Unveiling cellular and molecular aspects of ascending thoracic aortic aneurysms and dissections

An ascending thoracic aortic aneurysm (ATAA) is a localized dilation in the proximal segment of the aorta. Aortic aneurysms represent weakened areas within the aorta, posing a significant risk of tearing or rupturing and resulting in severe, potentially life-threatening internal bleeding. If left untreated, ATAA can lead to severe complications such as aortic dissection (ATAAD) and rupture, with mortality rates of 50% within 24 h, including 21% mortality in patients who arrived alive in the hospital (Fig. 1) [137]. Known risk factors for ATAA development include advanced age > 65 years, systemic hypertension, and male sex (Fig. 2) [40]. Indeed, ATAA is diagnosed more frequently in men, which also is reflected in 70% of individuals with ATAAD [152]. On the contrary, ATAA severity has been indicated to be worse in women compared to men with faster aneurysm growth [32] and increased in-hospital mortality rates [134]. The estimated incidence of ATAA ranges from 5–10 per 100,000 individuals/year [37, 156], with a currently increasing trend [107]. Compared to abdominal aortic aneurysm (AAA), ATAA exhibits different flow patterns [176], regional variations [185], and developmental origins [193].

Notably, prophylactic treatment options of ATAA are limited to aortic surgery in which a diameter threshold of 5.5 cm and aortic diameter growth rate ≥ 0.3 cm/year are recognized as cut-off values [37]. However, it is recognized that most patients develop ATAAD before reaching these thresholds (Fig. 1) [75, 161, 163].

Therefore, clinical biomarkers, representing early detection of patients at risk of ATAAD are highly anticipated. From an etiological point of view, less than 30% of all ATAA cases are genetically triggered, and thus more than 70% are sporadic or degenerative [35]. It is tempting to speculate that unknown (epi)genetic factors play a key role in initiating and progressing degenerative ATAA.

In patients with bicuspid aortic valves (BAV) or genetic mutations, ATAA is more commonly observed at a younger age [198]. Heritable ATAA has been associated with over 15 genes, including those encoding ECM-proteins such as fibrillin 1 (FBN1), type III collagen (COL3A1), or transforming growth factor-β (TGF-β) receptor proteins such as TGFBR1 and TGFBR2 [204]. It also involves VSMC proteins such as smooth muscle cell actin (ACTA2), myosin heavy chain 11 (MYH11), myosin light chain kinases (MYLK), and protein kinase cGMP-dependent type 1 (PRKG1) [159].

Furthermore, arteries are exposed to wall shear stress (WSS), which is induced by blood flow and exerted at the valvular and vascular endothelial layers [10]. Unequal distribution of WSS at the outer curvature of the ascending aorta has been associated with degenerative ATAA [171]. Blood pressure also exerts a key hemodynamic influence, i.e., wall stress, on the integrity of aortic wall tissue [61].

In the medial layer, vascular smooth muscle cells (VSMC) play an important role in vascular remodeling, exhibiting characteristic plasticity to adapt to changing flow and pressure conditions, e.g., by switching between a contractile and a synthetic phenotype [177]. VSMCs are further supported by the extracellular matrix (ECM) which plays a crucial role in regulating mechanical behavior and resilience, providing elasticity, and imparting arterial wall strength [2]. Once the ECM–VSMC network is disrupted, mechanosensing and mechano-signaling are impaired, leading to VSMC phenotype switching [153]. A compromised structure and function of the ECM leads to mechanical abnormalities and functional changes at the tissue level associated with aortic disease [208]. Progressive loss of arterial wall strength eventually culminates in the development of ATAA and even ATAAD.

There is an urgent need to identify novel biomarkers to screen patients at high-risk for ATAAD. Here, we summarize the current literature on the pathophysiology of ATAA and ATAAD with a focus on ECM–VSMC dysfunction. We highlight gaps in current diagnostic approaches, as well as recommend potential clinical biomarkers that may contribute to advancing our understanding of the development of early-stage ATAA, ultimately to predict and prevent morbidity and mortality associated with ATAAD.

