Potential Phenotype-Genotype Correlation in Marfan Syndrome: When Less is More?

Losartan metabolite EXP3179 is a unique blood pressure-lowering AT1R antagonist with direct, rapid endothelium-dependent vasoactive properties

BACKGROUND AND PURPOSE

Losartan is an anti-hypertensive angiotensin II (ANGII) type 1 receptor (AT1R) blocker (ARB) with many unexpected therapeutic properties, even in non-blood pressure (BP)-related diseases. Administered as a prodrug, losartan undergoes serial metabolism into EXP3179, a metabolite alleged to lack AT1R-blocking properties, and EXP3174, the dominant AT1R antagonist. Having observed that losartan can decrease vascular tone in mice with low AT1R expression and inhibit Marfan aortic widening at very high doses, we investigated whether EXP3179 may have unique, AT1R-independent effects on vascular tone and endothelial function.

EXPERIMENTAL APPROACH

We compared the AT1R blocking capabilities of EXP3179 and EXP3174 using AT1R-expressing cell lines. Their BP lowering and vasoactive properties were studied in normal, hypertensive and transgenic rodents, and ex vivo wire myography.

KEY RESULTS

We observed that both EXP3179 and EXP3174 can fully block (100%) AT1R signaling in vitro and significantly decrease BP in normotensive and spontaneously hypertensive rats. Only EXP3179 prevented PE-induced contraction by up to 65% (p < 0.01) in L-NAME and endothelium removal-sensitive fashion. Use of transgenic mice revealed that these effects involve the eNOS/caveolin-1 axis and the endothelium-dependent hyperpolarization factor (EDHF).

CONCLUSION AND IMPLICATIONS

We provide direct structure-activity evidence that EXP3179 is a BP-lowering AT1R blocker with unique endothelial function-enhancing properties not shared with losartan or EXP3174. The major pharmacological effects of losartan in patients are therefore likely more complex than simple blockade of AT1R by EXP3174, which helps rationalize its therapeutic and prophylactic properties, especially at very high doses. Reports relying on EXP3179 as an AT1R-independent losartan analogue may require careful re-evaluation.

A novel variant in fibrillin-1 is responsible for early-onset familial thoracic aortic aneurysms in Marfan patients

Background

Marfan syndrome (MFS) is an inherited connective tissue disorder that affects the skeletal, ocular, and cardiovascular system. The disease’s severity and clinical manifestations vary greatly due to pathogenic variants which, combined with a lack of research on the correlation between MFS’s genotype and phenotype, make MFS a challenging disease to diagnose. This study aims to further the understanding of MFS by shedding light on the clinical manifestation of a novel variant in fibrillin-1 (FBN1)—the protein responsible for the genetic defects that lead to MFS.

Methods

A patient was diagnosed with MFS by combining a clinical examination (based on the 2010 revision to Ghent nosology criteria) with a targeted next-generation sequence analysis. The functional analysis of the causal mutation and the clinical details of the affected patient were then analyzed.

Results

The FBN1 heterozygous variant c.5081_5082insT, which is known to delete large fragments from amino acids 1702 to 2871, was found in the proband patient and her son. The two also displayed the skeletal and cardiovascular manifestations of MFS. In addition, the 14-year-old son was identified as having a dilated aortic bulb at the same rupture site of the proband’s dissection, and the proband’s mother also died at age 32 due to aortic dissection.

Conclusions

The FBN1 variant c.5081_5082insT (p.Leu1694fs*9) is a pathogenic mutation that can cause MFS patients to experience early-onset familial thoracic aortic aneurysms (TAA). We hope that this discovery can provide further insight into the treatment of MFS patients with truncating variants in exons 42-65.

A novel splicing mutation in Marfan syndrome

BACKGROUND

Marfan syndrome (MFS) is a connective tissue disease involving multiple organs and systems such as cardiovascular, skeletal, and ocular systems and is also an autosomal dominant inheritance disorder.

METHOD

A 30-year-old woman was rushed into the hospital owing to sudden persistent pain in the abdomen and died suddenly 2 days later. To find the real cause of death, a forensic autopsy was conducted owing to suspected medical malpractice, and the diagnosis of MFS was made in accordance with the 2010 revised Ghent nosology. By sequencing the gene of Marfan, aneurysm, and related disorders, a novel splicing mutation in the fibrillin-1 gene (FBN1) was detected. For the clinical characteristic findings (wrist and thumb sign) of the daughter, we recommend genetic analysis for the family. To better understand the role of the variant in the disease, we also investigated functional validation of this mutation.

RESULTS

According to the autopsy findings, the cause of death was acute cardiac tamponade caused by aortic rupture. DNA sequencing revealed a novel splicing mutation, c.5672-2delA, which was also detected in her daughter (II2). The functional validation of this mutation showed the base deletion at the same site in the PCR products using cDNA as a template. It is suggested that this mutation may cause abnormal spliceosome during transcription and may encode abnormal protein.

CONCLUSION

A novel pathogenic splicing mutation (c.5672-2delA) was confirmed. Present work enriches the profile mutations in FBN1 associated with MFS and stresses the importance of postmortem genetic analysis in such cases.

