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Fibrillin

Fibrillins are a family of large, cysteine-rich extracellular matrix glycoproteins that assemble into microfibrils, providing structural integrity and elasticity to connective tissues while also regulating the bioavailability of growth factors such as transforming growth factor-β (TGF-β) and bone morphogenetic proteins (BMPs).^[1] Encoded by three genes—FBN1, FBN2, and FBN3—these proteins are approximately 350 kDa in size and consist of modular domains including calcium-binding epidermal growth factor-like (cbEGF) repeats, TGF-β-binding (TB) domains, and unique regions such as proline- or glycine-rich segments that vary by isoform.^[1] In humans, fibrillin-1 and fibrillin-2 predominate, with fibrillin-3 expression limited to fetal tissues and certain species; fibrillin-1, in particular, is ubiquitously expressed in elastic and non-elastic tissues like the aorta, skin, and ocular ligaments.^[2] The assembly of fibrillins into beaded microfibrils begins at the cell surface, where monomers are secreted and processed by furin-like proprotein convertases before undergoing N-terminal and C-terminal interactions to form head-to-tail dimers, followed by lateral associations and disulfide cross-linking.^[1] This process depends on accessory molecules, including fibronectin for initiation, heparin for stabilization, and latent TGF-β-binding proteins (LTBPs) for integration into the matrix.^[2] Once formed, these microfibrils serve as scaffolds for elastin deposition in elastic fibers or act independently in non-elastic tissues, contributing to tissue resilience and homeostasis.^[1] Beyond their structural roles, fibrillins function as signaling hubs by sequestering and modulating growth factors; for instance, fibrillin-1 binds LTBP-1 to control TGF-β activation, influencing cell proliferation, migration, and differentiation.^[2] Dysregulation of fibrillin assembly or function, often due to mutations, leads to heritable connective tissue disorders known as fibrillinopathies, including Marfan syndrome (caused by FBN1 variants leading to aortic aneurysms and skeletal abnormalities) and congenital contractural arachnodactyly (linked to FBN2 mutations affecting joint contractures).^[1] Emerging research also implicates fibrillins in non-heritable conditions, such as fibrosis in chronic kidney disease and certain cancers, where altered TGF-β signaling promotes pathological remodeling.^[2]

Overview

Definition and Role

Fibrillins are a family of large, cysteine-rich glycoproteins, approximately 350 kDa in size, that constitute key components of the extracellular matrix (ECM). These proteins assemble into microfibrils with diameters of 10-12 nm, forming the structural backbone of these supramolecular structures found in both elastic and non-elastic tissues.^[2]^[3]^[1] The primary role of fibrillins is to provide mechanical support and structural integrity to connective tissues, enabling resilience and elasticity in dynamic environments. In tissues such as skin, blood vessels, ligaments, and lungs, fibrillin-based microfibrils act as force-bearing scaffolds that organize and stabilize the ECM, facilitating proper tissue function and homeostasis.^[2]^[3]^[1] In humans, three isoforms of fibrillin are expressed—fibrillin-1 (encoded by FBN1), fibrillin-2 (FBN2), and fibrillin-3 (FBN3)—each contributing to microfibril formation with tissue-specific distributions. These proteins exhibit evolutionary conservation across vertebrates and even broader metazoans, underscoring their fundamental importance in ECM architecture and development.^[3]^[1]^[4]

Discovery and Historical Context

The discovery of fibrillin emerged in the mid-1980s through studies on the ultrastructure of elastic fibers in connective tissues. Researchers Lynn Y. Sakai, Douglas R. Keene, and Erkki Engvall utilized rotary shadowing electron microscopy to examine microfibrils associated with elastic fibers in human connective tissues, using samples from cultured human fibroblasts, identifying a novel 350 kDa glycoprotein as a primary structural component of these 10-nm extracellular assemblies. This protein, initially termed fibrillin, was isolated from cultured fibroblasts and characterized as cysteine-rich, with immunolocalization confirming its presence in elastic fiber networks across various tissues.^[5] Building on this initial characterization, the 1990s marked significant advances in understanding fibrillin's genetic basis and clinical relevance. In 1991, Brendan Lee and colleagues cloned the human FBN1 gene encoding fibrillin-1 and mapped it to chromosome 15q21.1, establishing a direct linkage to Marfan syndrome through genetic analysis of affected families.^[6] This breakthrough shifted focus from fibrillin's structural role in microfibrils—first noted in the 1980s—to its implications in heritable connective tissue disorders, prompting extensive mutation screening.^[6] Concurrently, studies revealed fibrillin-1's calcium-binding properties, with Paul A. Handford's team demonstrating in 1995 that its epidermal growth factor-like domains form rigid, linear structures upon calcium coordination, essential for microfibril stability.^[7] By the 2000s, research illuminated fibrillin's regulatory functions beyond structural support. Seminal work by Isogai et al. in 2007 showed that fibrillin-1 sequesters latent transforming growth factor-β (TGF-β) in the extracellular matrix by interacting with latent TGF-β-binding protein-1 (LTBP-1), thereby modulating TGF-β bioavailability and signaling in tissues.^[8] This discovery integrated fibrillin into growth factor homeostasis, explaining pathological TGF-β dysregulation in conditions like Marfan syndrome and expanding its significance in connective tissue biology.

