An Epilepsy-Related ARX Polyalanine Expansion Modifies Glutamatergic Neurons Excitability and Morphology Without Affecting GABAergic Neurons Development

Epileptic encephalopathies comprise a heterogeneous group of severe infantile disorders for which the pathophysiological basis of epilepsy is inaccurately clarified by genotypephenotype analysis. Because a deficit of GABA neurons has been found in some of these syndromes, notably in patients with X-linked lissencephaly with abnormal genitalia, epilepsy was suggested to result from an imbalance in GABAergic inhibition, and the notion of interneuronopathy was proposed. Here, we studied the impact of a polyalanine expansion of aristaless-related homeobox (ARX) gene, a mutation notably found in West and Ohtahara syndromes. Analysis of Arx((GCG)7/Y) knock-in mice revealed that GABA neuron development is not affected. Moreover, pyramidal cell migration and cortical layering are unaltered in these mice. Interestingly, electrophysiological recordings show that hippocampal pyramidal neurons displayed a frequency of inhibitory postsynaptic currents similar to wild-type (WT) mice. However, these neurons show a dramatic increase in the frequency of excitatory inputs associated with a remodeling of their axonal arborization, suggesting that epilepsy in Arx((GCG)7/Y)mice would result from a glutamate network remodeling. We therefore propose that secondary alterations are instrumental for the development of disease-specific phenotypes and should be considered to explain the phenotypic diversity associated with epileptogenic mutations.