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.
Wang, C. C., Ortiz-González, X. R., Yum, S. W., Gill, S. M., White, A., Kelter, E., … Rasband, M. N. (2018). βIV Spectrinopathies Cause Profound Intellectual Disability, Congenital Hypotonia, and Motor Axonal Neuropathy. American Journal of Human Genetics, 102(6), 1158–1168. https://doi.org/10.1016/j.ajhg.2018.04.012