Hyperekplexia: A forgotten diagnosis clinched by next-generation sequencing
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/neuroindia.NI_851_16
Source of Support: None, Conflict of Interest: None
Hyperekplexia is a rare early neonatal onset, potentially treatable, neurological disorder, characterized by a triad of immediate neonatal-onset stiffness, an exaggerated startle reflex in response to tactile or auditory stimuli followed by short periodical generalized stiffness. It is a monogenic genetically heterogeneous condition which can be potentially life threatening due to apneic episodes and is usually misdiagnosed as seizures. Here, we report two female siblings with hyperekplexia who were being treated by multiple antiepileptic medications for seizure-like episodes. Hyperekplexia was diagnosed by next-generation sequencing, which has emerged as a powerful diagnostic tool over the last few years.
Keywords: GLRA1, hyperekplexia, next-generation sequencing, seizure
Congenital hyperekplexia is a rare, potentially treatable neuromotor disorder with episodic stiffness and excessive startle response to auditory and tactile stimuli. Due to its seizure-like presentation, it is usually misdiagnosed. Approximately 200 genetically proven cases have been described in literature since the first gene of effect (GLRA1) was described.,, We report the case study of two sisters from a consanguineous family with autosomal recessive form of hyperekplexia, which was identified by exome sequencing.
Two female siblings born to third-degree consanguineous parents were being followed-up since the last 14 years. The 17-year old younger sister had mild intellectual disability, learning difficulties, and seizures (controlled by medication). The phenotype was similar, albeit milder in the 21-year old elder sister. The younger girl had presented to us at the age of 7 days with infantile spasms and exaggerated startle responses when she and her elder sibling were extensively investigated and put on multiple antiepileptic medications.
Now, the parents approached us again concerned about the recurrence of this condition in their family. The sequencing of clinically relevant genes (clinical exome sequencing) by the method of next-generation sequencing (NGS) of both the sisters was done using the Illumina sequencing platform.
NGS, also called as massively parallel sequencing and high-throughput sequencing, is the non-Sanger-based sequencing in which billions of deoxyribonucleic acid (DNA) strands can be sequenced in parallel, leading to a high throughput, and faster and cheaper sequencing of genome. NGS can be whole genome or exome sequencing. For most clinical purposes, clinical exome, which involves sequencing of the protein coding part of DNA (exons) is preferred, as we performed in our patients.
DNA extracted from blood was used to perform targeted gene capture using a custom capture kit. The libraries were sequenced to mean >80–100 × coverage on Illumina sequencing platform. The sequences obtained were aligned to human reference genome (GRCh37/hg19) using the Burrows-Wheeler Aligner (BWA) program and analyzed using Picard and Genome analysis toolkit (GATK)-Lite toolkit to identify variants in the exome relevant to clinical indication. Annotation of the variant was performed against the Ensembl release 75 gene model. Clinically relevant mutations were annotated using published variants in the literature and a set of variant databases including ClinVar, Online Mendelian inheritance in Man (OMIM), genome-wide association studies (GWAS), human gene mutation database (HGMD), and SwissVar. Only nonsynonymous and splice site variants found in the genes relevant to the clinical symptoms were used for clinical interpretation.
A homozygous 3’ splice site variation in the GLRA1 gene (chr5:151235945; C>T) that affects the invariant CT acceptor splice site of exon 5 (c.477-1G>A; ENST00000455880) was detected [Figure 1]. The amplification and direct Sanger sequencing of the GLRA1 gene of the proband as well as her similarly affected sister revealed the same homozygous mutation, i.e., c.477-1G>A. The Sanger sequencing of the unaffected father and brother revealed a heterozygous mutation, i.e., c.477-1G>A. GLRA1(OMIM*138491). These mutations are associated with hyperekplexia-1 (OMIM#149400). This 3'splice variation is considered pathogenic because it is not present in the 1000 genomes database and has a minor allele frequency of less than 0.001% in the ExAC database. It is predicted to be damaging by the pathogenicity prediction software such as Mutation Taster and Human Splice Finder, and the region is conserved across species. Moreover, as discussed, the presence of this variant in both affected sisters and its absence in the unaffected family members further corroborate our findings.
