Silencing of Anticoagulant Protein C Evokes Low-Incident but Spontaneous Atherothrombosis in Apolipoprotein E–Deficient Mice—Brief Report

Objective— Murine atherosclerosis models do not spontaneously develop atherothrombotic complications. We investigated whether disruption of natural anticoagulation allows preexisting atherosclerotic plaques to progress toward an atherothrombotic phenotype. Approach and Results— On lowering of plasma protein C levels with small interfering RNA (siProc) in 8-week Western-type diet–fed atherosclerotic apolipoprotein E–deficient mice, 1 out of 4 mice displayed a large, organized, and fibrin- and leukocyte-rich thrombus on top of an advanced atherosclerotic plaque located in the aortic root. Although again at low incidence (3 in 25), comparable thrombi at the same location were observed during a second independent experiment in 9-week Western-type diet–fed apolipoprotein E–deficient mice. Mice with thrombi on their atherosclerotic plaques did not show other abnormalities and had equally lowered plasma protein C levels as siProc-treated apolipoprotein E–deficient mice without thrombi. Fibrinogen and thrombin–antithrombin concentrations and blood platelet numbers were also comparable, and plaques in siProc mice with thrombi had a similar composition and size as plaques in siProc mice without thrombi. Seven out of 25 siProc mice featured clots in the left atrium of the heart. Conclusions— Our findings indicate that small interfering RNA–mediated silencing of protein C in apolipoprotein E–deficient mice creates a condition that allows the occurrence of spontaneous atherothrombosis, albeit at a low incidence. Lowering natural anticoagulation in atherosclerosis models may help to discover factors that increase atherothrombotic complications.