Environmental Sensitivity to Trichloroethylene (TCE) in the Developing Heart

Abstract

Epidemiological reports first suggested a specific teratogenicity of trichloroethylene (TCE) in drinking water but due to other potential contaminants, uncertain exposure levels and selection of controls, these reports were controversial. Early animal studies explored the effects of high doses of TCE on heart development and a drinking water model was established that showed the ability of TCE to produce cardiac malformations. However, these doses were well above typical levels of environmental exposure and the significance of TCE as a cardiac teratogen remained controversial. Using molecular and functional measures of cardiac development, several laboratories began to examine the effects of TCE in animal models at relevant exposure levels. This work demonstrated a non-monotonic dose response where significantly more effects on gene expression and cardiac function were seen just above the current maximum contamination level (5 parts per billion) than at much higher dose levels. An examination of early heart valve formation showed that formation of valve progenitors was impaired. Molecular studies pointed towards changes in expression of several muscle genes involved in calcium homeostasis and myocardial contraction. Further studies showed that calcium-mediated contraction in the heart was impaired and that this corresponded to changes in intracellular calcium flux and cardiac output. To explore the non-monotonic dose curve, expression of phase I metabolic enzymes were examined. The early heart was found to be a specific site of cytochrome p450 expression prior to the development of the liver and CYP2C expression was elevated with TCE exposure. Surprisingly, the CYP expression pattern replicated the non-monotonic dose response and did not explain it. This suggests that a metabolite of TCE is likely to be the teratogen. To explore the teratogenicity of TCE, microarray data from exposed chick hearts were analyzed by utilization of interactome analysis. This analysis identified a subset of highly-linked genes that were perturbed by TCE exposure. The most centrally linked gene in the interactome was the transcription factor HNF4a. Though not previously known in the heart, HNF4a expression was confirmed. Its level of expression was unchanged with TCE exposure but its level of phosphorylation was altered. Ongoing studies are investigating the hypothesis that HNF4a is a proximal target of TCE exposure and that misregulation of gene expression by this transcription factor is a significant mediator of congenital heart defects produced by TCE.