Genetic and molecular mechanism for distinct clinical phenotypes conveyed by allelic truncating mutations implicated in FBN1

BACKGROUND

The molecular and genetic mechanisms by which different single nucleotide variant alleles in specific genes, or at the same genetic locus, cause distinct disease phenotypes often remain unclear. Allelic truncating mutations of FBN1 could cause either classical Marfan syndrome (MFS) or a more complicated phenotype associated with Marfanoid-progeroid-lipodystrophy syndrome (MPLS).

METHODS

We investigated a small cohort, encompassing two classical MFS and one MPLS subjects from China, whose clinical presentation included scoliosis potentially requiring surgical intervention. Targeted next generation sequencing was performed on all the participants. We analyzed the molecular diagnosis, clinical features, and the potential molecular mechanism involved in the MPLS subject in our cohort.

RESULTS

We report a novel de novo FBN1 mutation for the first Chinese subject with MPLS, a more complicated fibrillinopathy, and two subjects with more classical MFS. We further predict that the MPLS truncating mutation, and others previously reported, is prone to escape the nonsense-mediated decay (NMD), while MFS mutations are predicted to be subjected to NMD. Also, the MPLS mutation occurs within the glucogenic hormone asprosin domain of FBN1. In vitro experiments showed that the single MPLS mutation p.Glu2759Cysfs*9 appears to perturb proper FBN1 protein aggregation as compared with the classical MFS mutation p.Tyr2596Thrfs*86. Both mutations appear to upregulate SMAD2 phosphorylation in vitro.

CONCLUSION

We provide direct evidence that a dominant-negative interaction of FBN1 potentially explains the complex MPLS phenotypes through genetic and functional analysis. Our study expands the mutation spectrum of FBN1 and highlights the potential molecular mechanism for MPLS.

Human Marfan and Marfan-like Syndrome associated mutations lead to altered trafficking of the Type II TGFβ receptor in Caenorhabditis elegans

The transforming growth factor-β (TGFβ) family plays an important role in many developmental processes and when mutated often contributes to various diseases. Marfan syndrome is a genetic disease with an occurrence of approximately 1 in 5,000. The disease is caused by mutations in fibrillin, which lead to an increase in TGFβ ligand activity, resulting in abnormalities of connective tissues which can be life-threatening. Mutations in other components of TGFβ signaling (receptors, Smads, Schnurri) lead to similar diseases with attenuated phenotypes relative to Marfan syndrome. In particular, mutations in TGFβ receptors, most of which are clustered at the C-terminal end, result in Marfan-like (MFS-like) syndromes. Even though it was assumed that many of these receptor mutations would reduce or eliminate signaling, in many cases signaling is active. From our previous studies on receptor trafficking in C. elegans, we noticed that many of these receptor mutations that lead to Marfan-like syndromes overlap with mutations that cause mis-trafficking of the receptor, suggesting a link between Marfan-like syndromes and TGFβ receptor trafficking. To test this hypothesis, we introduced three of these key MFS and MFS-like mutations into the C. elegans TGFβ receptor and asked if receptor trafficking is altered. We find that in every case studied, mutated receptors mislocalize to the apical surface rather than basolateral surface of the polarized intestinal cells. Further, we find that these mutations result in longer animals, a phenotype due to over-stimulation of the nematode TGFβ pathway and, importantly, indicating that function of the receptor is not abrogated in these mutants. Our nematode models of Marfan syndrome suggest that MFS and MFS-like mutations in the type II receptor lead to mis-trafficking of the receptor and possibly provides an explanation for the disease, a phenomenon which might also occur in some cancers that possess the same mutations within the type II receptor (e.g. colon cancer).

Precise Therapy for Thoracic Aortic Aneurysm in Marfan Syndrome: A Puzzle Nearing Its Solution

Marfan Syndrome (MFS) is a rare connective tissue disorder, resulting from mutations in the fibrillin-1 gene, characterized by pathologic phenotypes in multiple organs, the most detrimental of which affects the thoracic aorta. Indeed, thoracic aortic aneurysms (TAA), leading to acute dissection and rupture, are today the major cause of morbidity and mortality in adult MFS patients. Therefore, there is a compelling need for novel therapeutic strategies to delay TAA progression and counteract aortic dissection occurrence. Unfortunately, the wide phenotypic variability of MFS patients, together with the lack of a complete genotype-phenotype correlation, have represented until now a barrier hampering the conduction of translational studies aimed to predict disease prognosis and drug discovery. In this review, we will illustrate available therapeutic strategies to improve the health of MFS patients. Starting from gold standard surgical overtures and the description of the main pharmacological approaches, we will comprehensively review the state-of-the-art of in vivo MFS models and discuss recent clinical pharmacogenetic results. Finally, we will focus on induced pluripotent stem cells (iPSC) as a technology that, if integrated with preclinical research and pharmacogenetics, could contribute in determining the best therapeutic approach for each MFS patient on the base of individual differences. Finally, we will suggest the integration of preclinical studies, pharmacogenetics and iPSC technology as the most likely strategy to help solve the composite puzzle of precise medicine in this condition.

