Genetic Risk Factors of Thrombosis.

Abstract SCI-43

Venous thrombosis is a common episodic disease with a steep age gradient. Interactions between various risk factors determine the development of the disease, and the proportion of variance attributable to genetic factors may be as high as 50-60%. There are six (moderately) strong genetic risk factors. First there are heterozygous deficiencies of the natural anticoagulants protein C, protein S, and antithrombin. These deficiency states are quite rare in the general population (in all races) and their genetic architecture is complex with hundreds of documented mutations. The risk for the development of venous thrombosis may be increased 10-20-fold in these deficiency states. There is no consistent evidence that deficiencies of other members of the anticoagulant systems - such as thrombomodulin, EPCR, and heparin co-factor II - are also strong risk factors for venous thrombosis, possibly because these natural anticoagulants are associated with other episodic or chronic diseases. Secondly there are three genetic factors associated with an increase, directly or indirectly, in the procoagulant potential of the coagulation system: blood group non-O, factor V Leiden and prothrombin G20201A. The genetic architecture of these risk factors is extremely simple. The prevalence in the general Caucasian population is modest for prothrombin G20210A and factor V Leiden; in other races these two risk factors are extremely rare. The increase in thrombotic risk is about 3-fold or 7-fold for prothrombin G20210 and Factor V Leiden respectively. Blood group non-O is the most common of the prothrombotic genetic risk factors and approximately doubles the risk of venous thrombosis, and may do so in all races. In addition to these six ‘classical’ risk factors, a growing list of weak genetic risk factors has been discovered. Almost without exception, these weak risk factors are common single nucleotide polymorphisms in coagulation factor genes - e.g. those encoding for fibrinogen, factor XIII, factor IX, et cetera - that have a small effect on gene function, and consequently a small effect on thrombotic risk. This list of weak but common risk factors is expected to grow considerably in the near future as the large-scale genome wide association studies that are currently under way deliver their results. Moreover, deep resequencing studies are expected to start soon, whether based on a candidate gene approach or genome-wide, which will yield unprecedented insight in the extend to which rare genetic variation determines individual thrombotic risk.