TUBA1A mutations identified in lissencephaly patients dominantly disrupt neuronal migration and impair dynein activity

Abstract

Tubulinopathies are severe human brain malformations associated with mutations in tubulin genes. Despite the identification of many tubulin mutations in patients, we do not understand how these mutations impact the microtubule cytoskeleton, how the changes to microtubule function lead to brain malformations, or how different tubulin isotypes regulate microtubules to support normal neurodevelopment. TUBA1A -tubulin is the most commonly affected tubulin isotype in tubulinopathy patients. Heterozygous mutations in TUBA1A have been identified in patients with diverse cortical malformations including microlissencephaly, lissencephaly, pachygyria, and polymicrogyria. Here we focus on mutations affecting the conserved arginine at position 402 (R402), which account for 30% of all reported TUBA1A mutations in patients. We demonstrate that exogenous expression of TUBA1A-R402C and TUBA1A-R402H patient alleles is sufficient to dominantly disrupt cortical neuron migration in the developing mouse brain, recapitulating the human lissencephaly phenotype. Intriguingly, ectopic expression of TUBA1A-R402C/H alleles does not alter morphology, axonal trafficking, or microtubule polymerization rates in cultured neurons, but does lead to subtle changes in axonal microtubule orientation. Further, we find that budding yeast -tubulin with analogous R402C and R402H mutations assembles into microtubules but disrupts the activity of the microtubule motor dynein. The level of dynein impairment scales with abundance of R402 mutant -tubulin in the cell. Together, our results support a model in which tubulinopathy mutations at R402 poison the microtubule network in young neurons by creating defective binding sites for dynein at the microtubule surface.