The Multi-Factorial Threshold Model of Thrombotic Risk

The Multi-Factorial Threshold Model of Thrombotic Risk Richard A. Marlar, PhD and Dorothy M. Adcock, MD The incidence of venous thrombosis in the US is between two and three million per year, resulting in 60,000 deaths. There are a number of risk factors that increase the risk of developing thrombosis and these thrombogenic factors may be genetic, acquired, or physiological. Many risk factors are still unknown. These factors induce thrombus formation through a number of different mechanisms such as activation or up-regulation of the coagulation system, down-regulation of endogenous coagulation regulatory mechanisms, impairment in the fibrinolytic mechanism, and endothelial cell perturbation. The mechanisms or causes of thrombosis involve numerous components including plasma proteins, platelets, and blood vessels. Volume 17, Number 6 June 2003 Objective: The reader will be able to discuss the theory of the multi-factorial threshold model of thrombotic risk and site examples of the interaction of risk factors that lead to thrombosis. Introduction During the last 10-15 years, a number of genetic defects have been found in families with a notable history of venous thrombosis, suggesting a cause/effect relationship. These defects can be grouped into a clinically related genetic disorder termed hereditary thrombotic disease (HTD) or thrombophilia. The best and most robust studies of the genetic influence in thrombosis have been conducted in large families with established risk as a result of an inherited disorder. Hemostatic Balance The hemostatic system attains its fine balance due to the interplay of proteins that serve to promote clot formation and those proteins that induce clot lysis or fibrinolysis. Although the traditional coagulation cascade has been modified in the past decade to reflect the importance of tissue factor as a major component, the formation of a thrombus is essentially accomplished through a series of reactions involving enzymatic factors and their cofactors. Each of the prothrombotic factors is modulated by naturally occurring anticoagulants which serve to regulate thrombus development. Concept of Risk Factors The concept of thrombotic risk in the development of a thrombotic event is contributing to the multi-hit theory in which genetic, environmental, acquired, and unchangeable factors play a major role. This theory parallels the multiple risk-factor concept of coronary heart disease. Each thrombogenic risk factor has a different risk potential, defined as the ability or capacity of the factor to contribute to the thrombotic etiology. The degree of this risk potential varies depending on the factor and its in-

CLINICAL HEMOSTASIS REVIEW 7700 E. Wrightstown Road,Ste.106 Tucson, AZ 85715 520.722.0797 chr@coagulation.com EDITOR IN CHIEF Rebecca Jensen, MT(ASCP) CLINICAL ADVISORS Dorothy M. Adcock, MD Alexander Duncan, MD, ChB H. James Day, MD, FACP Don W. Hill, MD, FACP CONTRIBUTOR Lynne Stevens, MT(ASCP) ASSITANT TO THE EDITOR Kendra Sagar Clinical Hemostasis Review is published by Esoterix Coagulation, Inc. and is circulated to selected physicians and laboratorians. Copyright 2003. Esoterix Coagulation is part of the ESOTERIX, Inc. family of laboratories providing exoteric testing in numerous disease corridors. The opinions expressed in the articles are those of the author(s) and do not necessarily reflect the opinions or recommendations of the advertisers, editors, or publisher. The publisher reserves copyright and renewal on all published material and such material may not be reproduced in whole or in part without written permission from the publisher. Consult the full prescribing information on any drugs or devices discussed. All correspondence should be directed to the attention of the Editor, Clinical Hemostasis Review, 7700 E. Wrightstown Road, Ste. 106, Tucson, AZ 85715. Subscription Rate: $65.00/year, 14 issues, prepaid. Outside the USA additional postage is required: Canada $20.00/year, all other destinations $50.00/year. Single copies $7.00. Subscriptions not accompanied by payment will be assessed a billing charge. To assure prompt delivery of your issues, please notify us of any address corrections. Six weeks are required to effect a change. Missing copies not received by mail will be provided free of charge if we are notified no later than two months after the issue date. After this deadline, we will charge subscribers $5.00 per issue. ISSN 0894-1025 teraction with other factors. Most factors interact synergistically to cause a significant increase in risk compared to each factor alone. In some instances, factors may not interact to increase risk or may even interact to lessen thrombotic risk. Multiple genetic risk factors with varying risk potentials are found in the more severely affected individuals and families. While genetic risk factors are constant throughout the person s life, risk potential associated with each factor may increase with age, as increasing age is an important risk factor for thrombus development. Genetic Factors The emerging concept is that a patient with HTD has at least one and probably more genetic risk factors for thrombosis, thus increasing the potential for developing thrombosis. In fact, the prevalence of some of the genetic alterations or deficiencies of coagulation proteins increases the odds that a given individual will have multiple deficiencies. These