Engineered vascular grafts lend unique insight to pathophysiology of aortic aneurysms.

Yang et al.1 generate tissue engineered blood vessels from hiPSC-derived smooth muscle cells harboring a mutation found in Loeys-Dietz syndrome. In vitro and in vivo data from these vessels provide new insight into the molecular physiology of aortic aneurysms and may create a paradigm for understanding a suite of vascular diseases.

Validating a Curvature-Based Marker of Cervical Carotid Tortuosity for Risk Assessment in Heritable Aortopathies.

BACKGROUND: Cervical arterial tortuosity is associated with adverse outcomes in Loeys-Dietz syndrome and other heritable aortopathies. METHODS AND RESULTS: A method to assess tortuosity based on curvature of the vessel centerline in 3-dimensional space was developed. We measured cervical carotid tortuosity in 65 patients with Loeys-Dietz syndrome from baseline computed tomography angiogram/magnetic resonance angiogram and all serial images during follow-up. Relations between baseline carotid tortuosity, age, aortic root diameter, and its change over time were compared. Patients with unoperated aortic roots were assessed for clinical end point (type A aortic dissection or aortic root surgery during 4 years of follow-up). Logistic regression was performed to assess the likelihood of clinical end point according to baseline carotid tortuosity. Total absolute curvature at baseline was 11.13±5.76 and was relatively unchanged at 8 to 10 years (fold change: 0.026±0.298, P=1.00), whereas tortuosity index at baseline was 0.262±0.131, with greater variability at 8 to 10 years (fold change: 0.302±0.656, P=0.818). Baseline total absolute curvature correlated with aortic root diameter (r=0.456, P=0.004) and was independently associated with aortic events during the 4-year follow-up (adjusted odds ratio [OR], 2.64 [95% CI, 1.02-6.85]). Baseline tortuosity index correlated with age (r=0.532, P<0.001) and was not associated with events (adjusted OR, 1.88 [95% CI, 0.79-4.51]). Finally, baseline total absolute curvature had good discrimination of 4-year outcomes (area under the curve=0.724, P=0.014), which may be prognostic or predictive. CONCLUSIONS: Here we introduce cervical carotid tortuosity as a promising quantitative biomarker with validated, standardized characteristics. Specifically, we recommend the adoption of a curvature-based measure, total absolute curvature, for early detection or monitoring of disease progression in Loeys-Dietz syndrome.

Bioengineered vascular grafts with a pathogenic TGFBR1 variant model aneurysm formation in vivo and reveal underlying collagen defects.

Thoracic aortic aneurysm (TAA) is a life-threatening vascular disease frequently associated with underlying genetic causes. An inadequate understanding of human TAA pathogenesis highlights the need for better disease models. Here, we established a functional human TAA model in an animal host by combining human induced pluripotent stem cells (hiPSCs), bioengineered vascular grafts (BVGs), and gene editing. We generated BVGs from isogenic control hiPSC-derived vascular smooth muscle cells (SMCs) and mutant SMCs gene-edited to carry a Loeys-Dietz syndrome (LDS)-associated pathogenic variant (TGFBR1A230T). We also generated hiPSC-derived BVGs using cells from a patient with LDS (PatientA230T/+) and using genetically corrected cells (Patient+/+). Control and experimental BVGs were then implanted into the common carotid arteries of nude rats. The TGFBR1A230T variant led to impaired mechanical properties of BVGs, resulting in lower burst pressure and suture retention strength. BVGs carrying the variant dilated over time in vivo, resembling human TAA formation. Spatial transcriptomics profiling revealed defective expression of extracellular matrix (ECM) formation genes in PatientA230T/+ BVGs compared with Patient+/+ BVGs. Histological analysis and protein assays validated quantitative and qualitative ECM defects in PatientA230T/+ BVGs and patient tissue, including decreased collagen hydroxylation. SMC organization was also impaired in PatientA230T/+ BVGs as confirmed by vascular contraction testing. Silencing of collagen-modifying enzymes with small interfering RNAs reduced collagen proline hydroxylation in SMC-derived tissue constructs. These studies demonstrated the utility of BVGs to model human TAA formation in an animal host and highlighted the role of reduced collagen modifying enzyme activity in human TAA formation.

Unraveling the genetic collagen connection: clinical and therapeutic insights on genetic connective tissue disorders.

