Antithrombin deficiency is a blood disorder characterized by the tendency to form clots in the veins (thrombosis). An inherited tendency to thrombosis is known as thrombophilia. Antithrombin is a substance in the blood that limits the blood’s ability to clot (coagulation) and is the primary inhibitor of thrombin, which is required for the development of blood clots; it also is the primary inhibitor of two clotting factors, factor Xa and factor IXa, that are required for the generation of thrombin. In people with congenital antithrombin deficiency, there is a reduced amount of this substance in the blood due to a genetic abnormality. Antithrombin deficiency may also be acquired; in such cases, the disorder may be reversible with resolution/improvement in the disease process responsible for the deficiency.
Pathophysiology
Antithrombin is a plasma glycoprotein consisting of 432 amino acid residues integral in the regulation of the coagulation process during bleeding. Antithrombin most notably binds to serine proteases factor II (thrombin), factor IXa, and factor Xa which inhibits the blood clotting process involved in the coagulation cascade pathway. As part of the normal physiological response to bleeding, platelets circulating the plasma become initially activated by multiple factors produced from endothelial cells to aggregate and form a plug. Circulating fibrinogen is then converted into fibrin by thrombin through a series of protease activations, which constitute the reactions of the coagulation cascade pathway. Fibrin acts to stabilize the initial platelet-created plug which determines the completion of the clot formation.[rx]
Antithrombin is part of a family of serine protease inhibitors known as serpins. Serpins generally consist of a highly conserved structure of amino acid chains organized into three beta-sheets, nine alpha sheets, and a reactive center loop (RCL) designated as the sequence of amino acids which serve as the reactive site for protease interaction[rx]. The RCL loop in antithrombin exists along the amino acid chain sequence at the 393 arginine residue and 394 serine residue near the carboxyl-terminal of the amino acid sequence. It is the region within the antithrombin that creates the antithrombin-protease complex for inhibition.[rx]
Synthesis of antithrombin occurs primarily in the liver initially as an immature protein chain made up of 464 amino acid residues. The amino acid chain is then cleaved at the N-terminal by 32 amino acids, thereby creating a mature 432 amino acid sequence protein. The mature protein contains three disulfide bonds that intermolecularly link six cysteine residues together. This configuration allows for four potential glycosylation sites to exist within the molecule.[rx] Depending on the number of occupied glycosylation sites, anti-thrombin further subcategorized into two isoforms: alpha antithrombin and beta antithrombin.[rx]
Alpha antithrombin refers to antithrombin in which oligosaccharides bind all four glycosylation sites. It is the predominant configuration of antithrombin, presenting as around 90% of the antithrombin in the plasma.[rx] Beta antithrombin, however, refers to antithrombin with three of the four sites occupied, with the oligosaccharide chain at Asn135 missing.[rx] This change in configuration increases the affinity of beta antithrombin binding to heparin at a designated heparin-binding domain.[rx] The binding of heparin dramatically increases the affinity for antithrombin to bind to serine protease, enhancing the functional efficiency of antithrombin to inhibit clot formation.
Mechanism
Antithrombin has two specific binding sites- the reactive site consisting of the reactive center loop (RCL) which binds proteases such as thrombin, factor Xa, IXa, and the heparin-binding domain which, as the name suggests, binds heparin.[rx]
The reactive center loop of antithrombin located at the arginine residue at 393 and the serine residue at 394 near the carboxyl-terminal functions to bind to the active site of proteases through a complex mechanism involving a conformational change of the antithrombin reactive site.[8] Strands from beta-sheet A of the antithrombin site separate halfway along their length and the RCL subsequently rearranges at the point of entry into sheet A along with various other conformational changes, which serve to increase the mobility of the RCL. The increased mobility provides a docking site for the protease, which in turn creates an irreversible complex.[rx]
The inhibitor-protease complex is then rapidly removed from the circulation no more than 5 minutes after formation, which removes thrombin from the circulation, disrupting the clotting effect of the coagulation cascade. While the exact mechanism is still uncertain, evidence may suggest receptors on hepatocytes to be involved in the removal of the complex from the plasma.[rx]
The formation of the antithrombin-protease complex, while irreversible, is a naturally slow and inefficient reaction. The process can be rapidly increased up to 1000-fold with the presence of sulfated polysaccharides in the form of heparin and heparan sulfate.[8] Heparin contains a unique pentasaccharide sequence within its glycosaminoglycan chain consisting of negatively charged sulfate groups, which is responsible for its high affinity towards the antithrombin heparin-binding domain.
