Congenital heart defects: trapping the genetic culprits.

More children die from congenital heart disease (CHD) each year than are diagnosed with cancer. Although CHD is the most common human birth defect, the etiology of the vast majority of CHDs remains unknown. The difficulty in elucidating the molecular and genetic bases for CHD reflects the complex nature of this and other birth defects. Most CHD occurs sporadically, but the recurrence risk in subsequent pregnancies of parents with one affected child is '5%, or 5-fold higher than in other pregnancies. For some types of CHD, the recurrence risk can be as high as 10% to 15%.1 Thus, a genetic predisposition to CHD is likely present in affected individuals, but a second or third insult may occur during embryogenesis to contribute to the cardiac defects observed clinically. This possibility is similar to the multiple-hit theory in the pathogenesis of cancer and could come in the form of environmental or other genetic factors that accentuate effects of predisposing genetic mutations. Evidence for this theory in CHD comes from patients with similar genetic mutations who display widely varying forms of CHD or no CHD at all.2,3 This complexity and the often lethal nature of CHD have resulted in a paucity of large human families amenable to genetic linkage analysis,4,5 limiting the usefulness of classical genetic analyses. The study by Lee et al6 in this issue of Circulation Research provides evidence that a more systematic approach directed at identifying genes important in cardiac formation may yield insight into the bases of CHD. Lyons and his colleagues have harnessed the powerful genetics available in mice and performed a large-scale screen to search for genes that are activated during the critical periods of cardiogenesis. By combining embryonic stem (ES) cell technology and a retroviral-based gene trap approach, lacZ was randomly incorporated into intronic regions of individual genes in ES cells.7,8 As a result, cells that normally transcribed the gene in which lacZ had been trapped were marked by the color blue. Subsequent differentiation of ES cells into embryoid bodies containing beating cardiomyocytes allowed for rapid screening of targeted genes expressed in the heart. Sequencing genes that had trapped lacZ could lead to identification of novel genes active during cardiac development or demonstrate cardiac expression of previously known genes. As an added bonus, genes containing lacZ were usually disrupted by insertion of the lacZ transgene, resulting in ES cells containing heterozygous mutations of individual genes. Such ES cell clones could be used to generate mice heterozygous for disrupted genes with lacZ “knocked in” to specific loci. Through this approach, Lee et al6 demonstrated that the previously known gene implicated in neural tube formation, jumonji (jmj),9 was expressed at high levels in the developing heart. Taking advantage of the insertion of lacZ into the jmj gene through the screen described above, these investigators generated mouse mutants homozygous for the jmj gene disruption (jmj) to determine its functional role during cardiogenesis. jmj animals died soon after birth from cardiac and respiratory defects.6 The cardiac defects shared some similarity to the types of CHD seen in humans. All animals had ventricular septal defects (VSDs), the most common cardiac defect in humans. Most also had an abnormality of alignment of the outflow tract, known as doubleoutlet right ventricle (DORV), in which the aorta and pulmonary artery arise from the right ventricle. Malalignment of the outflow tract results in an obligatory VSD because of displacement of the conal part of the ventricular septum at the level of the great vessels. This, too, is a common defect seen in tertiary care centers and is often associated with other defects of alignment of the inflow and outflow tracts. Demonstration that the gene trap method was effective in identifying a gene necessary for normal cardiac development provides hope that this approach may be useful in a systematic search for genes involved in cardiogenesis. Like many mouse knockout strategies, the gene trap method falls short of elucidating the functional pathways involving jmj. Although Lee et al6 show that jmj is a nuclear protein, its cellular role remains unclear. Whether it functions in regulating transcription, as suggested, remains to be determined and will require alternative approaches. Similarly, defects in cell proliferation are suggested but are yet to be documented in jmj mice. Beyond a cellular role, the precise role of jmj in cardiogenesis remains unknown. DORV has been documented in numerous mouse mutants and likely results from abnormalities affecting one of two broad aspects of cardiogenesis: cardiac looping and cardiac neural crest development. The process of cardiac looping is a critical event in the development of a 4-chambered heart and is necessary for proper alignment of atrioventricular and ventriculoarterial connections.10 Abnormalities of looping often result in DORV, as evidenced by the high incidence of DORV in patients with heterotaxy syndrome in which left-right asymmetry and The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Departments of Pediatrics (Cardiology) and Molecular Biology, University of Texas Southwestern Medical Center, Dallas, Tex. Correspondence to Deepak Srivastava at the Departments of Pediatrics (Cardiology) and Molecular Biology, University of Texas Southwestern Medical Center, 6000 Harry Hines Blvd, Dallas, TX 75235-9148. E-mail [email protected] (Circ Res. 2000;86:917-918.) © 2000 American Heart Association, Inc.