In clinical practice, the characterization of ATAA is predominantly confined to diameter measurements. The aortic diameter is the principal decision-making criterion for surgical intervention within the Multidisciplinary Aortic Team. According to the latest clinical guidelines (EACTS/STS) for aortic diseases published in 2024, surgical replacement of the aorta is recommended when the aortic diameter is greater than 5.5 cm [37]. In high-risk patients with the presence of connective tissue disorders (e.g., Marfan or Loeys-Dietz), earlier intervention is recommended (≥ 4.5 cm). Patients with low-risk are monitored by imaging every year for timely detection of the surgical threshold (Fig. 1). However, up to 96% of ATAAD occurs in vessels with diameters below the surgical interventional threshold (< 5.5 cm) and 60% of ATAAD occurs in aortas with normal diameters (< 4 cm) [180]. Also, most patients with ascending aortic aneurysms of > 4 cm show little to no further growth during annual follow-up [1]. This is a major unmet clinical challenge for cardiologists and cardiac surgeons in assessing and managing ATAA.

Recently, several new parameters such as aortic elongation and aortic volume have been identified as potentially important predictors of ATAAD [74, 87]. Although these parameters alongside other morphologic characteristics of the vasculature such as the vertebral artery tortuosity show promising results [147], they still represent patients at a later stage of disease development and earlier detection of patients at risk requires a holistic approach implementing multi-scale analysis including vessel wall characterization.

In recent years, imaging modalities such as computed tomography (CT) combined with positron emission tomography (PET), using fluorodeoxyglucose (18F-FDG) or 18F-sodium fluoride (18F-NaF) administration provide geometrical, molecular, and functional information of aortic disease [56]. Several studies showed promising results regarding aortitis [80, 81], aneurysm growth and future clinical events [103], atherosclerosis [179], and detecting malignant tumors of the aorta [199]. Photon Counting-CT is another promising technique, which is still in clinical trial, yet has proven its clinical value regarding improved spatial resolution, and optimized spectral imaging [221]. Furthermore, this method offers precise tissue characterization and improved perfusion imaging while minimizing radiation exposure.

Blood-flow characteristics play a key role in ATAA formation, with effects on endothelial cell (EC) homeostasis and the response of VSMCs. Therefore, a functional assessment of detailed hemodynamic measurements is required to investigate flow characteristics and biomechanical forces. Four-dimensional flow magnetic resonance imaging (4D)-flow MRI, where phase-contrast methods are used to encode blood flow velocities along all dimensions in the aorta [42, 110], has been introduced as a powerful non-invasive technique in cardiovascular imaging for the assessment of local WSS [30]. WSS, which refers to the force per unit area exerted by moving fluid in the vessel, can be estimated as a product of wall shear rate (WSR) and local blood viscosity. Yet, it has not been validated as a clinical screening tool. One of the limitations of 4D flow MRI is insufficient spatial resolution which may underestimate WSS values and affect the accuracy of flow patterns [86]. Broader application of 4D flow MRI has been impeded by long scan times, costs, and data processing and analysis requiring special software. These obstacles hinder both reproducibility and clinical application [131]. Nevertheless, some longitudinal studies showed that 4D flow MRI can be used as a predictive tool to distinguish low and high WSS in ATAA patients with BAV [14, 20, 67, 144] or without BAV [171] compared to healthy volunteers.

Recently, the left ventricular outflow tract angle (LVOT-angle) has been a region of interest in ATAA pathology. Aortas less aligned to the axis of the heart were associated with ATAA [97]. Another study showed a positive correlation between LVOT-aortic angle and WSS on the outer curvature, indicating that increased LVOT-aortic angles (> 58.5º) were linked to elevated levels of WSS [184]. Geometrical biomarkers combined with flow patterns may improve the prediction of ATAA at risk or those who need an earlier intervention.

In recent years, major progress has been made in unraveling gene mutations as molecular markers for predisposition to ATAA. This has been by the identification of a variety of single nucleotide polymorphisms (SNPs) from genome-wide association studies (GWAS) suggestive of having a role in ATAA pathophysiology [165]. Also, genetic mutations in ATAA pathophysiology including systemic features are frequently classified by clinicians as a syndromic disease with clear connective tissue anomalies [129]. In the absence of these features, gene mutations in ATAA are often described as causative of a monogenetic connective tissue disorder, affecting proteins encoding for VSMC contractile apparatus or ECM of the aortic wall (Table 2) [213].

Aging is the biggest risk factor for impaired cardiovascular health, with cardiovascular disease being the cause of death in 40% of individuals over 65 years old. The remodeling of the human thoracic aorta correlates with aging [115, 140] and ATAA related to aging is often labeled as a degenerative disease. As a main component of the vessel wall, elastin fulfills a key role in the remodeling process. Elastin content progressively decreases with a half-life of some 75 years in humans [203]. Several studies reported that with progressing age, alterations in the quantity and quality of elastin and collagen cause a decrease in total arterial compliance [208, 225]. Aging is associated with the destruction of interlaminar fibrillar elastic structures as well as a decreased amount of medial VSMCs [208]. This results in a reduction or loss of elastic and vasoactive function of the vascular system [58]. Loss of aortic elasticity is not only related to damage of elastin but also changes in collagen content. As individuals age, collagen type I remains consistently predominant, while the quantity of collagen type III declines gradually from the heart to the distal portion of the aorta [133].

