Factor V is an essential component in the blood coagulation cascade. Inherited or acquired deficiencies in factor V are rare causes of bleeding disorders.
Factor V deficiency is also known as Owren disease. Dr. Paul Owren identified this defect in Norway in 1943. Using relatively primitive technology, he was able to deduce the existence of a fifth component required for fibrin formation, which he named factor V, thus beginning the era of Roman numerology for coagulation factors.
Dr. Owren's work defined factor V as the activity in normal plasma that corrected the prothrombin time (PT) of the plasma in a patient with factor V deficiency. Factor V deficiency has also been called parahemophilia, since hemarthrosis can occur with severe deficiencies and with increased bleeding time. [1, 2]
Factor V is an essential component in the blood coagulation cascade. Factor V is synthesized in the liver and possibly in megakaryocytes. Factor V circulates in an inactive form. During coagulation, factor V is converted to the active cofactor, factor Va, via limited proteolysis by the serine protease a-thrombin. Factor Va and activated factor Xa form the prothrombinase complex. The prothrombinase complex is responsible for the rapid conversion of the zymogen prothrombin to the active serine protease a-thrombin. [3, 4] Thrombin cleaves fibrinogen to form fibrin, leading to the ultimate step in coagulation, the formation of a fibrin clot.  See images below.
Inherited factor V deficiency is a rare condition that is associated with an abnormal factor V plasma level. Acquired factor V deficiency is a rare clinical condition in which the development of antibodies to factor V (factor V inhibitors) leads to hemorrhagic complications of varying severity. The addition of normal plasma cannot correct the prolonged PT and activated partial thromboplastin time (aPTT). Factor V inhibitors can occur after surgery, childbirth, autoimmune diseases, and with certain neoplasms.
Factor V Leiden is a completely different inherited disorder that involves a single point mutation in the factor V gene. Factor V activity levels in patients with factor V Leiden are normal.  Proteolytic inactivation of factor Va and factor VIIIa by activated protein C (APC) normally limits clot formation; however, factor V Leiden resists inactivation by APC.
Only 150 cases of congenital factor V deficiency have been reported worldwide since 1943. Homozygous factor V deficiency is rare, occurring in approximately 1 per million population.
Delev et al presented 39 German patients with factor V deficiency.  In 36 cases with an identifiable causative mutation, 20 patients were heterozygous for the mutation, whereas 9 were homozygous, 6 were compound heterozygous, and 1 proband was pseudohomozygous.  There were no mutations found in the remaining 3 patients.
The investigators identified 33 uniquely different mutations of a total 42 genetic mutations: 19 missense mutations, 8 nonsense mutations, 4 small deletions, and 2 splice site mutations.  Of the 33 unique mutations, 23 were novel sequence variations not previously reported, and all changes found in exon 13 led to null alleles as nonsense mutations or small deletions.
The severity of factor V deficiency varies from bruising to lethal hemorrhage.
No apparent racial predilection for factor V deficiency exists.
Factor V deficiency affects males and females with equal frequency.
Factor V deficiency affects all ages. The age at presentation indirectly varies with the severity of disease.
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