BackgroundThe volume overload of isolated mitral regurgitation (MR) in the dog results in left ventricular (LV) dilatation and interstitial collagen loss. To better understand the mechanism of collagen loss, we performed a gene array and overlaid regulated genes into ingenuity pathway analysis. Methods and ResultsGene arrays from LV tissue were compared in 4 dogs before and 4 months after MR. Cine-magnetic resonance- derived LV end-diastolic volume increased 2-fold (P-0.005), and LV ejection fraction increased from 41% to 53% (P-0.007). LV interstitial collagen decreased 40% (P-0.05) compared with controls, and replacement collagen was in short strands and in disarray. Ingenuity pathway analysis identified Marfan syndrome, aneurysm formation, LV dilatation, and myocardial infarction, all of which have extracellular matrix protein defects and/or degradation. Matrix metalloproteinase-1 and -9 mRNA increased 5- (P-0.01) and 10-fold (P-0.003), whereas collagen I did not change and collagen III mRNA increased 1.5-fold (P-0.02). However, noncollagen genes important in extracellular matrix structure were significantly downregulated, including decorin, fibulin 1, and fibrillin 1. In addition, connective tissue growth factor and plasminogen activator inhibitor were downregulated, along with multiple genes in the transforming growth factor- signaling pathway, resulting in decreased LV transforming growth factor-1 activity (P-0.03). ConclusionsLV collagen loss in isolated, compensated MR is chiefly due to posttranslational processing and degradation. The downregulation of multiple noncollagen genes important in global extracellular matrix structure, coupled with decreased expression of multiple profibrotic factors, explains the failure to replace interstitial collagen in the MR heart. (Circulation. 2009;119:2086-2095.) © 2009 American Heart Association, Inc.