Association Analysis of<i> MET</i> Gene Polymorphism with Papillary Thyroid Carcinoma in a Chinese Population

Ning, Lifeng; Yu, Yaqin; Liu, Xiaoli; Ai, Lizhe; Zhang, Xin; Rao, Wenwang; Shi, Jieping; Sun, Hui; Yu, Qiong

doi:https://doi.org/10.1155/2015/405217

International Journal of Endocrinology

On this page

Abstract Introduction Methods Results Discussion References Copyright Related Articles

Research Article | Open Access

Volume 2015 | Article ID 405217 | https://doi.org/10.1155/2015/405217

Association Analysis of MET Gene Polymorphism with Papillary Thyroid Carcinoma in a Chinese Population

Lifeng Ning,^1,2Yaqin Yu,¹Xiaoli Liu,³Lizhe Ai,¹Xin Zhang,¹Wenwang Rao,¹Jieping Shi,¹Hui Sun,³and Qiong Yu¹

Academic Editor: Diego Russo

Received10 Jun 2015

Revised16 Oct 2015

Accepted27 Oct 2015

Published15 Nov 2015

Abstract

To investigate the association of MET SNPs with gender disparity in thyroid tumors, as well as the metastasis and prognosis of patients, 858 patients with papillary thyroid carcinoma (PTC), 556 patients with nodular goiter, and 896 population-based normal controls were recruited. The genotyping of MET SNPs was carried out using the Sequenom MassARRAY system. The distribution of MET SNPs (rs1621 and rs6566) was different among groups. Gender stratification analysis revealed a significant association between the rs1621 genotype and PTC in female patients (), but not in male patients (). For female patients, the rs1621 AG genotype was significantly higher in patients with PTC than in normal controls () and revealed an increasing risk of PTC (OR: 1.465, 95% CI: 1.118–1.92). However, association analysis of the rs1621 genotype with metastasis and prognosis revealed no significant correlation in both male and female patients. The findings of our study showed that polymorphism of SNP locus rs1621 in MET gene may be associated with gender disparity in PTC. Higher AG genotypes in rs1621 were correlated with PTC in female patients, but not in male patients.

1. Introduction

Papillary thyroid carcinoma (PTC) is the most common form of thyroid malignancy, which generally has a good prognosis and accounts for approximately 80–85% of all thyroid carcinomas [1–3]. However, this type of cancer may cause distant metastasis and be more aggressive in older patients [4]. Established risk factors for PTC include ionizing radiation, positive family history, and thyroid nodular disease [5]; but these factors do not appear to account for the increasing incidence of PTC [6]. Studies have shown that age, gender, Hashimoto’s thyroiditis, thyroid-stimulating hormone concentrations, solitary nodularity, and anti-thyroglobulin antibodies positivity are known risk factors for PTC development [7, 8]. Other studies have proposed that genetic factors may also contribute to the risk of PTC [9, 10].

Gender difference in incidence, aggressiveness, and prognosis has been well-established in thyroid cancer. The incidence of thyroid cancer has been reported to be three to five times more frequent in women, and this gender difference is particularly obvious for women of reproductive age [11]. Gender disparity in thyroid cancer has also been known to be specific to the histologic subtype of thyroid cancer, with the more commonly differentiated thyroid cancer of follicular cell origin including PTC in women. However, the potential reason for this disparity is poorly understood. Genetic analysis such as single-nucleotide polymorphism analysis has been suggested to be helpful in better understanding the molecular basis for gender disparity in thyroid and other cancers [12].

The cellular mesenchymal-epithelial transition (MET) factor is a plasma membrane tyrosine kinase receptor that has low activity in normal tissues but is dysregulated in many tumor types [13]. It can be activated in tumor cells through mutation, amplification, and overexpression [14]. The dysregulated activation of MET kinase may correlate with the aggressiveness of tumor growth and metastasis [15]. The role of MET mutation in human cancer was first established in papillary renal carcinoma [16]. Mutations of the MET protooncogene have also been described in several other types of human cancers and were suggested to correlate with tumor metastasis [17–19]. Therefore, the present study was designed to investigate the association of MET single-nucleotide polymorphisms (SNPs) with the gender disparity of thyroid tumors, as well as the metastasis and prognosis of PTC in the Chinese population.

