Biallelic truncating variants in cause syngnathia in humans.

Background: Syngnathia is an ultrarare craniofacial malformation characterised by an inability to open the mouth due to congenital fusion of the upper and lower jaws. The genetic causes of isolated bony syngnathia are unknown.

Methods: We used whole exome and Sanger sequencing and microsatellite analysis in six patients (from four families) presenting with syngnathia. We used CRISPR/Cas9 genome editing to generate and germline mutant zebrafish, and performed craniofacial cartilage analysis in homozygous mutants.

Results: We identified homozygous truncating variants in vestigial-like family member 2 () in all six patients. Two alleles were identified: one in families of Turkish origin and the other in families of Moroccan origin, suggesting a founder effect for each. A shared haplotype was confirmed for the Turkish patients. The family of genes encode cofactors of TEAD transcriptional regulators. is regionally expressed in the pharyngeal arches of model vertebrate embryos, and morpholino-based knockdown of in zebrafish has been reported to cause defects in development of pharyngeal arch cartilages. However, we did not observe craniofacial anomalies in or homozygous mutant zebrafish nor in fish with double knockout of and . In mice, which are known to present a skeletal muscle phenotype, we did not identify defects of the craniofacial skeleton.

Conclusion: Our results suggest that although loss of leads to a striking jaw phenotype in humans, other vertebrates may have the capacity to compensate for its absence during craniofacial development.