Structural and functional failure of fibrillin‑1 in human diseases (Review)

Re-evaluation of a Fibrillin-1 Gene Variant of Uncertain Significance Using the ClinGen Guidelines

Background

Marfan syndrome (MFS) is caused by fibrillin-1 gene (FBN1) variants. Mutational hotspots and/or well-established critical functional domains of FBN1 include cysteine residues, calcium-binding consensus sequences, and amino acids related to interdomain packaging. Previous guidelines for variant interpretation do not reflect the features of genes or related diseases. Using the Clinical Genome Resource (ClinGen) FBN1 variant curation expert panel (VCEP), we re-evaluated FBN1 germline variants reported as variants of uncertain significance (VUSs).

Methods

We re-evaluated 26 VUSs in FBN1 reported in 161 patients with MFS. We checked the variants in the Human Genome Mutation Database, ClinVar, and VarSome databases and assessed their allele frequencies using the gnomAD database. Patients' clinical information was reviewed.

Results

Four missense variants affecting cysteines (c.460T>C, c.1006T>C, c.5330G>C, and c.8020T>C) were reclassified as likely pathogenic and were assigned PM1_strong or PM1. Two intronic variants were reclassified as benign by granting BA1 (stand-alone). Four missense variants were reclassified as likely benign. BP5 criteria were applied in cases with an alternate molecular basis for disease, one of which (c.7231G>A) was discovered alongside a pathogenic de novo COL3A1 variant (c.1988G>T, p.Gly633Val).

Conclusions

Considering the high penetrance of FBN1 variants and clinical variability of MFS, the detection of pathogenic variants is important. The ClinGen FBN1 VCEP encompasses mutational hotspots and/or well-established critical functional domains and adjusts the criteria specifically for MFS; therefore, it is beneficial not only for identifying pathogenic FBN1 variants but also for distinguishing these variants from those that cause other connective tissue disorders with overlapping clinical features.

The extracellular matrix glycoprotein fibrillin-1 in health and disease

Fibrillin-1 (FBN1) is a large, cysteine-rich, calcium binding extracellular matrix glycoprotein encoded by FBN1 gene. It serves as a structural component of microfibrils and provides force-bearing mechanical support in elastic and nonelastic connective tissue. As such, mutations in the FBN1 gene can cause a wide variety of genetic diseases such as Marfan syndrome, an autosomal dominant disorder characterized by ocular, skeletal and cardiovascular abnormalities. FBN1 also interacts with numerous microfibril-associated proteins, growth factors and cell membrane receptors, thereby mediating a wide range of biological processes such as cell survival, proliferation, migration and differentiation. Dysregulation of FBN1 is involved in the pathogenesis of many human diseases, such as cancers, cardiovascular disorders and kidney diseases. Paradoxically, both depletion and overexpression of FBN1 upregulate the bioavailability and signal transduction of TGF-β via distinct mechanisms in different settings. In this review, we summarize the structure and expression of FBN1 and present our current understanding of the functional role of FBN1 in various human diseases. This knowledge will allow to develop better strategies for therapeutic intervention of FBN1 related diseases.

Clinical and genetic findings in Chinese families with congenital ectopia lentis

BACKGROUND

Congenital ectopia lentis (EL) refers to the congenital dysplasia or weakness of the lens suspensory ligament, resulting in an abnormal position of the crystalline lens, which can appear as isolated EL or as an ocular manifestation of a syndrome, such as the Marfan syndrome. The fibrillin-1 protein encoded by the FBN1 gene is an essential component of the lens zonules. Mutations in FBN1 are the leading causes of congenital EL and Marfan syndrome. Owing to the complexity and individual heterogeneity of FBN1 gene mutations, the correlation between FBN1 mutation characteristics and various clinical phenotypes remains unclear.

METHODS

This study describes the clinical characteristics and identifies possible causative genes in eight families with Marfan syndrome or isolated EL using Sanger and whole-exome sequencing.

