Thrombus stability explains the factor V Leiden paradox: a mouse model.

Humans carrying the factor V Leiden (FVL) variant have a fivefold increased risk for venous thrombosis. However, incidence of deep vein thrombosis (DVT) is proportionally greater than that of pulmonary embolism (PE) in these individuals. This is known as the FVL paradox. We hypothesized that the rate of initial DVT development is similar in FVL and noncarriers, but thrombi in FVL carriers are more stable and develop into a clinically significant DVT more often than in noncarriers. To test this, we induced thrombi in the femoral vein of wild-type (WT), heterozygous (F5L/+), and FVL homozygous (F5L/L) mice. Using intravital microscopy, thrombus size and embolization were visualized and emboli in the lungs were quantified. Compared with WT, femoral vein thrombi in F5L/+ and F5L/L mice were larger and embolized less. Total and large embolic events, the percentage of thrombus that embolized, and PE burden were significantly decreased in F5L/L mice. This suggests that in noncarriers (reflected by WT), a minor injury initially resulting in a small DVT tends to remain small and asymptomatic because of the embolization of the otherwise growing thrombus. Alternatively, the same insult in people with FVL (reflected by F5L/L) leads to thrombus growth as a result of less embolization, and thus symptomatic DVT development.