Early onset or syndromic epilepsy

Gene: GLRA2

Green List (high evidence)

The rating of this gene has been updated to Green and the mode of inheritance set to 'X-LINKED: hemizygous mutation in males, monoallelic mutations in females may cause disease (may be less severe, later onset than males)' following NHS Genomic Medicine Service approval.

Created: 1 Feb 2023, 9:39 a.m. | Last Modified: 1 Feb 2023, 9:39 a.m.

Panel Version: 3.29

Green List (high evidence)

Associated with relevant phenotype in OMIM, but not associated with phenotype in Gen2Phen. PMID: 35294868 reports eight GLRA2 variants in affected females (n=8) and males (n=5). The variants in the females were de novo and c.887C>T,
p.Thr296Met (NC_000023.10, chrX: g.14627284C>T) was present in six individuals (PMID: 35294868, table 2) and was found to have a gain-of-function effect, which is in contrast to c.754C>T, p.Arg252Cys and c.407A>G, p.Asn136Ser (PMID: 2637014). All of the 13 GLRA2 variant carriers in PMID: 35294868 had developmental delay/intellectual disability and epilepsy was evident in 7/13 of the cases (PMID: 35294868, table 2). Supportive functional studies were also presented.

Created: 25 Aug 2022, 2:14 p.m. | Last Modified: 25 Aug 2022, 2:14 p.m.

Panel Version: 2.578

Comment on list classification: There is enough evidence for this gene to be rated GREEN at the next major review.

Created: 25 Aug 2022, 1:45 p.m. | Last Modified: 25 Aug 2022, 1:45 p.m.

Panel Version: 2.578

I don't know

Heterozygous or hemizygous pathogenic GLRA2 variants cause Intellectual developmental disorder, X-linked, syndromic, Pilorge type (# 301076) as summarized in a recent OMIM entry.

The phenotype is characterized by DD with variably impaired intellectual development, behavioral abnormalities (autistic features in some), variable ocular findings (nystagmus, strabismus, oculomotor apraxia) and seizures in some [ 6/13 in Ref4 ].

GLRA2 encodes the α2 subunit that is expressed in embryonic and perinatal CNS with expression decreasing after birth.

Animal models support the role of the gene in CNS.

Studies have been performed for several of the variants reported to date (in all cases missense and a htz deletion of the last 2 exons).

As summarized by OMIM, most affected females carry de novo htz missense GoF variants, and most affected males inherited hemizygous LoF ones.

XCI has not been studied in most htz (affected/unaffected) females (with the exception of the del in Ref2, see also Ref3).

Details provided below.

(Note: Most articles refer to variants using HGVS nomenclature while few without incl. the signal peptide eg. p.Arg350Leu corresponding to Arg323Leu).

Consider inclusion in the current panel with amber/green rating.


[1]----
Piton et al (2011 - PMID: 20479760) sequenced 111 X-linked synaptic genes in a cohort of 142 individuals with ASD and identified a female (S00125) harboring Arg350Leu (NM_002063 chrX:14618871 G/T), inherited from her mother (no clinical information provided). Functional evaluation of the variant was performed in a later publication (Ref3), providing additional clinical details on the proband.

[2]----
Pilorge et al (2016 - PMID: 26370147) review the role of glycine receptors (GlyRs). These typically consist of pentameric combinations of alpha (α1-α4) and beta (β) subunits and form a pore that controls transmembrane flux of chloride. GlyRs can be formed either as homomers comprising five α subunits or as heteromers of α and β subunits (in 2:3 or 3:2 stoichiometry). Each subunit has an N-terminal extracellular domain with the ligand-binding site and 4 transmembrane domains. GLRA2 encodes the α2 subunit that is expressed in embryonic and perinatal CNS with expression decreasing after birth. The authors discuss the role of glycine as inhibitory neurotransmitter in adult CNS and depolarizing/excitatory action in immature neurons, as well as the role of GlyR α2 in proliferation and neuronal migration during cortical development.

