Dysregulation of the immune response in TGF-ß signalopathies.

The transforming growth factor-ß (TGF-ß) family of cytokines exerts pleiotropic functions during embryonic development, tissue homeostasis and repair as well as within the immune system. Single gene defects in individual component of this signaling machinery cause defined Mendelian diseases associated with aberrant activation of TGF-ß signaling, ultimately leading to impaired development, immune responses or both. Gene defects that affect members of the TGF-ß cytokine family result in more restricted phenotypes, while those affecting downstream components of the signaling machinery induce broader defects. These rare disorders, also known as TGF-ß signalopathies, provide the unique opportunity to improve our understanding of the role and the relevance of the TGF-ß signaling in the human immune system. Here, we summarize this elaborate signaling pathway, review the diverse clinical presentations and immunological phenotypes observed in these patients and discuss the phenotypic overlap between humans and mice genetically deficient for individual components of the TGF-ß signaling cascade.

Generation and validation of an iPSC line (BBANTWi008-A) from a Loeys-Dietz Syndrome type 3 patient.

Loeys-Dietz Syndrome (LDS) is an autosomal dominant connective tissue disorder. The major hallmark of LDS is thoracic aortic aneurysm and dissection (TAAD). We generated an induced pluripotent stem cell (iPSC) line of a severely affected LDS patient carrying a pathogenic SMAD3 p.Arg287Gln variant. Peripheral blood mononuclear cells were reprogrammed using non-integrating Sendai viral vectors. The autonomous pluripotency state of the resulting iPSC model was proven by the presence of pluripotency markers, trilineage differentiation potential and absence of the Sendai vector backbone. This iPSC line can be used to study and/or therapeutically target the cellular pathomechanisms of SMAD3-related LDS.

TGF-ß Signaling-Related Genes and Thoracic Aortic Aneurysms and Dissections.

Transforming growth factor-ß (TGF)-ß signaling plays a crucial role in the development and maintenance of various organs, including the vasculature. Accordingly, the mutations in TGF-ß signaling pathway-related genes cause heritable disorders of the connective tissue, such as Marfan syndrome (MFS), Loeys-Dietz syndrome (LDS), and Shprintzen-Goldberg syndrome (SGS), and these syndromes may affect skeletal, ocular, pulmonary, and cardiovascular systems. Aortic root aneurysms are common problems that can result in aortic dissection or rupture, which is the leading cause of sudden death in the natural history of MFS and LDS, and recent improvements in surgical treatment have improved life expectancy. However, there is currently no genotype-specific medical treatment. Accumulating evidence suggest that not only structural weakness of connective tissue but also increased TGF-ß signaling contributes to the complicated pathogenesis of aortic aneurysm formation, but a comprehensive understanding of governing molecular mechanisms remains lacking. Inhibition of angiotensin II receptor signaling and endothelial dysfunction have gained attention as a possible MFS treatment strategy, but interactions with TGF-ß signaling remain elusive. Heterozygous loss-of-function mutations in TGF-ß receptors 1 and 2 (TGFBR1 and TGFBR2) cause LDS, but TGF-ß signaling is activated in the aorta (referred to as the TGF-ß paradox) by mechanisms yet to be elucidated. In this review, we present and discuss the current understanding of molecular mechanisms responsible for aortopathies of MFS and related disorders.

Cross-ancestry genome-wide association analysis of corneal thickness strengthens link between complex and Mendelian eye diseases.

Central corneal thickness (CCT) is a highly heritable trait associated with complex eye diseases such as keratoconus and glaucoma. We perform a genome-wide association meta-analysis of CCT and identify 19 novel regions. In addition to adding support for known connective tissue-related pathways, pathway analyses uncover previously unreported gene sets. Remarkably, >20% of the CCT-loci are near or within Mendelian disorder genes. These included FBN1, ADAMTS2 and TGFB2 which associate with connective tissue disorders (Marfan, Ehlers-Danlos and Loeys-Dietz syndromes), and the LUM-DCN-KERA gene complex involved in myopia, corneal dystrophies and cornea plana. Using index CCT-increasing variants, we find a significant inverse correlation in effect sizes between CCT and keratoconus (r = -0.62, P = 5.30 × 10-5) but not between CCT and primary open-angle glaucoma (r = -0.17, P = 0.2). Our findings provide evidence for shared genetic influences between CCT and keratoconus, and implicate candidate genes acting in collagen and extracellular matrix regulation.

