Marfan syndrome: definition

Marfan syndrome (MFS) is an autosomal dominant disorder with pleiotropic features, including skeletal abnormalities, ectopia lentis and aortic root dilatation. The main causal gene for MFS is FBN1, encoding fibrillin-1, a large glycoprotein that is a main component of extracellular microfibrils. Prognosis is determined mainly by aortic complications (dissection or death), after progressive dilatation of the aortic root. Diagnosis criteria have changed over time; international criteria were proposed in 1988 and refined in 1996, to increase specificity and to integrate genetic testing . The current nosology is based on the Ghent criteria, which define major and minor manifestations in different systems (Table [not provided]) . In this setting, the diagnosis of MFS requires at least two major criteria and the involvement of at least one other body system (i.e., three criteria in total). In the presence of an FBN1 mutation or when MFS is diagnosed in a first-degree relative, only one major criterion and the involvement of another body system is required . These Ghent criteria have excellent specificity for FBN1 mutation recognition, because its detection is possible in 95% of patients who fulfil these criteria . However, a mutation in the FBN1 gene is not pathognomonic of MFS and may generate a large array of phenotypes that overlap with MFS (familial ectopia lentis, Shprintzen-Goldberg syndrome, other fibrillinopathies) . On the other hand, some features of MFS can also be present in patients with mutations in the gene coding for transforming growth factor beta (TGFβ) receptor 2 (TGFBR2), who present with MFS type 2 . Lastly, mutations in the gene coding for TGFβ receptor 1 (TGFBR1) may also lead to overlapping syndromes . Therefore, defining clear frontiers for MFS, fulfilling the Ghent criteria and differentiating from other clinical conditions that overlap with MFS (MFS type 2, Loeys-Dietz syndrome, familial thoracic aortic aneurysm, Ehlers-Danlos vascular syndrome ) can sometimes be challenging. In the near future, a revised nosology will emerge, which will focus on the features and criteria that distinguish MFS from other disorders; in the meantime, however, the Ghent criteria remain the reference and will be used for classification of patients in the current study.

Current medical management

The aortic root diameter at the sinuses of Valsalva is considered to be the best predictor of the occurrence of an aortic event . Patients with an absolute aortic diameter > 50 mm, an aortic ratio > 1.3 (observed/expected diameters) or a z score > 3 (the z score is the number of standard deviations above the mean) are considered at high risk for catastrophic complications, based on data obtained more than 10 years ago, before the publication of the Ghent criteria . In addition, rapid growth of the aortic root (> 0.5 cm/year) and a family history of dissection are also predictors of poor outcome in patients with MFS. Therefore, routine monitoring of the aortic diameter is necessary for determining the best time for surgery.

Associated medical therapy is mandatory and includes avoidance of isometric exercises, sports limitation and preventive medical therapy with beta-blockers or calcium channel blockers, inducing bradycardia . Indeed, the use of beta-adrenergic blockade to decrease the haemodynamic stress on the ascending aorta has been suggested since the 1970s . The first randomized, open-label trial was published in 1994 ( Fig. 1 ). In this study, the rate of change of aortic root diameter and clinical outcomes (aortic regurgitation, aortic dissection, surgery, heart failure and death) were compared between 32 patients assigned randomly to receive propanolol and 38 controls. At baseline, the absolute aortic diameters were larger in the treated group than in the non-treated group (34.6 vs 30.2 mm) but the aortic ratios (observed/expected diameters) were similar (1.4 vs 1.3, not significant). After a decade of follow-up, the mean change in aortic ratios over time was significantly lower in the treatment group (0.023/year) than in the control group (0.084/year). No statistically significant difference was observed between groups in event-free survival, but the rate of clinical events was higher in the control group than in the treatment group. Retrospective studies in children have demonstrated the beneficial role of beta-blockers , with a reduction in the rate of change in aortic diameter of 0.16 mm/year and a decrease in the number of aortic complications .

Beneficial effect of beta-blocker therapy on the aortic root diameter (from Shores et al. ). The Y-axis represents the ratio of observed aortic diameter/normal aortic diameter. Arrows correspond to patients not taking the drug.

From a theoretical point of view, the efficacy of beta-blockers relies on their haemodynamic properties, reducing the force of left ventricular ejection (i.e., dp/dt) by negative inotropy and the number of impulses due to bradycardia, particularly during stress and exercise. Most series also demonstrate an increase in indexes of arterial wall compliance (which is basically decreased in patients with MFS ) with beta-blockers . Nevertheless, the absence of an increase in aortic compliance with beta-blocker therapy has been observed in patients with marked aortic enlargement or increased weight, stressing the importance of starting treatment early in the course of the disease, with dose optimization .

Marfan syndrome in 2010: physiopathology

In MFS, histological observations of the aortic wall classically show medial degeneration, with smooth muscle cell disappearance, disorganization of elastic fibres and accumulation of mucopolysaccharides. These abnormalities are not specific to MFS, as they can be observed in the aortic wall of aneurysms with other aetiologies, such as those related to bicuspid aortic valves or even degenerative aneurysms .

These histological abnormalities led to hypotheses being generated on the pathogenesis of MFS aortic disease. Mutations in the FBN1 gene encoding fibrillin-1 result in an abnormal protein and enhanced proteolytic degradation of fibrillin-1. This protein is an essential component of the microfibrils that play a role in extracellular matrix structure and regulation, elastic fibre organization and cell adhesion . Hence, the presence of structurally abnormal microfibrils would weaken the extracellular matrix, allowing progressive aortic dilatation. In this hypothesis, all features associated with MFS are secondary to the weakening of the extracellular matrix in different tissues, leading to increased growth of bones, hernia, cutaneous striaes, etc.

