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.