Thyroid Hormone Receptor α Mutations Cause Heart Defects in Zebrafish

Background:Mutations of thyroid hormone receptor alpha 1 (TR alpha 1) cause resistance to thyroid hormone (RTH alpha). Patients exhibit growth retardation, delayed bone development, anemia, and bradycardia. By using mouse models of RTH alpha, much has been learned about the molecular actions of TR alpha 1 mutants that underlie these abnormalities in adults. Using zebrafish models of RTH alpha that we have recently created, we aimed to understand how TR alpha 1 mutants affect the heart function during this period. Methods:In contrast to human and mice, thethragene is duplicated,thraaandthrab, in zebrafish. Using CRISPR/Cas9-mediated targeted mutagenesis, we created C-terminal mutations in each of two duplicatedthragenes in zebrafish (thraa 8-bp insertionorthrab 1-bp insertionmutations). We recently showed that these mutant fish faithfully recapitulated growth retardation as found in patients and thramutant mice. In the present study, we used histological analysis, gene expression profiles, confocal fluorescence, and transmission electron microscopy (TEM) to comprehensively analyze the phenotypic characteristics of mutant fish heart during development. Results:We found both a dilated atrium and an abnormally shaped ventricle in adult mutant fish. The retention of red blood cells in the two abnormal heart chambers, and the decreased circulating blood speed and reduced expression of contractile genes indicated weakened contractility in the heart of mutant fish. These abnormalities were detected in mutant fish as early as 35 days postfertilization (juveniles). Furthermore, the expression of genes associated with the sarcomere assembly was suppressed in the heart of mutant fish, resulting in abnormalities of sarcomere organization as revealed by TEM, suggesting that the abnormal sarcomere organization could underlie the bradycardia exhibited in mutant fish. Conclusions:Using a zebrafish model of RTH alpha, the present study demonstrated for the first time that TR alpha 1 mutants could act to cause abnormal heart structure, weaken contractility, and disrupt sarcomere organization that affect heart functions. These findings provide new insights into the bradycardia found in RTH alpha patients.