MicroRNA-203 inhibits cell proliferation by repressing ΔNp63 expression in human esophageal squamous cell carcinoma
- Yang Yuan†1,
- Zhi-Yong Zeng†2,
- Xiao-Hong Liu†1,
- De-Jun Gong1,
- Jing Tao3,
- He-Zhong Cheng3Email author and
- Sheng-Dong Huang1Email author
© Yuan et al; licensee BioMed Central Ltd. 2011
Received: 1 November 2010
Accepted: 7 February 2011
Published: 7 February 2011
This study was performed to investigate the effect of microRNA-203 (miR-203) and ΔNp63 on cell proliferation and the functional connection between miR-203 and ΔNp63 in ESCC.
We employed 2 human ESCC cell lines, Eca109 and TE-1, as the model system. The effect of miR-203 and ΔNp63 on cell proliferation was determined in cells transfected with miR-203 mimic and ΔNp63 small interfering RNA (siRNA), respectively. The regulation of ΔNp63 expression in ESCC cells by miR-203 was studied by luciferase reporter assay, RT-PCR and western blot analysis in cells transfected with miR-203. The effect of ΔNp63 re-expression on miR-203 induced inhibition of cell proliferation was studied by cell proliferation assay in cells cotransfected with miR-203 and pcDNA-ΔNp63 plasmid (without the 3'-UTR of ΔNp63).
We found that both miR-203 and ΔNp63 siRNA signicantly inhibited cell proliferation in ESCC. MiR-203 could down-regulate endogenous ΔNp63 expression at the posttranscriptional level. Moreover, re-expression of ΔNp63 in cells transfected with miR-203 significantly attenuated the miR-203 induced inhibition of cell proliferation.
Our data implied that miR-203 could inhibit cell proliferation in human ESCC through ΔNp63-mediated signal pathway. Therefore, we propose that miR-203 might be used as a therapeutic agent for human ESCC.
Esophageal cancer is one of the most lethal human cancers mainly located in China, Japan, and southeast Africa . According to the etiologic and pathologic characteristics, it could be divided into two main forms, esophageal squamous cell carcinoma (ESCC) and esophageal adenocarcinoma (EAC). Previous reports showed that the incidence of ESCC is much higher than that of EAC in the above mentioned areas [1, 2], and the 5-year survival rate of ESCC after surgery ranges from 14% - 22% [3, 4]. Much effort has been spent on the study of the biological behavior of ESCC cells to develop effective treatment strategies. Although some oncogenes and tumor suppressor genes were reported to be associated with the development of ESCC [5–7], few specific molecules regulating the initiation and progression of ESCC have been identified. Conceivably, elucidation of the molecular pathways involved in the cell proliferation of ESCC will provide important clues for the development and evaluation of novel anticancer therapies.
MicroRNAs, a class of small non-coding RNAs, have been identified as a new kind of gene expression regulators through targeting the 3'-untranslated region (UTR) of mRNAs for translational repression, degradation or both [8–10]. In the recent years, mounting data suggest that microRNAs are involved in essential tumor cell biological processes, such as proliferation, invasion, and apoptosis [9, 11, 12].
It was reported that MicroRNA-203 (miR-203) located in a region at chromosome 14 , which contains a high density of microRNAs (including about 12% of the known human microRNA gene), exhibited significantly down-regulated expression in some tumors such as head and neck squamous cell carcinomas , hematopoietic malignancy  and colon cancer . Subsequent studies showed that the expression level of miR-203 is inversely correlated with the capacity of cell proliferation in human head and neck squamous cell carcinoma , hepatocellular carcinoma , chronic myelogenous leukemia and B cell leukemia . In addition, ΔNp63, an important oncogene regulating cell proliferation in some tumors , was recently identified as a target gene of miR-203 in human epithelial precursor cells, as well as human head and neck squamous cell carcinoma cells [14, 18]. These findings suggested that there might be a functional connection between miR-203 and ΔNp63 in cell proliferation regulation.
In human ESCC, genome-wide microRNA expression profile assay showed that the expression level of miR-203 was significantly down-regulated in tumor tissue compared with the matched normal tissue [19–21]. In contrast, the expression level of ΔNp63 was significantly higher in tumor tissue than in the matched normal tissue [22, 23]. However, the effect of miR-203 and ΔNp63 on the proliferation of ESCC cells, as well as the functional relationship between miR-203 and ΔNp63 in ESCC cells has not been documented. Since certain microRNAs may show different functions in different type of tumors , the role of miR-203 in the regulation of cell proliferation of ESCC warrants investigation.