Molecular Structure

General Architecture

Fibrillins are large, modular glycoproteins that serve as the primary structural components of extracellular microfibrils, with each monomer consisting of approximately 2,800 to 3,000 amino acids and a molecular weight ranging from 300 to 350 kDa.^[9]^[10] These proteins feature a linear arrangement of multiple domains, enabling their incorporation into supramolecular assemblies that provide tensile strength and elasticity to connective tissues.^[4] Fibrillins assemble into characteristic "bead-on-a-string" microfibrils through specific N- and C-terminal interactions, which facilitate head-to-tail polymerization and create periodic structures with a spacing of 55-60 nm between beads.^[11]^[12] These microfibrils, typically 10-12 nm in diameter, exhibit a hollow cross-section and form hierarchical networks that integrate with other extracellular matrix components to support tissue architecture.^[13] The conformation of fibrillins is highly dependent on calcium ions, which bind to approximately 40-50 sites per monomer—primarily within calcium-binding epidermal growth factor-like (cbEGF) domains—to stabilize an extended, rod-like structure essential for microfibril integrity and resistance to proteolysis.^[14]^[15] In the absence of calcium, the protein adopts a more compact form, but ion binding promotes rigid interdomain packing with affinities in the nanomolar to micromolar range, facilitating proper alignment during assembly.^[15] Recent cryogenic electron microscopy (cryo-EM) studies have resolved fibrillin microfibril structures to approximately 10 Å, revealing that up to eight monomers interweave to form a pseudo-eightfold symmetric core within the periodic beads.^[11] These insights highlight the macromolecular scale of organization, with each 57 nm repeat encompassing a mass of about 2.55 MDa, underscoring the role of terminal interactions in propagating the fibril's polarity and mechanical properties.^[11]

Domains and Post-Translational Modifications

Fibrillin proteins exhibit a highly modular architecture dominated by repeating domains that confer structural versatility and interactive capabilities. The core building blocks include epidermal growth factor-like (EGF-like) domains, transforming growth factor-β binding protein-like (TB) domains, and distinctive hybrid motifs. In fibrillin-1, the most extensively studied isoform, this composition comprises 47 EGF-like domains, 7 TB domains, and 2 hybrid domains located at the N-terminus, with the EGF-like and TB domains arranged in tandem repeats that form an elongated rod-like structure.^[16] Over 90% of the EGF-like domains (specifically 43 out of 47) are calcium-binding variants (cbEGF), which feature a consensus sequence enabling high-affinity calcium coordination.^[11] The cbEGF domains play a pivotal role in modulating fibrillin's mechanical properties by binding calcium ions, which rigidifies the interdomain interfaces and promotes a linear extension under tensile forces, essential for microfibril elasticity. In contrast, the TB domains, characterized by eight conserved cysteines, facilitate key intermolecular interactions, including binding to elastin precursors and latent TGF-β binding proteins (LTBPs), thereby integrating fibrillin into elastic fiber networks. The hybrid domains, combining elements of both EGF-like and TB motifs, contribute to unique N-terminal interactions that may influence assembly initiation, though their precise roles remain under investigation.^[1]^[17] Post-translational modifications are critical for fibrillin's maturation, secretion, and functionality, with extensive glycosylation and disulfide bond formation being predominant. Fibrillin-1 undergoes both N-linked glycosylation at asparagine residues within consensus sequences (Asn-X-Ser/Thr) in many EGF-like domains and O-linked glycosylation on serine and threonine residues, which protect against proteolysis, aid in proper folding, and facilitate cellular trafficking. These modifications occur primarily in the endoplasmic reticulum and Golgi apparatus, contributing to the protein's high molecular weight (approximately 350 kDa) and solubility.^[18] Disulfide bonds, formed by cysteine residues organized as six per EGF-like domain and eight per TB domain, stabilize the compact three-dimensional structure of each domain through characteristic pairings (e.g., 1-3, 2-4, 5-6 in cbEGF domains), preventing misfolding and enabling the protein's resilience in the extracellular matrix.^[16] These modifications collectively ensure fibrillin's stability and its ability to withstand physiological stresses within tissues.

