MMP9 and thoracic aortic aneurysm: Overexpression of MMP2 in smooth muscle cells and MMP9 in neutrophils and macrophages can cause degenerative changes in the middle layer of the artery in aortic aneurysms (29–31), and knockout of MMP2 and MMP9 can inhibit the development of thoracic aortic aneurysms (30).