Iron-sulfur (Fe-S) cluster deficiencies have been linked to pulmonary hypertension (PH). An important regulator of Fe-S cluster biogenesis, BolA Family Member 3 (BOLA3) regulates oxidative mitochondrial processes and synthesis of lipoic acid, a key post-translational moiety controlling glycine biosynthesis. Mutations in BOLA3 result in multiple mitochondrial dysfunction syndrome, a fatal disorder associated with PH. However, mechanisms by which BOLA3 drives this disease are not defined.
In hypoxic pulmonary arterial endothelial cells (PAECs) as well as lungs from human Group 1 and Group 3 PH patients and multiple rodent models of PH, BOLA3 expression was downregulated via a HIF-2α-HDAC-mediated epigenetic axis. In vitro studies in PAECs showed that BOLA3 deficiency decreased Fe-S integrity, impairing mitochondrial complexes and lipoate-contained 2-oxoacid dehydrogenases with consequential increase in glycolysis and alteration in mitochondrial respiration. Via either RNA knockdown or naturally occurring human genetic mutation, BOLA3-deficient cells also showed down-regulated expression of lipoate-dependent glycine cleavage system H protein (GCSH), with elevated accumulation of intracellular glycine. Downstream of altered oxidative metabolism and glycine levels, BOLA3 deficiency increased endothelial proliferation and vasoconstriction, while reducing apoptosis and angiogenesis. RNA-mediated knockdown of endothelial BOLA3 and virus-mediated overexpression of BOLA3 in mouse vasculature in vivo demonstrated that BOLA3 deficiency promotes PH with increased pulmonary vascular remodeling and worsened hemodynamics. Importantly, chronic in vivo supplementation of glycine reversed the protective effects of forced expression of BOLA3 in PH, thus proving that glycine upregulation is essential for the control of PH by BOLA3 deficiency.
BOLA3 deficiency plays a critical role in promoting endothelial dysfunction and PH via impairment of mitochondrial oxidative respiration and glycine metabolism. These results provide molecular insights for clinical associations of PH with hyperglycinemic syndromes and mitochondrial disorders and identify novel metabolic targets involved in epigenetics, Fe-S cluster biogenesis and glycine homeostasis for diagnostic and therapeutic development