Biosynthesis and Assembly

Gene Expression and Transcription

The fibrillin genes in humans consist of three principal members: FBN1, FBN2, and FBN3, each exhibiting a conserved genomic architecture that reflects their shared evolutionary origin and functional roles in extracellular matrix assembly. The FBN1 gene is located on chromosome 15q21.1, spanning approximately 230 kb and comprising 65 exons with a complex intron-exon organization that includes several large introns, particularly in the 5' region. Similarly, FBN2 resides on chromosome 5q23-31 and FBN3 on chromosome 19p13.3-13.2, with FBN2 featuring 65 exons and FBN3 66 exons spanning about 85 kb, both with analogous exon-intron boundaries that preserve modular domains encoding cysteine-rich repeats and other structural motifs. This structural similarity across the family facilitates coordinated regulation and protein functionality in connective tissues.^[17]^[19]^[20] Transcriptional regulation of fibrillin genes involves multiple promoters and enhancers that enable tissue-specific expression and responsiveness to environmental cues. The FBN1 promoter region contains CpG islands and binding sites for transcription factors that drive basal expression, while distal enhancers correlate with high activity in mesenchymal-derived tissues such as the aorta and heart valves. Mechanical stress, such as cyclic stretch in vascular smooth muscle cells, upregulates FBN1 transcription, highlighting its role in adapting to biomechanical demands in load-bearing tissues. The promoters of all three genes utilize overlapping regulatory elements that respond to transforming growth factor-β (TGF-β) signaling, which modulates microfibril homeostasis.^[21]^[22]^[23] Expression patterns of fibrillin genes are temporally and spatially distinct, aligning with developmental stages and tissue requirements. FBN1 exhibits ubiquitous expression in adult connective tissues, with particularly elevated levels in elastic structures like the aorta, ligaments, and ciliary zonules, supporting lifelong matrix integrity. In contrast, FBN2 is predominantly expressed during embryogenesis, showing high abundance in skin, perichondrium, and periosteum, where it contributes to early tissue patterning and remodeling. FBN3 displays more restricted expression, peaking in fetal tissues such as lung, brain, and kidney, with immunolocalization in developing bronchi, glomeruli, and perichondrium, suggesting a specialized role in organogenesis. These patterns underscore the genes' complementary contributions to matrix maturation.^[17]^[24]^[25] Alternative splicing of fibrillin pre-mRNAs generates minor isoforms that fine-tune protein diversity, though the predominant transcripts are highly conserved. For FBN1, the primary mRNA transcript measures approximately 9.7-10 kb, encoding the full-length profibrillin-1 precursor, with tissue- and development-specific inclusion of exons like those in the 5' region contributing to isoform variation in fibroblasts and vascular cells. Similar splicing events occur in FBN2 and FBN3, yielding variants that may alter calcium-binding or growth factor interactions, but without disrupting the core ~10 kb transcript structure essential for microfibril incorporation.^[26]^[27]^[28]

Microfibril Formation and Interactions

Fibrillins are synthesized and secreted by fibroblasts and other connective tissue cells as ~350 kDa proprotein monomers into the extracellular space.^[29] These monomers undergo proteolytic processing by furin-like proprotein convertases, which cleave C-terminal and N-terminal propeptides—such as the RKRR↓STNET site at the C-terminus and RAKR↓R45GGG at the N-terminus—either immediately before or shortly after secretion, yielding mature ~320 kDa forms essential for subsequent matrix incorporation.^[29] The C-terminal propeptide cleaved from fibrillin-1 is known as asprosin, a 140-amino-acid hormone that promotes hepatic glucose release and appetite stimulation during fasting.^[30] This cleavage is calcium-dependent and facilitates initial assembly at the cell surface, where the processed fibrillin molecules begin to polymerize into higher-order structures.^[31] Microfibril assembly proceeds through a hierarchical multimerization process starting with head-to-tail dimerization of mature fibrillin monomers, mediated primarily by interactions between the N-terminal domains and supported by the first hybrid domain.^[29] These dimers form via disulfide bonding at specific cysteines (e.g., Cys204 in the hybrid domain) and are further stabilized by transglutaminase cross-links, such as between residues 580 in the N-terminus and 2312 in the C-terminus.^[29] Subsequent lateral aggregation of these dimers into tetramers and higher-order multimers occurs extracellularly, creating the periodic bead-on-a-string architecture of microfibrils, with the C-terminal regions contributing to bead-like structures that align along the fibril axis.^[12] During this process, microfibril-associated glycoproteins are incorporated; for instance, MAGP-1 binds to the N-terminal region of fibrillin-1 and localizes to the bead regions, potentially stabilizing dimers through heterotypic disulfide bonds or cross-links.^[32] Similarly, MFAP-5 associates with fibrillin in elastin-containing microfibrils, contributing to the structural integrity and signaling functions of the network in tissues like skin and vessels.^[33] Fibrillin microfibrils interact with other extracellular matrix components to integrate into broader tissue architectures. In elastic tissues, microfibrils serve as a scaffold for the deposition of tropoelastin precursors, which cross-link via lysyl oxidase to form mature elastin fibers, with fibrillin-1 providing the initial template for this co-assembly.^[3] Additionally, fibrillins associate with latent TGF-β binding proteins (LTBPs), particularly LTBP-1 and LTBP-4, through binding sites in their N-terminal and central regions, facilitating the sequestration and latency of TGF-β in the matrix and influencing microfibril organization during development.^[34] These interactions ensure that microfibrils not only provide mechanical support but also modulate local growth factor bioavailability.^[4]

