ADAMTSL4 mutations result in recessively inherited isolated ectopia lentis, a dysgenesis of the fibrillin-1–rich zonule of Zinn. This research shows that ADAMTSL4 binds fibrillin-1 microfibrils and accelerates their biogenesis, thus providing a potential underlying mechanism for this disorder.
ADAMTSL4 mutations cause autosomal recessive isolated ectopia lentis (IEL) and ectopia lentis et pupillae. Dominant FBN1 mutations cause IEL or syndromic ectopia lentis (Marfan syndrome and Weill-Marchesani syndrome). The authors sought to characterize recombinant ADAMTSL4 and the ocular distribution of ADAMTSL4 and to investigate whether ADAMTSL4 influences the biogenesis of fibrillin-1 microfibrils, which compose the zonule.
ADAMTSL4 was expressed by the transfection of HEK293F cells. Protein extracts and paraffin sections from human eyes were analyzed by Western blot analysis and by immunoperoxidase staining, respectively. Immunofluorescence was used to evaluate fibrillin-1 deposition in the ECM of fetal bovine nuchal ligament cells after culture in ADAMTSL4-conditioned medium or control medium. Confocal microscopy was performed to investigate ADAMTSL4 and fibrillin-1 colocalization in these cultures.
Western blot analysis identified ADAMTSL4 as a glycoprotein in HEK293F cells and as a major band of 150 kDa in ocular tissues including ciliary body, sclera, cornea, and retina. Immunoperoxidase staining showed a broad ocular distribution of ADAMTSL4, associated with both cells and fibrillar ECM. When cultured in ADAMTSL4-containing medium, fetal bovine nuchal ligament cells showed accelerated fibrillin-1 deposition in ECM. ADAMTSL4 colocalized with fibrillin-1 microfibrils in the ECM of these cells.
ADAMTSL4 is a secreted glycoprotein that is widely distributed in the human eye. Enhanced fibrillin-1 deposition in the presence of ADAMTSL4 and colocalization of ADAMTSL4 with fibrillin-1 in the ECM of cultured fibroblasts suggest a potential role for ADAMTSL4 in the formation or maintenance of the zonule.
ADAMTS-like proteins are related to ADAMTS metalloproteases by their similarity to ADAMTS ancillary domains. Here, we have characterized ADAMTSL5, a novel member of the superfamily with a unique modular organization that includes a single C-terminal netrin-like (NTR) module. Alternative splicing of ADAMTSL5 at its 5′ end generates two transcripts that encode different signal peptides, but the same mature protein. These transcripts differ in their translational efficiency. Recombinant ADAMTSL5 is a secreted, N-glycosylated 60 kDa glycoprotein located in the subcellular matrix, on the cell-surface, and in the medium of transfected cells. RT-PCR and western blot analysis of adult mouse tissues showed broad expression. Western blot analysis suggested proteolytic release of the NTR module in transfected cells as well as in some mouse tissues. Immunostaining during mouse organogenesis identified ADAMTSL5 in musculoskeletal tissues such as skeletal muscle, cartilage and bone, as well as in many epithelia. Affinity-chromatography demonstrated heparin-binding of ADAMTSL5 through its NTR-module. Recombinant ADAMTSL5 bound to both fibrillin-1 and fibrillin-2, and co-localized with fibrillin microfibrils in the extracellular matrix of cultured fibroblasts, but without discernible effect on microfibril assembly. ADAMTSL5 is the first family member shown to bind both fibrillin-1 and fibrillin-2. Like other ADAMTS proteins implicated in microfibril biology through identification of human and animal mutations, ADAMTSL5 could have a role in modulating microfibril functions.
ADAMTS; ADAMTS-like; netrin-like module; fibrillin microfibril; heparin; alternative splicing
Geleophysic dysplasia is an autosomal recessive disorder characterized by short stature, brachydactyly, thick skin and cardiac valvular anomalies often responsible for an early death. Studying six geleophysic dysplasia families, we first mapped the underlying gene to chromosome 9q34.2 and identified five distinct nonsense and missense mutations in ADAMTSL2 (a disintegrin and metalloproteinase with thrombospondin repeats–like 2), which encodes a secreted glycoprotein of unknown function. Functional studies in HEK293 cells showed that ADAMTSL2 mutations lead to reduced secretion of the mutated proteins, possibly owing to the misfolding of ADAMTSL2. A yeast two-hybrid screen showed that ADAMTSL2 interacts with latent TGF-β–binding protein 1. In addition, we observed a significant increase in total and active TGF-β in the culture medium as well as nuclear localization of phosphorylated SMAD2 in fibroblasts from individuals with geleophysic dysplasia. These data suggest that ADAMTSL2 mutations may lead to a dysregulation of TGF-β signaling and may be the underlying mechanism of geleophysic dysplasia.
The extracellular glycoprotein fibrillin-1 forms microfibrils that act as the template for elastic fibers. Most mutations in fibrillin-1 cause Marfan syndrome with severe cardiovascular and ocular symptoms, and tall stature. This is in contrast to mutations within a heparin-binding TB domain (TB5), which is downstream of the arg-gly-asp cell adhesion domain, which can cause Weill-Marchesani syndrome (WMS) or Acromicric (AD) and Geleophysic Dysplasias (GD). WMS is characterized by short limbs, joint stiffness and ocular defects, whilst fibrillin-1 AD and GD have severe short stature, joint defects and thickened skin. We previously showed that TB5 binds heparin. Here, we show that the corresponding region of fibrillin-2 binds heparin very poorly, highlighting a novel functional difference between the two isoforms. This finding enabled us to map heparin/heparan sulfate binding to two sites on fibrillin-1 TB5 using a mutagenesis approach. Once these sites were mapped, we were able to investigate whether disease-causing mutations in this domain disrupt binding to HS. We show that a WMS deletion mutant, and five AD and GD point mutants all have disrupted heparin binding to TB5. These data provide insights into the biology of fibrillins and the pathologies of WMS, AD and GD.