RESULTS

Eight FBN1 mutations were identified in these families, of which three (c.5065G > C, c.1600 T > A, and c.2210G > C) are reported for the first time. Based on in silico analyses, we hypothesized that these mutations may be pathogenic by affecting the fibrillin-1 protein structure and function.

CONCLUSION

These findings expand the number of known mutations involved in EL and provide a reference for the research on their genotype and phenotype associations.

Genotype-phenotype correlations of marfan syndrome and related fibrillinopathies: Phenomenon and molecular relevance

Marfan syndrome (MFS, OMIM: 154700) is a heritable multisystemic disease characterized by a wide range of clinical manifestations. The underlying molecular defect is caused by variants in the FBN1. Meanwhile, FBN1 variants are also detected in a spectrum of connective tissue disorders collectively termed as ‘type I fibrillinopathies’. A multitude of FBN1 variants is reported and most of them are unique in each pedigree. Although MFS is being considered a monogenic disorder, it is speculated that the allelic heterogeneity of FBN1 variants contributes to various manifestations, distinct prognoses, and differential responses to the therapies in affected patients. Significant progress in the genotype–phenotype correlations of MFS have emerged in the last 20 years, though, some of the associations were still in debate. This review aims to update the recent advances in the genotype-phenotype correlations of MFS and related fibrillinopathies. The molecular bases and pathological mechanisms are summarized for better support of the observed correlations. Other factors contributing to the phenotype heterogeneity and future research directions were also discussed. Dissecting the genotype-phenotype correlation of FBN1 variants and related disorders will provide valuable information in risk stratification, prognosis, and choice of therapy.

Defining the Versican Interactome in Lung Health and Disease

The extracellular matrix (ECM) imparts critical mechanical and biochemical information to cells in the lungs. Proteoglycans are essential constituents of the ECM and play a crucial role in controlling numerous biological processes, including regulating cellular phenotype and function. Versican, a chondroitin sulfate proteoglycan required for embryonic development, is almost absent from mature, healthy lungs and is re-expressed and accumulates in acute and chronic lung disease. Studies using genetically engineered mice show that the versican-enriched matrix can be pro- or anti-inflammatory depending on the cellular source or disease process studied. The mechanisms whereby versican develops a contextual ECM remain largely unknown. The primary goal of this review is to provide an overview of the interaction of versican with its many binding partners, the "versican interactome," and how through these interactions, versican is an integrator of complex extracellular information. Hopefully, the information provided in this review will be used to develop future studies to determine how versican and its binding partners can develop contextual ECMs that control select biological processes. While this review focuses on versican and the lungs, what is described can be extended to other proteoglycans, tissues, and organs.

Cysteine Substitution and Calcium-Binding Mutations in FBN1 cbEGF-Like Domains Are Associated With Severe Ocular Involvement in Patients With Congenital Ectopia Lentis

Purpose: To investigate the clinical manifestations of congenital ectopia lentis (CEL) in patients with fibrillin (FBN1) calcium-binding epidermal growth factor (cbEGF)-like mutations.

Design: Retrospective cohort study.

Methods: Consecutive 68 CEL probands with FBN1 cbEGF-like mutations were recruited, mostly comprising Marfan syndrome (MFS) patients. Patients were classified into the cysteine group (n = 43), calcium (Ca²⁺)-binding group (n = 13) or the others (n = 12) according to their genotypes. Ocular biometrics, morbidities and visual performance were compared among different mutation groups. Linear regression was used to evaluate the risk factors for axial length (AL) elongation.

Results: With age-adjustment, cysteine substitution and Ca²⁺-binding mutations positively contributed to AL elongation (standardized coefficient: 0.410 and 0.367, p = 0.008 and 0.017, respectively). In addition, cataract formation was more frequently detected in patients with Ca²⁺-binding mutations (observed n = 3, expected n = 1.0; p = 0.036). Patients with cysteine substitutions had the poorest preoperative visual acuity among the three groups (p = 0.012) and did not recover as well as other patients. More MFS diagnoses were made in patients with cysteine substitutions (observed n = 16, expected n = 12.6), while ectopia lentis syndrome was detected more often in patients with cbEGF-like mutations out of the functional regions (observed n = 6, expected n = 2.5; p = 0.023).

