Genetic disorders of cardiac morphogenesis. The DiGeorge and velocardiofacial syndromes.

The phenotype associated with a 22q11 deletion is highly variable and still under investigation. Of particular interest to cardiologists and cardiac developmental biologists is the finding that many patients with a 22q11 deletion have conotruncal cardiac defects and aortic arch anomalies. Despite the phenotypic variability, the vast majority of patients have a similar large deletion spanning approximately 2 megabases. The low-frequency repeated sequences at either end of the commonly deleted region may be responsible for the size of the deletion and account for the instability of this chromosomal region. Molecular studies of patients with the DGS/VCFS phenotype and unique chromosomal rearrangements have allowed a minimal critical region for the disease to be defined. Multiple genes have been identified in the minimal critical and larger deleted region. These genes are being investigated for their potential role in the disease pathophysiology by screening for mutations in nondeleted patients with the phenotype and by analysis of the pattern of expression in the developing mouse embryo. Further experimentation in the mouse mammalian model system will be of great utility to help determine whether haploinsufficiency of one critical gene or several genes within the DGCR results in the disease phenotype. Modifying factors, both genetic and environmental, must also be considered. Further investigation into the disease mechanism leading to the DGS/VCFS phenotype will hopefully further our understanding of cardiac development and disease.