Hereditary connective tissue disorders include more than 200 conditions affecting different organs and tissues, compromising the biological role of the extracellular matrix through interference in the synthesis, development, or secretion of collagen and/or its associated proteins. The clinical phenotype includes multiple signs and symptoms, usually nonspecific but of interest to rheumatologists because of musculoskeletal involvement. The patient´s journey to diagnosis is long, and physicians should include these disorders in their differential diagnoses of diseases with systemic involvement. In this review, insights for the diagnosis and treatment of osteogenesis imperfecta, hypermobility spectrum disorder/Ehlers-Danlos syndrome, Marfan, Loeys-Dietz, and Stickler syndromes are presented.

Functional analysis of cell lines derived from SMAD3-related Loeys-Dietz syndrome patients provides insights into genotype-phenotype relation.

RATIONALE: Pathogenic (P)/likely pathogenic (LP) SMAD3 variants cause Loeys-Dietz syndrome type 3 (LDS3), which is characterized by arterial aneurysms, dissections and tortuosity throughout the vascular system combined with osteoarthritis. OBJECTIVES: Investigate the impact of P/LP SMAD3 variants with functional tests on patient-derived fibroblasts and vascular smooth muscle cells (VSMCs), to optimize interpretation of SMAD3 variants. METHODS: A retrospective analysis on clinical data from individuals with a P/LP SMAD3 variant and functional analyses on SMAD3 patient-derived VSMCs and SMAD3 patient-derived fibroblasts, differentiated into myofibroblasts. RESULTS: Individuals with dominant negative (DN) SMAD3 variant in the MH2 domain exhibited more major events (66.7% vs. 44.0%, P = 0.054), occurring at a younger age compared to those with haploinsufficient (HI) variants. The age at first major event was 35.0 years [IQR 29.0-47.0] in individuals with DN variants in MH2, compared to 46.0 years [IQR 40.0-54.0] in those with HI variants (P = 0.065). Fibroblasts carrying DN SMAD3 variants displayed reduced differentiation potential, contrasting with increased differentiation potential in HI SMAD3 variant fibroblasts. HI SMAD3 variant VSMCs showed elevated SMA expression and altered expression of alternative MYH11 isoforms. DN SMAD3 variant myofibroblasts demonstrated reduced extracellular matrix formation compared to control cell lines. CONCLUSION: Distinguishing between P/LP HI and DN SMAD3 variants can be achieved by assessing differentiation potential, and SMA and MYH11 expression. The differences between DN and HI SMAD3 variant fibroblasts and VSMCs potentially contribute to the differences in disease manifestation. Notably, myofibroblast differentiation seems a suitable alternative in vitro test system compared to VSMCs.

Cardiovascular pathology, inheritance and prognosis in a familial cohort of Loeys-Dietz type III.

INTRODUCTION: Loeys-Dietz syndrome (LDS) is a heritable disease that is the result of dysregulation of the transforming growth factor beta (TGFß) pathway. The pathogenic variants associated with the condition are linked to aortic aneurysms and dissections along with other cardiovascular and non-cardiovascular abnormalities. LDS type III is associated with pathogenic variants in the SMAD3 gene responsible for signally in the TGFß pathway. Most of the current knowledge of LDS stems from studies of LDS I and II patient with limited data on large cohorts of LDS III patients. We sought to identify the prevalence and course of cardiovascular diseases in a large familial cohort of LDS III patients and also to compare these findings with a previously described cohort of similar size with the identical pathogenic variant. METHODS: The cohort was identified by systematic genetic screening of a familial cohort identified through a single proband. Data was collected from retrospective chart review of patients identified to be affected by the syndrome. RESULTS: Screening of 97 patients identified 19 patients (16 through genetic testing and 3 through phenotypic screening of untested direct descendants of genetically positive individuals). The prevalence of cardiovascular abnormalities was 84%. There was significant intrafamilial phenotypic variability within the cohort with the predominant cardiovascular abnormality being mitral valve disease followed by aortic disease. 92% of patients >18 years of age had osteoarthritis which is a further hallmark of LDS III. CONCLUSION: LDS III sets itself apart from the more widely studied LDS types I and II cardiovascular phenotypes by presenting later in life and tending to be more strongly associated with mitral valve disease.

Loeys-Dietz syndrome with a novel in-frame SMAD3 deletion diagnosed as a result of postpartum aortic dissection: A case report.