Conclusion: Compared with patients with cbEGF-like mutations out of functional regions, patients with cysteine substitutions or Ca²⁺-binding mutations had longer ALs with age adjustment, poorer ocular involvement, visual performance, and systematic manifestations.

Oxidative Stress in Plasma from Patients with Marfan Syndrome Is Modulated by Deodorized Garlic Preliminary Findings

Marfan syndrome (MFS) is a genetic disorder of connective tissue that affects the fibrillin-1 protein (FBN-1). It is associated with the formation of aneurysms, damage to the endothelium and oxidative stress (OS). Allium sativum (garlic) has antioxidant properties; therefore, the goal of this study was to show the antioxidant effect of deodorized garlic (DG) on antioxidant enzymes and OS markers in the plasma of patients with MFS. The activity of antioxidant enzymes such as extracellular superoxide dismutase (EcSOD), peroxidases, glutathione peroxidase (GPx), gluthatione-S-tranferase (GST), and thioredoxin reductase (TrxR) was quantified, and nonenzymatic antioxidant system markers including lipid peroxidation (LPO), carbonylation, nitrates/nitrites, GSH, and vitamin C in plasma were determined in patients with MFS before and after treatment with DG. The results show that DG increased the activity of the EcSOD, peroxidases, GPx, GST, TrxR (p ≤ 0.05) and decrease LPO, carbonylation, and nitrates/nitrites (p ≤ 0.01). However, glutathione was increased (p = 0.01) in plasma from patients with MFS. This suggests that treatment with garlic could lower the OS threshold by increasing the activity of antioxidant enzymes and could help in the prevention and mitigation of adverse OS in patients with MFS.

Proteo-Trancriptomic Analyses Reveal a Large Expansion of Metalloprotease-Like Proteins in Atypical Venom Vesicles of the Wasp Meteorus pulchricornis (Braconidae)

Meteorus pulchricornis (Ichneumonoidea, Braconidae) is an endoparasitoid wasp of lepidopteran caterpillars. Its parasitic success relies on vesicles (named M. pulchricornis Virus-Like Particles or MpVLPs) that are synthesized in the venom gland and injected into the parasitoid host along with the venom during oviposition. In order to define the content and understand the biogenesis of these atypical vesicles, we performed a transcriptome analysis of the venom gland and a proteomic analysis of the venom and purified MpVLPs. About half of the MpVLPs and soluble venom proteins identified were unknown and no similarity with any known viral sequence was found. However, MpVLPs contained a large number of proteins labelled as metalloproteinases while the most abundant protein family in the soluble venom was that of proteins containing the Domain of Unknown Function DUF-4803. The high number of these proteins identified suggests that a large expansion of these two protein families occurred in M. pulchricornis. Therefore, although the exact mechanism of MpVLPs formation remains to be elucidated, these vesicles appear to be “metalloproteinase bombs” that may have several physiological roles in the host including modifying the functions of its immune cells. The role of DUF4803 proteins, also present in the venom of other braconids, remains to be clarified.

The role of physical cues in the development of stem cell-derived organoids

Organoids are a novel three-dimensional stem cells’ culture system that allows the in vitro recapitulation of organs/tissues structure complexity. Pluripotent and adult stem cells are included in a peculiar microenvironment consisting of a supporting structure (an extracellular matrix (ECM)-like component) and a cocktail of soluble bioactive molecules that, together, mimic the stem cell niche organization. It is noteworthy that the balance of all microenvironmental components is the most critical step for obtaining the successful development of an accurate organoid instead of an organoid with heterogeneous morphology, size, and cellular composition. Within this system, mechanical forces exerted on stem cells are collected by cellular proteins and transduced via mechanosensing—mechanotransduction mechanisms in biochemical signaling that dictate the stem cell specification process toward the formation of organoids. This review discusses the role of the environment in organoids formation and focuses on the effect of physical components on the developmental system. The work starts with a biological description of organoids and continues with the relevance of physical forces in the organoid environment formation. In this context, the methods used to generate organoids and some relevant published reports are discussed as examples showing the key role of mechanosensing–mechanotransduction mechanisms in stem cell-derived organoids.

