Hyperekplexia phenotype due to compound heterozygosity for GLRA1 gene mutations

A NanoBiT assay to monitor membrane proteins trafficking for drug discovery and drug development

Internalization of membrane proteins plays a key role in many physiological functions; however, highly sensitive and versatile technologies are lacking to study such processes in real-time living systems. Here we describe an assay based on bioluminescence able to quantify membrane receptor trafficking for a wide variety of internalization mechanisms such as GPCR internalization/recycling, antibody-mediated internalization, and SARS-CoV2 viral infection. This study represents an alternative drug discovery tool to accelerate the drug development for a wide range of physiological processes, such as cancer, neurological, cardiopulmonary, metabolic, and infectious diseases including COVID-19.

Membrane protein trafficking is monitored using split nanoluciferase. Receptor internalization leads to complementation on the early endosome and a bioluminescent response, and is applied to receptor internalization/recycling, antibody-mediated internalization and SARS-CoV2 entry.

A novel nonsense autosomal dominant mutation in the GLRA1 gene causing hyperekplexia

We present a family with two members affected by hyperekplexia and two unaffected members. All exons in the glycine receptor alpha 1 subunit gene (GLRA1) were sequenced in all four family members. Our index patient harbored a novel nonsense mutation (p.Trp314*; rs867618642) in the transmembrane domain three of the GLRA1 and a novel missense variant in the NH₂-terminal part (p.Val67Met; rs142888296). After development of tolerance for the effective treatment with clobazam a drug holiday led to a sustained restoration of the treatment response.

Impaired Glycine Receptor Trafficking in Neurological Diseases

Ionotropic glycine receptors (GlyRs) enable fast synaptic neurotransmission in the adult spinal cord and brainstem. The inhibitory GlyR is a transmembrane glycine-gated chloride channel. The immature GlyR protein undergoes various processing steps, e.g., folding, assembly, and maturation while traveling from the endoplasmic reticulum to and through the Golgi apparatus, where post-translational modifications, e.g., glycosylation occur. The mature receptors are forward transported via microtubules to the cellular surface and inserted into neuronal membranes followed by synaptic clustering. The normal life cycle of a receptor protein includes further processes like internalization, recycling, and degradation. Defects in GlyR life cycle, e.g., impaired protein maturation and degradation have been demonstrated to underlie pathological mechanisms of various neurological diseases. The neurological disorder startle disease is caused by glycinergic dysfunction mainly due to missense mutations in genes encoding GlyR subunits (GLRA1 and GLRB). In vitro studies have shown that most recessive forms of startle disease are associated with impaired receptor biogenesis. Another neurological disease with a phenotype similar to startle disease is a special form of stiff-person syndrome (SPS), which is most probably due to the development of GlyR autoantibodies. Binding of GlyR autoantibodies leads to enhanced receptor internalization. Here we focus on the normal life cycle of GlyRs concentrating on assembly and maturation, receptor trafficking, post-synaptic integration and clustering, and GlyR internalization/recycling/degradation. Furthermore, this review highlights findings on impairment of these processes under disease conditions such as disturbed neuronal ER-Golgi trafficking as the major pathomechanism for recessive forms of human startle disease. In SPS, enhanced receptor internalization upon autoantibody binding to the GlyR has been shown to underlie the human pathology. In addition, we discuss how the existing mouse models of startle disease increased our current knowledge of GlyR trafficking routes and function. This review further illuminates receptor trafficking of GlyR variants originally identified in startle disease patients and explains changes in the life cycle of GlyRs in patients with SPS with respect to structural and functional consequences at the receptor level.

A Missense Mutation A384P Associated with Human Hyperekplexia Reveals a Desensitization Site of Glycine Receptors

