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TBX5 (NM_000192.3) encodes the T-box transcription factor 5 which is involved in the regulation of developmental processes and plays a role in heart development and specification of limb identity. TBX5 is located at 12q24.21 and consists of 8 coding exons. Heterozygous mutations in TBX5 have been associated with autosomal dominant Holt-Oram syndrome (HOS) that comprises upper limb and cardiac defects, typically septal and conduction defects (Li QY et al. Nat Genet. 1997;15:21-29; Basson CT et al. Nat Genet. 1997;15:30-35). Both familial and sporadic cases have been reported (Yang J et al. Am J of Med Genet. 2000;92:237-240) and the majority are thought to be due to haploinsufficiency of TBX5 (Basson CT et al. Nat Genet. 1997;15:30-35). However, gain-of-function TBX5 mutations have also been identified (Postma AV et al. Circ Res. 2008;102:1433-1442; Baban A et al. Am J Med Genet. 2014;164A:1419-1424).