Prognostic Value of Plasma Myeloperoxidase in ESRD Patients Angela Yee-Moon Wang, MD, PhD, FRCP,1* Christopher Wai-Kei Lam, PhD, FACB,2,3 Iris Hiu-Shuen Chan, PhD,2 Mei Wang, PhD,1* Siu-Fai Lui, FRCP,1 and John E. Sanderson, MD, FRCP1 Background: Myeloperoxidase (MPO) has been suggested to have a role in atherosclerosis through its strong oxidative capacity. We hypothesized that MPO level may predict clinical outcomes in patients with end-stage renal disease receiving long-term peritoneal dialysis (PD) therapy. Study Design: Prospective cohort study. Setting & Participants: 236 long-term PD patients were recruited from a single regional dialysis unit in Hong Kong between April 1999 and February 2001. Predictor: Level of plasma MPO, analyzed using a sandwich enzyme-linked immunosorbent assay. Outcome & Measurement: Mortality and fatal or nonfatal cardiovascular events at 3 years. Results: The distribution of MPO levels was skewed with a median of 31.8 g/L (25th-75th percentiles, 24.4-42.7). There were 69 deaths and 81 cardiovascular events. Adjusting for traditional and nontraditional risk factors and C-reactive protein, cardiac troponin T, and N-terminal pro-brain natriuretic peptide levels, a doubling in plasma MPO level was associated independently with a 46% (95% CI, 1.02-2.08; P ⫽ 0.04) and 60% (95% CI, 1.17-2.18; P ⫽ 0.003) increase in risks of mortality and cardiovascular events, respectively. Log2MPO showed significant additional predictive value for mortality (P ⫽ 0.04) and cardiovascular events (P ⫽ 0.005) when included in Cox regression models consisting of clinical, demographic, dialysis, echocardiographic, and biochemical parameters, as well as C-reactive protein, cardiac troponin T, and N-terminal pro-brain natriuretic peptide levels. Limitations: MPO was measured at a single time and did not reflect changes over time. Conclusions: These data suggest that plasma MPO level has significant independent and additional prognostic value beyond the standard clinical, biochemical, and echocardiographic parameters and is useful for outcome stratification in long-term PD patients. MPO may be an important mediator of increased cardiovascular risk in patients with end-stage renal disease and warrants further investigation. Am J Kidney Dis 56:937-946. © 2010 by the National Kidney Foundation, Inc. INDEX WORDS: Myeloperoxidase; end-stage renal disease; mortality; cardiovascular events; peritoneal dialysis.
M
yeloperoxidase (MPO) is a hemoprotein found in the azurophilic granules of neutrophils and, to a lower extent, in monocytes and macrophages and is involved mainly in innate immune defense and bactericidal activity.1 In addition, MPO functions as a major enzymatic catalyst for the initiation of lipid peroxidation, a pivotal process in atherogenesis, at sites of inflammation.2-4 MPO serves as a catalytic sink for nitric oxide, decreases nitric oxide bioavailability, and impairs nitric oxide–dependent vasodilatation.5,6 Plasma MPO level predicts nitric oxide– dependent flow-mediated dilatation in humans.7 In immunohistochemical studies, MPO and products of MPO-catalyzed oxidation reactions have been identified in human atherosclerotic lesions.1,8-11 Increased plasma MPO level predicts the severity of angiographic coronary artery disease12 and adverse cardiac outcomes in patients with acute coronary syndromes independent of C-reactive protein (CRP) level.13 Furthermore, patients with a polymorphism of the MPO gene, a G to A
transition at position ⫺463 of the promoter region, showed a 2-fold decrease in MPO expression and had markedly decreased angiographic evidence of coronary artery disease, nonfatal myocardial infarction, and cardiac death.14,15 More recently, a study of hemodialysis patients From the Departments of 1Medicine and Therapeutics and 2Chemical Pathology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong; and 3Macau Institute for Applied Research in Medicine and Health, Macau University of Science and Technology, Taipa, Macau. * Current affiliation: Department of Medicine, Queen Mary Hospital, University of Hong Kong, Hong Kong. Received January 14, 2010. Accepted in revised form May 5, 2010. Originally published online as doi:10.1053/j.ajkd. 2010.05.008 on July 20, 2010. Address correspondence to Angela Yee-Moon Wang, MD, PhD, FRCP, University Department of Medicine, The University of Hong Kong, Queen Mary Hospital, 102 Pokfulam Rd, Pokfulam, Hong Kong. E-mail:
[email protected] © 2010 by the National Kidney Foundation, Inc. 0272-6386/10/5605-0018$36.00/0 doi:10.1053/j.ajkd.2010.05.008
American Journal of Kidney Diseases, Vol 56, No 5 (November), 2010: pp 937-946
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reported an association between plasma MPO level and mortality.16 Patients with end-stage renal disease (ESRD) with the G to A transition in the MPO promoter had a lower prevalence of clinical cardiovascular disease.17 Thus, laboratory, clinical, and genetic studies have lent strong supporting evidence that MPO may have an important role in atherogenesis. Given this background, the primary objective of this study is to evaluate whether plasma MPO level has prognostic value for mortality and cardiovascular events beyond the standard clinical, biochemical, and echocardiographic parameters in patients on long-term peritoneal dialysis (PD) therapy.
