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Cardiogenesis involves a precisely orchestrated series of molecular and morphogenetic events that combines cell types from multiple lineages. Subtle perturbations of this process can result in life-threatening illnesses in the form of congenital heart defects. As the organ most essential for life, the heart is the first organ to form and functions to support the rapidly growing embryo before it becomes a four-chambered organ. The combination of the complex morphogenetic events necessary for cardiogenesis and the superimposed hemodynamic influences may contribute to the exquisite sensitivity of the heart to perturbations. This phenomenon is reflected in the estimated 10% incidence of severe cardiac malformations observed in early miscarriages. The fraction of congenital heart malformations that are capable of supporting the intrauterine circulation comprises the spectrum of congenital heart disease observed clinically.

Although congenital heart disease was classified in the 18th and 19th centuries based on embryologic considerations, the advent of palliative procedures and clinical management led to a descriptive nomenclature founded on anatomic and physiologic features that directed surgery and medical therapy. However, seemingly unrelated defects may share common embryologic origins from a mechanistic standpoint, and thus the etiology of various defects may be better understood from a developmental standpoint. Recent advances in genetics and molecular biology have stimulated a renaissance in studies designed to define an embryologic framework for understanding congenital heart disease. The ability to go beyond descriptions of the anatomical defects to developing an understanding of the genes responsible for distinct steps of cardiac morphogenesis is necessary for more directed therapeutic and preventive measures.

Although human genetic approaches are important in understanding congenital heart disease, detailed molecular analysis of cardiac development in humans has been difficult. The recognition that cardiac genetic pathways are highly conserved across vastly diverse species from flies to man has resulted in an explosion of information from studies in more tractable and accessible biological models. Chemical mutagenesis studies of the fruit fly (Drosophila) and of the simple vertebrate, zebra fish, have resulted in the identification of many genes that are required for normal cardiac development. Although genetic approaches are not feasible in chick embryos, they have four-chambered hearts and the embryos are easily accessible within the egg for surgical and molecular manipulation during cardiogenesis. Such approaches have been useful for defining the role of populations of cells during development. Finally, the laboratory mouse, a mammal with a cardiovascular system nearly identical to that in humans, has been an invaluable model system for understanding the mechanisms underlying human disease. Advances in technology have made it possible to mutate or to delete specific genes in the mouse genome, either globally or in a tissue-specific manner, and to study the effects of such mutations in mice heterozygous or homozygous for the disrupted gene of interest. Thus, each biological system offers unique opportunities to enhance understanding of cardiogenesis.

Mapping and identification of human congenital heart disease genes is equally important. Correlation of particular amino ...

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