The heparin-binding domain located on the surface of antithrombin, on the other hand, contains positively charged arginine and lysine which bind to the negatively charged domains of the heparin pentasaccharide sequence through partially an allosteric mechanism.[rx]
The successful binding of heparin activates a conformational change within the antithrombin, which increases its affinity towards protease, promotes the formation of the antithrombin-protease complex, and ultimately inhibits blood coagulation.[rx]
Allosteric activation induced by heparin to the antithrombin serpin structure has been the object of extensive study, and while the kinetics of the reaction is quite complex, generally the allosteric activation of antithrombin induces structural changes within the RCL, the heparin-binding site, and the hydrophobic core that constitutes the antithrombin. The exact interactions involved in conformational change are still a topic for research and revision.[rx]
Causes
Antithrombin deficiency may be inherited or acquired. Inherited AT deficiency increases the risk of blood clots; acquired AT deficiency often does not. Acquired AT deficiency is the consequence of some other disorder, usually involving the liver, kidneys, or treatment of certain types of blood disorders, e.g., leukemias with a drug called L-asparaginase. Low antithrombin levels may also be temporarily associated with some other conditions such as heparin therapy and disseminated intravascular coagulation usually due to a severe infection of the bloodstream, severe trauma, severe burns, or the presence of acute blood clots.
Hereditary antithrombin deficiency is caused by changes (mutations) in the SERPINC1 gene and many different mutations in this gene are responsible for individual cases of antithrombin deficiency.
Hereditary antithrombin deficiency is inherited as an autosomal dominant condition. Dominant genetic disorders occur when only a single copy of an altered gene is necessary for the appearance of the disease. Heterozygote is the term used to describe such a person. However, not every person who has the altered gene will develop a blood clot. Geneticists call this variable clinical penetrance. Thus, antithrombin deficiency is an autosomal dominant disorder with variable clinical penetrance. The altered gene can be inherited from either parent or very rarely can be the result of a new mutation in the affected individual. The risk of passing the altered gene from affected parent to offspring is 50% for each pregnancy. The risk is the same for males and females.
A person who inherits two altered genes, one from each of the parents is known as a homozygote. Homozygous babies with antithrombin deficiency seldom survive though there are rare cases with so-called type IIB mutations.
Diagnosis
A low blood level of antithrombin suggests that the patient may have antithrombin deficiency. However, it is important to keep in mind that many conditions can lower antithrombin levels (acute clots, heparin therapy, liver or kidney disease, etc.) without the patient having inherited antithrombin deficiency. Repeat testing should be done at a time when the patient is not ill, is not on heparin, and does not have related medical problems.
Treatment
Due to a lack of clinical studies, hematologists differ in their opinions regarding the treatment of antithrombin deficiency. Often, intravenous antithrombin concentrates are prescribed when surgery or infant delivery is close at hand. Antithrombin concentrates are also used to prevent venous clots when blood thinners (such as heparin) are not advisable because they may lead to an increased risk of bleeding. This is especially true for neuro-surgery and in a severe trauma or at the time of delivery.
For people with very low antithrombin levels, heparin may not work well if administered alone. This is called heparin resistance. For heparin to work properly an adequate amount of antithrombin must be present in the blood. If heparin treatment is ineffective, then antithrombin concentrate may be prescribed.
Women with antithrombin deficiency are at particularly high risk for developing clots during pregnancy or after delivery. Reports of the incidence of blot clots during pregnancy in women with antithrombin deficiency range from 3% to 50%. Many recommend the use of subcutaneous low molecular weight heparin injections during pregnancy for women with antithrombin deficiency.
Pregnant women with antithrombin deficiency are at a slightly increased risk of losing the fetus without treatment. Pregnancy loss is likely due to blood clots forming in the placenta and cutting off the blood supply and oxygen to the fetus.
As noted above, patients with antithrombin deficiency who undergo surgery are at increased risk of a thromboembolic event unless appropriate preventive measures are taken. The duration of treatment with blood thinners or antithrombin concentrate depends on the type of surgery. In some cases, treatment will last only a few days while in other instances treatment may last for several weeks.
A family in which one or more members have antithrombin deficiency should consult with a hematologist and genetic counselor, who can help the family understand and cope with the disorder.
Two different concentrates of antithrombin concentrate are available in the US. Antithrombin concentrate (Thrombate) is a highly purified and viral-safe product prepared from pooled normal human plasma. Its half-life in the circulation is approximately 2.8 to 4.8 days. Recombinant human antithrombin (Atryn), produced from the milk of transgenic goats, is also available. This product is only approved for use in high-risk situations (eg, surgery, childbirth) in patients with antithrombin deficiency.
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