A Novel TBX1 Variant Causing Hypoparathyroidism and Deafness

The TBX1 gene encodes the T-box 1 protein that is a transcription factor involved in development. Haploinsufficiency of the TBX1 gene is reported to cause features similar to DiGeorge syndrome. The TBX1 gene is located within the DiGeorge syndrome region, and studies support that the TBX1gene is responsible for most of the features of the phenotype of hemizygous deletion of chromosome 22q11.2. In this study, we report a family of 4 (a father with 3 children) who presented with congenital hypoparathyroidism and hypocalcemia, facial asymmetry, deafness, normal intelligence, and no cardiac involvement.

We performed whole genome sequencing, computational structural analysis of the mutants, and gene expression studies for all affected family members.

Whole genome sequencing revealed a paternal inherited novel heterozygous variant, c.1158_1159delinsT p.(Gly387Alafs*73), in the exon 9 isoform C TBX1 gene, causing a loss of nuclear localization sequence (NLS) and transactivation domain (TAD) with no change in gene expression and resulted in a DiGeorge-like phenotype.

A pathogenic variant in the TBX1 gene exon 9 C that predicted to cause a loss in the NLS region and most of TAD leads to variable features of hypoparathyroidism, distinctive facial features, deafness, and no cardiac involvement. In addition, our report and previous reports indicate the presence of a wide phenotypic spectrum of TBX1 genetic variants and the consistent absence of cardiac involvement in the case of pathogenic variants on exon 9 isoform C TBX1 gene.