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Myoclonus epilepsy and ataxia due to potassium channel mutation (MEAK) is caused by heterozygous KCNC1 mutations Volume 18, supplément 2, September 2016

Illustrations


  • Figure 1

The term ‘progressive myoclonus epilepsy’ (PME) (Minassian et al. 2016) provides us with a broad, but reliable, characterization for this distinct subgroup of seizure disorders. ‘Progressive’ depicts the gradual neurological decline seen in patients affected with a PME. ‘Myoclonus’ and ‘epilepsy’ summarize the other two cardinal clinical features observed in these individuals (Berkovic et al., 1986). Aetiologically, most PMEs are the result of genetic abnormalities. The vast majority are inherited in an autosomal recessive fashion, although some cases exhibit autosomal dominant or mitochondrial inheritance.

Despite the impressive range of specific forms of PME, their phenotypes are fairly similar, which makes accurate clinical diagnosis very challenging, especially in the early stages. However, significant heterogeneity exists in terms of the genetic causes of the subtypes of progressive myoclonus epilepsies.

As is the case with countless other diseases in the realm of neurology, a proportion of PME cases still have no identified genetic basis. In this context, a cohort of 84 unconfirmed, unrelated PME patients was extensively investigated in a recent multicentre study. With a view to trying to elucidate their molecular diagnoses, all cases underwent whole-exome sequencing, and Sanger sequencing was performed for a secondary cohort of 28. The most remarkable finding was the identification of a recurrent heterozygous de novo mutation in the KCNC1 gene, which encodes for a subunit of a specific potassium channel. The new PME subtype was named ‘myoclonus epilepsy and ataxia due to potassium channel mutation’ (MEAK) (Muona et al., 2015). Notably, prior to this study, KCNC1 had never been associated with any human disease. In addition, this group of researchers importantly demonstrated that de novo mutations play a significant role in the genesis of progressive myoclonus epilepsies.

This article aims to review the most recent PME subtype described: MEAK. We begin by introducing the micro-world of potassium channels, since an understanding of this is of fundamental importance. We then focus on how KCNC1 was identified to be associated with MEAK and discuss how the effects of such a mutation are translated into clinical signs and symptoms. Finally, the phenotypes of all the reported individuals with MEAK are discussed.

Potassium ion channels

Potassium ion channels are ubiquitous membrane proteins responsible for a range of cellular functions, including maintenance of membrane potential, regulation of cell volume, and electrical excitability modulation. The latter is particularly important for the membrane physiology of excitable cells, such as neurons. According to their functional properties, potassium channels can be categorized into several groups, such as voltage-gated potassium channels, calcium-activated potassium channels, and sodium-activated potassium channels, among others. Furthermore, each of these groups can be subdivided into families and subfamilies, based on molecular similarity (Sansom et al., 2002).

Voltage-gated potassium channels (Kv channels) play an essential role in the generation and propagation of electrical impulses in the nervous system. By enabling the selective flow of potassium ions through neuronal membranes (upon changes in transmembrane potentials), Kv channels help set the resting potential and degree of excitability of the membrane (repolarization), influence action potential waveforms and firing patterns, and modulate synaptic activity (Ried et al., 1993). Voltage-gated potassium channels are classified into four subfamilies: Kv1, Kv2, Kv3, and Kv4.

Kv3 channels, in particular, are known for their high activation threshold and fast activation and deactivation properties (Rudy & McBain, 2001). Kv3 channels are crucial components of the circuitry of neurons that are able to fire action potentials at high frequencies or follow high frequency inputs (Wang et al., 2007). The Kv3 subfamily is composed of four subunits, Kv3.1, Kv3.2, Kv3.3, and Kv3.4, which are encoded by four genes, KCNC1to KCNC4, respectively.

These four subunits assemble as either homomers or heteromers to form voltage-gated tetrameric potassiumion channels. Each of the four subunits consists of 6 transmembrane segments (S1-S6) with a re-entrant P-loop region. The transmembrane segments S1-S4 are referred to as the voltage-sensing domain, of which the primary voltage-sensing unit is S4. The segments S5-P-S6 represent the ion-conducting pore domain. Upon membrane de- or hyper-polarization, which is sensed by positively charged arginine residues at the S4 segment, the S4 segment undergoes the largest reorientation, leading to channel opening and generation of transient gating currents (Aggarwal & MacKinnon, 1996; Chanda & Bezanilla, 2008) (figure 1).

Within the KCNC gene family, KCNC3 has been previously reported as a human disease associated gene. KCNC3 mutations have been recognized as a cause of spinocerebellar ataxia type 13 (SCA13) (Herman-Bert et al., 2000; Middlebrooks et al., 2013). Only very recently, KCNC1 mutations have been identified as a cause of human disease: myoclonus epilepsy and ataxia due to potassium channel mutation (MEAK).

