SAHA (Vorinostat) Corrects Inhibitory Synaptic Deficits Caused by Missense Epilepsy Mutations to the GABA Receptor γ2 Subunit.

The GABA receptor (GABAR) α1 subunit A295D epilepsy mutation reduces the surface expression of α1β2γ2 GABARs via ER-associated protein degradation. Suberanilohydroxamic acid (SAHA, also known as Vorinostat) was recently shown to correct the misfolding of α1 subunits and thereby enhance the functional surface expression of α1β2γ2 GABARs. Here we investigated whether SAHA can also restore the surface expression of γ2 GABAR subunits that incorporate epilepsy mutations (N40S, R43Q, P44S, R138G) known to reduce surface expression via ER-associated protein degradation. As a control, we also investigated the γ2 epilepsy mutation that impairs gating without reducing surface expression. Effects of mutations were evaluated on inhibitory postsynaptic currents (IPSCs) mediated by the major synaptic α1β2γ2 GABAR isoform. Recordings were performed in neuron-HEK293 cell artificial synapses to minimise contamination by GABARs of undefined subunit composition. Transfection with α1β2γ2 , α1β2γ2 , α1β2γ2 and α1β2γ2 subunits produced IPSCs with decay times slower than those of unmutated α1β2γ2 GABARs due to the low expression of mutant γ2 subunits and the correspondingly high expression of slow-decaying α1β2 GABARs. SAHA pre-treatment significantly accelerated the decay time constants of IPSCs consistent with the upregulation of mutant γ2 subunit expression. This increase in surface expression was confirmed by immunohistochemistry. SAHA had no effect on either the IPSC kinetics or surface expression levels of α1β2γ2 GABARs, confirming its specificity for ER-retained mutant γ2 subunits. We also found that α1β2γ2 GABARs and SAHA-treated α1β2γ2 , α1β2γ2 and α1β2γ2 GABARs all mediated IPSCs that decayed at significantly faster rates than wild type receptors as temperature was increased from 22 to 40°C. This may help explain why these mutations cause febrile seizures (FS). Given that SAHA is approved by therapeutic regulatory agencies for human use, we propose that it may be worth investigating as a treatment for epilepsies caused by the N40S, R43Q, P44S and R138G mutations. Although SAHA has already been proposed as a therapeutic for patients harbouring the α1 epilepsy mutation, the present study extends its potential utility to a new subunit and four new mutations.