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Abstract
Marfan syndrome has changed over the last few years: new diagnostic criteria have been proposed, new clinical entities recognised and life expectancy increased. The role of fibrillin 1, which was initially thought to be mainly structural, has been shown to also be functional. The altered transforming growth factor β pathway is better understood, the importance of epigenetic factors has been demonstrated and recent data suggest that many of the observations made in Marfan syndrome can actually be made in thoracic aortic aneurysm from diverse aetiologies. Besides transforming growth factor β, the role of metalloproteinase, the fibrinolytic/coagulation system, is being suggested in the progression of the disease.
A relationship between the type of fibrillin 1 (FBN1) gene mutation and the mechanism for the disease (haplo-insufficiency vs negative dominance), as well as some genotype/phenotype correlations, has been observed, although the main challenge of recognising gene modifiers has yet to explain tremendous variability despite similar mutation.
This progress has led to new hopes for tomorrow's therapies, some of which are being tested in clinics, whereas others are still in the field of animal models.
Here we review some of the new data obtained in the understanding of the pathophysiology and genetics of this disease.
- Dissection
- Marfans
- cytokines
- gene expression
- growth factors
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Marfan syndrome (MFS) is an autosomal dominant genetic disease, with an estimated frequency of 5/10,000 and a neomutation rate of 20% of reported cases (ie, 20% of the affected patients have no parents affected). It affects both males and females equally worldwide and carries an increased risk of aortic dilatation, dissection and rupture, which are responsible for the increased mortality. The syndrome also affects the mitral valve and possibly the myocardium,1 eyes (sometimes leading to blindness), skeleton, skin and integuments, lungs and neurological system. These features appear and develop with age as they are usually the consequence of altered resistance of the tissues. The definition of the syndrome has evolved throughout the 20th century, illustrating the difficulty in diagnosis. MFS is usually related to mutations in the fibrillin 1 (FBN1) gene, encoding fibrillin 1, a major adhesive protein of the extracellular matrix, homopolymerised and organised as microfibrils. Great progress has been made in patient care, which has led to improved survival: it is generally recognised that 30 years of life has been gained during the last 30 years.2 Recent progress in understanding pathophysiology has led to great expectations for further improvement in the near future.
The evolution of MFS through the years
Professor A. Marfan was a French paediatrician of the late 19th century who described the princeps case of a 5-year-old girl (named Gabrielle) who had a specific skeletal phenotype associating ‘dolichostenomelia’ and ‘arachnodactyly’. She died at the age of 12 from tuberculosis. After this initial description, further aspects of the syndrome were recognised, notably ocular involvement and cardiac features with aortic enlargement and rupture. It is not obvious that the princeps case had this risk, that is, that Gabrielle had an aortic risk; however, since then, the terminology of MFS has remained associated with the aortic risk, while the skeletal features associated with little aortic risk have been recognised as congenital contractural arachnodactyly, or Beals syndrome, which is now known to be related to mutations in the FBN2 gene encoding fibrillin 2.