The extracellular matrix (ECM) plays a key role in tissue formation, homeostasis and repair, mutations in ECM components have catastrophic consequences for organ function and therefore, for the fitness and survival of the organism. Collagen, fibrillin and elastin polymers represent the architectural scaffolds that impart specific mechanic properties to tissues and organs. Fibrillin assemblies (microfibrils) have the additional function of distributing, concentrating and modulating local transforming growth factor (TGF)-β and bone morphogenetic protein (BMP) signals that regulate a plethora of cellular activities, including ECM formation and remodeling. Fibrillins also contain binding sites for integrin receptors, which induce adaptive responses to changes in the extracellular microenvironment by reorganizing the cytoskeleton, controlling gene expression, and releasing and activating matrix-bound latent TGF-β complexes. Genetic evidence has indicated that fibrillin-1 and fibrillin-2 contribute differently to the organization and structural properties of non-collagenous architectural scaffolds, which in turn translate into discrete regulatory outcomes of locally released TGF-β and BMP signals. Additionally, the study of congenital dysfunctions of fibrillin-1 has yielded insights into the pathogenesis of acquired connective tissue disorders of the connective tissue, such as scleroderma. On the one hand, mutations that affect the structure or expression of fibrillin-1 perturb microfibril biogenesis, stimulate improper latent TGF-β activation, and give rise to the pleiotropic manifestations in Marfan syndrome (MFS). On the other hand, mutations located around the integrin-binding site of fibrillin-1 perturb cell matrix interactions, architectural matrix assembly and extracellular distribution of latent TGF-β complexes, and lead to the highly restricted fibrotic phenotype of Stiff Skin syndrome. Understanding the molecular similarities and differences between congenital and acquired forms of skin fibrosis may therefore provide new therapeutic tools to mitigate or even prevent disease progression in scleroderma and perhaps other fibrotic conditions.
Mutations in the gene for fibrillin-1 (FBN1) have been shown to cause Marfan syndrome, an autosomal dominant disorder of connective tissue characterised by pleiotropic manifestations involving primarily the ocular, skeletal, and cardiovascular systems. Fibrillin-1 is a major component of the 10-12 nm microfibrils, which are thought to play a role in tropoelastin deposition and elastic fibre formation in addition to possessing an anchoring function in some tissues.
Fibrillin-1 mutations have also been found in patients who do not fulfil clinical criteria for the diagnosis of Marfan syndrome, but have related disorders of connective tissue, such as isolated ectopia lentis, familial aortic aneurysm, and Marfan-like skeletal abnormalities, so that Marfan syndrome may be regarded as one of a range of type 1 fibrillinopathies.
There appear to be no particular hot spots since mutations are found throughout the entire fibrillin-1 gene. However, a clustering of mutations associated with the most severe form of Marfan syndrome, neonatal Marfan syndrome, has been noted in a region encompassing exons 24 to 32. The gene for fibrillin-2 (FBN2) is highly homologous to FBN1, and mutations in FBN2 have been shown to cause a phenotypically related disorder termed congenital contractural arachnodactyly. Since mutations in the fibrillin genes are likely to affect the global function of the microfibrils, the term microfibrillopathy may be the most appropriate to designate the spectrum of disease associated with dysfunction of these molecules.
The understanding of the global and the molecular functions of the fibrillin containing microfibrils is still incomplete and, correspondingly, no comprehensive theory of the pathogenesis of Marfan syndrome has emerged to date. Many, but not all, fibrillin-1 gene mutations are expected to exert a dominant negative effect, whereby mutant fibrillin monomers impair the global function of the microfibrils. In this paper we review the molecular physiology and pathophysiology of Marfan syndrome and related microfibrillopathies.
Keywords: Marfan syndrome; fibrillin; microfibrillopathies
Fibrillin-1 is a ubiquitous extracellular matrix molecule that sequesters latent growth factor complexes. A role for fibrillin-1 in specifying tissue microenvironments has not been elucidated, even though the concept that fibrillin-1 provides extracellular control of growth factor signaling is currently appreciated. Mutations in FBN1 are mainly responsible for the Marfan syndrome (MFS), recognized by its pleiotropic clinical features including tall stature and arachnodactyly, aortic dilatation and dissection, and ectopia lentis. Each of the many different mutations in FBN1 known to cause MFS must lead to similar clinical features through common mechanisms, proceeding principally through the activation of TGFβ signaling. Here we show that a novel FBN1 mutation in a family with Weill-Marchesani syndrome (WMS) causes thick skin, short stature, and brachydactyly when replicated in mice. WMS mice confirm that this mutation does not cause MFS. The mutation deletes three domains in fibrillin-1, abolishing a binding site utilized by ADAMTSLIKE-2, -3, -6, and papilin. Our results place these ADAMTSLIKE proteins in a molecular pathway involving fibrillin-1 and ADAMTS-10. Investigations of microfibril ultrastructure in WMS humans and mice demonstrate that modulation of the fibrillin microfibril scaffold can influence local tissue microenvironments and link fibrillin-1 function to skin homeostasis and the regulation of dermal collagen production. Hence, pathogenetic mechanisms caused by dysregulated WMS microenvironments diverge from Marfan pathogenetic mechanisms, which lead to broad activation of TGFβ signaling in multiple tissues. We conclude that local tissue-specific microenvironments, affected in WMS, are maintained by a fibrillin-1 microfibril scaffold, modulated by ADAMTSLIKE proteins in concert with ADAMTS enzymes.