2. Material and Methods

2.1. Subjects

The study subjects comprised 858 patients with PTC (, 208 males/650 females), 556 patients with nodular goiter (NG, , 131 males/425 females), and 896 population-based normal controls (NC, , 219 males/677 females). All patients were recruited from the China-Japan Union Hospital of Jilin University during August 2012 to December 2014. Healthy individuals were collected from The First Hospital of Jilin University during the same period. Patients with PTC and NG were diagnosed according to the revised American Thyroid Association management guidelines for patients with thyroid nodules and differentiated thyroid cancers [20]. Age- and gender-matched control subjects were from the general population, who were free from thyroid diseases, diabetes, and other endocrine system diseases. All patients and control subjects came from the Han population of Northern China. This study was reviewed and approved by the Medical Ethics Committee of the School of Public Health, Jilin University; and written informed consent was obtained from all participants.

2.2. DNA Extraction and SNP Genotyping

SNP data located in the MET gene were downloaded from the HapMap database. Two tag SNPs of the gene, rs1621 and rs6566, were obtained through Haploview 4.2 (population: CHB, square cutoff: 0.8, MAF cutoff: 0.1, and ). Blood samples were collected from all participants, and the whole DNA genome was isolated using a DNA extraction kit (Beijing Kangwei Century Biotech Co., Ltd., China) according to standard protocols. The concentration and purity of the DNA samples were determined by a UV-260 spectrophotometer (Shimadzu, Kyoto, Japan). SNP genotyping was carried out using the Sequenom MassARRAY platform (San Diego, CA, USA). Primer sequences used were as follows: rs1621-F: ACGTTGGATGACCCTGAGCAGAACTTTGTG, rs1621-R: ACGTTGGATGGTAACCTACACCACATGCAC, and rs6566-F: ACGTTGGATGCTGGCAATAACCACTATCAG; rs6566-R: ACGTTGGATGACTGAATGGTACTTCGTATG. Polymerase chain reaction (PCR) analysis was performed with Taq DNA polymerase (Tiangen Biotech Co., Ltd., Beijing, China). PCR condition was 94°C for 15 minutes to perform a hot-start, followed by denaturing at 94°C for 20 seconds, annealing at 56°C for 30 seconds, extension at 72°C for one minute for 45 cycles, and incubation at 72°C for three minutes. Then, shrimp alkaline phosphatase (SAP) reaction was performed by incubating the PCR product with SAP (Sequenom, Inc., San Diego, CA, USA) at 37°C for 40 minutes, followed by inactivation at 85°C for five minutes. The iPLEX extension reaction was performed at 94°C for 30 seconds and for five seconds, followed by 40 cycles at 52°C for five seconds and five cycles at 80°C for five seconds and at 72°C for three minutes. The product was desalted by the addition of resin in a 384-dimple plate, mixed, resuspended, and centrifuged to separate the extension products from the resin. The completed products were analyzed using the MassARRAY Typer software version 4.0 (Sequenom, USA).

2.3. Statistical Analysis

All statistical analyses were conducted using the “genetics” and “dgc.genetics” packages running in the R software environment (version 3.0.2). Hardy-Weinberg equilibrium was examined in control samples by Pearson’s -test. The interaction between the MET gene and gender was examined by logistic regression using an additive model. All statistical tests were two-sided. A value < 0.05 was considered statistically significant.

3. Results

There was no apparent difference with respect to gender among the three groups (). The age of patients in the NG group was comparatively older than the age of patients in the PTC and NC groups (), while there was no significant difference between the PTC and NC groups (). The SNP distribution satisfied the Hardy-Weinberg equilibrium (). The allele and genotype frequencies of MET in the PTC, NG, and NC groups are listed in Table 1. MET SNPs (rs1621 and rs6566) were differentially distributed among the groups.

Interactions between MET SNPs (rs1621 and rs6566) and PTC were analyzed by stratifying the patients according to gender. The MET SNP rs1621 genotype was significantly associated with PTC in female subjects (), but not in their male counterparts (), as shown in Table 2. There was no significant association between rs6566 and PTC in both male and female genders (). Therefore, the genotype frequency of rs1621 was analyzed in the female and male part of the PTC, NG, and NC groups, respectively. Multiple comparison adjustments were made using Bonferroni correction (). Among female patients, the rs1621 AG genotype was significantly higher in patients with PTC when compared with normal controls, while the frequency of genotypes AA and GG was comparatively lower (). There was no significant difference in rs1621 genotype distribution between the NG and NC groups or between the PTC and NG groups ( and , resp.). By contrast, no significant difference was found among male patients.

