosis or lysosomal storage disorders. KCTD7 deficiency appears to cause an underlying autophagy-lysosome defect conserved in yeast, thereby assigning a biological role for KCTD7. Ann Neurol 2018;84:774-788.

Familial cases of progressive myoclonic epilepsy caused by maternal somatic mosaicism of a recurrent KCNC1 p.Arg320His mutation

PURPOSE

A recurrent de novo mutation in KCNC1 (c.959G > A, p.Arg320His) has been identified recently as one of the important genetic causes of progress myoclonic epilepsy (PME). The clinical phenotype resulting from this mutation has been named as myoclonus epilepsy and ataxia due to potassium channel mutation (MEAK). This finding carries important clinical implications in that autosomal dominant inheritance and de novo occurrence need to be considered when conducting genetic tests in patients with PME. We present two familial cases of MEAK in siblings with a recurrent p.Arg320His mutation in KCNC1.

METHOD

Whole exome sequencing and subsequent Sanger sequencing were performed for the cases and their parents.

RESULTS

A recurrent p.Arg320His mutation in KCNC1 was identified in the two brothers who showed characteristic features of MEAK: near normal early development, onset of myoclonus around 10 years of age, infrequent generalized tonic-clonic seizures, relatively mild cognitive impairment, and generalized epileptiform discharges. Interestingly, the asymptomatic mother was suspected as being mosaic for this mutation. This finding could lead to misleading inheritance patterns and make genetic diagnosis of PME more complicated.

CONCLUSIONS

Our familial MEAK cases show that consideration of parental mosaicism in addition to meticulous phenotyping is needed when conducting KCNC1 genetic testing.