SCN2A A1659V loss-of-function variant causes early infantile onset encephalopathy

Authors:
Lisastella Morinelli (1,2), Martina Albini (1,2), Loretta Ferrera (3,4), Alessandra Bianchi (1), Bruno Sterlini (1,2), Antonella Riva (3,4), Alessandra Ludovico (3), Giulia Lombardo (5), Francesca Madia (3), Raffaele Falsaperla (5), Federico Zara (3,4), Anna Corradi (1,2)

Organisations:
1: Department of Experimental Medicine, University of Genoa, Genoa, Italy
2: Department of Neuroscience and Brain Technologies, Fondazione Istituto Italiano di Tecnologia, Genoa, Italy
3: Unit of Medical Genetics, IRCCS Istituto Giannina Gaslini, Genoa, Italy
4: Department of Neurosciences, Rehabilitation, Ophthalmology, Genetics, Maternal and Child Health, University of Genoa, Genoa, Italy
5: General Pediatrics and Pediatric Acute and Emergency Unit, Neonatal Intensive Care Unit and Neonatal Accompaniment Unit, Azienda Ospedaliero-Universitaria Policlinico-San Marco, San Marco Hospital, University of Catania, Catania, Italy

Presenting author:
Lisastella Morinelli

Exposition date:
Friday, September 15, 2023

Exposition position:
068/1

Pathogenic variants in SCN2A, gene encoding for the α-subunit type 2 of the voltage-gated Sodium channel Nav1.2, are known to cause a phenotypic spectrum including epileptic encephalopathy and intellectual disability or autism without epilepsy. Gain-of-function (GOF) mutations’ resulting phenotypes may be treated with Sodium channels blockers, while loss-of-function conditions are non-respondent.

We focused on the effects of the pathogenic SCN2A variant c.2976C>T (p.A1659V) found, by Next Generation Sequencing, at a mosaic state in an infant affected by Developmental Epileptic Encephalopathy non respondent to carbamazepine.

To study the functional effects of this variant, we generated by site-directed mutagenesis a plasmid encoding for the A1659V mutated SCN2A and demonstrated that this mutation does not impact on channel expression and subcellular localization in HEK293 cells by Western blot and immunofluorescence assays.

Whole-cell patch-clamp recordings showed a reduced Na+ current in HEK293 cells with overexpression of the A1659V channel compared to the one generated by the wild-type channel. The quantitative analysis of A1659V channel activation properties showed a shift of the activation curve towards more negative potentials and a time constant slower than the one of the wild-type channel. These results explain the loss-of-function (LOF) mechanism underlying the epileptic phenotype of patients and help define the genotype-phenotype correlation with the ultimate goal of improving therapeutic strategies to treat epileptic syndromes.