The pathogenesis of aortopathy in Marfan syndrome and related diseases

Generation of Marfan syndrome-specific induced pluripotent stem cells harboring FBN1 mutations

Marfan syndrome (MFS) is a hereditary condition caused by mutations in the FBN1 gene. Genetic mutations in the FBN1 locus impact the function of the encoded protein, Fibrillin 1, a structural molecule forming microfibrils found in the connective tissue. MFS patients develop severe cardiovascular complications including thoracic aortic aneurysm and aortic dissection, which predispose them to an enhanced risk of premature death. Here, we generated two induced pluripotent stem cell (iPSC) lines harboring mutations in the FBN1 gene (p.C1942C>A and c.1954 T>C), directly derived from MFS patients. We have shown that both iPSC lines displayed expression of pluripotency markers, normal karyotype and ability of trilineage differentiation, representing a valuable tool for the identification of new therapeutic strategies for intervening in this disease.

Marfan and Loeys-Dietz aortic phenotype: A potential tool for diagnosis and management

Objective

In heritable aortic diseases, different vascular involvement may occur with potential variable implications in aortic dilation/dissection risk. This study aimed to analyze the aortic anatomy of individuals with Marfan syndrome and Loeys-Dietz syndrome to identify possible morphological differences.

Methods

Computed tomography and magnetic resonance imaging of the thoracoabdominal aorta from the proximal supra-aortic vessels to the femoral bifurcation level of 114 patients with Marfan and Loeys-Dietz syndromes and 20 matched control subjects were examined. Aortic diameters, areas, length, and tortuosity were measured in different aortic segments using specific vessel analysis software.

Results

Patients with Marfan syndrome showed a higher prevalence of ascending aorta and aortic root dilation (P = .011), larger and longer aortic roots (P = .013) with pear-shaped phenotype, larger isthmus/descending aorta diameter ratio (P = .015), and larger suprarenal aorta and iliac arteries. Patients with Loeys-Dietz syndrome showed longer indexed segments and a significantly longer arch (P = .006) with type 2/3 arch prevalence (P = .097). Measurement ratios analysis provided cut-off values (aortic root to ascending aorta length/aortic root diameter, aortic root/sinotubular junction, aortic root/ascending aorta diameter) differentiating patients with Marfan syndrome from patients with Loeys-Dietz syndrome, even in the early stage of the disease.

Conclusions

Both syndromes show peculiar anatomic patterns at different aortic levels irrespective of aortic dilation and disease severity. These features may represent the expression of different genetic mutations on aortic development, with a potential impact on prognosis and possibly contributing to better management of the diseases. The systematic adoption of whole body imaging with magnetic resonance or computed tomography should always be considered, because they allow a complete vascular assessment with practical indicators of differential diagnosis.

Genetic architecture of cardiac dynamic flow volumes

Cardiac blood flow is a critical determinant of human health. However, the definition of its genetic architecture is limited by the technical challenge of capturing dynamic flow volumes from cardiac imaging at scale. We present DeepFlow, a deep-learning system to extract cardiac flow and volumes from phase-contrast cardiac magnetic resonance imaging. A mixed-linear model applied to 37,653 individuals from the UK Biobank reveals genome-wide significant associations across cardiac dynamic flow volumes spanning from aortic forward velocity to aortic regurgitation fraction. Mendelian randomization reveals a causal role for aortic root size in aortic valve regurgitation. Among the most significant contributing variants, localizing genes (near ELN, PRDM6 and ADAMTS7) are implicated in connective tissue and blood pressure pathways. Here we show that DeepFlow cardiac flow phenotyping at scale, combined with genotyping data, reinforces the contribution of connective tissue genes, blood pressure and root size to aortic valve function.

Risk for acquired coronary artery disease in genetic vs. congenital thoracic aortopathy

Objective

Patients with Marfan syndrome (MFS) and patients with a bicuspid aortic valve (BAV) have a significantly increased risk to develop thoracic aortopathy. Both conditions share many pathophysiological mechanisms leading to aortic complications. Bicuspidy is known to have a low risk for acquired coronary artery sclerosis. The aim of this study is to determine the risk of coronary sclerosis in MFS patients.