The microenvironment is specified by cell-surface molecules, growth factors, and the extracellular matrix. Here we report genetic evidence that implicates fibrillin-1, a ubiquitous extracellular matrix molecule that sequesters latent growth factor complexes, as a key determinant in the local control of musculoskeletal and skin microenvironments. A novel mutation in fibrillin-1 demonstrates that modulation of the fibrillin microfibril scaffold can influence tissue microenvironments and result in the clinical features of Weill-Marchesani syndrome (WMS), including thick skin, short stature, and brachydactyly. Dysregulated WMS microenvironments diverge from Marfan pathogenetic mechanisms, which lead to broad activation of TGFβ signaling in multiple tissues.
We present here a family with a clinical phenotype resembling Marfan syndrome (MFS), and displaying joint contracture and episodes of knee joint effusions, but lacking the cardiovascular features of the syndrome. The phenotype of this family represents a unique mixture of connective tissue symptoms, some of which are found in classical MFS and some of which are typical of dominant ectopia lentis. Linkage analyses suggested a linkage (LOD score 2.4; theta = 0) between the phenotype of the family and a polymorphic marker in the vicinity of the fibrillin locus on chromosome 15 (FBN1). Furthermore, a novel transition mutation was identified in the FBN1 gene in all the affected members of the family. In contrast to the majority of fibrillin mutations reported so far, this mutation substitutes a cysteine for arginine, producing an extra cysteine in one of the non-calcium-binding EGF-like motifs of the fibrillin polypeptide, most probably disturbing the formation of one of the three disulfide bridges known to be essential for the normal conformation of this motif.
In systemic sclerosis (SSc), a common and etiologically mysterious form of scleroderma (defined as pathologic fibrosis of the skin), previously healthy adults acquire fibrosis of the skin and viscera in association with autoantibodies . Familial recurrence is extremely rare and causal genes have not been identified. While the onset of fibrosis in SSc typically correlates with the production of autoantibodies, whether they contribute to disease pathogenesis or simply serve as a marker of disease remains controversial and the mechanism for their induction is largely unknown . The study of SSc is hindered by a lack of animal models that recapitulate the etiology of this complex disease. To gain a foothold in the pathogenesis of pathologic skin fibrosis, we chose to study stiff skin syndrome (SSS), a rare but tractable Mendelian disorder that shows childhood onset of diffuse skin fibrosis with autosomal dominant inheritance and complete penetrance. We showed that SSS is caused by heterozygous missense mutations in the gene (FBN1) encoding fibrillin-1, the major constituent of extracellular microfibrils . Notably, SSS mutations all localize to the only domain in fibrillin-1 that harbors an Arg-Gly-Asp (RGD) motif needed to mediate cell-matrix interactions by binding to cell-surface integrins . Here we show that mouse lines that harbor analogous amino acid substitutions in fibrillin-1 recapitulate aggressive skin fibrosis that is prevented by integrin-modulating therapies and reversed by antagonism of the pro-fibrotic cytokine transforming growth factor β (TGFβ). Mutant mice show skin infiltration of pro-inflammatory immune cells including plasmacytoid dendritic, T helper, and plasma cells, and autoantibody production; these findings are normalized by integrin-modulating therapies or TGFβ antagonism. These data show that alterations in cell-matrix interactions are sufficient to initiate and sustain inflammatory and pro-fibrotic programs and highlight novel therapeutic strategies.
To identify the spectrum and frequency of five candidate genes in Chinese patients with congenital ectopia lentis (EL).
Forty consecutive and unrelated congenital probands with EL were collected and underwent ocular, skeletal, and cardiovascular examinations. Sanger sequencing was used to analyze all of the coding and adjacent regions of five candidate genes: FBN1, ADAMTS10, ADAMTSL4, TGFBR2, and CBS. Mutation analysis was performed to evaluate the pathogenic variants and to identify the cause of congenital EL.
The FBN1 gene screen revealed 25 pathogenic variants in 34 of the 40 families with congenital EL, including three novel (c.1955G>T, c.2222delA, and c.4381T>C) and 22 known mutations. The ADAMTSL10 gene screen revealed a compound heterozygous variant (c.1586G>A and c.2485T>A) in a family with Weill-Marchesani syndrome (WMS). In the remaining five probands, no pathogenic variant was detected in any of the five screened genes.
In this study, we identified three novel and 22 known mutations in FBN1 in 34 of 40 EL families. The results expand the mutation spectrum of the FBN1 gene and suggest that FBN1 mutations may be the major cause of congenital EL in Chinese patients.
