Abstract: Myxomatous mitral valve disease (MMVD),also known as degenerative mitral valve disease,is the result of valve degeneration.The disease is one of the most common heart diseases in dogs and causes serious consequences such as mitral regurgitation (MR) and left side congestive heart failure (CHF).The incidence of MMVD increases with age and has a high genetic characteristic in certain breeds of dogs.The pathological changes of MMVD are mainly mitral valve thickening,prolapse and elongation or rupture of the tendinous cord.The pathogenesis of the disease is still not completely clear,and the existing studies have obtained certain results from the aspects of tissue cell level,genetics and proteomics,and signaling pathways.Studies at the tissue and cell level have shown that MMVD is associated with the activation of valve interstitial cell (VIC) and the deposition of collagen and proteoglycan in the extracellular matrix (ECM) in dogs and other species.Genetic and proteomic studies have shown that miRNAs may be involved in the genetic regulation of MMVD,and blood circulating miRNAs are expected to be biomarkers of CHF in dogs with MMVD.From the molecular signaling pathway level,MMVD is closely related to many signaling pathways.Among them,the well-studied 5-hydroxytryptamine (5-HT) and transforming growth factor-β (TGF-β) pathways play important roles in the activation of VICs.At the same time,the mitral valve is also affected by the mechanical stress of pulling and blood flushing during the contraction and relaxation of the heart,and the two pathways interact with each other to promote the occurrence and development of MMVD.In this review,the epidemiological characteristics,pathological changes,clinical symptoms,diagnostic stages,therapeutic methods and disease-related mechanisms of MMVD in dogs were reviewed.

Key words: myxomatous mitral valve disease; mitral valve; valve interstitial cell; 5-hydroxytryptamine; transforming growth factor-β