Submission Details

T-Box transcription factor 4 (TBX4) is a part of the T-box gene family which plays an important role in the development of hind limb and pelvis. TBX4 is also expressed in lung mesenchyme during early embryogenesis and regulates lung branching morphogenesis. Mutations in TBX4 were first identified in small patella syndrome (SPS), which is characterized by small or no kneecaps and weak pelvic bones. Genetic evidence: TBX4 was first associated with pulmonary arterial hypertension (PAH) through identification of a 3.75 Mb microdeletion of 17q22q23.2 in a patient with multiple congenital anomalies and PAH (PMID: 21271665). However, the deletion region included 15 known genes, and therefore this case could not be scored. Subsequent publications identified intragenic TBX4 mutations in PAH. Two patients with insertions in TBX4 gene causing a premature stop codon were scored from PMID: 23592887. These patients had SPS and PAH phenotypes. Three nonsense mutations were scored from PMID: 27587546, and the patients had idiopathic or familial forms of PAH. Two nonsense mutations (one de novo) and one deletion were scored from PMID: 29631995, and the patients either had familial or idiopathic forms of PAH. These variants yielded a total score of 12 for genetic evidence. Two more recent studies PMID: 29650961 and PMID: 31727138 add a further 7 and 8 frameshift/stop-gain mutations respectively which were not scored, but indicate very strong replication over time. Experimental evidence: Expression of TBX4 and functional consequence of variants in the gene are not yet widely studied in relevance to PAH. PMID: 22876201 demonstrated strong expression of TBX4 in developing lung and trachea. PMID: 33385213 reported that in vitro knockdown of TBX4 resulted in decreased p-Smad1/5. Two patient-derived lymphoblast lines (one with a frameshift deletion and other with complete deletion of TBX4) also had significantly lower levels of p-Smad1/5 when compared to control cells. Smad phosphorylation is a key mediator of bone morphogenetic signaling, and is known to be attenuated in PAH, especially in cells with a BMPR2 mutation. Therefore, this study was scored for biochemical function, since the effect of reduced TBX4 levels on Smad phosphorylation is similar to that caused by haploinsufficiency for BMPR2, the most common cause of heritable PAH. Overall, the experimental evidence from both the studies yielded a score of 1. The total score including both genetic and experimental evidence was 13 and since there is replication over time, the evidence was finally classified as DEFINITIVE.