A crippling dwarfism was first described in the Miniature Poodle in Great Britain in 1956. Here, we resolve the genetic basis of this recessively inherited disorder. A case-control analysis (8∶8) of genotype data from 173 k SNPs revealed a single associated locus on CFA14 (Praw <10–8). All affected dogs were homozygous for an ancestral haplotype consistent with a founder effect and an identical-by-descent mutation. Systematic failure of nine, nearly contiguous SNPs, was observed solely in affected dogs, suggesting a deletion was the causal mutation. A 130-kb deletion was confirmed both by fluorescence in situ hybridization (FISH) analysis and by cloning the physical breakpoints. The mutation was perfectly associated in all cases and obligate heterozygotes. The deletion ablated all but the first exon of SLC13A1, a sodium/sulfate symporter responsible for regulating serum levels of inorganic sulfate. Our results corroborate earlier findings from an Slc13a1 mouse knockout, which resulted in hyposulfatemia and syndromic defects. Interestingly, the metabolic disorder in Miniature Poodles appears to share more clinical signs with a spectrum of human disorders caused by SLC26A2 than with the mouse Slc13a1 model. SLC26A2 is the primary sodium-independent sulfate transporter in cartilage and bone and is important for the sulfation of proteoglycans such as aggregan. We propose that disruption of SLC13A1 in the dog similarly causes undersulfation of proteoglycans in the extracellular matrix (ECM), which impacts the conversion of cartilage to bone. A co-dominant DNA test of the deletion was developed to enable breeders to avoid producing affected dogs and to selectively eliminate the mutation from the gene pool.
The Tight skin (Tsk) mutation is a duplication of the mouse fibrillin 1 (Fbn1) gene that results in a larger (418 kD) than normal (350 kD) protein; Tsk/+ mice display increased connective tissue, bone overgrowth, and lung emphysema. Lung emphysema, bone overgrowth, and vascular complications are the distinctive traits of mice with reduced Fbn1 gene expression and of Marfan syndrome (MFS) patients with heterozygous fibrillin 1 mutations. Although Tsk/+ mice produce equal amounts of the 418- and 350-kD proteins, they exhibit a relatively mild phenotype without the vascular complications that are associated with MFS patients and fibrillin 1–deficient mice. We have used genetic crosses, cell culture assays and Tsk-specific antibodies to reconcile this discrepancy and gain new insights into microfibril assembly. Mice compound heterozygous for the Tsk mutation and hypomorphic Fbn1 alleles displayed both Tsk and MFS traits. Analyses of immunoreactive fibrillin 1 microfibrils using Tsk- and species-specific antibodies revealed that the mutant cell cultures elaborate a less abundant and morphologically different meshwork than control cells. Cocultures of Tsk/Tsk fibroblasts and human WISH cells that do not assemble fibrillin 1 microfibrils, demonstrated that Tsk fibrillin 1 copolymerizes with wild-type fibrillin 1. Additionally, copolymerization of Tsk fibrillin 1 with wild-type fibrillin 1 rescues the abnormal morphology of the Tsk/Tsk aggregates. Therefore, the studies suggest that bone and lung abnormalities of Tsk/+ mice are due to copolymerization of mutant and wild-type molecules into functionally deficient microfibrils. However, vascular complications are not present in these animals because the level of functional microfibrils does not drop below the critical threshold. Indirect in vitro evidence suggests that a potential mechanism for the dominant negative effects of incorporating Tsk fibrillin 1 into microfibrils is increased proteolytic susceptibility conferred by the duplicated Tsk region.
elastic fibers; microfibrils; Tsk; Marfan syndrome; extracellular matrix
Homocystinuria is a genetic disorder resulting in elevated levels of homocysteine in plasma and tissues. Some of the skeletal and ocular symptoms such as long-bone overgrowth, scoliosis and ectopia lentis overlap with symptoms seen in Marfan syndrome. Marfan syndrome is caused by mutations in the extracellular matrix protein fibrillin-1. We previously showed that fibrillin-1 is a target for homocysteine and that the deposition of homocysteinylated fibrillin-1 in the extracellular matrix is compromised. Since the assembly of fibrillin-1 is critically dependent on fibronectin, we analyzed the consequences of fibronectin homocysteinylation and its interaction with fibrillin-1. Cellular fibronectin and proteolytic fragments were homocysteinylated and tested in various interaction assays with recombinant fibrillin-1 and heparin. Fibronectin homocysteinylation consistently compromised the fibronectin-fibrillin-1 interaction, while the interaction with heparin was not affected. Fibronectin homocysteinylation, but not cysteinylation, reduced the fibronectin dimers to monomers as shown by Western blotting. ELISA analyses of homocysteinylated fibronectin with three monoclonal antibodies demonstrated structural changes in the disulfide-containing FNI domains FNI2, FNI4 and FNI9. Using fluorescently labeled fibronectin, we studied the consequence of fibronectin homocysteinylation on assembly in cell culture. Modified fibronectin showed deficiencies in de-novo matrix incorporation and initial assembly. In conclusion, we define here characteristic structural changes of fibronectin upon homocysteinylation that translate into functional deficiencies in the fibronectin-fibrillin-1 interaction and in fibronectin assembly. Since fibronectin is a major organizer of various extracellular protein networks, these structural and functional alterations may contribute to the pathogenesis of homocystinuria and Marfan syndrome.