METHODS Study Design This is a prospective cohort study conducted in a single regional dialysis center in Hong Kong. Study participants were recruited between April 1999 and February 2001 and prospectively followed up for 3 years. The study protocol was approved by the Clinical Research Ethics Committee of the Chinese University of Hong Kong. All patients provided informed consent before study entry.
Research Participants Patients were considered eligible for study entry if they had ESRD and had been maintained on continuous PD treatment for 3 months or longer. Exclusion criteria included patients with underlying active malignancy, chronic liver disease, systemic lupus erythematosus with ongoing active lupus activity requiring immunosuppression, systemic vasculitis, chronic rheumatic heart disease, and congenital heart disease; patients who refused to give consent; or patients with incomplete data. Based on inclusion and exclusion criteria, 236 long-term PD patients were recruited, representing 82.1% of the total PD population in the center. All patients were dialyzed using conventional lactate-buffered glucose-based PD solutions. In patients who developed acute coronary syndrome, acute heart failure, peritonitis, exit-site infections, or other infective complications, all of the following baseline assessments were deferred for at least 1 month after complete resolution of the complication.
Biochemical Analysis A 20-mL fasting venous blood sample was collected at study baseline for measurement of blood hemoglobin and plasma MPO, cardiac troponin T (cTnT), N-terminal probrain natriuretic peptide (NT-pro-BNP), high-sensitivity CRP, albumin, urea, creatinine, calcium, phosphorus, and lipid profile. MPO was measured using a sandwich enzymelinked immunosorbent assay kit (Immunodiagnostik AG, www.immundiagnostik.com) with a lower detection limit of 1.6 g/L. Our study-specific intra- and interassay coeffi-
cients of variation were 8.5% and 10.2% at 4.8 g/L and 6.8% and 7.2% at 32.3 g/L, respectively. cTnT and NT-proBNP were quantified using electrochemiluminescence immunoassay (Elecsys 2010 analyzer; Roche Diagnostic GmbH, www.roche.de). High-sensitivity CRP was measured using chemiluminescence immunoassay with the IMMULITE Analyzer (Siemens Medical Solutions Diagnostics, www.medical. siemens.com). Albumin was measured using the bromcresol purple method, and total cholesterol and triglyceride, using enzymatic assay on the Hitachi 911 analyzer (Roche Diagnostics GmbH). High-density lipoprotein cholesterol was measured after precipitation of apolipoprotein B–containing lipoproteins with phosphotungstate, whereas low-density lipoprotein cholesterol was calculated using the Friedewald formula.18 Twenty-four–hour dialysate and simultaneous urine samples were collected for measurement of residual kidney function and dialysis adequacy. Residual glomerular filtration rate was measured as the average of 24-hour urine urea and creatinine clearance.19 Adequacy of dialysis was estimated by measuring total weekly urea clearance (Kt/V) and creatinine clearance using standard methods.20 Contribution of the PD and kidney components to total urea clearance was estimated separately.
Clinical Data Collection Demographic and clinical data were collected prospectively at baseline using direct patient inquiry and review of the computerized medical record system. Clinical atherosclerotic vascular disease was defined as the presence of coronary artery disease (indicated by previous myocardial infarction or stable or unstable angina with or without a history of coronary artery bypass surgery or percutaneous coronary intervention), cerebrovascular disease (indicated by a history of stroke, carotid endarterectomy, or stent placement), and peripheral vascular disease (indicated by the presence of intermittent claudication or leg pain at rest, together with clinical signs of peripheral vascular disease with or without a history of amputation or revascularization). Body mass index was calculated as body weight in kilograms divided by height in meters squared.
Echocardiographic Data Collection Two-dimensional echocardiography was performed using a GE-VingMed System 5 echocardiographic machine (GEVingMed Sound AB, www.vingmed.se) with a 3.3-mHz multiphase-array probe in patients lying in the left decubitus position by a single experienced cardiologist blinded to all clinical details of patients except body weight, height, age, and sex. The cardiologist performing echocardiography was not involved in the clinical management of patients. Echocardiographic data were recorded according to the guidelines of the American Society of Echocardiography.21 Left ventricular (LV) mass was calculated using the modified American Society of Echocardiography cube formula proposed by Devereux et al22 and indexed by body surface area.
