Disturbed NO signalling gives rise to congenital bicuspid aortic valve and aortopathy

Patients with a congenital bicuspid aortic valve (BAV), a valve with two instead of three aortic leaflets, have an increased risk of developing thoracic aneurysms and aortic dissection. The mechanisms underlying BAV-associated aortopathy are poorly understood. This study examined BAV-associated aortopathy in Nos3^−/− mice, a model with congenital BAV formation. A combination of histological examination and in-vivo ultrasound imaging was used to investigate aortic dilation and dissections in Nos3^−/− mice. Moreover, cell lineage analysis and single cell RNA sequencing were used to observe the molecular anomalies within vascular smooth muscle cells (VSMCs) of Nos3^−/− mice. Spontaneous aortic dissections was found in ascending aortas located at the sinotubular junction in ∼13% of Nos3^−/− mice. Moreover, Nos3^−/−mice were prone to develop aortic dilations in the mid- and distal-ascending aorta during early adulthood. Lower volumes of elastic fibres were found within vessel walls of the ascending aortas of Nos3^−/−mice as well as incomplete coverage of the aortic inner media by neural crest (NCC)-derived VSMCs. VSMCs of Nos3^−/− showed downregulation of 15 genes of which 7 were associated with aortic aneurysms and dissections in the human population. Elastin mRNA was most markedly downregulated, followed by Fibulin-5 expression, both primary components of elastic fibres. This study demonstrates that disrupted endothelial mediated NO signalling in mice causes next to a congenital BAV also aortic dilation and dissection as a consequence of inhibited elastic fibre formation in VSMCs within the ascending aorta of Nos3^−/−mice.