# Genetic and Molecular Characterization of Treacher Collins Syndrome in Three Mexican Families

## TL;DR

This study identifies genetic causes of Treacher Collins syndrome in three Mexican families and emphasizes the importance of genetic testing in underrepresented populations.

## Contribution

The study reports novel pathogenic variants in TCOF1 and POLR1D genes in Mexican families with Treacher Collins syndrome.

## Key findings

- A splice-site variant in TCOF1 (c.4345 + 1 G > A) was identified in Family 1.
- A frameshift variant in TCOF1 (c.226_227insC) was found in Family 2.
- A frameshift variant in POLR1D (c.290_291delAG) was identified in Family 3.

## Abstract

Treacher Collins syndrome (TCS) is a rare disorder within the group of mandibulofacial dysostoses, occurring in 1 in 50,000 live births. It is characterized by anomalies in the maxillary, mandibular, and stapes bones, among others. TCS is caused by pathogenic variants in the TCOF1, POLR1D, POLR1C, and POLR1B genes with autosomal dominant or recessive inheritance patterns. Genetic data from Latin American populations remain scarce. Eleven patients from three different families were recruited. Whole-exome sequencing (WES) was performed on the probands to identify genetic variants, followed by Sanger sequencing for variant validation and familial segregation analysis. Finally, three-dimensional protein structures of wild-type and mutant proteins were predicted. In Family 1, a heterozygous pathogenic splice-site variant in the TCOF1 gene, c.4345 + 1 G > A, was identified and inherited from her mother. In Family 2, a heterozygous pathogenic variant in the TCOF1 gene, c.226_227insC (p.R77fs), was identified and inherited from the paternal lineage. In Family 3, a heterozygous pathogenic POLR1D variant, c.290_291delAG (p.G99fs), was identified among multiple affected relatives; direct parent-of-origin could not be established due to unavailability of one parent, but segregation supports autosomal dominant transmission across three generations. All findings were validated by Sanger sequencing. Our findings highlight the utility of WES for the molecular diagnosis of TCS and underscore the importance of including underrepresented populations in genetic studies to improve diagnosis, genetic counseling, and perinatal planning in at-risk pregnancies.