It is an autosomal dominant condition in which the coagulation factor has a mutation and cannot be destroyed by activated protein C (aPC). The actual point mutation causes an arginine to be replaced with a glutamine at the 506th amino acid, the cleavage site for protein C.
As factor V cannot be inactivated, it continues to facilitate production of thrombin, and so thrombi form in the veins.
Symptoms and signs
Up to 30% of patients who present with venous thrombosis or pulmonary embolism have this mutation.
This disease can be diagnosed by watching the APTT (the time it takes for blood to clot) as activated protein C is added. With a normal patient, adding aPC increases the APTT. With patients suffering from Factor V Leiden, adding aPC will barely affect the time it takes for blood to clot.
There is also a simple genetic test that can be done for this disorder. The mutation (a 1691G?A substitution) removes a cleavage site of the restriction endonuclease MnlI, so simple PCR, treatment with MnlI, and then DNA electrophoresis will give a quick diagnosis.