Follow-up All patients were followed up prospectively for 3 years from the day of baseline assessments or until death. The follow-up period ended in February 2004. No patient was
Myeloperoxidase in Kidney Failure lost to follow-up. The outcomes evaluated were death from all causes and fatal or nonfatal cardiovascular event. For patients who developed multiple cardiovascular events, survival analysis was limited to the first event. Cardiovascular event included acute myocardial ischemic event, stroke, sustained atrial or ventricular arrhythmia, peripheral vascular disease, and sudden cardiac death defined and diagnosed clinically as unexpected natural death within 1 hour from symptom onset and without a prior condition that would appear fatal.23,24 Acute myocardial ischemia was diagnosed by the attending physician based on the presence of symptoms and serial electrocardiographic and cardiac enzyme changes in accordance with World Health Organization criteria. Cause of death and nature of the first cardiovascular event were established by the attending physicians, who had no knowledge of baseline plasma MPO results. In case of death out of the hospital, family members were interviewed by telephone to ascertain the circumstances surrounding death.
Statistical Analysis Continuous data are expressed as mean ⫾ standard deviation or median (25th, 75th percentiles) depending on the distribution, and categorical data, as percentage. Patients were stratified by tertiles of plasma MPO. Between-group comparisons were performed using 1-way analysis of variance, Kruskal-Wallis test, or 2 test, when appropriate. Bivariate correlation analysis was performed to evaluate correlations of different factors with plasma MPO levels. In view of the skewed distribution, plasma MPO level was log2-transformed before performing correlation analysis. Cumulative survival curves were generated using the Kaplan-Meier method, and between-group survival was compared using log-rank test. In this analysis, patients who underwent kidney transplant or transferred to hemodialysis therapy during the 3-year follow-up were censored at the time of transfer to alternative kidney replacement therapy. If a patient died within 3 months of transfer to hemodialysis therapy, he or she was not censored because the early mortality was considered to reflect health status during the period of failing PD treatment. We used a Cox proportional hazards model to estimate hazard ratios (HRs) of all-cause mortality and cardiovascular event in relation to log2MPO and with stepwise adjustment for traditional risk factors (age, male sex, smoking history, diabetes, background atherosclerotic vascular disease, hypertension, and low-density lipoprotein cholesterol level), kidney disease–related risk factors (serum albumin level, hemoglobin level, calciumphosphorus product, duration of dialysis therapy, residual glomerular filtration rate, and LV mass index), CRP level, NT-pro-BNP level, and cTnT level. We plotted scaled Schoenfeld residuals versus time for all variables and computed their correlations against time to confirm that all variables considered in the Cox regression analysis met the assumptions of proportional hazards. We performed receiver operating characteristic (ROC) curve analysis to investigate the predictive value for allcause mortality and first cardiovascular event at 3 years by adding CRP, cTnT, NT-pro-BNP, and log2MPO values to the model including standard clinical, demographic, biochemical, dialysis, and echocardiographic parameters. A risk score
939 for each participant based on each model was calculated by the sum of the products of the coefficient estimates from the Cox models for all-cause mortality and first cardiovascular event and the values of the variable for each patient. This risk score then was used in the ROC curve analysis to calculate the area under the curve, which is the Harrell global C statistic for censored responses.25 The incremental prognostic value of log2MPO over the standard clinical, demographic, biochemical, dialysis, and echocardiographic parameters was assessed using a modified stepwise procedure in 4 modeling steps. In brief, model 1 adjusted for traditional risk factors. Model 2 adjusted for factors in model 1 and serum albumin level, hemoglobin level, calcium-phosphorus product, duration of dialysis therapy, residual glomerular filtration rate, and LV mass index. Model 3 adjusted for factors in model 2 and CRP level. Model 4 adjusted for factors in model 3 and cTnT and NT-pro-BNP levels. A significant improvement in model prediction was based on the ⫺2 log likelihood ratio statistic, which followed a 2 distribution, and P value was based on the incremental value compared with the previous model. All statistical analysis was performed using SPSS, version 16.0 (SPSS Inc, www.spss.com). All P values were 2 tailed. P ⬍ 0.05 was considered to be statistically significant.