The authors previously (2010 - PMID: 20531469) identified a boy with ASD, language delay and low average IQ (verbal 93, performance 75, full-scale IQ 82) harboring a 142 kb microdeletion spanning the last 2 exons of GRLA2 (hg19 - chrX:14693216-14836199). This CNV was confirmed with qPCR and the breakpoints localized to intron 7 after sequencing. Reverse transcription of mRNA from blood revealed presence of a truncated transcript in the child suggestive of little or no NMD. In the mother, the non-truncated transcript was amplified. Further it was shown that the product leaded to incorporation of intron 7, with inclusion of 5 residues followed by a stop codon. The mother had a normal, non-skewed XCI. Previous testing had excluded an FMR1 expansion.

Screening of 400 males with ASD identified a further male with de novo missense SNV (NM_002063.3:c.458G>A / p.Arg153Gln). This child had non-syndromic autism, severe language delay, mild ID (fs IQ 63) and GTC seizures with onset at 18y. Previous testing incl. a normal karyotype, FMR1 analysis, and CMA. The boy had an older sister with ASD, not harboring the same GLRA2 variant (interpreted in the context of intrafamilial genetic heterogeneity for ASD).

The authors also studied a dn missense variant (NM_001118886.1:c.407A>G / p.Asn136Ser) previously reported in a proband with autism (11842.p2 - Iossifov et al, 2014 - PMID: 25363768).

In vitro studies demonstrated that the 3 aforementioned variants impaired GlyR2 α2 function:
- The authors generated constructs for wt, the deletion (of last 2 exons) and Arg153Gln and performed co-transfection with EGFP cDNA in Chinese hamster ovary (CHO) cells. While wt and Arg153Gln were observed at the plasma membrane of transfected cells, the del was undetectable at the cell surface and was mislocalized in the cytoplasm (as also expected by loss of the transmembrane domains).
- Upon isolation of biotinylated surface receptors and western blot, Arg153Gln was shown to result to 56% decreased surface expression compared to wt, while the intracellular fragment was also reduced by 32% suggesting impaired synthesis or degradation. Asn136Ser had 67% lower surface (and 15% lower intracellular) expression.
- Whole-cell patch clamp recordings of transfected CHO cells suggested that the minimum concentration of glycine to evoke whole-cell current was ~100 higher for Arg153Gln compared to wt. High concentrations of glycine were unable to evoke any current in the case of the deletion (due to loss of surface expression). Asn136Ser also reduced glycine sensitivity (14x increase in EC50).

Zebrafish studies for glra2 and the del or Arg153Gln variants:
Morpholino mediated knockdown of glra2 led to hyperbranching of spinal motor axons compared to ctrls. Co-injection of human wt mRNA with glra2 morpholino, rescued the aberrant branching phenotype which was not the case for the 2 variants.

Glra2 ko mouse model (also on chrX):
- Mutant mice (Glra2-/Y) had normal adult body, brain weight, were fertile and had a normal lifespan. They displayed no differences in locomotor activity, or social behavior compared to wt. They however exhibited impaired learning and memory in the novel object recognition task (spatial learning and memory in the novel location recognition task and Morris water maze were N).
- Long-term potentiation in prefrontal cortex after high frequency stimulation was significantly impaired in mutant mice compared to wt, overall supporting that impaired glycinergic signaling results in abnormal synaptic plasticity in this relevant for ASD region.

[3]----
Zhang et al (2017 - PMID: 28588452) determined the functional effects of Arg350Leu which was reported by Piton et al (Arg323Leu without the signal peptide).

The authors provide further clinical details on this female with autism, macrocephaly, loss of acquired words, seizures, mild motor delay and hypothyroidism. The mother of the, also carrier of the SNV, was reportedly unaffected.

The potency of glycine in activating recombinant homomeric α2 and heteromeric α2β receptors was examined by whole-cell patch-clamp recording (HEK293 cells).

In homo-/ and heteromeric receptors this variant resulted in small decrease in glycine sensitivity with peak currents not significantly different compared to wt (the latter suggestive of normal surface expression).

