Mutations in fibrillin-1 cause congenital scleroderma: stiff skin syndrome

Novel FBN1 intron variant causes isolated ectopia lentis via in-frame exon skipping

Mutations in fibrillin-1 (FBN1) cause various clinical conditions, such as Marfan syndrome (MFS). However, the genotype-phenotype relationships underlying MFS and other conditions relevant to FBN1 mutations have not been fully elucidated. We performed whole-exome sequencing on three participants, including an affected mother-daughter pair, in a three-generation Japanese family with isolated ectopia lentis (IEL). The sequencing identified a novel single-nucleotide variant (c.1327+3A>C) in intron 11 of FBN1 that was shared between the two patients. We confirmed the co-segregation of the variant with IEL in two additional affected relatives in the family. The Combined Annotation-Dependent Depletion score of the variant was 26.1, which was indicated by SpliceAI to influence splicing, with a score of 0.93. Reverse transcription-polymerase chain reaction (RT-PCR) of mRNAs isolated from peripheral blood mononuclear cells revealed aberrant bands in all four affected individuals. Subsequent sequencing revealed that these bands originated from FBN1 transcripts lacking exon 11. The causality of the variant in the skipping of exon 11, which results in an in-frame deletion of 60 amino acids corresponding to the "hinge" region of FBN1 protein, was confirmed in a minigene experiment. Interestingly, the same result was observed for a minigene for c.1327+1G>A, a variant previously identified in two unrelated EL families without MFS manifestations. These results suggest that the c.1327+3A>C mutation in FBN1 likely leads to IEL. The findings expand our knowledge of FBN1 and provide insights into FBN1-related diseases.

Protein O-Fucosyltransferases: Biological Functions and Molecular Mechanisms in Mammals

Domain-specific O-fucosylation is an unusual type of glycosylation, where the fucose is directly attached to the serine or threonine residues in specific protein domains via an O-linkage. O-fucosylated proteins play critical roles in a wide variety of biological events and hold important therapeutic values, with the most studied being the Notch receptors and ADAMTS proteins. O-fucose glycans modulate the function of the proteins they modify and are closely associated with various diseases including cancer. In mammals, alongside the well-documented protein O-fucosyltransferase (POFUT) 1-mediated O-fucosylation of epidermal growth factor-like (EGF) repeats and POFUT2-mediated O-fucosylation of thrombospondin type 1 repeats (TSRs), a new type of O-fucosylation was recently identified on elastin microfibril interface (EMI) domains, mediated by POFUT3 and POFUT4 (formerly FUT10 and FUT11). In this review, we present an overview of our current knowledge of O-fucosylation, integrating the latest findings and with a particular focus on its biological functions and molecular mechanisms.

Fibrillin-1 G234D mutation in the hybrid1 domain causes tight skin associated with dysregulated elastogenesis and increased collagen cross-linking in mice

Fibrillin-1, an extracellular matrix (ECM) protein encoded by the FBN1 gene, serves as a microfibril scaffold crucial for elastic fiber formation and homeostasis in pliable tissue such as the skin. Aside from causing Marfan syndrome, some mutations in FBN1 result in scleroderma, marked by hardened and thicker skin which limits joint mobility. Here, we describe a tight skin phenotype in the Fbn1G234D/G234D mice carrying a corresponding variant of FBN1 in the hybrid1 domain that was identified in a patient with familial aortic dissection. Unlike scleroderma, skin thickness and collagen fiber abundance do not change in the Fbn1G234D/G234D mutant skin. Instead, increased collagen cross-links were observed. In addition, short elastic fibers were sparsely located underneath the panniculus muscle layer, and an abundance of thin, aberrant elastic fibers was increased within the subcutaneous fascia, which may have tightened skin attachment to the underlying skeletal muscle. Structurally, Fbn1G234D/G234D microfibrils have a disrupted shoulder region that shares similarities with hybrid1 deletion mutant microfibrils. We then demonstrate the consequence of fibrillin-1 G234D mutation on dermal fibroblast functions. Mutant primary fibroblasts produce fewer elastic fibers, exhibit slower migration and increased cell stiffness. Moreover, secretome from mutant fibroblasts are marked by enhanced secretion of ECM, ECM-modifying enzymes, proteoglycans and cytokines, which are pro-tissue repair/fibrogenic. The transcriptome of mutant fibroblasts displays an increased expression of myogenic developmental and immune-related genes. Our study proposes that imbalanced ECM homeostasis due to a fibrillin-1 G234D mutation impacts fibroblast properties with potential ramifications on skin function.

Control of Epithelial Tissue Organization by mRNA Localization

mRNA localization to specific subcellular regions is common in mammalian cells but poorly understood in terms of its physiological roles1-6,7. This study demonstrates the functional importance of Net1 mRNA, which we find prominently localized at the dermal-epidermal junction (DEJ) in stratified squamous epithelia. Net1 mRNA accumulates at DEJ protrusion-like structures that interact with the basement membrane and connect to a mechanosensitive network of microfibrils. Disrupting Net1 mRNA localization in mouse epithelium alters DEJ morphology and keratinocyte-matrix connections, affecting tissue homeostasis. mRNA localization dictates Net1 protein distribution and its function as a RhoA GTPase exchange factor (GEF). Altered RhoA activity is in turn sufficient to alter the ultrastructure of the DEJ. This study provides a high-resolution in vivo view of mRNA targeting in a physiological context. It further demonstrates how the subcellular localization of a single mRNA can significantly influence mammalian epithelial tissue organization, thus revealing an unappreciated level of post-transcriptional regulation that controls tissue physiology.

Clinical, mechanistic, and therapeutic landscape of cutaneous fibrosis

When dysregulated, skin fibrosis can lead to a multitude of pathologies. We provide a framework for understanding the wide clinical spectrum, mechanisms, and management of cutaneous fibrosis encompassing a variety of matrix disorders, fibrohistiocytic neoplasms, injury-induced scarring, and autoimmune scleroses. Underlying such entities are common mechanistic pathways that leverage morphogenic signaling, immune activation, and mechanotransduction to modulate fibroblast function. In light of the limited array of available treatments for cutaneous fibrosis, scientific insights have opened new therapeutic and investigative avenues for conditions that still lack effective interventions.

Asprosin: its function as a novel endocrine factor in metabolic-related diseases

BACKGROUND AND PURPOSE

Asprosin was discovered as a new endocrine hormone originating from fibrillin-1 cleavage that plays a crucial role in various metabolic-related diseases, such as obesity, nonalcoholic fatty liver disease (NAFLD), diabetes, polycystic ovary syndrome (PCOS), and cardiovascular diseases. The purpose of this review is to describe the recent advancements of asprosin.

METHOD

Narrative review.

RESULT

This comprehensive review explores its tissue-specific functions, focusing on white adipose tissue, liver, hypothalamus, testis, ovary, heart, pancreas, skeletal muscle, and kidney.

