A Novel Missense Variant in DLG2 /PSD‐93 Disrupts Protein Folding and Binding In Neurodevelopmental Disorders

Abstract

The genetic basis of neurodevelopmental disorders (NDDs) such as autism spectrum disorder and intellectual disability is complex, involving multiple genes with mutations that can be rare, common, de novo or inherited. However, amidst this complexity, genomic studies continue to reveal a convergence of mutations in synaptic genes in NDDs. At excitatory synapses, in the vertebrate brain, the postsynaptic terminal contains highly conserved multiprotein complexes critical for synaptic signalling and behavior. These multiprotein complexes are organised by postsynaptic scaffold proteins, such as PSD-93 (encoded by the DLG2 gene). PSD-93 directly binds and anchors NMDA receptors to the membrane and assembles a downstream network of proteins to form the postsynaptic signalling machinery. We have identified a novel missense variant in DLG2 (R837N mutation in PSD-93) in a family with two brothers diagnosed with intellectual disability and autistic features. Here, we investigated the biological impact of this DLG2/PSD-93 mutation on protein structure and function. We show that the mouse PSD-93 R714N mutation (corresponding to human PSD-93 R837N) did not significantly alter PSD-93 protein expression nor density of PSD-93-positive synapses in mouse hippocampal neurons in vitro. However, protein structural analyses revealed that this mutation impairs PSD-93 protein folding and, as a consequence, decreases protein stability, and disrupts binding of PSD-93 to other key postsynaptic interactors. Our results show that the DLG2/PSD-93 R837N missense variant causes significant changes in the shape, stability and binding of the scaffold protein. This likely affects the structural capacity of PSD-93 to dynamically bind and reorganise the postsynaptic terminal and thus, potentially disrupts synaptic signalling and behaviour. Elucidating the functional impacts of pathogenic disease variants at the structural and molecular levels provide important biological insights into the dysfunctional mechanisms that underlie NDDs.