Isoforms

Fibrillin-1

Fibrillin-1 is encoded by the FBN1 gene, which is located on chromosome 15q21.1. This isoform is the predominant component of microfibrils in adult elastic tissues, comprising the majority of these structures and providing essential structural support. It is highly expressed in key connective tissues, including the walls of large arteries such as the aorta, the ciliary zonules of the eye, and the periosteum surrounding bones.^[35]^[36]^[17] A distinctive feature of fibrillin-1 is its robust calcium-binding capacity, facilitated by 43 calcium-binding epidermal growth factor-like (cbEGF) domains, each capable of coordinating one Ca²⁺ ion to stabilize the protein's structure and assembly into microfibrils. This calcium-dependent conformation is particularly vital for the integrity of microfibrils in ocular tissues, such as the ciliary zonules that maintain lens position, and in cardiovascular structures, where it contributes to vascular elasticity and resilience. Like other fibrillins, it features a modular domain architecture including cbEGF modules and transforming β-binding protein-like (TB) domains, but its specific arrangement enhances its role in these specialized tissues.^[17]^[4] Evolutionarily, fibrillin-1 represents the most conserved fibrillin isoform, with orthologs identified across jawed vertebrates, including mammals and birds, underscoring its fundamental role in connective tissue development and maintenance. Studies in model organisms highlight its indispensability; for instance, complete knockout of Fbn1 in mice results in embryonic lethality around E11.5 due to severe defects in yolk sac and embryonic vasculature, demonstrating its critical function in early vascular integrity.^[37]^[38]

Fibrillin-2

Fibrillin-2 is a large extracellular matrix glycoprotein encoded by the FBN2 gene, which is located on the long arm of human chromosome 5 at locus 5q23.3.^[39] This protein serves as a key structural component of microfibrils in connective tissues, particularly during early developmental stages. Unlike more persistent isoforms, fibrillin-2 expression peaks in embryonic and early postnatal periods, with prominent localization in elastic fiber-rich matrices of developing skin, cartilage, and perichondrium.^[40] Its transcript and protein levels are notably high in fetal connective tissues, such as the dermis and perichondrial regions, where it contributes to initial matrix organization, but decline markedly in adulthood as tissues mature.^[41] Fibrillin-2 features a modular structure with multiple calcium-binding epidermal growth factor-like (cbEGF) domains that confer stability to microfibrils.^[42] This isoform plays a specialized role in promoting early elastogenesis, particularly in the developing dermis, where it facilitates the deposition and alignment of elastic fibers essential for tissue extensibility.^[41] In growing tissues, fibrillin-2 supports joint and skin flexibility by integrating into nascent microfibril networks, enabling proper biomechanical properties during rapid developmental expansion.^[43] Fibrillin-2 can co-assemble with fibrillin-1 to form heterotypic transitional microfibrils, bridging early embryonic matrices to more stable adult structures.^[44] Studies in animal models underscore its developmental importance; Fbn2-null mice display skin fragility, evident as unusually thin and prone-to-tear dermis upon handling, and syndactyly.^[43]^[45] These phenotypes highlight fibrillin-2's non-redundant contributions to early tissue integrity and elasticity.^[45]

Fibrillin-3 and Fibrillin-4

Fibrillin-3 is encoded by the FBN3 gene located on chromosome 19p13.2.^[20] It exhibits restricted expression primarily during fetal development, with high levels observed in the lung, brain, kidney, skin, and skeletal muscle.^[46] ^[20] Immunolocalization studies have demonstrated its incorporation into extracellular microfibrils in developing skeletal elements, perichondrium, and other connective tissues, suggesting a role in supporting tissue architecture during embryogenesis.^[25] Fibrillin-3 assembles into 10-12 nm calcium-binding microfibrils that provide structural support, potentially contributing to both elastic and non-elastic networks in fetal tissues.^[47] ^[48] Unlike fibrillin-1 and fibrillin-2, which are more broadly expressed throughout life, fibrillin-3 constitutes a minor fraction of total fibrillins, typically less than 10% in examined tissues, and its expression diminishes postnatally.^[49] The FBN3 gene remains functional in humans, though evolutionary analyses indicate partial pseudogene-like inactivation in rodents, highlighting debates on its conservation across species.^[20] Mutations in FBN3 are rare and primarily identified in small cohorts with developmental anomalies; for instance, a homozygous missense variant (p.Arg644Cys) has been linked to a Bardet-Biedl syndrome-like phenotype involving cognitive impairment, obesity, and skeletal issues.^[50] Additionally, polymorphisms such as a dinucleotide repeat in intron 55 have been associated with susceptibility to polycystic ovary syndrome through modulation of TGF-β signaling.^[20] Fibrillin-4 represents an additional isoform identified in non-human vertebrates, notably in zebrafish where the orthologous gene fbn4 (also denoted fbn2a) is located on chromosome 13.^[51] This isoform shows expression in both embryonic and adult stages, including vascular structures, notochord, skin-like tissues, and bone precursors, with a protein structure featuring approximately 40 EGF-like domains—fewer than the 46-47 in human fibrillins.^[52] It contributes to microfibril formation in vascular and dermal contexts, supporting elasticity and tissue integrity.^[53] Fibrillin-4 is less abundant overall, comprising under 10% of fibrillins in zebrafish models, and recent studies from 2022-2025 have implicated it in periodontal-like and vascular microfibril assembly. Knockout of fbn4 in zebrafish disrupts fin development and overall morphogenesis, underscoring its preliminary role in appendage formation and extracellular matrix stability.^[54]

