**5. Genetics and hemodynamics in BAV aortopathy**

Analyses of the genetic contribution to BAV and its associate aortopathy in a large family-based study suggested that genetics and the presence of BAV independently influence ascending aortic diameter [50]. These investigations highlight the first and oldest hypothesis of BAV-associated AscAA development, namely the hemodynamic hypothesis [51]. Indeed, several studies have suggested a contribution of BAV-associated impaired flow to aneurysm development [52, 53]. A plethora of radiological tools, such as low-sensitive cardiac magnetic resonance imaging with full volumetric coverage of the ascending aorta, have allowed multiple flow-specific investigations on BAV aortopathy in the previous decade. As such, it is clear that hemodynamic alterations are intrinsic to BAV, even in the absence of severe valve disease, by virtue of its anatomy. Hemodynamic alterations in the presence of a BAV include flow jets, eccentric helical flow, and increased ascending aortic wall-shear stress. Observations of increased wall-shear stress and intramural stresses underpin the hypothesis that these hemodynamic alterations indeed participate in AscAA development and progression [54]. Not only are disturbances in ascending aortic flow pathognomic of BAV, but signs of flow-dependent cellular and histological changes have also been observed. Grewal et al. have published data suggesting a jet-associated phenotypic switch of the inner ascending aortic media by virtue of hemodynamic alterations [55]. Similarly, in support of a hemodynamic component in ascending aortopathy is a spatial differential expression of matrix proteins and smooth muscle cell depletion compared to a more circumferentially homogenous Marfan-or TAVassociated aorta [56, 57]. Furthermore, aortic endothelial cells isolated from BAV patients exhibit differential expression of flow-related *KLF2, KLF4, PECAM1,* and *CDH5* compared with TAV ECs [58]. Still, as explored by Gauer et al., the expression of eNOS synthase does not differ between different ascending aortic regions in BAV, despite being subject to different hemodynamic forces and wall-shear stress [59]. Another topic obscuring the influence of hemodynamics on BAV aortopathy is the remedy of BAV through aortic valve repair and the possible progression of ascending aortic dilatation. There are conflicting reports on the pace at which the BAV aorta continues to dilate following aortic valve repair, showing both a faster growth and a normal rate of dilatation [60, 61]. There are still no investigations on transcatheter aortic valve replacement in BAV patients and the continued dilatation of the ascending aorta.

The second hypothesis of BAV development, i.e., the genetic hypothesis, is gaining more favor, and indeed genome-wide association studies are finding noncoding variants of genes like *GATA4* associating with BAV [62] Albeit GATA4 deletion hampers endothelial-to-mesenchymal transition in transfected cells, and are through this mechanism believed to influence BAV-development [62]. GATA4-variants and mutations, while not functionally investigated, are further implicated in BAV development by a well-described association with congenital heart defects [63–65]. While GATA4 is associated with the presence of a BAV, genetic variants of ROBO4 – a gene associated

with endothelial cell performance and function [47, 48], are also implicated in both BAV development and its associated aortopathy [46]. Even still, variants of SMAD6 are associated with BAV-associated aortopathy [66].

Of note, exploration of genetic causes of nonfamilial BAV aortopathy cannot be disentangled from the concomitant hemodynamic alterations, and as such, both genetic and an altered hemodynamic are likely to contribute to disease development. This way, the current state of the literature warrants an integration of genetic and molecular factors being investigated in association with hemodynamic factors.

#### **6. Degenerative ascending aortic aneurysm**

While degenerative AscAA manifests with the same clinical manifestation as the BAV-associated AscAA, i.e., a dilated aorta, it is histomorphologically vastly different [67]. A comprehensive global gene expression analysis was conducted on ascending aortic intima-media obtained from both nondilated and dilated ascending aortas of 131 patients with BAV and TAV (i.e., degenerative AscAA), showing significant molecular disparities in the underlying pathophysiology between BAV and TAV- aortopathy [11]. The degenerative form of AscAA is marked by fibrotic, inflammatory, and degenerative changes [7, 68]. The histopathology of degenerative AscAA was first described by Erdheim in 1929 as idiopathic cystic medial necrosis [69]. Although this descriptor of the changes has been abandoned by contemporary science, the changes describe the loss of VSMCs with the subsequent accumulation of ECM components. Cystic medial necrosis is, by the current histopathological consensus on degenerative thoracic aortic disease [70], instead described as MEMA, whereby dead VSMCs deposit primarily collagens and proteoglycans to the site of injury [71]. Furthermore, one of the main load-bearing proteins, elastin [72], which, together with the VSMCs, make up the lamellar units of the vascular media, appear fragmented and thinned out with disease progression [70]. Furthermore, there are multiple reports of low-grade inflammation and infiltrated leukocytes into the aortic intima-media of degenerative AscAA [11].

Although the main histopathological characteristics of degenerative AscAA are described in the pathological consensus, structured histopathological studies reveal a degree of heterogeneity in AscAA tissue from patients undergoing ascending aortic repair. The degree of inflammatory activity and location of infiltrated leukocytes, i.e., subintimal or mid-media, varies without known associations to patient characteristics [11]. Noteworthy is also the reported experience of surgeons treating aortic diseases, where degenerative AscAA differ significantly from descending aortic aneurysm or abdominal aortic aneurysms (AAA), in particular with respect to the absence of mural thrombi or macroscopic signs of atherosclerosis [73]. Taken together, this indicates, in our minds, that the degenerative changes described in the pathological consensus apply to most AscAA patients despite some reports of heterogeneity as pronounced inflammation or microscopic atherosclerotic lesions.

