Experimental Evidence of The Genetic Hypothesis on The Etiology of Bicuspid Aortic Valve Aortopathy

Abstract

Bicuspid aortic valve (BAV) is the most prevalent human congenital cardiac malformation, predisposing to clinically relevant valvulopathies and aorthopathies. BAV aortopathy consists in aortic dilatation caused by medial cystic necrosis, characterized by elastic lamina (EL) disruption and smooth muscle cell (SMC) apoptosis. Although the pathogenetic mechanisms remain unknown for most patients, abnormal expression of several proteins such as TGFb, fibrillin, eNOS, and metalloproteases (MMP) have been assessed in affected patients. The so called genetic and hemodynamic hypotheses have been advanced to explain the etiology of BAV aortopathy. The former proposes that genetic defects cause abnormal formation of aortic valve primordia leading to BAV, together with structural alterations of the ascending aorta predisposing to aortopathy. The latter points to hemodynamic alterations produced by the abnormal valve morphology as the leading cause of aortic wall degeneration. Currently, no direct experimental evidence supports the genetic hypotheses for BAV aortopathy etiology. The only spontaneous model of BAV disease is an inbred strain of Syrian hamsters (T strain) with low penetrant (40%) BAV, in which conspicuous dilatations of the ascending aorta have not been detected. To identify possible alterations of the ascending aorta in this model, we have performed anatomical (diameter), histomorphological (elastic fiber waviness and apoptosis), and molecular (RT‐qPCR and protein expression and activity) quantitative analyses of the aorta of animals with normal and bicuspid valves belonging to both affected (T) and control strains. The diameter of the aorta was significantly higher in animals of the T strain, regardless of the aortic valve morphology. Medial EL fragmentation was not found in any specimen, but EL waviness was significantly increased in the convexity of the aorta of normal and BAV animals of the T strain. SMC apoptosis was also detected in the convex aortic region of the T strain, independently of the aortic valve morphology. eNOS protein expression and activity were significantly increased in the aorta of control animals. RT‐qPCR revealed increased expression of Tgf‐β and Fibrillin‐2 in the aorta of animals of the T strain, while Fibrillin‐1 expression did not change, resulting in a reduction in the Fibrillin‐1/Fibrillin‐2 expression ratio. Strain and aortic valve morphology did not affect Mmp‐9 expression, whereas Mmp‐2 transcripts were elevated only in animals with BAV. This animal model of BAV exhibits histomorphological and molecular alterations of the ascending aorta, which appear in isogenic animals irrespective of the aortic valve phenotype. Thus, this is a direct experimental evidence supporting the genetic association between BAV and predisposition to aortic dilatation. We hypothesize that T strain constitutes a model for a population of patients with predisposition to BAV aortopathy, in which defective expression of Tgf‐β and Fibrillin‐2 may alter the structure of the EL and cause an increase in cell...