βIV Spectrinopathies Cause Profound Intellectual Disability, Congenital Hypotonia, and Motor Axonal Neuropathy

βIV spectrin links ankyrinG (AnkG) and clustered ion channels at axon initial segments (AISs) and nodes of Ranvier to the axonal cytoskeleton. Here, we report bi-allelic pathogenic SPTBN4 variants (three homozygous and two compound heterozygous) that cause a severe neurological syndrome that includes congenital hypotonia, intellectual disability, and motor axonal and auditory neuropathy. We introduced these variants into βIV spectrin, expressed these in neurons, and found that 5/7 were loss-of-function variants disrupting AIS localization or abolishing phosphoinositide binding. Nerve biopsies from an individual with a loss-of-function variant had reduced nodal Na⁺ channels and no nodal KCNQ2 K⁺ channels. Modeling the disease in mice revealed that although ankyrinR (AnkR) and βI spectrin can cluster Na⁺ channels and partially compensate for the loss of AnkG and βIV spectrin at nodes of Ranvier, AnkR and βI spectrin cannot cluster KCNQ2- and KCNQ3-subunit-containing K⁺ channels. Our findings define a class of spectrinopathies and reveal the molecular pathologies causing nervous-system dysfunction.