Functions

Mechanical Properties

Fibrillin microfibrils play a crucial role in the biomechanics of elastic tissues by conferring reversible extensibility and tensile strength. These structures enable tissues such as arteries to withstand cyclic mechanical loads during physiological processes like the heartbeat, with microfibrils capable of extending up to 50% strain without permanent deformation.^[55] This extensibility arises from the modular arrangement of fibrillin molecules, which allows for controlled unfolding and refolding under stress, thereby maintaining tissue compliance.^[56] Biomechanical characterization using techniques like molecular combing has revealed that isolated fibrillin-rich microfibrils, such as those from zonular filaments, possess a Young's modulus in the range of 78–98 MPa, indicating their function as stiff reinforcing elements within softer extracellular matrices.^[57] Calcium binding is essential for this stiffness, as depletion of Ca²⁺ leads to a contraction in resting length by approximately 50% and a reduction in microfibril rigidity, primarily through destabilization of interdomain interactions.^[58] Such assays highlight how fibrillin's mechanical properties scale from molecular to supramolecular levels, supporting load-bearing without failure. Emerging evidence also suggests roles in mechanotransduction, where microfibrils act as sensors of tissue forces, influencing cellular behavior in dynamic environments like the periodontal ligament.^[18] At the tissue level, fibrillin microfibrils contribute to aortic compliance by facilitating elastic recoil and distributing tensile forces during systolic expansion and diastolic relaxation.^[59] Disruptions in fibrillin assembly or integrity compromise this compliance, increasing the propensity for aneurysmal dilation under repeated hemodynamic stress. Recent cryo-electron microscopy (cryo-EM) structures of native microfibrils have elucidated their molecular architecture, revealing long-range structural effects of mutations that influence growth factor binding sites and microfibril stability.^[11]

Regulation of Growth Factors

Fibrillins play a critical role in modulating the bioavailability of transforming growth factor-β (TGF-β) family members by sequestering them within extracellular microfibrils, thereby controlling their presentation to cell surface receptors and influencing downstream signaling pathways. Specifically, fibrillin-1 and fibrillin-2 bind to latent TGF-β-binding proteins (LTBPs), particularly LTBP-1, through high-affinity interactions between the N-terminal regions of fibrillins (Kd ≈ 20–90 nM) and the C-terminal domains of LTBPs, forming large latent complexes (LLCs) that incorporate TGF-β into the extracellular matrix.^[60]^[61]^[62] These complexes maintain TGF-β in a latent state by preventing its dissociation from the latency-associated peptide (LAP), thus inhibiting premature activation and excessive Smad2/3 signaling.^[60] The TB5 domain of fibrillin-1 contains specific motifs that further enhance this latency by directly interacting with TGF-β, distinguishing it from other isoforms in fine-tuning sequestration efficiency.^[61] Activation of sequestered TGF-β occurs through regulated mechanisms independent of fibrillin's structural integrity, primarily involving cell surface integrins or proteolytic cleavage. Integrins such as αvβ6 bind the RGD motif on LAP, generating mechanical traction on the anchored LLC via LTBP-fibrillin connections, which deforms LAP and liberates active TGF-β for receptor binding.^[62]^[63] Alternatively, proteases like matrix metalloproteinases (MMPs) cleave LTBP, releasing the complex from microfibrils and facilitating TGF-β maturation, with LTBP-1 playing a key role in both pathways.^[62] Fibrillins also sequester bone morphogenetic proteins (BMPs) and growth differentiation factors (GDFs), such as BMP-7 and GDF-5, via binding of their prodomains to fibrillin N-termini (Kd ≈ 5–34 nM), storing these factors in microfibrils for controlled release during tissue remodeling.^[64] This regulatory function has profound physiological impacts, balancing TGF-β and BMP signaling to prevent pathological outcomes. Proper sequestration mitigates excessive TGF-β activity, which otherwise promotes fibrosis and aortic aneurysm formation through sustained Smad2/3 activation; disruptions, as seen in Marfan syndrome mouse models with mutant FBN1, increase TGF-β bioavailability by 2–3 fold, driving elastin degradation and vascular pathology.^[60]^[65] In bone morphogenesis, fibrillin-1 primarily dampens BMP signaling to ensure orderly osteoblast maturation, while fibrillin-2 inhibits TGF-β to support collagen deposition; loss of fibrillin-1 results in approximately 2-fold elevated BMP bioavailability, accelerating differentiation.^[61]^[64] Imbalances in these interactions thus contribute to connective tissue homeostasis, with fibrillin acting as a spatiotemporal gatekeeper for growth factor gradients.^[17]

