Synaptic dysfunction in neurodevelopmental disorders associated with autism and intellectual disabilities

Abstract

The discovery of the genetic causes of syndromic autism spectrum disorders and intellectual disabilities has greatly informed our understanding of the molecular pathways critical for normal synaptic function. The top-down approaches using human phenotypes and genetics helped identify causative genes and uncovered the broad spectrum of neuropsychiatric features that can result from various mutations in the same gene. Importantly, the human studies unveiled the exquisite sensitivity of cognitive function to precise levels of many diverse proteins. Bottom-up approaches applying molecular, biochemical, and neurophysiological studies to genetic models of these disorders revealed unsuspected pathogenic mechanisms and identified potential therapeutic targets. Moreover, studies in model organisms showed that symptoms of these devastating disorders can be reversed, which brings hope that affected individuals might benefit from interventions even after symptoms set in. Scientists predict that insights gained from studying these rare syndromic disorders will have an impact on the more common nonsyndromic autism and mild cognitive deficits.

Figure 1.

The promise of molecular medicine in genetically defined disorders of brain development.

Figure 2.

(A) Schematic of a neuron and axonal-dendritic synapse that depict examples of cellular localization of the various types of defects in ASD/ID. (B) A signaling pathway at the excitatory synapses that couples activity as registered by the release of glutamate to local control of protein synthesis. Disruption of the gene products indicated in the colored boxes greatly increases the risk of ASD/ID. Syndromic disorders with increased prevalence of ASD include Phelan-McDermid Syndrome (SHANK3); Noonan syndrome (RAF1, MEK1); Neurofibromatosis type 1 (NF1); Costello syndrome (H-Ras, MEK1); Cowden syndrome (PTEN); Cardio-facio-cutaneous (CFC) syndrome (MEK1/2); Tuberous sclerosis complex (TSC1/2); Fragile X syndrome (FMRP); Angelman syndrome (AS UBE3a); Rett syndrome (RTT–MeCP2); and Rubinstein-Taybi syndrome (RTS–CREB binding protein, p300). Rare, nonsyndromic ASDs include NLGN3/4 and NRXN1; ID/ASD: SHANK2.

Figure 3.

Gain or loss of function of individual genes often yields an overlapping behavioral phenotype in humans that includes ASD and ID. The same appears to be true of a physiological process such as local synaptic protein synthesis, in which too much or too little can be manifest in similar ways. We propose that optimal synaptic function occurs within a limited dynamic range, and the pathophysiology at both ends of this range can cause autistic behavior and intellectual disability.