factors include deficiencies of protein C (PC), protein S (PS), and antithrombin (AT), as well as factor V Leiden (APC Resistance), prothrombin G20210A mutations, and congenital or acquired hyperhomocysteinemia. When deciphering an individual s risk for developing thrombosis, it is critical that an accurate and complete family history be obtained. It is important to ask the causes of death of blood relatives in order to determine whether an inherited coagulation disorder may be present. The patient should be questioned specifically about a history of blood clots, problems with the legs (such as deep venous thrombosis) or lungs ( such as pulmonary embolus) and whether there is a history of recurrent miscarriage, stillbirth, or infants that are small for gestational age. Also ascertaining whether blood relatives have been prescribed blood thinners may be helpful to assess the potential of venous thromboembolic disorders among the patient s family members. The patient s personal history of thrombosis is particularly important in determining risk of recurrence. A previous finding of venous thrombosis or clinical conditions associated with thrombosis should be determined while obtaining the patient s history. It is important to ascertain any lifestyle or acquired risk factors that may be present. However, despite a thorough evaluation of the patient s personal history, thrombotic disorders can remain asymptomatic in an individual for many years. The potential and sometimes fatal consequence of deep vein thrombosis and pulmonary embolism gives emphasis to early detection of thrombotic risk. Abnormalities of the Naturally Occurring Anticoagulant Systems Abnormalities of any of the naturally occurring anticoagulant systems such as a deficiency or dysfunctional molecule, can increase thrombotic potential. The two most important anticoagulant systems that control the clotting cascade include the protein C system and the antithrombin system. Many abnormalities have been identified within the protein C system that are associated with HTD including quantitative and qualitative deficiencies of PC, quantitative and qualitative deficiencies of PS, deficiencies of thrombomodulin, and genetic alterations in the factor V molecule that make it resistant to inactivation by activated PC. Homozygous deficiency of either PC or PS, a rare occurrence, results in lethal thrombotic disease in the neonate unless treated. Heterozygous PC and PS deficiency cause 8% and 12% of HTD respectively. The prevalence of PC deficiency is estimated at 1 in 500 individuals while the prevalence of PS deficiency is believed to be similar. The odds ratio of thrombosis is estimated to be 14 for PC deficiency and 11 for PS deficiency. Assays to detect PC or PS deficiencies are readily available in many special coagulation laboratories. Screening for PC deficiency is best accomplished with a PC activity assay while PS deficiency is best screened using a free PS antigen assay. Rarely, individuals may have a dysfunctional molecule and this can be determined by comparing results of an activity an antigen assay. Alterations to the factor V molecule that result in resistance to activated protein C, represent one of the most common abnormalities associated with HTD. The most frequently identified alteration is called factor V Leiden and is found in approximately 90% of individuals with APC resistance. The frequency of the mutant allele in Western countries is approximately 2%-7%. Factor V Leiden is primarily restricted to Caucasians and is most prominent in individuals of Scandinavian and northern European 2 CLINICAL HEMOSTASIS REVIEW / JUNE 2003

descent. The mutation is relatively rare in Africans, Australians, Southeast Asians, and in the Native American Indian population. Heterozygous factor V Leiden is associated with a 5- to 10-fold increased risk of thrombosis, while homozygous individuals have a 50- to 100-fold increase. Many coagulation laboratories screen for APC resistance using a plasma based assay, and a number of special coagulation laboratories can detect the genetic mutation for factor V Leiden. Abnormalities of AT related to HTD are the result of quantitative deficiencies of the protein or dysfunctional forms of the molecule. The prevalence of AT abnormalities in the general population is 1 in 5,000. AT abnormalities constitute an uncommon cause of HTD accounting for only 1% of individuals. However, the odds ratio for thrombosis in those with AT deficiency is approximately 24; a greater thrombotic risk than is seen in PC or PS deficiency. Assays to detect AT deficiency are available in most coagulation laboratories. An activity assay should be performed as the screening assay to test for AT deficiency. Other Causes of Thrombophilia A polymorphism of the prothrombin gene, prothrombin G20210A, is also a common risk factor for HTD. The prevalence of this variant is approximately 2.8% in the Caucasian population. Therefore, this abnormality is second only to factor V Leiden as a genetic risk factor for venous thrombosis. The odds ratio for developing thrombosis is estimated at 3%. The prothrombin G20210A mutation can be detected with genetic assays available through most special coagulation laboratories. The increased risk of thrombosis is thought to be due to elevations of prothrombin levels secondary to the mutation. Assays of prothrombin activity, however, have not been used successfully to screen for the G20210A