Hariyadarshi Pannu

Mutations in Transforming Growth Factor-β Receptor Type II Cause Familial Thoracic Aortic Aneurysms and Dissections

Background—A genetic predisposition for progressive enlargement of thoracic aortic aneurysms leading to type A dissection (TAAD) is inherited in an autosomal-dominant manner in up to 19% of patients, and a number of chromosomal loci have been identified for the condition. Having mapped a TAAD locus to 3p24–25, we sequenced the gene for transforming growth factor-&bgr; receptor type II (TGFBR2) to determine whether mutations in this gene resulted in familial TAAD. Methods and Results—We sequenced all 8 coding exons of TGFBR2 by using genomic DNA from 80 unrelated familial TAAD cases. We found TGFBR2 mutations in 4 unrelated families with familial TAAD who did not have Marfan syndrome. Affected family members also had descending aortic disease and aneurysms of other arteries. Strikingly, all 4 mutations affected an arginine residue at position 460 in the intracellular domain, suggesting a mutation “hot spot” for familial TAAD. Despite identical mutations in the families, assessment of linked polymorphisms suggested that these families were not distantly related. Structural analysis of the TGFBR2 serine/threonine kinase domain revealed that R460 is strategically located within a highly conserved region of this domain and that the amino acid substitutions resulting from these mutations will interfere with the receptor’s ability to transduce signals. Conclusion—Germline TGFBR2 mutations are responsible for the inherited predisposition to familial TAAD in 5% of these cases. Our results have broad implications for understanding the role of TGF-&bgr; signaling in the pathophysiology of TAAD.

Mutations in Smooth Muscle Alpha-Actin (ACTA2) Cause Coronary Artery Disease, Stroke, and Moyamoya Disease, Along with Thoracic Aortic Disease

The vascular smooth muscle cell (SMC)-specific isoform of alpha-actin (ACTA2) is a major component of the contractile apparatus in SMCs located throughout the arterial system. Heterozygous ACTA2 mutations cause familial thoracic aortic aneurysms and dissections (TAAD), but only half of mutation carriers have aortic disease. Linkage analysis and association studies of individuals in 20 families with ACTA2 mutations indicate that mutation carriers can have a diversity of vascular diseases, including premature onset of coronary artery disease (CAD) and premature ischemic strokes (including Moyamoya disease [MMD]), as well as previously defined TAAD. Sequencing of DNA from patients with nonfamilial TAAD and from premature-onset CAD patients independently identified ACTA2 mutations in these patients and premature onset strokes in family members with ACTA2 mutations. Vascular pathology and analysis of explanted SMCs and myofibroblasts from patients harboring ACTA2 suggested that increased proliferation of SMCs contributed to occlusive diseases. These results indicate that heterozygous ACTA2 mutations predispose patients to a variety of diffuse and diverse vascular diseases, including TAAD, premature CAD, ischemic strokes, and MMD. These data demonstrate that diffuse vascular diseases resulting from either occluded or enlarged arteries can be caused by mutations in a single gene and have direct implications for clinical management and research on familial vascular diseases.

Genetic Basis of Thoracic Aortic Aneurysms and Dissections: Focus on Smooth Muscle Cell Contractile Dysfunction

Thoracic aortic aneurysms leading to type A dissections (TAAD) can be inherited in isolation or in association with genetic syndromes, such as Marfan syndrome and Loeys-Dietz syndrome. When TAAD occurs in the absence of syndromic features, it is inherited in an autosomal dominant manner with decreased penetrance and variable expression, the disease is referred to as familial TAAD. Familial TAAD exhibits significant clinical and genetic heterogeneity. The first genes identified to cause TAAD were FBN1, TGFBR2, and TGFBR1. The identification and characterization of these genes suggested that increased TGF-beta signaling plays a role in pathogenesis. The recent discovery that mutations in the vascular smooth muscle cell (SMC)-specific beta-myosin (MYH11) and alpha-actin (ACTA2) can also cause this disorder has focused attention on the importance of the maintenance of SMC contractile function in preserving aortic structure and preventing TAAD.