Successive expert meetings have been held, which, over time, defined more and more precisely what should be coined ‘Marfan syndrome’ in the present day.3–5 Molecular genetics (ie, mutation of the FBN1 gene) has been repeatedly proposed as a key finding for the diagnosis of the disease, although FBN1 gene mutations were only identified 20 years ago.6 At that time, it was the syndrome, that is, the association of different sets of features, that made the diagnosis in the first ‘Berlin’ nosology.3 An FBN1 gene mutation was only one criterion among others that appeared in the revisited (‘Ghent’) nosology,4 and FBN1 gene mutations were demonstrated only in a minority of patients. Subsequently, we showed that when the diagnosis is based on clinical features, mutations can be found in the FBN1 gene and TGFBR2 gene in some cases (the entity is then coined ‘Marfan syndrome type 2’ or MFS2).7 8
Over the years, the number of patients screened by molecular genetics has increased tremendously because molecular biology methods became amenable to the study of large genes, more readily available and less expensive,9 so that a classification based on molecular genetics could be envisaged. However, one should keep in mind that knowing the molecular defect is not enough to precisely stratify the aortic risk of a patient: intrafamilial variability illustrates the fact that the same molecular defect may have different consequences in different individuals. Besides, the clinical spectrum associated with a mutation in the FBN1 gene varies widely from features limited to one system (eg, isolated thoracic aortic aneurysm (TAA), or ectopia lentis or skeletal features) to complete a classic MFS.10
Phenotypical variability is even greater in MFS2 patients (with a mutation in the TGFBR2 gene): some of them do not have any manifestation of the disease, whereas others have severe forms of the Loeys–Dietz syndrome. This new syndrome associates aortic involvement with, among other things, facial dysmorphology, arterial tortuosity and velvety skin.11 Mutations in the TGFBR2 gene have also been associated with familial forms of TAAs.12
In this context, a new set of diagnostic criteria has been recently proposed to replace the first Ghent nosology (box 1)5: MFS1 is considered as the classic MFS related to a mutation in the FBN1 gene. It comprises the mutation of the FBN1 gene associated with aortic dilatation, or/and ectopia lentis and/or systemic features (box 1). When no mutation in the FBN1 gene is known in the patient, an aortic dilatation associated with an ectopia lentis and/or systemic features also makes the diagnosis of MFS. In case of a parent affected by MFS, ectopia lentis and/or aortic dilatation and/or systemic features are enough to make the diagnosis.
Reproduced from Loeys et al5: new Ghent criteria
A. Diagnostic criteria: Ao, aortic dilatation; FBN1, mutation in the FBN1 gene; MFS, Marfan syndrome; EL, ectopia lentis; Syst, systemic score (see B); ELS, ectopia lentis syndrome; MVP, mitral valve prolapse
In the absence of family history:
Ao (Z≥2) AND
FBN1 = MFS
EL = MFS
Syst (≥7pts) = MFS
EL with or without Syst AND
FBN1 with known Ao = MFS
FBN1 not known with Ao or no FBN1= ELS
Ao (Z< 2) AND Syst (≥5) without EL = MASS
MVP AND Ao (Z<2) AND Syst (<5) without EL = MVPS
In the presence of family history:
Ao (Z≥2 above 20 years old, ≥3 below 20 years) = MFS
EL = MFS
Syst (≥7 pts) = MFS
B. Systemic score: maximum total, 20 points; score ≥7 indicates systemic involvement
Pectus
Carinatum: 2
Excavatum or chest asymmetry: 1
Wrist AND thumb sign: 3
Wrist OR thumb sign: 1
Scoliosis or thoracolumbar kyphosis: 1
No severe scoliosis and ↓ US/LS AND ↑ arm span/height: 1
Protrusio acetabuli: 2
↓ Elbow extension: 1
Hindfoot deformity: 2
plain pes planus: 1
Facial features (3/5): 1
Dolichocephaly, enophthalmos, downslanting palpebral fissures, malar hypoplasia, retrognathia)
Dural ectasia: 2
Pneumothorax: 2
Skin striae: 1
Myopia >3 diopters: 1
Mitral valve prolapse (all types): 1
MFS2 is phenotypically indistinguishable from the classic MFS but related to mutations in the TGFBR2 gene (it can also be coined Loeys Dietz Syndrome (LDS) according to the new nosology); ectopia lentis is used in the absence of aortic dilatation but in the presence of an FBN1 gene mutation, and Loeys–Dietz syndrome is used for severe forms related to TGFBR2 or TGFBR1 mutations associated with facial abnormalities and other specific features.5 Familial forms of TAA can also be related to mutations in these genes, and incomplete phenotypes, with moderate skeletal features, the absence of cardinal signs of MFS or LDS but mutations in FBN1, are coined MASS (Mitral, Aortic, Skeletal, Skin) in the last nosology.
Whatever the jungle of these diagnoses and whatever the mutated gene involved, the main common feature of all these diseases remains the aortic risk, which mandates regular echocardiographic follow-up.