RESULTS Baseline patient characteristics are listed in Table 1. Causes of ESRD were chronic glomerulonephritis in 75 (31.8%), diabetic nephropathy in 58 (24.6%), hypertensive nephrosclerosis in 31 (13.1%), polycystic kidney disease in 12 (5.1%), tubulointerstitial disease in 7 (3.0%), obstructive uropathy in 13 (5.5%), and not known in 40 patients (16.9%). Median plasma MPO level was 31.8 g/L (25th-75th percentiles, 24.4-42.7). Patients were stratified by tertiles of MPO, namely lower (MPO ⬍26.4 g/L), middle (MPO of 26.4-37.7 g/L), and upper tertiles (MPO ⱖ37.7 g/L). Comparisons across tertiles of plasma MPO are listed in Table 1. Log2MPO showed no significant association with use of -blockers (P ⫽ 0.2), statins (P ⫽ 0.4), and angiotensin-converting enzyme inhibitors/angiotensin receptor blockers (P ⫽ 0.4). Using bivariate partial correlation analysis controlling for other variables, log2MPO significantly correlated with CRP level (partial r ⫽ 0.224; P ⫽ 0.001), age (partial r ⫽ 0.185; P ⫽ 0.008), residual kidney function (partial r ⫽ ⫺0.164; P ⫽ 0.02), male sex (partial r ⫽ ⫺0.139; P ⫽ 0.05), and LV mass index (partial r ⫽ ⫺0.135; P ⫽ 0.05). Other variables, including diabetes, background atherosclerotic vascular disease, hemoglobin level, serum albumin level, low-density lipoprotein cholesterol
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Wang et al Table 1. Baseline Characteristics Plasma MPO Tertile Total (N ⴝ 236)
Lower (n ⴝ 78)
Middle (n ⴝ 80)
Upper (n ⴝ 78)
P for Trend
Men/women
122:114
49:29
38:42
35:43
0.05
Age (y)
56 ⫾ 12
53 ⫾ 13
56 ⫾ 11
58 ⫾ 10
0.008
26 (15.0, 50.8)
23 (10.8, 39.5)
35 (17.0, 63.3)
30 (18.0, 50.3)
Diabetes mellitus
73 (30.9)
19 (24.4)
26 (32.5)
28 (35.9)
0.3
Positive smoking history
88 (37.3)
29 (37.2)
34 (42.5)
25 (32.1)
0.4
Background AVD
55 (23.3)
15 (19.2)
21 (26.3)
19 (24.4)
0.6
Background CAD
47 (19.9)
15 (19.2)
16 (20.0)
16 (20.5)
0.9
Background HF
91 (38.6)
33 (42.3)
32 (40.0)
26 (33.3)
0.5
Body mass index (kg/m2)
23.1 ⫾ 3.4
22.5 ⫾ 3.0
23.1 ⫾ 3.3
23.5 ⫾ 3.7
0.2
Systolic BP (mm Hg)
147 ⫾ 17
147 ⫾ 17
148 ⫾ 16
146 ⫾ 18
0.8
Diastolic BP (mm Hg)
82 ⫾ 10
83 ⫾ 11
82 ⫾ 9
82 ⫾ 10
0.6
Duration on dialysis (mo)
0.006
Hemoglobin (g/dL)
9.22 ⫾ 1.71
9.20 ⫾ 1.71
9.11 ⫾ 1.67
9.35 ⫾ 1.77
0.7
Serum albumin (g/dL)
2.86 ⫾ 0.51
2.87 ⫾ 0.51
2.85 ⫾ 0.59
2.86 ⫾ 0.43
0.9 0.4
Ca ⫻ P (mg2/dL2)
54 ⫾ 17
54 ⫾ 19
55 ⫾ 19
52 ⫾ 13
LDL cholesterol (mg/dL)
128 ⫾ 39
126 ⫾ 35
131 ⫾ 41
125 ⫾ 40
0.6
HDL cholesterol (mg/dL)
48 ⫾ 17
50 ⫾ 15
48 ⫾ 18
47 ⫾ 17
0.5
Triglycerides (mg/dL)
183 ⫾ 129
160 ⫾ 83
186 ⫾ 127
202 ⫾ 162
0.1
Plasma MPO (g/L)
31.8 (24.4, 42.7)
23 (20.3, 24.4)
31.8 (28.8, 33.3)
48 (42.7, 57.2)
⬍0.001
C-Reactive protein (mg/L)
2.66 (0.91, 8.23)
1.29 (0.67, 2.84)
3.14 (0.64, 6.33)
5.64 (2.03, 17.48)
⬍0.001
cTnT (g/L)
0.06 (0.01, 0.15)
0.03 (0.01, 0.13)
0.07 (0.01, 0.16)
0.07 (0.01, 0.17)
0.2
NT-pro-BNP (pg/mL)
5,842 (1,950, 18,335)
4,098 (1,558, 14,336)
