GLRB is the third major gene of effect in hyperekplexia

Abstract

Glycinergic neurotransmission is a major inhibitory influence in the CNS and its disruption triggers a paediatric and adult startle disorder, hyperekplexia. The postsynaptic α(1)-subunit (GLRA1) of the inhibitory glycine receptor (GlyR) and the cognate presynaptic glycine transporter (SLC6A5/GlyT2) are well-established genes of effect in hyperekplexia. Nevertheless, 52% of cases (117 from 232) remain gene negative and unexplained. Ligand-gated heteropentameric GlyRs form chloride ion channels that contain the α(1) and β-subunits (GLRB) in a 2α(1):3β configuration and they form the predominant population of GlyRs in the postnatal and adult human brain, brainstem and spinal cord. We screened GLRB through 117 GLRA1- and SLC6A5-negative hyperekplexia patients using a multiplex-polymerase chain reaction and Sanger sequencing approach. The screening identified recessive and dominant GLRB variants in 12 unrelated hyperekplexia probands. This primarily yielded homozygous null mutations, with nonsense (n = 3), small indel (n = 1), a large 95 kb deletion (n = 1), frameshifts (n = 1) and one recurrent splicing variant found in four cases. A further three cases were found with two homozygous and one dominant GLRB missense mutations. We provide strong evidence for the pathogenicity of GLRB mutations using splicing assays, deletion mapping, cell-surface biotinylation, expression studies and molecular modelling. This study describes the definitive assignment of GLRB as the third major gene for hyperekplexia and impacts on the genetic stratification and biological causation of this neonatal/paediatric disorder. Driven principally by consanguineous homozygosity of GLRB mutations, the study reveals long-term additive phenotypic outcomes for affected cases such as severe apnoea attacks, learning difficulties and developmental delay.

Ethnicity can predict GLRA1 genotypes in hyperekplexia

“…We recently described the clinical features of hyperekplexia associated with GLRB mutations 2. Their reported ethnicities make an interesting comparison, 5/10 cases are of Indian origin, only two are Caucasian and there was the first kindred with Chinese ethnicity.…”

“…Classic (hereditary) hyperekplexia is a glycine synaptopathy caused by recessive and dominant mutations in the α1 and β-subunits of the inhibitory glycine receptor (GlyR; GLRA1 and GLRB )1 2 and the presynaptic glycine transporter 2 (GlyT2; SLC6A5 ) 3. Symptoms are present at birth, and the phenotype includes recurrent apnoea attacks and developmental delay and intellectual disability in some 4.…”

GABA/Glutamate synaptic pathways targeted by integrative genomic and electrophysiological explorations distinguish autism from intellectual disability

“…The p.Arg102Lysfs*6 variation in GLRB was absent in control databases representing more than 7500 individuals (1000 Genomes Project, NHLBI GO Exome Sequencing Project databases), whereas only one truncating single nucleotide variation was found from Exome Sequencing Project in 4294 European American individuals tested, 27 suggesting that such mutations in GLRB are , a rare neuromotor disorder associated with apnoea, learning difficulties and speech delay. 28,29 Heterozygous mutations in GLRB would therefore not be considered as pathogenic per se. However, the GLRB gene encodes the ß sub-unit of the postsynaptic glycinergic receptor expressed at inhibitory synapses, which directly interacts with gephyrin and collybistin proteins that both bind to NLGN4X protein (Figure 1c).…”

Genetics of Pediatric Epilepsy

“…It manifests both dominant and recessive inheritance of mutations in presynaptic and postsynaptic glycinergic genes (GLRA1, SLC6A5/GlyT2, and GLRB). 80 There have been a number of other suspected gene loci associated with other reflex epilepsies, but definitive confirmation of such loci remains the subject of ongoing research. …”