Molecular biology
MFS as an aneurysmal disease
Dilatation and rupture of the aorta are related to a progressive (aneurysms) or acute (dissections) degradation of the insoluble extracellular matrix proteins of the arterial wall, mainly, elastin and collagens, which give solidity to the arterial wall. This is responsible for the medial areas of mucoid degeneration (also formerly misnamed cystic medial necrosis), characterised by the local enrichment of alcianophilic glycosaminoglycans, vacuoles secondary to the local disappearance of smooth muscle cells (SMC) and degradation of extracellular proteins, including disorganisation of adhesive proteins such as fibronectin and fibrillin and the rupture of insoluble elastin and collagen (figure 1). The pathology of aneurysm and/or dissection does not differ in MFS as compared to other genetic or non-genetic aetiologies but appears in younger patients.
Immunohistochemistry (fluorescence) of localised area of mucoid degeneration showing the localised breakdown of elastin fibres (autofluorescent blue) (A), disorganised fibronectin in this area (green ≠ autofluorescence of elastin) (B), disorganised fibrillin in this area (red) (C) and fusion of the fluorescences showing the colocalisation of fibronectin (green) and fibrillin (red) in areas of mucoid degeneration (elastin blue) (D).
Role of proteases
Elastin and collagens are the main insoluble and hydrophobic components of the wall, giving it a strong support for resisting blood pressure (elementary contention function). Elastin is involved in wall elasticity, is the main structural component of resistance to dilatation and is degraded in aneurysmal diseases. Collagen is the main structural component of resistance to rupture and is altered in dissection and rupture. Therefore, there is a tremendous interest in defining the panel of proteases involved in extracellular matrix degradation in MFS and related diseases. The abundance and activity of matrix metalloproteinases (MMPs) have been shown to be related to TAA formation in numerous studies.13 We have identified MMP-7 (matrilysin) and MMP-3 (stromelysin) as MMPs preferentially localised within the areas of mucoid degeneration,14 which probably results from their particular affinity for sulphated glycosaminoglycans (figure 2).
Accumulation of MMP-3 and MMP-7 in areas of mucoid degeneration within the media of aorta. MMP, matrix metalloproteinase.
In contrast, the data concerning serine protease activities present in aneurysm are scarce. We reported the presence of thrombin within the areas of mucoid degeneration due, once again, to its affinity for glysosaminoglycans.15 Interestingly enough, we recently explored the role of the plasminergic system in aneurysms of the ascending aorta, including MFS.16 Besides the activation of plasminogen after its binding to fibrin (fibrinolytic system), plasminogen could be activated by the plasminogen activators (PAs) expressed by mesenchymal cells. Activated plasmin released by the interaction between plasminogen and cell-derived PAs, catalysed by membrane proteins, leads to fibronectin degradation, cell detachment17 and apoptosis.18 On the other hand, plasmin is able to activate MMPs; to degrade adhesive fibronectin, fibrillin and so on and, therefore, to provoke the release and activation of transforming growth factor β (TGF-β) of its matrix storage sites (cf infra). We reported that plasminogen is transferred better from plasma to an aneurysmal wall than to a normal aortic wall, that tissue PA and urokinase-type PA are more expressed in an aneurysmal wall than in a normal one and, therefore, that generation of plasmin is enhanced in aneurysmal walls as compared to normal walls, leading to an increase in TGF-β bioavailability16 in aneurysm of the ascending aorta. Since plasmin generation could participate in cell disappearance, MMP activation and TGF-β release, the fibrinolytic system is probably an important target for preventing dilatation.19 In parallel, plasmin is also involved in dissecting pathology. In particular, circulating plasmin–antiplasmin complex and fibrin degradation product have been proposed as markers of acute dissection, but this is probably due to the fibrinolysis of the clot in the false channel.20 Nevertheless, the participation of tissue plasmin is not excluded.
Pathophysiology of FBN1
What is fibrillin?
Fibrillin 1 is a large molecule coded only by one gene, located on chromosome 15q21.1 and made of 65 exons. This molecule is distributed in the skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea and ciliary zonules. It can polymerise (head-to-tail assembly of fibrillin monomers) in the presence of calcium-constituting microfibrils. It interacts with a large number of the extracellular matrix components, such as integrins, fibronectins, fibulins, TGF-β binding proteins (latent TGF-β binding protein, or LTBP) and insoluble elastin and collagens. In the aorta, fibrillin participates in the structuring of elastin on the extracellular matrix side and in the anchorage of SMCs by fibronectin intermediate on the cellular side. It contains 47 epidermal growth factor-like domains, including 43 binding calcium, and seven TGF-β binding protein-like domains.21 As with fibronectin, fibrillin is highly sensitive to proteolysis by serine proteases such as plasmin.
