Pregestational maternal diabetes is a noninherited factor associated with a fivefold increase in congenital heart defects (CHDs). The second heart field (SHF) progenitors, marked by Isl1, drive the heart tube extension during looping morphogenesis and cardiac chamber formation. The underlying mechanism of diabetes-induced CHDs is unknown but one mechanism may involve the inhibition of Isl1+ SHF progenitor-driven cardiogenesis by maternal diabetes. During the last decade, we have focused on the biology and regenerative capability of cardiac progenitors in CHD patients. It is critical to determine the biological effects of diabetes in vivo and high glucose in vitro on Isl1+ progenitors during embryogenesis and postnatally in order to maximize their regenerative and protective potentials in CHD patients. Therefore, our overarching hypothesis that hyperglycemia of maternal diabetes induces Isl1+ SHF progenitor dysfunction during the critical period of cardiac development through heightened oxidative stress, activation of the major UPR sensor IRE1? and its downstream transcription factor XBP1, which is responsible for DNA hypermethylation and SHF gene silencing leading to repression of RNA methyltransferase METTL14 and m6A RNA methylation. Suppressing cellular stress or modulating DNA/RNA methylation ameliorates defects in SHF progenitors, CHD formation and potential regenerative capacity of these progenitors. Aim 1 will determine whether hyperglycemia of maternal diabetes induces Isl1+ SHF progenitor dysfunction during heart development through oxidative stress. We hypothesize that diabetes causes mitochondrial dysfunction and during cardiac morphogenesis through the induction of oxidative stress and that mitigation of oxidative stress by superoxide dismutase 1 (SOD1) alleviates CHD formation in diabetic pregnancy. Aim 2 will determine the role of the major UPR sensor IRE1? and its downstream effector XBP1 in Isl1+ SHF progenitors leading to CHDs in diabetic pregnancy. We will test the hypothesis that oxidative stress is responsible for ER stress and UPR in Isl1+ SHF progenitors and that suppressing the ER stress-UPR pathway by inactivating either the major UPR sensor IRE1? or its downstream transcription factor XBP1 reduces diabetes-induced CHDs. Aim 3 will determine whether DNA methyltransferases-suppressed RNA methylation in Isl1+ SHF progenitors contributes to diabetes- induced CHDs and the therapeutic implications of these progenitors. We expect that that increased DNA methylation represses RNA methyltransferase-like 14 (METTL14) and RNA N(6)-methyladenosine (m6A) essential for Isl1+ progenitor function and that reducing DNA methylation or restoring RNA methylation specifically in Isl1+ progenitors reduces CHDs and increases the therapeutic values of these cells.