Intracellular and Extracellular Markers of Lethality in Osteogenesis Imperfecta: A Quantitative Proteomic Approach

Osteogenesis imperfecta (OI) is a heritable disorder that mainly affects the skeleton. The inheritance is mostly autosomal dominant and associated to mutations in one of the two genes, COL1A1 and COL1A2, encoding for the type I collagen α chains. According to more than 1500 described mutation sites and to outcome spanning from very mild cases to perinatal-lethality, OI is characterized by a wide genotype/phenotype heterogeneity. In order to identify common affected molecular-pathways and disease biomarkers in OI probands with different mutations and lethal or surviving phenotypes, primary fibroblasts from dominant OI patients, carrying COL1A1 or COL1A2 defects, were investigated by applying a Tandem Mass Tag labeling-Liquid Chromatography-Tandem Mass Spectrometry (TMT LC-MS/MS) proteomics approach and bioinformatic tools for comparative protein-abundance profiling. While no difference in α1 or α2 abundance was detected among lethal (type II) and not-lethal (type III) OI patients, 17 proteins, with key effects on matrix structure and organization, cell signaling, and cell and tissue development and differentiation, were significantly different between type II and type III OI patients. Among them, some non-collagenous extracellular matrix (ECM) proteins (e.g., decorin and fibrillin-1) and proteins modulating cytoskeleton (e.g., nestin and palladin) directly correlate to the severity of the disease. Their defective presence may define proband-failure in balancing aberrances related to mutant collagen.

Human Microfibrillar-Associated Protein 4 (MFAP4) Gene Promoter: A TATA-Less Promoter That Is Regulated by Retinol and Coenzyme Q10 in Human Fibroblast Cells

Elastic fibers are one of the major structural components of the extracellular matrix (ECM) in human connective tissues. Among these fibers, microfibrillar-associated protein 4 (MFAP4) is one of the most important microfibril-associated glycoproteins. MFAP4 has been found to bind with elastin microfibrils and interact directly with fibrillin-1, and then aid in elastic fiber formation. However, the regulations of the human MFAP4 gene are not so clear. Therefore, in this study, we firstly aimed to analyze and identify the promoter region of the human MFAP4 gene. The results indicate that the human MFAP4 promoter is a TATA-less promoter with tissue- and species-specific properties. Moreover, the promoter can be up-regulated by retinol and coenzyme Q10 (coQ10) in Detroit 551 cells.

Effects of fibrillin mutations on the behavior of heart muscle cells in Marfan syndrome

Marfan syndrome (MFS) is a systemic disorder of connective tissue caused by pathogenic variants in the fibrillin-1 (FBN1) gene. Myocardial dysfunction has been demonstrated in MFS patients and mouse models, but little is known about the intrinsic effect on the cardiomyocytes (CMs). In this study, both induced pluripotent stem cells derived from a MFS-patient and the line with the corrected FBN1 mutation were differentiated to CMs. Several functional analyses are performed on this model to study MFS related cardiomyopathy. Atomic force microscopy revealed that MFS CMs are stiffer compared to corrected CMs. The contraction amplitude of MFS CMs is decreased compared to corrected CMs. Under normal culture conditions, MFS CMs show a lower beat-to-beat variability compared to corrected CMs using multi electrode array. Isoproterenol-induced stress or cyclic strain demonstrates lack of support from the matrix in MFS CMs. This study reports the first cardiac cell culture model for MFS, revealing abnormalities in the behavior of MFS CMs that are related to matrix defects. Based on these results, we postulate that impaired support from the extracellular environment plays a key role in the improper functioning of CMs in MFS.

Involvement of Matrix Metalloproteinase 9 in Vertebral Arterial Dissection With Posterior Circulation Ischemic Stroke

Background

Spontaneous vertebral arterial dissection (VAD) is an important cause of posterior circulation ischemic stroke (PCS), but its pathogenesis remains elusive. Matrix metalloproteinase 9 (MMP‐9) is a gelatinase involved in inflammation process and several vascular diseases, such as aorta dissection, but its role in VBD is unclear yet. The present study aimed to determine the association between serum MMP‐9 level and VAD‐related PCS.

