Dravet Syndrome-Associated Mutations In Gabra1, Gabrb2 And Gabrg2 Define The Genetic Landscape Of Defects Of Gaba(A) Receptors

Dravet syndrome is a rare, catastrophic epileptic encephalopathy that begins in the first year of life, usually with febrile or afebrile hemiclonic or generalized tonic-clonic seizures followed by status epilepticus. De novo variants in genes that mediate synaptic transmission such as SCN1A and PCDH19 are often associated with Dravet syndrome. Recently, GABAA receptor subunit genes (GABRs) encoding alpha 1 (GABRA1), beta 3 (GABRB3) and gamma 2 (GABRG2), but not beta 2 (GABRB2) or beta 1 (GABRB1), subunits are frequently associated with Dravet syndrome or Dravet syndrome-like phenotype. We performed next generation sequencing on 870 patients with Dravet syndrome and identified nine variants in three different GABRs. Interestingly, the variants were all in genes encoding the most common GABAA receptor, the alpha 1 beta 2 gamma 2 receptor. Mutations in GABRA1 (c.644T>C, p. L215P; c.640C>T, p. R214C; c.859G>A; V287I; c.641G>A, p. R214H) and GABRG2 (c.269C>G, p. T90R; c.1025C>T, p. P342L) presented as de novo cases, while in GABRB2 two variants were de novo (c.992T>C, p. F331S; c.542A>T, p. Y181F) and one was autosomal dominant and inherited from the maternal side (c.990_992del, p.330_331del). We characterized the effects of these GABR variants on GABAA receptor biogenesis and channel function. We found that defects in receptor gating were the common deficiency of GABRA1 and GABRB2 Dravet syndrome variants, while mainly trafficking defects were found with the GABRG2 (c.269C>G, p. T90R) variant. It seems that variants in alpha 1 and beta 2 subunits are less tolerated than in gamma 2 subunits, since variant alpha 1 and beta 2 subunits express well but were functionally deficient. This suggests that all of these GABR variants are all targeting GABR genes that encode the assembled alpha 1 beta 2 gamma 2 receptor, and regardless of which of the three subunits are mutated, variants in genes coding for alpha 1, beta 2 and gamma 2 receptor subunits make them candidate causative genes in the pathogenesis of Dravet syndrome.