Insights into elastic fiber fragmentation: Mechanisms and treatment of aortic aneurysm in Marfan syndrome

Toward a rational therapeutic for elastin related disease: Key considerations for elastin based regenerative medicine strategies

Elastin is a connective tissue protein, produced from the ELN gene, that provides elasticity and recoil to tissues that stretch, such as the large arteries of the body, lung parenchyma, skin, ligaments and elastic cartilages. It is produced as a soluble monomer, tropoelastin, that when cross-linked in the extracellular space generates a polymer that is extraordinarily stable, with a predicted half-life of >70 years. Although data suggest ongoing elastin transcription, it is rare to see new elastin deposited outside of its tight developmental window. Consequently, elastin-related disease comes about primarily in one of three scenarios: (1) inadequate elastin deposition, (2) production of poor-quality elastic fibers, or (3) increased destruction of previously deposited elastin. By understanding the pathways controlling elastin production and maintenance, we can design new therapeutics to thwart those abnormal processes. In this review, we will summarize the diseases arising from genetic and environmental alteration of elastin (Williams syndrome, supravalvar aortic stenosis, autosomal dominant cutis laxa, and ELN-related vascular and connective tissue dysfunction) and then describe the mechanisms controlling elastin production and maintenance that might be manipulated to generate novel therapeutics aimed at these conditions. We will end by summarizing existing therapeutic strategies targeting these disease mechanisms before outlining future approaches that may better solve the challenges associated with elastin based regenerative medicine.

Expression of Elastin, F-Box and WD-40 Domain-Containing Protein 2, Fibrillin-1, and Alpha-Smooth Muscle Actin in Utilized Blood Vessels for explant culture-A New 3D in Vitro Vascular Model from Bovine Legs

Elastic fibers of the internal and external elastic laminae maintain blood vessel shapes. Impairment of smooth muscle cell function leads to vascular disease development. F-box and WD-40 domain-containing protein 2 (FBXW2) is associated with elastic fibers and osteocalcin expression for bone regeneration in the periosteum. Here, it is hypothesized that FBXW2 has different roles in periosteum and blood vessels. Furthermore, if FBXW2 would be a component of elastic fiber of blood vessels, FBXW2 would be expressed where the well-known components elastin and fibrillin-1 are expressed. For this purpose, explant culture of blood vessels from bovine legs were performed for 5 weeks. It was found that elastin and FBXW2 were expressed within the elastic laminae, whereas fibrillin-1 was expressed around them. After explant culture, elastin and FBXW2 sustained the shape of the elastic fibers in the elastic lamina, whereas the fibrillin-1-rich layer became wide range and encompass toward intima and adventitia layers. Hematoxylin Eosin staining and immunohistochemistry of alpha-smooth muscle actin (α-SMA) revealed weakened media layer after 5 weeks culture. Although fibrillin-1 is a well-known component of elastic fibers and elastin, this study revealed that the location of fibrillin-1 is different from that of elastin, whereas FBXW2 is present in the same region as elastin from day 0 to week 5. In blood vessels, fibrillin-1 fibers around the elastic lamina may be oxytalan fibers. Thus, the proposed 3D in vitro model in this study is useful for identifying the mechanisms of vascular degradation.

Sex- and region-specific differences in microstructural remodeling and passive biomechanics of the aorta correlate with aneurysm propensity in a mouse model of severe Marfan syndrome