The predisposition for scleroderma, defined as fibrosis and hardening of the skin, is poorly understood. We report that stiff skin syndrome (SSS), an autosomal dominant congenital form of scleroderma, is caused by mutations in the sole Arg-Gly-Asp (RGD) sequence-encoding domain of fibrillin-1 that mediates integrin binding. Ordered polymers of fibrillin-1 (termed microfibrils) initiate elastic fiber assembly and bind to and regulate the activation of the pro-fibrotic cytokine transforming growth factor β (TGFβ). Altered cell-matrix interactions in SSS accompany excessive microfibrillar deposition, impaired elastogenesis, and increased TGFβ concentration and signaling in the dermis. The observation of similar findings in systemic sclerosis (SSc), a more common acquired form of scleroderma, suggests broad pathogenic relevance.
Fibrillin-1, the major component of extracellular microfibrils that associate with insoluble elastin in elastic fibers, is mainly synthesized during development and postnatal growth and is believed to guide elastogenesis. Mutations in the fibrillin-1 gene cause Marfan syndrome, a multisystem disorder characterized by aortic aneurysms and dissections. The recent finding that early deficiency of elastin modifies vascular aging has raised the possibility that fibrillin-1 deficiency could also contribute to late-onset pathology of vascular remodeling. To address this question, we examined cardiovascular function in 3 week-old, 6 month-old, and 24 month-old mice that are heterozygous for a hypomorphic structural mutation of fibrillin-1 (Fbn1+/mgΔ mice). Our results indicate that Fbn1+/mgΔ mice, particularly those that are 24 month-old, are slightly more hypotensive than wild-type littermates. Additionally, aneurysm and aortic insufficiency were more frequently observed in aging Fbn1+/mgΔ mice than in the wild-type counterparts. We also noted substantial fragmentation and decreased number of elastic lamellae in the aortic wall of Fbn1+/mgΔ mice, which were correlated with an increase in aortic stiffness, a decrease in vasoreactivity, altered expressions of elastic fiber-related genes, including fibrillin-1 and elastin, and a decrease in the relative ratio between tissue elastin and collagen. Collectively, our findings suggest that the heterozygous mgΔ mutation accelerates some aspects of vascular aging and eventually lead to aortic manifestations resembling those of Marfan syndrome. Importantly, our data also indicate that vascular abnormalities in Fbn1+/mgΔ mice are opposite to those induced by elastin haploinsufficiency during aging that affect blood pressure, vascular dimensions and number of elastic lamellae.
elastic lamellae; fibrillin-1; Marfan syndrome; arterial aging; aneurysm; transgenic mice
Fibrillins are the major constituent of tissue microfibrils, which form the ocular zonule. In Marfan syndrome (MFS), FBN1 mutations lead to ectopia lentis. The goal of this work was to investigate zonule composition and formation in fibrillin-deficient and wild-type mice.
Immunofluorescence staining of eyes from wild-type, Fbn1-deficient, and Fbn2-deficient mice, as well as other species, was performed using monospecific fibrillin 1 and fibrillin 2 antibodies. The zonule of Fbn1-deficient and Fbn2-deficient mice was studied by electron microscopy. Microfibril formation in vitro was evaluated by immunofluorescence microscopy of cultured nonpigmented ciliary epithelial cells and fibroblasts.
A zonule was present in both Fbn1-deficient and Fbn2-deficient mouse eyes. Immunofluorescence demonstrated that the zonule of Fbn1-deficient mice, wild-type mice, rats, and hamsters contained fibrillin 2. The zonule of Fbn2−/− mice contained fibrillin 1. Fibrillin 1 and fibrillin 2 colocalized in microfibrils formed in human nonpigmented ciliary epithelium cultures. Like fibrillin 1, fibrillin 2 microfibril assembly was fibronectin dependent and initiated by cell surface punctate deposits that elongated to form microfibrils.
These data suggest that fibrillin 1 assembly and fibrillin 2 assembly share similar mechanisms. Microfibril composition depends substantially on the local levels of fibrillin isoforms and is not highly selective in regard to the isoform. This raises the intriguing possibility that the zonule could be strengthened in MFS by inducing fibrillin 2 expression in ciliary epithelium. The presence of fibrillin 2 in the murine zonule and an intact zonule in Fbn1-knockout mice may limit the utility of rodent models for studying ectopia lentis in MFS.
Rodent zonule contains both fibrillin 1 and fibrillin 2.
zonule; fibrillin; Marfan syndrome
Hereditary periodic fever syndromes are characterized by recurrent episodes of fever and inflammation with no known pathogenic or autoimmune cause. In humans, several genes have been implicated in this group of diseases, but the majority of cases remain unexplained. A similar periodic fever syndrome is relatively frequent in the Chinese Shar-Pei breed of dogs. In the western world, Shar-Pei have been strongly selected for a distinctive thick and heavily folded skin. In this study, a mutation affecting both these traits was identified. Using genome-wide SNP analysis of Shar-Pei and other breeds, the strongest signal of a breed-specific selective sweep was located on chromosome 13. The same region also harbored the strongest genome-wide association (GWA) signal for susceptibility to the periodic fever syndrome (praw = 2.3×10−6, pgenome = 0.01). Dense targeted resequencing revealed two partially overlapping duplications, 14.3 Kb and 16.1 Kb in size, unique to Shar-Pei and upstream of the Hyaluronic Acid Synthase 2 (HAS2) gene. HAS2 encodes the rate-limiting enzyme synthesizing hyaluronan (HA), a major component of the skin. HA is up-regulated and accumulates in the thickened skin of Shar-Pei. A high copy number of the 16.1 Kb duplication was associated with an increased expression of HAS2 as well as the periodic fever syndrome (p<0.0001). When fragmented, HA can act as a trigger of the innate immune system and stimulate sterile fever and inflammation. The strong selection for the skin phenotype therefore appears to enrich for a pleiotropic mutation predisposing these dogs to a periodic fever syndrome. The identification of HA as a major risk factor for this canine disease raises the potential of this glycosaminoglycan as a risk factor for human periodic fevers and as an important driver of chronic inflammation.