CONCLUSION

Asprosin is a multifaceted protein with tissue-specific roles in various physiological and pathological processes. Further research is needed to fully understand the mechanisms and potential of asprosin as a therapeutic target. These insights could provide new directions for treatments targeting metabolic-related diseases.

Childhood-onset progressive skin fibrosis and joint immobility

A 28-year-old woman presented with generalized hardening of the skin, joint immobility, shortness of breath and lower back pain since the age of 7 years.

Causative role of a novel intronic indel variant in FBN1 and maternal germinal mosaicism in Marfan syndrome

BACKGROUND

Marfan syndrome (MFS) is an autosomal dominant connective tissue disease with wide clinical heterogeneity, and mainly caused by pathogenic variants in fibrillin-1 (FBN1).

METHODS

A Chinese 4-generation MFS pedigree with 16 family members was recruited and exome sequencing (ES) was performed in the proband. Transcript analysis (patient RNA and minigene assays) and in silico structural analysis were used to determine the pathogenicity of the variant. In addition, germline mosaicism in family member (Ι:1) was assessed using quantitative fluorescent polymerase chain reaction (QF-PCR) and short tandem repeat PCR (STR) analyses.

RESULTS

Two cis-compound benign intronic variants of FBN1 (c.3464-4 A > G and c.3464-5G > A) were identified in the proband by ES. As a compound variant, c.3464-5_3464-4delGAinsAG was found to be pathogenic and co-segregated with MFS. RNA studies indicated that aberrant transcripts were found only in patients and mutant-type clones. The variant c.3464-5_3464-4delGAinsAG caused erroneous integration of a 3 bp sequence into intron 28 and resulted in the insertion of one amino acid in the protein sequence (p.Ile1154_Asp1155insAla). Structural analyses suggested that p.Ile1154_Asp1155insAla affected the protein's secondary structure by interfering with one disulfide bond between Cys1140 and Cys1153 and causing the extension of an anti-parallel β sheet in the calcium-binding epidermal growth factor-like (cbEGF)13 domain. In addition, the asymptomatic family member Ι:1 was deduced to be a gonadal mosaic as assessed by inconsistent results of sequencing and STR analysis.

CONCLUSIONS

To our knowledge, FBN1 c.3464-5_3464-4delGAinsAG is the first identified pathogenic intronic indel variant affecting non-canonical splice sites in this gene. Our study reinforces the importance of assessing the pathogenic role of intronic variants at the mRNA level, with structural analysis, and the occurrence of mosaicism.

Spontaneous Left Main Coronary Artery Dissection in a Male

Spontaneous coronary artery dissection (SCAD) is one of the causes of acute coronary syndrome (ACS) that is increasingly recognized in young to middle-aged women without typical coronary risk factors. This case report describes a 46-year-old male with a rare presentation of SCAD involving the left main (LM) coronary artery. The patient underwent an emergency coronary angiogram for high-risk ACS and had percutaneous coronary intervention (PCI) of LM due to active ischemia and hemodynamic instability. The extension of intramural hematoma after the LM coronary artery stent confirmed the initial suspicion of SCAD. The diagnosis of SCAD is crucial, as its management differs from other causes of ACS. Coronary angiography is the gold standard for diagnosing SCAD, with adjunctive imaging using optical coherence tomography (OCT) and intravascular ultrasound (IVUS). In this patient, his physical examination findings and further imaging raised a suspicion for systemic connective tissue disease. Genetic analysis was executed, but no reportable variants in any of the 29 genes studied were identified. This case highlights the importance of recognizing SCAD as a potential cause of ACS even in men and emphasizes the findings during coronary angiography that can aid in an accurate diagnosis and appropriate management.

Noncanonical WNT5A controls the activation of latent TGF-β to drive fibroblast activation and tissue fibrosis

Transforming growth factor β (TGF-β) signaling is a core pathway of fibrosis, but the molecular regulation of the activation of latent TGF-β remains incompletely understood. Here, we demonstrate a crucial role of WNT5A/JNK/ROCK signaling that rapidly coordinates the activation of latent TGF-β in fibrotic diseases. WNT5A was identified as a predominant noncanonical WNT ligand in fibrotic diseases such as systemic sclerosis, sclerodermatous chronic graft-versus-host disease, and idiopathic pulmonary fibrosis, stimulating fibroblast-to-myofibroblast transition and tissue fibrosis by activation of latent TGF-β. The activation of latent TGF-β requires rapid JNK- and ROCK-dependent cytoskeletal rearrangements and integrin αV (ITGAV). Conditional ablation of WNT5A or its downstream targets prevented activation of latent TGF-β, rebalanced TGF-β signaling, and ameliorated experimental fibrosis. We thus uncovered what we believe to be a novel mechanism for the aberrant activation of latent TGF-β in fibrotic diseases and provided evidence for targeting WNT5A/JNK/ROCK signaling in fibrotic diseases as a new therapeutic approach.

Segmental Skin Induration in a Boy

An 11-year-old boy presented with progressive unilateral skin hardening on his right thigh and buttock for 7 years. What is your diagnosis?

Multi-organ phenotypes in mice lacking latent TGFβ binding protein 2 (LTBP2)

BACKGROUND

Latent TGFβ binding protein-2 (LTBP2) is a fibrillin 1 binding component of the microfibril. LTBP2 is the only LTBP protein that does not bind any isoforms of TGFβ, although it may interfere with the function of other LTBPs or interact with other signaling pathways.

RESULTS

Here, we investigate mice lacking Ltbp2 (Ltbp2-/- ) and identify multiple phenotypes that impact bodyweight and fat mass, and affect bone and skin development. The alterations in skin and bone development are particularly noteworthy since the strength of these tissues is differentially affected by loss of Ltbp2. Interestingly, some tissues that express high levels of Ltbp2, such as the aorta and lung, do not have a developmental or homeostatic phenotype.

CONCLUSIONS

Analysis of these mice show that LTBP2 has complex effects on development through direct effects on the extracellular matrix (ECM) or on signaling pathways that are known to regulate the ECM.

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.

Extracellular matrix molecules associated with lymphatic vessels in health and disease

Lymphatic vessels (LyVs), responsible for fluid, solute, and immune cell homeostasis in the body, are closely associated with the adjacent extracellular matrix (ECM) molecules whose structural and functional impact on LyVs is currently more appreciated, albeit not entirely elucidated. These molecules, serving as a platform for various connective tissue cell activities and affecting LyV biology should be considered also as an integral part of the lymphatic system. Any alterations and changes in ECM molecules over the course of disease impair the function and structure of the LyV network. Remodeling of LyV cells, which are components of lymphatic vessel walls, also triggers alterations in ECM molecules and interstitial tissue composition. Therefore, in this review we aimed to present the current knowledge on ECM in tissues and particularly on molecules surrounding lymphatics in normal conditions and in disease.