Analysis of multigenerational families with thoracic aortic aneurysms and dissections due to TGFBR1 or TGFBR2 mutations

Background: Mutations in the transforming growth factor β receptor type I and II genes (TGFBR1 and TGFBR2) cause Loeys–Dietz syndrome (LDS), characterised by thoracic aortic aneurysms and dissections (TAAD), aneurysms and dissections of other arteries, craniosynostosis, cleft palate/bifid uvula, hypertelorism, congenital heart defects, arterial tortuosity, and mental retardation. TGFBR2 mutations can also cause TAAD in the absence of features of LDS in large multigenerational families, yet only sporadic LDS cases or parent–child pairs with TGFBR1 mutations have been reported to date. Methods: The authors identified TGFBR1 missense mutations in multigenerational families with TAAD by DNA sequencing. Clinical features of affected individuals were assessed and compared with clinical features of previously described TGFBR2 families. Results: Statistical analyses of the clinical features of the TGFBR1 cohort (n = 30) were compared with clinical features of TGFBR2 cohort (n = 77). Significant differences were identified in clinical presentation and survival based on gender in TGFBR1 families but not in TGFBR2 families. In families with TGFBR1 mutations, men died younger than women based on Kaplan–Meier survival curves. In addition, men presented with TAAD and women often presented with dissections and aneurysms of arteries other than the ascending thoracic aorta. The data also suggest that individuals with TGFBR2 mutations are more likely to dissect at aortic diameters <5.0 cm than individuals with TGFBR1 mutations. Conclusion: This study is the first to demonstrate clinical differences between patients with TGFBR1 and TGFBR2 mutations. These differences are important for the clinical management and outcome of vascular diseases in these patients.

TGFBR2 mutations alter smooth muscle cell phenotype and predispose to thoracic aortic aneurysms and dissections

AIMS Transforming growth factor-β (TGF-β) signaling is critical for the differentiation of smooth muscle cells (SMCs) into quiescent cells expressing a full repertoire of contractile proteins. Heterozygous mutations in TGF-β receptor type II (TGFBR2) disrupt TGF-β signaling and lead to genetic conditions that predispose to thoracic aortic aneurysms and dissections (TAADs). The aim of this study is to determine the molecular mechanism by which TGFBR2 mutations cause TAADs. METHODS AND RESULTS Using aortic SMCs explanted from patients with TGFBR2 mutations, we show decreased expression of SMC contractile proteins compared with controls. Exposure to TGF-β1 fails to increase expression of contractile genes in mutant SMCs, whereas control cells further increase expression of these genes. Analysis of fixed and frozen aortas from patients with TGFBR2 mutations confirms decreased in vivo expression of contractile proteins relative to unaffected aortas. Fibroblasts explanted from patients with TGFBR2 mutations fail to transform into mature myofibroblasts with TGF-β1 stimulation as assessed by expression of contractile proteins. CONCLUSIONS These data support the conclusion that heterozygous TGFBR2 mutations lead to decreased expression of SMC contractile protein in both SMCs and myofibroblasts. The failure of TGFBR2-mutant SMCs to fully express SMC contractile proteins predicts defective contractile function in these cells and aligns with a hypothesis that defective SMC contractile function contributes to the pathogenesis of TAAD.

Severe aortic and arterial aneurysms associated with a TGFBR2 mutation.

Background A 24-year-old man presented with previously diagnosed Marfans syndrome. Since the age of 9 years, he had undergone eight cardiovascular procedures to treat rapidly progressive aneurysms, dissection and tortuous vascular disease involving the aortic root and arch, the thoracoabdominal aorta, and brachiocephalic, vertebral, internal thoracic and superior mesenteric arteries. Throughout this extensive series of cardiovascular surgical repairs, he recovered without stroke, paraplegia or renal impairment.Investigations CT scans, arteriogram, genetic mutation screening of transforming growth factor β receptors 1 and 2.Diagnosis Diffuse and rapidly progressing vascular disease in a patient who met the diagnostic criteria for Marfans syndrome, but was later rediagnosed with Loeys–Dietz syndrome. Genetic testing also revealed a de novo mutation in transforming growth factor β receptor 2.Management Regular cardiovascular surveillance for aneurysms and dissections, and aggressive surgical treatment of vascular disease.

Genetic basis of thoracic aortic aneurysms and aortic dissections

Ascending thoracic aortic aneurysms leading to type A dissections (TAAD) can occur in association with a genetic syndrome, such as Marfan syndrome (MFS), or as an autosomal dominant disorder in the absence of syndromic features, termed familial TAAD. Familial TAAD demonstrates genetic heterogeneity, and linkage studies have identified three TAAD loci at 5q13‐14 (TAAD1), 11q23 (FAA1), and 3p24‐25 (TAAD2). The underlying genetic heterogeneity of TAAD is reflected in the phenotypic variation associated with familial TAAD with respect to age of onset, progression, penetrance, and association with additional cardiac and vascular features. Recently, mutations in the TGFBR2 gene have been identified as the cause of disease linked to the TAAD2 locus, supporting the hypothesis that dysregulation of TGFβ signaling is a mechanism leading to aneurysms and dissections. The recent identification of the TGFβ pathway as a key target in the molecular pathogenesis of TAAD has opened new avenues for future genetic and therapeutic research.