In light of the previous reports, we hypothesized that miR-203 might regulate the proliferation of ESCC cells through the ΔNp63-mediated signal pathway. Therefore, using 2 human ESCC cell lines (Eca109 and TE-1) as a model system, here we set out to investigate the effect of miR-203 and ΔNp63 on the proliferation of ESCC cell, as well as the regulation of the expression of ΔNp63 by miR-203 in ESCC. Our results suggest that miR-203 may inhibit the cell proliferation in ESCC through the ΔNp63-mediated signal pathway.
Human ESCC cell lines Eca109 and TE-1 were purchased from the Shanghai Institute of Biochemistry and Cell Biology (Shanghai, China). Cells were maintained in RPMI1640 (Invitrogen) supplemented with 10% fetal bovine serum (Invitrogen), 100 U/ml penicillin and 100 μg/ml streptomycin, within a humidified atmosphere containing 5% CO2 at 37°C.
1 × 106 cells cultured in a well of 6-well cell culture plate were transiently transfected with 50 pmol of miR-203 double-stranded mimics (or control microRNA) and ΔNp63 siRNA oligonucleotide duplexes (or control siRNA) using Lipofectamine 2000 (Invitrogen) according to the manufacturer's protocol, respectively. Transfection efficiency was optimized using 6-carboxyfluorescein-labeled microRNA (or siRNA) at approximately 80% in Eca109 and TE-1 cells.
The sequences of miR-203 were:
A scrambled microRNA with no homology to any known human microRNA was used as negative control:
The siRNA oligonucleotides targeting ΔNp63 were designed as previously described :
Anti-sense: 5'- UACAUACUGGGCAUGGCUGUU-3'.
A scrambled siRNA with no homology to any known human mRNA was used as negative control:
All microRNA mimics or siRNA oligonucleotide duplexes were synthesized by Genephama Biotech (Shanghai, China).
Quantitative Reverse Transcriptase Polymerase Chain Reaction (qRT-PCR)
Total RNA was extracted from 1 × 105 cells using the RNeasy RNA Mini Kit (Qiagen). First strand cDNA was synthesized using powerscipt reverse transcriptase (Clontech). The following gene-specific primer pairs were used for quantitative PCR:
ΔNp63: Forward, 5'-GGGTGAGCGTGTTATTGATGCT-3';
GAPDH: Forward, 5'-GCTGAGTATGTCGTGGAGTC-3';
PCR was performed using a Fast Start Master SYBR Green Kit (Roche) on a LightCycler (Roche). The expression level of ΔNp63 mRNA was analyzed using RealQuant software (Roche) and normalized to that of GAPDH mRNA.
Cellular proteins were prepared using cell lysis buffer (50 mM Tris-HCl, pH 8.0, 1% NP-40, 2 mM EDTA, 10 mM NaCl, 2 mg/ml aprotinin, 5 mg/ml leupeptin, 2 mg/ml pepstatin, 1 mM DTT, 0.1% SDS and 1 mM phenylmethylsulfonyl fluoride). Equal amounts of protein (50 μg) were separated by 10% SDS PAGE and then transferred to nitrocellulose membranes (NY, USA) by electroblotting. The membranes were blocked with 5% BSA in TBST (10 mM Tris-HCl, pH 8.0, 150 mM NaCl, and 0.05% Tween 20) for 1 hr, and then incubated with mouse anti-human ΔNp63 antibody (Santa Cruz) overnight at 4°C before subsequent incubation with horseradish peroxidase-conjugated goat anti-mouse antibody (BD) for 1 hr at 37°C. Protein was visualized using enhanced chemiluminescence reagent (Santa Cruz). The expression level of ΔNp63 protein was analyzed using LabWork 4.0 program (UVP) and normalized to that of β-actin protein.
Single-cell suspension was prepared using trypsin treatment. Cells were then seeded into 6-well cell culture plates (200 cells/well) and incubated for 2 weeks at 37°C. Then, cells were washed twice with PBS and stained with a mixture of 6.0% glutaraldehyde and 0.5% crystal violet for 1 hour at 37°C. The plates were air-dried at room temperature. Colony forming efficiency was calculated as the percentage of plated cells that formed colonies.
