TRIO is among the genes most frequently impacted by damaging de novo mutations and rare variants in individuals with autism spectrum disorder (ASD), severe intellectual disability (ID), schizophrenia (SCZ), bipolar disorder (BPD), and encephalopathic epilepsy (EE). TRIO encodes a large protein with three catalytic domains—two guanine nucleotide exchange factor domains (GEF1 and GEF2) and a kinase domain—as well as additional lipid- and protein-interaction motifs. In neurons, TRIO localizes to axons, dendrites and synapses, where it acts downstream of cell surface receptors to control axon and dendrite outgrowth and pathfinding, synapse development and synaptic transmission1.
An intriguing feature of TRIO biology is that distinct patterns of single nucleotide variants are associated with different conditions2–4. Specifically, (i) de novo missense mutations and rare variants that impair TRIO GEF1 function are enriched in individuals with ASD and ID, suggesting that decreased TRIO GEF1 function underlies the changes in TRIO-associated ASD; (ii) rare variants in SCZ that create nonsense mutations are found throughout the TRIO gene and are likely loss-of-function due to nonsense-mediated mRNA decay; and (iii) rare variants predicted to disrupt function of the GEF2 and kinase domains are found in BPD and EE.
Building on these observations, Anthony Koleske and colleagues hypothesize that TRIO mutations found in ASD drive biochemical changes that are distinct from TRIO mutations in other neurodevelopmental and psychiatric disorders and yield distinct anatomical, physiological and behavioral phenotypes. This hypothesis leads to fundamental questions regarding TRIO function in ASD, including: (i) what are the key biochemical signaling events that are disrupted by ASD-related TRIO mutations during development?; (ii) at what developmental time period and in what brain regions/circuits are these events disrupted?; and (iii) how do these specific alterations in TRIO-regulated signaling events contribute to specific anatomical and behavioral phenotypes?
In pilot studies, Koleske’s team have demonstrated that unbiased proteomic analyses can be used to identify proteins and signaling events altered by reduced Trio function5. Their preliminary data also show that Dscam, Ank2 and Stxbp1—all high-risk ASD genes—are reduced in the Trio haploinsufficient mouse cortex, enabling them to address how altered Trio interacts functionally with the gene products of other ASD risk genes. They also assessed anatomical, electrophysiological and behavioral phenotypes that resulted from reduced Trio function in cortical and hippocampal excitatory neurons of mice and showed that some of these phenotypes can be rescued by restoration of a Trio-dependent pathway (involving phosphodiesterase 4A5 signaling) that was identified in the proteomic analysis5.
Koleske and colleagues have also generated and are characterizing an isogenic set of CRISPR/Cas-engineered mice bearing a Trio de novo missense mutation (K1431M) found in individuals with ASD and other conditions. In the current project, they plan to systematically use these mouse models to address the aforementioned fundamental questions about Trio function as it relates to ASD and other neurodevelopmental disorders.