Open Access

Association of apolipoprotein A1 -75 G/A polymorphism with susceptibility to the development of acute lung injury after cardiopulmonary bypass surgery

  • Jie Tu1,
  • Bingdong Zhang1Email author,
  • Yanhua Chen1,
  • Beiwei Liang1,
  • Dongke Liang1,
  • Guofeng Liu1 and
  • Fang He1
Lipids in Health and Disease201312:172

https://doi.org/10.1186/1476-511X-12-172

Received: 17 October 2013

Accepted: 28 October 2013

Published: 11 November 2013

Abstract

Introduction

Apolipoprotein A1 (apoA1) is the major apoprotein constituent of high density lipoprotein (HDL) which exerts innate protective effects in systemic inflammation. However, its role in the acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) has not been well studied. The objective of this study was to investigate the potential association between APOA1 -75 G/A polymorphism and the development of ALI after cardiopulmonary bypass (CPB) surgery.

Materials and methods

A hospital-based case–control study was conducted in patients with ALI (n = 300), patients without ALI (n = 300) and healthy controls (n = 300). Polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) assay was applied to assess the APOA1 -75 G/A genotypes.

Results

Patients with ALI had a significantly higher frequency of APOA1 -75 AA genotype [odds ratio (OR) =1.75, 95% confidence interval (CI) = 1.04, 2.92; P = 0.03] than patients without ALI. APOA1 -75 AA genotype (OR =3.47, 95% CI = 1.60, 7.52; P = 0.002) and A allele (OR =1.92, 95% CI = 1.24, 2.96; P = 0.003) were the significant independent prognostic factors for the 30-day survival rate of patients with ALI after CPB surgery.

Conclusion

Our study suggested that APOA1 -75 AA genotype was associated with a higher ALI risk after CPB surgery. Patients with the APOA1 -75 AA genotype and A allele had higher 30-day mortality of ALI after CPB surgery. Additional studies are needed to confirm this finding.

Keywords

Apolipoprotein A1 Acute lung injury Cardiopulmonary bypass Gene polymorphism

Introduction

Acute lung injury (ALI) is a common complication after cardiopulmonary bypass (CPB) surgery. ALI remains the main cause of mortality after CPB surgery [1]. The main causes of ALI after CPB surgery have been identified, including ischemia reperfusion injury, endotoxemia, primary pulmonary disease, surgical injury, and the systemic inflammatory reaction initiated by the contact of the blood leukocytes with the artificial surface of the bypass circuit [25]. Improved understanding of disease heterogeneity through use of evolving biologic, genomic, and genetic approaches should provide major new insights into pathogenesis of ALI [6].

Apolipoprotein A1 (apoA1) is the major apoprotein constituent of high density lipoprotein (HDL) which exerts innate protective effects in systemic inflammation [7]. However, its role in the acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) has not been well studied. One variant resides in the APOA1 gene, which involves a guanine to adenine transition 75 base pairs (bp) upstream from the start of transcription (G–75A) and destroys a site for the Msp I restriction enzyme. A strong association was found between the G to A substitution at -75 bp with serum HDL and apoA1 levels [8]. The objective of this study was to investigate the potential association between APOA1 -75 G/A polymorphism and the development of ALI after cardiopulmonary bypass (CPB) surgery.

Materials and methods

Study population

From January 2008 to January 2013, a hospital-based case–control study was conducted in patients with ALI (n = 300), patients without ALI (n = 300) after CPB surgery and healthy controls (n = 300) in the Institute of cardiovascular Diseases of the First Affiliated Hospital of Guangxi Medical University. All subjects were collected from the same geographic region. Surgery type included valvular surgery, coronary artery bypass graft (CABG) and aortic surgery. The healthy control subjects were matched with the patients for age and sex. Healthy control subjects were recruited from the First Affiliated Hospital of Guangxi Medical University where they were attending a clinic for routine examination. Patients who met diagnostic criteria of acute lung injury at 24 hours after surgery were allocated to ALI group; those without ALI were allocated to without ALI group. ALI was defined as PaO2/FiO2 < 300 mm Hg; and bilateral pulmonary infiltrates on chest radiograph in the absence of acute pulmonary edema after left cardiac failure or other nonlung pathology 24 hours after surgery, and a left atrial pressure lower than 18 mm Hg [9]. Patients were excluded if they met the following criteria: immunodeficiency, autoimmune disease, or immunosuppressive therapy; tuberculosis, chronic obstructive pulmonary disease (COPD), or other chronic pulmonary diseases; liver dysfunction or chronic renal disease; bleeding disorders; anemia; postoperative pericardial tamponade requiring reoperation; postoperative low cardiac output syndrome, or acute pulmonary edema after left cardiac failure. The Ethical Committee of the First Affiliated Hospital of Guangxi Medical University approved the study protocols, and all participants gave written informed consent according to the Declaration of Helsinki.

