Deciphering the Molecular Mechanisms Underlying Inherited and Sporadic Brain Arteriovenous Malformations

About This Grant

SUMMARY Brain arteriovenous malformations (bAVMs) are composed of abnormal connections between arteries and veins that lack an intervening capillary network. As a result, high-pressure blood from feeding arteries shunts directly into veins. These vascular lesions undergo extensive remodeling, resulting in a tangle of enlarged blood vessels that are prone to rupture. Accordingly, bAVMs are the leading cause of hemorrhagic stroke in children and young adults. Current treatment options for bAVMs, including surgery, embolization or radiation, all pose a significant risk of disability or death, and these options are not available for ~20% of bAVM patients due to excessive risk. Due to these complications, alternative treatment strategies with lower morbidities, such as targeted pharmacological therapies, are required. Most bAVMs occur sporadically without a family history of the disease. Using whole exome sequencing, we identified somatic, activating mutations in the gene KRAS, which encodes a small GTPase involved in receptor tyrosine kinase signaling. These mutant variants are restricted to the endothelium and result in constitutive KRAS activity. Using mouse and zebrafish models of endothelial-specific expression of mutant KRAS, we demonstrated these patient variants drive bAVM development in vivo. Intriguingly, our recent work showed that endothelial cell cycle is dysregulated in AVMs in the inherited disease Hereditary Hemorrhagic Telangiectasia (HHT). HHT arises from decreased BMP9/10 signaling, which disrupts endothelial quiescence. Notably, CDK4/6 inhibition restores endothelial cell cycle control and prevents AVMs in a neonatal murine model of HHT. Whether sporadic bAVMs, which arise in adults, also feature dysregulated endothelial cell cycle dynamics is unknown, although our preliminary data suggest mutant KRAS promotes cell cycle progression in vivo and in vitro. If sporadic bAVMs (driven by KRAS activation of MEK-ERK activity) and the inherited syndrome HHT (where decreased BMP9/10 signaling increases VEGF signaling) act on shared downstream targets, then activating KRAS variants may act as a “second-hit” in HHT to drive bAVMs. Furthermore, drugs that disrupt their common effectors may be potential non-surgical therapeutic options for both inherited and sporadic bAVM. We will utilize our expertise in animal models of bAVMs, imaging, cell biology, signaling and single-cell RNA sequencing, to define the morphological, cellular, and molecular changes that are involved in the initiation and maintenance of sporadic and inherited bAVMs. Our proposal will determine: 1) if the cell cycle is dysregulated in the endothelium of bAVMs in a model of HHT; 2) the cell-autonomous and non-cell autonomous mechanisms driving bAVM progression in sporadic bAVM; 3) whether Kras gain-of-function represents a novel “second-hit” in HHT patients; and 4) the requirement for cell cycle dysregulation for AVM initiation and maintenance in our pre-clinical animal models. Together, these studies will expand our understanding of bAVM pathogenesis and will determine if normalizing endothelial cell cycle dynamics may be a rational therapeutic target to pursue in human patients with both inherited and sporadic bAVMs.