Hyperekplexia, an inherited neuronal disorder characterized by exaggerated startle responses with unexpected sensory stimuli, is caused by dysfunction of glycinergic inhibitory transmission. From analysis of newly identified human hyperekplexia mutations in the glycine receptor (GlyR) α1 subunit, we found that an alanine-to-proline missense mutation (A384P) resulted in substantially higher desensitization level and lower agonist sensitivity of homomeric α1 GlyRs when expressed in HEK cells. The incorporation of the β subunit fully reversed the reduction in agonist sensitivity and partially reversed the desensitization of α1^A384P The heteromeric α1^A384Pβ GlyRs showed enhanced desensitization but unchanged agonist-induced maximum responses, surface expression, main channel conductance, and voltage dependence compared with that of the wild-type α1β (α1^WTβ) GlyRs. Coexpression of the R392H and A384P mutant α1 subunits, which mimic the expression of the compound heterozygous mutation in a hyperekplexia patient, resulted in channel properties similar to those with α1^A384P subunit expression alone. In comparison, another human hyperekplexia mutation α1^P250T, which was previously reported to enhance desensitization, caused a strong reduction in maximum currents in addition to the altered desensitization. These results were further confirmed by overexpression of α1^P250T or α1^A384P subunits in cultured neurons isolated from SD rats of either sex. Moreover, the IPSC-like responses of cells expressing α1^A384Pβ induced by repeated glycine pulses showed a stronger frequency-dependent reduction than those expressing α1^WTβ. Together, our findings demonstrate that A384 is associated with the desensitization site of the α1 subunit and its proline mutation produced enhanced desensitization of GlyRs, which contributes to the pathogenesis of human hyperekplexia.SIGNIFICANCE STATEMENT Human startle disease is caused by impaired synaptic inhibition in the brainstem and spinal cord, which is due to either direct loss of GlyR channel function or reduced number of synaptic GlyRs. Considering that fast decay kinetics of GlyR-mediated inhibitory synaptic responses, the question was raised whether altered desensitization of GlyRs will cause dysfunction of glycine transmission and disease phenotypes. Here, we found that the α1 subunit mutation A384P, identified from startle disease patients, results in enhanced desensitization and leads to rapidly decreasing responses in the mutant GlyRs when they are activated repeatedly by the synaptic-like simulation. These observations suggest that the enhanced desensitization of postsynaptic GlyRs could be the primary pathogenic mechanism of human startle disease.

The Intracellular Loop of the Glycine Receptor: It's not all about the Size

The family of Cys-loop receptors (CLRs) shares a high degree of homology and sequence identity. The overall structural elements are highly conserved with a large extracellular domain (ECD) harboring an α-helix and 10 β-sheets. Following the ECD, four transmembrane domains (TMD) are connected by intracellular and extracellular loop structures. Except the TM3–4 loop, their length comprises 7–14 residues. The TM3–4 loop forms the largest part of the intracellular domain (ICD) and exhibits the most variable region between all CLRs. The ICD is defined by the TM3–4 loop together with the TM1–2 loop preceding the ion channel pore. During the last decade, crystallization approaches were successful for some members of the CLR family. To allow crystallization, the intracellular loop was in most structures replaced by a short linker present in prokaryotic CLRs. Therefore, no structural information about the large TM3–4 loop of CLRs including the glycine receptors (GlyRs) is available except for some basic stretches close to TM3 and TM4. The intracellular loop has been intensively studied with regard to functional aspects including desensitization, modulation of channel physiology by pharmacological substances, posttranslational modifications, and motifs important for trafficking. Furthermore, the ICD interacts with scaffold proteins enabling inhibitory synapse formation. This review focuses on attempts to define structural and functional elements within the ICD of GlyRs discussed with the background of protein-protein interactions and functional channel formation in the absence of the TM3–4 loop.

Disturbances of Ligand Potency and Enhanced Degradation of the Human Glycine Receptor at Affected Positions G160 and T162 Originally Identified in Patients Suffering from Hyperekplexia

Ligand-binding of Cys-loop receptors is determined by N-terminal extracellular loop structures from the plus as well as from the minus side of two adjacent subunits in the pentameric receptor complex. An aromatic residue in loop B of the glycine receptor (GlyR) undergoes direct interaction with the incoming ligand via a cation-π interaction. Recently, we showed that mutated residues in loop B identified from human patients suffering from hyperekplexia disturb ligand-binding. Here, we exchanged the affected human residues by amino acids found in related members of the Cys-loop receptor family to determine the effects of side chain volume for ion channel properties. GlyR variants were characterized in vitro following transfection into cell lines in order to analyze protein expression, trafficking, degradation and ion channel function. GlyR α1 G160 mutations significantly decrease glycine potency arguing for a positional effect on neighboring aromatic residues and consequently glycine-binding within the ligand-binding pocket. Disturbed glycinergic inhibition due to T162 α1 mutations is an additive effect of affected biogenesis and structural changes within the ligand-binding site. Protein trafficking from the ER toward the ER-Golgi intermediate compartment, the secretory Golgi pathways and finally the cell surface is largely diminished, but still sufficient to deliver ion channels that are functional at least at high glycine concentrations. The majority of T162 mutant protein accumulates in the ER and is delivered to ER-associated proteasomal degradation. Hence, G160 is an important determinant during glycine binding. In contrast, T162 affects primarily receptor biogenesis whereas exchanges in functionality are secondary effects thereof.

