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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: (HGNC:9107) and (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.