Shar-Pei dogs have two unique features: a breed defining “wrinkled” skin phenotype and a genetic disorder called Familial Shar-Pei Fever (FSF). The wrinkled phenotype is strongly selected for and is the result of excessive hyaluronan (HA) deposited in the skin. HA is a molecule that may behave in a pro-inflammatory manner and create a “danger signal” by being analogous to molecules on the surface of pathogens. FSF is characterized by unprovoked episodes of fever and/or inflammation and resembles several human autoinflammatory syndromes. Here we show that the two features are connected and have the same genetic origin, a regulatory mutation located close to a HA synthesizing gene (HAS2). The mutation is a 16.1 Kb duplication, the copy number of which correlates with HAS2 expression and disease. We suggest that the large amount of HA responsible for the skin condition predisposes to sterile fever and inflammation. HAS2 was previously not known to associate with autoinflammatory disease, and this finding is of wide interest since approximately 60% of human patients with periodic fever syndrome remain genetically unexplained. This investigation also demonstrates how strong artificial selection may affect not only desired and selected phenotypes, but also the health of domestic animals.
Mutations in FBN1 cause a range of overlapping but distinct conditions including Marfan syndrome (MFS), Weill-Marchesani syndrome (WMS), familial thoracic aortic aneurysms/dissections (FTAAD), acromicric dysplasia (AD), and geleophysic dysplasia (GD). Two forms of acromelic dysplasia, AD and GD, characterized by short stature, brachydactyly, reduced joint mobility, and characteristic facies, result from heterozygous missense mutations occurring in exons 41 and 42 of FBN1; missense mutations in these exons have not been reported to cause MFS or other syndromes. Here we report on probands with MFS and WMS who have heterozygous FBN1 missense mutations in exons 41 and 42, respectively. The proband with WMS has ectopia lentis, short stature, thickened pinnae, tight skin, striae atrophicae, reduced extension of the elbows, contractures of the fingers and toes, and brachydactyly and has a missense mutation in exon 42 of FBN1 (c.5242T>C ;p.C1748R). He also experienced a previously unreported complication of WMS, an acute thoracic aortic dissection. The second proband displays classic characteristics of MFS, including ectopia lentis, skeletal features and aortic root dilatation, and has a missense mutation in exon 41 of FBN1 (c.5084G>A; p.C1695Y). These phenotypes provide evidence that missense mutations in exons 41 and 42 of FBN1 lead to MFS and WMS in addition to AD and GD and also suggest that all individuals with pathogenic FBN1 mutations in these exons should be assessed for thoracic aortic disease and ectopia lentis. Further studies are necessary to elucidate the factors responsible for the different phenotypes associated with missense mutations in these exons of FBN1.
geleophysic dysplasia; acromicric dysplasia; aortic aneurysm; aortic dissection; ectopia lentis
A new connective tissue protein, which we call fibrillin, has been isolated from the medium of human fibroblast cell cultures. Electrophoresis of the disulfide bond-reduced protein gave a single band with an estimated molecular mass of 350,000 D. This 350-kD protein appeared to possess intrachain disulfide bonds. It could be stained with periodic acid-Schiff reagent, and after metabolic labeling, it contained [3H]glucosamine. It could not be labeled with [35S]sulfate. It was resistant to digestion by bacterial collagenase. Using mAbs specific for fibrillin, we demonstrated its widespread distribution in the connective tissue matrices of skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea, and ciliary zonule. Electron microscopic immunolocalization with colloidal gold conjugates specified its location to a class of extracellular structural elements described as microfibrils. These microfibrils possessed a characteristic appearance and averaged 10 nm in diameter. Microfibrils around the amorphous cores of the elastic fiber system as well as bundles of microfibrils without elastin cores were labeled equally well with antibody. Immunolocalization suggested that fibrillin is arrayed periodically along the individual microfibril and that individual microfibrils may be aligned within bundles. The periodicity of the epitope appeared to match the interstitial collagen band periodicity. In contrast, type VI collagen, which has been proposed as a possible microfibrillar component, was immunolocalized with a specific mAb to small diameter microfilaments that interweave among the large, banded collagen fibers; it was not associated with the system of microfibrils identified by the presence of fibrillin.
Hereditary footpad hyperkeratosis (HFH) represents a palmoplantar hyperkeratosis, which is inherited as a monogenic autosomal recessive trait in several dog breeds, such as e.g. Kromfohrländer and Irish Terriers. We performed genome-wide association studies (GWAS) in both breeds. In Kromfohrländer we obtained a single strong association signal on chromosome 5 (praw = 1.0×10−13) using 13 HFH cases and 29 controls. The association signal replicated in an independent cohort of Irish Terriers with 10 cases and 21 controls (praw = 6.9×10−10). The analysis of shared haplotypes among the combined Kromfohrländer and Irish Terrier cases defined a critical interval of 611 kb with 13 predicted genes. We re-sequenced the genome of one affected Kromfohrländer at 23.5× coverage. The comparison of the sequence data with 46 genomes of non-affected dogs from other breeds revealed a single private non-synonymous variant in the critical interval with respect to the reference genome assembly. The variant is a missense variant (c.155G>C) in the FAM83G gene encoding a protein with largely unknown function. It is predicted to change an evolutionary conserved arginine into a proline residue (p.R52P). We genotyped this variant in a larger cohort of dogs and found perfect association with the HFH phenotype. We further studied the clinical and histopathological alterations in the epidermis in vivo. Affected dogs show a moderate to severe orthokeratotic hyperplasia of the palmoplantar epidermis. Thus, our data provide the first evidence that FAM83G has an essential role for maintaining the integrity of the palmoplantar epidermis.