Its structure is remarkable due to the important percentage of cysteine residues responsible for the formation of disulphide bonds within the molecule, influencing its three-dimensional conformation: growth factor-like domains-like motifs have six conserved cysteine residues that form three disulphide bonds—between C1 and C3, C2 and C4, and C5 and C6.
Molecular consequences of a mutated FBN1 gene
Two main hypotheses have been proposed to explain how a mutation in the FBN1 gene can be responsible for the development of clinical features:
Haplo-insufficiency: the abnormal fibrillin molecule is not synthesised or is rapidly destroyed so that the allele including the mutated FBN1 gene is functionally not transcripted. The disease is probably the consequence of the decreased level of normal fibrillin 1 available within the cell. The main argument supporting this hypothesis is derived from a mouse model KI for a C1039G mutation. This mutation is responsible for the development of a Marfan phenotype in the mouse, including aortic dilatation in the heterozygous animals: introduction of the WT transgene (responsible for an increased production of normal FBN1) onto the C1039G heterozygous background rescues the vascular phenotype in this mouse model of MFS, indicating that an increased level of normal fibrillin can compensate here for abnormal fibrillin production.22
Negative dominance: abnormal fibrillin 1 molecules, which are going to polymerise with other fibrillin 1 molecules, could be responsible for the abnormal behaviour of the resulting polymer, although some of the fibrillin 1 molecules included in the resulting polymer are normal (poisoning effect). In theory, one abnormal molecule of fibrillin 1 within the entire polymer may be sufficient for the alteration of the properties of the entire molecule. This is supported by the observation that the level of FBN1 present in the tissue of patients with MFS is much lower than the 50% anticipated by the absence of production of one allele23: this observation suggests that the abnormal polymer is removed.
Actually, it is possible that some mutations act through haplo-insufficiency, while others have a dominant negative effect. This was suggested by some phenotype/genotype correlations we observed in a large cohort of probands10 (see below).
Pathophysiology of TGF-β in MFS
Besides the structural role of fibrillin 1 recognised earlier, subsequent studies have associated the disease with altered cell signalling notably through the TGF-β pathway. The involvement of this pathway appeared elegant since fibrillin 1 possesses seven TGF-β binding protein-like domains (also identified in LTBP proteins that bind TGF-β), indicating a possible interaction between fibrillin 1 and the TGF-β pathway (figure 3).
TGF-β pathway. Pro-TGF-β is dimerised into small latent complex, which, in combination with LTBP, constitutes the large latent complex. This complex is linked to microfibrils through sequence complementarity between LTBP and microfibrils (including fibrillin 1), and TGF-β is stored in an inactive state. When there is activation (through proteolysis for example), TGF-β is released, is activated and can bind to its receptors on the smooth muscle cell. TGFBR1 and TGFBR2 work together, the TGFBR2 fixing the TGF-β, phosphorylating the TGFBR1, which phosphorylates Smad-2. P-Smad-2 is then translocated into the nucleus where it activates the transcription of numerous genes including connective tissue growth factor (CTGF). TGF-β also acts through non-Smad pathways. TGF-β, transforming growth factor β; LTBP; latent TGF-β binding protein; SMC, smooth muscle cell.
