reactive oxygen species ( ros ) are considered to play an important role in vascular disease [ 28 ] by influencing the process of atherosclerosis of cerebral arteries [ 911 ] . the cytosolic components of p47 , p67 , p40 , rac-1 , nox2 , and p22 ( 2 membrane - bound subunits ) are involved in nicotinamide adenine dinucleotide phosphate ( nadph ) oxidase , an important source of superoxide . the gene encodes p22 and is located on chromosome 16q24 , containing some genetic variants [ 1416 ] . recently , an increasing number of studies have investigated the association of nadph oxidase c242 t polymorphism and icvd . inoue et al .   reported that t allele in the p22 gene presented association with a low risk for coronary artery disease . in contrast , some studies suggested that the t allele in the p22 gene is a risk factor of coronary artery disease . published meta - analyses have evaluated the association of p22 gene polymorphism and icvd and various studies have focused on this topic , but the results were conflicting . thus , we carried out this meta - analysis to reassess the possible association between p22 gene c242 t polymorphism and icvd . systematic searches of the electronic databases embase , pubmed , chinese biological medical literature database ( cbm ) , and chinese national knowledge infrastructure ( cnki ) from january 2000 to december 2013 were performed . the search strategy was as follows :  nadph oxidase  ;  p22phox gene  ; study inclusion criteria were : ( 1 ) studies on the association between nadph oxidase c242 t polymorphism and icvd ; ( 2 ) study type : case - control or cohort ; ( 3 ) healthy people as the control group ; ( 4 ) raw data was available . studies were excluded if they were : ( 1 ) duplicate publications . studies that deviated from hardy - weinberg equilibrium ( hwe ) were not eliminated . the following information was extracted from each included study by 2 authors ( p. li , c. qin ) : country , first author , year , journal , number of cases and controls , allele frequencies , genotype frequencies , ethnicity , experimental design , diagnoses of icvd . the association between p22phox gene c242 t polymorphism and icvd risk was measured by odds ratio ( or ) and 95% confidence intervals ( cis ) . heterogeneity was evaluated by the chi - squared test and the inconsistency index ( i ) . quantification of the heterogeneity was calculated by the i metric , which is independent of the number of studies in the meta - analysis . a random effects model was used if significant heterogeneity existed ; otherwise , fixed effects model was used . the publication bias between studies was assessed by the funnel plot and egger s test . systematic searches of the electronic databases embase , pubmed , chinese biological medical literature database ( cbm ) , and chinese national knowledge infrastructure ( cnki ) from january 2000 to december 2013 were performed . the search strategy was as follows :  nadph oxidase  ;  p22phox gene  ; study inclusion criteria were : ( 1 ) studies on the association between nadph oxidase c242 t polymorphism and icvd ; ( 2 ) study type : case - control or cohort ; ( 3 ) healthy people as the control group ; ( 4 ) raw data was available . studies were excluded if they were : ( 1 ) duplicate publications . studies that deviated from hardy - weinberg equilibrium ( hwe ) were not eliminated . the following information was extracted from each included study by 2 authors ( p. li , c. qin ) : country , first author , year , journal , number of cases and controls , allele frequencies , genotype frequencies , ethnicity , experimental design , diagnoses of icvd . the association between p22phox gene c242 t polymorphism and icvd risk was measured by odds ratio ( or ) and 95% confidence intervals ( cis ) . heterogeneity was evaluated by the chi - squared test and the inconsistency index ( i ) . quantification of the heterogeneity was calculated by the i metric , which is independent of the number of studies in the meta - analysis . a random effects model was used if significant heterogeneity existed ; otherwise , fixed effects model was used . the publication bias between studies was assessed by the funnel plot and egger s test . we tried to contact the author for the raw data but no response came back , which limited our quantitative synthesis . hence , a total of 8 studies [ 19,2329 ] involving 2045 cases and 2102 controls were eligible according to our selection criteria . three articles were written in chinese [ 2729 ] and the rest were in english . of all these articles , six [ 19,23,24,2729 ] were in asian populations and two   were in european populations . a total of 2045 cases and 2102 controls in the included studies were analyzed in our meta - analysis . pooling the data of the 8 included studies , the overall results showed that p22phox gene c242 t had no association with icvd risk in the following genetic models : dominant model ( or=0.74 , 95% ci 0.541.02 , p=0.064 ) and recessive model ( or=1.40 , 95% ci 0.892.19 , p=0.146 ) . the overall results of the allelic model ( or=1.33 , 95% ci 1.001.77 , p=0.048 ) was significantly associated with icvd risk ( figure 1 ) . a fixed effects model was used because no heterogeneity existed in the recessive model ( i=31.4% , p=0.2 ) . if significant inter - study