Marfan Syndrome: Enhanced Diagnostic Tools and Follow-up Management Strategies

Marfan syndrome (MFS) is a rare inherited autosomic disorder, which encompasses a variety of systemic manifestations caused by mutations in the Fibrillin-1 encoding gene (FBN1). Cardinal clinical phenotypes of MFS are highly variable in terms of severity, and commonly involve cardiovascular, ocular, and musculoskeletal systems with a wide range of manifestations, such as ascending aorta aneurysms and dissection, mitral valve prolapse, ectopia lentis and long bone overgrowth, respectively. Of note, an accurate and prompt diagnosis is pivotal in order to provide the best treatment to the patients as early as possible. To date, the diagnosis of the syndrome has relied upon a systemic score calculation as well as DNA mutation identification. The aim of this review is to summarize the latest MFS evidence regarding the definition, differences and similarities with other connective tissue pathologies with severe systemic phenotypes (e.g., Autosomal dominant Weill-Marchesani syndrome, Loeys-Dietz syndrome, Ehlers-Danlos syndrome) and clinical assessment. In this regard, the management of MFS requires a multidisciplinary team in order to accurately control the evolution of the most severe and potentially life-threatening complications. Based on recent findings in the literature and our clinical experience, we propose a multidisciplinary approach involving specialists in different clinical fields (i.e., cardiologists, surgeons, ophthalmologists, orthopedics, pneumologists, neurologists, endocrinologists, geneticists, and psychologists) to comprehensively characterize, treat, and manage MFS patients with a personalized medicine approach.

Segmental stiff skin syndrome treated with secukinumab

Segmental stiff skin syndrome is a rare fibrosing scleroderma-like disorder characterized by progressive indurations of the skin leading to joint contractures, decreased mobility, and pain. Treatment options are limited; we report a patient that showed improvement with anti-IL17 biologic therapy.

Molecular genetic etiology by whole exome sequence analysis in cases with familial type 1 diabetes mellitus without HLA haplotype predisposition or incomplete predisposition

OBJECTIVES

Familial transmission is observed in approximately 10% of cases with type 1 diabetes mellitus (T1DM). The most important gene determining susceptibility is the human leukocyte antigen complex (HLA) located on chromosome 6. More than 50 susceptible loci are associated with T1DM susceptibility have been identified in genes other than HLA. In this study, it was aimed to investigate the molecular genetic etiology by whole-exome sequence (WES) analysis in cases with familial T1DM with no or weakly detected HLA tissue type susceptibility. We aimed to identify new genes responsible for the development of type 1 diabetes and to reveal new genes that have not been shown in the literature before.

METHODS

Cases with at least one T1DM diagnosis in first-degree relatives were included in the study. In the first step, HLA DQ2 and DQ8 loci, which are known to be associated with T1DM susceptibility, were investigated by. In the second step, the presence of variants that could explain the situation was investigated by WES analysis in patients who were negative for both HLA DQ2 and HLA DQ8 haplotypes, HLA DQ2 negative, HLA DQ8 positive, and HLA DQ2 positive and HLA DQ8 negative patients.

RESULTS

The mean age and duration of diabetes of the 30 cases (Girl/Male: 17/13) were 14.9 ± 6 and 7.56 ± 3.84 years, respectively. There was consanguineous marriage in 5 (16%) of the families. As a result of filtering all exome sequence analysis data of two cases with DQ2 (DQB1*02) (-) and DQ8 (DQB1*03:02) (-), seven cases with DQ2 (DQB1*02) (+) and DQ8 (DQB1*03:02) (-), and one case with DQ2 (DQB1*02) (-) and DQ8 (DQB1*03:02) (+), seven different variants in seven different genes were detected in five cases. The pathogenicity of the detected variants were determined according to the "American College of Medical Genetics and Genomics (ACMG)" criteria. These seven variants detected were evaluated as high-score VUS (Variants of unknown/uncertain significance). In the segregation study conducted for the mutation in the POLG gene detected in case 5, this variant was detected in the mother of the case and his brother with T1DM. Segregation studies are ongoing for variants detected in other affected individuals in the family.

CONCLUSIONS

In conclusion, in this study, seven different variants in seven different genes were detected in five patients by WES analysis in familial T1DM patients with no or weak HLA tissue type susceptibility. These seven variants detected were evaluated as high-score VUS. POLG might be a novel candidate gene responsible for susceptibility to T1DM. Non-HLA genes directly responsible for the development of T1DM were not detected in any of the cases.

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.

Dermal extracellular matrix molecules in skin development, homeostasis, wound regeneration and diseases

The extracellular matrix (ECM) is a dynamic structure that surrounds and anchors cellular components in tissues. In addition to functioning as a structural scaffold for cellular components, ECMs also regulate diverse biological functions, including cell adhesion, proliferation, differentiation, migration, cell-cell interactions, and intracellular signaling events. Dermal fibroblasts (dFBs), the major cellular source of skin ECM, develop from a common embryonic precursor to the highly heterogeneous subpopulations during development and adulthood. Upon injury, dFBs migrate into wound granulation tissue and transdifferentiate into myofibroblasts, which play a critical role in wound contraction and dermal ECM regeneration and deposition. In this review, we describe the plasticity of dFBs during development and wound healing and how various dFB-derived ECM molecules, including collagen, proteoglycans, glycosaminoglycans, fibrillins and matricellular proteins are expressed and regulated, and in turn how these ECM molecules play a role in regulating the function of dFBs and immune cells. Finally, we describe how dysregulation of ECM matrix is associated the pathogenesis of wound healing related skin diseases, including chronic wounds and keloid.

Classification and rising medication therapy in stiff skin syndrome: A case report and literature review

Stiff skin syndrome (SSS) is a rare disorder characterized by skin induration and limited joint mobility in the absence of visceral, musculoskeletal, vascular, or immunologic abnormalities. Distinctive subsets of SSS could be distinguished by various manifestation and mechanism, which accounts for the high heterogeneity in SSS cases. Although rehabilitation training remains the mainstay of management, rising medications has drawn awareness in recent years, owing to the potential efficacy. Nevertheless, experience was limited, especially in widespread SSS. We report on a 5-year-old girl with widespread SSS, whose lesion stopped progressing after combination therapy by mycophenolic acid (MPA) and losartan (LST) in addition to rehabilitation exercise. Despite limited experience, a combined therapy of MPA and LST seems to be effective in retarding progression of widespread SSS.

The critical role of the TB5 domain of Fibrillin-1 in endochondral ossification

Mutations in the Fibrillin-1 (FBN1) gene are responsible for the autosomal dominant form of Geleophysic Dysplasia (GD), which is characterized by short stature and extremities, thick skin, and cardiovascular disease. All known FBN1 mutations in GD patients are localized within the region encoding the TB5 (TGF-β binding protein-like 5) domain of this protein. Herein, we generated a knock-in mouse model, Fbn1Y1698C by introducing the p.Tyr1696Cys mutation from a GD patient into the TB5 domain of murine Fbn1 to elucidate the specific role of this domain in endochondral ossification. We found that both Fbn1Y1698C/+ and Fbn1Y1698C/Y1698C mice exhibited a reduced stature reminiscent of the human GD phenotype. The Fbn1 point mutation introduced in these mice affected the growth plate formation owing to abnormal chondrocyte differentiation such that mutant chondrocytes failed to establish a dense microfibrillar network composed of fibrillin-1. This original Fbn1 mutant mouse model offers new insight into the pathogenic events underlying GD. Our findings suggest that the etiology of GD involves the dysregulation of the ECM composed by abnormal fibrillin-1 microfibril network impacting the differentiation of the chondrocytes.

