Cellular and Molecular Drivers of Acute Aortic Dissections

Thoracic aortic aneurysms (TAAs) progressively enlarge over time until an acute tear in the intimal layer at the sinotubular junction leads to an acute type A aortic dissection (AAD). These AADs cause sudden death in up to 50% of afflicted individuals, and another 20% die despite emergent surgical repair of the aorta.25 Less acutely deadly type B AADs originate in the descending thoracic aorta just distal to the branching of the subclavian artery and typically occur without significant aortic enlargement. Surgical repair of TAAs to prevent type A AADs is recommended at an aortic diameter -of 5.0 to 5.5 cm. Paradoxically, the majority of patients presenting with type A AADs have diameters smaller than 5.5 cm at presentation.27 The overarching goal of this TNE is to prevent dissection-related mortality and morbidity by identifying cellular and molecular events that trigger AADs and use these data to discover biomarkers to predict and therapeutics to prevent AADs. The major risk factors for AADs are increased biomechanical forces on the aorta, primarily due to hypertension (HTN), and genetic variants. Altered genes predisposing to AAD lead to decreased smooth muscle cell (SMC) contraction and survival, altered extracellular matrix (ECM) integrity, and decreased canonical transforming growth factor-β (TGFβ) signaling.1-3 The genes inform the molecular pathogenesis of AADs: (1) the SMC-selective expression of some AAD-predisposing genes implicates a primary role of SMCs in AAD;2 (2) mutations in genes encoding proteins involved in canonical TGFβ signaling indicate loss of signaling predisposes to disease; (3) causative genes disrupt a prominent structural unit in the aorta potentially involved in mechanosensing: elastic fiber extensions that connect, via microfibrils, to integrin receptors and then to SMC contractile units. These units are established during postnatal growth of the aorta, due the fact that lifetime elastin is deposited in mice by 4 weeks of age. Both deletion of Tgfbr2 or exposure to the lysyl oxidase inhibitor β-aminoproprionitrile (BAPN) at 3 weeks confers a greater risk for AADs than later exposures, emphasizing this critical aortic developmental window to prevent AADs (Figure ).4 Thus, we postulate that altered SMC mechanosensing, due to either loss of SMC contraction, defective SMC-matrix connections, or HTN, is a driver of AADs.5, 6 Furthermore, evidence also supports a role for endothelial cell (EC) dysfunction in AAD. EC-specific deletion of the angiotensin II (Ang II) type Ia receptor attenuates AAD in multiple mouse models.7, 8 Finally, intimal tears that initiate type A and B AADs occur at two discrete locations and we have new evidence of increased EC permeability in these regions. Thus, we hypothesize that AADs are triggered by a complex series of cell-specific events, coupled with biomechanical forces from pulsatile hemodynamics. Specifically, genetic and environment factors can disrupt the proper formation of structural components for aortic SMC mechanosensing that are established during postnatal growth. Aberrant SMC signaling, due to either a lack of SMC connections to the ECM, disrupted SMC contraction, or increased forces across these connections, leads to altered SMC signaling, metabolism and survival and disruption of the ECM, ultimately triggering AADs. With disease progression, sub-intimal SMCs, blood flow, or an altered basement membrane triggers aberrant signaling in the overlying ECs, which in turn leads to ECs dysfunction and 2 contributes to AAD risk. It is important to note that underlying environmental and genetic factors may drive AADs through different signaling pathways. At the same time, we hypothesize that the proposed studies will reveal dominant and unifying cell-specific molecular pathways for AADs regardless of underlying cause. During the first year, Aim 1 will be pursue unbiased transcriptomic and proteomic analyses of acutely dissected ascending aortas from patients and “pre-dissection” aortas from mouse models to identify dominant cell-type-specific signaling changes. LeMaire, Shen, and Milewicz have established single cell RNA sequencing (scRNA-seq) analyses of human and mouse aortas and Redondo, Dichek and Susuki have established proteomics pipelines. Milewicz, Redondo, Dougherty and Shen have established mouse models of AADs, and Dichek will establish a Tgfbr2 mutation mouse model of AADs. Saeys will provide the bioinformatic expertise to analyze SMC and EC signaling and interactions associated with AADs using the generated datasets. Aim 2 will pursue in depth assessment of cellular and molecular pathways triggering dissection in clinically relevant mouse models. To illustrate the augmented information gained by combining the expertise in this TNE, members generated preliminary data on BAPN-induced AADs (Figure): Milewicz and Daugherty independently replicated the phenotype, identified sex differences and histologic changes; Humphrey pursued biomechanical and transmission electron microscopy (TEM) analyses; Dichek provided proteomics analyses; Ghaghada is completing imaging studies to localize intimal tears; Saeys analyzed scRNAseq and proteomics datasets and will quantify the TEM changes; and Milewicz initiated treatment trials to prevent AADs. The BAPN model will be further assessed in Year 1, including time-dependent changes in aorta with progression to AAD, mechanistically interrogated identified signaling pathways through genetic manipulation and chemicals targeting signaling pathways to augment or prevent AADs, and assess the role of EC dysfunction through special transcriptomics, in vivo imaging for EC dysfunction and genetic manipulation. The most clinically relevant mouse models of AADs as informed by Aim 1 data will be similarly interrogated in the subsequent years of the grant. Aim 3 will translate the data from Aim 1 and 2 by: assess therapeutic(s) that prevent AADs in a large animal model; pursue plasma proteomic analyses and imaging of patients at high risk for AADs to validate potential biomarkers for AADs. Finally, the TNE members are committed to use this collaborative project to train junior investigators and to provide timely release of generated data and models to the scientific community. The assembled international team has the knowledge, expertise and passion to identify the complex cell-specific molecular alterations that trigger AADs with the critical goal of preventing AAD associated premature deaths.