heterogeneity existed in the dominant model ( i=57.2% , p=0.022 ) and allelic model ( i=58.9% , p=0.017 ) , the random effects model was used to calculate the pooled or . to explore the source of inter - study heterogeneity , a total of 6 studies [ 19,23,24,2729 ] in asian populations and 2 studies   in european populations were identified . no relevant association was found in p22 gene c242 t polymorphism and icvd in both asian populations ( or=0.64 , 95% ci 0.411.00 , p=0.05 ) and european populations ( or=0.97 , 95% ci 0.661.43 , p=0.88 ) for dominant model ( figure 1a ) . there was no heterogeneity in european populations ( i=0% , p=0.873 ) , but significant heterogeneity was discovered in asian populations ( i=69% , p=0.007 ) . in the 6 asian population studies , 3 studies [ 2729 ] investigated people of chinese descent and the other three   investigated people of japanese descent . thus , the 6 asian population studies were divided into 2 subgroups for further analysis . the results indicated no heterogeneity in chinese populations ( i=36.6% , p=0.207 ) but heterogeneity existed in japanese populations ( i=69.5% , p=0.038 ) ( figure 2 ) . results of subgroup analysis for recessive model indicated a pertinent association in p22 gene c242 t polymorphism and icvd in european people ( or=2.13 , 95% ci 1.064.26 , p=0.034 ) but no relevant association in asian populations ( or=0.98 , 95% ci 0.531.81 , p=0.948 ) was found ( figure 1b ) . however , no significant association was found between p22 gene c242 t polymorphism and icvd in chinese ( or=0.94 , 95%ci : 0.146.36 , p=0.949 ) or japanese populations ( or=0.98 , 95% ci 0.511.88 , p=0.962 ) ( figure 3 ) . no inter - study heterogeneity existed in either chinese ( i=24.7% , p=0.249 ) or japanese populations ( i=0% , p=0.401 ) . p22 gene c242 t polymorphism had no association with icvd in asian ( or=1.51 , 95% ci 0.982.32 , p=0.061 ) and european descent ( or=1.18 , 95% ci : 0.881.59 , p=0.266 ) ( figure 1c ) . no heterogeneity existed in european populations ( i=0% , p=0.58 ) , but moderate heterogeneity was found in asian populations ( i=69.9% , p=0.005 ) . further analysis of asian groups for an allelic model showed significant association between p22 gene c242 t polymorphism and icvd risk in chinese populations ( or=2.36 , 95% ci1.015.53 , p=0.048 ) , but no significant association existed in japanese population ( or=1.20 , 95% ci 0791.84 , p=0.386 ) ( figure 4 ) . sensitivity analysis was carried out by removing 1 individual study each time to evaluate the stability of the meta - analysis and explore the source of heterogeneity . results indicated evidence of quite obvious impact on the overall pooled or for the dominant model , recessive model , and allelic model , which demonstrated the meta - analysis results were stable . the i - square and p values of heterogeneity test of the results altered when 1 study was removed . was removed ( dominant model : i=40% , p=0.125 ; allelic model : i=35.7% , p=0.156 ) , which indicated that this study might contribute to the inter - study heterogeneity ( figure 5 ) . results showed that the pooled ors were stable when studies that deviated from hwe were excluded . we tried to contact the author for the raw data but no response came back , which limited our quantitative synthesis . hence , a total of 8 studies [ 19,2329 ] involving 2045 cases and 2102 controls were eligible according to our selection criteria . three articles were written in chinese [ 2729 ] and the rest were in english . of all these articles , six [ 19,23,24,2729 ] were in asian populations and two   were in european populations . a total of 2045 cases and 2102 controls in the included studies were analyzed in our meta - analysis . pooling the data of the 8 included studies , the overall results showed that p22phox gene c242 t had no association with icvd risk in the following genetic models : dominant model ( or=0.74 , 95% ci 0.541.02 , p=0.064 ) and recessive model ( or=1.40 , 95% ci 0.892.19 , p=0.146 ) . the overall results of the allelic model ( or=1.33 , 95% ci 1.001.77 , p=0.048 ) was significantly associated with icvd risk ( figure 1 ) . a fixed effects model was used because no heterogeneity existed in the recessive model ( i=31.4% , p=0.2 ) . if significant inter - study heterogeneity existed in the dominant model ( i=57.2% , p=0.022 ) and allelic model ( i=58.9% , p=0.017 ) , the random effects model was used to calculate the pooled or . to explore the source of inter - study heterogeneity , a total of 6 studies [ 19,23,24,2729 ] in asian populations and 2 studies   in european populations were identified . no relevant association was found in p22 gene c242 t polymorphism and icvd in both asian populations ( or=0.64 , 95% ci 0.411.00 , p=0.05 ) and european populations ( or=0.97 , 95% ci 0.661.43 , p=0.88 ) for dominant model ( figure 1a ) . there was no heterogeneity in european populations ( i=0% , p=0.873 ) , but significant heterogeneity was discovered in asian populations ( i=69% , p=0.007 ) . in the 6 asian population studies , 3 studies [ 2729 ] investigated people of chinese descent and the other three   investigated people of japanese descent . thus , the 6 asian population studies were divided into 2 subgroups for further analysis . the results indicated no heterogeneity in chinese