Disturbed neuronal ER-Golgi sorting of unassembled glycine receptors suggests altered subcellular processing is a cause of human hyperekplexia

Recent studies on the pathogenic mechanisms of recessive hyperekplexia indicate disturbances in glycine receptor (GlyR) α1 biogenesis. Here, we examine the properties of a range of novel glycine receptor mutants identified in human hyperekplexia patients using expression in transfected cell lines and primary neurons. All of the novel mutants localized in the large extracellular domain of the GlyR α1 have reduced cell surface expression with a high proportion of receptors being retained in the ER, although there is forward trafficking of glycosylated subpopulations into the ER-Golgi intermediate compartment and cis-Golgi compartment. CD spectroscopy revealed that the mutant receptors have proportions of secondary structural elements similar to wild-type receptors. Two mutants in loop B (G160R, T162M) were functional, but none of those in loop D/β2-3 were. One nonfunctional truncated mutant (R316X) could be rescued by coexpression with the lacking C-terminal domain. We conclude that a proportion of GlyR α1 mutants can be transported to the plasma membrane but do not necessarily form functional ion channels. We suggest that loop D/β2-3 is an important determinant for GlyR trafficking and functionality, whereas alterations to loop B alter agonist potencies, indicating that residues here are critical elements in ligand binding.

Hyperekplexia: Treatment of a Severe Phenotype and Review of the Literature

Hyperekplexia is a rare disorder caused by autosomal dominant or recessive modes of inheritance and characterized by episodes of exaggerated startle. Five causative genes have been identified to date. The syndrome has been recognized for decades and due to its rarity, the literature contains mostly descriptive reports, many early studies lacking molecular genetic diagnoses. A spectrum of clinical severity exists. Severe cases can lead to neonatal cardiac arrest and death during an episode, an outcome prevented by early diagnosis and clinical vigilance. Large treatment studies are not feasible, so therapeutic measures continue to be empiric. A marked response to clonazepam is often reported but refractory cases exist. Herein we report the clinical course and treatment response of a severely affected infant homozygous for an SLC6A5 nonsense mutation and review the literature summarizing the history and genetic understanding of the disease as well as the described comorbidities and treatment options.

Glycine receptor mouse mutants: model systems for human hyperekplexia

Human hyperekplexia is a neuromotor disorder caused by disturbances in inhibitory glycine-mediated neurotransmission. Mutations in genes encoding for glycine receptor subunits or associated proteins, such as GLRA1, GLRB, GPHN and ARHGEF9, have been detected in patients suffering from hyperekplexia. Classical symptoms are exaggerated startle attacks upon unexpected acoustic or tactile stimuli, massive tremor, loss of postural control during startle and apnoea. Usually patients are treated with clonazepam, this helps to dampen the severe symptoms most probably by up-regulating GABAergic responses. However, the mechanism is not completely understood. Similar neuromotor phenotypes have been observed in mouse models that carry glycine receptor mutations. These mouse models serve as excellent tools for analysing the underlying pathomechanisms. Yet, studies in mutant mice looking for postsynaptic compensation of glycinergic dysfunction via an up-regulation in GABAA receptor numbers have failed, as expression levels were similar to those in wild-type mice. However, presynaptic adaptation mechanisms with an unusual switch from mixed GABA/glycinergic to GABAergic presynaptic terminals have been observed. Whether this presynaptic adaptation explains the improvement in symptoms or other compensation mechanisms exist is still under investigation. With the help of spontaneous glycine receptor mouse mutants, knock-in and knock-out studies, it is possible to associate behavioural changes with pharmacological differences in glycinergic inhibition. This review focuses on the structural and functional characteristics of the various mouse models used to elucidate the underlying signal transduction pathways and adaptation processes and describes a novel route that uses gene-therapeutic modulation of mutated receptors to overcome loss of function mutations.

The impact of human hyperekplexia mutations on glycine receptor structure and function

Hyperekplexia is a rare neurological disorder characterized by neonatal hypertonia, exaggerated startle responses to unexpected stimuli and a variable incidence of apnoea, intellectual disability and delays in speech acquisition. The majority of motor defects are successfully treated by clonazepam. Hyperekplexia is caused by hereditary mutations that disrupt the functioning of inhibitory glycinergic synapses in neuromotor pathways of the spinal cord and brainstem. The human glycine receptor α1 and β subunits, which predominate at these synapses, are the major targets of mutations. International genetic screening programs, that together have analysed several hundred probands, have recently generated a clear picture of genotype-phenotype correlations and the prevalence of different categories of hyperekplexia mutations. Focusing largely on this new information, this review seeks to summarise the effects of mutations on glycine receptor structure and function and how these functional alterations lead to hyperekplexia.

New hyperekplexia mutations provide insight into glycine receptor assembly, trafficking, and activation mechanisms

