Prothrombin (Factor II) Deficiency

Prothrombin (also known as factor II, or FII) deficiency remains one of the rarest coagulation disorders of the rare bleeding disorders (RBDs). Quick first described a deficiency of prothrombin in 1947 after evaluating clotting abnormalities in two unrelated families.¹ In 1955 and 1962, Quick further classified these families as having two distinct types of prothrombin deficiencies, which he named hypoprothrombinemia and pseudo-hypoprothrombinemia, based upon the differences in coagulation assays observed in these two families despite similarly prolonged prothrombin times.^2,3 In 1969, Shapiro described the first prothrombin defect, Prothrombin Cardeza, which is characterized by low FII activity but normal FII antigen level.⁴

Thrombin, which is derived from the Greek word thrombos (meaning “to clot”), is a serine protease that is synthesized in the liver in the presence of vitamin K as the inactive zymogen prothrombin. Prothrombin is cleaved by factor Xa on the surface of platelets to generate the active enzyme a-thrombin. Prothrombin is a 72 kDa glycoprotein with a plasma concentration of 100 µg/ml (1-2 µM) and a half-life of approximately 3 days (70 hours).^5,6

In the propagation phase of coagulation, the prothrombinase complex plays a critical role in prothrombin activation. Factor Xa in the presence of calcium and factor Va on a phospholipid surface form the prothrombinase complex that cleaves FII at two peptide bonds to form active thrombin.^7,8 Thrombin subsequently cleaves fibrinogen to generate fibrin resulting in stable fibrin clot formation.

Thrombin plays a key role in multiple pathways of the coagulation cascade to induce thrombin generation including: 1) activation of platelet aggregation through protease-activated receptors (PARs) and glycoprotein 1b binding, 2) activation of factors V, VIII, and XI, which further potentiates prothrombin activation, 3) activation of factor XIII that crosslinks the fibrin clot, and 4) stabilization of clot formation through the activation of thrombin-activatable fibrinolysis inhibitor (TAFI).⁹

Regulation of thrombin is critical for preventing overstimulation of the clotting cascade and pathogenic thrombosis. Antithrombin is a small 58 kDa glycoprotein produced by the liver that exerts anti-FII and anti-factor X activity when coupled to heparin.¹⁰  Additionally, heparin cofactor II and nexin I inhibit thrombin catalytic activity and limit clot formation. Antithrombin, heparin cofactor II, and nexin I are all members of the serine protease inhibitor (serpin) superfamily. Glycosaminoglycans such as heparin increase the activation of these serpins.¹¹ Thrombin interacts with the endothelial protein thrombomodulin, which activates protein C in the anticoagulant pathway. Activated protein C subsequently inactivates factors Va and VIIIa, reducing clot formation. Thrombin also interacts with endothelial cells to release tissue plasminogen activator (tPA) via PARs that mediate the conversion of plasminogen to plasmin to enhance thrombolysis.¹²

Additionally, thrombin has also been shown to activate endothelial cells, vascular smooth muscle cells regulating vascular development and modeling, monocytes, T lymphocytes, fibroblasts, and mast cells via PARs.^5,13

The prothrombin gene (20,210 bp) is located on chromosome 11 and consists of 10 exons and 8 introns. FII activity levels of 20-40% are considered hemostatic. No living patients with truly unmeasurable levels of prothrombin activity have ever been identified, and complete absence of prothrombin appears to be incompatible with life.¹⁴ Additionally, a mouse knockout model of FII deficiency resulted in embryonic or neonatal lethality.^15,16

Inherited prothrombin deficiency is expressed in one of two forms: either a quantitative disorder, described as hypoprothrombinemia or type I deficiency, which is characterized by concordant low FII coagulation activity and antigen levels of the prothrombin protein; or a qualitative disorder, referred to as dysprothrombinemia or type II deficiency, and characterized by a discrepant low FII activity level but normal or near-normal FII antigen levels.

Missense mutations account for the majority of disease-causing mutations in prothrombin deficiency.¹⁷ Missense mutations that cause hypoprothrombinemia are primarily due to protein that is misfolded; as a result, prothrombin is either not secreted or is rapidly cleared.¹⁸ Nonsense mutations that lead to a stop codon and single nucleotide deletions are also encountered.

Two groups of mutations cause dysprothrombinemia: the first group of mutations is characterized by defects in prothrombin activation, while the second group is characterized by defects in the thrombin protein itself. The first group consists of prothrombin variants with defective cleavage of prothrombin by factor Xa, including Prothrombin Barcelona and Clamart as well as variants that are unable to bind calcium (e.g.  Prothrombin San Juan). These prothrombin mutations result in abnormal, slow prothrombin activation, but fully-functional thrombin is obtained once formed.^6,19 The second group consists of prothrombin mutations that result in either a thrombin defect that disrupts interactions with fibrinogen (i.e. Prothrombin Quick and Himi) or a defect in the catalytic region of thrombin (Prothrombin Tokushima).⁶

Some abnormal prothrombin variants may resist the inhibitory actions of antithrombin (termed antithrombin resistance), resulting in prolonged thrombin activity and a predilection for thrombophilia.²⁰ These prothrombin variants tend to have a mutation in exon 12 of the prothrombin gene, a region of thrombin that interacts with antithrombin. At least five prothrombin variants in 7 families have been described as presenting with venous thrombosis in the absence of another known pro-thrombotic defect, slightly decreased or borderline low FII activity, FII antigen levels that are higher than the FII activity level, and a family history of venous thrombosis.¹⁰

The well-known prothrombin 20210 guanine-to-adenine mutation (G20210A) is associated with an increased risk of thrombosis. The G20210A mutation is located in the 3’ untranslated region of the prothrombin gene, and results in higher levels of prothrombin.²¹ This mutation was first described in 1996, and is found in 2-3% of Caucasians and in 4-8% of individuals who present with their first venous thromboembolism (VTE).^22,23 Heterozygotes with this mutation have a 3-5 fold increased risk of VTE, with homozygotes having an even higher risk.²⁴

Prothrombin deficiency affects 1 in 2 million people, with higher rates reported in regions with increased rates of consanguinity.^17,25 A higher prevalence of patients of Latin/Hispanic origin with prothrombin deficiency is observed, especially with the dysprothrombinemias. Patients from areas such as Barcelona, Padua, Segovia, and Puerto Rico account for ~70% of all patients with FII defects.⁵ The significance of this association is unknown.