TGF-β is a growth factor involved in matrix and antiprotease secretion, is stored as an inactive form via its binding to LTBP and is involved in the interaction between LTBP, fibrillin, fibulins and fibronectin extracellular microfibrils. In response to microfibril proteolytic injury, active TGF-β is released from its microfibril storage and induces via its receptors the secretion of extracellular matrix proteins, antiproteases and others, therefore participating in tissue repair. A cause-and-effect relationship between an alteration of the TGF-β pathway and the clinical features of MFS was first reported by Neptune et al24 in Fbn1mgΔ transgenic mice and by Mizuguchi et al7 in MFS patients who showed that the MFS2 disease gene was the TGFBR2 gene encoding the TGF-β type 2 receptor. Subsequently, the effect of activation of TGF-β pathway in the prolapsing mitral valve of the Fbn1C1039G KI mouse model of MFS was reported by Ng et al.25 They reported on the increased P-Smad-2, interpreted as a marker of activation of the TGF-β pathway, in the mitral valve: TGF-β, after binding to its receptors (TGFBR1 and TGFBR2), induces the phosphorylation of intracellular Smad-2, which will be translocated to the nucleus to induce transcription of specific genes (figure 3). They also demonstrated that the use of TGF-β antibodies in vivo limited mitral valve abnormalities in this mouse model; this supports the causative role of increased TGF-β in the development of mitral abnormalities. In the same mouse model, aortic dilatation occurs in heterozygous mice, and increased P-Smad-2 was observed in the aortic wall of KI mice compared to normals.26 Here also, TGF-β neutralising antibodies were able to decrease the level of P-Smad-2 within the aortic wall and prevented abnormal aortic dilatation. Actually, similar results were obtained by the same team in the lungs and skeletal muscles of the same Marfan mouse model, suggesting that the activation of TGF-β was also the main abnormality responsible for these anomalies.27 The idea that the increased TGF-β pathway was responsible for the main anomalies associated with MFS, rather than an abnormal structural protein directly leading to weakening of the extracellular matrix and tissues, has therefore emerged.
TGF-β stimulation also leads to the activation of Smad-independent pathways, such as Ras, Rhoa and TGF-β activated kinase 1 (TAK1)/MEK kinase 1 (MEKK1), and there is no indication that these pathways are activated in MFS, as would have been anticipated if increased TGF-β activity were present. Actually, we were not able to find any activation of these non-Smad pathways in the aortic wall of patients with MFS, nor were we able to find increased messenger RNA (mRNA) levels for TGF-β1 within the aortic wall.28
However, increased amounts of TGF-β1 protein were retained within the aortic wall and released by aneurysmal aortic tissue of diverse aetiologies in response to proteolytic injury of the microfibril network. Due to their structural similarity to TGF-β (seven TFG-β-like domains in fibrillin), fibrillin fragments, generated by fibrillin proteolysis by plasmin, may displace and activate TGF-β. This was associated with enhanced LTBP-1 protein and LTBP-1 mRNA (indicating increased LTBP1 synthesis) within the aortic wall, supporting the idea that TGF-β was trapped within the extracellular matrix by its physiological ligand LTBP.
In contrast, we were able to demonstrate an increase in P-Smad-2 in SMCs from the aortic wall in patients with MFS and in patients with TAAs from other aetiologies (figure 4).28 Actually, we and others have also reported increased P-Smad-2 in the aortic wall of patients with TAA secondary to the mutation in the TGF-β receptor, despite the fact that the mutation in the TGF-β receptor alters (blocks) the transmission of the signal 7 29; this suggests that an increased Psmad-2 within the aortic wall is not secondary to an increased TGF-β activation in human aorta. Furthermore, no clear association in localisation could be found between the Smad-2 nuclear levels and the TGF-β extracellular staining in the aortic wall of patients with aneurysmal aorta, also suggesting the absence of a direct link between the two observations.28
The proportion of P-Smad-2-positive nuclei is increased in the media of thoracic aorta, whatever the aetiology is.28 BAV, bicuspid aortic valve.