mutation. Hyperhomocysteinemia is an important risk factor for both venous and arterial thrombosis. The risk associated with elevated homocysteine levels is graded, that is, the higher the homocysteine level, the greater the risk. A number of polymorphisms affecting enzymes involved in homocysteine metabolism may lead to hyperhomo-cysteinemia. There are two polymorphisms that affect methylenetetrahydrofolate reductase (MTHFR). The C677T polymorphism causes the enzyme to be thermolabile. This polymorphism can be seen in the heterozygous state in about 50% of the population and the homozygous state can be observed in 5% of the general population. Heterozygous MTHFR C677T is not thought to increase thrombotic risk. Individuals that are homozygous for the mutation may develop hyperhomocysteinemia if they are deficient in folate or certain B vitamins. Another mutation in MTHFR is the A1298C mutation. This polymorphism is thought to increase thrombotic risk only when in the heterozygous state and in the presence of the heterozygous MTHFR C677T mutation. These mutations can be detected by a number of special coagulation laboratories. Acquired Risk Factors Acquired thrombotic risk factors, along with environmental and unchangeable factors, may contribute synergistically with genetic factors to establish the overall risk potential for each person. There is a myriad of acquired risk factors and each can be either transient or long standing. Acquired factors include underlying disease states such as antiphospholipid antibody syndrome, as well as transient factors such as surgery or pregnancy as shown in Table 1. Some prescribed medications such as oral contraceptives or chemotherapeutic agents also confer a risk for thrombosis. Acquired and environmental factors may increase the risk periodically. An important consideration is that these risk factors may be in part controlled by the patient s lifestyle. In some circumstances, acquired risk factors can be anticipated and treated to avoid contributing to thrombotic risk. Environmental Factors Environmental factors or lifestyle factors such as smoking, prolonged bed rest, immobility, or poor nutritional habits that lead to obesity can contribute to an increased risk of thrombosis. Changes in patient lifestyle such as cessation of smoking can decrease thrombotic risk in a relatively rapid fashion. One environmental cause of thrombotic risk that has recently been afforded considerable press is airline travel. Although first named economy! Antiphospholipid antibody syndrome! Atrial fibrillation! Congestive heart failure! Diabetes mellitus! Estrogen therapy! Heparin-induced thrombocytopenia! Hypertension! Hyperviscosity! Immobilization! Lupus anticoagulant! Malignancy! Myeloproliferative disorders! Nephrotic syndrome! Oral contraceptive therapy! Paroxysmal nocturnal hemoglobinuria! Postoperative state! Pregnancy! Thrombotic thrombocytopenic purpura! Trauma Table 1. Acquired Risk Factors Associated with Increased Risk of Thrombosis syndrome, the risk of thrombosis is attributed to the pooling of blood in lower extremities that can occur with immobility or long periods of time sitting in one position. Thus, this risk factor can occur at any classification of airline travel and also can be seen following long car trips as well or prolonged periods sitting at a desk. Periods of immobility such as that found during travel, following trauma, or during brief illness are associated with approximately a three-fold increase in risk of venous thromboembolic disease according to Eekoff, et al. Since our global economy increasingly requires transcontinental travel, enhancing the likelihood of travel-induced thrombosis, it is probable that clinicians will be presented with such cases. Unchangeable Factors Lastly, two primary unchangeable factors, age and sex, are known to contribute to thrombotic risk. Advancing age after the age of 40 increases thrombotic risk for both genders. The influence of gender on thrombotic risk is primarily hormone related. The risk of developing venous thrombosis is increased in women on estrogen therapy and also during pregnancy. In 4 CLINICAL HEMOSTASIS REVIEW / JUNE 2003

Genetic Risk Factors Multiple risk factors can interact to exceed the thrombotic threshold and thrombosis occurs Acquired Risk Factors Relative Thrombotic Risk Thrombotic Threshold Physiological Risk Factors Figure 1. The Risk of Thrombosis in an Individual as Interpreted by the Thrombotic Thereshold Trait Theory fact, the most common cause of maternal death in pregnancy is venous thrombosis. Thrombotic Threshold Trait Many patients, even those with inherited deficiencies of coagulation proteins, experience thrombosis or thromboembolic events only when a combination of risk factors exists. Genetic, environmental, and acquired factors may interact in a synergistic manner, substantially increasing the thrombotic potential at any given time within the patient s life. The interaction of numerous risk factors to produce a phenotype of thrombosis is a concept familiar to geneticists, and is termed multi-factorial trait. Multi-factorial trait is the combined effect of multiple genes (and possibly acquired factors) to produce the observed phenotype, which in this case is thrombosis. Dr. Marlar and colleagues have combined the theories of multi-factorial trait with HTD and propose a new concept termed Thrombotic Threshold Trait (TTT). The TTT concept is a more accurate