Marfan syndrome (MFS) is a connective tissue disorder caused by mutations in the gene that encodes fibrillin-1, a glycoprotein necessary for elastic fiber assembly and stability in the large elastic arteries. MFS is associated with aortic aneurysms that typically occur in the proximal ascending aorta and have worse outcomes in males. Mechanisms for the sex- and region-specific differences in aneurysm development and outcomes are unknown. We quantified aortic geometry, microstructural remodeling, and passive biomechanics of the thoracic ascending, thoracic descending, abdominal suprarenal, and abdominal infrarenal aorta in 4 months old male and female Fbn1mgR/mgR (a model of severe MFS) and littermate wild-type mice to determine correlations between aortic geometry, microstructural remodeling, biomechanics, and aneurysmal dilation. We showed that aneurysmal dilation was strongly correlated with unloaded thickness, microstructural remodeling including loss of elastic fibers, deposition of collagen fibers, and decrease in cell nuclei number, and mechanical metrics including physiologic and ex vivo circumferential material stiffness. A multivariable mixed model showed that unloaded thickness, elastic fiber degradation, and ex vivo material stiffness predicted aneurysmal dilation with an adjusted R2 = 0.8818. Our results highlight the potential of geometric, microstructural remodeling, and biomechanical metrics to serve as physical biomarkers for personalized aortic aneurysm diagnosis and management in MFS. STATEMENT OF SIGNIFICANCE: Marfan syndrome (MFS) is a genetic disease associated with aortic aneurysms that have distinct sex- and region-specific outcomes. The mechanisms driving these variations are unclear. We used a severe MFS mouse model (Fbn1mgR/mgR) to explore differences in microstructural remodeling and passive wall mechanics along the aortic length in males and females. We correlated these changes with aneurysm severity, as quantified by aortic dilation. We found that sex- and region-specific alterations in unloaded thickness, microstructural remodeling, and passive mechanical properties of the aortic wall play a critical role in aortic dilation. Our findings showed that mechanical metrics, particularly ex vivo material stiffness, may serve as biomarkers for the diagnosis and management of aortic aneurysms.

Mitochondrial Dysfunction: A New Hallmark in Hereditable Thoracic Aortic Aneurysm Development

Thoracic aortic aneurysms (TAAs) pose a significant health burden due to their asymptomatic progression, often culminating in life-threatening aortic rupture, and due to the lack of effective pharmacological treatments. Risk factors include elevated hemodynamic stress on the ascending aorta, frequently associated with hypertension and hereditary genetic mutations. Among the hereditary causes, Marfan syndrome is the most prevalent, characterized as a connective tissue disorder driven by FBN1 mutations that lead to life-threatening thoracic aortic ruptures. Similarly, mutations affecting the TGF-β pathway underlie Loeys-Dietz syndrome, while mutations in genes encoding extracellular or contractile apparatus proteins, such as ACTA2, are linked to non-syndromic familial TAA. Despite differences in genetic origin, these hereditary conditions share central pathophysiological features, including aortic medial degeneration, smooth muscle cell dysfunction, and extracellular remodeling, which collectively weaken the aortic wall. Recent evidence highlights mitochondrial dysfunction as a crucial contributor to aneurysm formation in Marfan syndrome. Disruption of the extracellular matrix-mitochondrial homeostasis axis exacerbates aortic wall remodeling, further promoting aneurysm development. Beyond its structural role in maintaining vascular integrity, the ECM plays a pivotal role in supporting mitochondrial function. This intricate relationship between extracellular matrix integrity and mitochondrial homeostasis reveals a novel dimension of TAA pathophysiology, extending beyond established paradigms of extracellular matrix remodeling and smooth muscle cell dysfunction. This review summarizes mitochondrial dysfunction as a potential unifying mechanism in hereditary TAA and explores how understanding mitochondrial dysfunction, in conjunction with established mechanisms of TAA pathogenesis, opens new avenues for developing targeted treatments to address these life-threatening conditions. Mitochondrial boosters could represent a new clinical opportunity for patients with hereditary TAA.

The transcriptomic landscape of monosomy X (45,X) during early human fetal and placental development

Monosomy X (45,X) is associated with Turner syndrome and pregnancy loss in humans, but the underlying mechanisms remain unclear. We therefore undertook an exploratory study of the transcriptomic landscape of clinically relevant human fetal 45,X tissues (including pancreas, liver, kidney, skin, placenta) with matched 46,XX and 46,XY control samples between 11 and 15 weeks post conception (n = 78). Although most pseudoautosomal region 1 (PAR1) genes are lower in monosomy X tissues, we also found reduced expression of several key genes escaping X inactivation (e.g., KDM5C and KDM6A), several ancestral X-Y gene pairs, and potentially clinically important transcripts such as genes implicated in ascending aortic aneurysm. In contrast, higher expression of an autosomal, long non-coding RNA (OVCH1-AS1) is seen in all 45,X tissues. In the placenta, lower expression of CSF2RA is demonstrated, likely contributing to immune dysregulation. Taken together, these findings provide insights into the biological consequences of a single X chromosome during early human development and potential insights in genetic mechanisms in Turner syndrome.