All these data question the simple cause-and-effect relationship that has been proposed between TGF-β activation and aortic root dilatation in MFS but widen the potential importance of the TGF-β pathway alteration in the aortic aneurysm disease: it is suggested that, actually, increased Smad-2 within the aortic wall is related to a ‘common pathway’ observed in all forms of TAA, which could either be responsible for (as suggested by the beneficial effect of neutralising antibodies) or be responsive to (as would have been anticipated by the known profibrotic and antiproteolytic effects of TGF-β) the dilatation of the aorta. This last hypothesis is also compatible with the correlation observed between increased Smad-2 level and the degree of elastic fibre fragmentation that we observed in an aortic aneurysmal wall of diverse aetiologies.28
Dissociation between the activation of TGF-β and an increase in Smad-2 signalling was further supported by recent experiments.30 We could demonstrate that (1) increased P-Smad-2 was specific to SMC, that is, not present in fibroblasts obtained from the aortic wall, despite the fact that all cells coming from the same aortic wall should be submitted to the same TGF-β stimulation; (2) increased P-Smad-2 was associated with increased Smad-2 RNA level within SMC (which also was not present in the fibroblasts coming from the same aortic wall) and (3) this deregulation of the Smad-2 pathway within the SMC was heritable, that is, increased P-Smad-2 concentration was maintained during SMC culture, despite the absence of TGF-β within the culture milieu, indicating an epigenetic control of increased Smad-2 within the SMC) (figure 5). Actually, this epigenetic control was further suggested by using chromatin immunoprecipitation, showing alterations of the histones linked to the promoter of the Smad-2 gene within the SMC.30 These last observations were made in SMC derived from an aneurysmal aortic wall from various aetiologies (ie, patients with MFS but also patients with aortic aneurysms from other aetiologies) compared to SMC derived from normal human aorta.
Heritability and cell specificity of the Smad-2 overexpression and activation in thoracic aortic SMC. Smad-2 and connective tissue growth factor (CTGF) mRNA levels are quantified over three successive passages between passages 3 and 5. SMCs were cultured in free-serum SMC medium 24 h before RNA extraction. Increased Smad-2 and CTGF expression is observed in aneurysmal SMCs compared with controls, whatever the passage (*p=0.001) is from.30 This observation is cell specific (ie, non-present in fibroblasts). BAV, bicuspid aortic valve; SMC, smooth muscle cell; mRNA, messenger RNA.
As a conclusion regarding these observations, we can say that increased P-Smad-2 within the SMC of an aortic aneurysmal wall is observed, whatever the aetiology of the aneurysm, and that its relation to TGF-β activation is not clearly established. Actually, this may also be a compensatory mechanism induced within the SMC of aneurysmal wall independent from the aetiology of the aneurysm.
In clinics, because of the promising observations made in the KI mouse model for MFS, numerous trials are in progress in different countries to test the potential benefit of losartan in patients with MFS. This molecule may exert a beneficial effect not only through metabolic modulation as suggested in the model (blocking the TGF-β pathways) but also through modifications in haemodynamics, decreasing aortic tension by decreasing blood pressure and rebound wave due to its vasodilatory effect.
Clinical data
Spectrum of the disease
A lot of efforts have also been made recently to better describe the pathologies associated with FBN1 mutations, as well as related phenotypes, and to look for possible phenotype/genotype relationships. A large cooperative study collecting mutations from more than 1000 probands has allowed some observations.
When the missense mutation alters the number of cysteines (either producing or substituting a cysteine), ectopia lentis is more frequent.10 In patients with MFS, the amount of fibrillin is reduced in the zonules and capsules, and here, structural incompetence of the abnormal fibrillin may be more important than interaction with TGF-β.31
A premature termination codon in FBN1, probably responsible for the absence of synthesis of the fibrillin 1 protein, is associated with more severe skeletal and skin phenotypes than in-frame mutations, suggesting that haplo-insufficiency is likely to be the main mechanism involved here (see above).
The main phenotype/genotype correlation observed is that of a severe phenotype associated with mutations located within exons 24–32 (‘neonatal region of the gene’), which are more frequent in children than in adults10 32: it includes not only neonatal forms of MFS but also more severe phenotypes than that of MFS patients carrying a mutation in another region of the FBN1 gene even in non-neonatal forms.10 Interestingly, when a premature termination codon is present in this region (probably with no synthesis of abnormal fibrillin 1 protein, leading to haplo-insufficiency), the clinical spectrum is less severe, but this type of mutation is under-represented in this region.33 This may suggest that a dominant negative effect (abnormal protein poisoning the formed polymer) is responsible for the more severe forms of the disease including the neonatal MFS, whereas haplo-insufficiency leads to less severe phenotypes. However, even in this specific region, the great phenotypical variability observed in patients with the same mutation illustrates the importance of factors other than the causal mutation in determining the severity of the clinical features (eg, modifier genes).