Animal Models of Systemic Sclerosis: Using Nailfold Capillaroscopy as a Potential Tool to Evaluate Microcirculation and Microangiopathy: A Narrative Review

Systemic sclerosis (SSc) is an autoimmune disease with three pathogenic hallmarks, i.e., inflammation, vasculopathy, and fibrosis. A wide plethora of animal models have been developed to address the complex pathophysiology and for the development of possible anti-fibrotic treatments. However, no current model comprises all three pathological mechanisms of the disease. To highlight the lack of a complete model, a review of some of the most widely used animal models for SSc was performed. In addition, to date, no model has accomplished the recreation of primary or secondary Raynaud’s phenomenon, a key feature in SSc. In humans, nailfold capillaroscopy (NFC) has been used to evaluate secondary Raynaud’s phenomenon and microvasculature changes in SSc. Being a non-invasive technique, it is widely used both in clinical studies and as a tool for clinical evaluation. Because of this, its potential use in animal models has been neglected. We evaluated NFC in guinea pigs to investigate the possibility of applying this technique to study microcirculation in the nailfold of animal models and in the future, development of an animal model for Raynaud’s phenomenon. The applications are not only to elucidate the pathophysiological mechanisms of vasculopathy but can also be used in the development of novel treatment options.

Targeting TGF-β signal transduction for fibrosis and cancer therapy

Transforming growth factor β (TGF-β) has long been identified with its intensive involvement in early embryonic development and organogenesis, immune supervision, tissue repair, and adult homeostasis. The role of TGF-β in fibrosis and cancer is complex and sometimes even contradictory, exhibiting either inhibitory or promoting effects depending on the stage of the disease. Under pathological conditions, overexpressed TGF-β causes epithelial-mesenchymal transition (EMT), extracellular matrix (ECM) deposition, cancer-associated fibroblast (CAF) formation, which leads to fibrotic disease, and cancer. Given the critical role of TGF-β and its downstream molecules in the progression of fibrosis and cancers, therapeutics targeting TGF-β signaling appears to be a promising strategy. However, due to potential systemic cytotoxicity, the development of TGF-β therapeutics has lagged. In this review, we summarized the biological process of TGF-β, with its dual role in fibrosis and tumorigenesis, and the clinical application of TGF-β-targeting therapies.

The fibrillinopathies: new insights with focus on the paradigm of opposing phenotypes for both FBN1 and FBN2

Different pathogenic variants in the fibrillin‐1 gene (FBN1) cause Marfan syndrome and acromelic dysplasias. Whereas the musculoskeletal features of Marfan syndrome involve tall stature, arachnodactyly, joint hypermobility, and muscle hypoplasia, acromelic dysplasia patients present with short stature, brachydactyly, stiff joints, and hypermuscularity. Similarly, pathogenic variants in the fibrillin‐2 gene (FBN2) cause either a Marfanoid congenital contractural arachnodactyly or a FBN2‐related acromelic dysplasia that most prominently presents with brachydactyly. The phenotypic and molecular resemblances between both the FBN1 and FBN2‐related disorders suggest that reciprocal pathomechanistic lessons can be learned. In this review, we provide an updated overview and comparison of the phenotypic and mutational spectra of both the “tall” and “short” fibrillinopathies. The future parallel functional study of both FBN1/2‐related disorders will reveal new insights into how pathogenic fibrillin variants differently affect the fibrillin microfibril network and/or growth factor homeostasis in clinically opposite syndromes. This knowledge may eventually be translated into new therapeutic approaches by targeting or modulating the fibrillin microfibril network and/or the signaling pathways under its control.

Elastic Fibre Proteins in Elastogenesis and Wound Healing

As essential components of our connective tissues, elastic fibres give tissues such as major blood vessels, skin and the lungs their elasticity. Their formation is complex and co-ordinately regulated by multiple factors. In this review, we describe key players in elastogenesis: fibrillin-1, tropoelastin, latent TGFβ binding protein-4, and fibulin-4 and -5. We summarise their roles in elastogenesis, discuss the effect of their mutations on relevant diseases, and describe their interactions involved in forming the elastic fibre network. Moreover, we look into their roles in wound repair for a better understanding of their potential application in tissue regeneration.

Matrix Stiffening Enhances DNCB-Induced IL-6 Secretion in Keratinocytes Through Activation of ERK and PI3K/Akt Pathway

Matrix stiffness, a critical physical property of the cellular environment, is implicated in epidermal homeostasis. In particular, matrix stiffening during the pathological progression of skin diseases appears to contribute to cellular responses of keratinocytes. However, it has not yet elucidated the molecular mechanism underlying matrix-stiffness-mediated signaling in coordination with chemical stimuli during inflammation and its effect on proinflammatory cytokine production. In this study, we demonstrated that keratinocytes adapt to matrix stiffening by increasing cell–matrix adhesion via actin cytoskeleton remodeling. Specifically, mechanosensing and signal transduction are coupled with chemical stimuli to regulate cytokine production, and interleukin-6 (IL-6) production is elevated in keratinocytes on stiffer substrates in response to 2,4-dinitrochlorobenzene. We demonstrated that β1 integrin and focal adhesion kinase (FAK) expression were enhanced with increasing stiffness and activation of ERK and the PI3K/Akt pathway was involved in stiffening-mediated IL-6 production. Collectively, our results reveal the critical role of matrix stiffening in modulating the proinflammatory response of keratinocytes, with important clinical implications for skin diseases accompanied by pathological matrix stiffening.

BMP antagonists in tissue development and disease

Bone morphogenic proteins (BMPs) are important growth regulators in embryogenesis and postnatal homeostasis. Their tight regulation is crucial for successful embryonic development as well as tissue homeostasis in the adult organism. BMP inhibition by natural extracellular biologic antagonists represents the most intensively studied mechanistic concept of BMP growth factor regulation. It was shown to be critical for numerous developmental programs, including germ layer specification and spatiotemporal gradients required for the establishment of the dorsal-ventral axis and organ formation. The importance of BMP antagonists for extracellular matrix homeostasis is illustrated by the numerous human connective tissue disorders caused by their mutational inactivation. Here, we will focus on the known functional interactions targeting BMP antagonists to the ECM and discuss how these interactions influence BMP antagonist activity. Moreover, we will provide an overview about the current concepts and investigated molecular mechanisms modulating BMP inhibitor function in the context of development and disease.

