Understanding Cardiac Development Through the Perspective of Gene Regulation and Gene Manipulation

Cardiac abnormalities induced by zinc deficiency are associated with alterations in the expression of genes regulated by the zinc-finger transcription factor GATA-4

Zinc (Zn) deficiency during pregnancy results in a wide variety of developmental abnormalities. The objective of this study was to determine if expression of cardiac developmental genes regulated by Zn-finger transcription factors could be modulated during dietary Zn deficiency. Rats were fed 0.5 (low Zn) or 90 (controls) microg Zn/g diet throughout pregnancy. Fetal development was examined and RNA isolated at gestation day (GD) 13 and 20. Cardiac abnormalities were detected at GD 20 in 82% of fetuses from dams fed low Zn diets compared with only 2% in controls. Cardiac developmental gene expression regulated by the Zn-finger transcription factor, GATA-4, was measured by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). In GD 13 and 20 hearts, two genes critical for heart development, alpha-myosin heavy chain (alpha-MHC) and cardiac troponin I (cTnI), were down-regulated in Zn-deficient fetuses. Expression of alpha-MHC was 66 and 40% lower at GD 13 and 20, respectively, in fetuses from dams fed low Zn diets compared with fetuses from control dams (p<0.05). Fetal cardiac TnI RNA levels were reduced 40 and 45% at GD 13 and 20 in the Zn-deficient group compared with controls (p<0.05). Fetal cardiac transcript levels of GATA-4 and MHox, a gene regulated by a helix-loop-helix transcription factor, whose expressions are not Zn-dependent, were unaffected by diet. These data indicated that alterations in gene regulation might be an underlying mechanism of cardiac abnormalities. Dysfunction of other Zn-dependent transcription factors may be an integral part of the extensive teratogenesis associated with Zn deficiency.

What cardiovascular defect does my prenatal mouse mutant have, and why?

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.