ATM and pulmonary arterial hypertension: To determine whether there is a cause-and-effect relationship between DNA damage and persistent PAH, we reduced the level of the DNA damage sensor ATM in human PAEC by ATM siRNA vs. control siRNA and subjected the cells to normoxia for 72 h or to reoxygenation (0.5% hypoxia for 48 h followed by room air for 24 h).