Abstract 3392: A Novel TBX5 Mutation Leads to Enhanced DNA-Binding in a Family with Holt-Oram Syndrome and Paroxysmal Atrial Fibrillation
Background: Holt-Oram syndrome (HOS) is a heart-hand syndrome clinically characterized by upper limb- and cardiac malformations. Mutations in TBX5, a member of the T-box transcription factor family, were shown to underlie this syndrome. Here we describe a HOS family of 13 members with paroxysmal atrial fibrillation and demonstrate that a novel TBX5 mutation co-segregates with affected family members, displays enhanced DNA binding affinity, and doubles the Cx40 promoter activity.
Methods and results: Affected family members are characterized by autosomal dominant radial luxation, carpal fusion, scapular dysplasia and variable heart defects (ASD/VSD) without thumb malformations. The majority of the affected family members, including those without (haemodynamically significant) structural heart defects, also developed paroxysmal atrial fibrillation. Sequencing of TBX5 revealed a novel mutation (p.Gly125Arg), in all affected family members, located in the conserved T-box binding element and absent from over 200 controls. Trafficking experiments showed that the mutant TBX5 protein resides in the nucleus. Subsequent EMSA studies demonstrated that the DNA binding affinity of the mutant TBX5 is higher than wildtype TBX5. In contrast to the lack of an effect of the TBX5 mutant on the Nppa promoter, e.g. unaltered activation and dose-dependent transactivation by either NKX2.5 or GATA4, the TBX5 mutant nearly doubles basal activity of the Cx40 promoter in comparison to wildtype TBX5
Conclusions: We describe a novel TBX5 mutation in a family with features of HOS. Contrary to previously published mutations, the underlying p.Gly125Arg TBX5 mutant shows a higher DNA-binding activity, which suggests that the effect of this mutation is through a gain of function mechanism. As the developing heart is highly sensitive to the level of TBX5 gene expression, an increase in the DNA binding affinity of the TBX5 protein may thus be sufficient to elicit HOS. Interestingly, the majority of the affected family members display paroxysmal atrial fibrillation. Taken together with the fact that the mutant TBX5 doubles the activity of the Cx40 promoter, this could hint at a possible role of TBX5 in the development of (paroxysmal) atrial fibrillation in this family.