Expanding the genotype and phenotype spectrum of SYT1-associated neurodevelopmental disorder

Synaptotagmin-1 (SYT1) is a critical mediator of neurotransmitter release in the central nervous system. Previously reported missense SYT1 variants in the C2B domain are associated with severe intellectual disability, movement disorders, behavioural disturbance and EEG abnormalities. Here, we expand the genotypes and phenotypes and identify discriminating features of this disorder. We describe 22 individuals with 15 de novo missense SYT1 variants. Evidence for pathogenicity is discussed, including ACMG criteria, known structure-function relationships, and molecular dynamics simulations. Quantitative behavioural data for 14 cases are compared to other monogenic neurodevelopmental disorders. Four variants lie in the C2A domain with the remainder in the C2B. We classify 6 variants as pathogenic, 4 as likely pathogenic, and 5 as variants of uncertain significance. Prevalent clinical phenotypes include delayed developmental milestones, ophthalmic problems, movement disorders and sleep disturbance. Discriminating behavioural characteristics were severity of motor and communication impairment, presence of motor stereotypies and mood instability. SYT1 variants associated with neurodevelopmental disorder extend beyond previously reported regions, and the phenotypic spectrum encompasses a broader range of severity than initially reported. This work guides diagnosis and molecular understanding of this rare neurodevelopmental disorder, and highlights a key role for SYT1 function in emotional regulation, motor control and emergent cognitive function.