lung cancer is a leading cause of cancer mortality in the usa and other developed countries.1 the two major forms of the disease are non - small cell lung cancer and small cell lung cancer , which account for 85% and 15% of all lung cancers , respectively.2 despite advances in preliminary detection and standard treatment with combination chemotherapy and radiotherapy , the prognosis for patients with lung cancer has not improved significantly in the last 20 years,3,4 and survival rates have changed little over the past 2 decades .
currently , an enormous amount of research is aimed at understanding the molecular and cellular biology of lung cancer ; however , much more work is needed to better understand the correlation between gene regulation and lung cancer .
growing evidence shows that aberrant hypermethylation in 5-cpg islands in the promoter regions is a major mechanism for silencing tumor suppressor or other cancer - associated genes in many kinds of human cancer.58 loss of function in cancer suppressor genes may hinder inhibition of growth of cancer cells , which leads to malignant transcription and translation during replication of dna . a number of genes , including the cyclin - dependent kinase inhibitor ( p16 ) , the tumor suppressor gene ras association domain family protein 1a , kelch - like ech - associating protein 1 , the dna repair gene mgmt , and the cystic fibrosis transmembrane conductance regulator are demonstratively methylated in lung cancer.912 cdh1
, a member of the transmembrane glycoprotein family , also known as cadherin-1 , cam 120/80 , epithelial cadherin ( e - cadherin ) , or uvomorulin , is encoded by the cdh1 gene ( 16q22.1).13 cdh1 is a calcium - dependent cell - cell adhesion glycoprotein containing three domains , ie , five extracellular cadherin repeats , a transmembrane region , and a highly conserved cytoplasmic tail.14 
 cdh1 as a tumor suppressor gene plays an essential role in maintaining cell adhesion and adherent junctions in normal tissues .
cdh1 expression is frequently absent in a variety of epithelial tumors , and loss of normal intercellular junctions results in promotion of cancer invasion and metastasis and is correlated with several types of cancers.1517 however , the association between and clinicopathological significance of cdh1 promoter hypermethylation and lung cancer remains unclear . in this study , we systematically investigate studies of cdh1 promoter hypermethylation and lung cancer , and validate the association between cdh1 promoter hypermethylation and lung cancer using meta - analysis methods .
in addition , we summarize these findings and discuss tumor suppressor function , as well as the clinicopathological significance of cdh1 in lung cancer .
a systematic literature searching was performed using pubmed , embase , and the web of science up to august 13 , 2014 without any language restrictions .
the following keywords and terms were used : [ methylation or dna methylation or hypermethylation or de - methylation ] and [ cdh1 or cadherin-1 or cam 120/80 or epithelial cadherin ( e - cadherin ) or uvomorulin ] and [ lung cancer or lung carcinoma or lung tumor ] .
the published scientific articles were restricted to english language , and conference abstracts were excluded due to lack of sufficient data .
titles , abstracts , and key words in the articles were initially evaluated for inclusion criteria .
details and additional information were identified and collected from the full text of these articles .
a study included for meta - analysis needed to have : evaluated the correlation between cdh1 methylation and lung cancer ; included a clinical cohort and controls ; included at least three patients and controls ; used methylation - specific polymerase chain reaction or quantitative methylation - specific polymerase chain reaction to examine cdh1 methylation and expression ; and used tissue data rather than blood data .
studies that did not meet our inclusion criteria were excluded . when the same groups of patients were reported in multiple papers ,
two researchers independently collected the information and extracted the data regarding authorship , year , source of publication , inclusion criteria , cdh1 methylation frequency , sex status , smoking history , pathological type , clinical staging , degree of differentiation , lymph node metastasis , epidermal growth factor receptor status , and prognosis in patients and controls .
methodological evaluation was assessed by the researchers according to the remark ( reporting recommendations for tumor marker prognostic studies ) guidelines and elcwp ( european lung cancer working party ) score.18,19 the meta - analysis was performed using reviewer manager 5 ( cochrane collaboration , oxford , uk ) . pooled odds ratios ( ors ) and confidence intervals ( cis ) were calculated to assess the correlation between cdh1 methylation and lung cancer .
cochran s q test and i 
 2 were used to assess heterogeneity among the studies.20 a q test showing p<0.05 or an i 
 2 test > 50% indicated significant heterogeneity and a fixed - effects model was used to calculate the parameters .
otherwise , a random - effects model was used to pool data and attempt to identify potential sources of heterogeneity based on subgroup analyses.21,22 publication bias was assessed by begg s test and use of funnel plots.23 the analysis of meta - regression and publication bias was performed using stata version 10.0 ( statacorp , college station , tx , usa ) .
a systematic literature searching was performed using pubmed , embase , and the web of science up to august 13 , 2014 without any language restrictions .
the following keywords and terms were used : [ methylation or dna methylation or hypermethylation or de - methylation ] and [ cdh1 or cadherin-1 or cam 120/80 or epithelial cadherin ( e - cadherin ) or uvomorulin ] and [ lung cancer or lung carcinoma or lung tumor ] .
