Platelet disorders and inherited or acquired deficiencies of hemostatic factors (eg, factor VIII, factor IX, or von Willebrand factor [vWF]) lead to excessive bleeding, as is widely recognized. Widespread experience with the use of thrombolytic agents in acute myocardial infarction currently indicates that excess plasmin, generated by thrombolytic drugs, increases bleeding risk. However, the fact that a deficiency of alpha2-plasmin inhibitor (alpha 2-PI, a2-PI), a physiologic inhibitor of fibrinolysis, can lead to excessive bleeding is not widely appreciated.
To date, only 15 cases of congenital homozygous alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) and 7 molecular defects of the alpha 2-PI gene have been reported. The first reported case involved a 25-year-old Japanese homozygous male born of consanguineous parents.1 He had a lifelong history of severe bleeding, starting with bleeding from the umbilical cord at birth. The patient experienced hematomas, prolonged bleeding from cuts and after dental extraction, and muscle and joint bleeds following minor trauma.1 Central nervous system (CNS) bleeding has also been described in a Dutch patient who was homozygously deficient.2
In 3 homozygous patients (sisters) from another Japanese family, bleeding was milder, with umbilical bleeding at birth followed by hematomas, gingival bleeding, and epistaxis without joint bleeding. The levels of alpha 2-PI were undetectable in all of the patients.
Most reported heterozygous patients did not have clinically significant bleeding, although some had a bleeding disorder. Currently, the reasons for variability in bleeding manifestations in heterozygous persons with alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) are unclear.
Alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) is the most important physiologic inhibitor of plasmin, which is the principal protease of the fibrinolytic pathway. Plasminogen activators convert the zymogen plasminogen to the active enzyme plasmin, which then hydrolyzes susceptible arginine and lysine bonds in a variety of proteins.3,4,5
Plasmin has a broad range of actions. Plasmin not only degrades fibrin, which is its principal substrate, but it also degrades fibrinogen, factors V and VIII, proteins involved in platelet adhesion (glycoprotein I and vWF), platelet aggregation (glycoprotein IIb/IIIa) and maintenance of platelet aggregates (thrombospondin, fibronectin, histidine-rich glycoprotein), and the attachment of platelets and fibrin to the endothelial surface.
A positive feedback mechanism exists whereby plasmin acts to further increase the generation of plasmin by converting Glu-plasminogen to Lys-plasminogen; Lys-plasminogen is more susceptible to activation by plasminogen activators. In addition, other noncoagulation proteins, such as complement, growth hormone, corticotropin, and glucagon, are substrates for plasmin. Therefore, the reasons for the bleeding disorder that develops due to the actions of excess unfettered and unneutralized plasmin are easily comprehended.
Alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) belongs to the serpin family of inhibitors, is synthesized by the liver, and is present in plasma as a single-chain protein in approximately half the concentration of plasminogen. Two forms of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) are present in blood; 70% of alpha 2-PI binds plasminogen and has inhibitory activity, whereas the remaining 30% is in a nonbinding form. The nonbinding form is a degradation product of the binding form and has little inhibitory activity.
A small amount of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) present in platelets contributes to inhibition of fibrinolysis in platelet-containing thrombi. Activated factor XIII (FXIIIa) cross-links alpha 2-PI to the a-chains of fibrin(ogen), thus making a cross-linked fibrin clot more resistant to lysis by plasmin.
Alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) reacts very rapidly with plasmin to form a stable plasmin-inhibitor complex. This interaction is central to the physiologic control of fibrinolysis and irreversibly inhibits plasmin activity, which in turn, partially degrades alpha 2-PI. The plasmin-alpha 2-PI complex is cleared more rapidly from the circulation. The half-life of the complex is approximately 12 hours compared with the longer half-life of 3 days for native alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI).
Alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) performs several functions. Alpha 2-PI inhibits free plasmin rapidly and more readily than fibrin-bound plasmin. Alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) is cross-linked to fibrin, thus conferring resistance to degradation by plasmin, and it interferes with the adsorption of plasminogen to fibrin. As a result, recent clots are more susceptible than older clots to degradation by plasmin.
