SPP1 and atrial fibrillation: Promoter methylation induced by hypoxia results in a 50% decrease in PLK2 expression in atrial fibrillation (AF) patients; In canine tachycardia, PLK2 expression is decreased in tissues of atria, but not ventricles; Drug inhibition or KO of PLK2 leads to cardiac fibroblasts displaying myofibroblast phenotype; PLK2 KO mouse heart fibroblasts secretes inflammatory OPN; The concentration of OPN in peripheral blood of AF patients with myocardial fibrosis is significantly higher than that of patients with sinus rhythm and AF patients without fibrosis.