Clinical Significance

Associated Disorders

Fibrillin-related disorders, collectively known as fibrillinopathies, encompass a spectrum of connective tissue diseases primarily caused by mutations in the FBN1, FBN2, and other fibrillin genes. The most prominent is Marfan syndrome, an autosomal dominant condition resulting from heterozygous mutations in FBN1, characterized by tall stature, arachnodactyly, ectopia lentis (lens dislocation), and life-threatening aortic root aneurysms and dissections.^[66] It affects approximately 1 in 5,000 individuals worldwide, with no ethnic or gender predilection.^[67] Congenital contractural arachnodactyly (CCA), also autosomal dominant and caused by mutations in FBN2, presents with multiple joint contractures at birth, arachnodactyly, scoliosis, and tall stature, but notably lacks the ocular and cardiovascular manifestations seen in Marfan syndrome.^[68] Clinical features often improve with age, though musculoskeletal issues like pes planus and mitral valve prolapse may persist.^[69] Other fibrillinopathies include acromicric dysplasia and geleophysic dysplasia, both linked to specific FBN1 mutations in the TGFβ-binding protein-like domain 5; acromicric dysplasia features short stature, stiff joints, and pseudomuscular build, while geleophysic dysplasia is more severe with "happy" facial features, tracheal stenosis, and cardiac involvement.^[70] Isolated ectopia lentis, another FBN1-associated condition, involves lens dislocation without systemic skeletal or cardiovascular features and follows autosomal dominant inheritance.^[71] Over 3,000 pathogenic FBN1 variants have been reported in databases as of 2025, with certain mutations leading to neonatal lethal forms of Marfan syndrome exhibiting profound skeletal deformities such as severe arachnodactyly and contractures, alongside pulmonary hypoplasia causing respiratory failure.^[72]^[73]

Pathophysiology and Mutations

Mutations in the fibrillin genes, particularly FBN1, underlie a spectrum of connective tissue disorders by disrupting the assembly and function of microfibrils in the extracellular matrix. Missense mutations are the most prevalent type, accounting for approximately 55-60% of pathogenic variants in FBN1, with a significant proportion occurring in calcium-binding epidermal growth factor-like (cbEGF) domains.^[74] These often involve cysteine substitutions that impair disulfide bond formation, leading to misfolded proteins and defective microfibril incorporation. For instance, substitutions like C1977Y in cbEGF30 destabilize domain structure and reduce secretion efficiency. Nonsense mutations, comprising about 10-20% of cases, trigger haploinsufficiency through nonsense-mediated decay, resulting in reduced fibrillin-1 levels and impaired microfibril deposition. A notable example is the FBN1 G234D variant in the hybrid1 domain, identified in 2024 studies, which alters domain folding and promotes dysregulated elastogenesis, as demonstrated in mouse models exhibiting tight skin phenotypes. Pathogenic mechanisms primarily involve dominant-negative effects, where mutant fibrillin-1 incorporates into microfibrils, poisoning their assembly and stability. This is exacerbated in cbEGF missense variants, which compromise multimerization and reduce microfibril elasticity, contributing to tissue fragility; a substantial proportion of Marfan syndrome cases feature missense mutations in EGF-like domains, impairing overall microfibril stability. Failed sequestration of growth factors, particularly TGF-β, due to defective microfibril networks leads to excessive signaling and downstream dysregulation of extracellular matrix remodeling. Additionally, some variants confer proteolysis resistance, as seen in certain classical Marfan syndrome mutations like N548I, where altered conformations protect against cathepsin degradation, potentially prolonging dysfunctional protein presence in tissues. Recent advances have elucidated long-range structural impacts of mutations. A 2023 cryo-EM study revealed that deletions in FBN1 associated with Weill-Marchesani syndrome induce conformational rearrangements in microfibrils, propagating effects over 100 nm to block latent TGF-β-binding protein interactions and enhance TGF-β bioavailability.^[11] In 2025, zebrafish models of FBN1 disruption confirmed that loss-of-function variants compromise tissue integrity, particularly in cardiovascular and skeletal structures, by failing to support proper microfibril network formation during development.^[75] These insights highlight how mutations propagate beyond local domains to drive systemic pathophysiology.