7,258 (2,672, 21,406)
5,380 (1,860, 20,192)
0.5
LV mass index (g/m2)
224 ⫾ 84
222 ⫾ 79
235 ⫾ 95
216 ⫾ 78
0.4
53 ⫾ 9
54.6 ⫾ 7.6
52.1 ⫾ 10.8
52.6 ⫾ 8.6
0.2
Total weekly Kt/V
1.81 ⫾ 0.46
1.85 ⫾ 0.46
1.78 ⫾ 0.35
1.82 ⫾ 0.54
0.6
Peritoneal dialysis Kt/V
LV ejection fraction (%)
1.52 ⫾ 0.36
1.46 ⫾ 0.36
1.58 ⫾ 0.37
1.53 ⫾ 0.36
0.09
Total weekly CCr (L/wk/1.73 m2)
57 ⫾ 22
59 ⫾ 23
53 ⫾ 16
58 ⫾ 26
0.2
Residual GFR (mL/min/1.73 m2)
0.61 (0, 1.92)
0.95 (0.2, 2.4)
0.27 (0, 1.37)
0.15 (0, 1.98)
0.003
Note: Continuous data expressed as mean ⫾ standard deviation or median (25th, 75th percentiles); categorical data expressed as number (percentage). Conversion factors for units: hemoglobin in g/dL to g/L, ⫻10; serum albumin in g/dL to g/L, ⫻10; Ca ⫻ P in mg2/dL2 to mmol2/L2, ⫻0.0806; LDL cholesterol in mg/dL to mmol/L, ⫻0.02586; HDL cholesterol in mg/dL to mmol/L, ⫻0.02586; triglycerides in mg/dL to mmol/L, ⫻0.01129. Abbreviations; AVD, atherosclerotic vascular disease; BP, blood pressure; Ca ⫻ P, calcium-phosphorus product; CAD, coronary artery disease; CCr, creatinine clearance; cTnT, cardiac troponin T; GFR, glomerular filtration rate; HDL, high-density lipoprotein; HF, heart failure; Kt/V, urea clearance; LDL, low-density lipoprotein; LV, left ventricular; MPO, myeloperoxidase; NT-pro-BNP, N-terminal pro– brain natriuretic peptide.
level, and duration of dialysis therapy had no significant correlation with log2MPO. During follow-up, 69 patients died, 25 patients underwent kidney transplant, and 24 patients were switched to long-term hemodialysis therapy. Causes of death included 6 acute myocardial ischemic events, 12 strokes, 19 sudden cardiac deaths, 2 heart failure episodes, 4 peripheral vascular diseases, 1 ventricular arrhythmia, 9 peritonitis episodes, 11 other infections, 2 malignancies (carcinoma of lung and pancreas), and 3 dialysis therapy terminations. Baseline median plasma MPO level was significantly higher in
patients who died versus those who survived the 3-year follow-up (35.6 [25th-75th percentiles, 27.1-47.5] vs 29.1 g/L [25th-75th percentiles, 23.6-38.4], respectively; P ⫽ 0.002). Eighty-one patients experienced one or more cardiovascular events. The first cardiovascular event included 25 strokes, 25 acute myocardial ischemic events, 6 peripheral vascular disease events, 10 sustained arrhythmias, and 15 sudden cardiac deaths. Baseline median plasma MPO level was significantly higher in patients who had one or more cardiovascular events versus those with no cardiovascular event during fol-
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Figure 1. Kaplan-Meier estimates of (A) overall survival probability and (B) fatal or nonfatal cardiovascular event–free survival probability of patients stratified by tertiles of plasma myeloperoxidase (MPO). Lower tertile, MPO ⱕ26.4 g/L; middle tertile; MPO, ⬍26.4 to ⬍37.7 g/L; upper tertile, MPO ⱖ37.7 g/L.