the published scientific articles were restricted to english language , and conference abstracts were excluded due to lack of sufficient data .
titles , abstracts , and key words in the articles were initially evaluated for inclusion criteria .
details and additional information were identified and collected from the full text of these articles .
a study included for meta - analysis needed to have : evaluated the correlation between cdh1 methylation and lung cancer ; included a clinical cohort and controls ; included at least three patients and controls ; used methylation - specific polymerase chain reaction or quantitative methylation - specific polymerase chain reaction to examine cdh1 methylation and expression ; and used tissue data rather than blood data .
studies that did not meet our inclusion criteria were excluded . when the same groups of patients were reported in multiple papers , only the most recent and complete paper was selected to avoid overlap .
two researchers independently collected the information and extracted the data regarding authorship , year , source of publication , inclusion criteria , cdh1 methylation frequency , sex status , smoking history , pathological type , clinical staging , degree of differentiation , lymph node metastasis , epidermal growth factor receptor status , and prognosis in patients and controls .
methodological evaluation was assessed by the researchers according to the remark ( reporting recommendations for tumor marker prognostic studies ) guidelines and elcwp ( european lung cancer working party ) score.18,19
the meta - analysis was performed using reviewer manager 5 ( cochrane collaboration , oxford , uk ) .
pooled odds ratios ( ors ) and confidence intervals ( cis ) were calculated to assess the correlation between cdh1 methylation and lung cancer .
cochran s q test and i 
 2 were used to assess heterogeneity among the studies.20 a q test showing p<0.05 or an i 
 2 test > 50% indicated significant heterogeneity and a fixed - effects model was used to calculate the parameters .
otherwise , a random - effects model was used to pool data and attempt to identify potential sources of heterogeneity based on subgroup analyses.21,22 publication bias was assessed by begg s test and use of funnel plots.23 the analysis of meta - regression and publication bias was performed using stata version 10.0 ( statacorp , college station , tx , usa ) .
the process followed to select the papers used in this report is shown in figure 1 .
ninety papers were identified by electronic database searching and 20 further papers by manual searching .
eighty - five papers were excluded for being duplicate publications , being an irrelevant title or abstract , or being a relevant title and abstract but not published in the english language .
after retrieval of the full - text articles , eleven papers were excluded for not having a large enough study population or for being reviews .
thirteen further studies were screened out ( involving 866 cases and 757 controls ) based on the inclusion and exclusion criteria in the pooled analysis .
the characteristics of the studies are shown in table 1.2436 six papers were used to study the correlation between cdh1 methylation and sex status .
five papers provided smoking history data , allowing an investigation of the influence of smoking on cdh1 methylation .
three studies investigated the effect of clinical stage , ie , stage i , ii , iii , or iv . one paper discussed lymph node metastasis and another discussed degree of differentiation . analyzing tissue samples from 657 patients and 593 controls , the mean frequency of cdh1 methylation was 32% ( range 8.33% to 66.32% ) in tumor tissue and 9% ( range 0.00%27.78% ) in tissue from controls .
this result indicates that the occurrence of cdh1 methylation is higher in tumor tissue than in normal tissue . using the fixed model , meta - analysis showed that 657 cases and 593 controls from 12 studies were pooled or as shown in figure 2 ( or 3.89 , 95% ci 2.875.27 , p<0.00001 ) .
these findings indicate that cdh1 methylation is a key molecular event in tumor tissue but not in normal tissue .
the results also show heterogeneity across the included studies ( i 
 2 is 61% , ie , more than 50% ) . given this significant heterogeneity , a subgroup analysis was performed to investigate sex status , smoking history , pathological type , clinical stage , differentiation degree , and lymph node metastasis to observe the relationship between cdh1 methylation and clinical characteristics ( see figure 3 ) .
however , there was no correlation between cdh1 promoter methylation and any of these factors ( sex status , or 0.78 , 95% ci 0.411.50 , p=0.46 ; smoking history , or 0.97 , 95% ci 0.531.79 , p=0.93 ; pathological type , or 0.97 , 95% ci 0.591.60 , p=0.91 ; clinical stage , or 1.48 , 95% ci 0.812.68 , p=0.2 ; lymph node metastasis , or 0.68 , 95% ci 0.133.63 , p=0.65 ; and differentiation degree , or 1.01 , 95% ci 0.343.02 , p=0.99 ) .
the or ranged from 0.78 to 3.89 , which was not a significant change , suggesting that the results of our meta - analysis were not significantly unstable .
the funnel plot shown in figure 4 is partially symmetric , indicating low publication bias regarding cdh1 methylation in lung cancer .
hypermethylation of tumor suppressor genes and hypomethylation of oncogenes are two essential components of the molecular mechanism involved in the epigenomic regulation of initiation and progression of cancer . as a tumor suppressor gene ,
cdh1 maintains cell - cell adhesion and keeps epithelial cells arranged in normal arrangement and layer . in vitro studies
demonstrate that loss of expression or function of cdh1 can activate transcription factors associated with epithelial - mesenchymal transition , leading to metastasis of cancer cells.37 
 cdh1 methylation has been detected in several types of carcinoma , including breast cancer , gastric cancer , and lung cancer.3840 a comprehensive evaluation of markers of methylation in lung cancer is needed to better understand the relationship between cdh1 methylation and lung cancer .