OBJECTIVE: Loeys-Dietz syndrome (LDS) is a rare, autosomal dominant connective tissue disorder which can aggressively affect the aortic vasculature. Limited information is available regarding its impact on pregnancy and postpartum outcomes. CASE REPORT: A pregnant 38-year-old nulliparous woman with mild aortic regurgitation and family history of aortic aneurysms presented with an aortic root measuring 49 mm. Despite concerns of an underlying connective tissue disorder, a definitive diagnosis was not reached. She delivered under strict blood pressure control, developed intractable uterine atony, and underwent uterine artery embolization. On the second postpartum day, aortic dissection was incidentally diagnosed, and aortic root replacement surgery was performed. Genetic testing revealed a novel in-frame SMAD3 deletion [NM_005902.4: c.703_708del, (p.Ile235_Ser236del)], leading to a diagnosis of LDS type 3. CONCLUSION: This case highlights the high postpartum aortic dissection risk in women with LDS, emphasizing the importance of early diagnosis in pregnant women with few clinical symptoms.

Familial visceral branch artery aneurysms in Loeys-Dietz syndrome.

Loeys-Dietz syndrome (LDS) is an autosomal dominant heritable disorder due to pathogenic variants in one of several genes involved in TGF-ß (transforming growth factor-beta) signalling. LDS is associated with aortic aneurysm and dissection. LDS may also lead to extra-aortic aneurysms, the majority of which occur in the head and neck vasculature. Visceral aneurysms are uncommon, and no cases of distal superior mesenteric artery (SMA) branch aneurysms in patients with LDS have been reported. Three related females with TGFBR1-related LDS developed distal SMA branch artery aneurysms involving the ileocolic and jejunal arteries. Endovascular or surgical intervention was performed in each. The presence and severity of arterial, craniofacial, and cutaneous features of LDS in these patients are variable. TGFBR1-related LDS may rarely lead to SMA branch artery aneurysms that can develop later in life. Surgical and endovascular procedures can successfully treat these aneurysms, but data to guide size thresholds and optimal treatment strategies are lacking.

Arterial tortuosity in pediatric Loeys-Dietz syndrome patients.

Loeys-Dietz syndrome (LDS) is an autosomal connective tissue disorder commonly presenting with hypertelorism, bifid uvula, aortic aneurysms, and arterial tortuosity. The aim of the present study was to investigate differences in tortuosity index (TI) between genotypes of LDS, possible progression over time and its use as an adjunctive prognostic tool alongside aortic dimensions to aid timely surgical planning in pediatric patients. A retrospective observational study of pediatric LDS patients referred to our center (November 2012-February 2021) was conducted. Using magnetic resonance angiography (MRA) with 3D maximum intensity projection volume-rendered angiogram, arterial TI was measured. Twenty three patients had genetically confirmed LDS with at least one head and neck MRA and 19 had no less than one follow-up MRA available. All patients presented arterial tortuosity. Patients with TGFBR2 variants had greater values of TI compared to patients with TGFB2 variants (p = 0.041). For patients who did not undergo surgery (n = 18), z-scores at the level of the sinus of Valsalva showed a significant correlation with vertebral TI (rs = 0.547). There was one death during follow-up. This study demonstrates that patients with LDS and TGFBR2 variants have greater values of TI than patients with TGFB2 variants and that greatest values of TI are associated with increased aortic root z-scores. Furthermore, as TI decreases over time, less frequent neuroimaging follow-up can be considered. Nevertheless, additional studies are needed to better define more accurate risk stratification and long-term surveillance in these patients.

Iatrogenic aortic dissection during aortic root replacement in an older Loeys-Dietz syndrome type III patient with no family history of aortic disease: a case report.

BACKGROUND: Iatrogenic aortic dissection during cardiac surgery is a rare but critical complication. At present, no strategies have been developed to prevent it. We herein report a case of intraoperative aortic dissection during aortic root replacement in an older patient with Loeys-Dietz syndrome type III who had no family history of aortic disease. CASE PRESENTATION: A 60-year-old man was admitted to the hospital for Stanford type B acute aortic dissection and given conservative treatment. He was found to have aortic root dilatation and severe aortic regurgitation. Thus, elective Bentall procedure was performed. Postoperative computed tomography showed new Stanford type A aortic dissection that may have developed due to aortic cannulation during surgery. The patient was given conservative treatment and successfully discharged to home at postoperative day 34. Although he had no family history of aortic disease, a genetic test revealed an unreported SMAD3 frameshift mutation (c.742_749dup, p. Gln252ThrfsTer7), and the patient was diagnosed with Loeys-Dietz syndrome type III. CONCLUSION: In patients with connective tissue disorder, aortic manipulations may become the cause of critical complications. Avoiding the use of invasive techniques, such as cannulation and cross-clamping, and implementing treatment strategies, such as perfusion from other sites than the aorta and open distal anastomosis, can prevent these complications, and may be useful treatment modalities. The possibility of connective tissue disease should be considered even if the patient is older and has no family history of aortic disease.

