1. Introduction

• GABRB3 gene is a recently identified gene located in 15q12 chromosome and encodes encodes the β3-subunit of the GABA-A receptor, a ligand-gated chloride channel.
• The gene is believed to share a role in inhibitory GABAergic synapses, GABA iron-gated channel function, and possible cellular response to histamine.
• The β3 subunit is expressed in cerebral grey matter, thalami, hippocampi, and cerebellum, among other structures.
• GABA-A receptors mediate fast inhibitory neurotransmission in the central nervous system (CNS).
• Faulty GABRB3 function is linked to several neurological disorders and clinical syndromes. However, the spectrum of such disorders is not yet well known
• Mutations in GABRB3 are emerging as a significant cause of early infantile epileptic encephalopathy (EIEE), a severe developmental and epileptic encephalopathy.

2. GABA-A Receptor: Structure and Function

• Composition: Pentameric assembly of subunits (α, β, γ, δ, etc.). Typical GABA-A receptors have:
  • 2 α-subunits
  • 2 β-subunits (e.g., GABRB3)
  • 1 γ-subunit
• Ion Channel:
  • Formed by transmembrane domains M1–M4 of each subunit.
  • The M2 domain lines the ion channel pore.
• Function:
  • GABA binding leads to chloride influx (hyperpolarization → neuronal inhibition).
  • In immature neurons, GABA causes chloride efflux (depolarization → excitation) due to NKCC1 transporter activity​

3. Clinical Relevance of GABRB3

• Associated Disorders:
  1. Early infantile epileptic encephalopathy (EIEE):
     • Severe seizure disorders starting <10 months of age.
     • Variable EEG abnormalities, neurodevelopmental delays.
  2. Angelman Syndrome:
     • Chromosomal deletions involving 15q11.2-q12 (where GABRB3 is located).
     • Clinical features: Epilepsy, intellectual disability, ataxia, and happy demeanor.
  3. Childhood Absence Epilepsy:
     • Polymorphisms in GABRB3 reduce receptor transcriptional activity.
  4. Autism Spectrum Disorder (ASD):
     • GABRB3 variants are implicated in ASD pathogenesis.

4. GABRB3 Mutations and Epilepsy

• Mutation Characteristics:
  • Typically de novo missense mutations in highly conserved regions.
  • Most mutations occur in:
    • M2 transmembrane domain (critical for ion flux).
    • Extracellular loops (e.g., near the amino terminus)​
• Reported Mutations:
  • Mutations affect receptor gating, chloride conductance, and cellular trafficking​
  • Table of key mutations (Papandreou A et al 2016):

Key Point: The p.Thr287Ile mutation, located in the M2 domain, impacts ion pore function and likely reduces chloride flux, causing hyperexcitability​

5. Pathophysiology

• GABRB3 mutations disrupt the function of GABA-A receptors by:

  1. Impaired chloride ion flow: Decreased inhibitory signaling → epileptogenesis.
  2. Hyperglycosylation: Abnormal protein folding and trafficking​
  3. Haploinsufficiency: Reduced expression leads to loss of receptor function.

• Mechanisms of Hyperexcitability:

  • Reduced inhibition in mature neurons.
  • Increased excitation in immature neurons due to chloride efflux.

6. Phenotypic Spectrum of GABRB3 Mutations

GABRB3 mutations demonstrate a wide phenotypic range, including:

1. Febrile Seizures (FS): Simple FS to genetic epilepsy with FS plus (GEFS+).
2. Generalized Epilepsies:
   • Early-onset absence epilepsy (EOAE).
   • Myoclonic-atonic epilepsy (MAE).
3. Severe Epileptic Encephalopathies (EEs):
   • West Syndrome (WS).
   • Lennox-Gastaut syndrome (LGS).
   • Early-onset epileptic encephalopathy (EOEE).
   • Dravet Syndrome (DS)-like phenotypes.