DNA extraction and genotyping

The commercially available Qiagen kit (QIAGEN Inc., Valencia, CA, USA) was used to extract DNA from peripheral blood leukocytes. The APOA1 -75 G/A genotypes were analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay. Based on the GenBank reference sequence, the PCR primers were as follows: forward-5′- AGGGACAGAGCTGATCC-TTGAACTCTTAG -3′ and reverse-5′- TTAGGGGACACCTACCCGTACAGGAAGAGCA -3′. DNA was denaturanted at 94°C for 5 minutes, followed by 35 cycles of denaturation at 94°C for 1 minute, annealing at 60°C for 0.5 minute, and extension at 72°C for 0.5 minutes, with a final extension step of 5 minutes at 72°C. A total volume of 10 μl containing 20 U Msp I was added directly to the PCR product and digested at 37°C overnight. After electrophoresis, the digested products were visualized on a 9% polyacrylamide gel with ethidium bromide staining.

Statistical analysis

We used Statistical Analysis System software (Version 8; SAS Institute Inc., Cary, NC, USA) to perform all of the statistical analyses. The x2 test was used to test for deviation of genotype frequencies from Hardy-Weinberg equilibrium and to compare the genotype distributions among patients with ALI, patients without ALI and healthy controls. We applied multivariate logistic regression to calculate crude and adjusted odds ratios (OR) and 95% confidence intervals (CI) for the association between the genotypes and the development of ALI after CPB surgery. A P-value was considered significant at a level of < 0.05.

Results

Clinical data of patients with or without ALI after CPB surgery were listed in Table 1. Univariate study was performed to identify that age (P = 0.005), NYHA (P = 0.02), obesity (P = 0.03), peripheral vascular disease (P = 0.03), duration of operation (P = 0.003), and duration of CPB (P = 0.04) were associated with ALI after CPB surgery (Table 1). In addition, no significant differences were found between the patients with or without ALI after CPB surgery in gender, ASA, LVEF, previous cardiac surgery, hypertension, diabetes mellitus, transfusion, duration of cross-clamp, and surgery type (Table 1).
Table 1

Clinical data of patients with or without ALI after CPB surgery

Variable

ALI

Without ALI

P

Total no.

300

300

 

Age (years)

59.1 ± 10.6

56.8 ± 9.4

0.005

Gender (Male/Female)

205/95

201/99

0.73

ASA

2.77 ± 0.71

2.67 ± 0.65

0.07

NYHA

2.69 ± 0.67

2.57 ± 0.55

0.02

LVEF (%)

59.68 ± 7.94

60.68 ± 8.04

0.13

Previous cardiac surgery (%)

45 (15.0)

30 (10.0)

0.07

Hypertension (%)

75 (25.0)

70 (23.3)

0.63

Diabetes mellitus (%)

26 (8.7)

24 (8.0)

0.77

Obesity (%)

70 (23.3)

48 (16.0)

0.03

Peripheral vascular disease (%)

43 (14.3)

26 (8.7)

0.03

Transfusion (mL)

694.3 ± 103.7

687.3 ± 99.8

0.40

Duration of operation (min)

231.7 ± 90.1

210.5 ± 82.3

0.003

Duration of CPB (min)

106.7 ± 43.7

99.5 ± 41.5

0.04

Duration of cross-clamp (min)

62.1 ± 32.4

60.7 ± 31.9

0.59

Surgery

   

  Valvular surgery (%)

176 (58.7)

183 (61.0)

0.56

  CABG surgery (%)

99 (33.0)

102 (34.0)

0.80

  Aortic surgery (%)

25 (8.3)

15 (5.0)

0.11

ALI, Acute lung injury; CPB, Cardiopulmonary bypass; ASA, American Society of Anesthesiology; NYHA, New York Heart Association; LVEF, Left ventricular ejection fraction; CABG, Coronary artery bypass graft.