Differences in rs1621 genotype frequencies in female patients in each group were assessed by an additive model of logistic regression using the major allele as a reference. AG genotype revealed an increasing risk of PTC in female patients (OR: 1.465, 95% CI: 1.118–1.92), as shown in Table 3, indicating an interaction between gender and MET SNP (rs1621) in PTC patients. Further analysis of the association between the genotype frequencies of MET SNP (rs1621) and PTC metastasis or prognosis of patients did not reveal a significant correlation in both male and female patients (Table 4).

4. Discussion

MET polymorphisms are associated with cancer risk [21, 22]. The presence of adenosine (A) at SNP rs1621 has been reported to increase the risk of cancer development [23]. SNP rs1621 in the seed-matching sequence of MET has been suggested to affect the activity of miR-199a, which mediates the downregulation of the MET gene through targeting the 3′-UTR [24]. Furthermore, SNP rs1621 was also selected to investigate its effect on breast cancer risk [25]. SNP rs6566, located in MET, has also been investigated for gastric cancer risk [26]. In this study, these two tag SNPs (rs1621 and rs6566) were obtained by Haploview 4.2 (population: CHB, square cutoff: 0.8, MAF cutoff: 0.1, and ). Our study revealed a significant gender difference in the genotype frequencies of MET SNP rs1621 among patients with PTC and NG and NC. Multiple comparisons between groups further confirmed the statistical difference in rs1621 genotype frequency for female patients in the PTC and NC groups. Genotype AG in rs1621 increased the risk of PTC in female patients, while there was no significant difference between groups for male patients. A higher AG genotype frequency was the significant risk factor for PTC in female patients, compared with male patients. However, no significant association was found between rs1621 genotype frequency and the metastasis and prognosis of PTC in both male and female patients.

Disparity between genders in incidence, disease aggressiveness, therapy responsiveness, and prognosis has long been observed in a variety of gender-nonspecific cancers [27]. As the most common cancer of the endocrine system, thyroid cancer has a well-established gender disparity in incidence, aggressiveness, and prognosis. It has been reported to be the seventh most common malignancy in women, but not among the most common 15 cancers in men [28]. Several hypotheses have been proposed for gender differences in thyroid cancer initiation and progression. Reproductive, menstrual, and environmental factors have been hypothesized to account for this gender-specific disparity [12]. However, other studies have also indicated that dietary, environmental, and reproductive factors, as well as frequent activating somatic mutations, do not appear to contribute to this difference in PTC, the most common type of thyroid cancer [29]. Female gender hormones have been suggested to play a crucial role in the carcinogenesis of cancers [30], while hormonal exposure did not affect the innate characteristics of the tumor [31]. Gender disparity in cancers has been postulated to be due to yet unidentified molecular factors. The identification of these factors may help us better understand the biological behavior of cancers, which is essential in the development of effective strategies for cancer diagnosis and treatment.

Characterized by early metastasis and multifocal involvement, PTC exhibits a highly invasive behavior [32]. The MET protooncogene encodes a tyrosine kinase receptor that is known to influence cell invasion. It has been demonstrated to be significantly overexpressed in PTC [33] and has been suggested to play an important role in PTC invasion and metastasis [34]. MET mutations have been reported in various types of cancers and were found to be correlated with tumor metastasis [17–19, 35]. However, other studies also revealed low mutation frequencies [36] or the mutation was not correlated with the progression of the disease [37]. In thyroid cancer, MET alteration has been reported to be relatively frequent in differentiated types [38]. MET SNPs rs1621 and rs6566 were reported in our study, and stratification analysis according to gender revealed an increased risk of PTC in female patients with the AG genotype in rs1621, which has been previously reported to correlate with chronic rhinosinusitis [39]. By contrast, there was no significant association in male patients. MET single-nucleotide variants have been identified in PTC with or without distant metastases and were suggested to correlate with the aggressive behavior of the disease [40]. Therefore, the association between rs1621 and tumor metastasis, as well as the prognosis of patients, was analyzed in PTC patients. Our results revealed that there was no obvious correlation between rs1621 and the metastasis and prognosis of PTC patients in either the male or female gender. Further studies are needed to uncover the gender disparity of MET SNP rs1621 in PTC patients.