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[24]
A bioinformatics framework for genotype–phenotype correlation in ...
The FBN1 gene, located on chromosome 15q21.1, is about 236 kb in size and contains 65 exons. The gene is transcribed in a 9.7 kb mRNA which encodes a 2871 amino ...
[25]
Conservation of 5′-upstream region of the FBN1 gene in primates
Mar 19, 2008 · FBN1 is 237 kb in size and is located on chromosome 15q21. FBN1 ... Exon M contains the putative initiating methionine, a Kozak consensus sequence ...
[26]
Assembly of Microfibrils - Madame Curie Bioscience Database - NCBI
This chapter summarizes the current knowledge of fibrillin assembly into microfibrils and discusses this understanding in the light of FBN1 mutations.
[27]
C-terminal propeptide is required for fibrillin-1 secretion and blocks ...
Just before or after secretion (1), the propeptide is cleaved by furin. At the cell surface, heparan sulfate proteoglycans (HS) bind fibrillin at multiple ...
[28]
The Microfibril-Associated Glycoproteins (MAGPs) and the ...
The microfibril-associated glycoproteins MAGP-1 and MAGP-2 are extracellular matrix proteins that interact with fibrillin to influence microfibril function.
[29]
Microfibrillar-associated protein 5 is linked with markers of obesity ...
Aug 29, 2011 · MFAP5 is associated with microfibrils in elastin networks in a number of tissues, and its function may be related to cell signaling during ...
[30]
Latent TGF-β binding protein 4 promotes elastic fiber assembly by ...
Feb 4, 2013 · Here we show that latent TGF-β binding protein 4 (LTBP-4) potentiates formation of elastic fibers through interacting with fibulin-5, a tropoelastin-binding ...Latent Tgf-β Binding... · Sign Up For Pnas Alerts · Results
[31]
2200 - Gene ResultFBN1 fibrillin 1 [ (human)] - NCBI
Sep 27, 2025 · This gene encodes a member of the fibrillin family of proteins. The encoded preproprotein is proteolytically processed to generate two proteins.
[32]
Novel and recurrent FBN1 mutations causing Marfan syndrome in ...
Dec 13, 2022 · ... protein (2), which is widely expressed in the aorta, tendons, periosteum of the bones, and ciliary zonules of the eye (3). Although the risk ...
[33]
NCBI Orthologs - FBN1 - NIH
FBN1 - fibrillin 1. This gene encodes a member of the fibrillin family of proteins. The encoded preproprotein is proteolytically processed to generate two ...Missing: evolutionary conservation
[34]
2201 - Gene ResultFBN2 fibrillin 2 [ (human)] - NCBI
Sep 13, 2025 · The protein encoded by this gene is a component of connective tissue microfibrils and may be involved in elastic fiber assembly.
[35]
Structure and expression of fibrillin-2, a novel microfibrillar ... - NIH
Finally, immunohistochemistry revealed that the fibrillins co-distribute in elastic and non-elastic connective tissues of the developing embryo, with ...
[36]
https://pmc.ncbi.nlm.nih.gov/articles/PMC9792469/
[37]
Fibrillin-1 and Fibulin-2 Interact and Are Colocalized in Some Tissues
In relaxed tissues, the fibrillin periodicity appears to match the periodic banding pattern of collagen fibers (Sakai et al., 1986). These observations suggest ...Materials And Methods · Results · Discussion<|control11|><|separator|>
[38]
Fibrillins 1 and 2 Perform Partially Overlapping Functions during ...
In contrast to Fbn1−/− mice, Fbn2 null mice had a well developed and morphologically normal aortic wall. However, virtually all Fbn1−/−;Fbn2−/− embryos and ...
[39]
Regulation of limb patterning by extracellular microfibrils
Examination of newborn Fbn2−/− mice revealed contractures of carpal, metacarpal, and phalangeal joints in the forelimbs (Fig. 2 a); upon mechanical extension, ...Missing: knockout | Show results with:knockout
[40]
Entry - *608529 - FIBRILLIN 3; FBN3 - OMIM - (OMIM.ORG)
RNA dot blot analysis showed highest expression in fetal lung, brain, and kidney, and much lower expression in all fetal and adult tissues examined. RT-PCR ...
[41]
Differential expression of fibrillin-3 adds to microfibril variety in ...
Like FBN2, the highest expression levels of FBN3 were found in fetal tissues, with only low levels in postnatal tissues. Immunolocalization demonstrated ...
[42]
FBN3 - Fibrillin-3 - Homo sapiens (Human) | UniProtKB | UniProt
Gene Ontology (GO) annotations organized by slimming set. Aspect, Term ... Proteomes. Identifier. UP000005640. Component. Chromosome 19. Organism-specific ...
[43]
Fibrillin Assembly Requires Fibronectin | Molecular Biology of the Cell
Nov 26, 2008 · Fibrillins constitute the major backbone of multifunctional microfibrils in elastic and nonelastic extracellular matrices.
[44]
Genetic models of fibrillinopathies - PMC - PubMed Central - NIH
It is likely that the ancestral teleost had 3 distinct fibrillin genes, corresponding to human FBN1, FBN2, and FBN3. In most species at least 80% of the ...Missing: FBN4 | Show results with:FBN4
[45]
Case Report: A Novel Homozygous Missense Variant of FBN3 ...
Fibrillin proteins are extracellular matrix glycoproteins assembling into microfibrils. FBN1, FBN2, and FBN3 encode the human fibrillins and mutations in FBN1 ...Missing: isoforms | Show results with:isoforms
[46]
fbn2 - ZFIN Gene
fbn2 Nomenclature History; Previous Names. fbn2a (1); fbn4 (1); fibrillin 4 (1) ... Human ortholog(s) of this gene implicated in congenital contractural ...
[47]
