Haploinsufficiency, de novo and biallelic missense variants in CSMD2 are associated with neurodevelopmental disorders.

CUB and Sushi domains-containing CSMD proteins are involved in neuron development. Variants in the CSMD1-3 paralogs were GWAS-associated with cognition, attention-deficit hyperactivity disorder (ADHD), schizophrenia and brain structure, suggesting that these genes could be linked to neurodevelopmental disorders (NDD). Indeed, rare biallelic variants in all three CSMD paralogs were recently associated with epilepsy, while rare biallelic variants in CSMD1 were also linked with intellectual disability and cortical malformations. We identified nine individuals from eight families carrying CSMD2 biallelic missense variants in developmental delay/intellectual disability cohorts. Affected individuals presented with autism spectrum disorder (ASD), ADHD and brain anomalies. Consistent with gnomAD metrics that suggest CSMD2 haploinsufficiency, we also report two families segregating monoallelic CSMD2 frameshift variants, which complement a described de novo truncation variant in an ASD proband. We concomitantly report two phenotypically overlapping individuals with de novo CSMD2 missense variants and compare them to twelve de novo CSMD2 cases identified in published trio analyses. UK Biobank analyses showed nominally significant enrichment of ADHD, ASD and neuroticism in volunteers carrying at least one rare CSMD2 variant. In silico modelling and immunofluorescence assays showed that mutated residues in affected individuals with biallelic and de novo missense variants are clustering in two regions and that the encoded proteins showed impaired membrane localization and an increased cytoplasmic aggregation. Zebrafish crispant larvae for the orthologous csmd2 presented with microcephaly, smaller cerebellum, and reduced Purkinje cell size. Larvae also displayed decreased swimming velocity, diminished connectivity between the optic tecta, and abnormal peripheral neuronal branching. We demonstrate that rare, likely pathogenic variants in CSMD2 are associated with both autosomal recessive and dominant forms of NDD further substantiating the functional significance of CSMD paralogs in neurogenesis/synapse formation.