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- PATIENTS AND METHODS
Systemic lupus erythematosus (SLE) is an autoimmune disease of unknown etiology that involves multiple organ systems. Arterial and venous thromboembolism is a well-known clinical entity in SLE, with a prevalence >10%. This prevalence may even exceed 50% in high-risk patients (1, 2). Thrombosis in SLE occurs through 3 major conditions: hypercoagulability, premature atherosclerosis, and vasculitis (3). Hypercoagulability is most commonly secondary to the presence of lupus anticoagulant (LAC) and anticardiolipin antibodies (aCL) (3); the interaction between the antibodies to phospholipid–protein complexes and antigen targets on endothelial cells, platelets, or components of coagulation cascade can mediate the vascular injury. Nevertheless, the pathogenesis of the thrombotic tendency and vessel damage in SLE is not yet completely clarified. In recent years, research has focused on the role of other disease-associated risk factors and predisposing conditions such as hypertension, corticosteroid treatment, hyperhomocysteinemia, decreased protein S concentration, and different hemostatic markers (4–6). Endothelial cells may be injured by high levels of plasma homocysteine (7, 8), and several studies have demonstrated an association between mild hyperhomocysteinemia and occlusive vascular disease (9). Thromboembolic disease is now viewed as a multicausal model, and the thrombotic event seems to be the result of gene–gene and gene–environment interactions (10). Inherited thrombophilia is defined as a genetically determined tendency for development of thromboembolism; the overall prevalence of thrombophilic traits in the general population is ∼10% and the risk of a first thrombotic event increases in the presence of combined defects (10, 11).
This study was carried out to investigate the thrombotic tendency in patients with SLE by evaluating congenital or acquired abnormalities associated with an increased risk of venous and/or arterial thrombosis.
- Top of page
- PATIENTS AND METHODS
Thrombosis is a common manifestation in patients with SLE. The identification of thrombophilic risk factors in these patients is clinically useful in determining those in whom thrombosis would be more likely to occur (18). Several blood parameters have been proposed to predict the occurrence of thrombosis, including the presence of LAC, aCL, and anti–β2-glycoprotein I antibodies. In contrast, it is noteworthy that approximately 40% of adults with SLE who are negative for aPL antibodies are diagnosed with thrombosis (19, 20). Thus, the precise mechanism(s) responsible for thrombosis in these patients remains unclear.
In our study, we examined the presence of thromboembolic risk factors in patients with SLE with or without a history of thrombosis, and we found modifications regarding several parameters analyzed. We documented alterations responsible for or markers of hypercoagulability, including raised levels of fibrinogen, homocysteine, and D-dimer, as well as increased levels of natural anticoagulant protein, such as protein C and antithrombin.
Of interest, it has been recently demonstrated that increased thrombin generation accompanied by heightened activity of antithrombin occurs in venous blood of patients with SLE and APS (21). The authors suggest that in these patients, the increased activities of antithrombin III are secondary to augmented thrombin generation and serve to protect these patients from frequent thromboembolic episodes (21).
In the current study, no significant decrease in the plasma concentration of the total protein S was observed. Tomas et al (22) reported that in patients with SLE, protein C, antithrombin, and total protein S antigenic levels were in the normal range, but other authors have observed alterations in these protein systems (23, 24). Therefore, our results do not exclude a possible role of these proteins in the pathogenesis of thrombosis; further studies could elucidate the coagulation picture in patients with SLE.
D-dimer levels significantly increased in our patients compared with controls. In particular, assuming 0.55 μg/ml (the highest value found in our controls) as cutoff, we documented high concentrations of D-dimer in 15 (26%) of 57 patients, with a higher percentage in patients with (43%) than in those without (17%) a history of thrombosis. Elevated levels of D-dimer are usually detectable during the thrombotic events; thus, raised concentrations of D-dimer in our patients could indicate a subclinical activation of a blood coagulation system without overt thrombotic manifestations. This result suggests that both a prethrombotic state and a compensatory fibrinolytic process secondary to subclinical intravascular coagulation might coexist in SLE, as hypothesized by other authors (4). The careful followup of patients with elevated D-dimer will make clear the correctness of this hypothesis.
Homocysteine is a nonessential amino acid produced during normal metabolism, and hyperhomocysteinemia is frequently associated with arterial and venous thrombosis. Prothrombotic activities may be attributable to either direct toxic effect on endothelium or indirect effects (9).
