Genetic, molecular and mouse model investigations of broad neurodevelopmental impact of deleterious variants of the TREX mRNA export complex subunits.

mRNA export is mediated by the highly conserved multisubunit TREX (Transcription-Export) complex. We studied >60 deleterious variants in THOC2 subunit of TREX in neurodevelopmental disorders (NDD) with intellectual disability (ID) as the core phenotype. We also identified variants in other TREX subunits THOC1,4,5,6 and 7 in children with NDDs. We show that THOC2 protein stability is reduced in patient cells with missense variants and splicing and deletion variants lead to truncated C-terminal RNA binding domain. We generated a CRISPR-Cas9-edited Thoc2 Exon37-38 deleted mouse (del37-38) model inspired by a patient with ID, speech delay, hypotonia, and microcephaly. Like the patient, the del37-38 male mice are proportionally smaller and lighter (~15%) compared to their wild-type littermates. We observed a truncated Thoc2 protein that accumulates in the del37-38 male mice brain, eye, and lungs. Behavioral testing (n=14 mice/genotype) using multiple protocols revealed significant deficit in spatial learning and working memory. Histological investigations of E18.5 embryonic and adult mouse brains show significantly compressed cortical (ventricular and marginal zones of E18.5 mice brain) and corpus callosum architecture. In vitro primary cortical neuron and neural stem cell (NSC) cultures from E18.5 embryonic brains of del37-38 males show shorter primary axons and sub-optimal neural migration. Intriguingly, NSCs from del37-38 male mice show higher proliferation rate and premature differentiation but significantly increased cell death. Our combined patient cell, molecular, and Thoc2 mouse model data suggest novel function(s) and specific neurodevelopmental impact of compromised TREX mRNA export complex function.