description of the interplay between the genetic, acquired, and physiological factors in families with a high incidence of thrombosis as shown in Figure 1. The details and interactions of the genetic risk factors and the acquired factors have not been clearly defined as yet, and many of these risk factors still remain unknown. In the general theory of TTT, all of the risk factors (genetic, acquired, and physiological) interact to give a relative risk potential. As shown in Figure 1, if this risk potential is above a threshold, then any acute stimulus will trigger the thrombotic process, and a thrombus will form. The thrombotic risk factors can be divided as follows: 1) type of risk (genetic, acquired, etc.), 2) level of risk (high risk, moderate risk, low risk), and 3) interactive effect (synergistic, null, or negating). Some of the risk factors may include hemostatic factors, while others may be cellular or non-hemostatic mechanisms. An individual may have significant underlying risk to form a thrombus, but an initiating stimulus is still necessary to induce the thrombotic process. The underlying risk factors either enhance the process of thrombus formation or inhibit the regulatory mechanisms. The importance of each risk factor and its interaction is beginning to be elucidated by epidemiological studies, and further refinement of the TTT theory is currently being investigated. It is the role of the clinical laboratory to determine the genetic markers of thrombotic risk (either with DNAbased or plasma-based testing), while the clinical evaluation (personal and family history) is used to determine the acquired and environmental risk factors, as well as to determine the potential for and/or cause of a thrombotic episode. Examples of Multi-factorial Theory Since the prothrombin gene G20210A mutation and factor V Leiden JUNE 2003 / CLINICAL HEMOSTASIS REVIEW 5

are relatively common in Caucasian populations, combined genetic defects are increasingly recognized. Researchers are currently investigating whether some genetic factors may in fact co-segregate. Furthermore environmental factors such as travel, or concomitant drug therapies such as oral contraceptives have demonstrated a synergistic influence on thrombotic risk in persons with genetic mutations. One of the more common observances of the multi-factorial threshold model is the increased thrombotic risk seen in women with factor V Leiden mutation who are also taking oral contraceptives. The risk of thrombosis is 35- to 50-fold greater in women on oral contraceptives that have either factor V Leiden or prothrombin gene G20210A mutation. The greatest risk of thrombosis is during the first year of oral contraceptive administration. Conclusions Physicians should become increasingly aware of the convergence of environmental, genetic, and acquired risk factors as contributors to thrombotic disease. The discovery of risk factors in an asymptomatic individual does not preclude the need for observation of the individual and certainly suggests that the individual should be educated regarding the signs and symptoms of pathologic blood clot formation. As mutations of hemostatic proteins continue to be identified, more individuals will potentially be acknowledged as having increased thrombotic risk and receive prompt treatment should thrombosis develop or appropriate prophylactic therapy to avoid thrombosis during times of increased risk. Acknowledgements This work was supported in part by a grant from the Department of Veteran s Affairs (MERIT Review to RAM). References Adcock DM, Fink L, Marlar RA. A laboratory approach to the evaluation of hereditary hypercoagulability. Am J Clin Path. 1997;108:434. Adcock DM. Hereditary thrombotic disease: A primer-part I. Clinical Hemostasis Review. 1999;13:10. Bovill EG, et al. Hereditary thrombophilia as a model for multigenic disease. Thromb Haemost. 1999;82:662. Bovill EG. Hereditary thrombophilia: A model for multigenic disease. Presented at Hemostasis and Thrombosis. 2000. Phildadelphia, PA. DeStefano V, Finazzi G, Mannucci PM. Inherited thrombophila: Pathogenesis, clinical syndromes, and management. Blood. 1996;87:3531. Goldhaber SZ. Venous thromboembolism: Clinical impact and multifactorial etiology. In: Goldhaber SZ, Hirsh J, Marder VJ, Salzman EW, Eds. Hemostasis and thrombosis: Basic principles and clinical practice. Philadelphia, PA: JB Lippincott; 1994;2543-1561. Lewis R. Multi-factorial traits. In: Human genetics: Concepts and applications. Dubuque, Iowa: WC Brown Publishers; 1994:93-109. Marlar RA, et al. Thrombotic threshold trait: Concept of a multi-hit theory for hereditary thrombotic disease. Clinical Hemostasis Review. 1998;12:12. Marlar RA. Hereditary thrombotic disease: New concepts and cost effective lab testing. 1998;1. Nowak-Gottl U, et al. Risk of recurrent venous thrombosis in children with combined prothrombotic risk factors. Blood. 2001;97:858. Review Analysis. Clin Appl Thromb Hemost. 1996;2:227-236. Ridker PM, eds. Thrombosis and Thromboembolism. New York: Marcel Dekker, Inc. 2002:211-25. Zoller B, Berntsdotter A, Garcia de Frutos P, Dahlback B. Resistance to activated protein C as an additional genetic risk factor in hereditary deficiency of protein S. Blood. 1995;85 (12):3518. Adcock DM, Marlar RA. Laboratory evaluation of hereditary thrombotic disease: Interpretation of assay results. Clinical Hemostasis Review. 1998;12: 13. 6 CLINICAL HEMOSTASIS REVIEW / JUNE 2003