Patients with ALI had a significantly higher frequency of APOA1 -75 AA genotype (OR =1.75, 95% CI = 1.04, 2.92; P = 0.03) than patients without ALI (Table 2). APOA1 -75 AA genotype (OR =3.47, 95% CI = 1.60, 7.52; P = 0.002) and A allele (OR =1.92, 95% CI = 1.24, 2.96; P = 0.003) were the significant independent prognostic factors for the 30-day survival rate of patients with ALI after CPB surgery (Table 3).
Table 2

Genotype and allele frequencies of apolipoprotein A1 -75 G/A polymorphism among patients with or without ALI after CPB surgery and controls

 

ALI (n = 300)

Without ALI (n = 300)

Healthy controls (n = 300)

ALI vs without ALI

ALI vs healthy controls

Without ALI vs healthy controls

    

OR (95% CI)

P value

OR (95% CI)

P value

OR (95% CI)

P value

Genotype

GG

132(44.0)

144(48.0)

138(46.0)

1.00(Reference)

 

1.00(Reference)

 

1.00(Reference)

 

GA

120(40.0)

126(42.0)

129(43.0)

1.04(0.74,1.47)

0.83

0.97(0.69,1.37)

0.87

0.94(0.67,1.31)

0.70

AA

48(16.0)

30(10.0)

33(11.0)

1.75(1.04,2.92)

0.03

1.52(0.92,2.52)

0.10

0.87(0.50,1.51)

0.62

Allele

G

384(64.0)

414(69.0)

405(67.5)

1.00(Reference)

 

1.00(Reference)

 

1.00(Reference)

 

A

216(36.0)

186(31.0)

195(32.5)

1.25(0.99,1.59)

0.07

1.17(0.92,1.48)

0.20

0.93(0.73,1.19)

0.58

ALI, Acute lung injury; CPB, Cardiopulmonary bypass; vs, versus; OR, Odds ratio; CI, Confidence interval.

Table 3

Effect of apolipoprotein A1 -75 G/A polymorphism on the 30-day survival rate of patients with ALI after CPB surgery

 

Non-survivors (n = 50)

Survivors (n = 250)

OR (95% CI)

P value

Genotype

    

GG

18(36.0)

114(45.6)

1.00(Reference)

 

GA

15(30.0)

105(42.0)

0.91(0.43,1.89)

0.79

AA

17(34.0)

31(12.4)

3.47(1.60,7.52)

0.002

Allele

    

G

51(51.0)

333(66.6)

1.00(Reference)

 

A

49(49.0)

167(33.4)

1.92(1.24,2.96)

0.003

ALI, Acute lung injury; CPB, Cardiopulmonary bypass; OR, Odds ratio; CI, Confidence interval.

Discussion

Some studies have been performed to find an association of genetic polymorphism and ALI [10]. A prospective case–control study found that -607C/C genotype in IL-18 gene played a pivotal role in the development of ALI after CPB surgery in Chinese Han population [11]. Another case–control study found that the IL-6 -572 polymorphism was associated with ALI after CPB surgery [12]. Several studies have suggested that pre-B-cell colony-enhancing factor (PBEF) gene polymorphisms were associated with susceptibility to and prognosis of ALI [13, 14]. The plasminogen activator inhibitor-1 (PAI-1) 4G allele was associated with worse outcome in ALI/ARDS [15]. A prospective cohort demonstrated that the AC genotype at position -1221 in the NQO1 gene caused decreased transcription and was associated with a lower incidence of ALI following major trauma [16]. In a nested case–control study, patients with the NRF2 -617 A allele had a significantly higher risk for developing ALI after major trauma [17]. A case–control study found that myosin light chain kinase (MYLK) genetic variants were associated with increased risk of sepsis-associated ALI [18].

