Introduction
Nearly 150 years ago, Virchow postulated that thrombosis was caused by changes in
the flow of blood, the vessel wall, or the composition of blood. This concept created
the foundation for subsequent investigation of hereditary and acquired hypercoagulable
states. This review will focus on an example of the use of modern genetic epidemiologic
analysis to evaluate the multigenic pathogenesis of the syndrome of juvenile thrombophilia.
Juvenile thrombophilia has been observed clinically since the time of Virchow and
is characterized by venous thrombosis onset at a young age, recurrent thrombosis,
and a positive family history for thrombosis. The pathogenesis of juvenile thrombophilia
remained obscure until the Egeberg observation, in 1965, of a four generation family
with juvenile thrombophilia associated with a heterozygous antithrombin deficiency
subsequently identified as antithrombin Oslo (G to A in the triplet coding for Ala
404).1,2 The association of a hereditary deficiency of antithrombin III with thrombosis appeared
to support the hypothesis, first put forward by Astrup in 1958, of a thrombohemorrhagic
balance.3 He postulated that there is a carefully controlled balance between clot formation
and dissolution and that changes in conditions, such as Virchow’s widely encompassing
triad, could tip the balance toward thrombus formation.
The importance of the thrombohemorrhagic balance in hypercoagulable states has been
born out of two lines of investigation: evidence supporting the tonic activation of
the hemostatic mechanism and the subsequent description of additional families with
antithrombin deficiency and other genetically abnormal hemostatic proteins associated
with inherited thrombophilia. Assessing the activation of the hemostatic mechanism
in vivo is achieved by a variety of measures, including assays for activation peptides
generated by coagulation enzyme activity. Activation peptides, such as prothrombin
fragment1+2, are measurable in normal individuals, due to tonic hemostatic activity and appear
elevated in certain families with juvenile thrombophilia.4
In the past 25 years since Egeberg’s description of antithrombin deficiency, a number
of seemingly monogenic, autosomal dominant, variably penetrant hereditary disorders
have been well established as risk factors for venous thromboembolic disease. These
disorders include protein C deficiency, protein S deficiency, antithrombin III deficiency,
the presence of the factor V Leiden mutation, and the recently reported G20210A prothrombin
polymorphism.5,6 These hereditary thrombophilic syndromes exhibit considerable variability in the
severity of their clinical manifestations. A severe, life-threatening risk for thrombosis
is conferred by homozygous protein C or protein S deficiency, which if left untreated,
leads to death.7,8 Homozygous antithrombin III deficiency has not been reported but is also likely to
be a lethal condition. Only a moderate risk for thrombosis is conferred by the homozygous
state for factor V Leiden or the G20210A polymorphism.9,10 In contrast to homozygotes, the assessment of risk in heterozygotes, with these single
gene disorders, has been complicated by variable clinical expression in family members
with identical genotypes.11 Consideration of environmental interactions has not elucidated the variability of
clinical expression. Consequently, it has been postulated that more than one genetic
risk factor may co-segregate with a consequent cumulative or synergistic effect on
thrombotic risk.12
A number of co-segregating risk factors have been described in the past few years.
Probably the best characterized interactions are between the common factor V Leiden
mutation, present in 3% to 6% of the Caucasian population,13,14 and the less common deficiencies of protein C, protein S, and antithrombin III. The
factor V Leiden mutation does not, by itself, confer increased risk of thrombosis.
The high prevalence of the mutation, however, creates ample opportunity for interaction
with other risk factors when present.
The G20210A prothrombin polymorphism has a prevalence of 1% to 2% in the Caucasian
population and, thus, may play a similar role to factor V Leiden. A number of small
studies have documented an interaction of G20210A with other risk factors.15-17 A limited evaluation of individuals with antithrombin III, protein C, or protein
S deficiency revealed a frequency of 7.9% for the G20210A polymorphism, as compared
to a frequency of 0.7% for controls.18 The G20210A polymorphism was observed in only 1 of the 6 protein C-deficient patients.18
In the present state, the elucidation of risk factors for venous thromboembolic disease
attests to the effectiveness of the analytical framework constructed from the molecular
components of Virchow’s triad, analyzed in the context of the thrombohemorrhagic balance
hypothesis. Two investigative strategies have been used to study thromobophilia: clinical
case-control studies and genetic epidemiologic studies. The latter strategy has gained
considerable utility, based on the remarkable advances in molecular biology over the
past two decades. Modern techniques of genetic analysis of families offer important
opportunities to identify cosegregation of risk factors with disease.19 The essence of the genetic epidemiologic strategy is the association of clinical
disease with alleles of specific genes. It is achieved either by the direct sequencing
of candidate genes or by demonstration of linkage to genetic markers.