Cell population doubling time
Cells were plated into 6-well plates (1 × 104 cells/well) and cultured at 37°C. Cell population doubling time (PDT) was calculated using the following equation: PDT (hr) = (log2 × t)/(logNt - logN0), where t = time in culture (hr), Nt = final cell count, N0 = original cell count.
Cell cycle assay
Cells were fixed in 70% ethanol for 2 hr at 4°C. After washing with PBS, cells were treated with RNaseA (50 μg/ml) and stained with propidium iodide (25 μg/ml) for 30 min at 37°C. Samples were analyzed using an FACSCalibur flow cytometer (BD Biosciences) and distribution of cell-cycle phases was determined using Modfit Software (BD Biosciences). The proliferative index was calculated as the percentage of cells in S/G2/M-phase.
Cells were stained with annexin V-FITC and propidium iodide using the ANNEXIN V-FITC Kit (Beckman) according to the manufacturer's protocol and subjected to flow cytometric analysis. Viable cells were unstained by annexin V or propidium iodide, early apoptotic cells were stained by annexin V but not propidium iodide, and late apoptotic cells were stained by annexin V and propidium iodide. The apoptotic index was calculated as the percentage of annexin V+/propidium iodide- cells.
Luciferase reporter assay
The full-length 3'-UTR of ΔNp63 mRNA containing the miR-203 binding site was amplified by PCR (Forward: 5'-ggggagctcatataagaactcttgcagtct-3'; Reverse: 5'-gggaagcttggtgtacattcttctagaac-3'). Mutant ΔNp63 3'-UTR, which carried a substitution of four nucleotides (CTTT to GAAA) within the core binding sites of ΔNp63 3'-UTR (14), was obtained using overlapping extension PCR. Normal (or mutant) ΔNp63 3'-UTR was cloned into the SacI-HindIII site of the pMIR-REPORT luciferase vector (Biosystems) and named as Luc-ΔNp63 (or Luc-ΔNp63-mut). Then, 1 × 106 cells were cotransfected with 50 pmol of microRNA-203 (or control microRNA), 1 μg of Luc-ΔNp63 (or Luc-ΔNp63-mut) plasmid, and 1 μg of pMIR-REPORT β-Gal vector using Lipofectamine 2000. The Luciferase activity was examined at 48 hr posttransfection using the luciferase assay kit (Clontech) and normalized to β-galactosidase activity.
Data are presented as mean ± SEM. Statistical significance was tested using SPSS11.0 software, using t tests for 2-group comparisons. A P value less than 0.05 is considered statistically significant.
MiR-203 could inhibit the proliferation of ESCC cell lines
Repressing ΔNp63 expression could inhibit the proliferation of ESCC cell
MiR-203 posttranscriptionally down-regulates ΔNp63 expression by targeting the 3' untranslated region of ΔNp63
We further detected the expression of ΔNp63 protein and mRNA by western blot and qRT-PCR in Eca109 and TE-1 cells transfected with miR-203 (or control microRNA). As shown in Figure 3C and 3D, the expression of ΔNp63 protein was decreased by approximately 80% in cells transfected with miR-203 as compared to the cells treated with control microRNA at 48 hr posttransfection (0.08 ± 0.02 vs. 0.62 ± 0.10 for Eca109, P < 0.05; 0.13 ± 0.03 vs. 0.80 ± 0.11 for TE-1, P < 0.05). However, the expression of ΔNp63 mRNA showed no significant difference between the 2 groups (Figure 3E). These results indicate that the 3'-UTR of ΔNp63 mRNA is a functional target of miR-203 in ESCC cells.
The inhibition of the proliferation of ESCC cells by miR-203 could be significantly attenuated by the re-expressing of ΔNp63
It was known that microRNAs could regulate a variety of cellular pathways by affecting the expression of multiple types of target genes and the alteration of microRNAs expression might contribute to human carcinogenesis [9, 11, 12]. Thus, an understanding of the specific microRNAs involved in the process of tumor development would provide valuable insight for the diagnosis and treatment of patients with tumor. Here, we have demonstrated that miR-203 could down-regulate the proliferation of ESCC cells, probably through the ΔNp63-mediated signal pathway. Our data suggest that re-expressing miR-203 might benefit the treatment of ESCC.
