Introduction:
Reduced oxygen delivery in complex congenital heart disease (CHD) can lead to brain white matter (WM) injury in utero. Currently, no treatment exists. Tetrahydrobiopterin (BH4) is a cofactor for neuronal nitric oxide synthase, and in its absence, toxic peroxynitrite production is favored. Hypoxia activates nitric oxide synthase, which reduces BH4 availability.
Hypothesis:
Decreased BH4 levels underlie WM injury in the fetus with CHD, and treatment with BH4 will reduce this injury.
Methods:
Mice were divided into three groups: normoxic controls (Nx), hypoxic (Hx), and hypoxic with BH4 treatment (Hx-BH4). Hx and Hx-BH4 mice were kept at 10.5% FiO2 from postnatal day 3 to 11--a period of WM development equivalent to the 3rd trimester in humans. Brain BH4 levels were quantified and compared between Nx (n=11) and Hx (n=12). Densities of cells expressing CNP (marker of oligodendrocytes--cells responsible for myelination) and Caspase3 (apoptosis marker) were quantified in three WM regions and compared between groups (n=3-6 each). Western blot detected myelin basic protein (myelin marker).
Results:
Brain BH4 levels were depleted in Hx compared to Nx (-38.4%, p=0.02). CNP+ oligodendrocytes increased after Hx compared to Nx (Fig.1a), consistent with hypoxia-induced proliferation seen previously. BH4 treatment did not limit this proliferation (Fig.1a). Hx had increased WM apoptosis (Fig.1b), which decreased with BH4 treatment (Fig.1b). Remarkably, there was no difference in WM caspase3+ cells between Nx and Hx-BH4 (Fig.1b). There was no difference in effect across WM region. Finally, loss of myelin with hypoxia was mitigated by BH4 treatment (Fig.1c).
Conclusions:
Our results show that suboptimal BH4 levels influence hypoxic WM injury. BH4 treatment of phenylketonuria is safe during pregnancy, thus maternal BH4 therapy is feasible. Our data demonstrate that repurposing BH4 for use during fetal brain development has potential to limit WM injury in CHD.
Influence of Hypoxia on Nitric Oxide Synthase Activity and Gene Expression in Children With Congenital Heart Disease