Generation of an induced pluripotent stem cell line from a Loeys-Dietz syndrome patient with transforming growth factor-beta receptor-2 gene mutation.

Loeys-Dietz syndrome (LDS) is an autosomal-dominant connective tissue disorder, commonly caused by genetic mutation of transforming growth factor-beta receptor (TGFBR)-1 or TGFBR2. This study describes the generation of human induced pluripotent stem cells (hiPSCs) from peripheral blood mononuclear cells obtained from an LDS patient with TGFBR2 mutation (R193W). Analysis confirmed the cells had a normal karyotype, expressed typical pluripotency markers, had the ability to differentiate into all three germ layers in vivo, and retained the TGFBR2 mutation from the derived hiPSCs. This iPSC line represents a potentially useful tool for investigating LDS disease mechanisms.

TGF-ß signalopathies as a paradigm for translational medicine.

This review focusses on impact of a better knowledge of pathogenic mechanisms of Marfan and related disorders on their treatment strategies. It was long believed that a structural impairment formed the basis of Marfan syndrome as deficiency in the structural extracellular matrix component, fibrillin-1 is the cause of Marfan syndrome. However, the study of Marfan mouse models has revealed the strong involvement of the transforming growth factor-ß signalling pathway in the pathogenesis of Marfan. Similarly, this pathway was demonstrated to be key in the pathogenesis of Loeys-Dietz and Shprintzen-Goldberg syndrome. The elucidation of the underlying pathogenic mechanisms has led to new treatment strategies, targeting the overactive TGF-ß pathway. Various clinical trials are currently investigating the potential new treatment options. A meta-analysis will contribute to a better understanding of the various trial results.

Dysregulated TGF-ß signaling alters bone microstructure in a mouse model of Loeys-Dietz syndrome.

Loeys-Dietz syndrome (LDS) is a connective tissue disorder characterized by vascular and skeletal abnormalities resembling Marfan syndrome, including a predisposition for pathologic fracture. LDS is caused by heterozygous mutations in the genes encoding transforming growth factor-ß (TGF-ß) type 1 and type 2 receptors. In this study, we characterized the skeletal phenotype of mice carrying a mutation in the TGF-ß type 2 receptor associated with severe LDS in humans. Cortical bone in LDS mice showed significantly reduced tissue area, bone area, and cortical thickness with increased eccentricity. However, no significant differences in trabecular bone volume were observed. Dynamic histomorphometry performed in calcein-labeled mice showed decreased mineral apposition rates in cortical and trabecular bone with normal numbers of osteoblasts and osteoclasts. Mechanical testing of femurs by three-point bending revealed reduced femoral strength and fracture resistance. In vitro, osteoblasts from LDS mice demonstrated increased mineralization with enhanced expression of osteoblast differentiation markers compared with control cells. These changes were associated with impaired TGF-ß1-induced Smad2 and Erk1/2 phosphorylation and upregulated TGF-ß1 ligand mRNA expression, compatible with G357W as a loss-of-function mutation in the TGF-ß type 2 receptor. Paradoxically, phosphorylated Smad2/3 in cortical osteocytes measured by immunohistochemistry was increased relative to controls, possibly suggesting the cross-activation of TGF-ß-related receptors. The skeletal phenotype observed in the LDS mouse closely resembles the principal structural features of bone in humans with LDS and establishes this mouse as a valid in vivo model for further investigation of TGF-ß receptor signaling in bone.

Aortic aneurysms in Loeys-Dietz syndrome - a tale of two pathways?

Loeys-Dietz syndrome (LDS) is a connective tissue disorder that is characterized by skeletal abnormalities, craniofacial malformations, and a high predisposition for aortic aneurysm. In this issue of the JCI, Gallo et al. developed transgenic mouse strains harboring missense mutations in the genes encoding type I or II TGF-ß receptors. These mice exhibited several LDS-associated phenotypes. Despite being functionally defective, the mutated receptors enhanced TGF-ß signaling in vivo, inferred by detection of increased levels of phosphorylated Smad2. Aortic aneurysms in these LDS mice were ablated by treatment with the Ang II type 1 (AT1) receptor antagonist losartan. The results from this study will foster further interest into the potential therapeutic implications of AT1 receptor antagonists.