It was reported that the expression of miR-203 was significantly down-regulated in some tumors, including head and neck squamous cell carcinomas , hematopoietic malignancy  and colon cancer . Moreover, it was reported that inhibition of miR-203 expression could significantly increase the proliferation of Hela cells , whilst re-expression of miR-203 could inhibit the proliferative capacity of cells in human head and neck squamous cell carcinoma , hepatocellular carcinoma , chronic myelogenous leukemia and B cell leukemia . These findings suggest that miR-203 might function as a tumor suppressor gene in a variety of tumors. In the case of esophageal cancer, genome-wide microRNA expression profile analysis revealed that the expression level of miR-203 was 2- to 10-fold lower in tumor than in the matched normal tissues [19–21]. However, the effect of miR-203 on the cell proliferation in human ESCC has not been reported. In the present study, we found that the proliferative capacity of ESCC cells transfected with miR-203 was significantly lower than that of cells transfected with control microRNA, suggesting that miR-203 could inhibit the proliferative capacity of ESCC cells.
Previous studies indicated that ΔNp63, an alternative splice variant of p63 gene lacking TA domain , is the major isotype expressed in a variety of human squamous cell carcinoma including ESCC , and that the expression level of ΔNp63 in tumor tissues was significantly higher than in the matched normal tissues [22, 27]. In our pilot study, we found ΔNp63 was highly expressed, whilst TAp63 was hardly detectable in Eca109 and TE-1 (Additional file 1, Figure S2). Here, we demonstrated that repressing ΔNp63 expression by siRNA could significantly inhibit the proliferation of ESCC cell lines, implying that ΔNp63 played a positive role in ESCC cell proliferation. Our findings, in combination with the previous reports that ΔNp63 could promote the cell proliferation in head and neck squamous cell carcinoma  as well as lung squamous cell carcinoma , suggest that ΔNp63 may function as an oncogene in human squamous cell carcinoma.
Recently, using bioinformatic analysis, Lena et al.  and Yi et al.  independently reported that the 3'-UTR of ΔNp63 contain the miR-203 binding site. Subsequent studies showed that miR-203 could repress the expression of ΔNp63 and inhibit cell proliferation in human epithelial precursor cells as well as human head and neck squamous cell carcinoma cells, suggesting that miR-203 is a key molecule controlling the ΔNp63-mediated cell proliferation in some normal and tumor cells [14, 18]. However, whether miR-203 regulates the expression of ΔNp63 in ESCC has not been identified before. Here, we showed that miR-203 could significantly inhibit ΔNp63 protein expression without changing the expression level of ΔNp63 mRNA, suggesting that miR203 negatively regulated the expression of ΔNp63 at the posttranscriptional level in ESCC. Moreover, we demonstrated that re-expressing ΔNp63 in miR-203 transfected ESCC cells could significantly attenuate miR-203 induced inhibition of cell proliferation. Taken together, our results suggest that miR-203 may function as a tumor suppressor by regulating ΔNp63-mediated signal pathways in human ESCC.
However, we noticed that the proliferative capacity of the ESCC cells cotransfected with miR-203 and pcDNA-ΔNp63 plasmid, though much higher than that of the cells cotransfected with miR-203 and empty pcDNA plasmid, is still significantly lower than that of the cells cotransfected with control microRNA and empty pcDNA plasmid (Additional file 1, Figure S3). This result, combined with the fact that the expression level of ΔNp63 protein in the cells cotransfected with miR-203 and pcDNA-ΔNp63 was significantly higher than that in cells cotransfected with control microRNA and empty pcDNA plasmid (Additional file 1, Figure S4), suggest that miR-203 might regulate the proliferation of ESCC cells through multiple target genes. In this respect, the underlying mechanisms of miR-203 in regulating the proliferation of ESCC cell warrant further investigation.
In summary, we have demonstrated that miR-203 and ΔNp63 play an important role in ESCC cell proliferation regulation. MiR-203 can significantly inhibit the proliferation of ESCC cell through the ΔNp63-mediated signal pathway. Based on these findings, we propose that miR-203 might be used as a therapeutic agent for ESCC.
This work was supported by the National Natural Science Foundation of China (grant 30872552) and the Shanghai Municipal Natural Science Foundation (grant 8140902201 and 10140902300).
