First Genotype-Phenotype Study in TBX4 Syndrome: Gain-of-Function Mutations Causative for Lung Disease.

Abstract

<b>Rationale:</b> Despite the increased recognition of <i>TBX4</i> (T-BOX transcription factor 4)-associated pulmonary arterial hypertension (PAH), genotype-phenotype associations are lacking and may provide important insights. <b>Objectives:</b> To compile and functionally characterize all <i>TBX4</i> variants reported to date and undertake a comprehensive genotype-phenotype analysis. <b>Methods:</b> We assembled a multicenter cohort of 137 patients harboring monoallelic <i>TBX4</i> variants and assessed the pathogenicity of missense variation (<i>n</i> = 42) using a novel luciferase reporter assay containing T-BOX binding motifs. We sought genotype-phenotype correlations and undertook a comparative analysis with patients with PAH with <i>BMPR2</i> (Bone Morphogenetic Protein Receptor type 2) causal variants (<i>n</i> = 162) or no identified variants in PAH-associated genes (<i>n</i> = 741) genotyped via the National Institute for Health Research BioResource-Rare Diseases. <b>Measurements and Main Results:</b> Functional assessment of <i>TBX4</i> missense variants led to the novel finding of gain-of-function effects associated with older age at diagnosis of lung disease compared with loss-of-function effects (<i>P</i> = 0.038). Variants located in the T-BOX and nuclear localization domains were associated with earlier presentation (<i>P</i> = 0.005) and increased incidence of interstitial lung disease (<i>P</i> = 0.003). Event-free survival (death or transplantation) was shorter in the T-BOX group (<i>P</i> = 0.022), although age had a significant effect in the hazard model (<i>P</i> = 0.0461). Carriers of <i>TBX4</i> variants were diagnosed at a younger age (<i>P</i> < 0.001) and had worse baseline lung function (FEV₁, FVC) (<i>P</i> = 0.009) than the <i>BMPR2</i> and no identified causal variant groups. <b>Conclusions:</b> We demonstrated that <i>TBX4</i> syndrome is not strictly the result of haploinsufficiency but can also be caused by gain of function. The pleiotropic effects of <i>TBX4</i> in lung disease may be in part explained by the differential effect of pathogenic mutations located in critical protein domains.