Summary
The presence of the 20210A allele of the prothrombin (PT) gene has recently been shown
to be a risk factor for venous thromboembolism. This is probably mediated through
increased plasma prothrombin levels. The aim of this study was to compare the prevalence
of the prothrombin 20210A allele in control subjects and in subjects with recognised
thrombophilia and to establish whether the additional inheritance of the PT 20210A
allele is associated with an increased risk of venous thromboembolism. 101 subjects
with a history of venous thromboembolism and diagnosed as having either factor V Leiden
(R506Q) or heritable deficiencies of protein C, protein S or antithrombin were studied.
The prevalence of the PT 20210A allele in this group was compared with the results
obtained for 150 control subjects. In addition, the relationships were examined between
genetic status and the number of documented thromboembolic episodes, and between plasma
prothrombin levels and possession of the PT 20210A allele. 8 (7.9%) of the 101 patients
were also heterozygous for the PT 20210A allele. This compares with 0.7% in the control
subjects (p = 0.005). After exclusion of patients on warfarin, the mean plasma prothrombin
of 113 subjects without 20210A was 1.09 U/ml, as compared with 1.32 U/ml in 8 with
the allele (p = 0.0002). Among the 101 patients with either factor V Leiden, protein
S deficiency, protein C deficiency or antithrombin deficiency, the age adjusted mean
(SD) number of venous thromboembolic episodes at diagnosis was 3.7 (1.5) in those
with the PT 20210A allele, as compared with 1.9 (1.1) in those without (p = 0.0001).
We have demonstrated that the prevalence of the PT 20210A allele is significantly
greater in subjects with venous thrombosis and characterised heritable thrombophilia
than in normal control subjects and that the additional inheritance of PT 20210A is
associated with an increased risk of venous thromboembolism. We have also confirmed
that plasma prothrombin levels are significantly greater in subjects possessing the
PT 20210A compared with those who do not.