Inspired by a patient
Whitman MC, Engle EC. Ocular congenital cranial dysinnervation disorders (CCDDs): insights into axon growth and guidance. Hum Mol Genet. 2017 Aug 1;26(R1):R37-R44.
Unraveling the genetics of the paralytic strabismus syndromes known as congenital cranial dysinnervation disorders (CCDDs) is both informing physicians and their patients and broadening our understanding of development of the ocular motor system. Genetic mutations underlying ocular CCDDs alter either motor neuron specification or motor nerve development, and highlight the importance of modulations of cell signaling, cytoskeletal transport, and microtubule dynamics for axon growth and guidance. Here we review recent advances in our understanding of two CCDDs, congenital fibrosis of the extraocular muscles (CFEOM) and Duane retraction syndrome (DRS), and discuss what they have taught us about mechanisms of axon guidance and selective vulnerability. CFEOM presents with congenital ptosis and restricted eye movements, and can be caused by heterozygous missense mutations in the kinesin motor protein KIF21A or in the β-tubulin isotypes TUBB3 or TUBB2B. CFEOM-causing mutations in these genes alter protein function and result in axon growth and guidance defects. DRS presents with inability to abduct one or both eyes. It can be caused by decreased function of several transcription factors critical for abducens motor neuron identity, including MAFB, or by heterozygous missense mutations in CHN1, which encodes α2-chimaerin, a Rac-GAP GTPase that affects cytoskeletal dynamics. Examination of the orbital innervation in mice lacking Mafb has established that the stereotypical misinnervation of the lateral rectus by fibers of the oculomotor nerve in DRS is secondary to absence of the abducens nerve. Studies of a CHN1 mouse model have begun to elucidate mechanisms of selective vulnerability in the nervous system.
Monique M. Ryan, Elizabeth C. Engle, in Neuromuscular Disorders of Infancy, Childhood, and Adolescence (Second Edition), 2015
A series of recurrent dominant missense mutations in TUBB3 result in variable phenotypes that correlate with the specific mutation, and include CFEOM1 and CFEOM without bilateral ptosis and/or with some ability to elevate one or both eyes above midline, referred to as CFEOM3. In addition to CFEOM, patients with specific TUBB3 mutations may have intellectual or social disabilities, facial weakness, vocal cord paralysis, finger contractures, progressive sensorimotor polyneuropathy, Kallmann syndrome, and/or cyclic vomiting. In particular, the TUBB3 E410K syndrome may be misdiagnosed as atypical Moebius syndrome. Magnetic resonance imaging reveals oculomotor nerve hypoplasia that can be accompanied by dysgenesis of the corpus callosum, anterior commissure, corticospinal tracts, and/or basal ganglia. A knock-in disease mouse model revealed axon guidance defects of both cranial nerves and central axon tracts. TUBB3 encodes the neuron-specific beta-tubulin isotype III, which is one of multiple beta-tubulin isotypes that heterodimerize with alpha-tubulin isotypes to form microtubules. The disease-associated missense mutations have been shown to impair tubulin heterodimer formation to varying degrees, although folded mutant heterodimers can still polymerize into microtubules. Modeling each CFEOM-causing mutation in yeast tubulin demonstrated that all alter dynamic instability, whereas a subset disrupts the interaction of microtubules with kinesin motors. Similarly, a heterozygous missense mutation in a second beta-tubulin isotype, TUBB2B, has been reported to segregate with CFEOM and polymicrogyria. This mutation also incorporates into the microtubule network, where it alters microtubule dynamics and can reduce kinesin localization.195 The specific molecular relationship between KIF21A-CFEOM1, TUBB3-CFEOM3, and TUBB2B-CFEOM3, and the reason that the developing oculomotor nerve is particularly vulnerable to these disease mutations, remain to be elucidated.
Huang H, Yang T, Shao Q, Majumder T, Mell K, Liu G. Human TUBB3 Mutations Disrupt Netrin Attractive Signaling. Neuroscience. 2018 Mar 15;374:155-171.
Heterozygous missense mutations in human TUBB3 gene result in a spectrum of brain malformations associated with defects in axon guidance, neuronal migration and differentiation. However, the molecular mechanisms underlying mutation-related axon guidance abnormalities are unclear. Recent studies have shown that netrin-1, a canonical guidance cue, induced the interaction of TUBB3 with the netrin receptor deleted in colorectal cancer (DCC). Furthermore, TUBB3 is required for netrin-1-induced axon outgrowth, branching and pathfinding. Here, we provide evidence that TUBB3 mutations impair netrin/DCC signaling in the developing nervous system. The interaction of DCC with most TUBB3 mutants (eight out of twelve) is significantly reduced compared to the wild-type TUBB3. TUBB3 mutants R262C and A302V exhibit decreased subcellular colocalization with DCC in the growth cones of primary neurons. Netrin-1 increases the interaction of endogenous DCC with wild-type human TUBB3, but not R262C or A302V, in primary neurons. Netrin-1 also increases co-sedimentation of DCC with polymerized microtubules (MTs) in primary neurons expressing the wild-type TUBB3, but not R262C or A302V. Expression of either R262C or A302V not only suppresses netrin-1-induced neurite outgrowth, branching and attraction in vitro, but also causes defects in spinal cord commissural axon (CA) projection and pathfinding in ovo. Our study reveals that missense TUBB3 mutations specifically disrupt netrin/DCC-mediated attractive signaling.