Lipoprotein(a) (Lp(a)) is composed of a low density lipoprotein (LDL)-like particle with apoB-100 linked to the apolipoprotein(a) component. Elevated levels relate to increased risk of arterial disease.[1, 2] Lp(a) has structural homology to plasminogen,[3] may compete for binding to fibrin, inhibit tissue plasminogen activator, impair fibrinolysis and promote thrombosis by conveying a hypercoagulable state. Disparate data exist, however, in terms of whether Lp(a) relates to increased incidence of VTE, as some[4–6] but not all[7–9] studies have reported positive associations. To address whether circulating Lp(a) levels impact the development of VTE, we assessed its relationship to incident VTE. In addition, more than 90% of the variation in plasma lipoprotein concentrations may be attributable to the apolipoprotein(a) gene [10] and two common polymorphisms in the LPA gene, rs3798220 and rs10455872 may account for at least 40% of variation in Lp(a) levels[2]. Therefore, the relation of these polymorphisms to VTE was assessed.
We assessed these questions in the Women’s Health Study.[11] In brief, this randomized trial evaluated the risks and benefits of low-dose aspirin and vitamin E in the primary prevention of cardiovascular disease and cancer.[11] During the run-in period, 28,345 women provided blood samples, plasma aliquots of which were stored in liquid nitrogen (−150 to −180°C) until biochemical analyses. Over a median follow-up of 14.4 years, VTE was collected and confirmed. Deep vein thrombosis was confirmed by a positive report of venous ultrasound or venography, and pulmonary embolism confirmed by positive angiogram, chest computed tomography scan or ventilation-perfusion scan with 2 mismatched defects. VTE was classified as unprovoked if it occurred in the absence of any recent trauma, hospitalization, surgery within 3 preceding months of the event and in the absence of a malignant condition that was diagnosed before or up to 3 months after the event, and as provoked if it occurred in a patient with cancer or if it occurred during or shortly after trauma, hospitalization, or surgery.
Lp(a) measurement was performed in a core laboratory certified by the NHLBI /CDC Prevention Lipid Standardization Program. We used the only commercially available assay, not affected by Kringle IV-Type 2 repeats, as assessed by a NHLBI and International Federation of Clinical Chemistry standardization group.[12] The turbidimetric assay used reagents and calibrators from Denka Seiken (Niigata, Japan) on the Hitachi 917 analyzer (Roche Diagnostics - Indianapolis, IN). The intraassay coefficients of variation at Lp(a) concentrations of 17.6 and 58.1 mg/dL were 3.6 and 1.5%, respectively. Genotyping of the two polymorphisms were performed as described previously for the rs3798220 polymorphism[13] and imputed for rs10455872 with a r2 value of 0.52 (MaCH R2>0.3). [14] The population reported in this evaluation is comprised of the Women’s Health Study cohort who were Caucasian, and who were successfully assayed for Lp(a) and polymorphisms in Lp(a) (n=21,483).
To evaluate the association between plasma Lp(a) levels and VTE, we divided the population according to quintiles of Lp(a) levels and assessed the hazard ratios of future VTE in Cox proportional hazard models, comparing quintiles 2 through 5 to the lowest (referent) quintile. Hazard ratios were estimated in models adjusting for age only, and then in models adjusting for baseline covariates important to the development of venous thrombosis: age (years), smoking (current vs. other), body mass index (World Health Organization categories <25, 25–29.9, ≥30 kg/m2), hormone therapy status (current vs. other at baseline), physical activity level (four categories of number of times exercise/week) and randomization treatment arms. Models were run both with and without BMI with similar results.
At study entry, the women were on average 54.2 years old (standard deviation 7.1), with body mass index of 25.9 (SD 5.0) kg/m2. 2,499 women (11.6%) smoked, 538 women (2.5%) had history of diabetes, 5,271 women (24.5%) had history of hypertension. The median Lp(a) level was 10.4 mg/dL. 439 women developed venous thromboembolism (259 DVT, 115 pulmonary emboli; 65 women with both events), over a median follow-up time period of 14.4 years. Of these, 241 were provoked, 186 unprovoked and 12 of indeterminate type. For women with multiple events, time to development of the first was considered.
The table presents the age and multivariable-adjusted hazard ratios for future VTE according to quintiles of Lp(a). There was no relationship seen between quintiles of Lp(a) and development of total, unprovoked or provoked VTE (adjusted HR for total VTE was 1.04 [95% CI 0.77–1.41]; for unprovoked VTE 0.96 [95% CI 0.61–1.50]; and for provoked VTE 1.17 [95% CI 0.77–1.79] all comparing highest to lowest quintile of Lp(a)).
