A novel syndrome caused by the E410K amino acid substitution in the neuronal β-tubulin isotype 3

Abstract

Missense mutations in TUBB3, the gene that encodes the neuronal-specific protein β-tubulin isotype 3, can cause isolated or syndromic congenital fibrosis of the extraocular muscles, a form of complex congenital strabismus characterized by cranial nerve misguidance. One of the eight TUBB3 mutations reported to cause congenital fibrosis of the extraocular muscles, c.1228G>A results in a TUBB3 E410K amino acid substitution that directly alters a kinesin motor protein binding site. We report the detailed phenotypes of eight unrelated individuals who harbour this de novo mutation, and thus define the 'TUBB3 E410K syndrome'. Individuals harbouring this mutation were previously reported to have congenital fibrosis of the extraocular muscles, facial weakness, developmental delay and possible peripheral neuropathy. We now confirm by electrophysiology that a progressive sensorimotor polyneuropathy does indeed segregate with the mutation, and expand the TUBB3 E410K phenotype to include Kallmann syndrome (hypogonadotropic hypogonadism and anosmia), stereotyped midface hypoplasia, intellectual disabilities and, in some cases, vocal cord paralysis, tracheomalacia and cyclic vomiting. Neuroimaging reveals a thin corpus callosum and anterior commissure, and hypoplastic to absent olfactory sulci, olfactory bulbs and oculomotor and facial nerves, which support underlying abnormalities in axon guidance and maintenance. Thus, the E410K substitution defines a new genetic aetiology for Moebius syndrome, Kallmann syndrome and cyclic vomiting. Moreover, the c.1228G>A mutation was absent in DNA from ∼600 individuals who had either Kallmann syndrome or isolated or syndromic ocular and/or facial dysmotility disorders, but who did not have the combined features of the TUBB3 E410K syndrome, highlighting the specificity of this phenotype-genotype correlation. The definition of the TUBB3 E410K syndrome will allow clinicians to identify affected individuals and predict the mutation based on clinical features alone.

Figure 1

The TUBB3 c.1228G>A mutation (p.E410K) arose de novo in Pedigrees ZY and AER. (A) Schematic structures of Pedigrees ZY and AER. Squares denote male subjects, circles denote female subjects and filled symbols indicated by the black triangles denote Probands III and VI. The absence or presence of the c.1228G>A mutation is indicated by a G or A, respectively. (B) Chromatograms from a control individual (top) and a proband showing the c.1228G>A heterozygous mutation (bottom). The nucleotide substitution is represented by the double peak and indicated by a red arrow. The corresponding normal and mutated amino acid residues are indicated with brackets under each codon triplet.

Figure 2

Growth parameters for individuals harbouring the TUBB3 E410K substitution. Stature (A and D), weight (B and E) and head circumference (C and F) for age for the male subjects (top row) and female subjects (bottom row). The grey standard curves represent the 3rd, 25th, 50th, 75th and 97th percentiles. Stature and weight standard curves created with percentile data from Centers for Disease Control National Health Statistics, available at http://www.cdc.gov/growthcharts/clinical_charts.htm#Set2. Head circumference standard curves created with data from (Rollins et al., 2010). Blue and orange arrows indicate the start of testosterone therapy for hypogonadotropic hypogonadism for Subjects VI and VII, respectively.

Figure 3

3D morphometric analysis of individuals with the TUBB3 E410K substitution. 3D photographs were obtained for five subjects, and frontal (A–C) and profile (D–F) views of three subjects are shown. Generally, patients with the TUBB3 E410K substitution are noted to have ptosis, globe infraduction and exotropia, a short nose, a short smooth philtrum, midface flattening and a mask-like face. (A and D) Subject is noted to have ptosis, upslanting palpebral fissures and epicanthal folds. He has a short upturned nose and midface flattening. His philtrum is smooth. (B and E) Subject is noted to have ptosis, upslanting palpebral fissures and epicanthal folds. Her nose is short, non-protruding and broad. She has a flat nasal bridge and short smooth philtrum. (C and F) Subject is noted to have ptosis and facial asymmetry. He has a short nose and midface flattening. His philtrum is smooth, and his ears are slightly low set and posteriorly rotated.

Figure 4

Midline sagittally acquired (A–F) and coronal reformations (G–L) of volumetric T₁-weighted magnetic resonance images demonstrate abnormalities of the corpus callosum, pituitary gland, olfactory sulci and olfactory bulbs in individuals with the TUBB3 E410K substitution. (A) Corpus callosum (cc) and pituitary gland (pit) in a control individual. (B–F) The TUBB3 E410K substitution produces variable corpus callosum abnormalities, including diffuse thinning (C and F), hypoplasia of the posterior body (B), rostral dysgenesis with posterior hypoplasia (E) and complete agenesis (D). The pituitary gland appears small in all subjects, as can be seen in Kallmann syndrome. (G) Olfactory sulci (black arrowhead, os) and olfactory bulbs (white arrowheads, ob) in a control individual. (H–L) Olfactory sulcus agenesis is seen in all individuals with the TUBB3 E410K substitution except Subject III, who has unilateral olfactory sulcus dysgenesis (single abnormal olfactory sulcus is indicated by the black arrowhead) (I). Most individuals with the TUBB3 E410K substitution demonstrate bilateral olfactory bulb dysgenesis (highlighted by white arrowheads) (H–L). Subject VI (K) has left olfactory bulb hypoplasia (white arrowhead). The right olfactory bulb is not visible by neuroimaging. Voxel size, 1.1458 × 1.1458 × 1.0000 mm.

Figure 5

Oblique reformations of sagittally acquired volumetric T₁-weighted images oriented to the plane of selected cranial nerves demonstrate oculomotor and facial nerve hypoplasia in TUBB3 E410K subjects. (A) Plane of images displayed in (B–G). (B) Oculomotor nerves in a control individual are indicated by yellow arrows (arrowhead perpendicular to and touching the nerve). (C–G) Hypoplastic oculomotor nerves are visualized bilaterally in Subject VI (F) and unilaterally in Subject VII (G), but not in Subjects II, III and IV (C–E). (H) Plane of images displayed in I–N. (I) Vestibulocochlear nerves (purple arrowheads) and facial nerve (blue arrows) in a control individual. (J–N) Vestibulocochlear nerves (purple arrowheads) are visualized bilaterally in all subjects. Facial nerves are not visible by neuroimaging in these individuals, indicating absence or severe hypoplasia.