Human eye movement disorders help identify molecular and genetic etiologies of novel neurological syndromes

The development of cranial motoneurons appears to be particularly sensitive to mutations in genes that can be widely expressed throughout the nervous system and other organs. By identifying and characterizing different eye movement disorders in humans, we are beginning to reveal important functions for these genes beyond that of ocular motoneurons. First, using a positional cloning approach, we have identified homozygous truncating mutations in HOXA1 in three genetically isolated populations in the Middle East and American southwest. Affected individuals have a pleiotropic spectrum of phenotypes including horizontal gaze abnormalities, facial weakness, deafness, hypoventilation, skull deformities, autism, mental retardation, internal carotid artery malformations, and conotruncal heart defects. Importantly, our results demonstrate a new function for HOXA1 in vascular patterning that had not been previously reported in Hoxa1 -/- mice. We also demonstrate that HOXA1 is necessary for proper cognitive and behavioral development in humans, intriguing considering that the canonical CNS expression domain of Hoxa1 has a rostral boundary in the brainstem. Next, we report that eight heterozygous missense mutations in TUBB3, encoding the neuron specific β-tubulin isotype III, result in several human nervous system disorders now collectively called the TUBB3 syndromes. Each mutation causes the oculomotility disorder CFEOM3, while subsets result in intellectual and behavioral impairments, facial paralysis, and/or sensorimotor axonal polyneuropathy. MRI reveals a spectrum of brain malformations including hypoplastic oculomotor nerves, corpus callosum and anterior commissure dysgenesis, and basal ganglia dysmorphisms. We show that all mutant TUBB3 monomers fold and form heterodimers that can polymerize into microtubules. Introducing each mutation into yeast β-tubulin stabilizes microtubules and alters dynamic instability. A subset of these mutations also disrupts the interaction of microtubules with kinesin motors. Infecting dissociated neurons with each TUBB3 mutation can alter their differentiation and the morphology of microtubules in growth cones, and a mouse knock-in disease model has nervous system phenotypes consistent with axon guidance defects. This allelic series of human mutations in TUBB3 highlights residues in β-tubulin that are important for microtubule function and helps define the role of TUBB3 in neuronal development and maintenance.