[Clinical and genetic analysis of a patient with Loeys-Dietz syndrome due to variant of TGFBR2 gene].

OBJECTIVE: To explore the genetic basis of a patient with clinically suspected Loeys-Dietz syndrome (LDS). METHODS: A child who had presented at Beijing Anzhen Hospital in September 2018 was selected as the study subject. Clinical data and family history of the patient were collected, along with peripheral blood samples of the proband and his parents. Whole exome sequencing (WES) was carried out through next-generation sequencing. RESULTS: Candidate variants were searched through bioinformatic analysis focusing on genes associated with hereditary aortic aneurysms. Candidate variant was verified by Sanger sequencing. The patient was found to have cardiovascular abnormalities including early-onset aortic dilatation and coarctation, and LDS syndrome was suspected. WES revealed that he has harbored a heterozygous c.1526G>T missense variant of the TGFBR2 gene. The same variant was not found in either parent and was predicted as likely pathogenic (PM1+PM2_Supporting+ PM6+PP3+PP4) based on the guidelines from the American College for Medical Genetics and Genomics (ACMG). CONCLUSION: The TGFBR2 c.1526G>T variant probably underlay the LDS in this patient and was unreported previously in China. Above finding has enriched the mutational spectrum of the TGFBR2 gene associated with the LDS and provided a basis for the genetic counseling for the patient.

A novel pathogenic variant located just upstream of the C-terminal Ser423-X-Ser425 phosphorylation motif in SMAD3 causing Loeys-Dietz syndrome.

OBJECTIVE: Loeys-Dietz syndrome (LDS) is a heritable disorder of connective tissue closely related to Marfan syndrome (MFS). LDS is caused by loss-of-function variants of genes that encode components of transforming growth factor-ß (TGF-ß) signaling; nevertheless, LDS type 1/2 caused by TGFBR1/2 pathogenic variants is frequently found to have paradoxical increases in TGF-ß signaling in the aneurysmal aortic wall. Here, we present a Japanese LDS family having a novel SMAD3 variant. METHODS: The proband was tested via clinical, genetic, and histological analyses. In vitro analysis was performed for pathogenic evaluation. RESULTS: The novel heterozygous missense variant of SMAD3 [c.1262G>A, p.(Cys421Tyr)], located just upstream of the C-terminal Ser423-X-Ser425 phosphorylation motif, was found in this instance of LDS type 3. This variant led to reduced phospho-SMAD3 (Ser423/Ser425) levels and transcription activity in vitro; however, a paradoxical upregulation of TGF-ß signaling was evident in the aortic wall. CONCLUSIONS: Our results revealed the presence of TGF-ß paradox in this case with the novel loss-of-function SMAD3 variant. The precise mechanism underlying the paradox is unknown, but further research is warranted to clarify the influence of the SMAD3 variant type and location on the LDS3 phenotype as well as the molecular mechanism leading to LDS3 aortopathy.

Cardiovascular involvement and prognosis in Loeys-Dietz syndrome.