- Enzinger PC, Mayer RJ: Esophageal cancer. N Engl J Med. 2003, 349: 2241-2252. 10.1056/NEJMra035010.View ArticlePubMedGoogle Scholar
- Li JY: Epidemiology of esophageal cancer in China. Natl Cancer Inst Monogr. 1982, 62: 113-120.PubMedGoogle Scholar
- Gamliel Z, Krasna MJ: Multimodality treatment of esophageal cancer. Surg Clin North Am. 2005, 85: 621-630. 10.1016/j.suc.2005.01.011.View ArticlePubMedGoogle Scholar
- Pfaffl MW: A new mathematical model for relative quantification in real-time RT-PCR. Nucleic Acids Res. 2001, 29: e45-10.1093/nar/29.9.e45.View ArticlePubMedPubMed CentralGoogle Scholar
- Gratas C, Tohma Y, Barnas C, Taniere P, Hainaut P, Ohgaki H: Up-regulation of Fas (APO-1/CD95) ligand and down-regulation of Fas expression in human esophageal cancer. Cancer Res. 1998, 58: 2057-2062.PubMedGoogle Scholar
- Mizushima T, Nakagawa H, Kamberov YG, Wilder EL, Klein PS, Rustgi AK: Wnt-1 but not epidermal growth factor induces beta-catenin/T-cell factor-dependent transcription in esophageal cancer cells. Cancer Res. 2002, 62: 277-282.PubMedGoogle Scholar
- Yang L, Leung AC, Ko JM, Lo PH, Tang JC, Srivastava G, Oshimura M, Stanbridge EJ, Daigo Y, Nakamura Y, Tang CM, Lau KW, Law S, Lung ML: Tumor suppressive role of a 2.4 Mb 9q33-q34 critical region and DEC1 in esophageal squamous cell carcinoma. Oncogene. 2005, 24: 697-705. 10.1038/sj.onc.1208179.View ArticlePubMedGoogle Scholar
- Ambros V: microRNAs: tiny regulators with great potential. Cell. 2001, 107: 823-826. 10.1016/S0092-8674(01)00616-X.View ArticlePubMedGoogle Scholar
- Esquela-Kerscher A, Slack FJ: Oncomirs - microRNAs with a role in cancer. Nat Rev Cancer. 2006, 6: 259-269. 10.1038/nrc1840.View ArticlePubMedGoogle Scholar
- Hobert O: Gene regulation by transcription factors and microRNAs. Science. 2008, 319: 1785-1786. 10.1126/science.1151651.View ArticlePubMedGoogle Scholar
- Chen CZ, Li L, Lodish HF, Bartel DP: MicroRNAs modulate hematopoietic lineage differentiation. Science. 2004, 303: 83-86. 10.1126/science.1091903.View ArticlePubMedGoogle Scholar
- Croce CM, Calin GA: miRNAs, cancer, and stem cell division. Cell. 2005, 122: 6-7. 10.1016/j.cell.2005.06.036.View ArticlePubMedGoogle Scholar
- Bueno MJ, Perez de Castro I, Gomez de Cedron M, Santos J, Calin GA, Cigudosa JC, Croce CM, Fernandez-Piqueras J, Malumbres M: Genetic and epigenetic silencing of microRNA-203 enhances ABL1 and BCR-ABL1 oncogene expression. Cancer Cell. 2008, 13: 496-506. 10.1016/j.ccr.2008.04.018.View ArticlePubMedGoogle Scholar
- Lena AM, Shalom-Feuerstein R, Rivetti di Val Cervo P, Aberdam D, Knight RA, Melino G, Candi E: miR-203 represses 'stemness' by repressing DeltaNp63. Cell Death Differ. 2008, 15: 1187-1195. 10.1038/cdd.2008.69.View ArticlePubMedGoogle Scholar
- Schetter AJ, Leung SY, Sohn JJ, Zanetti KA, Bowman ED, Yanaihara N, Yuen ST, Chan TL, Kwong DL, Au GK, Liu CG, Calin GA, Croce CM, Harris CC: MicroRNA expression profiles mediated with prognosis and therapeutic outcome in colon adenocarcinoma. JAMA. 2008, 299: 425-436. 10.1001/jama.299.4.425.PubMedPubMed CentralGoogle Scholar