The palms and soles of mammals are covered by the palmoplantar epidermis, which has to bear immense mechanical forces and has therefore a special composition in comparison to the epidermis on regular skin. We studied a Mendelian disease in dogs, termed hereditary footpad hyperkeratosis (HFH). HFH affected dogs develop deep fissures in the paw pads, which are the consequence of a pathological thickening of the outermost layer of the epidermis. We mapped the disease causing genetic variant in the Kromfohrländer and Irish Terrier breeds to a 611 kb interval on chromosome 5. HFH affected Kromfohrländer and Irish Terriers shared the same haplotype indicating descent from a common founder. We re-sequenced the genome of an affected dog and compared it to genome sequences of 46 control dogs. The HFH affected dog had only one private non-synonymous variant in the critical interval, a missense variant of the FAM83G gene. We genotyped this variant in more than 500 dogs and found perfect association with the HFH phenotype. Our data very strongly suggest that the FAM83G variant is causative for HFH. FAM83G is a protein with unknown biochemical function. Our study thus provides the first link between this protein and the palmoplantar epidermis.
Infantile cortical hyperostosis (Caffey disease) is characterized by spontaneous episodes of subperiosteal new bone formation along 1 or more bones commencing within the first 5 months of life. A genome-wide screen for genetic linkage in a large family with an autosomal dominant form of Caffey disease (ADC) revealed a locus on chromosome 17q21 (LOD score, 6.78). Affected individuals and obligate carriers were heterozygous for a missense mutation (3040C↠T) in exon 41 of the gene encoding the α1(I) chain of type I collagen (COL1A1), altering residue 836 (R836C) in the triple-helical domain of this chain. The same mutation was identified in affected members of 2 unrelated, smaller families with ADC, but not in 2 prenatal cases and not in more than 300 chromosomes from healthy individuals. Fibroblast cultures from an affected individual produced abnormal disulfide-bonded dimeric α1(I) chains. Dermal collagen fibrils of the same individual were larger, more variable in shape and size, and less densely packed than those in control samples. Individuals bearing the mutation, whether they had experienced an episode of cortical hyperostosis or not, had joint hyperlaxity, hyperextensible skin, and inguinal hernias resembling symptoms of a mild form of Ehlers-Danlos syndrome type III. These findings extend the spectrum of COL1A1-related diseases to include a hyperostotic disorder.
Monoclonal antibodies to fibrillin 1 (MP340), a component of elastin-associated microfibrils, were used to screen cDNA libraries made from bovine nuchal ligament mRNA. One of the selected clones (cL9; 1.2 kb) hybridized on Northern (RNA) blotting with nuchal ligament mRNA to two abundant mRNAs of 9.0 and 7.5 kb, which were clearly distinct from fibrillin mRNA (10 kb). Further library screening and later reverse transcription PCR by the rapid amplification of cDNA ends (RACE) technique resulted in the isolation of additional overlapping cDNAs corresponding to about 6.7 kb of the mRNA. The encoded protein exhibited sequence similarity of around 80% with a recently identified human protein named latent transforming growth factor beta 1 (TGF-beta 1)-binding protein 2 (LTBP-2), indicating that the new protein was bovine LTBP-2. This was confirmed by the specific localization of bovine LTBP-2 cDNA probes to human chromosome 14q24.3, which is the locus of the human LTBP-2 gene. The domain structure of bovine LTBP-2 is very similar to that of the human LTBP-2, containing 20 examples of 6-cysteine epidermal growth factor-like repeats, 16 of which have the consensus sequence for calcium binding, together with 4 examples of 8-cysteine motifs characteristic of fibrillins and LTBP-1. A 4-cysteine sequence which is unique to bovine LTBP-2 and which has similarity to the 8-cysteine motifs was also present. Antibodies raised to two unique bovine LTBP-2 peptides specifically localized in tissue sections to the elastin-associated microfibrils, indicating that LTBP-2 is closely associated with these structures. Immunoblotting experiments identified putative LTBP-2 isoforms as a 260-kDa species released into the medium by cultured elastic tissue cells and as larger 290- and 310-kDa species in tissue extracts. A major proportion of tissue-derived LTBP-2 required treatment with 6 M guanidine for solubilization, indicating that the protein was strongly bound to the microfibrils. Most of the guanidine-solubilized LTBP-2 appeared to be monomeric, indicating that it was not involved in disulfide-bonded aggregation either with itself or with latent TGF-beta. Additional LTBP-2 was resistant to solubilization with 6 M guanidine but was readily extracted with a reductive saline solution. This treatment is relatively specific for solubilization of microfibrillar constituents including fibrillin 1 and microfibril-associated glycoprotein. Therefore, it can be inferred that some LTBP-2 is bound covalently to the microfibrils by reducible disulfide linkages. The evidence suggests that LTBP-2 has a direct role in elastic fiber structure and assembly which may be independent of its growth factor-binding properties. Thus, LTBP-2 appears to share functional characteristics with both LTBP-1 and fibrillins.
