HYPERCOAGULABLE STATES: GENETIC TESTING FOR PROTHROMBIN 20210 G>A MUTATION

Risk factors for venous thromboembolism may be inherited or acquired. A variety of genetic risk factors have been described, the most prominent being activated protein C resistance caused by Factor V Leiden. The prothrombin molecule has recently been targeted as an additional genetic risk factor because of the discovery of a mutation in its 3'-untranslated region. Prothrombin is the inactive precursor of thrombin. Thrombin is the central protease of the clotting cascade, activating both blood clotting proteins and platelets; thrombin is the key effector enzyme in the development of vascular thromboses. The 21kb prothrombin gene is located on chromosome 11 and consists of 14 exons as well as both 5' and 3' untranslated regions. The prothrombin gene variant is a G to A transition at nucleotide position 20210 and is located in the 3' untranslated region (1, 2). The location of the mutation has led to the hypothesis that it has an affect on translational regulation by increasing either translational efficiency or mRNA stability. One consequence of the mutation would therefore be an increase in the prothrombin plasma levels, and in fact, elevated prothrombin levels have been found in heterozygous carriers, along with increased thrombin formation. These changes would be likely to induce a hypercoagulable state (1, 3), and indeed, this genetic variant of prothrombin has been found to be associated with an increased risk of deep vein thrombosis (DVT) and perhaps arterial thrombosis as well. The 20210 G>A mutation frequency among northern Europeans presenting with a first DVT was 5.8% - 7.1%, cmopared with only 1.8% - 2.6% in normal controls (1, 4, 5). Individuals of northern European descent who are heterozygous for the 20210 G>A mutation have a 2.6 to 4.2-fold increased risk for developing DVT compared with wild-type controls. In patients presenting with recurrent DVT or with a family history of DVT, the frequency of the 20210 G>A mutation increases to 18% (1).

Several other clinical settings have recently been shown to have a higher frequency of the 20210 G>A mutation. Although the percentage of women who use oral contraceptives and also carry the 20210 G>A mutation is relatively low, such women have a 20-fold increased risk of cerebral-vein thrombosis (6). The frequency of the 20210 G>A mutation was found to be 40% in individuals presenting with idiopathic portal vein thrombosis as compared with 4.8% in controls; by contrast, factor V Leiden gene frequency was not increased in portal vein thrombosis patients (7). Analysis of studies investigating the risk of coronary arterial disease, specifically myocardial infarction (MI), found that the 20210 G>A mutation conveyed a moderate increased risk of MI (2.5-fold higher) in both men and women (3). When risk factors such as smoking or obesity were present, the 20210 G>A mutation confered even greater risk of MI. However, an increased odds ratio for MI due to the prothrombin mutation has not been found in all studies, and these positive studies must be confirmed by larger population analysis.

Homozygosity for the prothrombin 20210 G>A mutation is far less common than the heterozygous carrier state, and data are limited as to thrombosis risk (venous and arterial) in affected patients. However, since homozygosity for the 20210 G>A mutation further increases the thrombin generated in blood to levels even greater than those found in heterozygous patients, homozygosity is likely to increase the risk of venous and/or arterial thrombosis (8). One study in non-Caucasian populations in the United States found the 20210 G>A mutation frequency in African-Americans to be 0.2% (9). In Brazilians of African heritage, the frequency in two studies ranged from 0.67% to 2% (10,11). In native Americans, the only other ethnic group studied so far, the 20210 G>A mutation has not been found. The prothrombin 20210 G>A mutation may be found in association with other inherited hypercoagulable risk factors, including Factor V Leiden, so-called doubly heterozygous states. In a recent paper, patients carrying both the prothrombin and Factor V mutations (12) tended to develop thrombosis at an earlier age, as well as repeated episodes of DVT.

