Transposition of the great arteries. Some gleams of light to pathogenetic knowledge

Transposition of the great arteries (TGA) is a frequent and severe cardiac defect. In patients with this malformation, diagnostic and surgical results and the long-term prognosis significantly improved in the last years. From the embryological point of view there are two main theories: 1) the anomalous infundibular rotation, and 2) the anomaly of the aortico-pulmonary septum. Both of them still present important limits. Moreover, TGA is difficult to reproduce by animal experiments, but interesting data, using retinoid acid in pregnant rats, are nowadays available, as well as there are interesting data from the epidemiologic studies on human teratologic agents. TGA is rarely associated with genetic syndromes and with additional extracardiac anomalies. A few cases are in relation with DiGeorge syndrome with deletion of chromosome 22q11. On the contrary TGA is significantly prevalent, in association with other cardiac and extracardiac anomalies, in children with lateralization defects, heterotaxy and asplenia syndrome (right isomerism). However in patients with heterotaxy and polysplenia syndrome (left isomerism) TGA is significantly more rare. In mice with mutation of Smad2 and NODAL, two genes involved in the lateralization process, some cases of TGA, with or without right isomerism of the lungs, were reported. Moreover, in families with heterotaxy some cases with congenitally corrected TGA were reported and a new gene associated with heterotaxy, CRYPTIC, can present mutations in patients with "isolated" TGA. A recent study on familiar recurrence of TGA shows in the same family some cases of TGA and of corrected TGA so that a monogenic inheritance (autosomic dominant or recessive) with variable phenotypic expression can be suggested. The normal righthand spiralization of the heart is genetically determined in cases of situs solitus and d-loop of the ventricles. This pattern is not present in cases of TGA presenting a parallel position of the great arteries. On the basis of these observations and according to new epidemiologic and genetic data some cases of TGA should be classified in the group of the anomalies of lateralization and ventricular loop. The mystery is still present but perhaps some gleams of light are appearing.