Both the affected sisters had the characteristic clinical features of hyperekplexia in the form of stiffness which gradually waned off, and the most characteristic exaggerated startle response, without loss of consciousness, to auditory and tactile stimuli. Yet the clinical diagnosis of hyperekplexia was missed. The similarity of hyperekplexia with seizures can be cited for the somewhat surprisingly late description of this clinically recognizable entity that was initially reported in 1958 by Kirstein and Silfverskiold. There are only two reports of cases with hyperkplexia from India suggesting that the lack of awareness of the disorder may be the cause of its infrequent diagnosis.,
Hyperekplexia is an early neonatal-onset paroxysmal neurological disorder characterized by a triad of immediate neonatal-onset stiffness of prolonged periods which increases on handling and disappears during sleep, an excessive and unduly exaggerated startle reflex in response to tactile or auditory stimuli, followed by short periodical generalised stiffness in which no voluntary movement is possible. Shiang et al., in 1993 were the first to show that hyperekplexia is caused by hereditary mutations in the GLRA1 gene that encodes the α1 subunit of the inhibitory human glycine receptor chloride channel. The other causative genes are SLC6A5, which encode the presynaptic glycine transporter type-2 channel, and GLRB, which encodes the β subunit of the inhibitory human glycine receptor chloride channel., No other phenotypes are associated with the mutation of GLRA1, SLC6A5, or GLRB. In the largest cohort study of hyperekplexia till now, 62.8% cases were GLRA1 mutation positive, 24.7% SLC6A5 mutation positive, and 12.4% GLRB mutation positive.
The classic startle response in hyperekplexia is characterized by forceful closure of eyes, rising of bent arms over the head, and flexion of the neck, trunk, elbows, hips, and knees. The frequency of startle responses varies in individuals, and nervousness, anxiety, and fatigue are common precipitation factors for the startle response. These can be observed during examination by simple gentle nose tapping in affected children. The stiffness and startle episodes pose a threat to life as they can lead to injuries, hypoxic episodes, brain injury, and sudden infant death syndrome due to laryngospasm and cardiac arrest. The stiffness usually diminishes post infancy; however, the exaggerated startle responses can persist into adulthood leading to morbidity and mortality. The mainstay of treatment in hyperekplexia has been clonazepam, an allosteric potentiator of GABAA receptors, which is efficacious for patients with GLRA1, GLRB, and SLC6A5 mutations. Other antiepileptic drugs have shown variable efficacy. Vigevano manoeuvre can stop the attacks of tonic neonatal cyanosis, which consists of forced flexion of the head and legs towards the trunk. The intellectual disability and learning difficulties demonstrated in some patients with hyperekplexia are not due to apneic attacks but may represent subtle alterations in neuronal migration and targeting.
Missense, nonsense mutations and homozygous deletion of the first seven GLRA1 exons in GLRA1 have been identified in autosomal dominant, recessive, or compound heterozygote hyperekplexia. To our knowledge, no splice site variant has been reported as a causative factor of hyperekplexia. The novel homozygous splice site variant identified in both of our patients [homozygous 3’ splice site variation in the GLRA1 gene (chr5:151235945; C>T)] that affects the invariant CT acceptor splice site of exon 5 was predicted to be damaging (alteration within the used splice site is likely to disturb the normal splicing) by software prediction tools. This splice site change leads to loss of extracellular domain of transmembrane protein and probable alteration of other transmembrane complexes of glycine receptors. Such alterations of the transmembrane proteins have been reported in various recessive forms of hyperekplexia.
Though we have not undertaken functional studies, we consider the above variant to be likely pathogenic due to strong clinical correlation, the variant being present in the homozygous form in both the affected sisters, and absence of the variant in the homozygous form in unaffected family members.
This case emphasizes the power of exome sequencing in the diagnosis of genetic disorders of heterogeneous etiologies and reinforces the fact that hyperekplexia can be easily missed; hence, a high index of suspicion should be maintained when parents report abnormally stiff babies with exaggerated startle responses or seizures with no loss of consciousness. The timely diagnosis of hyperekplexia will allow families to take adequate preventive measures; moreover, prenatal diagnosis can be provided to families when mutation is identified in the proband.
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