At the other end of the spectrum of phenotypes associated with FBN1 mutations are patients without complete MFS features, in which a precise diagnosis is difficult. Actually, only a minority (5%) of patients with a pathogenic MFS mutation would not fit the first Ghent criteria,34 but this percentage increases to 21% of the adult population of probands with a pathogenic FBN1 mutation when only the clinical features are used for diagnosis (ie, not using the presence of an FBN1 mutation for diagnosis, to mimic the clinical situation of a patient seen for the first time for diagnostic evaluation).
This raises the problem of when to look for a mutation in the FBN1 gene in a patient suspected of presenting MFS and can be estimated by compiling the data obtained in the population in which FBN1 gene screening is performed.35 The efficacy of FBN1 gene screening in patients with classic MFS probands was high (73%) but low for patients referred to as having possible MFS (14%). The best predictor of the ability to identify a mutation in the FBN1 gene was the presence of features in at least three organ systems including one major Ghent criterion (aortic dilatation or dissection, ectopia lentis, dural ectasia), with specific emphasis put on ectopia lentis. Without these features, the probability of finding a pathogenic mutation in the FBN1 gene was very low and probably even overestimated in this study in which mutation analysis was decided on a case-to-case basis by the biologist. Similar conclusions are obtained when the international cohort of probands with FBN1 gene mutations is studied.36
The efficiency of molecular screening for TGFBR1 and TGFBR2 mutations also depends on the clinical presentation and remains low with all phenotypes (below 5%) apart from LDS, in which the detection rate is close to 90%.37
Aortic dilatation
Aortic dilatation is one of the major criteria for the diagnosis of MFS in all published criteria. It predominates at the level of the sinuses of Valsalva,8 which increases progressively over time, and the number of patients with aortic dilatation increases with age.9 Aortic dilatation is a late marker of alteration of the aortic wall, reflecting the remodelling of the aortic wall. In reality, even before aortic diameter is increased beyond the normal range, some differences of the aortic wall's physical properties can be evidenced in humans, including a loss of elasticity, responsible for an increase in central pulse pressure, due also to both earlier and greater rebound waves. Furthermore, a relationship has been established between central pulse pressure and dilatation of the aortic root.38 This pathophysiological scheme is the basis for the use of β-blockade, initially proposed to decrease dp/dt, even though this effect has not been confirmed in patients with major aortic dilatation.39 β-Blockade may also exert a beneficial effect through other mechanisms—bradycardia that decreases the rate of stretch of the aortic wall and decreased blood pressure—although the hypotensive effect of β-blockade in normotensive patients is probably minor (but has never been evaluated in a randomised study). Similarly, this reasoning is the basis for replacement of a β-blocker by a bradycardic calcium antagonist in case of intolerance, the avoidance of stressful sports and the reporting of deleterious effects of obstructive sleep apnoea in some patients.40
Aortic loss of compliance is present on the entire aorta but appears to be limited to the aorta, despite the fact that fibrillin 1 is present in all arteries.38 The reason for this observation is unclear and may be due to the fact that our techniques lack the ability to recognise a mild modification or to the fact that due to the radius of the artery, the tension applied on the arteries of medium size may not be sufficient to reveal this anomaly. Whatever the reason for this observation is, it parallels the clinical picture in that, in the classic MFS related to FBN1 mutation, only the aorta is prone to dilatation and dissection, and dissection of other vessels is a consequence of the extension of the dissection of the aorta. On the other hand, it should be stressed that all aortas are prone to dissection in patients with MFS, and dissection of the descending aorta can occur even in patients in whom ascending aortic diameter is within normal limits.41
Aortic dilatation is a key diagnostic criterion. As such, it should be evaluated precisely and compared to normal values. The most frequent nomogram used is that of Roman et al,42 but it tends to overestimate aortic dilatation in children, and so we had to develop alternative nomograms for children.43 Besides being a diagnostic criterion, aortic diameter is the most powerful predictive marker of dissection and rupture recognised in patients with MFS and, therefore, the basis for proposing prophylactic aortic root replacement surgery. It reflects the importance of the aortic wall abnormality and determines the stress applied to the wall according to the law of Laplace. Understanding this mechanism is important to determine which diameter (rough value or diameter normalised by size or body surface area (BSA)) should be the best parameter for aortic surgery timing. Our policy is to propose surgery when aortic diameter is above 50 mm.44
