Vulnerability of the Developing Heart to Oxygen Deprivation as a Cause of Congenital Heart Defects

Background-The heart develops under reduced and varying oxygen concentrations, yet there is little understanding of oxygen metabolism in the normal and mal-development of the heart. Here we used a novel reagent, the ODD-Luc hypoxia reporter mouse (oxygen degradation domain, ODD) of Hif-1 alpha fused to Luciferase (Luc), to assay the activity of the oxygen sensor, prolyl hydroxylase, and oxygen reserve, in the developing heart. We tested the role of hypoxia-dependent responses in heart development by targeted inactivation of Hif-1 alpha. Methods and Results-ODD-Luciferase activity was 14-fold higher in mouse embryonic day 10.5 (E10.5) versus adult heart and liver tissue lysates. ODD-Luc activity decreased in 2 stages, the first corresponding with the formation of a functional cardiovascular system for oxygen delivery at E15.5, and the second after birth consistent with complete oxygenation of the blood and tissues. Reduction of maternal inspired oxygen to 8% for 4 hours caused minimal induction of luciferase activity in the maternal tissues but robust induction in the embryonic tissues in proportion to the basal activity, indicating a lack of oxygen reserve, and corresponding induction of a hypoxia-dependent gene program. Bioluminescent imaging of intact embryos demonstrated highest activity in the outflow portion of the E13.5 heart. Hif-1 alpha inactivation or prolonged hypoxia caused outflow and septation defects only when targeted to this specific developmental window. Conclusions-Low oxygen concentrations and lack of oxygen reserve during a critical phase of heart organogenesis may provide a basis for vulnerability to the development of common septation and conotruncal heart defects.