Table.
Hazard ratios of future venous thromboembolic events among initially healthy women according to Lp(a) quintiles at study entry
| Lp(a) median (range), mg/dL |
Quintile 1 1.90 (0.10–3.40) |
Quintile 2 5.40 (3.50–7.40) |
Quintile 3 10.40 (7.50–14.90) |
Quintile 4 23.80 (15.00–43.50) |
Quintile 5 65.20 (43.60–229.40) |
P trend‡ |
|---|---|---|---|---|---|---|
| All VTE (n=439) | ||||||
| Number of VTE events (% with events in quintile) | 84 (1.9%) | 91 (2.2%) | 85 (2.0%) | 92 (2.2%) | 87 (2.0%) | |
| Person-years of follow-up | 59,656 | 57,455 | 58,358 | 58,011 | 57,999 | |
| Incidence Rate (95% CI)* | 1.41 (1.18, 1.74) | 1.58 (1.33, 1.95) | 1.46 (1.22, 1.80) | 1.59 (1.34, 1.95) | 1.50 (1.26, 1.85) | |
| Age-adjusted HR (95% CI) | 1 | 1.14 (0.85, 1.53) | 1.02 (0.75, 1.38) | 1.10 (0.82, 1.48) | 1.03 (0.77, 1.40) | 0.92 |
| Fully-adjusted† HR (95% CI) | 1 | 1.13 (0.84, 1.53 | 1.06 (0.78, 1.44 | 1.13 (0.84, 1.52) | 1.04 (0.77, 1.41) | 0.89 |
| Unprovoked VTE (n=186)§ | ||||||
| Number of Unprovoked Venous Thromboembolic Events |
39 (0.9%) | 32 (0.8%) | 41 (1.0%) | 37 (0.9%) | 37 (0.9%) | |
| Incidence Rate (95% CI)* | 0.65 (0.50, 0.89) | 0.56 (0.42, 0.79) | 0.70 (0.54, 0.95) | 0.63 (0.49, 0.88) | 0.64 (0.49, 0.88) | |
| Age-adjusted HR (95% CI) | 1 | 0.86 (0.54, 1.37 | 1.06 (0.69, 1.65) | 0.96 (0.61, 1.50 | 0.95 (0.61, 1.49) | 0.93 |
| Fully-adjusted† HR (95% CI) | 1 | 0.84 (0.53, 1.35) | 1.09 (0.70, 1.70) | 1.00 (0.63, 1.56) | 0.96 (0.61, 1.50) | 0.95 |
| Provoked VTE (n=241) | ||||||
| Number of Unprovoked Venous Thromboembolic Events |
41 (0.9%) | 58 (1.4%) | 42 (1.0%) | 52 (1.2%) | 48 (1.1%) | |
| Incidence Rate (95% CI)* | 0.69 (0.53, 0.93) | 1.01 (0.81, 1.31) | 0.72 (0.56, 0.97) | 0.90 (0.71, 1.18) | 0.83 (0.65, 1.10) | |
| Age-adjusted HR (95% CI) | 1 | 1.49 (1.00, 2.22) | 1.03 (0.67, 1.58) | 1.27 (0.84, 1.91) | 1.16 (0.77, 1.76) | 0.99 |
| Fully-adjusted† HR (95% CI) | 1 | 1.51 (1.01, 2.26) | 1.09 (0.71, 1.68) | 1.30 (0.86, 1.96) | 1.17 (0.77, 1.79) | 0.95 |
Incident events per 1000 person-years.
Fully-adjusted models refer to covariates of age, smoking, body mass index, hormone therapy status, exercise level and randomization treatment arms.
Tests for trends across quintiles of Lp(a) were addressed by entering a single ordinal term for each quintile based on the median value for Lp(a) within each quintile.
Type of VTE was not assigned in 12 individuals who developed VTE.
In sensitivity analyses, additional models were run without adjustment for BMI and with and without adjustment for alcohol use, without significant change.
Additional analyses was performed to evaluate for evidence of threshold effects between increasing levels of Lp(a) and risk of future VTE, we utilized prespecified cutoffs at the 25th, 50th, 75th, 90th, 95th and 99th percentiles of baseline Lp(a) levels, calculating hazard risks for individuals with Lp(a) levels exceeding each of these thresholds. Only extremely high values (≥99th percentile or 130.6 mg/dL) showed relationship to provoked VTE (adjusted HR 2.55 [95% CI 1.13–5.74]) compared to those with levels <99th percentile, with marginal relationship to total VTE (adjusted HR 1.88 [95% CI 0.93– 3.78)).
