Felix Boschann

Pathogenic variants in TRIO , encoding the guanine nucleotide exchange factor, are associated with two distinct neurodevelopmental delay phenotypes: gain‐of‐function missense mutations within the spectrin repeats are causative for a severe developmental delay with macrocephaly (MIM: 618825 ), whereas loss‐of‐function missense variants in the GEF1 domain and truncating variants throughout the gene lead to a milder developmental delay and microcephaly (MIM: 617061 ). In three affected family members with mild intellectual disability/NDD and microcephaly, we detected a novel heterozygous TRIO variant at the last coding base of exon 31 (NM_007118.4:c.4716G>A). RNA analysis from patient‐derived lymphoblastoid cells confirmed aberrant splicing resulting in the skipping of exon 31 (r.4615_4716del), leading to an in‐frame deletion in the first Pleckstrin homology subdomain of the GEF1 domain: p.(Thr1539_Lys1572del). To test for a distinct gestalt, facial characteristics of the family members and 41 previously published TRIO cases were systematically evaluated via GestaltMatcher. Computational analysis of the facial gestalt suggests a distinguishable facial TRIO‐ phenotype not outlined in the existing literature.