Genes Associated with Thoracic Aortic Aneurysm and Dissection: 2019 Update and Clinical Implications

Thoracic aortic aneurysm is a typically silent disease characterized by a lethal natural history. Since the discovery of the familial nature of thoracic aortic aneurysm and dissection (TAAD) almost 2 decades ago, our understanding of the genetics of this disorder has undergone a transformative amplification. To date, at least 37 TAAD-causing genes have been identified and an estimated 30% of the patients with familial nonsyndromic TAAD harbor a pathogenic mutation in one of these genes. In this review, we present our yearly update summarizing the genes associated with TAAD and the ensuing clinical implications for surgical intervention. Molecular genetics will continue to bolster this burgeoning catalog of culprit genes, enabling the provision of personalized aortic care.


Introduction
This review presents an annual update to the article "Genes Associated with Thoracic Aortic Aneurysm and Dissection: Update and Clinical Implications" originally published in 2017 and updated in 2018 in AORTA. 1,2 We have updated the list of genes with identified genetic variants predisposing individuals to a thoracic aortic aneurysm or dissection (TAAD) in ►Table 1, and the recommendation for individualized surgical interventions for specific genetic mutations is presented in ►Fig. 1.
Thoracic aortic aneurysm (TAA) affects 1% of the general population 3 and its natural history is to enlarge an average of 0.14 cm per year. 4 Prior to often lethal dissection or rupture, TAAs are usually asymptomatic. However, if identified and treated with appropriate blood pressure control and surgical intervention, life expectancy is improved.
Report of inherited TAAD in the 1990s 5 has led to the discovery and understanding of genetic and molecular mechanisms of TAAD. 6 To date, variants in 37 genes have been associated with TAAD (►Table 1; ►Fig. 1). These genes explain approximately 30% of the familial nonsyndromic Table 1 Genes associated with syndromic and nonsyndromic thoracic aortic aneurysm and/or dissection, associated vascular characteristics, and size criteria for elective surgical intervention (any gene newly reported during the past year to be associated with TAAD is highlighted in red)  Note: It is important to note that since mutations in many of these genes are rare and have only recently been implicated in TAAD, there is a lack of adequate prospective clinical studies. Therefore, it is difficult to establish threshold diameters for the intervention of TAAs, and each individual must be considered on a case by case basis, taking into account the rate of change in aneurysm size (>0.5 cm per year is considered rapid), any family history of aortic dissection at diameters< 5.0 cm, and the presence of significant aortic regurgitation, which are all indications for early repair if present; A " þ " symbol in the syndromic TAAD column indicates that mutations in the gene have been found in patients with syndromic TAAD (same for the nonsyndromic TAAD column). A " À " symbol in the syndromic TAAD column indicates that mutations in the gene have not been found in patients with syndromic TAAD (same for the nonsyndromic TAAD column); A reference is provided for each of the associated vascular characteristics not reported in the OMIM entry for that gene. There are no data to set threshold diameters for surgical intervention for EDS Type IV. 51 The Canadian guidelines recommend surgery for aortic root sizes of 4.0-5.0 cm and ascending aorta sizes of 4. Park et al recently demonstrated that Col5a2 haploinsufficiency increased the incidence and severity of AAA and led to aortic arch ruptures and dissections in an angiotensin II-induced aneurysm mouse model. 37 In an earlier paper, Park  Galvin et al demonstrated that Madh6, which encodes SMAD6, mutant mice exhibited defects in cardiac valve formation, outflow tract septation, vascular tone, and ossification but no aneurysm development was observed. 104 s TGFB3 knockout mice die at birth from cleft palate, 88 but minor differences in the position and curvature of the aortic arches of these mice compared with wild type mice have been described. 105 TAAD. 7 These genes encode proteins of the extracellular matrix, vascular smooth muscle cell contractile unit, or transforming growth factor β (TGF-β)-signaling pathways 8 and thus are essential to the structure and maintenance of the aortic wall. During 2018, several important studies were published that have enhanced our understanding of the pathogenesis of TAAD. Gould et al 9 performed whole-exome sequencing (WES) and targeted sequencing on 736 individuals with bicuspid aortic valve (BAV), non-syndromic ascending aortic aneurysm (AscAA), and 376 controls. 9 In 13 (1.8%) of the affected individuals a heterozygous ROBO4 mutation was identified, including two variants that segregated with disease among two affected families. 9 ROBO4 is well expressed in vascular endothelial cells and plays a role in endothelial barrier function. 9 In this study, its expression was found to be diminished in the resected aorta sample of an affected individual with AscAA. 9 To further test their hypothesis that ROBO4 variants lead to the disruption of endothelial performance at a cellular level, thus altering vascular permeability, the authors cultured human aortic endothelial cells and either silenced ROBO4 or expressed ROBO4 variants. They confirmed that ROBO4 abnormalities did indeed induce endothelial barrier dysfunction. Lastly, the authors created homozygous ROBO4 knockout mice and a knock-in mouse with an ROBO4 splice donor site mutation; the affected mice presented with a mix of aortic valve dysfunction (BAV and/or aortic regurgitation or stenosis) and AscAA, confirming their suspicion that a heterozygous mutation in ROBO4 can lead to a nonsyndromic presentation of BAV/AscAA. 9 Latent transforming growth factor binding proteins (LTBP), a family of extracellular matrix glycoproteins, have been shown to play a significant role in TGF-β regulation. 10 LTBP1, in particular, can bind to fibrillin-1 and inactivate TGF-β. 10,11 Quiñones-Pérez et al described a case series involving a three-generation family with TAA found to have a chromosome 2p22.3-p22.2 deletion involving LTBP1, amongst other genes. 10 Despite multiple genes being involved in the deletion, LTBP1 was considered the likely culprit given its relationship to TGF-β. In addition to TAA, the affected individuals displayed additional features of Marfan syndrome (MFS) and Loeys-Dietz syndrome, even though none of them met the criteria for diagnosis.
Mutations of the latent TGF-β binding protein-3 (LTBP3) gene have been associated with TAAD in a WES study of 271 individuals from unrelated families with heritable thoracic aortic disease (multiple affected family members) without a known genetic etiology for aortopathy. 12 In this study, compound heterozygous variants in one family and a homozygous insertion/deletion variant in LTBP3 in a second family were identified. Sequencing of 338 additional individuals with non-syndromic TAAD found nine additional heterozygous LTBP3 rare variants. The authors also demonstrated that LTBP3 knockout mice manifested enlarged aortic roots and ascending aortas compared with wild type mice. These findings demonstrate that individuals with LTPB3 are at increased risk for TAAD, in addition to the already established risk for skeletal and dental abnormalities. [12][13][14] Rare mutations in the Parkin-like E3 ubiquitin ligase Ariadne-1 (ARIH1) have been observed in patients with early-onset or familial TAAD. 15 AR1H1 encodes a protein of the LINC (linker of nucleoskeleton and cytoskeleton), a protein complex essential for anchoring myocyte nuclei to the cytoskeleton. 15 Aortic tissues from patients with these mutations exhibit affected nuclear morphology in vascular smooth muscle cells. It is well known there is an increased risk for BAV and TAA among individuals with Turner syndrome, although the precise etiology has thus far remained elusive. Corbitt et al 16 demonstrated that Turner syndrome patients with putatively-deleterious mutations in TIMP3 are associated with a greater incidence of BAV and TAA than the patients without TIMP3 variants. Hemizygosity for coincident TIMP1/ TIMP3 variants, synergistically increased the risk for BAV and TAA, 16 due to TIMP1's functional redundancy with TIMP3.
Numerous mutations of the myosin light chain kinase (MYLK) gene have been associated with TAAD. Shalata et al have identified an additional MYLK missense mutation in a single pedigree. 17 Myosin light chain kinase phosphorylates myosin regulatory light chains to facilitate actin-myosin generation of contraction. The mutation was shown to be functional, reducing kinase activity.
Insights to the pathogenesis of TAAD are as important as identifying TAAD variants. Nogi et al 18 found the protein expression of small GTP-binding protein GDP dissociation stimulator (SmgGDS) in aortic smooth muscle cells was decreased in TAAD patients compared with controls. 18 SmgGDS is encoded by the RAP1GDS1 gene and known to be involved in the contraction of vascular smooth muscle cell (VSMC). 18 Using a heterozygous SmgGDS þ/À mouse model, since the complete knockout (SmgGDS À/À ) was embryologically lethal, they observed that the downregulation of SmgGDS was causing "pathological phenotype changes in VSMC" via the angiotensin-II pathway. 18 Furthermore, they demonstrated that when SmgGDS was overexpressed in the SmgGDS þ/À , the mice had less aortic growth and fewer aortic ruptures, suggesting that SmgGDS could be used as a biomarker or a therapeutic agent.