Several other proteins are also involved in the complex process of regulation of fibrinolysis in vivo. Physiologically, the end result is that the hemostatic plug (fibrin and platelet clot) is protected from premature breakdown, leaving the fibrin meshwork intact so that it functions not only in hemostasis but also in wound repair as a scaffold for regenerating cells.
As the principal inhibitor of plasmin, alpha 2-PI plays a key role in the physiologic control of fibrinolysis by helping localize reactions to the sites where they are needed and by helping prevent systemic spillover. When the amount of plasmin generated exceeds the capacity of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) to neutralize plasmin (since, in plasma, plasminogen levels are twice those of alpha 2-PI) alpha 2-macroglobulin can function as a less efficient backup inhibitor.
Conceptually, alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) neutralizes plasmin at various sites of plasmin production, including in the fibrin clot, on the surface of cells, and in the fluid phase (For an excellent diagram showing these details, see Figure 2 in Castellino FJ, Ploplis VA. Plasminogen and streptokinase. In: Bachmann F, ed. Fibrinolytics and Antifibrinolytics. Berlin: Springer-Verlag; 2001:26-56.)6
Other inhibitors, such as antithrombin, alpha 1-antitrypsin, and C1 inactivator of complement, have in vitro antiplasmin activity, but these inhibitors may play only a minimal role in vivo.
In the absence of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI), plasmin degrades the primary hemostatic platelet-fibrin plug, thereby interfering with adequate primary hemostasis. Although fibrin formation is unimpaired, subsequent accelerated lysis of the formed fibrin plug (fibrinolysis) leads to the onset of delayed bleeding.
In pathologic states, in which there is an endogenous excessive activation of plasminogen or a secondary infusion of activators, such as tissue plasminogen activator (t-PA) and streptokinase, sudden generation of large amounts of plasmin overwhelms the neutralizing capacity of alpha 2-PI. In addition to degrading the primary fibrin-platelet plug, excess plasmin degrades circulating fibrinogen (fibrinogenolysis) and factors V and VIII, adding to the hemorrhagic diathesis.
Most patients with an inherited homozygous alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) have a clinically significant bleeding disorder that is characterized by prolonged bleeding and bruising following minor trauma and bleeding into the joints, similar to the manifestations seen in patients with hemophilia.
Gene knockout mouse models of alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) show the expected accelerated clot lysis, but the mice do not manifest the bleeding disorder that is seen in humans.
Very few cases of inherited alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) have been reported; therefore, data do not exist to determine the true frequency. In the next several years, as widespread high-throughput genomic testing becomes commonplace, the frequency of genetic defects will be known, and the frequency of these rare disorders can then be determined.
The frequency of acquired alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) depends on the frequency of the underlying disorders. As discussed in Causes, excessive bleeding can occur when alpha 2-PI levels are deficient.
Homozygous patients with alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) have severe bleeding that requires plasma therapy to limit the bleeding and to maintain plasma levels until the acute bleeding resolves.
Recurrent joint bleeds can lead to acute and chronic arthropathy, as occurs in severe hemophilia. Appropriate physical therapy, joint replacement, and treatment of chronic debilitating viral illnesses, such as hepatitis and acquired immunodeficiency syndrome (AIDS) and its sequelae, are needed in patients with alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency). Death may occur due to a CNS bleed or after major trauma.
Patients with an inherited homozygous alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) have a clinically significant bleeding disorder characterized by easy bruising, delayed onset of bleeding following trauma or surgery, menorrhagia, epistaxis, hematuria, and bleeding into joints, similar to the manifestations seen in patients with hemophilia.
The frequency of a bleeding disorder reportedly varies among patients who are heterozygous for alpha 2-PI deficiency and is characterized by a milder bleeding disorder in most heterozygotes, with a tendency to worsen with age.
The bleeding in patients with acquired disorders associated with alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) is described in Causes.
No ethnic predilection for alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) is known at this time because the overall number of reported cases is so small.
Alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) is inherited as an autosomal recessive trait.
Clinical manifestations of alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) may start at birth, with excess bleeding from the umbilical cord. Bleeding manifestations may start later in childhood, when trauma and minor cuts occur with increasing activity. Menorrhagia manifests following puberty in women.
