MMP9 and triple-A syndrome: ADAR1 promotes the stability of matrix metallopeptidase 2 (MMP2) and matrix metallopeptidase 9 (MMP9) mRNA in SMC by interacting with the RNA-binding protein HuR, increases the expression and activity of MMP2 and MMP9, leads to elastin degradation and medial degeneration in the vessel wall, and finally forms AAA.