AscAA in patients with tricuspid aortic valves can manifest as part of monogenic syndromes, where inherited genetic mutations play a significant role in the development of aortopathy. Mutations in genes encoding ECM proteins, such as FBN1 in Marfan syndrome, COL3A1 in vascular Ehlers–Danlos syndrome, and TGFBR1 and TGFBR2 in Loeys–Dietz syndrome, disrupt the structural integrity of the aortic wall [74]. These mutations lead to abnormal ECM synthesis, impaired collagen and elastin assembly, and risk of ascending aortic dilatation.

#### *Ascending Aortic Aneurysm in Relation to Aortic Valve Phenotype DOI: http://dx.doi.org/10.5772/intechopen.112883*

In the absence of monogenic diseases, molecular drivers of degenerative AscAA development may be matrix metalloproteinsases (MMP), TGFβ signaling pathway disruptions, and inflammation [11, 75]. MMPs are a family of enzymes involved in the breakdown of ECM components. Excessive MMP activity has been observed in AscAA, particularly MMP-2 [76], −3 [77], −9 [78], −14, and − 19 [79]. Increased MMP expression and degraded ECM structures, such as elastin, induce VSMC death, weakening the aortic wall and promoting aneurysm formation [80]. Dysregulated TGFβ signaling is central in AscAA in monogenic conditions like Marfan syndrome and Loeys–Dietz syndrome [81, 82] but may too contribute to disease progression in polygenic contexts. The disruption of TGFβ signaling results in increased production of TGFβ ligands, which, paradoxically, can lead to defective TGFβ signaling and impaired ECM maintenance and turnover [83]. Altered TGF-β signaling disrupts the balance between ECM synthesis and degradation, consequently leading to aneurysm development. Inflammatory processes also play a role in degenerative AscAA pathogenesis. Macrophages and T lymphocytes infiltrate the aortic wall, releasing pro-inflammatory cytokines and chemokines with reports of upregulated inflammatory genes [84]. This immune system activation contributes to chronic inflammation, subsequent oxidative stress, and ECM remodeling, thereby exacerbating degenerative characteristics and aneurysm growth [85–88].

The clear difference between degenerative and BAV-associated AscAA strengthens the idea that these diseases are indeed separate. Instead, degenerative AscAA appears more similar to AAA at a molecular level, although genetic analyses have demonstrated a limited overlap [74, 89].

Atherosclerotic processes are often implicated in AAA, with accumulation of lipids, immune cells, and ECM components within the abdominal aortic wall [90]. Lipid deposition and macrophage infiltration contributes to the release of pro-inflammatory cytokines and the production of reactive oxygen species, resulting in chronic inflammation, endothelial dysfunction, and vascular degeneration [91, 92]. Oxidative stress further promotes inflammation, ECM degradation, and apoptosis of vascular cells, exacerbating AAA progression [93]. Moreover, proteolytic enzymes, such as MMPs (in particular MMP-1, −2, −3, −9, −12, and − 13) and elastases, are secreted by immune and vascular SMCs leading to increased degradation of ECM components and weakening the abdominal aortic wall [80, 94]. Of note, in the context of differential mechanisms driving aneurysm development, the abdominal aorta is subject to different hemodynamic forces than the ascending aorta, including flow disturbances, pulsatile flow, and increased wall-shear stress. These hemodynamic forces induce endothelial dysfunction, inflammation, and arterial wall remodeling, contributing to AAA formation [95]. Of note, intracranial aneurysms (IAs) are also influenced by hemodynamic forces, specifically in the context of formation and rupture [96]. Regions of disturbed flow, such as bifurcations and curvatures, are particularly vulnerable [96]. Also, similarly to AscAA and AAA, proteolytic enzymes (e.g., MMP-2 and -9) are involved in the pathogenesis of IA [97].

Genetic predisposition also plays a significant role in IA development. Mutations in genes encoding components of the ECM, such as collagen type IV alpha-1 and alpha-2, have been identified in familial cases of IA [98, 99]. These mutations hamper the structural integrity of the intracranial arterial wall, causing a propensity to IA formation. Inflammation and immune responses also contribute to IA pathogenesis. Inflammatory cells infiltrate the arterial wall, releasing cytokines and promoting oxidative stress [100–102]. This leads to ECM degradation, smooth muscle cell apoptosis, and arterial wall remodeling, ultimately contributing to aneurysm formation and growth (**Figure 2**).

#### **Figure 2.**

*Matrix metalloproteinases associated with degenerative aneurysmal disease.*

It is important to note that while there are similarities in the molecular drivers between AscAA, IA, and AAA, there are also distinct differences. AscAA is often associated with monogenic connective tissue disorders or proteolytic enzyme degenerative effects, whereas IAs are more often influenced by polygenic genetic factors, hemodynamic forces, and inflammation [103]. AAA is influenced by atherosclerotic processes, proteolytic enzyme activity, oxidative stress, hemodynamic forces, inflammation, and ECM remodeling [74, 98]. It is important to note that while these molecular drivers are often associated with AscAA, IA, and AAA, there can be considerable heterogeneity in the underlying mechanisms between individual patients. Moreover, there may be overlapping molecular pathways and interactions, requiring further studies. Understanding the molecular drivers of aneurysmal disease is crucial for developing targeted therapies and interventions to prevent its progression and improve patient outcomes.
