Unusual white matter involvement in EAST syndrome associated with novel KCNJ10 mutations

EAST/SeSAME Syndrome and Beyond: The Spectrum of Kir4.1- and Kir5.1-Associated Channelopathies

In 2009, two groups independently linked human mutations in the inwardly rectifying K+ channel Kir4.1 (gene name KCNJ10) to a syndrome affecting the central nervous system (CNS), hearing, and renal tubular salt reabsorption. The autosomal recessive syndrome has been named EAST (epilepsy, ataxia, sensorineural deafness, and renal tubulopathy) or SeSAME syndrome (seizures, sensorineural deafness, ataxia, intellectual disability, and electrolyte imbalance), accordingly. Renal dysfunction in EAST/SeSAME patients results in loss of Na+, K+, and Mg2+ with urine, activation of the renin-angiotensin-aldosterone system, and hypokalemic metabolic alkalosis. Kir4.1 is highly expressed in affected organs: the CNS, inner ear, and kidney. In the kidney, it mostly forms heteromeric channels with Kir5.1 (KCNJ16). Biallelic loss-of-function mutations of Kir5.1 can also have disease significance, but the clinical symptoms differ substantially from those of EAST/SeSAME syndrome: although sensorineural hearing loss and hypokalemia are replicated, there is no alkalosis, but rather acidosis of variable severity; in contrast to EAST/SeSAME syndrome, the CNS is unaffected. This review provides a framework for understanding some of these differences and will guide the reader through the growing literature on Kir4.1 and Kir5.1, discussing the complex disease mechanisms and the variable expression of disease symptoms from a molecular and systems physiology perspective. Knowledge of the pathophysiology of these diseases and their multifaceted clinical spectrum is an important prerequisite for making the correct diagnosis and forms the basis for personalized therapies.

A case series of adult patients affected by EAST/SeSAME syndrome suggests more severe disease in subjects bearing KCNJ10 truncating mutations

EAST/SeSAME syndrome is a rare disease affecting the Central Nervous System (CNS), inner ear, and kidney. The syndrome is due to loss-of-function mutations in the KCNJ10 gene encoding the inward-rectifying potassium channel Kir4.1. EAST/SeSAME syndrome is mainly diagnosed during childhood with a tonic-clonic seizure being the usual first symptom. Due to a limited number of patients and recent identification of the disease, few data are available on the clinical progress of this disease in adulthood. In particular, neurologic and nephrological outcomes have not been reported. We present a case series of 4 adult patients harbouring homozygous missense mutation p.Ala167Val and homozygous frameshift mutations p.Asn232Glnfs*14 and p.Gly275Valfs*7. Effects of these mutations were predicted by in silico modelling and bioinformatic tools. Patients with truncating mutations were associated with more severe outcomes, both in tubulopathy severity and neurological symptomatology. Conversely, either missense or truncating mutations were correlated with similar severity of epilepsy, with a long free-of-event period up to 20 years old. No eGFR decline was documented. Modelling predicted that truncating mutations lead to complete Kir4.1 dysfunction. Finally, all patients had a mild increase in urinary protein excretion. Our study indicates that the prognosis of patients suffering from EAST/SeSAME syndrome is related to the severity of the mutation causing the disease. As predicted by in silico modelling, truncating mutations of KCNJ10 are associated with more severe disease, with recurrence of symptomatic hypokalemia and more severe neurological phenotype. The type of mutation should be considered for the therapy tailored to patients' phenotype.

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.

Role of Astrocytic Inwardly Rectifying Potassium (Kir) 4.1 Channels in Epileptogenesis

Astrocytes regulate potassium and glutamate homeostasis via inwardly rectifying potassium (Kir) 4.1 channels in synapses, maintaining normal neural excitability. Numerous studies have shown that dysfunction of astrocytic Kir4.1 channels is involved in epileptogenesis in humans and animal models of epilepsy. Specifically, Kir4.1 channel inhibition by KCNJ10 gene mutation or expressional down-regulation increases the extracellular levels of potassium ions and glutamate in synapses and causes hyperexcitation of neurons. Moreover, recent investigations demonstrated that inhibition of Kir4.1 channels facilitates the expression of brain-derived neurotrophic factor (BDNF), an important modulator of epileptogenesis, in astrocytes. In this review, we summarize the current understanding on the role of astrocytic Kir4.1 channels in epileptogenesis, with a focus on functional and expressional changes in Kir4.1 channels and their regulation of BDNF secretion. We also discuss the potential of Kir4.1 channels as a therapeutic target for the prevention of epilepsy.