Age at Onset:

• Median onset at 8.5 months (range: 1 day–36 months)​

7. Clinical Findings in GABRB3 Patients

• Seizure Types:
  • Febrile seizures, focal seizures, generalized tonic-clonic seizures (GTCS).
  • Infantile spasms (IS), myoclonic seizures, atonic seizures, and tonic seizures.
• Intellectual Disability (ID):
  • Mild to severe ID is common in EEs.
  • Patients with GEFS+ or EOAE may have normal intellect.
• Behavioral Features:
  • Hyperactivity, aggression, and autistic traits occur in ~36% of cases.
• MRI Abnormalities:
  • Hypomyelination.
  • Brain atrophy.
  • Heterotopias (e.g., bifrontal heterotopia in multifocal epilepsy cases).

8. Mutation Characteristics

1. Mutation Types:

   • Missense mutations dominate.
   • Truncating mutations and duplications are rare.

2. Recurrent Mutations:

   • Some mutations are recurrent across unrelated patients, such as:
     • p.Y302C: Associated with focal epilepsy and severe LGS.
     • p.R232Q: Linked to treatable multifocal epilepsy with moderate ID.
   • These recurrent sites suggest a hotspot for pathogenicity​

3. Inheritance:

   • ~64% are de novo mutations.
   • A smaller percentage are inherited in an autosomal dominant fashion.
   • Parental mosaicism may contribute to familial cases​

9. Functional Consequences of GABRB3 Mutations

Mutations in GABRB3 impair receptor function via:

1. Loss of GABA-Evoked Current:
   • Mutations like p.V37G, p.Y184H, and p.Y302C significantly reduce GABA-induced currents.
2. Reduced GABA Sensitivity:
   • Mutations such as p.Y302C and p.Y184H cause a rightward shift in dose-response curves, reflecting decreased GABA affinity​ .

Mechanism:

• GABAergic Disinhibition: Reduced chloride influx → Hyperexcitability → Epileptic seizures.

10. Diagnostic Tools

• Genetic Testing:
  • Multiple gene panels (e.g., 48-gene epilepsy panel).
  • Whole-exome sequencing (WES) for broader identification.
• EEG Findings:
  • Varied patterns: Generalized fast activity, multifocal spikes.
• Neuroimaging:
  • MRI is often normal in GABRB3-related epileptic encephalopathy.

11. Therapeutic Implications

• Current Management:
  • Antiepileptic drugs (AEDs): Sodium valproate, benzodiazepines.
  • Ketogenic diet
  • Seizure control remains challenging due to pharmacoresistance.
• Future Directions:
  • Targeted therapies to restore GABA-A receptor function.
  • Precision medicine approaches involving GABA modulation.

12. Research Gaps and Future Work

• Phenotypic Variability: Further studies to correlate mutation location with clinical severity.
• Disease Mechanism: Functional studies on chloride conductance, trafficking.
• Novel Therapies: Exploration of drugs targeting GABA-A receptor dysfunction in epileptic encephalopathies.

13. Conclusion

• GABRB3 mutations are an emerging cause of severe epileptic encephalopathies with broad phenotypic overlap with developmental disorders like autism and Angelman syndrome.
• GABRB3 mutations cause a spectrum of disorders from mild febrile seizures to severe epileptic encephalopathies.
• The disease mechanism involves GABAergic disinhibition through loss of receptor function or reduced GABA sensitivity.
• Advances in genetic testing allow early diagnosis and intervention.
• Recurrent mutations like p.Y302C and p.R232Q highlight the pleiotropic nature of GABRB3 variants.
• Advances in genetic sequencing will allow further identification and delineation of GABRB3-related disorders, improving diagnostic accuracy and paving the way for personalized therapies​

14. References

Papandreou A, McTague A, Trump N, Ambegaonkar G, Ngoh A, Meyer E etal (2016) GABRB3 mutations: a new and emerging cause of early infantile epileptic encephalopathy. Dev Med Child Neurol 58 (4):416-20. DOI: 10.1111/dmcn.12976 PMID: 26645412.

Absalom NL, Liao VWY, Johannesen KMH, Gardella E, Jacobs J, Lesca G etal (2022) Gain-of-function and loss-of-function GABRB3 variants lead to distinct clinical phenotypes in patients with developmental and epileptic encephalopathies. Nat Commun 13 (1):1822. DOI: 10.1038/s41467-022-29280-x PMID: 35383156.