In conclusion, our study revealed that the polymorphism of SNP locus rs1621 in the MET gene may be associated with gender disparity in PTC. Higher AG genotypes in rs1621 were correlated with PTC in female patients, but not in their male counterparts. However, SNP rs1621 was not correlated with metastasis and prognosis in both male and female PTC patients. Further studies are needed to better clarify the association between MET polymorphism and gender disparity in PTC and its potential mechanisms.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This work was supported by the Young Scholars Program of Norman Bethune Health Science Center of Jilin University (no. 2013202018).

References

C. L. Meinhold, E. Ron, S. J. Schonfeld et al., “Nonradiation risk factors for thyroid cancer in the US radiologic technologists study,” American Journal of Epidemiology, vol. 171, no. 2, pp. 242–252, 2010.
View at: Publisher Site | Google Scholar
T. Kondo, S. Ezzat, and S. L. Asa, “Pathogenetic mechanisms in thyroid follicular-cell neoplasia,” Nature Reviews Cancer, vol. 6, no. 4, pp. 292–306, 2006.
View at: Publisher Site | Google Scholar
R. A. DeLellis, “Pathology and genetics of thyroid carcinoma,” Journal of Surgical Oncology, vol. 94, no. 8, pp. 662–669, 2006.
View at: Publisher Site | Google Scholar
K. Pacak, D. C. Sweeney, L. Wartofsky et al., “Solitary cerebellar metastasis from papillary thyroid carcinoma: a case report,” Thyroid, vol. 8, no. 4, pp. 327–335, 1998.
View at: Publisher Site | Google Scholar
C. Leux, T. Truong, C. Petit, D. Baron-Dubourdieu, and P. Gúenel, “Family history of malignant and benign thyroid diseases and risk of thyroid cancer: a population-based case—control study in New Caledonia,” Cancer Causes and Control, vol. 23, no. 5, pp. 745–755, 2012.
View at: Publisher Site | Google Scholar
L. Dal Maso, C. Bosetti, C. La Vecchia, and S. Franceschi, “Risk factors for thyroid cancer: an epidemiological review focused on nutritional factors,” Cancer Causes and Control, vol. 20, no. 1, pp. 75–86, 2009.
View at: Publisher Site | Google Scholar
Y. Lun, X. Wu, Q. Xia et al., “Hashimoto's thyroiditis as a risk factor of papillary thyroid cancer may improve cancer prognosis,” Otolaryngology&Head and Neck Surgery, vol. 148, no. 3, pp. 396–402, 2013.
View at: Publisher Site | Google Scholar
T. Rago, E. Fiore, M. Scutari et al., “Male sex, single nodularity, and young age are associated with the risk of finding a papillary thyroid cancer on fine-needle aspiration cytology in a large series of patients with nodular thyroid disease,” European Journal of Endocrinology, vol. 162, no. 4, pp. 763–770, 2010.
View at: Publisher Site | Google Scholar
Q. Zhang, F. Song, H. Zheng et al., “Association between single-nucleotide polymorphisms of BRAF and papillary thyroid carcinoma in a Chinese population,” Thyroid, vol. 23, no. 1, pp. 38–44, 2013.
View at: Publisher Site | Google Scholar
F. Damiola, G. Byrnes, M. Moissonnier et al., “Contribution of ATM and FOXE1 (TTF2) to risk of papillary thyroid carcinoma in Belarusian children exposed to radiation,” International Journal of Cancer, vol. 134, no. 7, pp. 1659–1668, 2014.
View at: Publisher Site | Google Scholar
G. G. Chen, Q. Zeng, and G. M. K. Tse, “Estrogen and its receptors in cancer,” Medicinal Research Reviews, vol. 28, no. 6, pp. 954–974, 2008.
View at: Publisher Site | Google Scholar
R. Rahbari, L. Zhang, and E. Kebebew, “Thyroid cancer gender disparity,” Future Oncology, vol. 6, no. 11, pp. 1771–1779, 2010.
View at: Publisher Site | Google Scholar