Hyperekplexia is a syndrome of readily provoked startle responses, alongside episodic and generalized hypertonia, that presents within the first month of life. Inhibitory glycine receptors are pentameric ligand-gated ion channels with a definitive and clinically well stratified linkage to hyperekplexia. Most hyperekplexia cases are caused by mutations in the α1 subunit of the human glycine receptor (hGlyR) gene (GLRA1). Here we analyzed 68 new unrelated hyperekplexia probands for GLRA1 mutations and identified 19 mutations, of which 9 were novel. Electrophysiological analysis demonstrated that the dominant mutations p.Q226E, p.V280M, and p.R414H induced spontaneous channel activity, indicating that this is a recurring mechanism in hGlyR pathophysiology. p.Q226E, at the top of TM1, most likely induced tonic activation via an enhanced electrostatic attraction to p.R271 at the top of TM2, suggesting a structural mechanism for channel activation. Receptors incorporating p.P230S (which is heterozygous with p.R65W) desensitized much faster than wild type receptors and represent a new TM1 site capable of modulating desensitization. The recessive mutations p.R72C, p.R218W, p.L291P, p.D388A, and p.E375X precluded cell surface expression unless co-expressed with α1 wild type subunits. The recessive p.E375X mutation resulted in subunit truncation upstream of the TM4 domain. Surprisingly, on the basis of three independent assays, we were able to infer that p.E375X truncated subunits are incorporated into functional hGlyRs together with unmutated α1 or α1 plus β subunits. These aberrant receptors exhibit significantly reduced glycine sensitivity. To our knowledge, this is the first suggestion that subunits lacking TM4 domains might be incorporated into functional pentameric ligand-gated ion channel receptors.

Distinct phenotypes in zebrafish models of human startle disease

Startle disease is an inherited neurological disorder that causes affected individuals to suffer noise- or touch-induced non-epileptic seizures, excessive muscle stiffness and neonatal apnea episodes. Mutations known to cause startle disease have been identified in glycine receptor subunit (GLRA1 and GLRB) and glycine transporter (SLC6A5) genes, which serve essential functions at glycinergic synapses. Despite the significant successes in identifying startle disease mutations, many idiopathic cases remain unresolved. Exome sequencing in these individuals will identify new candidate genes. To validate these candidate disease genes, zebrafish is an ideal choice due to rapid knockdown strategies, accessible embryonic stages, and stereotyped behaviors. The only existing zebrafish model of startle disease, bandoneon (beo), harbors point mutations in glrbb (one of two zebrafish orthologs of human GLRB) that cause compromised glycinergic transmission and touch-induced bilateral muscle contractions. In order to further develop zebrafish as a model for startle disease, we sought to identify common phenotypic outcomes of knocking down zebrafish orthologs of two known startle disease genes, GLRA1 and GLRB, using splice site-targeted morpholinos. Although both morphants were expected to result in phenotypes similar to the zebrafish beo mutant, our direct comparison demonstrated that while both glra1 and glrbb morphants exhibited embryonic spasticity, only glrbb morphants exhibited bilateral contractions characteristic of beo mutants. Likewise, zebrafish over-expressing a dominant startle disease mutation (GlyR α1(R271Q)) exhibited spasticity but not bilateral contractions. Since GlyR βb can interact with GlyR α subunits 2-4 in addition to GlyR α1, loss of the GlyR βb subunit may produce more severe phenotypes by affecting multiple GlyR subtypes. Indeed, immunohistochemistry of glra1 morphants suggests that in zebrafish, alternate GlyR α subunits can compensate for the loss of the GlyR α1 subunit. To address the potential for interplay among GlyR subunits during development, we quantified the expression time-course for genes known to be critical to glycinergic synapse function. We found that GlyR α2, α3 and α4a are expressed in the correct temporal pattern and could compensate for the loss of the GlyR α1 subunit. Based on our findings, future studies that aim to model candidate startle disease genes in zebrafish should include measures of spasticity and synaptic development.

Startle syndromes

Startle refers to a sudden involuntary movement of the body in response to a surprising and unexpected stimulus. It is a fast twitch of facial and body muscles evoked by a sudden and intense tactile, visual, or acoustic stimulus. While startle can be considered to be a protective function against injury, startle syndromes are abnormal responses to startling events, consisting of three heterogeneous groups of disorders. The first is hyperekplexia, characterized by brisk and generalized startle in response to trivial stimulation. The major form of hereditary hyperekplexia has a genetic basis, frequently due to mutations in the α1 subunit of the glycine receptor (GLRA1) on chromosome 5q. In the second group, normal startle induces complex but stereotyped motor and/or behavioral abnormalities lasting several seconds, termed as startle epilepsy. It usually occurs in the setting of severe brain damage, particularly perinatal hypoxia. The third group is characterized by nonhabituating hyperstartling, provoked by loud noises, sudden commands, or gestures. The intensity of startle response tends to increase with frequency of stimulation, which often leads to injury. Interestingly, its occurrence is restricted to certain social or ethnic groups in different parts of the world, such as jumping Frenchmen of Maine among Franco-Canadian lumberjack communities, and Latah in Southeast Asia. So far, no neurological abnormalities have been reported in association with these neuropsychiatric startle syndromes. In this chapter, the authors discuss the clinical presentation, physiology, and the neuronal basis of the normal human startle as well as different groups of abnormal startle syndromes. The aim is to provide an overview of hyperstartling with some diagnostic hints and the distinguishing features among these syndromes.

