We described a boy with cochlear hypoplasia, multiple neurogenetic abnormalities (profound sensorineural hearing loss, balance disorder, disorder of oral motor function, torticollis, and mild developmental delay) and a homozygous deletion of chromosome 5q31.1 spanning 115,334 kb. The deletion predicted a nullisomy (homozygous loss) for the Neurogenin1 gene (NEUROG1, also called Neurogenic differentiation factor 3) and two other genes, DCNP1 and TIFAB. To our knowledge, there has been no previous report of a homozygous deletion including these genes. In humans, none of these genes (NEUROG1, DCNP1, TIFAB) was previously associated with the clinical signs observed in this patient, but findings in Xenopus and mice make NEUROG1 an excellent candidate gene .
The neurog1 and neurog2 genes and corresponding proteins have been well studied in vertebrates. In Xenopus, neurog1 was shown to be essential for the expression of a cascade of downstream proteins that induce neuronal differentiation. Overexpression of neurog1 induced a conversion of ectodermal cells into neurons during embryonic development . In mice, expression of neurog1 was found to be restricted to the nervous system. Neurog1 showed high expression levels in developing sensory but not in autonomic ganglia . In the central nervous system, neurog1 and neurog2 displayed overlapping expression patterns and functional redundancy in various regions of the brain . In the peripheral nervous system, neurog1 and neurog2 displayed specific properties. Neurog1 and neurog2 were required during different phases of neurogenesis and for the development of different classes of sensory neurons . The neurog1 protein was found to be essential for the development of proximal sensory ganglia and for neurons forming from the trigeminal and otic placodes . These and other observations in knock-out mice are in excellent agreement with our clinical and radiological findings in the patient, which included a truncation or severe hypoplasia of the vestibulo-cochlear (VIIIth cranial) nerve. In the neurog1−/− mouse embryos, similar malformations of peripheral neural structures with absence of the vestibular-cochlear ganglion and of all afferent, efferent, and autonomic nerve fibers of the VIIIth cranial nerve were reported . The efferent and autonomic fibers were thought to be lost as a secondary effect due to the absence of the afferents .
Furthermore, the anatomical deviations in the inner ear of the patient correspond to those in the knockout mice. The boy’s internal auditory canal was narrowed and the cochlea was hypoplastic with only one single widened cochlear turn. In the neurog1−/− mutant mice, the inner ear showed an overall reduction in size and the cochlea only had 1.25 turns, as opposed to 1.75 turns in the control littermates . Moreover, the patient presented with a balance disorder, and the vestibulo-cochlear system of the neurog1−/− mutant mice showed a distinct missing utriculosaccular duct with only a small saccular recess .
This patient and the neurog1−/− knockout mice also shared the severe disorder of oral motor function. The boy was unable to swallow and to chew food and showed increased salivation and speech difficulties, while all newborn neurog1−/− mice were unable to suckle, lacked milk in their stomachs and died within 12 hours after birth . Ma et al. suggested that the absence of the trigeminal ganglia and associated defects of the Vth nerve resulted in a lack of sensory innervation and neonatal lethality by interfering with suckling. Correspondingly, we assume a malfunction of the Vth cranial nerve in the boy that could be caused by lack of sensory innervation or a missing motor innervation due to defective development of the rhombencephalon, which is the origin of the motoric fibers of the Vth cranial nerve. Taken together, the anatomic and functional abnormalities in mice perfectly match the symptoms in this patient. Therefore, we strongly infer that Neurog1 is a neuronal determination gene for the cranial sensory neurons that give rise to cranial nerves V and VIII in numerous vertebrates including Xenopus, mouse and humans.
In contrast, we consider TIFAB and DCNP1 unlikely candidate genes for the abnormalities in this patient. TIFAB is highly expressed in spleen and inhibits TIFA-mediated cellular functions by inducing a conformational change in the TIFA protein . TIFAB impedes activation of NF-kappaB and acts as a negative regulator of TRAF6-induced cellular functions such as B cell proliferation and maturation of macrophages . DCNP1 is a genetic factor that has been reported to be involved in asthma susceptibility. A case–control study among subjects with asthma revealed an association of variants of DCNP1 with serum immunoglobulin E (IgE) levels . DCNP1 may also play a role in the pathogenesis of depressive disorders because it was shown to enhance corticotropin-releasing hormone expression in the hypothalamic paraventricular nucleus. Hence, DNCP1 interferes with hypothalamic stress response and may contribute to the pathogenesis of major depression .
The clinical findings in our proband perfectly match with those in patients with CCDD, especially of the HOXA1 spectrum: among other symptoms, these patients show severe bilateral sensory-neural hearing loss due to absence of the cochlear and vestibular apparatus, often accompanied by absence of the eighth cranial nerve . Every causative gene characterized in context of CCDDs is associated with neuronal development at the nuclear, brainstem, or peripheral nerve level [1, 2]. For example, the responsible gene in Duane Retraction Syndrome (CHN1) is involved in ocular motor axon path finding of the sixth nerve. Homozygous mutations in HOXA1 cause an early and profound brainstem patterning defect and heterozygous mutations in KIF21A, which are causative for congenital fibrosis of the extraocular muscles type 1, lead to disturbance of anterograde organelle transport in neuronal cells .
Taken together, the clinical and molecular genetic findings in our proband most closely match the term congenital cranial dysinnervation disorder and there are also analogies to Moebius syndrome and its variants (MIM 157900). In a large study of Moebius syndrome including 37 Dutch patients, the following clinical observations are consistent with those observed in our proband: feeding problems at birth due to insufficient suckling or swallowing, nasal dysarthria and delayed language development, congenital deafness, motor disabilities, and malformations of the extremities of variable severity. A lack of sensation of the lips, cheek, forehead and cornea in some patients indicated a defect of the sensory root of the trigeminal nerve . Other authors have described external ear deformities, deafness and pharyngeal involvement in patients with Moebius syndrome variants [21–23].
Moreover, the clinical observations made in our proband partially correspond with those described in patients harboring the homozygous HOXB1 c.619C > T mutation : the phenotype included bilateral facial palsy, hearing loss, and strabismus. Two affected brothers from consanguineous parents showed a “masked facies” without any facial movement, sensorineural hearing loss, feeding difficulties and speech delay. MR imaging of the older brother revealed bilateral absence of the facial nerve and bilateral cochlear malformation with abnormal tapering of the basal turn. In contrast to our case, the vestibulocochlear nerve was preserved on both sides. An affected brother and sister from another family not known to be consanguineous also presented with bilateral facial weakness and sensorineural hearing loss .
The human NEUROG1 maps within the interval of the DFNB60 locus for non-syndromic autosomal recessive hearing loss on 5q22-q31, but linkage data have excluded NEUROG1 from being causative in the DFNB60 patients (Michael S. Hildebrand and Richard J.H. Smith, Department of Otolaryngology - Head and Neck Surgery, University of Iowa, Iowa City, IA, USA, personal communication). However, given its large size (35 Mb, >100 genes), the 5q22-q31 area could well harbor more than one deafness gene.