This variant resulted in prolonged inhibitory postsynaptic currents (IPSCs) with ~2-fold slower rise and decay times, while IPSC amplitude did not differ significantly. Overall, the slowed decay times, prolongation of active periods and small but significantly increased conductance of mutant channels suggested that this variant exerts a gain-of-function effect.

The authors briefly cite a study by Cotton et al (2015, PMID: 25381334) providing evidence that GLRA2 escapes XCI in the vast majority of tissues and brain.

[4]----
Marcogliese et al (2022 - PMID: 35294868) functionally tested the effects of missense DNM observed in individuals with ASD diagnosis in Drosophila. The authors generated TG4 (MiMIC cassette) fly mutants for candidate ASD genes (creating LoF alleles for the respective genes). Using a GAL4/UAS system with human cDNA constructs for reference/variants they performed the rescue/overexpression assays to study the functional consequences.

Flies expressing human ref GLRA2 cDNA failed to copulate but exhibited normal movement. Flies for Asn136Ser (a variant reported by Iossifov et al, 2014 - PMID: 25363768) copulated similar to the TG4 mutant providing evidence for a LoF effect of this variant.

Upon GAL4/UAS expression and co-staining with neuronal (Elav) and glial (Repo) nuclear markers, GLRA2 was shown to be expressed in CNS with expression in a subset of neurons and in some glia.

Upon ubiquitous overexpression of human reference or variant cDNA, Asn136Ser also behaved as a LoF allele.

Based on the evidence on this gene, and following re-analyses of exome data, GeneMatcher collaborations etc, the authors identified 13 additional unrelated subjects harboring GLRA2 variants (8 females/5 males). These had DD/ID of variable severity (13/13) w/wo autistic features (in 4 or 5), microcephaly (4-5/13 all females), epilepsy (6/13 - both sexes) and ocular manifestations (10/13 - incl. nystagmus, strabismus, etc). Hypotonia/incoordination was observed in 7/13.

All females had dn missense variants (8/8, NM_001118886.1:c.887C>T/p.Thr296Met in 6/8, others: c.140T>C/p.Phe47Ser, c.777C>G/p.Ile259Met), while all males had inherited missense SNVs from their unaffected mothers (p.Arg252Cys, p.Ala288Thr, p.Pro396Thr, p.Pro400Leu, p.Arg445Gln).

The authors studied an variant which was recurrent in females (Thr296Met) and another found in a male (Arg252Cys). Upon overexpression, the latter behaved - similarly to Asn136Ser - as LoF allele, while Thr296Met did not differ significantly from reference.

Structural modeling suggested that Thr296 is adjacent to a residue important for keeping the ion pore in closed conformation.

Upon pnr-GAL4 (over)expression in the dorsolateral stripe in the notum, Thr296Met caused lethality, which was not the case for the reference. When expressed at lower levels, Thr296Met formation of melanized nodules in thorax, a phenotype not previously observed upon overexpression of ref/other variants.

The authors performed ERGs in fly eyes. They first used a pan-neuronal driver (nSyb-GAL) leading to GLRA2 ref / variant expression in pre-synaptic photoreceptors and post-synaptic neurons. A significant increase of "OFF" transients was observed for Thr296Met, suggesting increase in synaptic transmission and a GoF effect. Expression limited to pre-synaptic photoreceptors (Rh1-GAL4 driver) did not lead to significant differences compared to ref allele, while Arg252Cys was associated with decreased amplitudes of "OFF" transients, suggestive of decreased synaptic transmission and confirming a LoF effect.

Marcogliese et al conclude that reduced GLRA2 activity can lead to disease in males but can be tolerated in htz females (as was the case for asymptomatic mothers), while GoF variants leading to overactivation of the channel could be overrepresented in affected females.
Sources: Literature

Created: 21 Apr 2022, 8:44 a.m.

Mode of inheritance
X-LINKED: hemizygous mutation in males, monoallelic mutations in females may cause disease (may be less severe, later onset than males)

Phenotypes
Global developmental delay; Intellectual disability; Autism; Behavioral abnormality; Seizures; Microcephaly; Abnormality of eye movement

Publications

• 20531469
• 20479760
• 26370147
• 28588452
• 35294868

Mode of pathogenicity
Other