The APOA1 -75 G/A polymorphism has recently been linked to many other diseases. A comparative study found that carrying the APOA1 -75 A allele could confer a higher risk of hyperlipidemia in obese children [19]. A prospective case–control study found that the APOA1 -75 G/A polymorphism influenced cholesterol metabolism [20]. A study in healthy Tamilian volunteers of south India found that the APOA1 -75 G/A polymorphism was significantly associated with HDL-C levels [21]. A study found the APOA1 -75 G/A polymorphism was significantly associated with plasma triglyceride levels in men with coronary artery disease from the REGRESS study [22]. A case–control study found the APOA1 -75G/A promoter polymorphism was associated with variations in serum triglyceride concentrations in hypercholesterolemic individuals [23]. A case–control study suggested that a positive association was found between the APOA1 -75 A allele carriers and breast cancer risk [24]. A pilot study in a north Indian population suggested that the APOA1 -75 G allele might be susceptibility alleles for myocardial infarction [25]. A case–control study found an association of the APOA1 -75G/A promoter polymorphism with cognitive performance in multiple sclerosis [26]. A cohort study found that the APOA1 -75 G allele showed significant association with hypertension [27]. A case–control study found the APOA1 -75 G/A polymorphism was associated with gallstone disease [28]. A case–control study found the APOA1 -75 A allele was associated with an increased risk for Alzheimer’s disease [29]. A case–control study found the APOA1 -75 G/A polymorphism was significantly associated with lipid levels and coronary atherosclerosis disease [30].

Some limitations of this study should be noted. First of all, these results should be interpreted with caution because the population was only from China, which reduces the possibility of confounding from ethnicity, so it does not permit extrapolation of the results to other ethnic groups. Second, the sample size of this study is relatively small, which may not have enough statistical power to explore the real association. Third, this is a hospital based case control study, so the selection bias cannot be avoidable and the subjects may not be representative of the general population.

In conclusion, our study suggested that APOA1 -75 AA genotype was associated with a higher ALI risk after CPB surgery. Patients with the APOA1 -75 AA genotype and A allele had higher 30-day mortality of ALI after CPB surgery. Additional studies are needed to confirm this finding.

Acknowledgement

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Declarations

Authors’ Affiliations

(1)
Institute of cardiovascular Diseases, The First Affiliated Hospital of Guangxi, Medical University

