Co-segregation of thrombosis with the factor V Q506 mutation in an extended family with resistance to activated protein C

State-of-the-Art Review: Activated Protein C Resistance: Clinical Implications

The discovery of inherited resistance to activated protein C (APC) as a major risk factor for venous thrombosis has dramatically improved our understanding of the pathogenesis of venous thrombosis. In a majority of cases, APC resistance is associated with a single point mutation in the factor V gene (FV) that results in substitution of arginine, R, at position 506 by glutamine, Q. (FV:Q506). The mutation renders factor Va partially resistant to degradation by APC. A functional APC resistance test, which includes predilution of the patient plasma with factor V-deficient plasma, is found to be 100% sensitive and specific for the presence of FV:Q506and is useful as a screening assay. Carriers of the FV:Q506allele have increased thrombin generation, resulting in hypercoagulability and a lifelong increased risk of venous thrombosis. In Western countries, APC resistance due to the FV mutation is present in 20-60% of thrombosis patients and in 1-15% of healthy controls, whereas the mutation is virtually absent from ethnic groups other than Caucasians. This may explain the high incidence of venous thrombosis in Western countries. The thrombotic risk in APC-resistant individuals may be further increased by other genetic defects, e.g., protein C or protein S deficiency, and by exposure to circumstantial risk factors, e.g., oral contraceptives, pregnancy, immobilization, and surgery. The question is thus raised as to whether general screening for APC resistance before circumstantial risk factors occur is warranted in Western countries. Key Words: Factor V—APC resistance-Protein C-Protein S—Thrombosis—Mutation.

Factor V Leiden (R506Q) and risk of venous thromboembolism: a case-control study based on the Spanish population

Resistance to activated protein C (APC) is a frequent cause of thrombophilia. Most patients showing APC-resistance have a G to A mutation at codon 506 of the factor V that converts arginine to glutamine. This mutation is present in populations worldwide with frequencies ranging from 0.01 to 0.05. Genotyping of 150 control individuals from the Spanish population showed that 3.33% of them carried the mutation. Several studies have measured resistance to APC (following a classical functional assay) and have determined the factor V genotype in a number of thrombophilic patients, in an attempt to compare the predictive value of both laboratory methods. To assess the incidence of the factor V mutation among Spanish thrombophilic patients, we genotyped 51 of these. The frequency of mutation carriers rose from 3.33% in the controls to 53% in the patients. We found significant differences for the thrombosis-free survival curves and for the age at the first thrombotic event between patients who carried or did not carry the mutation. Analysis of relatives of 16 patients who carried the factor V mutation suggests the existence of additional genes that modulate the effect of the factor V gene in the development of venous thrombosis among carriers of the G to A mutation.

Arterial and venous thrombosis in two Italian families with the factor V Arg506-->Gln mutation

APC resistance, due to a point mutation in factor V at amino acid position Arg506, has been identified as a major cause of inherited thrombophilia. Here we report the presence of the factor V Arg506-->Gln mutation in 2 Italian families. In 1 family 3 subjects heterozygous and 2 subjects homozygous for the factor V Arg506-->Gln mutation were identified. The only subject who developed a thrombotic event was a 20-yr-old girl who was found to be homozygous for the factor V Arg506-->Gln mutation. In the second family 10 subjects were identified to be heterozygous for the factor V Arg506-->Gln mutation; among them 2 developed a thrombotic event. In the same family 2 individuals were found to be homozygous for the mutation: the first had a myocardial infarction at age 25 yr and the second suffered from multiple episodes of deep venous thrombosis and had a stroke at age 24 yr. These data show that the risk of developing deep venous thrombosis for the carriers of the factor V Arg506-->Gln mutation is high in the families investigated. Furthermore our data imply that the factor V Arg506-->Gln mutation in its homozygous form may relate to myocardial infarction and stroke.

Activated protein C resistance: from phenotype to genotype and clinical practice

The anticoagulant protein C system is an important regulator of the blood coagulation process. Its targets are the procoagulant cofactors factor Va and factor VIIIa, which are cleaved and inactivated by activated protein C, protein S and intact factor V working as cofactors. Genetic defects of protein C or protein S were, together with antithrombin III deficiency, the previously established major causes of familial venous thromboembolism. However, these abnormalities are found in less than 5-10% of patients with thrombosis. Inherited resistance to activated protein C was recently identified as a major risk factor for venous thromboembolism. The activated protein C-resistance phenotype is found in 20-60% of the patients with venous thrombosis, depending on selection criteria and on the prevalence of activated protein C-resistance in the population. The frequency of activated protein C-resistance is 2-10% in the normal populations studied so far. In more than 90% of cases, the molecular background for the activated protein C-resistance is a single point mutation in the factor V gene, which predicts substitution of an arginine at position 506 by a glutamine. Mutated factor V is activated by thrombin or factor Xa in the normal way, but impaired inactivation of mutated factor Va by activated protein C results in a life-long hypercoagulability. Owing to the high prevalence of activated protein C-resistance in the population, it occasionally occurs in patients with deficiency of protein S, protein C or antithrombin III. Individuals with combined defects suffer more severely from thrombosis, and often at a younger age, than those with single defects, suggesting thrombophilia to be a multigenetic disease.