In the present study, we found increased homocysteine levels in patients with SLE, which is consistent with results previously reported by other investigators (9, 25). When considering a cutoff level of 14.1 μmoles/liter as suggested by Petri et al (9) and Selhub et al (26), we found hyperhomocysteinemia in 8 (16%) of 57 of patients, which is almost the same percentage reported by these authors, who detected raised homocysteine concentrations in 15% of patients.
The total homocysteine concentration in the plasma of healthy individuals varies with age, sex, geographic area, and genetic factors. Some authors have recently proposed a lower cutoff for homocysteine, adjusted according to the top 95th percentile of the distribution of the control group (27). Based on this criterion, the prevalence of hyperhomocysteinemia was significantly higher in SLE patients with thrombosis. The 95th percentile of homocysteine levels in our control group was 10.7 μmoles/liter. According to this cutoff, 11 (52%) of 21 patients with and 10 (27.7%) of 36 patients without thrombosis showed raised levels of homocysteine.
With respect to the frequency of MTHFR genotypes, we found an homozygous mutant (+/+) in 25% of patients, a percentage higher than that reported by Fijnheer et al who demonstrated the homozygous mutant in 8% of their cohort of patients (25). However, in agreement with the results of these authors, we found no significant differences between patients with and without thrombosis. Therefore, the actual role of MTHFR gene mutation in SLE remains to be better elucidated.
The prothrombin mutation variant was present in 6 patients, which is higher than the prevalence in the control group (11% versus 4%). Recently, one study found that the presence of mutated allele of the prothrombin gene was higher in patients with APS when compared with controls, suggesting that prothrombin variant could increase the risk of thrombosis in these patients (18).
We investigated the presence of resistance to activated protein C in our SLE population. Resistance to activated protein C is defined as a decreased anticoagulant response to the activated protein C pathway (28). Hereditary APCr is caused by the Factor V Leiden mutation and it is currently regarded as the most frequent cause of familial thrombosis (11, 29–31); acquired APCr, a phenotypic APCr that occurs in the absence of the Factor V Leiden mutation, has been reported in patients with defined APS (27, 32–35). In our study, APCr was detected in 1 (2%) of 50 controls and 2 (3%) of 57 patients. Because all these subjects were positive for Factor V Leiden, they were not considered to have acquired APCr.
Male et al (35) determined the congenital or acquired APCr frequency in pediatric patients with SLE and its association with aPL. In their study, the authors reported that acquired APCr was present in 31% of pediatric patients with SLE, none of whom had Factor V Leiden. APCr was significantly associated with LAC and thrombotic events (35–37). Some authors have hypothesized that APCr may have been the result of thromboembolic events in some patients, reflecting an acute phase response (35, 38). A limitation of our study is the retrospective design; however, even in other studies, some patients had thrombotic events identified retrospectively. Also, in retrospective studies, APCr testing was not available for patients who were receiving warfarin therapy. Therefore, prospective studies would be required to test the effectiveness of using APCr to predict thrombosis in patients with SLE and to accurately define the mechanism of acquired APCr.
Our study confirms observations concerning the role of aCL at high titer in the development of thrombosis in patients with SLE, even though a positive correlation of aCL with thrombosis is well known (39, 40). In a previous study of 87 patients with SLE, we have demonstrated that IgG-aCL, when taken at any rate of positivity (low, medium, high titer), were not significantly associated with thrombosis. However, when only aCL at medium-high titer were taken into account, a significant association was demonstrated (20). In the present study, we confirm the association between thrombosis and IgG-aCL at medium-high titer.
Although hypercoagulability associated with the presence of aPL is one of the major factors responsible for thrombosis in patients with SLE, the risk of developing a thrombotic event in aPL-positive patients is likely to be enhanced by the presence of certain procoagulant alterations (3). Some risk factors can be directly related to SLE and/or its treatment, or can be attributable to other common hypercoagulable conditions (41). Indeed, patients with SLE have a thrombotic tendency that is not exclusively associated with aPL. Although in the present study we observed no significant association between a specific thrombophilic risk factor other than aPL and thrombosis, our results still demonstrate that patients with SLE had more than one potentially causative factor involved in thrombogenesis. We suggest that the coexistence of aPL and other risk factors can affect the expression of thrombosis in patients with SLE, and that the development of thrombosis is multifactorial. In fact, the risk of thrombotic events further increases in the presence of combined defects (10). Therefore, patients with SLE require closer followup for potential thrombotic complications to identify patients with more than one risk factor for thrombosis. This could be important because some risk factors, such as hyperhomocysteinemia, are potentially modifiable (27, 42).