References

  1. Hammermeister KE, Burchfiel C, Johnson R, Grover FL: Identification of patients at greatest risk for developing major complications at cardiac surgery. Circulation. 1990, 82: IV380-IV389.PubMedGoogle Scholar
  2. Wakayama F, Fukuda I, Suzuki Y, Kondo N: Neutrophil elastase inhibitor, sivelestat, attenuates acute lung injury after cardiopulmonary bypass in the rabbit endotoxemia model. Ann Thorac Surg. 2007, 83: 153-160. 10.1016/j.athoracsur.2006.08.023View ArticlePubMedGoogle Scholar
  3. Westaby S, Fleming J, Royston D: Acute lung injury during cardiopulmonary bypass, the role of neutrophil sequestration and lipid peroxidation. Trans Am Soc Artif Intern Organs. 1985, 31: 604-609.PubMedGoogle Scholar
  4. Bando K, Pillai R, Cameron DE, Brawn JD, Winkelstein JA, Hutchins GM, Reitz BA, Baumgartner WA: Leukocyte depletion ameliorates free radical-mediated lung injury after cardiopulmonary bypass. J Thorac Cardiovasc Surg. 1990, 99: 873-877.PubMedGoogle Scholar
  5. Asimakopoulos G, Smith PL, Ratnatunga CP, Taylor KM: Lung injury and acute respiratory distress syndrome after cardiopulmonary bypass. Ann Thorac Surg. 1999, 68: 1107-1115. 10.1016/S0003-4975(99)00781-XView ArticlePubMedGoogle Scholar
  6. Matthay MA, Zimmerman GA, Esmon C, Bhattacharya J, Coller B, Doerschuk CM, Floros J, Gimbrone MA, Hoffman E, Hubmayr RD: Future research directions in acute lung injury: summary of a National Heart, Lung, and Blood Institute working group. Am J Respir Crit Care Med. 2003, 167: 1027-1035. 10.1164/rccm.200208-966WSView ArticlePubMedGoogle Scholar
  7. Albahrani AI, Usher JJ, Alkindi M, Marks E, Ranganath L, Al-yahyaee S: ApolipoproteinA1–75 G/A (M1-) polymorphism and lipoprotein(a); anti- vs. pro-Atherogenic properties. Lipids Health Dis. 2007, 6: 19- 10.1186/1476-511X-6-19PubMed CentralView ArticlePubMedGoogle Scholar
  8. Saha N, Tay JS, Low PS, Humphries SE: Guanidine to adenine (G/A) substitution in the promoter region of the apolipoprotein AI gene is associated with elevated serum apolipoprotein AI levels in Chinese non-smokers. Genet Epidemiol. 1994, 11: 255-264. 10.1002/gepi.1370110304View ArticlePubMedGoogle Scholar
  9. Bernard GR, Artigas A, Brigham KL, Carlet J, Falke K, Hudson L, Lamy M, Legall JR, Morris A, Spragg R: The American-European Consensus Conference on ARDS. Definitions, mechanisms, relevant outcomes, and clinical trial coordination. Am J Respir Crit Care Med. 1994, 149: 818-824. 10.1164/ajrccm.149.3.7509706View ArticlePubMedGoogle Scholar
  10. Reddy AJ, Kleeberger SR: Genetic polymorphisms associated with acute lung injury. Pharmacogenomics. 2009, 10: 1527-1539. 10.2217/pgs.09.89PubMed CentralView ArticlePubMedGoogle Scholar
  11. Chen S, Xu L, Tang J: Association of interleukin 18 gene polymorphism with susceptibility to the development of acute lung injury after cardiopulmonary bypass surgery. Tissue Antigens. 2010, 76: 245-249. 10.1111/j.1399-0039.2010.01506.xView ArticlePubMedGoogle Scholar
  12. Wang JF, Bian JJ, Wan XJ, Zhu KM, Sun ZZ, Lu AD: Association between inflammatory genetic polymorphism and acute lung injury after cardiac surgery with cardiopulmonary bypass. Med Sci Monit. 2010, 16: CR260-CR265.PubMedGoogle Scholar
  13. Liu Y, Shao Y, Yu B, Sun L, Lv F: Association of PBEF gene polymorphisms with acute lung injury, sepsis, and pneumonia in a northeastern Chinese population. Clin Chem Lab Med. 2012, 50: 1917-1922.PubMedGoogle Scholar
  14. Bajwa EK, Yu CL, Gong MN, Thompson BT, Christiani DC: Pre-B-cell colony-enhancing factor gene polymorphisms and risk of acute respiratory distress syndrome. Crit Care Med. 2007, 35: 1290-1295. 10.1097/01.CCM.0000260243.22758.4FView ArticlePubMedGoogle Scholar
  15. Tsangaris I, Tsantes A, Bonovas S, Lignos M, Kopterides P, Gialeraki A, Rapti E, Orfanos S, Dimopoulou I, Travlou A, Armaganidis A: The impact of the PAI-1 4G/5G polymorphism on the outcome of patients with ALI/ARDS. Thromb Res. 2009, 123: 832-836. 10.1016/j.thromres.2008.07.018View ArticlePubMedGoogle Scholar
