Hemizygous deletion of chromosome 22q11 (del22q11) causes thymic, parathyroid, craniofacial and life-threatening cardiovascular birth defects in 1 in 4,000 infants. The del22q11 syndrome is likely caused by haploinsufficiency of TBX1, but its variable expressivity indicates the involvement of additional modifiers. Here, we report that absence of the Vegf164 isoform caused birth defects in mice, reminiscent of those found in del22q11 patients. The close correlation of birth and vascular defects indicated that vascular dysgenesis may pathogenetically contribute to the birth defects. Vegf interacted with Tbx1, as Tbx1 expression was reduced in Vegf164-deficient embryos and knocked-down vegf levels enhanced the pharyngeal arch artery defects induced by tbx1 knockdown in zebrafish. Moreover, initial evidence suggested that a VEGF promoter haplotype was associated with an increased risk for cardiovascular birth defects in del22q11 individuals. These genetic data in mouse, fish and human indicate that VEGF is a modifier of cardiovascular birth defects in the del22q11 syndrome.

Since the advent of mouse targeted mutations, gene traps, an escalating use of a variety of complex transgenic manipulations, and large-scale chemical mutagenesis projects yielding many mutants with cardiovascular defects, it has become increasingly evident that defects within the heart and vascular system are largely responsible for the observed in utero lethality of the embryo and early fetus. If a transgenically altered embryo survives implantation but fails to be born, it usually indicates that there is some form of lethal cardiovascular defect present. A number of embryonic organ and body systems, including the central nervous system, gut, lungs, urogenital system, and musculoskeletal system appear to have little or no survival value in utero (Copp, 1995). Cardiovascular abnormalities include the failure to establish an adequate yolk-sac vascular circulation, which results in early lethality (E8.5-10.5); poor cardiac function (E9.0-birth); failure to undergo correct looping and chamber formation of the primitive heart tube (E9.0-11.0); improper septation, including division of the common ventricle and atria and the establishment of a divided outflow tract (E11.0-13.0); inadequate establishment of the cardiac conduction system (E12.0-birth); and the failure of the in utero cardiovascular system to adapt to adult life (birth) and close the interatrial and aorta-pulmonary trunk shunts that are required for normal fetal life. Importantly, the developmental timing of lethality is usually a good indicator of both the type of the cardiovascular defect present and may also suggest the possible underlying cause/s. The purpose of this review is both to review the literature and to provide a beginner's guide for analysing cardiovascular defects in mouse mutants.

Na+/Ca2+ exchanger (Ncx‐1) is highly expressed in cardiomyocytes, is thought to be required to maintain a low intracellular Ca2+ concentration, and may play a role in excitation‐contraction coupling. Significantly, targeted deletion of Ncx‐1 results in Ncx1‐null embryos that do not have a spontaneously beating heart and die in utero. Ultrastructural analysis revealed gross anomalies in the Ncx1‐null contractile apparatus, but physiologic analysis showed normal field‐stimulated Ca2+ transients, suggesting that Ncx‐1 function may not be critical for Ca2+ extrusion from the cytosol as previously thought. Using caffeine to empty the intracellular Ca2+ stores, we show that the sarcoplasmic reticulum is not fully functional within the 9.5‐dpc mouse heart, indicating that the sarcoplasmic reticulum is unlikely to account for the unexpected maintenance of intracellular Ca2+ homeostasis. Using the Ncx1‐lacZ reporter, our data indicate restricted expression patterns of Ncx1 and that Ncx1 is highly expressed within the conduction system, suggesting Ncx1 may be required for spontaneous pacemaking activity. To test this hypothesis, we used transgenic mice overexpressing one of the two known adult Ncx1 isoforms under the control of the cardiac‐specific a‐myosin heavy chain promoter to restore Ncx1 expression within the Ncx1‐null hearts. Results indicate that the transgenic re‐expression of one Ncx1 isoform was unable to rescue the lethal null mutant phenotype. Furthermore, our in situ results indicate that both known adult Ncx1 isoforms are coexpressed within the embryonic heart, suggesting that effective transgenic rescue may require the presence of both isoforms within the developing heart.