low-up (35.4 [25th-75th percentiles, 26.8-50.8] vs 30.0 g/L [25th-75th percentiles, 23.4-38.0]; P ⫽ 0.001). Figure 1A and B shows Kaplan-Meier estimates of overall survival and cardiovascular event–free survival probability of patients stratified by tertiles of plasma MPO. On univariate analysis, HRs associated with doubling in plasma MPO level were 1.44 (95% confidence interval [CI], 1.08-1.91; P ⫽ 0.01) for all-cause mortality and 1.59 (95% CI, 1.21-2.08; P ⫽ 0.001) for first cardiovascular event. Table 2 lists the univariate Cox regression analysis for all-cause mortality and cardiovascular events and the associated area under the curve. Table 3 lists adjusted HRs associated with log2MPO in relation to all-cause mortality and first cardiovascular event in the multivariable Cox regression analysis. In patients without background atherosclerotic vascular disease (n ⫽ 181), univariate HRs for dou-
bling in plasma MPO level in relation to mortality and cardiovascular events were 1.51 (95% CI, 1.07-2.13; P ⫽ 0.02) and 1.70 (95% CI, 1.242.34; P ⫽ 0.001), respectively. Adjusting for variables (with P ⬍ 0.05 on univariate analysis), HRs associated with doubling in plasma MPO level in relation to mortality and cardiovascular event were 1.55 (95% CI, 1.03-2.35; P ⫽ 0.04) and 1.71 (95% CI, 1.19-2.47; P ⫽ 0.004), respectively. The additional predictive value of log2MPO for mortality and cardiovascular events at 3 years was investigated using ROC curve analysis (Table 4). Areas under the curve for mortality and cardiovascular events were highest when log2MPO was included in the model incorporating CRP, cTnT, and NT-pro-BNP levels in addition to the standard clinical, demographic, biochemical, dialysis, and echocardiographic parameters (model 4; Table 3). Figure 2A and B reports the additional predictive power of log2MPO for
942
Wang et al Table 2. Univariate Analysis of Factors in Relation to All-Cause Mortality and Cardiovascular Events All-Cause Mortality
Cardiovascular Events
Variables
HR (95% CI)
P
HR (95% CI)
P
Age (/1 y) Male sex Positive smoking history Diabetes Hypertension Background AVD Background CAD Background HF Duration of dialysis (/1 mo) Residual GFR (/1 mL/min/1.73 m2) Hemoglobin (/1 g/dL) Serum albumin (/1 g/dL) Ca ⫻ P (/1 mg2/dL2) LDL cholesterol (/1 mg/dL) LV mass index (/1 g/m2) C-Reactive protein (/1 mg/L) cTnT (/0.1 g/L) NT-pro-BNP (/1,000 pg/mL) Log2MPO (/1 g/L)
1.06 (1.03-1.08) 1.42 (0.88-2.29) 1.44 (0.90-2.33) 1.72 (1.07-2.78) 1.06 (0.54-2.07) 3.32 (2.05-5.36) 3.02 (1.85-4.95) 2.32 (1.45-3.74) 1.01 (0.001-1.02) 0.63 (0.49-0.80) 0.98 (0.85-1.13) 0.34 (0.21-0.55) 1.01 (0.99-1.02) 0.99 (0.98-1.00) 1.006 (1.004-1.009) 1.03 (1.02-1.04) 1.09 (1.05-1.13) 1.06 (1.04-1.08) 1.44 (1.08-1.91)
⬍0.001 0.1 0.1 0.03 0.9 ⬍0.001 ⬍0.001 0.001 0.03 ⬍0.001 0.7 ⬍0.001 0.5 0.002 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 0.01
1.05 (1.03-1.07) 1.26 (0.81-1.95) 1.46 (0.94-2.26) 2.05 (1.32-3.19) 1.14 (0.60-2.15) 3.23 (2.06-5.07) 2.10 (1.30-3.39) 1.76 (1.14-2.73) 1.004 (1.00-1.01) 0.78 (0.65-0.93) 0.85 (0.74-0.98) 0.43 (0.27-0.68) 1.01 (1.00-1.02) 0.99 (0.99-1.00) 1.006 (1.004-1.008) 1.02 (1.01-1.04) 1.13 (1.09-1.17) 1.05 (1.03-1.07) 1.59 (1.21-2.08)
⬍0.001 0.3 0.1 0.001 0.7 ⬍0.001 0.002 0.01 0.9 0.004 0.03 ⬍0.001 0.2 0.07 ⬍0.001 ⬍0.001 ⬍0.001 ⬍0.001 0.001
Abbreviations: AVD, atherosclerotic vascular disease; Ca ⫻ P, calcium-phosphorus product; CAD, coronary artery disease; cTnT, cardiac troponin T; GFR, glomerular filtration rate; CI, confidence interval; HF, heart failure; HR, hazard ratio; LDL, low-density lipoprotein; LV, left ventricular; MPO, myeloperoxidase; NT-pro-BNP, N-terminal pro– brain natriuretic peptide.
all-cause mortality and cardiovascular events using stepwise multivariable Cox regression analysis.
