Species-conserved SYNGAP1 phenotypes associated with neurodevelopmental disorders

Abstract

SYNGAP1 loss-of-function variants are causally associated with intellectual disability, severe epilepsy, autism spectrum disorder and schizophrenia. While there are hundreds of genetic risk factors for neurodevelopmental disorders (NDDs), this gene is somewhat unique because of the frequency and penetrance of loss-of-function variants found in patients combined with the range of brain disorders associated with SYNGAP1 pathogenicity. These clinical findings indicate that SYNGAP1 regulates fundamental neurodevelopmental processes that are necessary for brain development. Here, we describe four phenotypic domains that are controlled by Syngap1 expression across vertebrate species. Two domains, the maturation of cognitive functions and maintenance of excitatory-inhibitory balance, are defined exclusively through a review of the current literature. Two additional domains are defined by integrating the current literature with new data indicating that SYNGAP1/Syngap1 regulates innate survival behaviors and brain structure. These four phenotypic domains are commonly disrupted in NDDs, suggesting that a deeper understanding of developmental Syngap1 functions will be generalizable to other NDDs of known or unknown etiology. Therefore, we discuss the known molecular and cellular functions of Syngap1 and consider how these functions may contribute to the emergence of disease-relevant phenotypes. Finally, we identify major unexplored areas of Syngap1 neurobiology and discuss how a deeper understanding of this gene may uncover general principles of NDD pathobiology.

Keywords: Autism spectrum disorder; Circuits; Cognitive impairment; Epilepsy; Intellectual disability; Microcephaly; Neurodevelopment; SynGAP; Synapse; Syngap1.

Figure 1. Evidence that Syngap1 heterozygosity disrupts innate fear within the Elevated Plus Maze

The data shown here reflect a re-analysis of an EPM experiment previously published by our group (Ozkan et al., 2014). Mice were run in a standard 5 min elevated plus maze paradigm. Percent open arm entries were binned for each minute and calculated. A) Open arm entries for EMX1-WT and EMX1-Syngap1 Hets over five one-minute bins. Large overall differences in open arm entries between genotypes (F(1,57)=29.511, p=1.197E-6); RMANOVA. Posthoc comparisons show all bins, except for the first, were significantly different from each other; **p<0.01, ***p<0.001. B) Open arm duration for EMX1-Syngap1 Hets over five one-minute bins. Large overall differences in open arm entries between genotypes (F(1,57)=44.577, p=1.098E-8); RMANOVA. Posthoc comparisons show all bins, including the first, were significantly different from each other; **p<0.01, ***p<0.001. C) Collapsed, integrated heat maps for EMX1-WT and EMX1-Syngap1 Hets showing the cumulative location of animals at two times during the test.

Figure 2. Modeling height-related risk-taking in Syngap1 mice

A) Cliff avoidance task in adult conventional Syngap1 animals. We calculated the fraction of animals remaining on the platform over the course of a ten-minute testing period; chi square, X²(1)= 9.061, p<0.003. B) Modifications to cliff avoidance apparatus made for testing young mice. C) Risk-taking platform with quantifiable risk-taking postures in PND21 mice. D) Top-view heat map showing cumulative location of nose around the platform for each genotype. E) Quantification of risk-taking behaviors. left, number of edge departures during the test; t-test, p<0.001. right, cumulative fraction remaining on the apparatus as animals make their first full departure from the platform; chi square, X²(1)= 8.59, p<0.005.

Figure 3. Brain volume measurements in Syngap1 mice

A) Various MRI sections depicting absolute changes in brain volume in Syngap1 Het mice (n=18) relative to Syngap1 WT (n=16) at PND90. Blue color represents significance of reduced volume across brain areas thresholded at 5% FDR. B) Total brain volume in WT and Het mice. Bar graphs and relative data points depicting the absolute volume change in both mutants and wildtypes [t(27.680) = 4.319 p = .0002]; C) Evaluation of total brain volume analyzing male and female separately. Male [t(16) = 4.977, p = .00014]; Female [t(14) = 2.251, p =.041]

Figure 4. Syngap1 alternative splicing and resulting isoforms

A) Map showing alternative use of exons in N and C-terminal isoforms. N-terminal variants are constituted via use of different start codons in exon 1, 4 or 7. Exon 4 is present only in B-SynGAP. C-terminal isoforms originate from use of different splice acceptors in exon 19 and 21. Exon 20 is included only in γ isoform. B) Schematics of SynGAP isoforms & protein domains. A and B isoforms include full pleckstrin homology (PH) domain. In C-SynGAP, this domain is truncated. Core regions common to all isoforms include C2, GAP, Src Homology 3 (SH3) and coiled-coil (CC) domains. Multiple phosphorylation sites are present downstream of the GAP domain. In the C-terminus, α1 isoforms contain a type-1 PDZ ligand. Structure/function relationships of α2, β, γ isoforms remain largely unknown