Pathophysiological mechanisms of dominant and recessive GLRA1 mutations in hyperekplexia

Hyperekplexia is a rare, but potentially fatal, neuromotor disorder characterized by exaggerated startle reflexes and hypertonia in response to sudden, unexpected auditory or tactile stimuli. This disorder is primarily caused by inherited mutations in the genes encoding the glycine receptor (GlyR) alpha1 subunit (GLRA1) and the presynaptic glycine transporter GlyT2 (SLC6A5). In this study, systematic DNA sequencing of GLRA1 in 88 new unrelated human hyperekplexia patients revealed 19 sequence variants in 30 index cases, of which 21 cases were inherited in recessive or compound heterozygote modes. This indicates that recessive hyperekplexia is far more prevalent than previous estimates. From the 19 GLRA1 sequence variants, we have investigated the functional effects of 11 novel and 2 recurrent mutations. The expression levels and functional properties of these hyperekplexia mutants were analyzed using a high-content imaging system and patch-clamp electrophysiology. When expressed in HEK293 cells, either as homomeric alpha1 or heteromeric alpha1beta GlyRs, subcellular localization defects were the major mechanism underlying recessive mutations. However, mutants without trafficking defects typically showed alterations in the glycine sensitivity suggestive of disrupted receptor function. This study also reports the first hyperekplexia mutation associated with a GlyR leak conductance, suggesting tonic channel opening as a new mechanism in neuronal ligand-gated ion channels.

Recessive hyperekplexia mutations of the glycine receptor alpha1 subunit affect cell surface integration and stability

The human neurological disorder hyperekplexia is frequently caused by recessive and dominant mutations of the glycine receptor alpha1 subunit gene, GLRA1. Dominant forms are mostly attributed to amino acid substitutions within the ion pore or adjacent loops, resulting in altered channel properties. Here, the biogenesis of glycine receptor alpha1 subunit mutants underlying recessive forms of hyperekplexia was analyzed following recombinant expression in HEK293 cells. The alpha1 mutant S231R resulted in a decrease of surface integrated protein, consistent with reduced maximal current values. Decreased maximal currents shown for the recessive alpha1 mutant I244N were associated with protein instability, rather than decreased surface integration. The recessive mutants R252H and R392H encode exchanges of arginine residues delineating the intracellular faces of transmembrane domains. After expression, the mutant R252H was virtually absent from the cell surface, consistent with non-functionality and the importance of the positive charge for membrane integration. Surface expression of R392H was highly reduced, resulting in residual chloride conductance. Independent of the site of the mutation within the alpha1 polypeptide, metabolic radiolabelling and pulse chase studies revealed a shorter half-life of the full-length alpha1 protein for all recessive mutants as compared to the wild-type. Treatment with the proteasome blocker, lactacystin, significantly increased the accumulation of alpha1 mutants in intracellular membranes. These observations indicated that the recessive alpha1 mutants are recognized by the endoplasmatic reticulum control system, and degraded via the proteasome pathway. Thus, the lack of glycinergic inhibition associated with recessive hyperekplexia may be attributed to sequestration of mutant subunits within the endoplasmatic reticulum quality control system.

A novel GLRA1 mutation in a recessive hyperekplexia pedigree

We report the identification of a novel Y228C mutation within the M1 trans-membrane domain of the GLRA1 subunit of the glycine receptor responsible for a severe recessive hyperekplexia phenotype in a Kurdish pedigree.

A case of major form familial hyperekplexia: prenatal diagnosis and effective treatment with clonazepam

Hyperekplexia (OMIM 149400) is an uncommon neurologic disorder characterized by exaggerated response to sensitive stimuli. It may be sporadic or familial. The disease is usually caused by mutations in the inhibitory glycine receptor alpha1-subunit. The authors report a male patient who is affected by the major form of familial hyperekplexia. He is currently 5 years old and is being successfully treated with clonazepam. Prenatal diagnosis was made owing to prior identification of point mutation K276E in his affected mother. Early diagnosis avoided complex and prolonged differential diagnostic procedures and allowed for early and effective intervention on severe neonatal symptoms: hypertonia, episodes of cyanosis, apneic spells, and massive myoclonic jerks. During his first year of life, the patient was treated with cycles of phenobarbital and diazepam and achieved partial clinical response. Subsequent therapy with low-dose clonazepam was highly effective in reducing myoclonic jerks and exaggerated startle reaction, and unlike previously used drugs, it was decisive in reducing hypertonia.

Startle syndromes

Startle syndromes consist of three heterogeneous groups of disorders with abnormal responses to startling events. The first is hyperekplexia, which can be split up into the "major" or "minor" form. The major form of hyperekplexia is characterised by excessive startle reflexes, startle-induced falls, and continuous stiffness in the neonatal period. This form has a genetic basis: mutations in the alpha1 subunit of the glycine receptor gene, GLRA1, or related genes. The minor form, which is restricted to excessive startle reflexes with no stiffness, has no known genetic cause or underlying pathophysiological substrate. The second group of startle syndromes are neuropsychiatric, in which excessive startling and various additional behavioural features occur. The third group are disorders in which startling stimuli can induce responses other than startle reflexes, such as startle-induced epilepsy. Diagnosis of startle syndromes depends on clinical history, electromyographic studies, and genetic screening. Further study of these disorders may enable improved discrimination between the different groups.