Methods

Marfan syndrome patients with an aortic root dilatation, which were surgically treated between 1999 and 2017, were included and matched with BAV and tricuspid aortic valves (TAV) patients based on sex and age. Cardiovascular risk profiles were determined in all three groups. Coronary sclerosis was graded in all patients on coronary imaging (coronary angiography or computed tomography) using a coronary artery scoring method, which divides the coronaries in 28 segments and scores non-obstructive (20-49% sclerosis) and obstructive coronary sclerosis (>49% sclerosis) in each segment.

Results

A total of 90 matched patients (30 within each group) were included. MFS patients showed less cardiovascular risk factors compared to BAV and TAV patients. TAV patients had higher amounts of obstructive coronary sclerosis as compared to BAV patients (p = 0.039) and MFS patients (p = 0.032). No difference in non- and obstructive coronary artery disease (CAD) was found between the MFS and BAV population.

Conclusion

Marfan syndrome and bicuspid aortic valve patients have a significantly lower risk for, and prevalence of CAD as compared to TAV individuals.

Microvascular retinal changes in patients with Marfan syndrome

Purpose. To determine microvascular changes in patients with genetically proven Marfan syndrome.Methods. In a cross-sectional study, 32 eyes of 16 patients with genetically proven Marfan syndrome were evaluated using swept-source optical coherence tomography angiography (SS-OCTA). Patients were analyzed regarding lens status and systemic vascular disease. The foveal avascular zone (FAZ) and vessel density (VD) of the superficial and deep vascular plexus and central retinal thickness (CRT) were evaluated on SS-OCTA.Results. 44/56% patients presented without/with subluxation of the lens. 69% of patients had presence of mitral valve insufficiency, aortic dilatation or aneurysm of the aortic root. In patients with Marfan syndrome the mean area of the FAZ was 0.2 ± 0.1mm and the average VD of the superficial/deep vascular plexus was 36 ± 5%/22 ± 7%. In patients with subluxation of the lens FAZ area and perimeter were larger when compared to patients without subluxation of the lens (0.18 ± 0.08/0.28 ± 0.10 mm and 1.7 ± 0.4/2.3 ± 0.8; p = 0.02). VD of the superficial vascular plexus was reduced in patients with subluxation of the lens (on average 39 ± 3/33 ± 8; p = 0.01) together with an increased CRT in the inner segments of the ETDRS grid when compared to patients without subluxation of the lens. In patients with systemic vascular disease a larger FAZ area (0.19 ± 0.06/0.25 ± 0.1mm; p = 0.04) and reduced VD of the superficial vascular plexus in the central ETDRS grid (28 ± 7/21 ± 6; p = 0.02) was observed in comparison to patients without systemic vascular changes.Conclusions. In patients with Marfan syndrome SS-OCTA imaging revealed microvascular differences in patients with lens subluxation and/or systemic vascular disease.

Connective Tissue Disorders and Cardiovascular Complications: The Indomitable Role of Transforming Growth Factor-β Signaling

Marfan Syndrome (MFS) and Loeys-Dietz Syndrome (LDS) represent heritable connective tissue disorders that segregate with a similar pattern of cardiovascular defects (thoracic aortic aneurysm, mitral valve prolapse/regurgitation, and aortic dilatation with regurgitation). This pattern of cardiovascular defects appears to be expressed along a spectrum of severity in many heritable connective tissue disorders and raises suspicion of a relationship between the normal development of connective tissues and the cardiovascular system. With overwhelming evidence of the involvement of aberrant Transforming Growth Factor-beta (TGF-β) signaling in MFS and LDS, this signaling pathway may represent the common link in the relationship between connective tissue disorders and their associated cardiovascular complications. To further explore this hypothetical link, this chapter will review the TGF-β signaling pathway, the heritable connective tissue syndromes related to aberrant TGF-β signaling, and will discuss the pathogenic contribution of TGF-β to these syndromes with a primary focus on the cardiovascular system.