DISCUSSION This study is the first to show that plasma MPO level provides significant independent and
additional predictive value for long-term mortality and cardiovascular events beyond the standard clinical, demographic, biochemical, dialysis, and echocardiographic parameters, as well as CRP, cTnT, and NT-pro-BNP levels, in patients with ESRD on maintenance PD therapy. Despite
Table 3. Unadjusted and Adjusted HRs for All-Cause Mortality and Cardiovascular Events Associated With Log2MPO, by Stepwise Multivariable Cox Regression Models All-Cause Mortality Model
Covariates
HR (95% CI)
Univariate 1
⫺ Age, sex, diabetes, background AVD, background HF, smoking, hypertension, LDL cholesterol Model 1 ⫹ duration of dialysis ⫹ residual GFR ⫹ serum albumin ⫹ hemoglobin ⫹ Ca ⫻ P ⫹ LV mass index Model 2 ⫹ CRP Model 3 ⫹ NT-pro-BNP ⫹ cTnT
1.44 (1.08-1.91) 1.53 (1.10-2.11)
2
3 4
P
Cardiovascular Event HR (95% CI)
P
0.01 0.02
1.59 (1.21-2.08) 1.61 (1.19-2.19)
0.001 0.002
1.57 (1.14-2.17)
0.006
1.65 (1.22-2.22)
0.001
1.50 (1.07-2.10) 1.46 (1.02-2.08)
0.02 0.04
1.60 (1.17-2.18) 1.60 (1.17-2.18)
0.003 0.003
Note: Log2MPO is equivalent to doubling in plasma MPO level. Abbreviations: AVD, atherosclerotic vascular disease; Ca ⫻ P, calcium-phosphorus product; CI, confidence interval; CRP, C-reactive protein; cTnT, cardiac troponin T; GFR, glomerular filtration rate; HR, hazard ratio; HF, heart failure; LDL, low-density lipoprotein; LV, left ventricular; MPO, myeloperoxidase; NT-pro-BNP, N-terminal pro– brain natriuretic peptide.
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Table 4. Additional Predictive Value of Log2MPO and Other Cardiac Biomarkers in Predicting Mortality and Cardiovascular Events Model
All-Cause Mortality
Cardiovascular Events
Model 1: clinical, demographic, biochemical, dialysis, and echocardiographic parameters Model 2: model 1 ⫹ CRP Model 3: model 2 ⫹ cTnT ⫹ NT-pro-BNP Model 4: model 3 ⫹ log2MPO
0.843 (0.783-0.903)
0.780 (0.724-0.853)
0.838 (0.778-0.898) 0.854 (0.797-0.912) 0.861 (0.804-0.918)
0.784 (0.719-0.849) 0.791 (0.725-0.857) 0.810 (0.764-0.874)
Note: Data presented are based on ROC curve analysis with calculated AUCs or C statistics; 95% CIs are given in parentheses. Clinical and demographic parameters include age, diabetes, and background atherosclerotic vascular disease; biochemical parameters include hemoglobin level, low-density lipoprotein cholesterol level, and serum albumin level; dialysis parameters include duration of dialysis therapy and residual glomerular filtration rate; the echocardiographic parameter is left ventricular mass index. log2MPO is equivalent to a doubling in plasma MPO level. Abbreviations: AUC, area under the curve; CRP, C-reactive protein; cTnT, cardiac troponin T; NT-pro-BNP, N-terminal pro– brain natriuretic peptide; MPO, myeloperoxidase; ROC, receiver operating characteristic.
showing correlations with inflammation and residual kidney function, MPO level remained prognostically significant in multivariable Cox regression models independent of residual kidney function and CRP, cTnT, and NT-pro-BNP
Figure 2. Incremental predictive value of log2MPO (doubling in plasma myeloperoxidase [MPO] level) for (A) all-cause mortality and (B) fatal or nonfatal cardiovascular event in the stepwise multivariable Cox regression analysis. (A) The sequential addition of C-reactive protein (CRP), cardiac troponin T (cTnT), N-terminal pro– brain natriuretic peptide (NT-proBNP), and log2MPO significantly improved the predictive power of a model including clinical (diabetes mellitus, background atherosclerotic vascular disease, positive smoking history, hypertension, and duration of dialysis therapy), demographic (age and sex), biochemical (hemoglobin, serum albumin, calcium-phosphorus product, and low-density lipoprotein cholesterol values), dialysis (residual glomerular filtration rate), and echocardiographic parameters (left ventricular mass index). (B) The sequential addition of cTnT, NT-pro-BNP, and log2MPO, but not CRP, values significantly improved the predictive power of a model including clinical, demographic, biochemical, dialysis, and echocardiographic parameters.