Functional recovery of glycine receptors in spastic murine model of startle disease

Clinical variability is common in inherited gene defects of the central nervous system in humans and in animal models of human disorders. Here, we used the homozygous spastic (spa) mutant mice, which resemble human hereditary hyperekplexia, to determine the molecular remodeling of the spinal cord through the course of the disease, and develop a model for clinical disparity between littermates. The spa mutation is an insertion of a LINE-1 element in the gene for the beta subunit of the glycine receptor, Glrb. The insertion causes aberrant splicing in the beta subunit of glycine receptor gene with a consequent important reduction of glycine receptors. At young ages, all homozygous spa animals were spastic, showed loss of glycine receptors, increased expression of vesicular glycine/GABA transporter and NMDA receptors, induction of activated caspase3, and preferential loss of glycinergic interneurons consistent with neurotransmitter toxicity model. Those littermates that recovered from symptoms showed strong over-expression of the glycine receptor alpha 1 subunit (Glra1), and increased myelination and synaptic plasticity. Littermates that showed a deteriorating clinical course failed to over-express Glra1, and also showed relative loss of gephyrin (receptor clustering). These molecular changes were associated with a preferential loss of GABAergic interneurons, and extensive motorneuron loss. These data suggest that functional recovery is likely due to expression of homomeric glycine receptors, rescue from excitotoxicity, and subsequent neuronal remodeling. We propose that human patients with hyperekplexia show remodeling similar to that of the recovering spa mice, as human patients also show a lessening of symptoms as a function of age.

Recessive hyperekplexia due to a new mutation (R100H) in the GLRA1 gene

Hyperekplexia is commonly familial and with dominant transmission. The gene involved, GLRA1, encodes the alpha1 subunit of the glycine receptor. We describe 3 affected children homozygous for a new mutation, R100H. Both parents were heterozygous carriers; while the father was healthy, the mother has periodic limb movements during sleep. This suggests that Hys-100 could exhibit incomplete penetrance, but was linked to a severe classical form of hyperekplexia in homozygous.

Hereditary hyperekplexia caused by novel mutations of GLRA1 in Turkish families

BACKGROUND

Hyperekplexia, also known as startle disease or stiff-person syndrome, is a neurological condition characterized by neonatal hypertonia and a highly exaggerated startle reflex. Genetic studies have linked mutations in the gene encoding glycine receptor alpha1 (GLRA1) with hereditary hyperekplexia.

METHODS

We analyzed four Turkish families with a history of hyperekplexia. Genomic DNA was obtained from members of these families, and the entire coding sequence of GLRA1 was amplified by PCR followed by the sequencing of PCR products. DNA sequences were analyzed by direct observation using an electropherogram and compared with a published reference sequence.

RESULTS

We identified three novel mutations in GLRA1. These included a large deletion removing the first 7 of 9 exons, a single-base deletion in exon 8 that results in protein truncation immediately after the deletion, and a missense mutation in exon 7 causing a tryptophan-to-cysteine change in the first transmembrane domain (M1). These mutant alleles have some distinct features as compared to previously identified GLRA1 mutations. Our data provides further evidence for mutational heterogeneity in GLRA1. The new mutant alleles reported here should advance our understanding of the etiology of hyperekplexia.

Zebrafish bandoneon mutants display behavioral defects due to a mutation in the glycine receptor beta-subunit

Bilateral alternation of muscle contractions requires reciprocal inhibition between the two sides of the hindbrain and spinal cord, and disruption of this inhibition should lead to simultaneous activation of bilateral muscles. At 1 day after fertilization, wild-type zebrafish respond to mechanosensory stimulation with multiple fast alternating trunk contractions, whereas bandoneon (beo) mutants contract trunk muscles on both sides simultaneously. Similar simultaneous contractions are observed in wild-type embryos treated with strychnine, a blocker of the inhibitory glycine receptor (GlyR). This result suggests that glycinergic synaptic transmission is defective in beo mutants. Muscle voltage recordings confirmed that muscles on both sides of the trunk in beo are likely to receive simultaneous synaptic input from the CNS. Recordings from motor neurons revealed that glycinergic synaptic transmission was missing in beo mutants. Furthermore, immunostaining with an antibody against GlyR showed clusters in wild-type neurons but not in beo neurons. These data suggest that the failure of GlyRs to aggregate at synaptic sites causes impairment of glycinergic transmission and abnormal behavior in beo mutants. Indeed, mutations in the GlyR beta-subunit, which are thought to be required for proper localization of GlyRs, were identified as the basis for the beo mutation. These data demonstrate that GlyRbeta is essential for physiologically relevant clustering of GlyRs in vivo. Because GlyR mutations in humans lead to hyperekplexia, a motor disorder characterized by startle responses, the zebrafish beo mutant should be a useful animal model for this condition.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

