A novel GABAergic dysfunction in human Dravet syndrome

Objective Dravet syndrome is a rare neurodevelopmental disease, characterized by general cognitive impairment and severe refractory seizures. The majority of patients carry the gene mutation SCN1A, leading to a defective sodium channel that contributes to pathogenic brain excitability. A gamma-aminobutyric acid (GABAergic) impairment, as in other neurodevelopmental diseases, has been proposed as an additional mechanism, suggesting that seizures could be alleviated by GABAergic therapies. However, up to now the physiological mechanisms underlying the GABAergic dysfunction in Dravet syndrome are still unknown due to the scarce availability of this brain tissue. Here we studied, for the first time, human GABA(A)-evoked currents using cortical brain tissue from Dravet syndrome patients. Methods Results We transplanted in Xenopus oocytes cell membranes obtained from brain tissues of autopsies of Dravet syndrome patients, tuberous sclerosis complex patients as a pathological comparison, and age-matched controls. Additionally, experiments were performed on oocytes expressing human alpha 1 beta 2 gamma 2 and alpha 1 beta 2 GABA(A) receptors. GABA(A) currents were recorded using the two-microelectrodes voltage-clamp technique. Quantitative real-time polymerase chain reaction, immunohistochemistry, and double-labeling techniques were carried out on the same tissue samples. We found (1) a decrease in GABA sensitivity in Dravet syndrome compared to controls, which was related to an increase in alpha 4- relative to alpha 1-containing GABA(A) receptors; (2) a shift of the GABA reversal potential toward more depolarizing values in Dravet syndrome, and a parallel increase of the chloride transporters NKCC1/KCC2 expression ratio; (3) an increase of GABA(A) currents induced by low doses of cannabidiol both in Dravet syndrome and tuberous sclerosis complex comparable to that induced by a classical benzodiazepine, flunitrazepam, that still persists in gamma-less GABA(A) receptors. Significance Our study indicates that a dysfunction of the GABAergic system, considered as a feature of brain immaturity, together with defective sodium channels, can contribute to a general reduction of inhibitory efficacy in Dravet brain, suggesting that GABA(A) receptors could be a target for new therapies.