Distinct sub-clusters of developmental disorder-associated variants in the switch II region of RAC1

RAC1 is a signal transducer essential for neurodevelopment. Missense variants in RAC1 cause heterogeneous neurodevelopmental disorders whose features include intellectual disability and developmental delay. Individuals with RAC1 variants are categorized by head circumference into microcephalic, normocephalic and macrocephalic groups. We previously described a cohort of individuals with variants affecting the N-terminal part (Q61-R68) of switch II, a functional domain of RAC1. This cohort was normocephalic and their variants increased RAC1 signalling activity. Here, we report 15 new individuals with variants in switch II of RAC1. We describe clinical features for two individuals with variants affecting the N-terminal part of switch II (Q61-R68) and four individuals with variants in its C-terminal part (P69-Q74). All individuals exhibit intellectual disabilities and neuroradiological anomalies. Consistent with our previous study, individuals with variants in the N-terminal part of switch II were normocephalic. By contrast, individuals with variants in the C-terminal part of switch II exhibited microcephaly. Cell-based assays revealed that N- and C-terminal variants affect RAC1 function differently: N-terminal variants are activating, while C-terminal variants are dominant-negative. In a Drosophila model, we observe divergent effects on neuronal morphology, with switch II N-terminal variants increasing the complexity of dendritic arbors, while switch II C-terminal variants reduce dendritic complexity. Switch II N-terminal variants increase locomotor activity of adult Drosophila, while C-terminal variants had minimal effect, suggesting diverging effects on neuronal function and behaviour. We conclude that variants affecting N- and C-terminal parts of RAC1 switch II cause phenotypically and mechanistically distinct disorders.