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November 2006, Volume 56, Issue 11

Review Articles

Genetics of Cerebral Venous Thrombosis

Abstract

Cerebral venous thrombosis (CVT) is rare compared to arterial causes of stroke. It is often encountered in young patients and may occur in children and neonates. Predisposition to CVT also has a genetic basis and inherited thrombophilias are known to cause 22.4% of the CVT cases. Inherited thrombophilias should be suspected if a patient has recurrent CVT, is less than 45 years age, has a family history of venous thrombosis or has no apparent acquired risk factor. Factor V Leiden (FVL) is the most common genetic risk factor, followed by the prothrombin gene mutation G20210A. Other less common inherited venous thrombophilias include deficiencies of Protein S, Protein C and antithrombin III. FVL, the G20210A prothrombin gene mutation and a deficiency of protein S and C, cause a reduction in the control of thrombin generation. Deficiency of antithrombin causes a decreased neutralization of thrombin. Both these mechanisms are responsible for venous thrombosis. Inherited thrombophilias with concomitant acquired risk factors like surgery, trauma, prolonged immobilization, pregnancy and puerperium, oral contraceptives, antiphospholipid antibodies and hyperhomocysteinemia may increase the risk of CVT manifold. Similarly the co-inheritance of two or more known mutations also increases the risk markedly. FVL, prothrombin G20210A mutation, increased factor VIIIc, protein C & S deficiency and antithrombin III deficiency have all been reported to cause neonatal stroke due to CVT. Maternal and foetal testing is suggested when CVT occurs in neonates.

Introduction

Advancements in genetics over the last decade have led to the identification of a genetic basis of a number of neurological diseases. Although rare, predisposition to cerebral venous thrombosis (CVT) also has a genetic basis and inherited thrombophilias constitute 22.4 % of the CVT cases.1 Inherited thrombophilias should be suspected if a patient has CVT, recurrent venous thrombosis, is less than 45 years age, has a family history of venous thrombosis or has no apparent acquired risk factor. In this article we will discuss the genetic predisposition to CVT.

Background

Antithrombin 111 deficiency and dysfibrogenemia as causes of inherited venous sinus thrombosis were first reported in 1965,2,3 while protein C and protein S deficiency as a cause of venous thrombo-embolism were first described in 19814 and 1984.5 Few advances were made in the next ten years, until 1993, when resistance to activated protein C (APC-R) was discovered.6 This was found to be due to a substitution in the factor V gene (G1691A) that causes arginine in residue 506 of factor V protein to be replaced by glutamine (Arg 506Glu), giving rise to the protein called factor V Leiden (FVL).7 FVL reduces the rate of inactivation of factor Va, which leads to increased generation of thrombin. In addition FVL has diminished cofactor activity in the inactivation of factor VIIIa by activated protein C (APC) - causing APC-R. This results in the failure of APC to prolong the activated partial-thromboplastin time (APTT). Another recently identified is a mutation in the prothrombin gene(factor II), called the G20210A mutation8, causes an elevated level of prothrombin and an increases risk for venous thrombosis by promoting thrombin generation. Thus Factor V Leiden, the G20210A prothrombin gene mutation and a deficiency of protein S and C, cause a reduction in the control of thrombin generation. Deficiency of antithrombin causes a decreased neutralization of thrombin. Both these mechanisms are responsible for venous thrombosis.

Inheritance

The main inherited causes of CVT9 are listed in Table 1. All are inherited as an autosomal recessive trait.

Genetic risk and external factors

Patients with inherited thrombophilias are at an increased risk of CVT. The lifetime risk of venous thrombosis10 based on inherited factors is given in Table 2. The risk is increased manifold if there are concomitant external risk factors like surgery, trauma, prolonged immobilization, pregnancy and puerperium11, oral contraceptives12, antiphospholipid antibodies and hyperhomocysteinemia.13 The use of oral contraceptives (OCs) significantly increases the risk of CVT. Martinelli et al compared the prevalence of CVT in carriers of the G1691A and G20210A mutations who used OCs, and found that the use of OCs was more frequent among women with CVT (96%) than amongst controls.14 Women with inherited thrombophilias should avoid using OCs particularly if they have a past or family history of venous thrombosis. Hormone replacement therapy also increases the risk of venous thrombosis.13

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Inherited thrombophilias as a cause of foetal and neonatal CVT