BACKGROUND: Loeys-Dietz syndrome (LDS) is an inherited connective tissue disorder associated with aortic root enlargement and risk of thoracic aortic dissection (AD). Genetic examination is essential for diagnosis. AIMS: The study aimed at analysis of clinical data on cardiovascular involvement and management of LDS patients. METHODS: The study included carriers of LDS-associated genetic variants, identified between 2012 and 2022. Assessment of cardiovascular involvement was based on echocardiography and computed tomography angiography with quantitative assessment of arterial tortuosity. Involvement of other systems was also evaluated. We noted major cardiovascular events, including aortic events, defined as AD, elective aortic surgery, or otherwise unexplained sudden death. RESULTS: Thirty-four patients from 15 families were included, and five identified variants were novel. Probands' mean age was 41 years. Cardiovascular abnormalities, aortic involvement, aortic tortuosity, and tortuosity of cervical arteries were present in 79%, 71%, 68%, and 100% of carriers, respectively. First aortic events (9 A-type AD, 6 elective thoracic aortic surgeries, and one sudden death) occurred in 16 (47%) patients at a median age of 35 years. The youngest age at AD was 16 years, and 7 years for elective aneurysm repair. Second and third aortic events occurred in 9 and 4 patients, respectively. Eight patients (24%) experienced other major cardiovascular events. Aortic event-free survival was shorter in the presence of skin striae (P = 0.03), tended to be shorter in the presence of Marfanoid features (P = 0.06), and longer with TGFB2 variants (P = 0.06). CONCLUSIONS: LDS is associated with high burden of cardiovascular complications at a young age.

Anesthetic management of a child with Loeys-Dietz syndrome undergoing complete aortic arch replacement.

Loeys-Dietz syndrome (LDS) is a connective tissue disease related to ß-transforming growth factor mutations, which causes aneurysms formation, vascular tortuosity and skeletal manifestations. The prognosis is very poor, and mortality occurs at the age of 27 in patients without surgical treatment. Despite being diagnosed in childhood, is not usual surgical aortic replacement in children. We report a case of 12 years old child with LDS and multiple aneurysms in thoracic aorta, undergoing complete aortic arch replacement and our proposal for the anesthetic management, due to surgical complexity and implications in pediatric population.

El síndrome de Loeys-Dietz (SDL) es una enfermedad del tejido conectivo debida a mutaciones del factor de crecimiento transformador beta que provocan formación de aneurismas, malformaciones vasculares y esqueléticas. Tiene mal pronóstico y el fallecimiento sobreviene de media a los 27 años sin tratamiento quirúrgico. A pesar de diagnosticarse en la infancia, es infrecuente la cirugía en niños. Presentamos el caso de una niña de 12 años con SDL y aneurisma múltiple en aorta torácica, programada para recambio completo de arco aórtico, proponiendo estrategias para el manejo anestésico, dada la complejidad y las implicaciones de esta cirugía en la población pediátrica.

[Clinical and genetic characteristics of 12 cases of Loeys-Dietz syndrome].

OBJECTIVE: To summarize the clinical features and spectrum of genetic variants in 12 patients with Loeys-Dietz syndrome (LDS), and to explore the correlation between the type of genetic variants and clinical phenotypes. METHODS: Twelve patients suspected for LDS at Beijing Anzhen Hospital Affiliated to Capital Medical University from January 2015 to January 2022 were selected as the study subjects. Clinical data of the patients were collected. Genomic DNA was extracted from peripheral blood samples and subjected to genetic testing. Pathogenicity of candidate variants was analyzed. RESULTS: The clinical phenotypes of the 12 patients have mainly included cardiovascular, musculoskeletal, craniofacial, skin, ocular and other systemic signs. Four patients (patients 5-1, 5-2, 6, 7) have carried heterozygous missense variants of the TGFBR1 gene, 5 patients (patients 1-1, 1-2, 2, 3, 4) have carried heterozygous variants of the TGFBR2 gene, and 2 patients (patients 8-1, 8-2) had carried heterozygous frameshift variants of the TGFB3 gene. One patient (patient 9) had carried a heterozygous missense variant of the SMAD3 gene. Among these, TGFBR1 c.603T>G (p.1201M) and TGFB3 c.536delA (p.H179FS35) had not been reported previously. CONCLUSION: Variants of the TGFBR1, TGFBR2, SMAD3, TGFB2, TGFB3 and SMAD2 genes are mainly associated with LDS. The severity of the disease phenotype caused by the same variant may vary, whilst the clinical phenotype caused by different variant sites may be specific.

The Impact of Genetic Variability of TGF-Beta Signaling Biomarkers in Major Craniofacial Syndromes.