Aortic dilatation in Marfan syndrome: role of arterial stiffness and fibrillin-1 variants

OBJECTIVE

Marfan syndrome (MFS) is an autosomal dominant genetic disorder characterized by aortic root dilation and dissection and an abnormal fibrillin-1 synthesis. In this observational study, we evaluated aortic stiffness in MFS and its association with ascending aorta diameters and fibrillin-1 genotype.

METHODS

A total of 116 Marfan adult patients without history of cardiovascular surgery, and 144 age, sex, blood pressure and heart rate matched controls were enrolled. All patients underwent arterial stiffness evaluation through carotid-femoral pulse wave velocity (PWV) and central blood pressure waveform analysis (PulsePen tonometer). Fibrillin-1 mutations were classified based on the effect on the protein, into 'dominant negative' and 'haploinsufficient' mutations.

RESULTS

PWV and central pulse pressure were significantly higher in MFS patients than in controls [respectively 7.31 (6.81-7.44) vs. 6.69 (6.52-6.86) m/s, P = 0.0008; 41.3 (39.1-43.5) vs. 34.0 (32.7-35.3) mmHg, P < 0.0001], with a higher age-related increase of PWV in MFS (β 0.062 vs. 0.036). Pressure amplification was significantly reduced in MFS [18.2 (15.9-20.5) vs. 33.4 (31.6-35.2)%, P < 0.0001]. Central pressure profile was altered even in MFS patients without aortic dilatation. Multiple linear regression models showed that PWV independently predicted aortic diameters at the sinuses of Valsalva (ß = 0.243, P = 0.002) and at the sinotubular junction (ß = 0.186, P = 0.048). PWV was higher in 'dominant negative' than 'haploinsufficient' fibrillin-1 mutations [7.37 (7.04-7.70) vs. 6.60 (5.97-7.23) m/s, P = 0.035], although this difference was not significant after adjustment.

CONCLUSION

Aortic stiffness is increased in MFS, independently from fibrillin-1 genotype and is associated with diameters of ascending aorta. Alterations in central hemodynamics are present even when aortic diameter is within normal limits. Our findings suggest an accelerated arterial aging in MFS.

Identification of Novel Causal FBN1 Mutations in Pedigrees of Marfan Syndrome

Marfan syndrome (MFS) is an autosomal dominant genetic disorder of the connective tissue, typically characteristic of cardiovascular manifestations, valve prolapse, left ventricle enlargement, and cardiac failure. Fibrillin-1 (FBN1) is the causative gene in the pathogenesis of MFS. Patients with different FBN1 mutations often present more considerable phenotypic variation. In the present study, three affected MFS pedigrees were collected for genetic analysis. Using next-generation sequencing (NGS) technologies, 3 novel frameshift pathogenic mutations which are cosegregated with affected subjects in 3 pedigrees were identified. These novel mutations provide important diagnostic and therapeutic insights for precision medicine in MFS, especially regarding the lethal cardiovascular events.

Marfan syndrome: Report of a complex phenotype due to a 15q21.1 contiguos gene deletion encompassing FBN1, and literature review

Marfan syndrome (MFS) is an autosomal dominant connective tissue disorder that primarily involves skeletal, ocular, and cardiovascular systems with large inter- and intra-familial variability in terms of age of onset, severity, and aortic disease. The causal gene, FBN1, encodes for fibrillin 1, a multi-domain glycoprotein essential for many biological functions, including deposition and formation of elastic fibers. Reports describing chromosomal alterations involving FBN1 are rare, but in the last years their number has increased after copy number state analyses, such as multiplex ligation-dependent probe amplification and microarray-based comparative genomic hybridization, were adopted as routine diagnostic tools. Herein we report a patient with MFS and an atypical facial appearance and neuropsychiatric involvement likely not attributable to MFS due to a 15q21.1 deletion that involves part of FBN1 and 13 additional contiguous genes listed in OMIM. We compare his phenotype with those of the few patients described in the literature who share similar 15q11.2 deletions. This report expands the phenotype of patients with 15q11.2 deletion involving FBN1 and its contiguous genes, and suggests a possible role for these other genes in the pathogenesis of the observed unusual clinical signs that are not explained by FBN1 haploinsufficiency. © 2016 Wiley Periodicals, Inc.

Recent progress in understanding the natural and clinical histories of the Marfan syndrome

Over the past 4 decades, remarkable progress in understanding the cause, pathogenesis, and management of the MFS has led to an increase in life expectancy to near normal for most patients. Accompanying this increased life span has been the emergence of previously rare or unanticipated clinical problems. Despite much more detailed knowledge of the molecular, cellular, and tissue effects of a mutation in FBN1, targeted, effective therapy remains elusive. Until such precision medicine takes hold, management will depend on early diagnosis, regular scrutiny by imaging, chronic β-blockade, and perhaps ARBs, and prophylactic cardiothoracic surgery. Without question, MFS will remain a fertile subject for basic, translational, and clinical research for the foreseeable future.