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PATH42 Lineage, clinical, genetic, structural and cellular characterisation of a novel epilepsy mutation
  1. J A Johnston,
  2. J S Davies,
  3. K Baer,
  4. C L Hammond,
  5. J G L Mullins,
  6. T D Cushion,
  7. S K Chung,
  8. R H Thomas,
  9. H R Morris,
  10. C White,
  11. P E M Smith,
  12. M I Rees
  1. Wales Epilepsy Research Network, Swansea University, Swansea, UK
  1. Correspondence to jannjohnston{at}yahoo.co.uk

Abstract

Introduction We describe a novel GABAA receptor subunit mutation; GABRG2 R97X, segregating with febrile seizures (FS) in a borderline GEFS+ family; not found in 190 healthy controls. Structural modelling suggested the mutant polypeptide may traffic to the endoplasmic reticulum (ER), accumulate and be degraded.

Methods GABAA receptor subunits were cloned and PCR-directed mutagenesis performed. A neuronal phenotype cell line was cultured and magnetofected with GABRA1, GABRB2, and GABRG2 wild type (WT) or mutant receptor subunits. Surface and intracellular distribution of mutant and WT receptors were determined by co-labelling with anti-GABRG2 and anti-GABRB2,3 subunit antibodies with confocal microscopy.

Results WT GABRG2 fluorescence had a smooth distribution with clusters mainly detected on the cell surface and colocalised with GABRB2 immunoreactivity. Expression of GABRG2 R97X containing receptors resulted in less cell surface cluster immunoreactivity and more diffuse intracellular labelling. GABRG2 R97X immunoreactivity also clumped intracellularly around the nucleus in close proximity to the ER; validating the structural model.

Conclusion GABAA receptor dysfunction represents a putative epilepsy mechanism for GABRG2 R97X; which alters receptor composition and distribution by reducing expression and potentially compromising trafficking. The segregation with FS suggests the mutation may experience impaired temperature dependent trafficking and a reduced sensitivity to diazepam.

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