Oxidative Stress-Related Susceptibility to Aneurysm in Marfan's Syndrome

The involvement of highly reactive oxygen-derived free radicals (ROS) in the genesis and progression of various cardiovascular diseases, including arrhythmias, aortic dilatation, aortic dissection, left ventricular hypertrophy, coronary arterial disease and congestive heart failure, is well-established. It has also been suggested that ROS may play a role in aortic aneurysm formation in patients with Marfan's syndrome (MFS). This syndrome is a multisystem disorder with manifestations including cardiovascular, skeletal, pulmonary and ocular systems, however, aortic aneurysm and dissection are still the most life-threatening manifestations of MFS. In this review, we will concentrate on the impact of oxidative stress on aneurysm formation in patients with MFS as well as on possible beneficial effects of some agents with antioxidant properties. Mechanisms responsible for oxidative stress in the MFS model involve a decreased expression of superoxide dismutase (SOD) as well as enhanced expression of NAD(P)H oxidase, inducible nitric oxide synthase (iNOS) and xanthine oxidase. The results of studies have indicated that reactive oxygen species may be involved in smooth muscle cell phenotype switching and apoptosis as well as matrix metalloproteinase activation, resulting in extracellular matrix (ECM) remodeling. The progression of the thoracic aortic aneurysm was suggested to be associated with markedly impaired aortic contractile function and decreased nitric oxide-mediated endothelial-dependent relaxation.

CRISPR/Cas9 in zebrafish: An attractive model for FBN1 genetic defects in humans

Background

Mutations in the fibrillin‐1 gene (FBN1) are associated with various heritable connective tissue disorders (HCTD). The most studied HCTD is Marfan syndrome. Ninety percent of Marfan syndrome is caused by mutations in the FBN1 gene. The zebrafish share high genetic similarity to humans, representing an ideal model for genetic research of human diseases. This study aimed to generate and characterize fbn1^+/− mutant zebrafish using the CRISPR/Cas9 gene‐editing technology.

Methods

CRISPR/Cas9 was applied to generate an fbn1 frameshift mutation (fbn1^+/−) in zebrafish. F1 fbn1^+/− heterozygotes were crossed with transgenic fluorescent zebrafish to obtain F2 fbn1^+/− zebrafish. Morphological abnormalities were assessed in F2 fbn1^+/− zebrafish by comparing with the Tuebingen (TU) wild‐type controls at different development stages.

Results

We successfully generated a transgenic line of fbn1^+/− zebrafish. Compared with TU wild‐type zebrafish, F2 fbn1^+/− zebrafish exhibited noticeably decreased pigmentation, increased lengths, slender body shape, and abnormal cardiac blood flow from atrium to ventricle.

Conclusion

We generated the first fbn1^+/− zebrafish model using CRISPR/Cas9 gene‐editing approach to mimic FBN1 genetic defects in humans, providing an attractive model of Marfan syndrome and a method to determine the pathogenicity of gene mutation sites.

Marfan syndrome revisited: From genetics to the clinic

Marfan syndrome is an autosomal dominant connective tissue disease with an estimated incidence of 1 in 5000 individuals. In 90% of cases it is caused by mutations in the gene for fibrillin-1, the main constituent of extracellular microfibrils. Studies on animal models of Marfan syndrome have revealed that fibrillin-1 mutations interfere with local TGF-β signaling, in addition to impairing tissue integrity. The cardinal features involve the cardiovascular, ocular and skeletal systems. The diagnosis of Marfan syndrome is made according to the revised Ghent nosology. Early identification and appropriate management are critical for patients with Marfan syndrome, who are prone to the life-threatening cardiovascular complications of aortic aneurysms and aortic dissection. The standard treatment includes prophylactic beta-blockers in order to slow down dilation of the ascending aorta, and prophylactic aortic surgery. The success of current medical and surgical treatment of aortic disease in Marfan syndrome has substantially improved mean life expectancy, extending it above 72 years. This review aims to provide an overview of this hereditary disorder.

Functional characterization of common BCL11B gene desert variants suggests a lymphocyte-mediated association of BCL11B with aortic stiffness

