Molecular genetics of congenital heart disease. A problem of faulty septation.

The transition from the single circulation of the embryo to the double circulation of the neonatal and adult heart involves the transformation of the primitive heart tube through a complex morphogenetic process, resulting in completely separated right and left heart chambers and distinct pulmonary and systemic circulations. Septation of heart chambers starts at early stages in embryogenesis and is completed only at birth with the closure of the foramen ovale. Defects in heart septation, including atrial septal defects, atrioventricular canal septal defects, ventricular septal defects, and conotruncal septal defects, represent a major cause of congenital heart disease. The molecular basis of faulty heart septation is now the object of intensive investigation. Mutant genes coding for 2 transcription factors, TBX5 and NKX2.5, have been recently identified by positional cloning in families with high incidence of atrial or ventricular septal defects. Mutations of the TBX5 gene cause the Holt-Oram syndrome, a developmental disorder affecting the heart and the upper limb, the most frequent cardiac abnormalities being atrial and/or ventricular septal defects and conduction defects.1,2 Mutations of the homeobox transcription factor NKX2.5 cause nonsyndromic congenital heart disease, in particular, atrial septal defects and atrioventricular node dysfunction.3 Most mutations so far identified in the TBX5 and NKX2.5 genes induce the formation of truncated proteins resulting in haploinsufficiency. On the other hand, the atrioventricular canal septal defects frequently associated with Down syndrome are probably due to gene overexpression rather than deficiency, namely the presence of 3 copies of chromosome 21 genes. The study of rare patients with partial chromosome 21 trisomy and atrioventricular canal septal defects has allowed the definition of a 2.5-Mb critical region at 21q22.2-q22.3, which is responsible for cardiac malformations: this region should therefore contain one or more genes whose overexpression interferes with correct atrioventricular canal septation.4 Progress in disease gene discovery can be accelerated by the convergence of basic research in flies and mice with human molecular genetics. The role of the NKX2.5 gene in congenital heart disease is a case in point. A gene called tinman is essential for the development of the heart in Drosophila, and the inactivation of a murine homolog of tinman, Nkx2.5, causes arrested development of the primitive heart tube.5 The human NKX2.5 gene maps to chromosome 5q35 and when a locus in the very same position was mapped by linkage analysis in families with a high incidence of atrial septal defects, the NKX2.5 gene was an obvious candidate, and mutations of this gene were rapidly identified. The chick embryo is also a powerful model to identify candidate genes responsible for congenital heart disease, because experimental manipulations such as transplanting and backtransplanting specific tissues are feasible in the chick but not in the mammalian embryo. In this issue of Circulation Research, Margaret Kirby and her coworkers have made clever use of the chick embryo system to explore the role of Hira, a candidate gene for the DiGeorge syndrome (DGS), and the velocardiofacial syndrome (VCFS), in outflow septation.6