levels. Furthermore, MPO level had significant incremental predictive value in models incorporating clinical, demographic, biochemical, dialysis, and echocardiographic risk factors, as well as CRP, cTnT, and NT-pro-BNP levels. Our obser-
944
vations extend a previous study linking plasma MPO level and mortality in hemodialysis patients17 and add value in providing new insights into the pathogenesis of increased cardiovascular mortality in PD patients. Increased production of MPO-catalyzed 3-chlorotyrosine in dialysis patients has been related to increased oxidative stress,25 and 3-chlorotyrosine is present in human atherosclerotic aorta.9 Increased plasma MPO levels during hemodialysis also appears to reflect increased oxidative stress.26 Therefore, increased plasma MPO levels may contribute to increased cardiovascular mortality in patients with ESRD through leukocyte activation and increased oxidative stress. Although MPO and CRP are both inflammatory markers and are closely correlated in both the general and ESRD populations,17,27 increased plasma MPO levels may more specifically reflect increased oxidative stress. In contrast to CRP level, which remains increased for a long time, MPO has a much shorter half-life. A return of MPO level to baseline was observed within 1 week after acute coronary syndrome.13,28 These data suggest that recruitment and degranulation of neutrophils with release of MPO likely is the primary event, followed by release of other inflammatory proteins, such as CRP.28 CRP level may reflect vascular inflammation more, whereas MPO may be more a systemic marker of neutrophil activation and plaque instability. Buffon et al29 showed widespread systemic activation of neutrophils across the coronary vascular bed in patients with unstable angina independent of the site of culprit stenosis. Furthermore, systemic correlation was found between CRP level and either aortic or coronary sinus neutrophil MPO level.30 Thus, the role of CRP and MPO in predicting cardiovascular risk likely is complementary. No previous study has compared MPO levels in hemodialysis and PD patients. However, hemodialysis may induce more MPO activation through leukocyte activation at the dialysis membrane,29,31 and the degree of MPO activation may be related to the biocompatibility of dialyzer membranes.27 However, the inverse association between plasma MPO level and residual kidney function in PD patients is noteworthy. It is possible that this may be caused by increased oxidative stress and inflammation with worsening uremia. Human MPO is a 150-kDa
Wang et al
hemoprotein,32 and its increase with decrease in residual kidney function is unlikely to be explained by decreased kidney clearance alone. Additional study is needed to clarify this issue and compare MPO levels between hemodialysis and PD patients. Both cTnT and NT-pro-BNP recently have emerged as useful biomarkers for prognostication in patients with ESRD in that a cTnT level increase indicates subclinical myocardial necrosis, whereas an NT-pro-BNP level increase reflects increased LV wall stress.33 Studies of PD patients also have shown their usefulness in predicting mortality and cardiovascular events.34,35 However, in this study, plasma MPO level had no correlation with cTnT or NT-pro-BNP level, suggesting that MPO likely reflects a unique aspect of cardiovascular risk independent of cTnT and NT-pro-BNP. A previous study showed the stability of plasma MPO levels over time in the general population.12 Whether similar stability is observed in the ESRD population requires further evaluation. Multiple lines of experimental evidence suggest that MPO is proatherogenic, has powerful oxidative capacity, and contributes to inflammatory tissue injury.2-6 MPO has been linked to the development of atherosclerotic plaque, plaque instability,36,37 and production of oxidized lipids,38,39 all pivotal steps in atherosclerosis. Expression of human MPO in murine macrophages increased atherosclerosis in hypercholesterolemic mice.40 There also is clinical evidence to support an important link between MPO and cardiovascular disease in humans. Patients with MPO deficiency were protected against cardiovascular disease.41 A single plasma MPO measurement predicts future risk of coronary artery disease in apparently healthy individuals42 and early risk of myocardial infarction and risk of major adverse cardiac events in patients presenting to the emergency department with chest pain independent of levels of CRP and other inflammatory markers.41 Increased leukocyte and blood MPO levels also correlated with angiographically proven coronary artery disease independent of CRP level and Framingham risk score.12 Our findings therefore are consistent with previous studies of the general population and support the hypothesis that MPO has a significant contribution to increased cardiovascular mortality independent of CRP.
Myeloperoxidase in Kidney Failure
Although increased plasma MPO level predicts an increased risk of cardiovascular events and mortality over the long-term in PD patients, we did not observe a direct cross-sectional link between plasma MPO level and atherosclerotic vascular disease. This may seem contrary to our hypothesis. A previous study of the general population showed that increased plasma MPO level preceded the onset of coronary artery disease by years.42 Our study has several limitations. First, plasma MPO and other parameters were measured on a single occasion and did not reflect possible changes with time on dialysis therapy. However, given that this is a prognostic rather than etiologic study, a single time measurement also reproduces the typical situation of daily clinical practice. Second, our study included prevalent dialysis patients and may introduce survival bias. Further study is needed to elucidate the exact mechanistic link between MPO level and cardiovascular disease in uremia. In this study, the rule of 10 events per variable in the multivariable Cox regression analysis was relaxed to show adequate control of confounding43; however, we acknowledged that the models may be somewhat overfitted. In conclusion, our study has shown that plasma MPO level predicts long-term mortality and cardiovascular events in patients with ESRD beyond the standard clinical, dialysis, echocardiographic, and biochemical parameters, including CRP, cTnT, and NT-pro-BNP levels. MPO may be involved in the pathogenesis of increased cardiovascular mortality in PD patients.
ACKNOWLEDGEMENTS This study was presented as a free communication in Renal Week of the American Society of Nephrology, November 4-9, 2008, Philadelphia, PA. Support: This study was supported by Hong Kong Health Service Research Grant No. 6901023, of which Dr Wang is the principal investigator. Financial Disclosure: The authors declare that they have no relevant financial interests.
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