KCTD3 deficiency disrupts axon initial segment organization and neurite outgrowth in a neurodevelopmental disorder mouse model.

Pathogenic variants in human KCTD3 are associated with severe neurodevelopmental disorders characterized by early-onset seizures, developmental delay, hypotonia, and cerebellar hypoplasia, but the underlying mechanisms remain unclear. Here we show that KCTD3 regulates neuronal structural organization through DAAM1, a protein involved in actin cytoskeleton assembly. In Kctd3-deficient neurons, DAAM1 protein levels are reduced, the axon initial segment is abnormally organized, and neurite growth and growth cone morphology are impaired. These defects are rescued by DAAM1 overexpression. Systemic Kctd3 depletion also disrupts axon initial segment organization and motor axon innervation at neuromuscular junctions, leading to motor dysfunction and growth impairment in mice. These findings identify a KCTD3-DAAM1 pathway required for neuronal structural development and provide a framework for understanding how KCTD3 deficiency contributes to neurodevelopmental disorders.