Hyperekplexia (startle disease): a novel mutation (S270T) in the M2 domain of the GLRA1 gene and a molecular review of the disorder

BACKGROUND

We report on a novel mutation (S270T) in the M2 domain of the GLRA1 (alpha subunit of the glycine receptor) gene causing autosomal dominant hyperekplexia in a neonate, the mother and maternal uncle. All affected members showed the typical clinical features of the disorder. This novel S270T (T1188A) mutation is located in the boundary of the transmembrane M2 domain of the GLRA1 protein, close to other previously reported mutations. Mutations in this 'hot spot' domain of GLRA1 are frequent in autosomal dominant hyperekplexia but are not usually seen in the autosomal recessive form of the disease in which both the M1 and the carboxy terminal domains have been implicated.

METHODS

Genomic DNA was extracted by standard procedures from peripheral blood leukocytes and exon 6 of the GLRA1 gene was amplified using primers and PCR conditions. A complete sequence analysis of the fragment was performed. DNA sequences were analyzed both by direct observation of the electropherogram and by comparison with the published sequence.

RESULTS

The proband had metabolic acidosis, which was probably related to continuous contractions of somatic muscles and intractable hypertonia. Data seem to show a direct relationship between the mechanism of inheritance of the disorder and the location of the molecular defect. The patients showed almost all the clinical signs of hyperekplexia: exaggerated startle response, muscle hypertonia in response to unexpected tactile and/or auditory stimuli, hyperexcitability, and sudden falls.

Magnetic resonance spectroscopy of cerebral cortex is normal in hereditary hyperekplexia due to mutations in the GLRA1 gene

Excessive startling and stiffness in hereditary hyperekplexia has been attributed to lack of inhibition at either the cortical or brainstem level. Six patients with hereditary hyperekplexia (HH) and a confirmed mutation in the gene encoding the alpha(1) subunit of the glycine receptor (GLRA1) underwent single voxel (1)H magnetic resonance spectroscopy (MRS) of the brainstem and an area of frontal cortex and white matter using a method that allows absolute quantification of metabolites. The results of MRS were within normal limits, although there was a tendency for the neuronal marker N-acetyl aspartate to be reduced in the brainstem of patients compared with that in controls. Thus, we found no evidence to support a deficit in the cerebral cortex in patients with hereditary hyperekplexia due to mutations in the GLRA1 gene.

Hyperekplexia: a treatable neurogenetic disease

Hyperekplexia is primarily an autosomal dominant disease characterized by exaggerated startle reflex and neonatal hypertonia. It can be associated with, if untreated, sudden infant death from apnea or aspiration pneumonia and serious injuries and loss of ambulation from frequent falls. Different mutations in the alpha1 subunit of inhibitory glycine receptor (GLRA1) gene have been identified in many affected families. The most common mutation is Arg271 reported in at least 12 independent families. These mutations uncouple the ligand binding and chloride channel function of inhibitory glycine receptor and result in increased excitability in pontomedullary reticular neurons and abnormal spinal reciprocal inhibition. Three mouse models from spontaneous mutations in GLRA1 and beta subunit of inhibitory glycine receptor (GLRB) genes and two transgenic mouse models are valuable for the study of the pathophysiology and the genotype-phenotype correlation of the disease. The disease caused by mutation in GLRB in mice supports the notion that human hyperekplexia with no detectable mutations in GLRA1 may harbor mutations in GLRB. Clonazepam, a gamma aminobutyric acid (GABA) receptor agonist, is highly effective and is the drug of choice. It enhances the GABA-gated chloride channel function and presumably compensates for the defective glycine-gated chloride channel in hyperekplexia. Recognition of the disease will lead to appropriate treatment and genetic counseling.

Functional characterization of compound heterozygosity for GlyRalpha1 mutations in the startle disease hyperekplexia

The human disease hyperekplexia is characterized by excessive startle reactions to auditory and cutaneous stimuli. In its familial form, hyperekplexia has been associated with both dominant and recessive mutations of the GLRA1 gene encoding the glycine receptor alpha1 subunit (GlyRalpha1), which mediates inhibitory transmission in the spinal cord and brainstem. Here we have examined the functional consequences of two amino acid substitutions found in a compound heterozygous family, R252H and R392H, to investigate the mechanisms determining this inheritance pattern. When expressed in Xenopus laevis oocytes, both mutations were non-functional. Neither mutant affected the electrophysiological properties of wild type GlyRalpha1 when co-expressed. We introduced a green fluorescent protein tag to mutant subunits and found that both mutant proteins were detectable. Evidence that subcellular localization differed from wild type was significant for one of the mutants. Thus, an effective loss of functional GlyRalpha1-mediated current underlies hyperekplexia in this family, whereas a partial loss is asymptomatic.