RNF213 Rare Variants in an Ethnically Diverse Population With Moyamoya Disease

Background and Purpose— Moyamoya disease (MMD) is a rare, genetically heterogeneous cerebrovascular disease resulting from occlusion of the distal internal carotid arteries. A variant in the Ring Finger 213 gene (RNF213), altering arginine at position 4810 (p.R4810K), is associated with MMD in Asian populations. However, there are a lack of data on the role of RNF213 in patients with MMD of additional ethnicities and diasporic Asian populations. We investigate the contribution of RNF213 alterations to MMD in an ethnically diverse population based in the United States. Methods— We initially sequenced RNF213 exons 43, 44, and 45 (encoding the eponymous RING finger domain) and exon 60 (encoding p.R4810K) in 86 ethnically diverse patients with MMD. Comprehensive exome sequencing data from 24 additional patients with MMD was then analyzed to identify RNF213 variants globally. Segregation of variants with MMD and other vascular diseases was assessed in families. Results— RNF213 p.R4810K was identified in 56% (9/16) of patients with MMD of Asian descent and not in 94 patients of non-Asian descent. 3.6% (4/110) of patients had variants in the exons encoding the RING finger domain. Seven additional variants were identified in 29% (7/24) of patients with MMD who underwent exome sequencing. Segregation analysis supported an association with MMD for 2 variants and a lack of association with disease for 1 variant. Conclusions— These results confirm that alterations in RNF213 predispose patients of diverse ethnicities to MMD, and that the p.R4810K variant predisposes individuals of Asian descent in the United States to MMD.

MAT2A Mutations Predispose Individuals to Thoracic Aortic Aneurysms

Up to 20% of individuals who have thoracic aortic aneurysms or acute aortic dissections but who do not have syndromic features have a family history of thoracic aortic disease. Significant genetic heterogeneity is established for this familial condition. Whole-genome linkage analysis and exome sequencing of distant relatives from a large family with autosomal-dominant inheritance of thoracic aortic aneurysms variably associated with the bicuspid aortic valve was used for identification of additional genes predisposing individuals to this condition. A rare variant, c.1031A>C (p.Glu344Ala), was identified in MAT2A, which encodes methionine adenosyltransferase II alpha (MAT IIα). This variant segregated with disease in the family, and Sanger sequencing of DNA from affected probands from unrelated families with thoracic aortic disease identified another MAT2A rare variant, c.1067G>A (p.Arg356His). Evidence that these variants predispose individuals to thoracic aortic aneurysms and dissections includes the following: there is a paucity of rare variants in MAT2A in the population; amino acids Glu344 and Arg356 are conserved from humans to zebrafish; and substitutions of these amino acids in MAT Iα are found in individuals with hypermethioninemia. Structural analysis suggested that p.Glu344Ala and p.Arg356His disrupt MAT IIα enzyme function. Knockdown of mat2aa in zebrafish via morpholino oligomers disrupted cardiovascular development. Co-transfected wild-type human MAT2A mRNA rescued defects of zebrafish cardiovascular development at significantly higher levels than mRNA edited to express either the Glu344 or Arg356 mutants, providing further evidence that the p.Glu344Ala and p.Arg356His substitutions impair MAT IIα function. The data presented here support the conclusion that rare genetic variants in MAT2A predispose individuals to thoracic aortic disease.

Genetic basis of thoracic aortic aneurysms and dissections: potential relevance to abdominal aortic aneurysms.

Abstract:  Ascending thoracic aortic aneurysms leading to type A dissections (TAAD) have long been known to occur in association with a genetic syndrome such as Marfan syndrome (MFS). More recently, TAAD has also been demonstrated to occur as an autosomal dominant disorder in the absence of syndromic features, termed familial TAAD. Familial TAAD demonstrates genetic heterogeneity, and linkage studies have identified TAAD loci at 5q13‐14 (TAAD1), 11q23 (FAA1), 3p24‐25 (TAAD2), and 16p12.2‐13.13. The genetic heterogeneity of TAAD is reflected by variation in disease in terms of the age of onset, progression, penetrance, and association with additional cardiac and vascular features. The underlying genetic heterogeneity of TAAD is reflected in the phenotypic variation associated with familial TAAD with respect to age of onset, progression, penetrance, and association with additional cardiac and vascular features. Mutations in the TGFBR2 gene have been identified as the cause of disease linked to the 3p24‐25 locus, implicating dysregulation of TGF‐β signaling in TAAD. Mutations in myosin heavy chain (MYH11), a smooth muscle cell‐specific contractile protein, have been identified in familial TAAD associated with patent ductus arteriosus (PDA) linked to 16p12.2‐12.13. The identification of these novel disease pathways has led to new directions for future research addressing the pathology and treatment of TAAD.