Genetic Architecture of Susceptibility to PCB126-Induced Developmental Cardiotoxicity in Zebrafish

Polychlorinated dibenzo-p-dioxins and structurally related dioxin-like compounds (DLCs), including coplanar polychlorinated biphenyls (PCBs), are potent teratogens that persist in the environment and pose significant risk to human and ecological health. Amid the toxic effects associated with DLC exposure, a mounting body of evidence suggests that the cardiovascular system is one of the primary targets during development. Aberrant morphogenesis and dysfunction of the heart are observed following early-life stage exposure. Among vertebrates, this cardiotoxicity is evolutionarily conserved. The cardioteratogenic effects of DLCs have been most extensively studied in zebrafish. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 3,3′,4,4′,5-pentachlorobiphenyl (PCB126) arrests ventricular cardiomyocyte cell cycle (Antkiewicz et al., 2005), impairs atrioventricular and bulboventricular valve development (Grimes et al., 2008; Mehta et al., 2008), and prevents normal looping of the heart (Carney et al., 2006b). The hypoplastic heart is accompanied by a reduction in blood flow, pericardial edema, and a slowed heart beat with intermittent ventricular standstill (Antkiewicz et al., 2005). Morphological and functional deficits are also manifested in higher vertebrates following early-life stage DLC exposure. Chicken embryos exposed to dioxin and dioxin-like PCBs exhibit compromised endocardial cushion, thin ventricular walls, a reduction in cardiomyocyte proliferation, reduced ventricular function, and edema (Ivnitski et al., 2001; Walker and Catron, 2000). Mice exposed in utero exhibit a dose-related decrease in heart-to-body weight ratio, interventricular septal defects, significant decrease in cardiomyocyte proliferation, and diminished heart rate (Lin et al., 2001; Thackaberry et al., 2005b). Susceptibility to DLCs has been shown to vary within and among several piscine, avian, and mammalian species. Resistance to DLC-induced early-life stage effects has been observed in perch, tomcod, rainbow trout, and killifish populations residing in habitats with PCB-contaminated sediment (Carvalho et al., 2004; Forlin and Celander, 1995; Nacci et al., 1999; Wirgin et al., 2011; Yuan et al., 2006). Differences in susceptibility to DLC-induced cardiomyopathy between two breeds of chickens have been reported (Walker and Catron, 2000). Responsiveness to DLCs has also been shown to differ among lines of inbred mice (reviewed in Nebert, 1989). Moreover, in humans, differences in the inducibility of DLC-responsive genes have been reported (Micka et al., 1997). Susceptibility to environmental toxicants is determined by an individual’s genetic background, environment, gene × environment interactions, and epistasis. In the postgenomic era, the interindividual variation that exists in DLC-induced cardiotoxicity can be leveraged to elucidate the genetic factors influencing vulnerability. Genome-wide comparisons of phenotypically discordant individuals can be made to link phenotypic variation to its underlying genetic basis, offering new insights into toxicological mechanisms and modes of action. In this study, a quantitative trait loci (QTL) analysis of PCB126-induced developmental cardiotoxicity was performed in zebrafish (Danio rerio). Genetically divergent strains of zebrafish were screened for differences in sensitivity to PCB126, and the most sensitive and tolerant strains were then chosen for a genetic cross. A bulk segregant analysis (BSA) was performed, comparing extreme discordant phenotypes from the recombinant generation. Candidate loci co-segregating with sensitivity differences were then chosen for QTL analysis. Herein, multiple QTLs are reported, identified from a genome-wide scan for single- and multiple-gene effects, including additive and epistatic interactions.