although a large number of studies have demonstrated a possible relationship between cdh1 methylation and lung cancer , a meta - analysis can summarize the relevant studies and compare different subgroup characteristics . in this meta - analysis
we analyzed data from 13 studies that included 657 tumor tissue samples and 593 control samples .
the results show that the cdh1 methylation level in the cancer group was significantly higher than in the control group . the pooled or using the fixed - effect model was 3.89 ( 95% ci 2.875.27 versus the control group ) .
cdh1 methylation plays a key role in the induction of lung cancer due to silencing of the tumor suppressor gene cdh1 .
this conclusion is consistent with that of a previous study.36 since changes in cdh1 promoter hypermethylation are reversible , drug treatment promoting demethylation may be useful for delaying carcinogenesis and progression . treatment with 5-aza-2-deoxycytidine showed that migration of a549 cells decreased markedly upon restoration of cdh1.41 these preclinical studies show the therapeutic potential of restoration of tumor suppressor expression via epigenetic modulation .
we further determined the clinicopathological significance of cdh1 promoter hypermethylation in patients with lung cancer . for smoking history ,
the summary or was 0.97 ( 95% ci 0.531.79 ) in the 65 cases and 27 controls .
smoking may target other specific genes for methylation or mutation , for example , methylenetetrahydrofolate reductase.33 other subgroup meta - analysis was performed , including for sex status ( or 0.78 , 95% ci 0.411.50 ) , pathological type ( or 0.97 , 95% ci 0.591.60 ) , clinical stage ( or 1.48 , 95% ci 0.812.68 ) , lymph node metastasis ( or 0.68 , 95% ci 0.133.63 ) , and degree of differentiation ( or 1.01 , 95% ci 0.343.02 ) .
cdh1 methylation determined by clinical staging shows a slightly more significant association than the other subgroups .
interestingly , cdh1 methylation was detected much more frequently in stages iii and iv than in stages i and ii .
however , cdh1 methylation itself does not correlate with pathological type , sex status , lymph node metastasis , or degree of differentiation .
possible reasons for this finding might be the widely heterogeneous results for the subgroups or the lack of cases and controls in the subgroups .
other potentially significant factors may need to be investigated , such as patient age , tumor size , and biopsy sample control.42
cdh1 methylation might be a biomarker of lung cancer , with potential value in predicting the prognosis of the disease , and warrants further studies involving more clinical cases for meta - analysis in the future .
in addition , the potential variables on cdh1 methylation from different group database are still not clear due to the limitation of the statistical power of meta - analysis .
 backgroundcdh1 is a protein encoded by the cdh1 gene in humans . 
 mutations in this gene are linked with several types of cancer . 
 loss of cdh1 function contributes to the progression of cancer by increasing proliferation , invasion , and/or metastasis . 
 however , the association between and clinicopathological significance of cdh1 promoter methylation and lung cancer remains unclear . in this study 
 , we systematically reviewed the studies of cdh1 promoter methylation and lung cancer , and evaluated the association between cdh1 promoter methylation and lung cancer using meta - analysis methods.methodsa comprehensive search of the pubmed and embase databases was performed up to july 2014 . 
 the methodological quality of the studies was also evaluated . 
 the data were extracted and assessed by two reviewers independently . 
 analyses of pooled data were performed . 
 odds ratios ( ors ) were calculated and summarized.resultsfinally , an analysis of 866 patients with non - small cell lung cancer from 13 eligible studies was performed . 
 the cdh1 methylation level in the cancer group was significantly higher than in the controls ( or 3.89 , 95% confidence interval [ ci ] 2.875.27 , p<0.00001 ) . 
 however , there were no correlations between cdh1 promoter methylation and clinicopathological characteristics ( sex status , or 0.78 , 95% ci 0.411.50 , p=0.46 ; smoking history , or 0.97 , 95% ci 0.531.79 , p=0.93 ; pathological type , or 0.97 , 95% ci 0.591.60 , p=0.91 ; clinical staging , or 1.48 , 95% ci 0.812.68 , p=0.2 ; lymph node metastasis , or 0.68 , 95% ci 0.133.63 , p=0.65 ; or differentiation degree , or 1.01 , 95% ci 0.343.02 , p=0.99).conclusionthe results of this meta - analysis suggest that cdh1 methylation is associated with an increased risk of lung cancer . 
 cdh1 hypermethylation , which induces inactivation of the cdh1 gene , plays an important role in carcinogenesis and may serve as a potential drug target in lung cancer . 
 however , cdh1 methylation does not correlate with other factors , such as smoking history , clinical stage , pathological type , sex status , lymph node metastasis , or degree of differentiation . 
Introduction
Materials and methods
Publication selection
Inclusion and exclusion criteria
Data extraction and quality assessment
Data analysis
Results
Discussion
Conclusion