Expansion of NEUROD2 phenotypes to include developmental delay without seizures

Abstract

De novo heterozygous variants in the brain-specific transcription factor Neuronal Differentiation Factor 2 (NEUROD2) have been recently associated with early-onset epileptic encephalopathy and developmental delay. Here, we report an adolescent with developmental delay without seizures who was found to have a novel de novo heterozygous NEUROD2 missense variant, p.(Leu163Pro). Functional testing using an in vivo assay of neuronal differentiation in Xenopus laevis tadpoles demonstrated that the patient variant of NEUROD2 displays minimal protein activity, strongly suggesting a loss of function effect. In contrast, a second rare NEUROD2 variant, p.(Ala235Thr), identified in an adolescent with developmental delay but lacking parental studies for inheritance, showed normal in vivo NEUROD2 activity. We thus provide clinical, genetic, and functional evidence that NEUROD2 variants can lead to developmental delay without accompanying early-onset seizures, and demonstrate how functional testing can complement genetic data when determining variant pathogenicity.

Keywords: Neurod2; developmental delay; seizure.

Figure 1:

Location of reported variants in NEUROD2. Top panel shows location of all reported variants in relation to bHLH (green box) and NEURO bHLH (magenta box) domains. Bottom panel shows close-up of variants found in the bHLH domain, demonstrating the high degree of amino acid conservation across multiple species. Previously reported variants are in black text, variants reported in this paper are in red text. Adapted from Sega and others, 2019.

Figure 2:

Induction of ectopic neurons in vivo with NEUROD2 overexpression. Left panel with neuronal beta-tubulin staining showing normal neuronal configuration (top) and ectopic neuronal induction (bottom). Chart in right panel shows quantification of ectopic neuronal induction for wild type NEUROD2 and both patient variants tested. “n” denotes number of individual embryos scored for phenotype for the indicated variant. Bars at top denote significance of difference as compared to wild type (n.s. = not significant, *** = significant).