Abstract. Conotruncal defects (CTDs) account for ~30% of all types of congenital heart disease and contribute to increased morbidity and mortality rates. Increasing evidence suggests that genetic risk factors are involved in the pathogenesis of CTDs. Mutations in a number of genes, including the TBX1 gene that codes for a T-box transcription factor essential for normal cardiovascular development, may contribute to the development of CTD. CTDs are genetically heterogeneous and the genetic defects responsible for CTDs in the majority of patients remain unknown. The present study sequenced the coding regions and splicing junction boundaries of TBX1 in 136 patients with CTDs and 300 matched healthy individuals. The disease-causing potential of the identified TBX1 sequence variation was evaluated using MutationTaster, PolyPhen-2, SIFT and PROVEN software. The functional characteristics of the mutant TBX1 gene were defined using a dual-luciferase reporter assay system. A novel heterozygous TBX1 mutation, p.S233Y, was identified in a patient with transposition of the great arteries (TGA) and a ventricular septal defect. This mutation was absent in the 300 controls and altered the amino acid produced, serine, which is evolutionarily conserved across several species, and was predicted to be pathogenic in silico. Luciferase assays conducted in COS-7 cells demonstrated that the newly identified TBX1 mutation was associated with significantly diminished transcriptional activation of the ANF promoter compared with the wild-type TBX1. To the best of our knowledge, the present study is the first to associate a TBX1 loss-of-function mutation with enhanced susceptibility to TGA, which adds significant insight to the molecular mechanism of TGA.

Congenital heart disease (CHD) is the most common form of birth defect in humans, and remains a leading non‑infectious cause of infant mortality worldwide. An increasing number of studies have demonstrated that genetic defects serve a pivotal role in the pathogenesis of CHD, and mutations in >60 genes have been causally associated with CHD. CHD is a heterogeneous disease and the genetic basis of CHD in the majority of patients remains poorly understood. In the present study, the coding exons and flanking introns of the mesoderm posterior 1 (MESP1) gene, which encodes a basic helix‑loop‑helix transcription factor required for normal cardiovascular development, were sequenced in 178 unrelated patients with CHD. The available relatives of the index patient carrying an identified mutation and 200 unrelated, ethnically‑matched healthy individuals, who were used as controls, were genotyped for MESP1. The functional characteristics of the MESP1 mutation were determined using a dual‑luciferase reporter assay system. As a result, a novel de novo heterozygous MESP1 mutation, p.Q118X, was identified in an index patient with double outlet right ventricle (DORV) and a ventricular septal defect. The nonsense mutation was absent in the 400 reference chromosomes and the altered amino acid was completely conserved evolutionarily across species. Functional assays indicated that the mutant MESP1 protein had no transcriptional activity when compared with its wild‑type counterpart. The present study firstly provided experimental evidence supporting the concept that a MESP1 loss‑of‑function mutation may contribute to the development of DORV in humans, which presents a significant insight into the molecular pathogenesis of CHD. The results highlight the potential implications for the genetic counseling and personalized treatment of patients with CHD.