Angiotensin II-dependent TGF-ß signaling contributes to Loeys-Dietz syndrome vascular pathogenesis.

Loeys-Dietz syndrome (LDS) is a connective tissue disorder that is characterized by a high risk for aneurysm and dissection throughout the arterial tree and phenotypically resembles Marfan syndrome. LDS is caused by heterozygous missense mutations in either TGF-ß receptor gene (TGFBR1 or TGFBR2), which are predicted to result in diminished TGF-ß signaling; however, aortic surgical samples from patients show evidence of paradoxically increased TGF-ß signaling. We generated 2 knockin mouse strains with LDS mutations in either Tgfbr1 or Tgfbr2 and a transgenic mouse overexpressing mutant Tgfbr2. Knockin and transgenic mice, but not haploinsufficient animals, recapitulated the LDS phenotype. While heterozygous mutant cells had diminished signaling in response to exogenous TGF-ß in vitro, they maintained normal levels of Smad2 phosphorylation under steady-state culture conditions, suggesting a chronic compensation. Analysis of TGF-ß signaling in the aortic wall in vivo revealed progressive upregulation of Smad2 phosphorylation and TGF-ß target gene output, which paralleled worsening of aneurysm pathology and coincided with upregulation of TGF-ß1 ligand expression. Importantly, suppression of Smad2 phosphorylation and TGF-ß1 expression correlated with the therapeutic efficacy of the angiotensin II type 1 receptor antagonist losartan. Together, these data suggest that increased TGF-ß signaling contributes to postnatal aneurysm progression in LDS.

Hand and fibrillin-1 deposition abnormalities in Loeys-Dietz syndrome--expanding the clinical spectrum.

Loeys-Dietz syndrome (LDS) is an autosomal dominant connective tissue disorder characterized by hypertelorism, bifid uvula, cleft palate and arterial tortuosity. We report on a patient with LDS, bearing mutation in the TGFßR2 gene, whose prenatal examination demonstrated clenched fists and club feet, suggesting arthrogryposis multiplex congenita. Postnatal assessment showed digital abnormalities, including brachydactyly, camptodactyly, partial syndactyly and absent distal phalanges. With the lack of fibrillin-1 microfibril deposition as well as impaired and inadequate elastic fiber assembly in our patient's fibroblasts, we speculate that the skeletal abnormalities seen in this patient with LDS are the result of lack of these components in embryonal perichondrium and in blood vessels. We suggest that LDS should be included in the differential diagnosis of joint contractures seen pre and postnatally. Prenatal diagnosis of LDS would be important in parental counseling and early post natal diagnosis could prompt treatment before the development of detrimental vascular complications.

TGFß receptor mutations impose a strong predisposition for human allergic disease.

Transforming growth factor-ß (TGFß) is a multifunctional cytokine that plays diverse roles in physiologic processes as well as human disease, including cancer, heart disease, and fibrotic disorders. In the immune system, TGFß regulates regulatory T cell (Treg) maturation and immune homeostasis. Although genetic manipulation of the TGFß pathway modulates immune tolerance in mouse models, the contribution of this pathway to human allergic phenotypes is not well understood. We demonstrate that patients with Loeys-Dietz syndrome (LDS), an autosomal dominant disorder caused by mutations in the genes encoding receptor subunits for TGFß, TGFBR1 and TGFBR2, are strongly predisposed to develop allergic disease, including asthma, food allergy, eczema, allergic rhinitis, and eosinophilic gastrointestinal disease. LDS patients exhibited elevated immunoglobulin E levels, eosinophil counts, and T helper 2 (TH2) cytokines in their plasma. They had an increased frequency of CD4(+) T cells that expressed both Foxp3 and interleukin-13, but retained the ability to suppress effector T cell proliferation. TH2 cytokine-producing cells accumulated in cultures of naïve CD4(+) T cells from LDS subjects, but not controls, after stimulation with TGFß, suggesting that LDS mutations support TH2 skewing in naïve lymphocytes in a cell-autonomous manner. The monogenic nature of LDS demonstrates that altered TGFß signaling can predispose to allergic phenotypes in humans and underscores a prominent role for TGFß in directing immune responses to antigens present in the environment and foods. This paradigm may be relevant to nonsyndromic presentations of allergic disease and highlights the potential therapeutic benefit of strategies that inhibit TGFß signaling.