Alternative hypothesis have been proposed, however, based on observations made in a mouse model of MFS, which carries a pathogenic mutation in FBN1 . In this model, markers for activation of the TGFβ pathway (i.e., p-smad-2) were present in smooth muscle cells, suggesting activation. TGFβ molecules are cytokines, synthesized and secreted by smooth muscle cells as inactive precursors in the form of a large latent complex (that includes a pro-TGFβ molecule and the “large TGFβ-binding protein”), which is stored in the extracellular matrix . One proposed hypothesis is that abnormal fibrillin causes failure of the latent complex sequestration and excessive activation ( Fig. 2 ). Excessive TGFβ signalling would then cause increased smad-2 phosphorylation and nuclear localization, resulting in altered gene expression ( Fig. 3 ).

Relationship between transforming growth factor beta (TGFβ)-binding protein and extracellular microfibrils (from Isogai et al. ). TGFβ is synthesized as a precursor molecule containing a propeptide region. After it is synthesized, the TGFβ homodimer interacts with a latency-associated peptide (LAP; a protein derived from the N-terminal region of the TGFβ gene product), forming a complex called “small latent complex”. This complex remains in the cell until it is bound by another protein called latent TGFβ-binding protein (LTBP), forming a larger complex called “large latent complex”. This complex is secreted to the extracellular matrix.

The transforming growth factor beta (TGFβ) signalling pathway (modified from ten Dijke and Arthur ). ALK5: transforming growth factor beta receptor 1; TGFBR2: transforming growth factor beta receptor 2.

Experimental therapy

Experimental studies in the KI mouse have shown an absence of development of aortic aneurysm or myxomatous mitral valve in animals treated with TGFβ antibodies . Interestingly, a similar effect has been observed with the use of losartan, a blocker of the angiotensin II type 1 receptor (AT1) . Indeed, AT1 stimulation with angiotensin II activates the process of fibrosis and cell proliferation. These effects are mediated by TGFβ activation.

In the experimental study by Habashi et al. , comparison of aortic diameter growth between Marfan mice treated with placebo, propanolol or losartan showed that both beta-blockers and AT1 blockers (angiotensin II type 1 receptor blockers [ARBs]) reduced the rate of change in aortic diameter compared with placebo. However, regarding the histological abnormalities, only treatment with losartan reduced the disarray of the extracellular matrix significantly.

The primary results of non-randomized and small studies in humans are encouraging. Brooke et al. compared the progression of aortic root diameter in 18 children with MFS, before and after treatment with ARBs. After treatment with ARBs in addition to beta-blockers over 2 years, the rate of change of the Valsalva and sinotubular junction decreased significantly from 3.5 mm/year to 0.46 mm/year. However, this study had many limitations, including its retrospective nature, the selection of children at the time of maximal aortic growth, the absence of a control group of any kind and the normalization of the aortic diameter using unusual rules.

Although this hypothesis is very appealing, some unexplained results have been observed. Firstly, blocking only AT1, ARBs allow beneficial effects (fibrosis and cell proliferation inhibition) via signalling through the angiotensin II type 2 receptor (AT2), whereas angiotensin-converting enzyme inhibitors (ACEIs) reduce both AT1 and AT2 signalling. Ahismatos et al. randomized 17 adults with MFS to receive either an ACEI (perindopril) or placebo in association with beta-blocker therapy. The study showed that, after 6 months, the stiffness of the aorta and the aortic root diameters had decreased significantly in patients receiving the ACEI and the aortic diameter became smaller compared with placebo. Levels of TGFβ were reduced by ACEI therapy, suggesting that ACEIs, like ARBs, target the underlying tissue pathology, in addition to reducing haemodynamic stress. These results, obtained in only one centre in a very small number of patients (10 receiving placebo, seven receiving perindopril) will have to be reproduced by others before being accepted widely. However, they are supported by non-randomized data .

Secondly, the presence of p-samd-2 in smooth muscle cells has been observed in the aortic wall of patients with a TGFBR2 mutation blocking transmission of the signal and in aneurysms of various aetiologies; this is compatible with the release of TGFβ by the matrix when it is destroyed, regardless of the aetiology of the extracellular matrix alteration .

Hence the benefit of blockade of the renin-angiotensin system seems promising in MFS, but has not been demonstrated in humans. Multicentre trials are needed to address this question. A trial designed to test the efficacy of losartan vs atenolol is ongoing in the USA and aims to include 604 patients (children and adults). Another three-arm Italian trial is ongoing to evaluate the effect of losartan vs nebivolol vs a combination of both on the progression of aortic root dilatation .

Rationale for this trial

We have initiated this trial to evaluate the safety and benefit of losartan on aortic root growth in MFS when added to accepted standard therapy. Beta-blocker therapy has limitations (merely a haemodynamic effect, lack of target in the underlying tissue pathology, side effects), but remains the standard of medical care in MFS. Therefore, considering a trial without allowing beta-blocker therapy might be perceived to be unethical. Besides, while beta-blockers are exerting their haemodynamic effects, losartan should modify the physiology within the aortic wall; in other words, the pathophysiology indicates that their effects should be additive.