Craniofacial development is a complex process involving several signaling pathways, including the one regulated by the TGF-beta (TGF-ß) superfamily of growth factors. Isoforms of TGF-ß play a vital part in embryonic development, notably in craniofacial patterning. Consequently, pathogenic variants in their coding genes may result in a variety of orofacial and craniofacial malformations. Here, we review the impact of genetic variability of TGF-ß signaling biomarkers in major disorders, including palatal and lip clefts, dental anomalies, and craniofacial syndromes, such as the Loeys-Dietz syndrome (LDS) and Camurati-Engelmann disease. Cleft lip and cleft palate are associated with missense mutations in the TGFB1 and TGFB3 genes, while mutations in the LTBP3 gene encoding TGF-ß binding protein 3 may cause selective tooth agenesis. Oligodontia may also be caused by TGFB1-inactivating mutations and/or by variations in the GREM2 gene, which disrupt the activity of gremlin 2, a TGF-ß/bone morphogenetic protein (BMP4) signaling antagonist. CED may be caused by mutations in the TGFB1 gene, while the TGF-ß-related genetic background of LDS consists mostly of TGFBR1 and TGFBR2 mutations, which may also impact the above syndromes' vascular manifestations. The potential utility of the TGF-ß signaling pathway factors as biomarkers that correlate genetics with clinical outcome of craniofacial malformations is discussed.

Truncating variants in the penultimate exon of TGFBR1 escaping nonsense-mediated mRNA decay cause Loeys-Dietz syndrome.

Pathogenic variants in TGFBR1 are a common cause of Loeys-Dietz syndrome (LDS) characterized by life-threatening aortic and arterial disease. Generally, these are missense changes in highly conserved amino acids in the serine-threonine kinase domain. Conversely, nonsense, frameshift, or specific missense changes in the ligand-binding extracellular domain cause multiple self-healing squamous epithelioma (MSSE) lacking the cardiovascular phenotype. Here, we report on two novel variants in the penultimate exon 8 of TGFBR1 were identified in 3 patients from two unrelated LDS families: both were predicted to cause frameshift and premature stop codons (Gln448Profs*15 and Cys446Asnfs*4) resulting in truncated TGFBR1 proteins lacking the last 43 and 56 amino acid residues, respectively. These were classified as variants of uncertain significance based on current criteria. Transcript expression analyses revealed both mutant alleles escaped nonsense-mediated mRNA decay. Functional characterization in patient's dermal fibroblasts showed paradoxically enhanced TGFß signaling, as observed for pathogenic missense TGFBR1 changes causative of LDS. In summary, we expanded the allelic repertoire of LDS-associated TGFBR1 variants to include truncating variants escaping nonsense-mediated mRNA decay. Our data highlight the importance of functional studies in variants interpretation for correct clinical diagnosis.

Retinal detachment in Loeys-Dietz syndrome.

Loeys-Dietz syndrome (LDS) is an autosomal-dominant connective-tissue disorder with vascular and musculoskeletal abnormalities similar to Marfan syndrome. However, unlike Marfan, retinal detachment (RD) is rarely reported, and screening protocols do not currently feature ophthalmic assessment or RD counseling. We report a 5-generation family affected by LDS, where RD occurred in six eyes of four individuals. The proband was an 84-year-old male recently diagnosed with type-V LDS (TGFß3 pathogenic variant c.899G>A, p.(Arg300Gln)). Further investigation was undertaken into the family's medical history. The proband experienced bilateral rhegmatogenous RD at age 60, requiring emergency surgical repair. Other notable ophthalmic features include unusual keratometry, abnormal biometry, and severe hayfever requiring long-term sodium cromoglycate treatment. The proband's sister, father, and uncle had also experienced RDs, all prior to LDS diagnosis. This series demonstrates that RD risk may be significant in LDS, and on occasion the presenting clinical feature. We suggest ophthalmic examination should be added to the initial assessment LDS patients, and patients informed of the early warning symptoms of retinal detachment. As in Marfan syndrome, LDS patients may exhibit cornea plana and abnormal corneal topography, producing atypical biometry. They may also present with allergic conjunctivitis, and awareness of these signs might facilitate earlier diagnosis.

Chiari I Malformations and the Heritable Disorders of Connective Tissue.

The heritable disorders of connective tissue (HDCTs) are a heterogeneous group of inherited disorders caused by pathogenic variants in genes encoding a wide range of molecules involved in the structure and function of the extracellular matrix. Currently, more than 450 HDCTs are recognized. These include the Ehlers-Danlos syndrome (EDS), Marfan syndrome, Loeys-Dietz syndrome (LDS), Stickler syndrome, and a wide range of skeletal dysplasias. Recent evidence suggests that people with the HDCTs are at an increased risk of Chiari I malformation (CM1).