Bone lessons from Marfan syndrome and related disorders: fibrillin, TGF-B and BMP at the balance of too long and too short.

The extracellular matrix (ECM) is a complex entity with structural proteins (such as fibrillins, collagen, elastin), ground substance (proteoglycans), modifying enzymes (ADAMTS, PLOD, lysyloxidases (LOX)) and cytokines that regulate morphogenesis, growth, homeostasis and repair (transforming growth factor-beta [TGF-beta], bone morphogenic protein [BMP]). Over the last decade, the intimate relationship between structural proteins and these growth factors has emerged. The study of the extracellular matrix in human conditions and relevant mouse models is gradually unmasking the key role of these structural molecules in the regulation of the bio-availability of these growth factors. Major progress has been made in the study of the cardiovascular system (1) and the first clues in the skeletal system have emerged. (2) In this review, we will discuss the clinical, molecular, and pathogenic aspects of Marfan syndrome, Loeys-Dietz syndrome and related disorders with emphasis on the role of fibrillins and TGF-beta.

Transforming growth factor ß signaling perturbation in the Loeys-Dietz syndrome.

The transforming growth factor ß (TGFß) superfamily consists of multipotential secreting cytokines that mediate many key events in normal cellular growth and development, including differentiation, proliferation, motility, organization and death. TGFßs act as ligand for 3 classes of cell surface receptors, the transmembrane serine-threonine kinase receptors, TGFß receptor type I (TGFßRI) and type 2 (TGFßRII), and TGFßRIII receptors which include an ubiquitous extracellular ß-glycan and the membrane glycoprotein endoglin (CD105). Binding of TGFßs to their receptors initiates diverse cellular responses resulting in the phosphorilation of Smad proteins, which then translocate to the nucleus and regulate the transcription of target genes. Perturbation of TGFß signaling has been implicated in various human disorders including cancer, fibrosis and auto-immune diseases. Recently, mutations in TGFßR1 and TGFßR2 genes have been found in association with a continuum of clinical features with widespread vascular involvement. The extreme of clinical severity is represented by the Loeys-Dietz syndrome (LDS), an autosomal dominant disorder characterized by hypertelorism, bifid uvula, and/or cleft palate, and aggressive arteriopathy causing arterial tortuosity as well as life-threatening complications such as vascular aneurysms and dissections. Elastin disarray, loss of elastic fibre architecture and increased collagen expression in the arterial wall are the pathologic hallmark of LDS. In the present review article we will provide details on the activation of TGFß cascade, on the clinical features of LDS, as well as on the mechanisms of TGFß signaling perturbation leading to this condition and the potential role of the antagonism of TGFß activity in disease management.

Mitral valve disease in Marfan syndrome and related disorders.

Marfan syndrome (MFS) is a systemic disorder of the connective tissue with pleiotropic manifestations due to heterozygous FBN1 mutations and consequent upregulation of TGFß signaling in affected tissues. Myxomatous thickening and elongation of the mitral valve (MV) leaflets commonly occur in this condition. Investigation of murine models of this disease has led to improved understanding of the mechanisms that underlie many of the phenotypic features of MFS, including MV disease. Loeys-Dietz syndrome (LDS) is a related disorder due to heterozygous mutations in the genes encoding subunits of the TGFß receptor, and it may also involve the MV leaflets with similar elongation and thickening of the MV leaflets. Although the genetic basis and pathogenesis of nonsyndromic MV prolapse has been elusive to date, insights derived from monogenic disorders like MFS and LDS can be informative with regard to novel gene discovery and investigation into the pathogenesis of MV disease. This manuscript will review the prevalence of MV disease in MFS, its pathogenic basis as determined in mice with Fbn1 mutations, and ongoing studies that seek to better understand MV disease in the context of fibrillin-1 deficiency or excessive TGFß signaling.