Novel mutations in the KCNJ10 gene associated to a distinctive ataxia, sensorineural hearing loss and spasticity clinical phenotype

KCNJ10 encodes the inward-rectifying potassium channel (Kir4.1) that is expressed in the brain, inner ear, and kidney. Loss-of-function mutations in KCNJ10 gene cause a complex syndrome consisting of epilepsy, ataxia, intellectual disability, sensorineural deafness, and tubulopathy (EAST/SeSAME syndrome). Patients with EAST/SeSAME syndrome display renal salt wasting and electrolyte imbalance that resemble the clinical features of impaired distal tubular salt transport in Gitelman's syndrome. A key distinguishing feature between these two conditions is the additional neurological (extrarenal) manifestations found in EAST/SeSAME syndrome. Recent reports have further expanded the clinical and mutational spectrum of KCNJ10-related disorders including non-syndromic early-onset cerebellar ataxia. Here, we describe a kindred of three affected siblings with early-onset ataxia, deafness, and progressive spasticity without other prominent clinical features. By using targeted next-generation sequencing, we have identified two novel missense variants, c.488G>A (p.G163D) and c.512G>A (p.R171Q), in the KCNJ10 gene that, in compound heterozygosis, cause this distinctive EAST/SeSAME phenotype in our family. Electrophysiological characterization of these two variants confirmed their pathogenicity. When expressed in CHO cells, the R171Q mutation resulted in 50% reduction of currents compared to wild-type KCNJ10 and G163D showed a complete loss of function. Co-expression of G163D and R171Q had a more pronounced effect on currents and membrane potential than R171Q alone but less severe than single expression of G163D. Moreover, the effect of the mutations seemed less pronounced in the presence of Kir5.1 (encoded by KCNJ16), with whom the renal Kir4.1 channels form heteromers. This partial functional rescue by co-expression with Kir5.1 might explain the lack of renal symptoms in the patients. This report illustrates that a spectrum of disorders with distinct clinical symptoms may result from mutations in different parts of KCNJ10, a gene initially associated only with the EAST/SeSAME syndrome.

White matter and cerebellar involvement in alternating hemiplegia of childhood

OBJECTIVE

To determine whether brain volumetric and white matter microstructural changes are present and correlate with neurological impairment in subjects with alternating hemiplegia of childhood (AHC).

METHODS

In this prospective single-center study, 12 AHC subjects (mean age 22.9 years) and 24 controls were studied with 3DT1-weighted MR imaging and high angular resolution diffusion imaging at 3T. Data obtained with voxel-based morphometry and tract-based spatial statistics were correlated with motor impairment using the International Cooperative Ataxia Rating Scale (ICARS) and Movement and Disability sub-scales of Burke-Fahn-Marsden Dystonia Rating Scale (BFMMS and BFMDS).

RESULTS

Compared to healthy controls, AHC subjects showed lower total brain volume (P < 0.001) and white matter volume (P = 0.002), with reduced clusters of white matter in frontal and parietal regions (P < 0.001). No significant regional differences were found in cortical or subcortical grey matter volumes. Lower cerebellar subvolumes correlated with worse ataxic symptoms and global motor impairment in AHC group (P < 0.001). Increased mean and radial diffusivity values were found in the corpus callosum, corticospinal tracts, superior and inferior longitudinal fasciculi, subcortical frontotemporal white matter, internal and external capsules, and optic radiations (P < 0.001). These diffusion scalar changes correlated with higher ICARS and BFMDS scores (P < 0.001).

INTERPRETATION

AHC subjects showed prevalent white matter involvement, with reduced volume in several cerebral and cerebellar regions associated with widespread microstructural changes reflecting secondary myelin injury rather than axonal loss. Conversely, no specific pattern of grey matter atrophy emerged. Lower cerebellar volumes, correlating with severity of neurological manifestations, seems related to disrupted developmental rather than neurodegenerative processes.