Mutations in the glycine receptor alpha1 subunit (GLRA1) gene in hereditary hyperekplexia pedigrees: evidence for non-penetrance of mutation Y279C

Clinical features and genetic analysis of two siblings with startle disease in an Italian family: a case report

Background

Hyperekplexia also known as Startle disease is a rare neuromotor hereditary disorder characterized by exaggerated startle responses to unexpected auditory, tactile, and visual stimuli and generalized muscle stiffness, which both gradually subside during the first months of life. Although the diagnosis of Hyperekplexia is based on clinical findings, pathogenic variants in five genes have been reported to cause Hyperekplexia, of which GLRA1 accounts for about 80% of cases. Dominant and recessive mutations have been identified in GLRA1 gene as pathogenic variants in many individuals with the familial form of Hyperekplexia and occasionally in simplex cases.

Case presentation

In the present study, we describe clinical and genetic features of two Italian siblings, one with the major and one with the minor form of the disease. DNA samples from the probands and their parents were performed by NGS approach and validated by Sanger sequencing. The analysis of the GLRA1 gene revealed, in both probands, compound heterozygous mutations: c.895C > T or p.R299X inherited from the mother and c.587C > A or p.D98E inherited from the father.

Conclusions

Until now, these two identified mutations in GLRA1 have not been reported before as compound mutations. What clearly emerges within our study is the clinical heterogeneity in the same family. In fact, even though in the same pedigree, the affected mother showed only mild startle responses to unexpected noise stimuli, which might be explained by variable expressivity, while the father, showed no clear signs of symptomatology, which might be explained by non-penetrance. Finally, the two brothers have different form of the disease, even if the compound heterozygous mutations in GLRA1 are the same, showing that the same mutation in GLRA1 could have different phenotypic expressions and suggesting an underling mechanism of variable expressivity.

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.

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

Fine architecture and mutation mapping of human brain inhibitory system ligand gated ion channels by high-throughput homology modeling

The common architecture of the brain inhibitory system ligand-gated ion-channels was examined at the level of each of the subunits and in their assembled pentameric arrangements. Structural modeling of the GABAA receptor, GlyR1, and the serotonin receptor, 5HTR3A, was carried out on a multi-homolog basis employing a high-throughput homology modeling pipeline. The locations of all the known mutations of each of the subunits of the receptor subfamily were mapped upon their computed structures and structural relationships between patterns of mutations in different subunits were identified, resulting in the zoning of mutations to four specific regions of the common subunit structure. These classifications may be of value in discerning probable molecular mechanisms and functional manifestations of emerging mutations and polymorphisms, providing the foundation for a family-specific predictive algorithm that may allow researchers to focus experimental effort on the most probable molecular indicators of compromised receptor function and disease mechanism.

A novel GLRA1 mutation associated with an atypical hyperekplexia phenotype

Hyperekplexia (MIM #149400) is a rare neurological disorder characterized by an exaggerated startle response, infantile hypertonia and hyperreflexia without spasticity, a hesitant gait that usually improves by 3 years of age, and nocturnal myoclonus. Familial hyperekplexia is usually autosomal dominant resulting from mutations in the inhibitory glycine receptor subunit alpha 1 (GLRA1) gene on chromosome 5q. We identified a 3-generation family with progressively severe phenotypes of hyperekplexia. All affected family members were found to be heterozygous for a novel arginine271proline mutation in GLRA1. Long-term follow-up of the affected members of the third generation, now aged 6 and 7 years, reveals enhanced startle responses and persistent hypertonia of the extremities without clonus or a catch, tight heel cords and abnormal toe-walking gait, and plantar flexor reflexes. The 7-year-old child recently reponded well to a benzodiazepine. Future studies are warranted to examine whether this new missense mutation is solely responsible for this atypical phenotype.

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.

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.

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.

Statistical coassembly of glycine receptor alpha1 wildtype and the hyperekplexia mutant alpha1(P250T) in HEK 293 cells: impaired channel function is not dominant in the recombinant system

The mutant allele GLRA1 (P250T) of the glycine receptor alpha1 subunit gene underlies a dominant form of the human neurological disorder, hyperekplexia. The encoded substitution, residing within the short intracellular TM1-2 loop, leads to dramatically accelerated desensitization and reduced apparent ligand affinity. Here, dominant negative effects of the alpha1(P250T) subunit on receptor function were studied in a recombinant system. Coexpression of varying ratios of wildtype and mutant cDNA resulted in intermediate desensitization time constants and EC(50) values. The gradual transition of current response properties indicated random coassembly of receptor subunits. Different from the dominant trait of clinical hyperekplexia associated with GLRA1 (P250T), wildtype subunits dominated the functional properties of mixed receptor complexes in the recombinant system.