In patients with severe alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency), bleeding patterns are similar to those seen in patients with hemophilia, as follows:
Delayed onset of bleeding after minor trauma
Prolonged bleeding from cuts and wounds, including mucosal bleeding
Increased bruising and hematomas, including muscle bleeding
Bleeding into joints following trauma rather than spontaneous joint bleeding
Excessive postsurgical bleeding (may be a clue in milder cases)
Increased bleeding following ingestion of nonsteroidal anti-inflammatory drugs (NSAIDs)
Physical findings in individuals with alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) depend on the site of bleeding, as follows:
Joint bleeds resulting in pain, swelling, and limitation of joint movement (Acute and chronic arthropathy similar to arthropathy seen in patients with severe hemophilia can develop because of chronic joint bleeds.)
Epistaxis and other mucosal bleeding
Gastrointestinal tract bleeding
Menorrhagia starting at menarche
Family studies in the few cases reported thus far suggest that alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) is inherited as an autosomal recessive trait.
Inherited reductions or inherited functional deficiencies of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) are due to specific defects in the gene coding for alpha 2-plasmin inhibitor, which is located on chromosome 17. The full genomic sequence and the functional implications of all its regions are not currently fully known. The molecular defect has been characterized in a few families.
In a family with severe deficiency, a trinucleotide deletion led to the synthesis of a dysfunctional protein, which was retained within the cell.
In another family, trinucleotide duplication led to production of a dysfunctional protein that could not inhibit plasmin.
In a third family, a single base insertion in a codon near the 3′ end was the molecular basis for the transcription of an abnormal protein, which had abnormal intracellular transport leading to a plasma deficiency.
One specific polymorphism has been found in several white and Japanese persons and will help in the search for future defects.
Acquired causes of alpha 2-plasmin inhibitor deficiency (alpha 2-PI deficiency, a2-PI deficiency) reflect the frequency of the associated disease state. Specific clinical conditions that lead to a reduction in the level of alpha 2-plasmin inhibitor are as follows:
Alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) levels in ill neonates are lower than the reference range levels found in healthy full-term neonates and are similar to adult levels. However, the level of the plasmin-alpha 2-PI complex was increased in both healthy and ill neonates, with levels higher than those seen in adults.
Increased levels of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) contribute to inhibition of fibrinolysis during pregnancy. However, recognizing the significant role played by plasminogen activator inhibitor type 2 in dampening fibrinolysis is important. Plasminogen activator inhibitor type 2 is produced in increasing amounts by the placenta as the pregnancy advances.
In a study involving women in labor, t-PA levels increased starting early in labor and remained high after placental separation. However, after placental separation, an increase in plasmin–alpa 2-PI complex levels occurred together with an increase in fibrinopeptide A and thrombin-antithrombin complex levels, indicating activation of fibrinolysis before the development of a hypercoagulable state induced by placental separation.
Physiologic examples of increased local fibrinolysis include ovulation and the fluidity of menstrual blood loss. Patients with menorrhagia may have excessive local fibrinolysis and may benefit from antifibrinolytic therapy, but no relationship to reduced levels of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) has been proven in these patients.
The liver plays a central role in hemostasis. Synthesis of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) and other physiologically important inhibitors of hemostasis, synthesis of procoagulant, and clearance of activated coagulation factors are regulated by the liver. Severe liver disease is associated with reductions in alpha 2-plasmin inhibitor levels, and the reduction is probably a contributing factor in the well-recognized excessive fibrinolytic activity seen in some patients with liver disease.
Due to decreased synthesis of inhibitors, including alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI), and a decreased ability to clear activated coagulation factors, patients undergoing orthotopic liver transplantation have excess fibrinolytic activity, particularly during the anhepatic phase, which contributes to increased bleeding.
Systemic thrombolytic therapy
Patients receiving activators of fibrinolysis, such as t-PA or streptokinase, for the treatment of acute myocardial infarction or for extensive venous thromboembolic disease develop a systemic fibrinogenolytic state, with excess plasmin generation resulting from the use of pharmacologic doses of the activators.4,6,7,8 Reduced alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) levels are common after the use of these agents, with a greater reduction after streptokinase than after t-PA administration. The increased incidence of bleeding into the CNS in older patients and of large hematomas at invasive sites are the result of excess plasmin, which degrades all recent thrombi and cannot distinguish between a physiologic hemostatic plug and a pathologic thrombus.