Senescence-associated β-galactosidase (SA-β-Gal) activity is used as a tool for in vivo assessment of aging. The increase of SA-β-Gal is a result of increased lysosomal content, the expression of cyclin-dependent kinase inhibitors, the presence of DNA damage, or the presence of critically short telomere length [44, 63]. Telomere length provides a potential marker for an individual’s biologic age. Several studies suggested that telomerase plays a protective role in AAA [47] and ATAA [7]. Telomeres shortening, reduced telomerase function, and cellular senescence of VSMCs play a crucial role in the development of ATAAs. Significantly higher expressions of stress-induced senescence markers p16(INK4a) and p19(ARF) in telomerase-deficient mice were shown compared to wild-type mice [18]. Further, telomere shortening in human blood leukocytes reveals its use as a potential biomarker for ATAA [12].

Other important macromolecules contributing to the pathogenesis of ATAAD, are glycosaminoglycans (GAGs) and proteoglycans (PG), fundamental contributors to the structure and function of the aortic wall. There is contradictory data regarding changes in GAGs upon aging, most studies reported an increase in GAGs, often followed by a decrease upon further in an aging aorta [16, 154]. In ATAAD, multiple structural disruptions are reported as a result of localized GAG accumulation leading to increased interlamellar pressure [175]. Mitochondria also play an important role in aging. Tyrrell et al. demonstrated that with aging mitochondrial dysfunction may activate innate immune pathways including the TLR9, inflammasome, and STING pathways [210]. Recently, it was demonstrated in mice that the mitochondrial function of VSMCs is controlled by the ECM and drives the development of aortic aneurysms in Marfan syndrome [157].

Alteration in connective fibers within the aorta impairs the elastic recoil and reduces adhesive strength between the aortic wall layers. This may impair the functionality of aortic cells and subsequently lead to ATAA formation or aortic rupture in case pressure-induced wall stresses exceed this strength.

Sex differences play a significant role in the development, management, and clinical outcomes of aorta pathology. Biologically, women are protected against ATAA due to premenopausal levels of estrogen and therefore often present with ATAA at an older age than men [150]. Although ATAA is less prevalent in women, a recent epidemiologic study demonstrates that women have a 40% increased risk of mortality [152] and a threefold increased risk of ATAAD or rupture compared to men [43]. Although heritable ATAA growth rates were similar, ATAA growth rates were over three-fold higher in women than in men with degenerative ATAA [32]. These differences in sex etiology can be explained by anatomic differences such as aorta size and proportional dilation between genders. Despite the correction of aneurysm size to body size, acute aortic syndromes occur at smaller aneurysm sizes in women than in men with worse ATAA-related outcomes [55]. Thus, a smaller diameter of the aorta can progress more rapidly in women requiring close monitoring. In vitro and animal studies have indicated that estrogen can reduce collagen deposition and increase elastin in the aortic wall, potentially contributing to the prevention of TAA development [150, 168]. However, during and after menopause women exhibit a greater aortic stiffening and impairment of elastic properties, which correlates with declining levels of estrogen [215]. This may explain why women have a more progressed state of aortic disease and need to undergo surgery for ATAA at an older age [15].

Significant progress has been made over the last decades in understanding the pathophysiology of ATAA. However, important gaps remain in the early detection of acute aortic pathologies such as ATAAD. The autopsy reports indicate that up to 25% of patients with ATAAD die before diagnosis [158], and these cases often involve younger individuals [167]. Relying solely on aortic diameter for risk assessment is inadequate to distinguish between different pathologic processes with varying risks of acute complications.

Most of the recent literature on ATAA focused on identifying circulating biomarkers to improve diagnosis. While these biomarkers show promising results, their isolated use may lack specificity in indicating disease progression due to their involvement in multiple processes throughout the body. Understanding the patient-specific cellular and molecular mechanisms and integrating complementary diagnostic tools by combining circulating biomarkers, with advanced imaging tools, such as molecular imaging probes could enable direct visualization of biologic processes at specific anatomic locations.

Further investigation emphasizes the need for more personalized strategies to improve risk assessment such as integrating imaging data with genotypes and circulating biomarkers to identify patients at high-risk and guide surgical decision-making.