The fibrillin-1 RGD motif posttranscriptionally regulates ERK1/2 signaling and fibroblast proliferation via miR-1208

Fibrillin-1 is an extracellular matrix protein which contains one conserved RGD integrin-binding motif. It constitutes the backbone of microfibrils in many tissues, and mutations in fibrillin-1 cause various connective tissue disorders. Although it is well established that fibrillin-1 interacts with several RGD-dependent integrins, very little is known about the associated intracellular signaling pathways. Recent published evidence identified a subset of miRNAs regulated by fibrillin-1 RGD-cell adhesion, with miR-1208 among the most downregulated. The present study shows that the downregulated miR-1208 controls fibroblast proliferation. Inhibitor experiments revealed that fibrillin-1 RGD suppressed miR-1208 expression via c-Src kinase and the downstream JNK signaling. Bioinformatic prediction and experimental target sequence validation demonstrated four miR-1208 binding sites on the ERK2 mRNA and one on the MEK1 mRNA. ERK2 and MEK1 are critical proliferation-promoting kinases. Decreased miR-1208 levels elevated the total and phosphorylated ERK1/2 and MEK1/2 protein levels and the phosphorylated to total ERK1/2 ratio. Together, the data demonstrate a novel outside-in signaling mechanism explaining how fibrillin-1 RGD-cell binding regulates fibroblast proliferation.

Systemic sclerosis skin is a primed microenvironment for soft tissue calcification-a hypothesis

Calcinosis cutis, defined as sub-epidermal deposition of calcium salts, is a major clinical problem in patients with SSc, affecting 20-40% of patients. A number of recognized factors associated with calcinosis have been identified, including disease duration, digital ischaemia and acro-osteolysis. Yet, to date, the pathogenesis of SSc-related calcinosis remains unknown, and currently there is no effective disease-modifying pharmacotherapy. Following onset of SSc, there are marked changes in the extracellular matrix (ECM) of the skin, notably a breakdown in the microfibrillar network and accumulation of type I collagen. Our hypothesis is that these pathological changes reflect a changing cellular phenotype and result in a primed microenvironment for soft tissue calcification, with SSc fibroblasts adopting a pro-osteogenic profile, and specific driving forces promoting tissue mineralization. Considering the role of the ECM in disease progression may help elucidate the mechanism(s) behind SSc-related calcinosis and inform the development of future therapeutic interventions.

Segmental stiff skin syndrome (SSS): Clinical case and a brief review

Stiff skin syndrome (SSS) is a rare, scleroderma-like condition that is commonly characterised by stony hard skin and limited joint mobility, in the absence of visceral involvement or immunologic abnormalities. Depending on the distribution of the disease, this disorder can be further categorised into classic (widespread) SSS or its newly described segmental variant. Additional features of this syndrome may include hypertrichosis, lipodystrophy, dysmetria and scoliosis. In this report, we present the case of a patient with segmental SSS and we briefly review the current literature about the topic.

Proof-of-Concept: Antisense Oligonucleotide Mediated Skipping of Fibrillin-1 Exon 52

Marfan syndrome is one of the most common dominantly inherited connective tissue disorders, affecting 2–3 in 10,000 individuals, and is caused by one of over 2800 unique FBN1 mutations. Mutations in FBN1 result in reduced fibrillin-1 expression, or the production of two different fibrillin-1 monomers unable to interact to form functional microfibrils. Here, we describe in vitro evaluation of antisense oligonucleotides designed to mediate exclusion of FBN1 exon 52 during pre-mRNA splicing to restore monomer homology. Antisense oligonucleotide sequences were screened in healthy control fibroblasts. The most effective sequence was synthesised as a phosphorodiamidate morpholino oligomer, a chemistry shown to be safe and effective clinically. We show that exon 52 can be excluded in up to 100% of FBN1 transcripts in healthy control fibroblasts transfected with PMO52. Immunofluorescent staining revealed the loss of fibrillin 1 fibres with ~50% skipping and the subsequent re-appearance of fibres with >80% skipping. However, the effect of exon skipping on the function of the induced fibrillin-1 isoform remains to be explored. Therefore, these findings demonstrate proof-of-concept that exclusion of an exon from FBN1 pre-mRNA can result in internally truncated but identical monomers capable of forming fibres and lay a foundation for further investigation to determine the effect of exon skipping on fibrillin-1 function.

Assembly assay identifies a critical region of human fibrillin-1 required for 10-12 nm diameter microfibril biogenesis

The human FBN1 gene encodes fibrillin-1 (FBN1); the main component of the 10–12 nm diameter extracellular matrix microfibrils. Marfan syndrome (MFS) is a common inherited connective tissue disorder, caused by FBN1 mutations. It features a wide spectrum of disease severity, from mild cases to the lethal neonatal form (nMFS), that is yet to be explained at the molecular level. Mutations associated with nMFS generally affect a region of FBN1 between domains TB3-cbEGF18—the "neonatal region". To gain insight into the process of fibril assembly and increase our understanding of the mechanisms determining disease severity in MFS, we compared the secretion and assembly properties of FBN1 variants containing nMFS-associated substitutions with variants associated with milder, classical MFS (cMFS). In the majority of cases, both nMFS- and cMFS-associated neonatal region variants were secreted at levels comparable to wild type. Microfibril incorporation by the nMFS variants was greatly reduced or absent compared to the cMFS forms, however, suggesting that nMFS substitutions disrupt a previously undefined site of microfibril assembly. Additional analysis of a domain deletion variant caused by exon skipping also indicates that register in the neonatal region is likely to be critical for assembly. These data demonstrate for the first time new requirements for microfibril biogenesis and identify at least two distinct molecular mechanisms associated with disease substitutions in the TB3-cbEGF18 region; incorporation of mutant FBN1 into microfibrils changing their integral properties (cMFS) or the blocking of wild type FBN1 assembly by mutant molecules that prevents late-stage lateral assembly (nMFS).

Case Report: Pansclerotic Morphea-Clinical Features, Differential Diagnoses and Modern Treatment Concepts

Pansclerotic morphea (PSM) is a rare skin disease characterized by progressive stiffening of the skin with or without the typical superficial skin changes usually seen in morphea (localized scleroderma). Standard therapy, consisting of a combination of systemic glucocorticoids and methotrexate or mycophenolate mofetil, does rarely stop disease progression, which may lead to severe cutaneous sclerosis and secondary contractures. Little is known about the efficacy of newer biologicals such as abatacept, a fusion protein antibody against CTLA-4, or tocilizumab, a fully humanized IL-6R antibody, in the treatment of this pathology. We present the case of an 8 years old girl with an unusual, progressive stiffening of the skin, which was eventually diagnosed as pansclerotic morphea. A treatment with systemic glucocorticoids and methotrexate combined with tocilizumab led to a good clinical response within 2 months after initiation. In this paper, we discuss differential diagnoses to be considered and this new promising treatment option based on a case review of the literature.

