De Novo TRIO Missense Variants Disrupt Ras‐GEF Domains and Cause Congenital Ventriculomegaly and Hydrocephalus

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Congenital hydrocephalus (CH), characterized by congenital ventriculomegaly (CV), affects approximately 0.5–1 per 1000 live births and is a common cause of pediatric neurosurgical intervention, yet its genetic architecture remains incompletely defined. We report a child with syndromic CH requiring cerebrospinal fluid diversion who harbored a pathogenic de novo missense variant in TRIO (c.3232C > T; p.(Arg1078Trp)), a gene previously associated with autosomal dominant neurodevelopmental disorders featuring variable head circumference. This case prompted systematic evaluation of TRIO variation in our CV/CH cohort (2,697 patient–parent trios) using exome sequencing. We identified five additional unrelated probands with de novo TRIO variants, including two novel substitutions affecting the same residue within the Ras‐GEF1 domain (p.(Glu1299Lys) and p.(Glu1299Gly)), yielding significant gene‐level enrichment for protein‐damaging de novo variants (adjusted p = 6.12 × 10 −5 ). All affected individuals exhibited CV, frequently accompanied by developmental delay and additional structural brain abnormalities. In silico structural modeling predicted that associated variants destabilize critical TRIO Ras‐GEF domains required for Rho GTPase activation. Analysis of single‐nucleus transcriptomic data from the developing human neocortex revealed enrichment of TRIO expression in multipotent progenitor populations. A systematic literature review identified six additional individuals with TRIO de novo variants and reported CV or CH, including an unrelated patient with the same p.(Arg1078Trp) substitution. Together, these findings expand the phenotypic spectrum associated with pathogenic TRIO variation to include CV/CH and support TRIO as a clinically relevant gene in the genetic evaluation of syndromic CV/CH patients.

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