The recent genome-wide analysis of carotid–femoral pulse wave velocity (PWV) identified a significant locus within the 14q32.2 gene desert. Gene regulatory elements for the transcriptional regulator B-cell CLL/lymphoma 11B (BCL11B) are within this locus and an attractive target for the gene association. We investigated the functional impact of these gene desert SNPs on BCL11B transcript in human aorta to characterize further its role in aortic stiffness. To do this, we used a large repository of aortic tissues (n = 185) from an organ transplant program and assessed ex vivo stiffness of the aortic rings. We tested association of three lead SNPs from the GWAS meta-analysis with ex vivo aortic stiffness and BCL11B aortic mRNA expression: rs1381289 and rs10782490 SNPs associated significantly with PWV and showed allele-specific differences in BCL11B mRNA. The risk alleles associated with lower BCL11B expression, suggesting a protective role for BCL11B. Despite strong association, we could not detect BCL11B protein in the human aorta. However, qPCR for CD markers showed that BCL11B transcript correlated strongly with markers for activated lymphocytes. Our data confirm the significance of the 14q32.2 region as a risk locus for aortic stiffness and an upstream regulator of BCL11B. The BCL11B transcript detected in the human aorta may reflect lymphocyte infiltration, suggesting that immune mechanisms contribute to the observed association of BCL11B with aortic stiffness.

SMAD4 gene mutation increases the risk of aortic dilation in patients with hereditary haemorrhagic telangiectasia

BACKGROUND

Mutations in the genes ENG, ACVRL1 and SMAD4 that are part of the transforming growth factor-beta signalling pathway cause hereditary haemorrhagic telangiectasia (HHT). Mutations in non-HHT genes within this same pathway have been found to associate with aortic dilation. Therefore, we investigated the presence of aortic dilation in a large cohort of HHT patients as compared to non-HHT controls.

METHODS

Chest computed tomography of consecutive HHT patients (ENG, ACVRL1 and SMAD4 mutation carriers) and non-HHT controls were reviewed. Aortic root dilation was defined as a z-score>1.96. Ascending and descending aorta dimensions were corrected for age, gender and body surface area.

RESULTS

In total 178 subjects (57.3% female, mean age 43.9±14.9years) were included (32 SMAD4, 47 ENG, 50 ACVRL1 mutation carriers and 49 non-HHT controls). Aortopathy was present in a total of 42 subjects (24% of total). Aortic root dilatation was found in 31% of SMAD4, 2% of ENG, 6% of ACVRL1 mutation carriers, and 4% in non-HHT controls (p<0.001). The aortic root diameter was 36.3±5.2mm in SMAD4 versus 32.7±3.9mm in the non-SMAD4 group (p=0.001). SMAD4 was an independent predictor for increased aortic root (β-coefficient 3.5, p<0.001) and ascending aorta diameter (β-coefficient 1.6, p=0.04).

CONCLUSIONS

SMAD4 gene mutation in HHT patients is independently associated with a higher risk of aortic root and ascending aortic dilation as compared to other HHT patients and non-HHT controls.

Fibrillin-1 and alpha8 integrin are co-expressed in the glomerulus and interact to convey adhesion of mesangial cells

Fibrillin-1 is a microfibrillar extracellular matrix protein that was described to be a ligand for α8 integrin. α8 integrin is a matrix receptor specifically expressed in mesangial and smooth muscle cells of the kidney. In previous studies we detected glomerular expression of fibrillin-1. Moreover, fibrillin-1 promoted adhesion, migration, and proliferation of mesangial cells. We hypothesized that fibrillin-1 and α8 integrin might interact in the glomerulus, and thus, regulate mesangial cell properties. Our studies showed that fibrillin-1 and α8 integrin colocalize in the glomerular mesangium. Induction of experimental glomerulonephritis led to an increase of both fibrillin-1 and α8 integrin expression. In vitro studies revealed that mesangial cells deficient for α8 integrin adhere weaker to fibrillin-1 and migrate more easily on fibrillin-1 than wild-type mesangial cells. Baseline proliferation on fibrillin-1 is higher in α8 integrin-deficient mesangial cells, but the induction of proliferation is not different in α8 integrin-deficient and wild-type mesangial cells. We conclude that fibrillin-1 and α8 integrin interact, and thus, regulate mesangial cell adhesion and migration. The concomitant induction of both fibrillin-1 and α8 integrin in a self-limited model of glomerular injury points to a protective role of the interaction of fibrillin-1 with α8 integrin in the glomerulus resulting in reduced damage of the glomerular tuft as a consequence of firm adhesion of mesangial cells.