Reversion-inducing cysteine-rich protein with Kazal motifs and MT1-MMP promote the formation of robust fibrillin fibers

Fibrillins (FBNs) form mesh-like structures of microfibrils in various elastic tissues. RECK and FBN1 are co-expressed in many human tissues, suggesting a functional relationship. We found that dermal FBN1 fibers show atypical morphology in mice with reduced RECK expression (RECK-Hypo mice). Dermal FBN1 fibers in mice-lacking membrane-type 1-matrix metalloproteinase (MT1-MMP) show a similar atypical morphology, despite the current notion that MT1-MMP (a membrane-bound protease) and RECK (a membrane-bound protease inhibitor) have opposing functions. Our experiments using dermal fibroblasts indicated that RECK promotes pro-MT1-MMP activation, increases cell-associated gelatinase/collagenase activity, and decreases diffusible gelatinase/collagenase activity, while MT1-MMP stabilizes RECK in these cells. Experiments using purified proteins indicate that RECK and its binding partner ADAMTS10 keep the proteolytic activity of MT1-MMP within a certain range. These findings suggest that RECK, ADAMTS10, and MT1-MMP cooperate to support the formation of robust FBN1 fibers.

Elastic fibers during aging and disease

Elastic fibers are essential constituents of the extracellular matrix of higher vertebrates and endow several tissues and organs including lungs, skin and blood vessels with elasticity and resilience. During the human lifespan, elastic fibers are exposed to a variety of enzymatic, chemical and biophysical influences, and accumulate damage due to their low turnover. Aging of elastin and elastic fibers involves enzymatic degradation, oxidative damage, glycation, calcification, aspartic acid racemization, binding of lipids and lipid peroxidation products, carbamylation and mechanical fatigue. These processes can trigger an impairment or loss of elastic fiber function and are associated with severe pathologies. There are different inherited or acquired pathological conditions, which influence the structure and function of elastic fibers and microfibrils predominantly in the cardiorespiratory system and skin. Inherited elastic-fiber pathologies have a direct or indirect impact on elastic-fiber formation due to mutations in the fibrillin genes (fibrillinopathies), in the elastin gene (elastinopathies) or in genes encoding proteins that are associated with microfibrils or elastic fibers. Acquired elastic-fiber pathologies appear age-related or as a result of multiple factors impairing tissue homeostasis. This review gives an overview on the fate of elastic fibers over the human lifespan in health and disease.

Monogenic Immune Diseases Provide Insights Into the Mechanisms and Treatment of Chronic Graft-Versus-Host Disease

Chronic graft-versus-host disease (GvHD) has become a leading cause of morbidity and mortality following allogeneic hematopoietic stem cell transplantation (HSCT) and can burden patients with devastating and lifelong health effects. Our understanding of the pathogenic mechanisms underlying chronic GvHD remains incomplete and this lack of understanding is reflected by lack of clear therapeutic approaches to steroid refractory disease. Observations predominantly from mouse models and human correlative studies currently support a three phase model for the initiation and development of chronic GvHD: 1) early inflammation and tissue damage triggers the innate immune system. This leads to inflammatory cytokine/chemokine patterns that recruit effector immune cell populations; 2) chronic inflammation causes the loss of central and peripheral tolerance mechanisms leading to emergence of pathogenic B and T cell populations that promote autoimmune and alloimmune reactions; 3) the dysregulated immunity causes altered macrophage polarization, aberrant tissue repair leading to scarring and end organ fibrosis. This model has led to the evaluation of many new therapies aimed at limiting inflammation, targeting dysregulated signaling pathways and restoring tolerance mechanisms. However, chronic GvHD is a multisystem disease with complex clinical phenotypes and it remains unclear as to which cluster of patients will respond best to specific therapeutic strategies. However, it is possible to gain novel insights from immune-related monogenic diseases. These diseases either share common clinical manifestations, replicate steps from the three phase chronic GvHD model or serve as surrogates for perfectly targeted drugs being investigated in chronic GvHD therapy. In this review, we will summarize the evidence from these monogenic immune related diseases that provide insight into pathogenic pathways in chronic GvHD, rationales for current therapies and novel directions for future drug discovery.

Anesthetic implications of a pediatric patient with stiff skin syndrome: A case report

Stiff skin syndrome is a rare genetic disorder that is present in infancy or early childhood. It is characterized by hard, inflexible skin and limited joint mobility making anesthetic management of these patients challenging. Their limited neck flexibility and chest wall rigidity make intubation and mask ventilation difficult. Intraoperative positioning can be challenging due to joint contractures and potential entrapment peripheral neuropathy. Even though peripheral intravenous access can be relatively easy, central venous cannulation may be problematic due to the hard skin overlying the entry sites. Our case report details the anesthetic management and considerations of a pediatric patient with stiff skin syndrome.

The fibrillin microfibril/elastic fibre network: A critical extracellular supramolecular scaffold to balance skin homoeostasis

Supramolecular networks composed of fibrillins (fibrillin-1 and fibrillin-2) and associated ligands form intricate cellular microenvironments which balance skin homoeostasis and direct remodelling. Fibrillins assemble into microfibrils which are not only indispensable for conferring elasticity to the skin, but also control the bioavailability of growth factors targeted to the extracellular matrix architecture. Fibrillin microfibrils (FMF) represent the core scaffolds for elastic fibre formation, and they also decorate the surface of elastic fibres and form independent networks. In normal dermis, elastic fibres are suspended in a three-dimensional basket-like lattice of FMF intersecting basement membranes at the dermal-epidermal junction and thus conferring pliability to the skin. The importance of FMF for skin homoeostasis is illustrated by the clinical features caused by mutations in the human fibrillin genes (FBN1, FBN2), summarized as "fibrillinopathies." In skin, fibrillin mutations result in phenotypes ranging from thick, stiff and fibrotic skin to thin, lax and hyperextensible skin. The most plausible explanation for this spectrum of phenotypic outcomes is that FMF regulate growth factor signalling essential for proper growth and homoeostasis of the skin. Here, we will give an overview about the current understanding of the underlying pathomechanisms leading to fibrillin-dependent fibrosis as well as forms of cutis laxa caused by mutational inactivation of FMF-associated ligands.

A case of Myhre syndrome mimicking juvenile scleroderma

BACKGROUND

Myhre syndrome is a genetic disorder caused by gain of function mutations in the SMAD Family Member 4 (SMAD4) gene, resulting in progressive, proliferative skin and organ fibrosis. Skin thickening and joint contractures are often the main presenting features of the disease and may be mistaken for juvenile scleroderma.

CASE PRESENTATION

We report a case of a 13 year-old female presenting with widespread skin thickening and joint contractures from infancy. She was diagnosed with diffuse cutaneous systemic sclerosis, and treatment with corticosteroids and subcutaneous methotrexate recommended. There was however disease progression prompting genetic testing. This identified a rare heterozygous pathogenic variant c.1499 T > C (p.Ile500Thr) in the SMAD4 gene, suggesting a diagnosis of Myhre syndrome. Securing a molecular diagnosis in this case allowed the cessation of immunosuppression, thus reducing the burden of unnecessary and potentially harmful treatment, and allowing genetic counselling.