populations ( i=36.6% , p=0.207 ) but heterogeneity existed in japanese populations ( i=69.5% , p=0.038 ) ( figure 2 ) . results of subgroup analysis for recessive model indicated a pertinent association in p22 gene c242 t polymorphism and icvd in european people ( or=2.13 , 95% ci 1.064.26 , p=0.034 ) but no relevant association in asian populations ( or=0.98 , 95% ci 0.531.81 , p=0.948 ) was found ( figure 1b ) . however , no significant association was found between p22 gene c242 t polymorphism and icvd in chinese ( or=0.94 , 95%ci : 0.146.36 , p=0.949 ) or japanese populations ( or=0.98 , 95% ci 0.511.88 , p=0.962 ) ( figure 3 ) . no inter - study heterogeneity existed in either chinese ( i=24.7% , p=0.249 ) or japanese populations ( i=0% , p=0.401 ) . c242 t polymorphism had no association with icvd in asian ( or=1.51 , 95% ci 0.982.32 , p=0.061 ) and european descent ( or=1.18 , 95% ci : 0.881.59 , p=0.266 ) ( figure 1c ) . no heterogeneity existed in european populations ( i=0% , p=0.58 ) , but moderate heterogeneity was found in asian populations ( i=69.9% , p=0.005 ) . further analysis of asian groups for an allelic model showed significant association between p22 gene c242 t polymorphism and icvd risk in chinese populations ( or=2.36 , 95% ci1.015.53 , p=0.048 ) , but no significant association existed in japanese population ( or=1.20 , 95% ci 0791.84 , p=0.386 ) ( figure 4 ) . sensitivity analysis was carried out by removing 1 individual study each time to evaluate the stability of the meta - analysis and explore the source of heterogeneity . results indicated evidence of quite obvious impact on the overall pooled or for the dominant model , recessive model , and allelic model , which demonstrated the meta - analysis results were stable . the i - square and p values of heterogeneity test of the results altered when 1 study was removed . was removed ( dominant model : i=40% , p=0.125 ; allelic model : i=35.7% , p=0.156 ) , which indicated that this study might contribute to the inter - study heterogeneity ( figure 5 ) . results showed that the pooled ors were stable when studies that deviated from hwe were excluded . the incidence of icvd are known to be related to ethnic factors , which may reveal genetic or non - genetic differences . recently , more and more studies have focused on the relationship between p22 gene c242 t polymorphism and icvd risk . .   indicated no evidence of association between p22 gene c242 t polymorphism and icvd risk . the potential association between p22 gene c242 t polymorphism and icvd has aroused great interest from the public . a variety of studies have been devoted to this topic , but the results are conflicting . therefore , we conducted this meta - analysis to identify and reassess the association between p22 gene c242 t polymorphism and icvd . to our best knowledge , this is the most comprehensive meta - analysis thus far to assess the association between p22 gene c242 t polymorphism and icvd risk with 8 studies included , involving 2045 icvd cases and 2102 controls . results obtained from the allelic model confirmed a positive association between p22 gene c242 t polymorphism and icvd risk . the pooled results of the dominant model and recessive model indicated that p22 gene c242 t polymorphism was not associated with icvd , which is consistent with the previously published meta - analyses . to estimate the genetic effect on different population , according to the results , it seems to have a null hereditary effect in asian populations for the dominant model , recessive model , and allelic model . in european populations , no significant association of genetic effect however , p22 gene c242 t polymorphism was associated with icvd risk in the recessive model , which may indicate that european participants who inherited a recessive allele may have increased p22 gene c242 t polymorphism susceptibility . a further analysis in asian groups was also conducted , revealing a null genetic effect in chinese populations for the dominant and recessive model but a relevant association of icvd in the allelic model . no significant association of p22 gene c242 t polymorphism and icvd existed in japanese populations for the following genetic model : dominant model , recessive model and allelic model . heterogeneity is a potential dilemma and is inevitable when combining the overall results of meta - analyses . the i - square values fall within the range of 0100% , with higher values demonstrating a greater degree of heterogeneity . moderate heterogeneity was observed in the dominant and allelic genetic models except for the recessive model when pooling the overall results of included studies . to explore the source of heterogeneity , the results of subgroup analysis by ethnicity showed that no heterogeneity existed in european populations for dominant and allelic genetic models . a moderate degree of inter - study heterogeneity existed in asian populations for both dominant and allelic genetic models , which might contribute to the existence of heterogeneity . significant heterogeneity was observed in japanese populations in both dominant and allelic genetic models , but no heterogeneity was found in chinese populations in these 2 genetic models . sensitivity analysis was conducted by pooling the overall data of included studies with 1 individual study eliminated each time . heterogeneity existed when eliminating most of the included studies except for 1 study . when the study conducted by kuroda et al . was eliminated , the i - square and p - value for heterogeneity test clearly changed in both dominant and allelic genetic models ( figure 5 ) without statistical significance of heterogeneity . these findings indicated that inter - study heterogeneity might come from the study by kuroda et al . in the 3 japanese studies , 2 studies   selected healthy subjects who visited the hospital for regular check - ups as controls , while kuroda et al .   