Essential role for fibrillin-2 in zebrafish notochord and vascular ... - NIH
... gene fibrillin-4 (fbn4) (Fig. 6E). Compared to the three human fibrillins, zebrafish fibrillin-4 exhibits highest amino acid identity with human fibrillin-2 ( ...
[48]
Microfibril-associated glycoprotein 4 (Mfap4) regulates ... - Nature
Jul 16, 2020 · Here, we demonstrate that zebrafish mfap4 transcripts are detected embryonically, resolving to the macrophage lineage by 24 h post fertilization.
[49]
Systematic disruption of zebrafish fibrillin genes identifies a translational zebrafish model for Marfan syndrome
Insufficient relevant content. The provided text from https://www.biorxiv.org/content/10.1101/2025.06.21.659830v1 does not contain specific information on fbn4 or fbn2a knockout in zebrafish, effects on development (especially fin), or expression patterns. It only includes metadata, a forwarding message, and a CAPTCHA prompt, with no scientific details or full article text.
[50]
Steered molecular dynamic simulations reveal Marfan syndrome ...
Oct 8, 2020 · Metal ion dependency of microfibrils supports a rod-like conformation for fibrillin-1 calcium-binding epidermal growth factor-like domains.
[51]
Review Dissecting the Fibrillin Microfibril: Structural Insights into ...
Feb 8, 2012 · This review summarizes our current knowledge of microfibril structure, from individual fibrillin domains and the calcium-dependent tuning of pairwise ...Missing: general | Show results with:general
[52]
https://pmc.ncbi.nlm.nih.gov/articles/PMC3081706/
[53]
Role of Ca 2+ for the mechanical properties of fibrillin
Aug 9, 2001 · We find that Ca2+-depletion results in a 50% decrease in rest length and reduces the stiffness of fibrillin-rich microfibrils. At high ...Missing: reduction | Show results with:reduction
[54]
Discrete Contributions of Elastic Fiber Components to Arterial ...
Oct 22, 2009 · Mechanical tests confirm that elastin and fibrillin-1 impart elastic recoil and tensile strength to the aortic wall, respectively. Additional ex ...
[55]
Fibrillin-1 regulates the bioavailability of TGFβ1 - PMC - NIH
We have discovered that fibrillin-1, which forms extracellular microfibrils, can regulate the bioavailability of transforming growth factor (TGF) β1.
[56]
Fibrillin-1 and -2 differentially modulate endogenous TGF-β and ...
Sep 20, 2010 · TGF-β and BMP signaling are both abnormally high in Fbn1-null osteoblasts. Although neonatal lethality of Fbn1−/− mice limits performing ...
[57]
Latent TGF-β-binding proteins - PMC - NIH
LTBPs are ECM components interacting with fibrillin, forming latent complexes with TGFβ, and have roles in TGFβ regulation and microfibril organization.Ltbp-2 · Ltbp-4 · Figure 3. Tgfβ Independent...
[58]
Integrin α V β 6 -mediated activation of latent TGF-β requires the ...
We show that even though αvβ6 recognizes an RGD on LAP, LTBP-1 is required for αVβ6-mediated latent TGF-β activation. The domains of LTBP-1 necessary for ...
[59]
https://www.ahajournals.org/doi/10.1161/atvbaha.109.193227
[60]
Overexpression of Transforming Growth Factor-β Is Associated With ...
In this report we show that in subjects with MFS, there is increased expression of TGF-β and increased tissue hyaluronan content. Altered cellular protein ...
[61]
FBN1-Related Marfan Syndrome - GeneReviews® - NCBI Bookshelf
Apr 18, 2001 · Approximately 75% of individuals with Marfan syndrome have an affected parent; approximately 25% have a de novo FBN1 pathogenic variant.
[62]
Marfan Syndrome - StatPearls - NCBI Bookshelf
Jan 23, 2023 · Approximately one in every 3,000 to 5,000 individuals is affected.[4] The disease occurs worldwide, with no preference for race or gender. It ...
[63]
Congenital Contractural Arachnodactyly - GeneReviews - NCBI - NIH
Jan 23, 2001 · While many individuals with CCA have an affected parent, as many as 50% may have a de novo FBN2 pathogenic variant. If a parent of a proband has ...Summary · Diagnosis · Clinical Characteristics · Genetic Counseling
[64]
Entry - CONTRACTURAL ARACHNODACTYLY, CONGENITAL; CCA
Congenital contractural arachnodactyly (CCA) is a rare, autosomal dominant connective tissue disorder characterized by contractures, arachnodactyly, scoliosis, ...
[65]
Mutations in the TGFβ Binding-Protein-Like Domain 5 of FBN1 ... - NIH
All mutations were absent from alleles in 2000 ethnicity-matched controls and in the Marfan mutation database. All together, we identified a total of 16 ...<|control11|><|separator|>
[66]
Isolated ectopia lentis - Genetics - MedlinePlus
Mar 1, 2015 · Mutations in the FBN1 or ADAMTSL4 gene cause isolated ectopia lentis. ... When isolated ectopia lentis is caused by mutations in the FBN1 gene ...
[67]
[PDF] Primer on the Metabolic Bone Diseases and Disorders of Mineral ...
... FBN4 can result in skeletal features (arachnodactyly) as well as aneurysm and cutis laxa [49] , suggesting that defects in fibulin function may perturb ...
[68]
Phenotypic Diversity of Marfan Syndrome | JACC: Advances
Aug 8, 2025 · Over 3,000 pathogenic variants of the FBN1 gene have been identified, with missense mutations being the most common. However, even at the same ...
[69]
A novel large in-frame FBN1 deletion causes neonatal Marfan ...
We report on a full-term female neonate with postnatal characteristics suggestive of nMFS, including severe cardiovascular disease resulting in ...