- Furuta M, Kozaki KI, Tanaka S, Arii S, Imoto I, Inazawa J: miR-124 and miR-203 are epigenetically silenced tumor-suppressive microRNAs in hepatocellular carcinoma. Carcinogenesis. 2010, 31: 766-776. 10.1093/carcin/bgp250.View ArticlePubMedGoogle Scholar
- Moll UM, Slade N: p63 and p73: roles in development and tumor formation. Mol Cancer Res. 2004, 2: 371-386.PubMedGoogle Scholar
- Yi R, Poy MN, Stoffel M, Fuchs E: A skin microRNA promotes differentiation by repressing 'stemness'. Nature. 2008, 452: 225-229. 10.1038/nature06642.View ArticlePubMedPubMed CentralGoogle Scholar
- Feber A, Xi L, Luketich JD, Pennathur A, Landreneau RJ, Wu M, Swanson SJ, Godfrey TE, Litle VR: MicroRNA expression profiles of esophageal cancer. J Thorac Cardiovasc Surg. 2008, 135: 255-260. 10.1016/j.jtcvs.2007.08.055.View ArticlePubMedGoogle Scholar
- Mathe EA, Nguyen GH, Bowman ED, Zhao Y, Budhu A, Schetter AJ, Braun R, Reimers M, Kumamoto K, Hughes D, Altorki NK, Casson AG, Liu CG, Wang XW, Yanaihara N, Hagiwara N, Dannenberg AJ, Miyashita M, Croce CM, Harris CC: MicroRNA expression in squamous cell carcinoma and adenocarcinoma of the esophagus: associations with survival. Clin Cancer Res. 2009, 15: 6192-6200. 10.1158/1078-0432.CCR-09-1467.View ArticlePubMedPubMed CentralGoogle Scholar
- Kano M, Seki N, Kikkawa N, Fujimura L, Hoshino I, Akutsu Y, Chiyomaru T, Enokida H, Nakagawa M, Matsubara H: miR-145, miR-133a and miR-133b: Tumor suppressive miRNAs target FSCN1 in esophageal squamous cell carcinoma. Int J Cancer. 2010, 127: 2804-2814. 10.1002/ijc.25284.View ArticlePubMedGoogle Scholar
- Hu H, Xia SH, Li AD, Xu X, Cai Y, Han YL, Wei F, Chen BS, Huang XP, Han YS, Zhang JW, Zhang X, Wu M, Wang MR: Elevated expression of p63 protein in human esophageal squamous cell carcinomas. Int J Cancer. 2002, 102: 580-583. 10.1002/ijc.10739.View ArticlePubMedGoogle Scholar
- Koike M, Hibi K, Kasai Y, Ito K, Akiyama S, Nakao A: Molecular detection of circulating esophageal squamous cell cancer cells in the peripheral blood. Clin Cancer Res. 2002, 8: 2879-2882.PubMedGoogle Scholar
- Folini M, Gandellini P, Longoni N, Profumo V, Callari M, Pennati M, Colecchia M, Supino R, Veneroni S, Salvioni R, Valdagni R, Daidone MG, Zaffaroni N: miR-21: an oncomir on strike in prostate cancer. Mol Cancer. 2010, 9: 12-10.1186/1476-4598-9-12.View ArticlePubMedPubMed CentralGoogle Scholar
- Barbieri CE, Perez CA, Johnson KN, Ely KA, Billheimer D, Pietenpol JA: IGFBP-3 is a direct target of transcriptional regulation by DeltaNp63alpha in squamous epithelium. Cancer Res. 2005, 65: 2314-2320. 10.1158/0008-5472.CAN-04-3449.View ArticlePubMedGoogle Scholar
- Li SD, Zhang JR, Wang YQ, Wan XP: The role of microRNAs in ovarian cancer initiation and progression. J Cell Mol Med. 2010, 14: 2240-2249. 10.1111/j.1582-4934.2010.01058.x.View ArticlePubMedPubMed CentralGoogle Scholar
- Rocco JW, Leong CO, Kuperwasser N, DeYoung MP, Ellisen LW: p63 mediates survival in squamous cell carcinoma by suppression of p73-dependent apoptosis. Cancer Cell. 2006, 9: 45-56. 10.1016/j.ccr.2005.12.013.View ArticlePubMedGoogle Scholar
- The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1471-2407/11/57/prepub
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.