Abstracts

Rationale: Predicting clinical outcomes for patients based on their personal genomes is an emerging challenge. Members of a family segregating a mutation (R43Q) in the γ2 subunit of the GABAA receptor display febrile seizures and typical absence seizures with varying and incomplete penetrance. Here we analyse three different mouse models of GABAA γ2 gene dysfunction to investigate the basis of the phenotypic heterogeneity seen in the family. Methods: Spike and wave discharges (SWDs) and thermal seizure susceptibility were measured in heterozygous GABAA γ2 knock-out and GABAA γ2(R43Q) knock-in mice models crossed to different mouse strains. SWDs were measured on electroencephalograms recorded from freely moving mice for 4h. Thermal seizure susceptibility was determined by measuring the latency to the first tonic-clonic seizure of mice subjected to a constant 42oC temperature. Results: GABAA γ2 knock-out mice spent an identical total time having SWDs compared to the GABAA γ2(R43Q) knock-in mice (2.07 ± 0.16 vs 1.96 ± 0.22%, n=5 and 8, P=0.74). In the thermal stress assay, GABAA γ2(R43Q) knock-in mice developed seizures with a shorter latency than wild-type controls (P=0.0003). In contrast, there was no difference in the latency to seizures of GABAA γ2 knock-out compared to the wild-type littermates (P=0.9492). To test the influence of background genetics on seizure susceptibility we crossed the GABAA γ2(R43Q) knock-in mice into the seizure-prone DBA/2J and relatively seizure resistant C57Bl/6 strains. SWD expression was robust in the DBA/2J (n=40/40) but this seizure phenotype was never observed in the C57Bl/6 strain (n=0/10). In contrast, thermal seizure susceptibility was equally robust for the GABAA γ2(R43Q) knock-in in both strains (P<0.05 for both). Conclusions: Our results show that a single gene mutation can cause distinct seizure phenotypes through independent molecular mechanisms. Haploinsufficiency underlies the genesis of absence seizures, but cannot account for the thermal seizure susceptibility. A lack of effect of genetic background on thermal seizure susceptibility is consistent with the higher penetrance of febrile seizures compared to absence seizures seen in family members with the mutation. These mouse studies help to provide a conceptual framework within which clinical heterogeneity seen in genetic epilepsy can be explained.