In addition, we saw no relationship of the rs3798220 polymorphism in LPA with incident VTE (adjusted hazard ratios for overall VTE were 1.36 [95% CI 0.88–2.11]; unprovoked VTE 1.69 [95% CI 0.92–3.12]; and provoked VTE 1.18 [95% CI 0.62–2.22]). Similarly, there was no relationship of rs10455872 with incident VTE (adjusted hazard ratio was 1.04 [95% CI 0.72–1.50]; unprovoked VTE 1.12 [95% CI 0.64–1.94]; and provoked VTE 1.01 [95% CI 0.61–1.67]). For rs3798220, both additive and dominant models of inheritance were tested and Cox proportional hazard models were constructed using time to VTE and regressing on the LPA SNP. Rs10455872 was an imputed SNP that ranged from 0–2, with 2 indicating likely carriage of two minor variants; it was modeled as a continuous variable.
The data expand on varying evidence regarding plasma Lp(a) levels and VTE, as some [4–6, 15] but not all [7, 9, 16] studies have shown an association of Lp(a) with VTE. Our findings are concordant with those of the Longitudinal Investigation of Thromboembolism Etiology study (LITE) study that pooled the Atherosclerosis Risk in Communities Study (ARIC) and Cardiovascular Health Study (CHS) studies and found no relationship between Lp(a) levels (>30 mg/dL) and VTE[8].
Strengths of this study include its prospective nature and use of a reliable mass-based assay[12] to measure circulating levels of Lp(a). Some of the prior discrepant findings of studies regarding Lp(a) levels and VTE may be due in part to difficulty in measuring Lp(a). This study benefits from the well-characterized profiles of participants that allow adjustment for VTE risk factors such as smoking and hormone therapy. Limitations include the female-only nature of the cohort, that the cohort is comprised of health care workers which may limit its generalizability, and that there was single measurement of Lp(a) although levels are stable over time.[10]
This study shows that there is little evidence of relationship between quintiles of Lp(a) and total, unprovoked or provoked VTE. Analysis for threshold effects show only rare, extremely elevated values of Lp(a) (≥ 99th percentile, 130.6 mg/dL) may relate to VTE. In addition, the rs3798220 and rs10455872 polymorphisms in the LPA gene, while related to circulating Lp(a) levels, are not significantly related to venous disease. Unlike the reported relationships of Lp(a) with arterial disease, there was no evidence of relationship of circulating Lp(a) levels or Lp(a) polymorphisms with future venous thromboembolism in this large cohort.
Acknowledgements
Dr. Danik has received support from the National Heart, Lung and Blood Institute (HL-076443) and the American Heart Association (D005113). Dr. Glynn reports funding from NIH grant AG031061. Dr. Buring has received investigator-initiated research funding and support as Principal Investigator from the National Institutes of Health (the National Heart, Lung, and Blood Institute, the National Cancer Institute, and the National Institute of Aging) and Dow Corning Corporation; research support for pills and/or packaging from Bayer Heath Care and the Natural Source Vitamin E Association; honoraria from Bayer for speaking engagements; and serves on an external scientific advisory committee for a study by Procter & Gamble. Dr. Ridker reports that he currently or in the past five years has received research funding support from not-for-profit entities including the National Heart, Lung, and Blood Institute, the National Cancer Institute, the American Heart Association, the Doris Duke Charitable Foundation, the Leducq Foundation, the Donald W Reynolds Foundation, and the James and Polly Annenberg La Vea Charitable Trusts. Dr. Ridker also reports that currently or in the past five years he has received investigator-initiated research support from for-profit entities including Astra-Zeneca, Bayer, Bristol-Myers Squibb, Dade-Behring, Novartis, Pharmacia, Roche, Sanofi-Aventis, and Variagenics. Dr Ridker reports being listed as a coinventor on patents held by the Brigham and Women's Hospital that relate to the use of inflammatory biomarkers in cardiovascular disease and has served as a consultant to Schering-Plough, Sanofi/Aventis, AstraZeneca, Isis Pharmaceutical, Dade-Behring, and Interleukin genetics. The funding agencies played no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; and preparation, review, or approval of the manuscript.
Sources of funding: This work was supported by NIH grants AG031061, CA047988, HL043851, HL080467, HL099355, HL007575, HL-076443 and by a grant from the Donald W. Reynolds Foundation (Las Vegas, NV).
Footnotes
Conflict of Interest There are no additional conflicts to disclose.
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