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PurposeSynaptotagmin-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.MethodsWe 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.ResultsFour 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, abnormal EEG, movement disorders and sleep disturbance. Discriminating behavioural characteristics were severity of motor and communication impairment, presence of motor stereotypies and mood instability.ConclusionSYT1 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.

Pathogenic variants in HNRNPH1 were first reported in 2018. The reported individual, a 13 year old boy with a c.616C>T (p.R206W) variant in the HNRNPH1 gene, was noted to have overlapping symptoms with those observed in HNRNPH2-related X-linked intellectual disability, Bain type (MRXSB), specifically intellectual disability and dysmorphic features. While HNRNPH1 variants were initially proposed to represent an autosomal cause of MRXSB, we report an additional seven cases which identify phenotypic differences from MRXSB. Patients with HNRNPH1 pathogenic variants diagnosed via WES were identified using clinical networks and GeneMatcher. Features unique to individuals with HNRNPH1 variants include distinctive dysmorphic facial features; an increased incidence of congenital anomalies including cranial and brain abnormalities, genitourinary malformations, and palate abnormalities; increased incidence of ophthalmologic abnormalities; and a decreased incidence of epilepsy and cardiac defects compared to those with MRXSB. This suggests that pathogenic variants in HNRNPH1 result in a related, but distinct syndromic cause of intellectual disability from MRXSB, which we refer to as HNRNPH1-related syndromic intellectual disability.