Tubulins, and microtubule polymers into which they incorporate, play critical mechanical roles in neuronal function during cell proliferation, neuronal migration, and postmigrational development: the three major overlapping events of mammalian cerebral cortex development. A number of neuronally expressed tubulin genes are associated with a spectrum of disorders affecting cerebral cortex formation. Such "tubulinopathies" include lissencephaly/pachygyria, polymicrogyria-like malformations, and simplified gyral patterns, in addition to characteristic extracortical features, such as corpus callosal, basal ganglia, and cerebellar abnormalities. Epilepsy is a common finding in these related disorders. Here we describe two unrelated individuals with infantile-onset epilepsy and abnormalities of brain morphology, harboring de novo variants that affect adjacent amino acids in a beta-tubulin gene TUBB2A. Located in a highly conserved loop, we demonstrate impaired tubulin and microtubule function resulting from each variant in vitro and by using in silico predictive modeling. We propose that the affected functional loop directly associates with the alpha-tubulin-bound guanosine triphosphate (GTP) molecule, impairing the intradimer interface and correct formation of the alpha/beta-tubulin heterodimer. This study associates mutations in TUBB2A with the spectrum of "tubulinopathy" phenotypes. As a consequence, genetic variations affecting all beta-tubulin genes expressed at high levels in the brain (TUBB2B, TUBB3, TUBB, TUBB4A, and TUBB2A) have been linked with malformations of cortical development.
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The authors would like to thank the affected individuals and their families for participation in this study. We would also like to thank Owain Howell for providing immunofluorescent antibodies to perform the in vitro functional analysis. Research reported in this publication was supported by the (UK) National Institute of Social Care and Health Research (M.I.R., D.T.P.), the Wales Epilepsy Research Network (WERN – M.I.R., S.-K.C., D.T.P.) and Waterloo Foundation (M.I.R.), and by the (USA) National Institute for Neurological Disorders and Stroke (NINDS) of the National Institutes of Health under award numbers K08NS078054 (to A.R.P.), K12NS066098 (to L.E.S.), and R01NS050375 and R01NS058721 (to W.B.D.).