This study is the first to demonstrate that MYH11
mutations are a rare cause of familial TAAD. At the same time, we confirm that MYH11
are a common cause of familial TAAD associated with PDA. Two of three families with TAAD in conjunction with PDA were found to carry novel missense point mutations in MYH11
, whereas none of the 93 families with familial TAAD alone was found to have mutations in this gene. By structural analysis, the mutations found in the TAAD/PDA families are predicted to be deleterious to protein function. Similar to two previously reported families with MYH11
mutations, we observed variable expressivity of the mutant gene in these families, with some family member presenting with TAAD in conjunction with PDA, some with PDA or some with TAAD alone. In addition, similar to our observations with TGFBR2
), decreased penetrance of the mutation was observed in some adults, who had no known cardiovascular disease (). Finally, the aneurysm in patients with MYH11
mutations involved the ascending aorta, a location where a majority of force is placed on the aortic wall with each cardiac contraction and spared the sinuses of Valsalva, which is the location of aneurysms in Marfan patients and TAAD patients with TGFBR2
mutations () (21
Interestingly, two individuals with MYH11
mutations from TAA069 presented with premature occlusive arterial disease, resulting in coronary artery disease and peripheral vascular disease. Additionally, one individual from TAA027 who carried an MYH11
mutation was determined to have persistent and severe LR, occlusive disease. In addition to this evidence of occlusive disease in TAA027 and TAA069, death due to strokes occurred in three individuals who were carriers of MYH11
mutations in a previously reported family (11
), raising the intriguing possibility that occlusive vascular disease is associated with MYH11
It is notable that mutations in the paralogous cardiac-specific myosin heavy chain gene (MYH7
) are responsible for ~30–40% of HCM, an autosomal dominant disease characterized pathologically by the presence of myocyte disarray, myocyte hypertrophy and interstitial fibrosis (22
) Indeed, myocyte disarray, i.e. the presence of abnormally shaped cells that are disoriented with respect to one another, is considered a pathological hallmark of HCM (24
). These pathological findings in HCM are thought to occur secondary to the activation of specific trophic and mitotic factors in the heart, and studies demonstrating increased IGF-1 and TGF-β1 levels in HCM implicate these growth factors in disease progression (26
). Our data suggest that MYH11
mutations result in a similar pathology in SMCs forming the medial layer of blood vessels.
Our data demonstrate a specific aortic pathology associated with MYH11 mutations, which is distinct from pathology previously described for thoracic aneurysms. The pathological hallmark of HCM, myocyte disarray, is similar to the SMC disarray observed in the aortic media in two unrelated patients with MYH11 mutations. In contrast to HCM, there was no evidence of hypertrophy of aortic SMCs, but rather focal hyperplasia of SMCs was observed, both in the aortic media and vasa vasorum of patients with MYH11 mutations. In addition, there were areas of SMC loss and typical medial degeneration, which might be postulated to represent a later stage. Given that the aorta we were able to assess in detail had been prophylactically rather than emergently repaired in a young individual, overall greater numbers of SMCs were observed. Although terminally differentiated cardiac myocytes lack replicative capacity and respond to stress either by an increase in size leading to hypertrophy or by premature death leading to fibrosis, SMCs can switch between differentiated and prolif-erative phenotypes in response to intracellular or extracellular cues. Our data suggest that SMCs proliferate rather than hypertrophy as a result of MYH11 mutations, implying that although paralogous genes are altered between TAAD and HCM, there is a cell type-specific response to these alterations. Therefore, although some pathological alterations are similar, e.g. cellular disarray, there are differences observed between cardiomyocytes and SMCs, resulting from programmed responses in these cells.
Further similarities between HCM due to MYH7
mutations and aortic disease due to MYH11
mutations may be drawn when the stress induced pathways leading to disease are examined in the MYH11
patient. Previous data indicate that expression of mutant sacromeric proteins in cardiomyocytes in HCM impairs their mechanical performance, leading to compensatory hypertrophy (28
). Growth factors, including IGF-1 and TGF-β1, are upregulated in the myocardium of HCM patients and may contribute to the hypertrophy and interstitial fibrosis (27
). Ang II produced by a local RAS has been implicated in both promoting myocyte hypertrophy and increasing cardiac fibrosis, and its antagonist, losartan, inhibits load-induced cardiac hypertrophy and reverses cardiac interstitial fibrosis (29
Increased IGF-1 expression and production were found in the aortic media, vasa vasorum and explanted SMCs from a patient with an MYH11
mutation. IGF-1 expression is known to increase in response to cyclic stretch, possibly as an adaptive response to maintain vessel tone and function in response to impaired contraction (30
). Increased IGF-1 intiates two signaling pathways in SMCs—the phosphoinositide-3 kinase (PI3K) pathway leading to increased production of contractile proteins and the extracellular signal-regulated kinase (ERK) pathway, leading to increased proliferation. (31
). The focal nature of the SMC hyperplasia suggests that the presence of the mutant MYH11 protein alone is not sufficient to induce hyperplasia and a second, as yet unknown, event is needed to elicit this response.
The increased ACE
expression by explanted MYH11 mutant SMCs suggests that increased tissue production of Ang II may occur as part of disease process, augmented by the increased blood flow associated with the increased vascularity observed. Cytokine profiling and expression analysis of MYH11 mutant aortic tissue and SMCs indicated significant upregulation of MIP-1α and β. MIP-1α has also been shown to be upregulated in aortic explants from a mouse model of Ang II infusion, suggesting that MIP-1α is downstream of Ang II signaling (32
). Thus, the ACE
upregulation seen in mutant aortic SMCs appears to functionally result in enhanced tissue response to Ang II. There was no evidence of increased TGF-β1 production or signaling in the aorta carrying the MYH11 mutation, as assessed by the expression of the TGF-β responsive genes PAI-1
However, substantial crosstalk between the Ang II and TGF-β signaling pathways makes it difficult to assess the individual impact of these candidate pathways in this study.
Our findings further confirm the association of PDA with MYH11
mutations. Current data suggest that closure of the ductus arteriosus requires several events, including a postnatal constriction of the ductus caused by SMC contraction, partly in response to declining prostaglandin levels, followed by formation of the intimal cushions by SMC migration and endothelial/SMC proliferation (33
). This remodeling results in permanent ductus closure. The MYH11
mutations identified are predicted to decrease SMC contractility, thereby disrupting proper ductal closure. In addition, the aortic medial SMC disarray associated with MYH11
mutations suggests compromised SMC alignment and migration in response to cellular signals. Additional studies are required to determine the frequency of MYH11
alterations in children with isolated PDA and whether these mutations predict adult-onset vascular disease in these children.
The paucity of aortic samples available from patients with MYH11 mutations for pathological and functional analyses is an unavoidable limitation of this study. However, our observations of SMC disarray are based on findings in the ascending aorta of two unrelated patients, and the pathological parallels with HCM aid in validating these findings. The observation of focal fibromuscular dysplasia associated with these mutations is not only supported by the increased thickness of arteries and the presence of premature occlusive vessel diseases including CAD and LR in patients with MYH11 mutations in this study, but also by the increased occurrence of strokes in previously reported families with MYH11 mutations. Further studies are required to verify and fully understand the molecular consequences of MYH11 mutations leading to aortic and potentially other vascular diseases including strokes and CAD.