Systemic sclerosis (SSc) is a connective tissue disorder characterized by endothelial cell injury, autoimmunity and fibrosis. The following three fibrillin-1 alterations have been reported in SSc. (1) Fibrillin-1 microfibrils are disorganized in SSc dermis. (2) Fibrillin-1 microfibrils produced by SSc fibroblasts are unstable. (3) Mutations in the FBN1 gene and anti-fibrillin-1 autoantibodies have been reported in SSc. Fibrillin-1 microfibrils, which are abundantly produced by blood and lymphatic microvascular endothelial cells (B-MVECs and Ly-MVECs, respectively), sequester in the extracellular matrix the latent form of the potent profibrotic cytokine transforming growth factor β (TGF-β). In the present study, we evaluated the effects of SSc sera on the deposition of fibrillin-1 and microfibril-associated glycoprotein 1 (MAGP-1) and the expression of focal adhesion molecules by dermal B-MVECs and Ly-MVECs.
Dermal B-MVECs and Ly-MVECs were challenged with sera from SSc patients who were treatment-naïve or under cyclophosphamide (CYC) treatment and with sera from healthy controls. Fibrillin-1/MAGP-1 synthesis and deposition and the expression of αvβ3 integrin/phosphorylated focal adhesion kinase and vinculin/actin were evaluated by immunofluorescence and quantified by morphometric analysis.
Fibrillin-1 and MAGP-1 colocalized in all experimental conditions, forming a honeycomb pattern in B-MVECs and a dense mesh of short segments in Ly-MVECs. In B-MVECs, fibrillin-1/MAGP-1 production and αvβ3 integrin expression significantly decreased upon challenge with sera from naïve SSc patients compared with healthy controls. Upon challenge of B-MVECs with sera from CYC-treated SSc patients, fibrillin-1/MAGP-1 and αvβ3 integrin levels were comparable to those of cells treated with healthy sera. Ly-MVECs challenged with SSc sera did not differ from those treated with healthy control sera in the expression of any of the molecules assayed.
Because of the critical role of fibrillin-1 in sequestering the latent form of TGF-β in the extracellular matrix, its decreased deposition by B-MVECs challenged with SSc sera might contribute to dermal fibrosis. In SSc, CYC treatment might limit fibrosis through the maintenance of physiologic fibrillin-1 synthesis and deposition by B-MVECs.
Systemic sclerosis; blood and lymphatic microvascular endothelial cells; fibrillin-1; focal adhesion molecules; cyclophosphamide
Mice carrying the Tight skin (Tsk) mutation harbor a genomic duplication within the fibrillin-1 (Fbn 1) gene that results in a larger than normal in-frame Fbn 1 transcript. In this study, the consequences of the Tsk mutation for fibrillin-containing microfibrils have been examined. Dermal fibroblasts from Tsk/+ mice synthesized and secreted both normal fibrillin (∼330 kD) and the mutant oversized Tsk fibrillin-1 (∼450 kD) in comparable amounts, and Tsk fibrillin-1 was stably incorporated into cell layers. Immunohistochemical and ultrastructural analyses of normal and Tsk/+ mouse skin highlighted differences in the gross organization and distribution of microfibrillar arrays. Rotary shadowing of high Mr preparations from Tsk/+ skin demonstrated the presence of abundant beaded microfibrils. Some of these had normal morphology and periodicity, but others were distinguished by diffuse interbeads, longer periodicity, and tendency to aggregate. The presence of a structurally abnormal population of microfibrils in Tsk/+ skin was unequivocally demonstrated after calcium chelation and in denaturating conditions. Scanning transmission electron microscopy highlighted the presence of more mass in Tsk/+ skin microfibrils than in normal mice skin microfibrils. These data indicate that Tsk fibrillin-1 polymerizes and becomes incorporated into a discrete population of beaded microfibrils with altered molecular organization.
The microfibrillar glycoprotein fibrillin is linked to the Marfan syndrome, an autosomal dominant connective tissue disorder. In this study, fibrillin synthesis, deposition and assembly has been investigated in Marfan dermal fibroblast lines from two unrelated patients for whom distinct mutations in the fibrillin gene FBN1 have been identified. In patient NB, a point mutation has occurred which causes an amino acid substitution and the other patient (GK) has a deletion in one allele. The two cell lines were broadly comparable with respect to de novo fibrillin synthesis and its distribution between medium and cell layer compartments. Electrophoresis of fibrillin immunoprecipitates confirmed the presence of fibrillin in medium and cell layers. GK cells secreted an additional higher relative molecular mass fibrillin-immunoreactive component. The time-course of fibrillin secretion was similar for the two lines, but differences in fibrillin aggregation were apparent. Rotary shadowing electron microscopy of extracted cell layers demonstrated the presence of abundant and extensive microfibrils in NB cell layers. These were abnormal in their gross morphology in comparison to microfibrils isolated from control cultures. No periodic microfibrillar structures were isolated from GK cell layers. These studies underline the need to classify fibrillin defects in terms of biochemical and ultrastructural criteria. Examination of the effects of individual mutations on microfibril organization will be particularly informative in elucidating the relationship between microfibril dysfunction and the complex clinical manifestations of Marfan patients.