TBX5 variant with the novel phenotype of mixed-type total anomalous pulmonary venous return in Holt-Oram Syndrome and variable intrafamilial heart defects

Variants in T-box transcription factor 5 (TBX5) can result in a wide phenotypic spectrum, specifically in the heart and the limbs. TBX5 has been implicated in causing non-syndromic cardiac defects and Holt-Oram syndrome (HOS). The present study investigated the underlying molecular etiology of a family with heterogeneous heart defects. The proband had mixed-type total anomalous pulmonary venous return (mixed-type TAPVR), whereas her mother had an atrial septal defect. Genetic testing through trio-based whole-exome sequencing was used to reveal the molecular etiology. A nonsense variant was identified in TBX5 (c.577G>T; p.Gly193*) initially showing co-segregation with a presumably non-syndromic presentation of congenital heart disease. Subsequent genetic investigations and more complete phenotyping led to the correct diagnosis of HOS, documenting the novel association of mixed-type TAPVR with HOS. Finally, protein modeling of the mutant TBX5 protein that harbored this pathogenic nonsense variant (p.Gly193*) revealed a substantial drop in the quantity of non-covalent bonds. The decrease in the number of non-covalent bonds suggested that the resultant mutant dimer was less stable compared with the wild-type protein, consequently affecting the protein's ability to bind DNA. The present findings extended the phenotypic cardiac defects associated with HOS; to the best of our knowledge, this is the first association of mixed-type TAPVR with TBX5. Prior to the current analysis, the molecular association of TAPVR with HOS had never been documented; hence, this is the first genetic investigation to report the association between TAPVR and HOS. Furthermore, it was demonstrated that the null-variants reported in the T-box domain of TBX5 were associated with a wide range of cardiac and/or skeletal anomalies on both the inter-and intrafamilial levels. In conclusion, genetic testing was highlighted as a potentially powerful approach in the prognostication of the proper diagnosis.