# Congenital Temporomandibular Joint Ankylosis: Investigating Potential Genetic Etiologies with Whole Exome Sequencing

## TL;DR

This study uses whole exome sequencing to identify genetic variants possibly linked to congenital temporomandibular joint ankylosis in a patient with facial asymmetry.

## Contribution

The study reports the first case of congenital ankylosis with pathogenic variants in genes associated with craniofacial dysostosis syndromes.

## Key findings

- Three pathogenic variants were identified in TCOF1, POLR1B, and DHODH genes.
- Eight pathogenic variants were found in genes not previously linked to facial disorders.
- WES proved effective in identifying genetic factors for congenital TMJ ankylosis.

## Abstract

Background: Ankylosis of the temporomandibular joint (TMJ) is a rare developmental disorder that involves fibrous or bony fusion within the joint. It is a severe structural and functional disorder. Typically, the phenotype manifests as joint immobilization and results in facial deformity and trismus. To date, ankylosis is rarely diagnosed as congenital and its occurrence mechanism has not been thoroughly understood. We observed a female patient who as a newborn showed slight facial asymmetry and impaired mandibular retraction. In addition, non-uniform occlusal fissures were noted; the lower part of the left earlobe was slightly smaller than the right earlobe. The aim of the work was the identification of pathogenic variants in the genome related to ankylosis. Ankylosis has no known causative gene yet; thus, Whole Exome Sequencing (WES) was performed. Materials and Methods: We observed a female patient with facial asymmetry and impaired mandibular retraction from birth. No phenotypic abnormalities were noted on the head or elsewhere on the body. A diagnostic computed tomography (CT) scan of the head performed at five months of age led to the diagnosis of congenital zygomatic-coronoid ankylosis. Genomic DNA samples were subjected to WES. Library preparation was carried out using the Twist Library Preparation EF Kit 2.0, followed by target enrichment with the Twist Exome 2.0 Plus Comprehensive Exome. Sequencing reads were aligned to the human reference genome (GRCh38), and variant calling was performed using standard bioinformatics workflows. Variants were subsequently filtered, annotated, and interpreted using VariantStudio. Assessment of variant pathogenicity was primarily based on comparisons with public databases, including ClinVar and VarSome, and was supported by in silico prediction tools such as SIFT and PolyPhen-2. Results: In genes responsible for disorders of the I and II pharyngeal arches, three pathogenic variants were identified: in the genes TCOF1 and POLR1B, responsible for the development of Treacher Collins syndrome (TCS), and one in the DHODH gene, responsible for Miller syndrome. Additionally, in genes that have not been linked so far with rare facial disorders, 42 variants were identified, of which 8 are listed as pathogenic. We present the first described patient with congenital ankylosis, who, although showing no phenotypic features of these syndromes, has identified pathogenic variants in genes responsible for craniofacial dysostosis. Conclusions: Variants in TCOF1, POLR1B and DHODH may represent candidate genetic factors associated with susceptibility to ankylosis. WES analysis is an appropriate method in the case of patients with congenital diseases with unknown genetic origin. In this study we provide a comprehensive list of all identified pathogenic variants. This might be useful for scientists searching for the genetic background of skeletal system issues, one of which could be bone and fibrous tissue remodeling.