CONCLUSION

Myhre Syndrome is a rare genetic mimic of scleroderma that should be considered alongside several other monogenic diseases presenting with pathological fibrosis from early in life. We highlight this case to provide an overview of these genetic mimics of scleroderma, and highlight the molecular pathways that can lead to pathological fibrosis. This may provide clues to the pathogenesis of sporadic juvenile scleroderma, and could suggest novel therapeutic targets.

Delineation of a new fibrillino-2-pathy with evidence for a role of FBN2 in the pathogenesis of carpal tunnel syndrome

BACKGROUND

Although carpal tunnel syndrome (CTS) is the most common form of peripheral entrapment neuropathy, its pathogenesis remains largely unknown. An estimated heritability index of 0.46 and an increased familial occurrence indicate that genetic factors must play a role in the pathogenesis.

METHODS AND RESULTS

We report on a family in which CTS occurred in subsequent generations at an unusually young age. Additional clinical features included brachydactyly and short Achilles tendons resulting in toe walking in childhood. Using exome sequencing, we identified a heterozygous variant (c.5009T>G; p.Phe1670Cys) in the fibrillin-2 (FBN2) gene that co-segregated with the phenotype in the family. Functional assays showed that the missense variant impaired integrin-mediated cell adhesion and migration. Moreover, we observed an increased transforming growth factor-β signalling and fibrosis in the carpal tissues of affected individuals. A variant burden test in a large cohort of patients with CTS revealed a significantly increased frequency of rare (6.7% vs 2.5%-3.4%, p<0.001) and high-impact (6.9% vs 2.7%, p<0.001) FBN2 variants in patient alleles compared with controls.

CONCLUSION

The identification of a novel FBN2 variant (p.Phe1670Cys) in a unique family with early onset CTS, together with the observed increased frequency of rare and high-impact FBN2 variants in patients with sporadic CTS, strongly suggest a role of FBN2 in the pathogenesis of CTS.

Calpain 9 as a therapeutic target in TGFβ-induced mesenchymal transition and fibrosis

Fibrosis is a common pathologic outcome of chronic disease resulting in the replacement of normal tissue parenchyma with a collagen-rich extracellular matrix produced by myofibroblasts. Although the progenitor cell types and cellular programs giving rise to myofibroblasts through mesenchymal transition can vary between tissues and diseases, their contribution to fibrosis initiation, maintenance, and progression is thought to be pervasive. Here, we showed that the ability of transforming growth factor-β (TGFβ) to efficiently induce myofibroblast differentiation of cultured epithelial cells, endothelial cells, or quiescent fibroblasts is dependent on the induced expression and activity of dimeric calpains, a family of non-lysosomal cysteine proteases that regulate a variety of cellular events through posttranslational modification of diverse substrates. siRNA-based gene silencing demonstrated that TGFβ-induced mesenchymal transition of a murine breast epithelial cell line was dependent on induction of expression of calpain 9 (CAPN9), an isoform previously thought to be restricted to the gastrointestinal tract. Mice lacking functional CAPN9 owing to biallelic targeting of Capn9 were viable and fertile but showed overt protection from bleomycin-induced lung fibrosis, carbon tetrachloride-induced liver fibrosis, and angiotensin II-induced cardiac fibrosis and dysfunction. A predicted loss-of-function allele of CAPN9 is common in Southeast Asia, with the frequency of homozygosity matching the prediction of Hardy-Weinberg equilibrium. Together with the highly spatially restricted pattern of CAPN9 expression under physiologic circumstances and the heartiness of the murine knockout, these data provide a strong signature for tolerance of therapeutic strategies for fibrosis aimed at CAPN9 antagonism.

Pro-Fibrotic Phenotype in a Patient with Segmental Stiff Skin Syndrome via TGF-β Signaling Overactivation

Transforming growth factor β (TGF-β) superfamily signaling pathways are ubiquitous and essential for several cellular and physiological processes. The overexpression of TGF-β results in excessive fibrosis in multiple human disorders. Among them, stiff skin syndrome (SSS) is an ultrarare and untreatable condition characterized by the progressive thickening and hardening of the dermis, and acquired joint limitations. SSS is distinct in a widespread form, caused by recurrent germline variants of FBN1 encoding a key molecule of the TGF-β signaling, and a segmental form with unknown molecular basis. Here, we report a 12-year-old female with segmental SSS, affecting the right upper limb with acquired thickening of the dermis evident at the magnetic resonance imaging, and progressive limitation of the elbow and shoulder. To better explore the molecular and cellular mechanisms that drive segmental SSS, several functional studies on patient's fibroblasts were employed. We hypothesized an impairment of TGF-β signaling and, consequently, a dysregulation of the associated downstream signaling. Lesional fibroblast studies showed a higher phosphorylation level of extracellular signal-regulated kinase 1/2 (ERK1/2), increased levels of nuclear factor-kB (NFkB), and a nuclear accumulation of phosphorylated Smad2 via Western blot and microscopy analyses. Quantitative PCR expression analysis of genes encoding key extracellular matrix proteins revealed increased levels of COL1A1, COL3A1, AGT, LTBP and ITGB1, while zymography assay reported a reduced metalloproteinase 2 enzymatic activity. In vitro exposure of patient's fibroblasts to losartan led to the partial restoration of normal transforming growth factor β (TGF-β) marker protein levels. Taken together, these data demonstrate that in our patient, segmental SSS is characterized by the overactivation of multiple TGF-β signaling pathways, which likely results in altered extracellular matrix composition and fibroblast homeostasis. Our results for the first time reported that aberrant TGF-β signaling may drive the pathogenesis of segmental SSS and might open the way to novel therapeutic approaches.

Acromicric dysplasia with stiff skin syndrome-like severe cutaneous presentation in an 8-year-old boy with a missense FBN1 mutation: Case report and literature review

Background

Acromicric dysplasia is a rare heritable short‐stature syndrome with joint stiffness and varying degrees of cutaneous hardness. Stiff skin syndrome is a rare connective tissue disorder characterized by diffusely thick and hard skin from the time of birth. Heterozygous point mutations in the FBN1 have been proposed as the predominant cause of both diseases.

Methods

By performing skin biopsy, X‐ray imaging, electrocardiography, as well as whole‐genome sequencing and Sanger sequencing, we diagnosed an 8‐year‐old Chinese boy as acromicric dysplasia with severe skin stiffness caused by a heterogeneous mutation in the FBN1.

Results

The patient presented with skin tightness, wrist and ankle stiffness, short stature and limbs, several deformed joints in the extremities, cone‐shaped epiphyses, and distinct facial features. He also had a patent foramen ovale and frequent respiratory infections. Skin biopsy showed thickened dermis and excessive collagen aggregation. Alcian blue staining indicated dermal mucopolysaccharide deposition. Mutation analysis revealed a heterozygous missense mutation, c.5243G>A (p.Cys1748Tyr), in exon 42 of the FBN1.