The Molecular Diagnostics laboratory at Yale-New Haven Hospital offers genetic testing for the prothrombin 20210 G>A mutation. This test uses an allele-specific polymerase chain reaction (PCR) to determine whether individuals are normal (wild-type), heterozygous, or homozygous mutant for the prothrombin 20210 G>A mutation (12). Samples for prothrombin 20210 G>A mutation testing should be sent to the Clinical Immunology Laboratory (688-2440); the sample requirement for adults and children is a single lavender top tube of blood.

1. Poort SR, Rosendaal FR, Reitsma PH, Bertina RM. A common genetic variation in the 3'-untranslated region of the prothrombin gene is associated with elevated plasma prothrombin levels and an increase in venous thrombosis. Blood 88(10):3698-703, 1996.

2. Degen SJ, Davie EW. Nucleotide sequence of the gene for human prothrombin. Biochemistry 26:6165, 1987.

3. Franco RF, Trip MD, ten Cate H, van den Ende A, Prins MH, Kastelein JJ, Reitsma PH. The 20210 G-->A mutation in the 3'-untranslated region of the prothrombin gene and the risk for arterial thrombotic disease. British Journal of Haematology 104(1):50-4, 1999.

4. Brown K, Luddington R, Williamson D, Baker P, Baglin T. Risk of venous thromboembolism associated with a G to A transition at position 20210 in the 3'-untranslated region of the prothrombin gene. British Journal of Haematology 98(4):907-9, 1997.

5. Hillarp A, Zoller B, Svensson PJ, Dahlback B. The 20210 A allele of the prothrombin gene is a common risk factor among Swedish outpatients with verified deep venous. Thrombosis & Haemostasis. 78(3):990-2, 1997.

6. Martinelli I, Sacchi E, Landi G, Taioli E, Duca F, Mannucci PM. High risk of cerebral-vein thrombosis in carriers of a prothrombin-gene mutation and in users of oral. New England Journal of Medicine. 338(25):1793-7, 1998.

7. Chamouard P, Pencreach E, Maloisel F, Grunebaum L, Ardizzone JF, Meyer A, Gaub MP, Goetz J, Baumann R, Uring-Lambert B, Levy S, Dufour P, Hauptmann G, Oudet P. Frequent factor II G20210A mutation in idiopathic portal vein Gastroenterology 116(1):144-8, 1999.

8. Kyrle PA, Mannhalter C, Beguin S, Stumpflen A, Hirschl M, Weltermann A, Stain M, Brenner B, Speiser W, Pabinger I, Lechner K, Eichinger S. Clinical studies and thrombin generation in patients homozygous or heterozygous for the G20210A mutation in the prothrombin gene. Arteriosclerosis, Thrombosis & Vascular Biology 18(8):1287-91, 1998.

9. Dilley A, Austin H, Hooper WC, El-Jamil M, Whitsett C, Wenger NK, Benson J, Evatt B. Prevalence of the prothrombin 20210 G-to-A variant in blacks: infants patients with venous thrombosis, patients with myocardial infarction, and control. Journal of Laboratory & Clinical Medicine 132(6):452-5, 1998.

10. Rosendaal FR, Doggen CJ, Zivelin A, Arruda VR, Aiach M, Siscovick DS, Hillarp A, Watzke HH, Bernardi F, Cumming AM, Preston FE, Reitsma PH. Geographic distribution of the 20210 G to A prothrombin variant. Thrombosis & Haemostasis 79(4):706-8, 1998.

11. Arruda VR, Annichino-Bizzacchi JM, Goncalves MS, Costa FF. Prevalence of the prothrombin gene variant (nt20210A) in venous thrombosis and arterial disease. Thrombosis & Haemostasis 78(6):1430-3, 1997.

12. Ferraresi P, Marchetti G, Legnani C, Cavallari E, Castoldi E, Mascoli Ardissino D, Palareti G, Bernardi F. The heterozygous 20210 G/A prothrombin genotype is associated with early venous thrombosis in inherited thrombophilias and is not increased in frequency in artery disease. Arteriosclerosis, Thrombosis & Vascular Biology 17(11):2418-22, 1997.

John Greg Howe, Ph.D.