Bleeding after cardiopulmonary bypass surgery
Reduced alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) levels with reduced fibrinogen levels, increased fibrin split products, and higher levels of plasminogen activator inhibitor type 1 were found in mediastinal blood that was shed by patients who had again undergone exploratory surgery for excessive bleeding following open heart surgery and who had negative intraoperative findings. The high local fibrinolytic activity with reduction of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) levels was believed to be secondary to clot formation in the chest; irrigation and removal of the clots along with the use of inhibitors of fibrinolysis help reduce excess local fibrinolytic activity in the chest cavity
In a study, patients undergoing bypass surgery for coronary artery disease were evaluated prospectively, with the study group receiving aprotinin priming of the pump and an intravenous (IV) infusion during bypass surgery.9 Alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) levels were reduced in the control group, with a marked increase in fibrin split-product and plasmin–alpha 2-PI complex levels, indicating fibrinolysis activation secondary to coagulation activation. Aprotinin treatment effectively suppressed hyperfibrinolysis and reduced postoperative blood loss.9
Primary fibrinolysis during supraceliac aortic clamping
Excessive fibrinolysis was found within 20 minutes of clamping in patients undergoing supraceliac aortic clamping but not in patients undergoing infrarenal aortic clamping. Laboratory tests revealed the presence of a primary fibrinolytic state, as evidenced by a reduction in euglobulin lysis times (measure of total fibrinolytic activity in the absence of physiologic inhibitors within the testing system), increased t-PA levels, elevated ratios of t-PA to plasminogen activator inhibitor type 1, and reduced levels of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI). The supraceliac aortic clamping caused hepatocellular injury with prolonged circulation of t-PA, leading to a profibrinolytic state characterized by an excess generation of plasmin with alpha 2-plasmin inhibitor depletion.
Increased fibrinolytic activity of lung cancers has been documented over many years. In a series from Japan, 70 patients with both nonsmall cell and small cell lung cancer were studied. Increased levels of plasmin–alpha 2-PI complex had prognostic significance and predicted poor survival independent of other factors, such as histologic findings, age, sex, and presence of metastatic disease, compared with control subjects. Other studies of patients with lung cancer confirmed the presence of increased levels of plasmin–alpha 2-PI complex, although they found a correlation between higher values of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) and histologic findings and/or the extent of disease. Plasmin–alpha 2-PI complex might be useful as a marker to predict outcomes in patients with malignancies.
Arterial disease and atherosclerosis
One group of patients with intermittent claudication and another group with coronary artery spasm were found to have increased levels of plasmin–alpha 2-PI complex. In addition, the group with intermittent claudication had higher thrombomodulin levels, whereas the coronary artery spasm group had high levels of thrombin-antithrombin complex. The complex is a sign of activation of coagulation and fibrinolysis secondary to vascular injury.
Fibrinolytic parameters after severe trauma
In a prospective study of the fibrinolytic system in patients admitted with severe trauma, patients had a reduced level of alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) at admission, with increased levels of t-PA antigen and plasminogen activator inhibitor activity.
Enhanced fibrinolysis during hemodialysis
In a study of patients undergoing regular hemodialysis, plasminogen and alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) levels were reduced, with increased levels of plasmin–alpha 2-PI complex present before hemodialysis. Serial sampling during a hemodialysis session showed a continuous fall in alpha 2-plasmin inhibitor levels, with rising levels of plasmin–alpha 2-PI complex at the end of hemodialysis. t-PA activity and antigen levels rose concomitantly, but plasminogen activator inhibitor type 1 antigen levels dropped, without any further rise in the basal level of cross-linked fibrin degradation products. These findings suggest the presence of a hyperfibrinolytic state before hemodialysis, with further increase during hemodialysis.
Patients with acute promyelocytic leukemia are treated routinely with heparin for disseminated intravascular coagulation (DIC), but the hemostatic defect may be due to accelerated fibrinolysis resulting from the release of both t-PA and urokinase-type plasmin activator (u-PA) by leukemic cells. Reduced alpha 2-plasmin inhibitor (alpha 2-PI, a2-PI) levels have been used as a criterion to treat these patients with epsilon-aminocaproic acid (EACA; 6-aminohexanoic acid, Amicar) in combination with heparin, with improvement in bleeding and in abnormal laboratory test findings.