Conclusion

This is a report about acromicric dysplasia with stiff skin syndrome‐like severe cutaneous presentation caused by a single hotspot mutation, further revealing the gene pleiotropy of FBN1.

Mosaicism in genodermatoses

Genodermatoses are inherited disorders presenting with cutaneous manifestations with or without the involvement of other systems. The majority of these disorders, particularly in cases that present with a cutaneous patterning, may be explained in the context of genetic mosaicism. Despite the barriers to the genetic analysis of mosaic disorders, next-generation sequencing has led to a substantial progress in understanding their pathogenesis, which has significant implications for the clinical management and genetic counseling. Advances in paired and deep sequencing technologies in particular have made the study of mosaic disorders more feasible. In this review, we provide an overview of genetic mosaicism as well as mosaic cutaneous disorders and the techniques required to study them.

A Rare Mutation in SMAD9 Associated With High Bone Mass Identifies the SMAD-Dependent BMP Signaling Pathway as a Potential Anabolic Target for Osteoporosis

Novel anabolic drug targets are needed to treat osteoporosis. Having established a large national cohort with unexplained high bone mass (HBM), we aimed to identify a novel monogenic cause of HBM and provide insight into a regulatory pathway potentially amenable to therapeutic intervention. We investigated a pedigree with unexplained HBM in whom previous sequencing had excluded known causes of monogenic HBM. Whole exome sequencing identified a rare (minor allele frequency 0.0023), highly evolutionarily conserved missense mutation in SMAD9 (c.65T>C, p.Leu22Pro) segregating with HBM in this autosomal dominant family. The same mutation was identified in another two unrelated individuals both with HBM. In silico protein modeling predicts the mutation severely disrupts the MH1 DNA-binding domain of SMAD9. Affected individuals have bone mineral density (BMD) Z-scores +3 to +5, mandible enlargement, a broad frame, torus palatinus/mandibularis, pes planus, increased shoe size, and a tendency to sink when swimming. Peripheral quantitative computed tomography (pQCT) measurement demonstrates increased trabecular volumetric BMD and increased cortical thickness conferring greater predicted bone strength; bone turnover markers are low/normal. Notably, fractures and nerve compression are not found. Both genome-wide and gene-based association testing involving estimated BMD measured at the heel in 362,924 white British subjects from the UK Biobank Study showed strong associations with SMAD9 (PGWAS = 6 × 10-16 ; PGENE = 8 × 10-17 ). Furthermore, we found Smad9 to be highly expressed in both murine cortical bone-derived osteocytes and skeletal elements of zebrafish larvae. Our findings support SMAD9 as a novel HBM gene and a potential novel osteoanabolic target for osteoporosis therapeutics. SMAD9 is thought to inhibit bone morphogenetic protein (BMP)-dependent target gene transcription to reduce osteoblast activity. Thus, we hypothesize SMAD9 c.65T>C is a loss-of-function mutation reducing BMP inhibition. Lowering SMAD9 as a potential novel anabolic mechanism for osteoporosis therapeutics warrants further investigation. © 2019 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

A novel ADAMTS17 variant that causes Weill-Marchesani syndrome 4 alters fibrillin-1 and collagen type I deposition in the extracellular matrix

Weill-Marchesani syndrome (WMS) is a rare genetic disorder that affects the musculoskeletal system, the eye, and the cardiovascular system. Individuals with WMS present with short stature, joint contractures, thick skin, microspherophakia, small and dislocated lenses, and cardiac valve anomalies. WMS can be caused by recessive mutations in ADAMTS10 (WMS 1), ADAMTS17 (WMS 4), or LTBP2 (WMS 3), or by dominant mutations in fibrillin-1 (FBN1) (WMS 2); all genes encode secreted extracellular matrix (ECM) proteins. Individuals with WMS 4 due to ADAMTS17 mutations appear to have less severe cardiac involvement and present predominantly with the musculoskeletal and ocular features of WMS. ADAMTS17 is a member of the ADAMTS family of secreted proteases and directly binds to fibrillins. Here we report a novel pathogenic variant in ADAMTS17 that causes WMS 4 in an individual with short stature, brachydactyly, and small, spherical, and dislocated lenses. We provide biochemical and cell biological insights in the pathomechanisms of WMS 4, which also suggest potential biological functions for ADAMTS17. We show that the variant in ADAMTS17 prevents its secretion and we found intracellular accumulation of fibrillin-1 and collagen type I in patient-derived skin fibroblasts. In accordance, transmission electron microscopy revealed elastic fiber abnormalities, decreased collagen fibril diameters, and intracellular collagen accumulation in the dermis of the proband. Together, the data indicate a possible role for ADAMTS17 in the secretion of fibrillin-1 and collagen type I or in their early assembly in the pericellular matrix or the ECM.

The role of fibrillin and microfibril binding proteins in elastin and elastic fibre assembly

Fibrillin is a large evolutionarily ancient extracellular glycoprotein that assembles to form beaded microfibrils which are essential components of most extracellular matrices. Fibrillin microfibrils have specific biomechanical properties to endow animal tissues with limited elasticity, a fundamental feature of the durable function of large blood vessels, skin and lungs. They also form a template for elastin deposition and provide a platform for microfibril-elastin binding proteins to interact in elastic fibre assembly. In addition to their structural role, fibrillin microfibrils mediate cell signalling via integrin and syndecan receptors, and microfibrils sequester transforming growth factor (TGF)β family growth factors within the matrix to provide a tissue store which is critical for homeostasis and remodelling.

A disease-associated mutation in fibrillin-1 differentially regulates integrin-mediated cell adhesion

Fibrillins serve as scaffolds for the assembly of elastic fibers that contribute to the maintenance of tissue homeostasis and regulate growth factor signaling in the extracellular space. Fibrillin-1 is a modular glycoprotein that includes 7 latent transforming growth factor β (TGFβ)-binding protein-like (TB) domains and mediates cell adhesion through integrin binding to the RGD motif in its 4th TB domain. A subset of missense mutations within TB4 cause stiff skin syndrome (SSS), a rare autosomal dominant form of scleroderma. The fibrotic phenotype is thought to be regulated by changes in the ability of fibrillin-1 to mediate integrin binding. We characterized the ability of each RGD-binding integrin to mediate cell adhesion to fibrillin-1 or a disease-causing variant. Our data show that 7 of the 8 RGD-binding integrins can mediate adhesion to fibrillin-1. A single amino acid substitution responsible for SSS (W1570C) markedly inhibited adhesion mediated by integrins α5β1, αvβ5, and αvβ6, partially inhibited adhesion mediated by αvβ1, and did not inhibit adhesion mediated by α8β1 or αIIbβ3. Adhesion mediated by integrin αvβ3 depended on the cell surface expression level. In the SSS mutant background, the presence of a cysteine residue in place of highly conserved tryptophan 1570 alters the conformation of the region containing the exposed RGD sequence within the same domain to differentially affect fibrillin's interactions with distinct RGD-binding integrins.