<i>KIF21A</i>-associated peripheral neuropathy defined by impaired binding with TUBB3

<i>KIF21A</i> encodes a kinesin motor protein associated with isolated congenital fibrosis of the extraocular muscles (CFEOM), which occurs when the autoinhibitory interaction between its motor and third coiled-coil domains is disrupted. In this study, we describe a female child who is heterozygous for a novel de novo missense variant in <i>KIF21A</i> p.Leu664Pro, located in the second coiled-coil domain that was absent in her unaffected parents and in healthy population cohorts. She presented with progressive peripheral neuropathy, hypoplasia of the corpus callosum and strabismus in the absence of CFEOM. Protein modelling predicts that the KIF21A variant leads to significant alterations in its structure as well as binding with TUBB3. Co-immunoprecipitation data was consistent with decreased binding of KIF21A p.Leu664Pro to TUBB3 in vitro compared with reference. Taken together, we delineate a <i>KIF21A</i>-related phenotype defined by progressive peripheral neuropathy, brain anomalies, developmental delay and comitant strabismus potentially stemming from the disruption of the interaction between KIF21A and TUBB3.