Systematic phenotype and genotype characterization of Moebius syndrome

Abstract

Purpose: To explore the phenotypic spectrum and genetic etiologies of Moebius Syndrome (MBS), a rare neurological disorder defined by congenital, nonprogressive facial weakness and limitations in ocular abduction.

Methods: We applied strict diagnostic criteria and conducted clinical phenotyping of 149 individuals with MBS. Subsequently, we performed exome and/or genome sequencing on 67 of these individuals and 117 unaffected family members.

Results: All 149 individuals had sporadic MBS, with no recurrence within or across generations. Common co-occurring phenotypes included tongue hypoplasia (81.9%), micrognathia (66.4%), congenital talipes equinovarus (42.3%), major limb anomalies (31.5%), intellectual disability (30.9%), sleep difficulties (22.8%), and Poland anomaly (14.1%). Filtering for rare de novo or autosomal recessive single-nucleotide, insertion/deletion, and structural variants in the sequenced cohort yielded 173 single-nucleotide variant/indels in 113 genes. Although we prioritized 7 candidate genes with de novo variants and 5 with biallelic variants, no compelling recurrently mutated genes were identified. Similarly, we found no convincing variants in 2 putative genes previously implicated in MBS: PLXND1 (HGNC:9107) and REV3L (HGNC:9968).

Conclusion: We did not identify a strong or unifying germline genetic etiology for MBS. Future studies may explore alternative causes, including environmental exposures, somatic variants, and/or complex inheritance patterns affecting brainstem and organ embryogenesis.

Keywords: Congenital facial weakness; Cranial nerve; Exome sequencing; Genome sequencing; Moebius syndrome.

Figure 1

MBS definition and study workflow. A. Schematic of human right-sided CN6 (abducens nerve, green) and CN7 (facial nerve, purple) innervating the lateral rectus and muscles of facial expression, respectively (left). Photos of an individual with MBS highlighting bilateral restricted abduction with full vertical eye movements (right top) and bilateral facial weakness (right bottom). Note mask-like facies, lagophthalmos, asymmetry of nasolabial folds with attempted smile, and engagement of the platysma to increase facial movement. CN6, cranial nerve 6; CN7, cranial nerve 7; L, left; R, right; X, restricted or absent ocular motility. B. Graphical depiction of study enrollment and phenotype and genotype analyses workflow. CBCT, cone-beam computed tomography; dbGaP, the database of Genotypes and Phenotypes; DTI, diffusion tensor imaging; indel, small insertions and deletions less than 50 nucleotides in length; MBS, Moebius Syndrome; MRI, magnetic resonance imaging; PBMCs, peripheral blood mononuclear cells; SNP, single-nucleotide polymorphism; SNV, single-nucleotide variant; SV, structural variant; TGAC, The Genomic Ascertainment Cohort.

Figure 2

MBS clinical phenotypes. A. Representative facial images for both sexes across the lifespan reveal a predominant phenotype of asymmetric smiles, esotropia, and a spectrum of dysmorphic features including low-set ears, short nose, midface hypoplasia, underdeveloped nasolabial or forehead creases, narrow mouth with downturned corners, and micrognathia. B. Representative upper and lower limb anomalies, including transverse limb reduction defect/absent forearm and hand, brachydactyly, brachysyndactyly, absent digit(s), ectrodactyly, absent foot or toes, metatarsus adductus, and clubfoot. C. Severe phenotypes include lower cranial nerve anomalies, such as tongue hypoplasia/furrowed tongue, tracheostomy and/or gastrostomy tube for feeding, mild to moderate intellectual disability, and/or autism spectrum disorder. MBS, Moebius Syndrome.

Figure 3

Demographics and phenotypic spectrum of MBS cohort. A. Self-reported race, ethnicity, and sex distribution in the study cohort are presented in pie charts with slice sizes proportional to the respective frequencies. ASAB, assigned sex at birth. The study represents a large and diverse cohort of MBS individuals, with equal male/female distribution. B. Percent incidence of key phenotypic features in the cohort (n = 149) sorted by frequency. ADHD, attention-deficit hyperactivity disorder; BHGP, bilateral horizontal gaze paresis; CFW, congenital facial weakness; GI, gastrointestinal; GU, genitourinary; trach, tracheostomy. C. Clustering of participants given presence of 6 features (clubfoot, major limb anomalies, intellectual disability and/or autism, hearing loss, cleft palate or bifid uvula, and Poland anomaly), with features listed from highest to lowest frequency for the entire MBS cohort (N = 149). To visualize, probands with the most frequent characteristic were identified and sorted based on frequency of the remaining 6 characteristics in descending order, followed by the next highest frequent characteristic. Phenotype data are visualized using the pheatmap R package. Each column is a feature and each row an MBS proband, denoted by their code number. Filled blue boxes denote the presence of that phenotype feature in the proband, whereas filled gray boxes denote absence of available data for that feature in that proband. D. Offspring number per MBS proband are depicted. A total of 8 MBS individuals had 1 to 5 children each for a total of 16 offspring, all unaffected. E. The total number of siblings of MBS probands. A total of 160 full siblings were reported in the family pedigrees of MBS individuals in the cohort, all unaffected. Six individuals had a living unaffected fraternal twin, and 3 had a prenatal history positive for a vanishing fetus. F. Prenatal histories and environmental exposures during pregnancy by self-report. MBS, Moebius Syndrome.

Figure 4

Brain magnetic resonance imaging findings in MBS. Features of MBS on brain MRI from a 33-year-old female proband with MBS and clubfoot (F58-001, panels 1-6) compared with a 24-year-old individual with hemifacial myohyperplasia in the absence of MBS and intracranial abnormalities (panels 7-12). CN7 (short white arrows in 1-3 and 7-9) is hypoplastic in MBS (1-3) but is well seen in the control (7-9). The pontine tegmentum (black arrowhead in 4) is hypoplastic in F58-001, resulting in a concavity in the dorsal brainstem. CN6 cannot be identified in the MBS proband (long white arrows in 5) but is well seen traversing the prepontine cistern in the control (long white arrows in 11). The lateral rectus muscles are relatively small (gray arrows in 6) in MBS compared with control (gray arrows in 12). Imaging sequences: T2 weighted volume isotropic turbo spin echo images reformatted axial through the internal auditory canals (1,7) and oblique axial through the prepontine cisterns (5,11); sagittal reformats of 0.5 mm isotropic balanced fast field echo through the right (2,8) and left (3,9) IACs at sites of arrows in panels 1 and 7; midline sagittal image from 1mm isotropic 3D (4,10); coronal fast spin echo T2 through the orbits (6,12). Scale bars, 1 cm (white bars in 1-5 and 7-11). See also Supplemental Figure 3. MBS, Moebius Syndrome.