Conclusion
Advances in 2018 have increased our understanding of the pathogenesis of TAAD. The number of genes with genetic variants or mutations associated with TAAD has increased from 29 in our original 2017 report 2 to 37 in this 2019 update. Advances in genetic techniques and bioinformatics tools have enabled rapid progress in the genetic and molecular understanding of TAA. As the cost for genome sequencing decreases, we anticipate accelerating progress. With our greater understanding of the genetics of the individuals affected with TAAD and their specific genetic mutations or susceptibility variants, we can provide a personalized aortic care, tailoring surgical recommendations for each patient depending on their individual genetic profiles. Because most families that have multiple affected members with TAAD still have not had known genetic variants identified in the aortopathy genes, we expect many new genes harboring variants for TAAD will be discovered in the foreseeable future and thereby enhance our genetic dictionary. Furthermore, it is important to remind ourselves that every disease-causing mutation starts out as a variant of unknown significance (VUS). 19 Only after extensive functional studies it is possible to confidently state that a VUS is a disease-causing mutation. Such work requires multidisciplinary collaboration.
We will continue to report annual updates regarding the "TAA genetic dictionary" with updates to the Table and Figure below and provide suggested surgical intervention criteria for each identified mutation.

Funding
None.

Conflict of Interest
The authors declare no conflict of interest related to this article.