Aortic dilatation associated with TGF-β receptor mutations
Clinically, aortic dilatation associated with TGFBR2 mutations (LDS, or MFS2 or TAA) is indistinguishable from that observed in patients with a mutation in the FBN1 gene8: it predominates at the level of the sinuses of Valsalva, and the entire aortic wall is altered so that aortic dissection may occur at all levels. However, more aggressive vasculopathy including aortic dissections at a lower aortic diameter than that observed with FBN1 gene mutations and aneurysms on medium-size arteries has been reported so that we systematically perform complete imaging of arteries, including cerebral arteries with a nuclear magnetic resonance study. This aggressive vasculopathy appears to occur mainly in patients with severe forms and facial features of LDS.29 In our experience, when the patient is recognised and managed according to the usual rules proposed for MFS, the prognosis is similar to that of patients with a classic MFS related to an FBN1 mutation.8 In contrast, when the disease is not recognised, prognosis is much worse, due to the fact that an aortic event often reveals the disease in the case of TGFBR2 gene mutation carriers, as ophthalmological features are usually absent and skeletal features are less pronounced than those in patients with an FBN1 gene mutation (figure 6).
Mode of entry into the study of probands carrying a mutation in the TGFBR2 or FBN1 gene. Dissec, aortic dissection; TAA, thoracic aortic aneurysm; SD, sudden death; SKE, skeletal features; Mitral, mitral features; OPH, ophthalmological features; SKEOPH, both skeletal and ophthalmological features; PNO, pneumothorax. From Attias et al.8
Potential implications of the science for novel therapies
From a cardiological point of view, the new understanding of patients with MFS and related diseases has shifted the focus from haemodynamics to molecular biology. Although losartan is a vasodilatory drug and therefore decreases the stress applied on the aortic wall through both decreased arterial pressure and decreased rebound wave, it is now proposed with the aim of blunting the overstimulation of the TGF-β pathway. TGF-β antibodies that have also been shown to be useful in the mouse model may also represent a future option to be tested in patients. The benefit of ACE inhibitors is unknown in this model.
MFS may serve as a model for all TAAs, and many observations similar to those made in the mouse model have been made in an aortic wall coming from patients with aortic aneurysm from other aetiologies.
Some data also indicate that interacting with the molecular mechanisms responsible for the aortic wall destruction (ie, the MMPs) may be of value in diverse animal models of aortic aneurysm. Besides MMP, interacting with other proteolytic systems, such as serine proteases and the fibrinolytic system, may appear to be fruitful. Lastly, tomorrow's treatment will probably combine different approaches, one potentiating the other: as an example, a recent report supports the combination of losartan and doxycycline in the mouse model KI for fibrillin 1,45 and the ongoing losartan trial in France is evaluating the benefit of losartan+β-blockade against β-blockade alone.46
Conclusion
MFS has evolved from a pure clinical definition to a more comprehensive definition including genetics. One should keep in mind that a mutation provides incomplete prognostic information, as illustrated by intrafamilial variability. The scientific challenge of the coming years is to identify the genetic modifiers that could lead to the identification of prognostic factors. MFS has also evolved from a structural disease to a functional disease with the implication of the TGF-β pathway. However, the involvement of this cytokine pathway is still being unwound, as it appears that TAAs of all genetic origins share an alteration in the Smad-2 pathway.
Areas of progress for the future will include understanding intrafamilial variability, the role of and reasons for Smad-2 activation in TAAs in man and probably the description and recognition of the importance of other pathways (eg, MMP, fibrinolytic system, etc); this last point may be critical in the development of new therapies.
References
Footnotes
Funding This study was funded by the ANR.
Competing interests None to declare.
Provenance and peer review Commissioned; externally peer reviewed.