A Genomic Study of Myxomatous Mitral Valve Disease in Cavalier King Charles Spaniels

Cavalier King Charles spaniels (CKCSs) show the earliest onset and the highest incidence of myxomatous mitral valve disease (MMVD). Previous studies have suggested a polygenic inheritance of the disease in this breed and revealed an association with regions on canine chromosomes 13 and 14. Following clinical and echocardiographic examinations, 33 not-directly-related CKCSs were selected and classified as cases (n = 16) if MMVD was present before 5 years of age or as controls (n = 17) if no or very mild MMVD was present after 5 years of age. DNA was extracted from whole blood and genotyped with a Canine 230K SNP BeadChip instrument. Cases and controls were compared with three complementary genomic analyses (Wright’s fixation index—FST, cross-population extended haplotype homozygosity—XP-EHH, and runs of homozygosity—ROH) to identify differences in terms of heterozygosity and regions of homozygosity. The top 1% single-nucleotide polymorphisms (SNPs) were selected and mapped, and the genes were thoroughly investigated. Ten consensus genes were found localized on chromosomes 3-11-14-19, partially confirming previous studies. The HEPACAM2, CDK6, and FAH genes, related to the transforming growth factor β (TGF-β) pathway and heart development, also emerged in the ROH analysis. In conclusion, this work expands the knowledge of the genetic basis of MMVD by identifying genes involved in the early onset of MMVD in CKCSs.

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Abstract 9967: Altered TGF-β Signalling and Myofibroblast-like Cell Differentiation Contribute to Myxomatous Mitral Valve Disease

Circulation ◽

10.1161/circ.132.suppl_3.9967 ◽

2015 ◽

Vol 132 (suppl_3) ◽

Author(s):

Alex Sahagian ◽

Candice Dilworth ◽

Elizabeth Robertson ◽

Yaxin Lu ◽

Murat Kekic ◽

...

Keyword(s):

Mitral Valve ◽

Mitral Valve Disease ◽

Transforming Growth Factor ◽

Proteolytic Enzymes ◽

Disease Process ◽

Valve Disease ◽

Compensatory Mechanism ◽

Mitral Valve Leaflet ◽

Control Tissue ◽

Pathological Remodeling

Upregulated Transforming Growth Factor-beta (TGF-β) signaling has been implicated in the pathogenesis of myxomatous mitral valve disease (MMVD) in both humans and canines. Moreover, it has been postulated that the phenotypic transformation of valve interstitial cells (VICs) in the mitral valve into alpha-smooth muscle actin (α-SMA)-positive myofibroblast-like cells contribute to elevated levels of proteolytic enzymes such as matrix metalloproteinases (MMPs), which subsequently induce pathological remodeling of the extracellular matrix (ECM) in the mitral valve. We hypothesized that, in MMVD, upregulation of TGF-β signaling would induce the transformation of VICs into myofibroblast-like cells, which subsequently express increased levels of MMPs. Computational immunohistochemistry was used to quantify protein expression in mitral valve leaflet tissue from canines affected by MMVD (n=4) and from control canines (n=5). TGF-β and α-SMA expression was significantly increased (p<0.001) in MMVD tissue compared to control tissue. Moreover, a significant positive correlation was found between TGF-β expression and α-SMA expression (p<0.05) across the entire cohort. There were also significant increases in MMP-2 and MMP-9 expression (p<0.05) in MMVD tissue compared to control tissue. No change in MMP-3 expression was detected. These findings are consistent with abnormal TGF-β signaling inducing the differentiation of VICs into α-SMA-positive myofibroblast-like cells. Moreover, the increases in expression of MMP-2 and MMP-9 in MMVD indicate the likely role of these myofibroblast-like cells in mediating pathological remodeling of the ECM throughout the disease process. The paradoxical finding of no change in MMP-3 expression may represent a compensatory mechanism that attempts to limit further degenerative damage to the mitral valve. In conclusion, MMVD is characterized by TGF-β-induced differentiation of VICs into myofibroblast-like cells, which subsequently facilitate ECM degradation via the increased expression of MMPs. This supports the rationale for the use of angiotensin II receptor blockers, capable of attenuating TGF-β signaling, in the treatment of MMVD.

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