K. H. Jung, B. H. Park, and S.-S. Hong, “Progress in cancer therapy targeting c-Met signaling pathway,” Archives of Pharmacal Research, vol. 35, no. 4, pp. 595–604, 2012.
View at: Publisher Site | Google Scholar
L. Trusolino and P. M. Comoglio, “Scatter-factor and semaphorin receptors: cell signalling for invasive growth,” Nature Reviews Cancer, vol. 2, no. 4, pp. 289–300, 2002.
View at: Publisher Site | Google Scholar
P. C. Ma, G. Maulik, J. Christensen, and R. Salgia, “c-Met: structure, functions and potential for therapeutic inhibition,” Cancer and Metastasis Reviews, vol. 22, no. 4, pp. 309–325, 2003.
View at: Publisher Site | Google Scholar
L. Schmidt, F.-M. Duh, F. Chen et al., “Germline and somatic mutations in the tyrosine kinase domain of the MET proto-oncogene in papillary renal carcinomas,” Nature Genetics, vol. 16, no. 1, pp. 68–73, 1997.
View at: Publisher Site | Google Scholar
M. Jeffers, M. Fiscella, C. P. Webb, M. Anver, S. Koochekpour, and G. F. Vande Woude, “The mutationally activated Met receptor mediates motility and metastasis,” Proceedings of the National Academy of Sciences of the United States of America, vol. 95, no. 24, pp. 14417–14422, 1998.
View at: Publisher Site | Google Scholar
A. Lorenzato, M. Olivero, S. Patané et al., “Novel somatic mutations of the MET oncogene in human carcinoma metastases activating cell motility and invasion,” Cancer Research, vol. 62, no. 23, pp. 7025–7030, 2002.
View at: Google Scholar
M. F. Di Renzo, M. Olivero, T. Martone et al., “Somatic mutations of the MET oncogene are selected during metastatic spread of human HNSC carcinomas,” Oncogene, vol. 19, no. 12, pp. 1547–1555, 2000.
View at: Publisher Site | Google Scholar
D. S. Cooper, G. M. Doherty, B. R. Haugen et al., “Revised American thyroid association management guidelines for patients with thyroid nodules and differentiated thyroid cancer: the American Thyroid Association (ATA) guidelines taskforce on thyroid nodules and differentiated thyroid cancer,” Thyroid, vol. 19, no. 11, pp. 1167–1214, 2009.
View at: Publisher Site | Google Scholar
Y. Sunakawa, T. Wakatsuki, D. Yang et al., “Prognostic impact of the c-MET polymorphism on the clinical outcome in locoregional gastric cancer patients,” Pharmacogenetics and Genomics, vol. 24, no. 12, pp. 588–596, 2014.
View at: Publisher Site | Google Scholar
F. A. Schutz, M. M. Pomerantz, K. P. Gray et al., “Prospective analysis of genetic polymorphisms and risk of recurrence in renal cell cancer,” The Lancet Oncology, vol. 14, no. 1, pp. 81–87, 2013.
View at: Google Scholar
A. Levy and E. Freidman, “Methods for detecting an increased susceptibility to cancer,” Google Patents, 2009.
View at: Google Scholar
S. Duan, S. Mi, W. Zhang, and M. E. Dolan, “Comprehensive analysis of the impact of SNPs and CNVs on human microRNAs and their regulatory genes,” RNA Biology, vol. 6, no. 4, pp. 412–425, 2009.
View at: Google Scholar
S. Tchatchou, A. Jung, K. Hemminki et al., “A variant affecting a putative miRNA target site in estrogen receptor (ESR) 1 is associated with breast cancer risk in premenopausal women,” Carcinogenesis, vol. 30, no. 1, pp. 59–64, 2009.
View at: Publisher Site | Google Scholar
J. J. Yang, L. Y. Cho, K.-P. Ko et al., “Genetic susceptibility on caga-interacting molecules and gene-environment interaction with phytoestrogens: a putative risk factor for gastric cancer,” PLoS ONE, vol. 7, no. 2, Article ID e31020, 2012.
View at: Publisher Site | Google Scholar
N. U. Din, O. C. Ukoumunne, G. Rubin et al., “Age and gender variations in cancer diagnostic intervals in 15 cancers: analysis of data from the UK Clinical Practice Research Datalink,” PLoS ONE, vol. 10, no. 5, Article ID e0127717, 2015.
