Abstract Body: Moyamoya disease (MMD) is a rare cerebrovascular disorder that is characterized by steno-occlusive lesions in the distal internal carotid arteries (ICAs), which can lead to transient ischemic attacks or strokes. It is one of the most common causes of pediatric stroke, and importantly, pediatric patients present with no risk factors, such as hypertension, that could cause chronic vascular damage, suggesting that MMD may have a significant genetic component. The disease is established to be genetically heterogeneous, with multiple identified genes accounting for a small percentage of familial cases. Despite the identification of pathogenic variants linked to MMD, including a founder variant in the gene RNF213 that increases the risk for MMD in East Asian populations, the mechanism by which these pathogenic variants cause MMD remains unknown. Pathologic examination of internal carotid arteries from MMD patients shows lesions containing cells that stain positive for the smooth muscle cell (SMC) marker, smooth muscle α-actin (SMA). This suggests that SMCs move from the wall into the lumen and then proliferate forming occlusive lesions. Heterozygous ACTA2 p.R179 mutations predispose to thoracic aortic aneurysm and dissection (TAAD) and bilateral MMD-like occlusion of the distal ICAs. SMCs from a conditional knock-in mouse model, termed Acta2^SMC-R179C/+, are incompletely differentiated, leading to enhanced SMC migration and proliferation that drive MMD-like occlusive lesions. Furthermore, similar to stem cells, Acta2^SMC-R179C/+ SMCs have increased glycolysis and reduced oxidative respiration on Seahorse analyses. Treatment with nicotinamide riboside (NR) increases SMC differentiation, decreases migration, and restores oxidative respiration. To assess whether this SMC phenotype occurs with other MMD pathogenic variants, I assessed SMCs with a heterozygous RNF213 p. F4120L mutation (introduced in human iPSCs and differentiated), which is a mutation that leads to severe early-onset pediatric MMD. RNF213, p.F420L, iPSC-derived SMCs show evidence of incomplete differentiation. These data provide preliminary evidence that incomplete differentiation of SMCs is a common molecular mechanism across pathogenic variants linked to MMD.