chose control subjects from participants of an epidemiological study called the hisayama study . this indicated that the source of the control group might cause heterogeneity . and inter - study heterogeneity might come from various selection criteria of cases and controls , different sample size , and variation in diagnostic method of subtypes . many factors ( even the environmental factors ) could have led to the existence of heterogeneity . potential publication bias existed , although we performed careful search strategies for published studies and data extraction . relevant english and chinese studies were included , but articles in other languages , which might have contributed to the results , were probably missed . polymorphisms in p22 gene have been reported to be associated with nadph oxidase activity , which potentially contributes to the pathogenetic mechanisms of icvd . it has been reported that increased atherosclerosis had associations with increased ros production and increased activity of nadph oxidase subunits p22 and nox2 . mutation of ct may reduce susceptibility to cardiovascular diseases due to decreased oxidative stress in vasculature , and increased oxidative stress and nadh stimulated vascular superoxide production might possibly contribute to the development of atherosclerosis . specific risk factors , such as diabetes , may cause increased expression and activity of nadph oxidase in vascular disease . explored aspects of nadph oxidase c242 t polymorphism in pathogenesis of carotid atherosclerosis in patients with type 2 diabetes , and found no significant association of c242 t polymorphism in the nadph gene with clinical parameters , including plaque score and intima - media thickness of the carotid artery , using the 75 percentile of intima - media thickness as a cut - off value . factors like more frequent stable atherosclerotic plaques , lower cimt , and lower risk for complications might cause such phenomena . the presence of heterogeneity can result from various aspects such as sample population and selection criteria of cases and controls . secondly , only published articles were analyzed ; the unpublished significant findings or negative findings may also contribute to the overall results . in conclusion , this meta - analysis indicated a significant association between p22 gene c242 t polymorphism and icvd risk in the allelic genetic model . furthermore , no relevant association was found in asian populations for any of the 3 genetic models and european populations in the dominant model and allelic model . results of subgroup analysis revealed that p22 gene c242 t polymorphism had a significant association with icvd in european populations for the recessive model . future studies were needed for further analysis of the association between p22 gene c242 t polymorphism and icvd .
backgrounda growing number of studies on the associations between nicotinamide adenine dinucleotide phosphate ( nadph ) oxidase p22phox c242 t polymorphism and risk of ischemic cerebrovascular disease have recently been published , but the results remain inconsistent.material/methodswe performed an updated meta - analysis to evaluate this association . 
 eight case - control studies were included , involving 2045 cases and 2102 controls . 
 heterogeneity was assessed by the q test and the i2 statistic . 
 begg and egger s tests were conducted to evaluate publication bias . 
 odds ratio ( or ) was tested to identify the associations.resultssignificant associations between p22phox gene c242 t polymorphism and ischemic cerebrovascular disease ( icvd ) risk were observed in the allelic genetic model ( or=1.33 , 95% confidence interval [ ci ] 1.001.77 , p=0.048 ) . 
 no statistical significant association was found in the dominant model ( or=0.74 , 95% ci 0.541.02 , p=0.064 ) and recessive model ( or=1.40 , 95% ci 0.892.19 , p=0.146 ) . 
 subgroup analysis showed an association in european populations for recessive model ( or=2.13 , 95% ci 1.064.26 , p=0.034 ) and no significant evidence of association in asian populations was found ( dominant model : or=0.64 , 95% ci 0.411.00 , p=0.05 ; recessive model : or=0.98 , 95% ci 0.531.81 , p=0.948 ; allelic model : or=1.51 , 95% ci 0.982.32 , p=0.061).conclusionsp22phox gene c242 t polymorphism was associated with icvd risk in the allelic genetic model , as well as in european populations for recessive model . 
 no evidence showed association between p22phox gene c242 t polymorphism and icvd risk in the dominant model and recessive model . 
 furthermore , no association existed in asian populations for any of the 3 genetic models and european populations in the dominant model and allelic model .