Novel Insights into the Aortic Mechanical Properties of Mice Modeling Hereditary Aortic Diseases

OBJECTIVE

 Hereditary aortic diseases (hADs) increase the risk of aortic dissections and ruptures. Recently, we have established an objective approach to measure the rupture force of the murine aorta, thereby explaining the outcomes of clinical studies and assessing the added value of approved drugs in vascular Ehlers-Danlos syndrome (vEDS). Here, we applied our approach to six additional mouse hAD models.

MATERIAL AND METHODS

 We used two mouse models (Fbn1C1041G and Fbn1mgR ) of Marfan syndrome (MFS) as well as one smooth-muscle-cell-specific knockout (SMKO) of Efemp2 and three CRISPR/Cas9-engineered knock-in models (Ltbp1, Mfap4, and Timp1). One of the two MFS models was subjected to 4-week-long losartan treatment. Per mouse, three rings of the thoracic aorta were prepared, mounted on a tissue puller, and uniaxially stretched until rupture.

RESULTS

 The aortic rupture force of the SMKO and both MFS models was significantly lower compared with wild-type mice but in both MFS models higher than in mice modeling vEDS. In contrast, the Ltbp1, Mfap4, and Timp1 knock-in models presented no impaired aortic integrity. As expected, losartan treatment reduced aneurysm formation but surprisingly had no impact on the aortic rupture force of our MFS mice.

CONCLUSION

 Our read-out system can characterize the aortic biomechanical integrity of mice modeling not only vEDS but also related hADs, allowing the aortic-rupture-force-focused comparison of mouse models. Furthermore, aneurysm progression alone may not be a sufficient read-out for aortic rupture, as antihypertensive drugs reducing aortic dilatation might not strengthen the weakened aortic wall. Our results may enable identification of improved medical therapies of hADs.

The proprotein convertase FURIN is a novel aneurysm predisposition gene impairing TGF-β signalling

AIMS

Aortic aneurysms (AA) frequently involve dysregulation of transforming growth factor β (TGF-β)-signalling in the aorta. Here, FURIN was tested as aneurysm predisposition gene given its role as proprotein convertase in pro-TGF-β maturation.

METHODS AND RESULTS

Rare FURIN variants were detected by whole-exome sequencing of 781 unrelated aortic aneurysm patients and affected relatives. Thirteen rare heterozygous FURIN variants occurred in 3.7% (29) unrelated index AA patients, of which 72% had multiple aneurysms or a dissection. FURIN maturation and activity of these variants were decreased in vitro. Patient-derived fibroblasts showed decreased pro-TGF-β processing, phosphorylation of downstream effector SMAD2 and kinases ERK1/2, and steady-state mRNA levels of the TGF-β-responsive ACTA2 gene. In aortic tissue, collagen and fibrillin fibres were affected. One variant (R745Q), observed in 10 unrelated cases, affected TGF-β signalling variably, indicating effect modification by individual genetic backgrounds.

CONCLUSION

FURIN is a novel, frequent genetic predisposition for abdominal-, thoracic-, and multiple aortic or middle sized artery aneurysms in older patients, by affecting intracellular TGF-β signalling, depending on individual genetic backgrounds.

Age- and sex-specific biomechanics and extracellular matrix remodeling of the ascending aorta in a mouse model of severe Marfan syndrome

Thoracic aortic aneurysm (TAA) is associated with Marfan syndrome (MFS), a connective tissue disorder caused by mutations in fibrillin-1. Sexual dimorphism has been recorded for TAA outcomes in MFS, but detailed studies on the differences in TAA progression in males and females and their relationships to outcomes have not been performed. The aims of this study were to determine sex differences in the diameter dilatation, mechanical properties, and extracellular matrix (ECM) remodeling over time in a severe mouse model (Fbn1mgR/mgR = MU) of MFS-associated TAA that has a shortened life span. Male and female MU and wildtype (WT) mice were used at 1-4 mo of age. Blood pressure and in vivo diameters of the ascending thoracic aorta were recorded using a tail-cuff system and ultrasound imaging, respectively. Ex vivo mechanics and ECM remodeling of the aorta were characterized using a biaxial test system and multiphoton imaging, respectively. We showed that mechanical properties, such as structural and material stiffness, and ECM remodeling, such as elastic and collagen fiber content, correlated with diameter dilatation during TAA progression. Male MU mice had accelerated rates of diameter dilatation, stiffening, and ECM remodeling compared with female MU mice which may have contributed to their decreased life span. The correlation of mechanical properties and ECM remodeling with diameter dilatation suggests that they may be useful biomarkers for TAA progression. The differences in diameter dilatation and life spans in male and female MU mice indicate that sex is an important consideration for managing thoracic aortic aneurysm in MFS. NEW & NOTEWORTHY Using a mouse model (Fbn1mgR/mgR = MU) of severe thoracic aortic aneurysm in Marfan syndrome (MFS), we found that male MU aorta had an accelerated time line and increased amounts of dilatation, stiffening, and extracellular matrix (ECM) remodeling compared with female MU aorta that may have contributed to an increased risk of fatigue failure with cyclic loading over time and a reduced life span. We suggest that aortic stiffness may provide useful information for clinical management of aneurysms in MFS.