Major and minor form of hereditary hyperekplexia

Hyperekplexia is a hereditary neurological disorder characterized by excessive startle responses. Within the disorder two clinical forms can be distinguished. The major form is characterized by continuous generalized stiffness in the first year of life and an exaggerated startle reflex, accompanied by temporary generalized stiffness and falls, whereas in the minor form only excessive startle and hypnic jerks have been described. Mutations in the gene encoding the alpha-1 subunit of the glycine receptor (GLRA1) are responsible for the major form of hyperekplexia but no mutation was detected in patients with the minor form in the large Dutch pedigree originally described by Suhren and colleagues. Here we describe the genetic analysis of the GLRA1 gene of two English families in which both forms of hyperekplexia were present. Mutation analysis revealed no genetic defect in the GLRA1 gene in patients carrying either the minor or major forms. This is further evidence that the minor form of hyperekplexia is seldom due to a genetic defect in the GLRA1 gene.

The neuronal channelopathies

This review addresses the molecular and cellular mechanisms of diseases caused by inherited mutations of ion channels in neurones. Among important recent advances is the elucidation of several dominantly inherited epilepsies caused by mutations of both voltage-gated and ligand-gated ion channels. The neuronal channelopathies show evidence of phenotypic convergence; notably, episodic ataxia can be caused by mutations of either calcium or potassium channels. The channelopathies also show evidence of phenotypic divergence; for instance, different mutations of the same calcium channel gene are associated with familial hemiplegic migraine, episodic or progressive ataxia, coma and epilepsy. Future developments are likely to include the discovery of other ion channel genes associated with inherited and sporadic CNS disorders. The full range of manifestations of inherited ion channel mutations remains to be established.

Stiffness, spasticity, or both: a case report of stiff-person syndrome

Stiffness and spasticity are common neurologic symptoms that affect limb movements. We describe a patient who presented with ill-defined stiffness and an exaggerated startle response, who on serial examinations had variable degrees of stiffness and marked hyperreflexia but with plantar flexor signs. Stiff-person syndrome (SPS) was considered when axial stiffness became evident and was confirmed with highly elevated anti-GAD antibody titers. A favorable response to a short course of intravenous immunoglobulin treatment was sustained for more than 10 months, an unusual feature to the disease. We review the clinical features, pathologic mechanism, and treatment of this disorder.

Pseudotumor cerebri manifesting as stiff neck and torticollis

Stiff neck and torticollis are significant signs of neurologic disease. Nuchal rigidity is often associated with meningitis, subarachnoid hemorrhage, and posterior fossa tumor. Torticollis may be encountered in inflammatory disorders, such as cervical lymphadenitis, or it can be a sign of spinal cord syrinx or of central nervous system neoplasm. We report on three prepubertal children in whom stiff neck and torticollis were the presenting signs of pseudotumor cerebri. In all, the removal of 6-7 mL of cerebrospinal fluid led to prompt relief of symptoms and signs. We suggest that in the presence of unexplained stiff neck or torticollis in children, the optic discs should be examined to exclude pseudotumor cerebri.

Successful treatment of severe infantile hyperekplexia with low-dose clobazam

We report two cases of severe infantile hyperekplexia successfully treated with low-dose clobazam. The first case presented at 6 weeks of age with multiple episodes consisting of difficulty diapering because of stiffness and loud inspiratory noises followed by breath-holding in inspiration. She was diagnosed with hyperekplexia and started on clonazepam 0.05 mg daily. This was discontinued because of excessive sleepiness. The second case presented at 3 weeks of age with episodes of crying that would change in pitch and then abruptly stop, followed by leg and arm extension and stiffening. On occasion, there was cyanosis, and she received mouth-to-mouth resuscitation. She was diagnosed with hyperekplexia at 9 months of age. Both infants were treated with clobazam (0.25 and 0.3 mg/kg/day respectively), resulting in resolution of symptoms with no side effects. During treatment, both had minimal startle response to various stimuli and have now been successfully weaned from clobazam. Low-dose clobazam is effective in the treatment of hyperekplexia and is well tolerated in infants.

Paroxysmal dyskinesias as a paradigm of paroxysmal movement disorders

Paroxysmal dyskinesias are genetically and clinically heterogeneous. Paroxysmal kinesigenic choreoathetosis is frequently familial, with autosomal-dominant transmission. Benign infantile convulsions can be observed in these families and both diseases as linked to the pericentromeric region of chromosome 16. Two different forms of paroxysmal dystonic choreoathetosis are distinguished on clinical grounds, by the presence or absence of spasticity, and genetically, as they are linked with loci on different chromosomes. Among the paroxysmal disorders, these diseases may belong to the group of channelopathies.