Methods and Results

We recruited 149 patients with PCS, of which 30 were VAD and 119 had other determined etiologies (non‐VAD), and 219 non‐stroke individuals. Serum MMP‐9 was measured within 14 days from stroke onset. The age of VAD group was 59.6±15.0 years, which is similar to non‐stroke group (P=0.510) but significantly younger than non‐VAD group (69.9±14.0 years, P<0.001). Males and vascular risk factors were significantly more prevalent in VAD and non‐VAD groups than non‐stroke group (P<0.001). Multivariate logistic regression analysis adjusting potential confounders revealed that every 100 ng/mL of serum MMP‐9 level increment significantly predicted VAD (versus non‐stroke group: odds ratio (OR), 4.572; 95% CI, 2.240–9.333, P<0.001; versus non‐VAD group: OR, 1.819; 95% CI, 1.034–3.200, P=0.038).

Conclusions

Patients with VAD‐related PCS had higher levels of serum MMP‐9 at the acute stage of stroke compared with non‐stroke individuals and PCS of other causes, supporting the potential involvement of extracellular matrix‐degrading protease in the mechanism of VAD, which leads to ischemic events.

A positively selected FBN1 missense variant reduces height in Peruvian individuals

On average, Peruvian individuals are among the shortest in the world¹. Here we show that Native American ancestry is associated with reduced height in an ethnically diverse group of Peruvian individuals, and identify a population-specific, missense variant in the FBN1 gene (E1297G) that is significantly associated with lower height. Each copy of the minor allele (frequency of 4.7%) reduces height by 2.2 cm (4.4 cm in homozygous individuals). To our knowledge, this is the largest effect size known for a common height-associated variant. FBN1 encodes the extracellular matrix protein fibrillin 1, which is a major structural component of microfibrils. We observed less densely packed fibrillin-1-rich microfibrils with irregular edges in the skin of individuals who were homozygous for G1297 compared with individuals who were homozygous for E1297. Moreover, we show that the E1297G locus is under positive selection in non-African populations, and that the E1297 variant shows subtle evidence of positive selection specifically within the Peruvian population. This variant is also significantly more frequent in coastal Peruvian populations than in populations from the Andes or the Amazon, which suggests that short stature might be the result of adaptation to factors that are associated with the coastal environment in Peru.

Revisiting the Population Genetics of Human Height

CONTEXT

Recent advances in genetics and genomics present unique opportunities for enhancing knowledge of human physiology and disease susceptibility. An outstanding example of these new insights may be seen in the study of human height, of which it has been estimated that approximately 80% is genetically determined. Over the past decade, large-scale population analyses have led to the identification of novel variation in genes and loci individually associated with changes in adult height of as much as 2 cm.

OBJECTIVE

To assess these same variants in the genomes of 213 158 individuals compiled by the Genome Aggregation Database (GnomAD) consortium, representing different population groups from around the world.

RESULTS

The majority of these height-changing alleles are substantially less prevalent in GnomAD than found previously in other cohorts, with 4 of 5 amino acid substitution variants with the largest impact on adult height being more frequent in the European population than in other groups.

CONCLUSIONS

A larger-scale analysis of individuals from diverse backgrounds will be necessary to ensure a full and accurate understanding of the genetic underpinnings of human height throughout the world, and additional studies will be needed to discern the biochemical and molecular mechanisms governing the physiological processes that explain how these variant proteins might selectively impact the biology of the growth plate. Broader understanding of the genetics of height also should set the stage for more comprehensive investigation into the causes of prevalent polygenic human diseases.

Mutation analysis of FBN1 gene in two Chinese families with congenital ectopia lentis in northern China

AIM

To summarize the phenotypes and identify the underlying genetic cause of the fibrillin-1 (FBN1) gene responsible for congenital ectopia lentis (EL) in two Chinese families in northern China.