Tracheal Stenosis Associated With Operation for Pneumothorax With Marfan Syndrome: A Case Report

Marfan syndrome is a genetic disorder in which impaired protein leads to connective tissue weakness. We herein report a case of unexpected tracheal stenosis that was diagnosed just before an operation for a recurrent right pneumothorax with Marfan syndrome. A 16-year-old boy with bilateral repeated pneumothoraces associated with Marfan syndrome came to our emergency room complaining of dyspnea. A chest radiograph showed recurrent right pneumothorax. An operation was planned due to prolonged air leakage even after chest tube drainage. On induction of general anesthesia for repairing pneumothorax, a sudden difficulty occurred during manual ventilation, and the blood oxygen saturation temporarily decreased to 50%. Therefore, emergent intubation with a single-lumen tube was applied, which led back to full saturation. Bronchoscopy revealed a tortuous and flattened trachea. An endobronchial blocker tube was applied due to difficulty in double-lumen tube insertion, and bullectomy was accomplished without any other unexpected events. Patients with Marfan syndrome may have asymptomatic tracheal stenosis due to structural abnormalities and latent tracheomalacia, and general anesthesia could be a trigger to develop the symptoms. Surgeons should bear this in mind, cooperate with anesthesiologists well, and prepare for emergent intubation when managing patients with Marfan syndrome in the perioperative settings.

The Fbn1 gene variant governs passive ascending aortic mechanics in the mgΔlpn mouse model of Marfan syndrome when superimposed to perlecan haploinsufficiency

Introduction

Ascending thoracic aortic aneurysms arise from pathological tissue remodeling that leads to abnormal wall dilation and increases the risk of fatal dissection/rupture. Large variability in disease manifestations across family members who carry a causative genetic variant for thoracic aortic aneurysms suggests that genetic modifiers may exacerbate clinical outcomes. Decreased perlecan expression in the aorta of mgΔlpn mice with severe Marfan syndrome phenotype advocates for exploring perlecan-encoding Hspg2 as a candidate modifier gene.

Methods

To determine the effect of concurrent Hspg2 and Fbn1 mutations on the progression of thoracic aortopathy, we characterized the microstructure and passive mechanical response of the ascending thoracic aorta in female mice of four genetic backgrounds: wild-type, heterozygous with a mutation in the Fbn1 gene (mgΔlpn), heterozygous with a mutation in the Hspg2 gene (Hspg2+/-), and double mutants carrying both the Fbn1 and Hspg2 variants (dMut).

Results

Elastic fiber fragmentation and medial disarray progress from the internal elastic lamina outward as the ascending thoracic aorta dilates in mgΔlpn and dMut mice. Concurrent increase in total collagen content relative to elastin reduces energy storage capacity and cyclic distensibility of aortic tissues from mice that carry the Fbn1 variant. Inherent circumferential tissue stiffening strongly correlates with the severity of aortic dilatation in mgΔlpn and dMut mice. Perlecan haploinsufficiency superimposed to the mgΔlpn mutation curbs the viability of dMut mice, increases the occurrence of aortic enlargement, and reduces the axial stretch in aortic tissues.

Discussion

Overall, our findings show that dMut mice are more vulnerable than mgΔlpn mice without an Hspg2 mutation, yet later endpoints and additional structural and functional readouts are needed to identify causative mechanisms.