  16. Reddy AJ, Christie JD, Aplenc R, Fuchs B, Lanken PN, Kleeberger SR: Association of human NAD(P)H: quinone oxidoreductase 1 (NQO1) polymorphism with development of acute lung injury. J Cell Mol Med. 2009, 13: 1784-1791. 10.1111/j.1582-4934.2008.00581.xPubMed CentralView ArticlePubMedGoogle Scholar
  17. Marzec JM, Christie JD, Reddy SP, Jedlicka AE, Vuong H, Lanken PN, Aplenc R, Yamamoto T, Yamamoto M, Cho HY, Kleeberger SR: Functional polymorphisms in the transcription factor NRF2 in humans increase the risk of acute lung injury. FASEB J. 2007, 21: 2237-2246. 10.1096/fj.06-7759comView ArticlePubMedGoogle Scholar
  18. Gao L, Grant A, Halder I, Brower R, Sevransky J, Maloney JP, Moss M, Shanholtz C, Yates CR, Meduri GU: Novel polymorphisms in the myosin light chain kinase gene confer risk for acute lung injury. Am J Respir Cell Mol Biol. 2006, 34: 487-495. 10.1165/rcmb.2005-0404OCPubMed CentralView ArticlePubMedGoogle Scholar
  19. Toptas B, Gormus U, Ergen A, Gurkan H, Kelesoglu F, Darendeliler F, Bas F, Dalan AB, Izbirak G, Isbir T: Comparison of lipid profiles with APOA1 MspI polymorphism in obese children with hyperlipidemia. In Vivo. 2011, 25: 425-430.PubMedGoogle Scholar
  20. Smach MA, Edziri H, Charfeddine B, Ben Othman L, Lammouchi T, Ltaief A, Nafati S, Dridi H, Bennamou S, Limem K: Polymorphism in apoA1 influences high-density lipoprotein cholesterol levels but is not a major risk factor of alzheimer’s disease. Dement Geriatr Cogn Dis Extra. 2011, 1: 249-257. 10.1159/000329910PubMed CentralView ArticlePubMedGoogle Scholar
  21. Padmaja N, Kumar MR, Adithan C: Association of polymorphisms in apolipoprotein A1 and apolipoprotein B genes with lipid profile in Tamilian population. Indian Heart J. 2009, 61: 51-54.PubMedGoogle Scholar
  22. Souverein OW, Jukema JW, Boekholdt SM, Zwinderman AH, Tanck MW: Polymorphisms in APOA1 and LPL genes are statistically independently associated with fasting TG in men with CAD. Eur J Hum Genet. 2005, 13: 445-451. 10.1038/sj.ejhg.5201362View ArticlePubMedGoogle Scholar
  23. Sorkin SC, Forestiero FJ, Hirata MH, Guzman EC, Cavalli SA, Bertolami MC, Salazar LA, Hirata RD: APOA1 polymorphisms are associated with variations in serum triglyceride concentrations in hypercholesterolemic individuals. Clin Chem Lab Med. 2005, 43: 1339-1345.View ArticlePubMedGoogle Scholar
  24. Hamrita B, Ben Nasr H, Gabbouj S, Bouaouina N, Chouchane L, Chahed K: Apolipoprotein A1–75 G/A and +83 C/T polymorphisms: susceptibility and prognostic implications in breast cancer. Mol Biol Rep. 2011, 38: 1637-1643. 10.1007/s11033-010-0274-0View ArticlePubMedGoogle Scholar
  25. Dawar R, Gurtoo A, Singh R: Apolipoprotein A1 gene polymorphism (G-75A and C + 83 T) in patients with myocardial infarction: a pilot study in a north Indian population. Am J Clin Pathol. 2010, 134: 249-255. 10.1309/AJCPKPTXQ3QN1IFGView ArticlePubMedGoogle Scholar
  26. Koutsis G, Panas M, Giogkaraki E, Karadima G, Sfagos C, Vassilopoulos D: An APOA1 promoter polymorphism is associated with cognitive performance in patients with multiple sclerosis. Mult Scler. 2009, 15: 174-179.View ArticlePubMedGoogle Scholar
  27. Chen ES, Mazzotti DR, Furuya TK, Cendoroglo MS, Ramos LR, Araujo LQ, Burbano RR, de Arruda Cardoso Smith M: Apolipoprotein A1 gene polymorphisms as risk factors for hypertension and obesity. Clin Exp Med. 2009, 9: 319-325. 10.1007/s10238-009-0051-3View ArticlePubMedGoogle Scholar
  28. Dixit M, Choudhuri G, Saxena R, Mittal B: Association of apolipoprotein A1-C3 gene cluster polymorphisms with gallstone disease. Can J Gastroenterol. 2007, 21: 569-575.PubMed CentralPubMedGoogle Scholar
  29. Vollbach H, Heun R, Morris CM, Edwardson JA, McKeith IG, Jessen F, Schulz A, Maier W, Kolsch H: APOA1 polymorphism influences risk for early-onset nonfamiliar AD. Ann Neurol. 2005, 58: 436-441. 10.1002/ana.20593View ArticlePubMedGoogle Scholar
  30. Zou Y, Hu D, Yang X, Jia X, Wang L, Cui L, Liu X, Gao M, Wei Y, Xu Z: Relationships among apolipoprotein A1 gene polymorphisms, lipid levels and coronary atherosclerosis disease. Chin Med J (Engl). 2003, 116: 665-668.Google Scholar

Copyright

© Tu et al.; licensee BioMed Central Ltd. 2013

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.