Discovering KCNC1 mutations in myoclonus epilepsy and ataxia (MEAK)

Eighty-four clinically confirmed cases of PME without clear genetic aetiology from multiple centres (including Europe, North America, Asia, and Australia), were investigated using whole-exome sequencing. A recurrent heterozygous missense mutation, c.959G>A (p.Arg320His), in KCNC1 was identified in 11 unrelated patients (13.1 per cent). Sanger sequencing confirmed the mutation in two new cases, as well as in an affected sister and two affected children of one of the original 11 cases. In total, 16 cases, 13 of them unrelated, had the same KCNC1 mutation. The parents of all patients with the KCNC1 mutation were unaffected. Segregation analysis, where DNA was available for both parents, revealed that in each case the mutation occurred de novo. The family with four MEAK cases was further evaluated to rule out parental mosaicism (Muona et al., 2015). Based on a recently published mutation model (Samocha et al., 2014), the rate of this specific mutation was estimated at 1.75 × 10-7 mutations per person, representing an occurrence in one out of every 5,700,000 conceptions.

Pathogenicity of KCNC1 mutations

KCNC1 maps to chromosome 11, at 11p15.1, and encodes the Kv3.1 potassium channel (Ried et al., 1993). The c.959G>A mutation causes a substitution of histidine for arginine at codon 320 of the Kv3.1 protein (p.Arg320His). The targeted arginine is a highly evolutionary conserved residue in segment S4, which constitutes the main voltage sensor of Kv3 channels. Functional analysis showed that, upon membrane depolarization, mutant channels produced hardly detectable currents (Muona et al., 2015). Interestingly, experiments also indicated a dominant-negative effect of the mutant channel. When the mutant subunit and the wild-type protein were co-expressed at a ratio of 1:1, an 80 per cent decrease in the expected current was observed (Muona et al., 2015).

Previous studies have shown that loss of Kv3 function (such as that caused by p.Arg320His in KCNC1) disrupts the firing properties of fast-spiking neurons (Erisir et al., 1999; Issa et al., 2011; Rudy & McBain, 2001), affects neurotransmitter release (Sabatini & Regehr, 1997), and induces cell death (Irie et al., 2014). Specifically, in terms of Kv3.1 malfunctioning, the most affected neurons include inhibitory GABAergic interneurons (Gan & Kaczmarek, 1998; Rudy & McBain, 2001) and cerebellar neurons. Consequently, within the context of a p.Arg320His mutation, the physiology of both these types of neuronal cells would be compromised and could, eventually, degenerate. The implications of these outcomes are useful in attempting to explain the clinical scenario of subjects with MEAK. The first implication would be disinhibition following impaired firing of fast-spiking GABAergic interneurons, which is believed to contribute to myoclonus and tonic-clonic seizures. The second implication would be the dysfunction and degeneration of cerebellar neurons, which is hypothesized to contribute to motor impairment and ataxia.

MEAK phenotypes

The phenotypes of the 16 patients with MEAK were fairly similar. Early development was, as a rule, normal. The first symptom tended to be myoclonus, with an onset ranging from the age of 6 to 14 years. Myoclonus progressively worsened. Gait disturbances due to myoclonus led to the use of a walking aid or wheelchair by adolescence or early adulthood in most patients. Tonic-clonic seizures were present, albeit infrequent. Learning difficulties before seizure onset were not common. Mild cognitive decline subsequent to seizure onset, nonetheless, was shared by roughly half of the individuals. On electroencephalogram, these patients had generalized epileptiform discharges, with photosensitivity in some cases. Magnetic resonance imaging was either normal or showed cerebellar atrophy.

The early clinical presentation and evolution of MEAK resembles that of Unverricht-Lundborg disease (ULD). The age at onset, moderate-to-severe myoclonus, infrequent tonic-clonic seizures, and mild, if any, cognitive decline, are characteristic of both MEAK and ULD. As MEAK evolves, however, it might be clearly distinguished from ULD, as patients with MEAK usually suffer a more severe course. In terms of molecular diagnosis, however, the two entities are easily differentiated, irrespective of the time of diagnosis. While MEAK is caused by KCNC1 mutations, ULD is most frequently a result of mutations in the promoter region of CSTB, which leads to a massive reduction in levels of cystatin B, a lysosomal protease inhibitor (Rinne et al., 2002; Girard et al., 2013).

Conclusion

The identification of MEAK and its genetic basis have widened the phenotypic and genotypic spectra of PME. Given that pathogenic KCNC1 mutation is estimated to affect one in every 5,700,000 conceptions, many undetermined PME cases worldwide may now be solved. Diagnosing these individuals is likely to have a direct impact on counselling and education for patients and caregivers.

In addition, an emerging role for de novo mutations in progressive myoclonus epilepsies has been established. Finally, an understanding of the pathophysiology of MEAK provides singular insights into potential therapeutic interventions.

Disclosures

None of the authors have any conflict of interest to disclose.

The present manuscript is part of a Epileptic Disorders/Mariani Foundation supplement on Progressive Myoclonus Epilepsies, downloadable as a whole by visiting www.epilepticdisorders.com.