Abstract
Myxomatous mitral valve disease (MMVD) is the most prevalent cardiac disorder in small and toy breed dogs, with the Cavalier King Charles Spaniel (CKCS) showing exceptionally high predisposition and early onset of the disease. MMVD is characterized by progressive mitral valve degeneration, volume overload, and eventual development of congestive heart failure (CHF). Given the strong hereditary component in CKCS, considerable research has focused on elucidating the genetic basis of MMVD in this breed. This review article summarizes the current state of knowledge on the phenotypic features, inheritance, and candidate loci potentially responsible for early onset and severe course of the disease. The pathogenesis of the disease, its classification, and the effects of breeding programs aimed at reducing the occurrence of MMVD have been described. Key findings include associations between MMVD severity and polymorphisms in genes such as NEBL, ACE, CDK6, HEPACAM2, COL5A1, and FAH, as well as evidence implicating dysregulated TGF-β signaling, serotonin signaling, and extracellular matrix remodeling pathways. The current state of knowledge on the role of miRNA in the pathogenesis of MMVD was also summarized. Despite these findings, no specific high-penetrating mutation has been identified. MMVD is a complex, polygenic condition shaped by regulatory variants and breed-specific genetic bottlenecks. Comparative studies underscore the translational relevance of canine MMVD to human mitral valve disease, while genomic insights may be basis for the future selective breeding strategies and therapeutic approaches. Further large-scale, integrative studies combining genomics, transcriptomics, and functional validation are needed to clarify disease mechanisms and support targeted treatment in CKCS as well as the development of new breeding strategies and programs.