Aortic Involvement in Pediatric Marfan syndrome: A Review

Outlining specific protocols for the management of pediatric patients with Marfan syndrome has been challenging. This is mostly due to a dearth of clinical studies performed in pediatric patients. In Marfan syndrome, the major sources of morbidity and mortality relate to the cardiovascular system. In this review, we focus on aortic involvement seen in pediatric patients with Marfan syndrome, ranging from aortic dilatation to aortic rupture and heart failure. We discuss the histological, morphological, and pathogenetic basis of the cardiac manifestations seen in pediatric Marfan syndrome and use a specific case to depict our experienced range of cardiovascular manifestations. The survival for patients with Marfan syndrome may approach the expected survival for non-affected patients, with optimal management. With this potentiality in mind, we explore possible and actual management considerations for pediatric Marfan syndrome, examining both medical and surgical therapy modalities that can make the possibility of improved survival a reality.

Clinical utility of a next generation sequencing panel assay for Marfan and Marfan-like syndromes featuring aortopathy

Aortopathy can be defined as aortic dilation, aneurysm, dissection, and tortuosity. Familial aortopathy may occur secondary to fibrillin-1 (FBN1) mutations in the setting of Marfan syndrome, or may occur as a result of other genetic defects with different, but occasionally overlapping, phenotypes. Because of the phenotypic overlap and genetic heterogeneity of disorders featuring aortopathy, we developed a next generation sequencing (NGS) assay and comparative genomic hybridization (CGH) array to detect mutations in 10 genes that cause thoracic aortic aneurysms (TAAs). Here, we report on the clinical and molecular findings in 175 individuals submitted for aortopathy panel testing at ARUP laboratories. Ten genes associated with heritable aortopathies were targeted using hybridization capture prior to sequencing. NGS results were analyzed, and variants were confirmed using Sanger sequencing. Array CGH was used to detect copy-number variation. Of 175 individuals, 18 had a pathogenic mutation and 32 had a variant of uncertain significance (VUS). Most pathogenic mutations (72%) were identified in FBN1. A novel large SMAD3 duplication and FBN1 deletion were identified. Over half who had TAAs or other aortic involvement tested negative for a mutation, suggesting that additional aortopathy genes exist. We anticipate that the clinical sensitivity of at least 10.3% will rise with VUS reclassification and as additional genes are identified and included in the panel. The aortopathy NGS panel aids in the timely molecular diagnosis of individuals with disorders featuring aortopathy and guides proper treatment.

Molecular mechanisms of inherited thoracic aortic disease - from gene variant to surgical aneurysm

Aortic dissection is a catastrophic event that has a high mortality rate. Thoracic aortic aneurysms are the clinically silent precursor that confers an increased risk of acute aortic dissection. There are several gene mutations that have been identified in key structural and regulatory proteins within the aortic wall that predispose to thoracic aneurysm formation. The most common and well characterised of these is the FBN1 gene mutation that is known to cause Marfan syndrome. Others less well-known mutations include TGF-β1 and TGF-β2 receptor mutations that cause Loeys-Dietz syndrome, Col3A1 mutations causing Ehlers-Danlos Type 4 syndrome and Smad3 and-4, ACTA2 and MYHII mutations that cause familial thoracic aortic aneurysm and dissection. Despite the variation in the proteins affected by these genetic mutations, there is a unifying pathological end point of medial degeneration within the wall of the aorta characterised by vascular smooth muscle cell loss, fragmentation and loss of elastic fibers, and accumulation of proteoglycans and glycosaminoglycans within vascular smooth muscle cell-depleted areas of the aortic media. Our understanding of these mutations and their post-translational effects has led to a greater understanding of the pathophysiology that underlies thoracic aortic aneurysm formation. Despite this, there are still many unanswered questions regarding the molecular mechanisms. Further elucidation of the signalling pathways will help us identify targets that may be suitable modifiers to enhance treatment of this often fatal condition.

Genetic biomarkers in aortopathy

The field of aortopathy, in common with other genomic disorders, is undergoing a revolution. This is largely driven by the implementation of newer forms of genetic sequencing (massively parallel or next-generation sequencing). Advantages conferred by this technology include reduced costs, reduced sequencing time and the ability to simultaneously test multiple genes. This has a significant advantage in the identification of genes disrupted in heritable aortopathies. These advances are enabling scientists and clinicians to identify key molecular pathways; translating fundamental genetic findings into a better understanding of disease mechanisms is ultimately leading to effective treatments. In outlining contemporary knowledge of genetic biomarkers in aortopathy we seek to demonstrate that the era of genomically orientated decision-making is here.