View at: Publisher Site | Google Scholar
J. Ortega, C. Sala, B. Flor, and S. Lledo, “Efficacy and cost-effectiveness of the UltraCision harmonic scalpel in thyroid surgery: an analysis of 200 cases in a randomized trial,” Journal of Laparoendoscopic & Advanced Surgical Techniques Part A, vol. 14, no. 1, pp. 9–12, 2004.
View at: Publisher Site | Google Scholar
R. Rahbari, L. Zhang, and E. Kebebew, “Is there a molecular basis for cancer gender disparity?” Journal of Surgical Research, vol. 165, no. 2, pp. 220–221, 2011.
View at: Google Scholar
Q. Zeng, G. G. Chen, A. C. Vlantis, and C. A. van Hasselt, “Estrogen and apoptosis in thyroid cancer,” in Oncogene Proteins: New Research, pp. 289–309, Nova Science, 2008.
View at: Google Scholar
J. Nitzkorski, F. Zhu, C. Loveland-Jones et al., “Breast cancer histology and the influence of the hormonal milieu,” Journal of Surgical Research, vol. 165, no. 2, p. 220, 2011.
View at: Publisher Site | Google Scholar
M. J. Schlumberger, “Papillary and follicular thyroid carcinoma,” The New England Journal of Medicine, vol. 338, no. 5, pp. 297–306, 1998.
View at: Publisher Site | Google Scholar
C. A. T. D. S. Mitteldorf, J. M. De Sousa-Canavez, K. R. M. Leite, C. Massumoto, and L. H. Camara-Lopes, “FN1, GALE, MET, and QPCT overexpression in papillary thyroid carcinoma: molecular analysis using frozen tissue and routine fine-needle aspiration biopsy samples,” Diagnostic Cytopathology, vol. 39, no. 8, pp. 556–561, 2011.
View at: Publisher Site | Google Scholar
H. C. Nardone, A. F. Ziober, V. A. LiVolsi et al., “C-Met expression in tall cell variant papillary carcinoma of the thyroid,” Cancer, vol. 98, no. 7, pp. 1386–1393, 2003.
View at: Publisher Site | Google Scholar
E. H. Lim, S.-L. Zhang, J.-L. Li et al., “Using whole genome amplification (WGA) of low-volume biopsies to assess the prognostic role of EGFR, KRAS, p53, and CMET mutations in advanced-stage non-small cell lung cancer (NSCLC),” Journal of Thoracic Oncology, vol. 4, no. 1, pp. 12–21, 2009.
View at: Publisher Site | Google Scholar
F. Schmid, S. Burock, K. Klockmeier, P. M. Schlag, and U. Stein, “SNPs in the coding region of the metastasis-inducing gene MACC1 and clinical outcome in colorectal cancer,” Molecular Cancer, vol. 11, article 49, 2012.
View at: Publisher Site | Google Scholar
M. Gumustekin, A. Kargi, G. Bulut et al., “HGF/c-Met overexpressions, but not met mutation, correlates with progression of non-small cell lung cancer,” Pathology and Oncology Research, vol. 18, no. 2, pp. 209–218, 2012.
View at: Publisher Site | Google Scholar
V.-M. Wasenius, S. Hemmer, M.-L. Karjalainen-Lindsberg, N. N. Nupponen, K. Franssila, and H. Joensuu, “MET receptor tyrosine kinase sequence alterations in differentiated thyroid carcinoma,” The American Journal of Surgical Pathology, vol. 29, no. 4, pp. 544–549, 2005.
View at: Publisher Site | Google Scholar
R. Castano, Y. Bossé, L. M. Endam, A. Filali-Mouhim, and M. Desrosiers, “C-MET pathway involvement in chronic rhinosinusitis: a genetic association analysis,” Otolaryngology—Head and Neck Surgery, vol. 142, no. 5, pp. 665–671, 2010.
View at: Publisher Site | Google Scholar
G. Gandolfi, D. De Biase, V. Sancisi et al., “Deep sequencing of KIT, MET, PIK3CA, and PTEN hotspots in papillary thyroid carcinomas with distant metastases,” Endocrine-Related Cancer, vol. 21, no. 5, pp. L23–L26, 2014.
View at: Publisher Site | Google Scholar

Copyright

Copyright © 2015 Lifeng Ning et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

PDF Download Citation

Download other formats

Order printed copies

Views

1541

Downloads

954

Citations