Investigating Embryonic Gene Expression Changes in a Rat Model of Congenital Heart Defects and Longitudinal Effects on Postnatal Cardiac Structure and Function

Abstract

Congenital heart defects (CHD) are the most common birth defect, occurring in approximately 1% of all births and continue to be a significant cause of infant morbidity and mortality. Ventricular septal defects (VSD) account for the majority of CHDs and result from incomplete formation of the interventricular septum. To produce a population of offspring with VSD in a reproducible and dose-dependent manner we administered dimethadione (DMO) to pregnant Sprague-Dawley rats during the sensitive window of embryonic heart development. DMO, a discontinued anticonvulsant drug, is a potent heart teratogen; when administered in six doses it produces 74% incidence of VSD with poor postnatal viability, whereas four doses produces 43% VSD with greater postnatal survival. We aimed to use these models to test the hypothesis that in utero exposure to DMO interferes with normal gene expression patterning in the developing heart, resulting in CHDs which spontaneously resolve postnatally, yet exhibit maladaptive responses when challenged with isoproterenol (INE), a cardiac stressor. In Chapter 2 using the six dose model, we show that DMO exposure decreased mRNA expression of Gata4 in left ventricle and increased the expression of Myl2 in the atria, prior to formation of the ventricular septum. A critical review of the uses and technical considerations required for the use of echocardiography in developmental toxicity studies is provided in Chapter 3. This method of assessment was used in Chapter 4 to characterize the structural and functional development of male rat hearts exposed to four doses of DMO in utero. In this study, differences from control that were observed early postnatally, such as stroke volume, resolved by weaning, but re-emerged in adulthood. Using this cohort of body weight matched rats, we used increasing, intermittent doses of INE to stress the heart, as described in Chapter 5, and the cardiac effects were monitored via ultrasound. We observed decreased structural parameters in the DMO exposed rats, including left ventricular mass and diameter, and differences in function, evidenced by the changes in longitudinal fractional shortening. Taken together, these findings contribute to our understanding of how in utero chemical exposures may lead to VSDs and the importance of long-term follow up in spontaneously resolved VSDs and the potential for altered adaptation to cardiac stress in later life.