Approximately 25% of ischaemic cerebrovascular disease in children is due to CVT. In 70% an acquired etiology can be identified. In 30% no cause can be determined.15 It has recently been established that genetic thrombotic disorders are significant risk factors for neonatal stroke. Factor V Leiden, prothrombin G20210A mutation, increased factor VIIIc, protein C & S deficiency and antithrombin III deficiency have all been reported to cause neonatal stroke due to CVT.16 Table 3 lists coagulation profile testing suggested for mother and baby for neonatal stroke and neonatal CVT:

Table 3. Suggested testing for maternal/fetal Thrombotic disorders.
Protein S
Protein C
Antithrombin III
Factor V Leiden
Prothrombin G20210A
Lipoprotein (a)
Homocysteine
Anticardiolipin IgG/IgM (maternal only)

Conclusion

Cerebral venous thrombosis may cause significant neurological morbidity and mortality in young adults and children. FVL is the most common genetic risk factor, followed by the prothrombin gene mutation G20210A. Inherited thrombophilias with concomitant acquired risk factors may increase the risk of CVT manifold. Asymptomatic relatives of an affected patient may be tested and treated in high risk situations. Prophylactic treatment for asymptomatic carriers is not recommended as the risks outweigh the benefits. Maternal and foetal testing is suggested when CVT occurs in neonates.

References

1. Ferro JM, Canhao P, Stam J, Bousser MG, Barinagarrementeria F, et al. Prognosis ofCerebral vein and Dural Sinus Thrombosis: Results of the International Study on Cerebral Vein and Dural Sinus Thrombosis (ISCVT). Stroke 2004;35:664-70

2. Egeberg O. Inherited antithrombin III deficiency causing thrombophilias. Thromb Diath Haemorrh 1965; 13:516-30

3. Beck EA, Charache P, Jackson DP. A new inherited coagulation disorder caused by an abnormal fibrinogen ('fibrinogen Baltimore'). Nature 1965; 208:143-5.

4. Griffin JH, Evatt B, Zimmermann TS, Kleiss AJ, Wideman C. Defiency of protein C in congenital thrombotic disease. J Clin Invest 1981;68:1370-3.

5. Comp PC, Esmon CT. Recurrent venous thromboembolism in patients with a partial defiency of protein S. N Engl J Med 1984;311:1525-8.

6. Dahlback B, Carlsson M, Svensson PJ. Familial thrombophilias: a complex genetic disorder. Semin Hematol 1997; 34:256-64.

7. Bertina RM, Koeleman BP, Koster T, Rosendaal FR, Dirven RJ, De Ronade H, et al. Mutation in blood coagulation factor V associated with resistance to activated protein C. Nature 1994;369:64-7.

8. Poort SR, Rosendaal FR, Reitsma PH, Bertina RM. A common genetic variation in the 3'-untranslated region of the prothrombin levels and an increase in venous thrombosis. Blood 1996;88:3698-703.

9. Allan H Ropper, Robert H Brown Ed. Adams and Victor's Principles of Neurology. Eighth edition Mc Graw-Hill, New York, USA 2005; PP 709.

10. Reich LM, Bower M, Key NS. Role of the Genetist in testing and counseling for inherited thrombophilias. Gene Med. 2003;5:133-43.

11. Derex L, Philippeau F, Nighoghossian N, Trouillas P. Postpartum cerebral venous thrombosis, congenital protein C deficiency, and activated protein C resistance due to heterozygous factor V Leiden mutation. J Neurol Neurosurg Pshychiatry 1998;65:801-2.

12. Vandenbroucke JP, Koster T, Briet E, Reitsma PH,Bertina RM, Rosendaal FR. Increased risk of venous thrombosis in oral contraceptive users who are carriers of factor V Leiden mutation. Lancet 1994;344:1453-7.

13. Seligsohn U,Lubetsky A. Genetic Susceptibility to Venous thrombosis. N Engl J Med, 2001;2441222-31.

14. Martinelli I, Sacchi E,Landi G, Taioli E, Duca F, Mannucci PM. High risk of cerebral venous thrombosis in carriers of a prothrombin-gene mutation and in users of oral contraceptives. N Engl J Med, 1998; 338:1793-7.

15. Curry CJ, Holmes J, Catts ZA, et al. Hypercoaguable factors in pregnancy loss and neonatal stroke. Proc Greenwood Gen Ctr 2001. 20:92-3.

16. Mercuri E, Cowan F, Gupte G, Manning R, Laffan M, Rutherford M, et al. Prothrombotic disorders and abnormal neurodevelopment outcome in infants with neonatal cerebral infarction. Pediatrics 2001. 107:1400-4.

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