METHODS

A detailed family history and clinical data from all participants were collected by clinical examination. The candidate genes were captured and sequenced by targeted next-generation sequencing, and the results were confirmed by Sanger sequencing. Haplotyping was used to confirm the mutation sequence. Real-time PCR was used to determine the FBN1 messenger ribonucleic acid (mRNA) levels in patients with EL and in unaffected family members.

RESULTS

The probands and other patients in the two families were affected with congenital isolated EL. A heterozygous FBN1 mutation in exon 21 (c.2420_IVS20-8 delTCTGAAACAinsCGAAAG) was identified in FAMILY-1. A heterozygous FBN1 mutation in exon 14 (c.1633C>T, p.R545C) was identified in FAMILY-2. Each mutation co-segregated with the affected individuals in the family and did not exist in unaffected family members and 200 unrelated normal controls.

CONCLUSION

The insertion-deletion mutation (c.2420 IVS20-8delTCTGAAACA insCGAAAG) in the FBN1 gene is first identified in isolated EL. The mutation (c.1633C>T) in the FBN1 gene was a known mutation in EL patient. The variable phenotypes among the patients expand the phenotypic spectrum of EL in a different ethnic background.

Association of thoracic spine deformity and cardiovascular disease in a mouse model for Marfan syndrome

Aims

Cardiovascular manifestations are a major cause of mortality in Marfan syndrome (MFS). Animal models that mimic the syndrome and its clinical variability are instrumental for understanding the genesis and risk factors for cardiovascular disease in MFS. This study used morphological and ultrastructural analysis to the understanding of the development of cardiovascular phenotypes of the the mgΔ^loxPneo model for MFS.

Methods and results

We studied 6-month-old female mice of the 129/Sv background, 6 wild type (WT) and 24 heterozygous animals from the mgΔ^loxPneo model. Descending thoracic aortic aneurysm and/or dissection (dTAAD) were identified in 75% of the MFS animals, defining two subgroups: MFS with (MFS⁺) and without (MFS^-) dTAAD. Both subgroups showed increased fragmentation of elastic fibers, predominance of type I collagen surrounding the elastic fiber and fragmentation of interlaminar fibers when compared to WT. However, only MFS animals with spine tortuosity developed aortic aneurysm/dissection. The aorta of MFS⁺ animals were more tortuous compared to those of MFS^- and WT mice, possibly causing perturbations of the luminal blood flow. This was evidenced by the detection of diminished aorta-blood flow in MFS⁺. Accordingly, only MFS⁺ animals presented a process of concentric cardiac hypertrophy and a significantly decreased ratio of left and right ventricle lumen area.

Conclusions

We show that mgΔ^loxPneo model mimics the vascular disease observed in MFS patients. Furthermore, the study indicates role of thoracic spine deformity in the development of aorta diseases. We suggest that degradation of support structures of the aortic wall; deficiency in the sustenance of the thoracic vertebrae; and their compression over the adjacent aorta resulting in disturbed blood flow is a triad of factors involved in the genesis of dissection/aneurysm of thoracic aorta.

Cells respond to deletion of CAV1 by increasing synthesis of extracellular matrix

A range of cellular functions have been attributed to caveolae, flask-like invaginations of the plasma membrane. Here, we have used RNA-seq to achieve quantitative transcriptional profiling of primary embryonic fibroblasts from caveolin 1 knockout mice (CAV1-/- MEFs), and thereby to gain hypothesis-free insight into how these cells respond to the absence of caveolae. Components of the extracellular matrix were decisively over-represented within the set of genes displaying altered expression in CAV1-/- MEFs when compared to congenic wild-type controls. This was confirmed biochemically and by imaging for selected examples. Up-regulation of components of the extracellular matrix was also observed in a second cell line, NIH-3T3 cells genome edited to delete CAV1. Up-regulation of components of the extracellular matrix was detected in vivo by assessing collagen deposition and compliance of CAV1-/- lungs. We discuss the implications of these findings in terms of the cellular function of caveolae.