in recent years , genetic polymorphism in a variety of xenobiotic - metabolizing enzymes ( phase i and ii enzymes ) has been studied extensively . in general , these polymorphisms do not always lead to changes in protein expression and catalytic activities . for this reason , several polymorphic genes , encoded for enzymes involved in phase i and ii reactions , partially explain individual susceptibility to cancer . one of the enzymes is meh , which catalyzes xenobiotic biotransformation in mammalian tissues and supports detoxification capability of the organism . the meh ( ephx1 ec 3.3.2.3 ) is a smooth endoplasmic reticulum enzyme that catalyzes the hydrolysis of epoxides into trans - dihydrodiols and is responsible for the detoxification of pah [ 2 , 3 ] . two variant ephx1 alleles have been associated with altered meh activity , which is a substitution of histidine for tyrosine at residue 113 ( exon 3 polymorphism ) ; this decreases meh activity by approximately 40% , whereas substitution of arginine for histidine at residue 139 ( exon 4 polymorphism ) increases enzyme activity by approximately 25% . an epoxide is a three - membered cyclic ether , while its ring system is highly strained , and its oxygen - carbon bond is polarized . some reactive epoxides are responsible for electrophilic reactions , with critical biological targets such as dna and protein , leading to mutagenic , toxic , and carcinogenic effects . the meh is an important enzyme in the metabolism of environmental and man - made contaminants , as it can metabolize a broad array of epoxide - containing compounds . these include aliphatic epoxides ( butadiene oxide ) and polyaromatic oxides ( phenanthrene oxide ) [ 68 ] . the development of disease is an outcome of interactions between human genes and the environment . therefore , genetic variability may be responsible for individual susceptibility to carcinogenic factors in primary brain tumors . brain tumor etiology indicates that certain occupations involving exposure to polycyclic aromatic hydrocarbons ( or other aromatic hydrocarbons ) may be associated with increased risk . viral agents , household chemicals , and certain foods have not been established as causal . in addition , little is known about the interaction of genetic factors and environmental toxins in the genesis of brain tumors . a number of investigations have been conducted to examine the potential association of the meh genotype with altered susceptibility to disease incidence . however , negative or positive results have been observed in similar studies for genotypes that involve different genes . from this perspective , badr el - din et al . found benign brain tumors that showed a significantly higher frequency of il-10 - 1082 a / a and il-6 - 174 c / c homozygous genotypes , compared to controls . the sod3 ivs1 + 186c > t polymorphism is associated with a significantly increased risk of meningioma , while a possible increased risk of glioma was observed by rajaraman et al . . on the other hand , sima et al . found no significant association between the gstm1 and gstt1 homozygous deletion polymorphisms and risk of brain tumors . in some studies , ephx1 high - activity alleles have been associated with increased risk of various types of cancer . some studies also found that the variant ephx1 arg139 allele and gstp1*c allele confer protection against solvent - induced encephalopathy . the hypothesis of this study is that inherent genetic susceptibility to primary brain tumor is associated with meh gene polymorphisms . to investigate this , we analyzed epidemiologic and molecular data from a case - controlled study in the turkish population . we believe that there is a great need for large and well - designed epidemiological studies of potential genetic and environmental risk factors . cases included brain tumor patients ( n = 105 ) who attended the neurosurgery department at cumhuriyet university hospital in sivas . only those patients with newly diagnosed primary brain tumors ( without any previous radiotherapy and chemotherapy ) the diagnosis of brain tumor was histologically confirmed , and tumor types were classified according to who guidelines . age- and sex - matched controls were recruited mainly from patients ( without any previous cancer diagnosis , including radiotherapy or chemotherapy ) at the same hospital ( n = 150 ) . all study subjects agreed to participate and filled out a short questionnaire about occupation , tobacco use , alcohol consumption , and a family history of cancer . dna was extracted from whole blood by a salting out procedure when the samples reached the laboratory . two sequence variants of the ephx1 gene ( exon 3 and 4 ) were detected with the pcr and rflp assay . the target dna sequence in ephx1 gene exon 3 was amplified with primers f 5 ggg gtc ctg aat ttt gct cc 3 and r 5 caa tct tag tct tga agt gac ggt 3 , and in ephx1 gene exon 4 the primers used were f 5 ggg gta cca gac ctg acc gt 3 and r 5 aac acc ggg cca ccc ttg gc 3. pcr was carried out in a volume of 50  l with 50  ng of dna , 50  pmol of exon 3 or 4 primer , 5  l dntps , 4  l mgci2 , 5  l pcr buffer , and 0.5  l taq dna polymerase . the amplification of exon 3 was an initial denaturation at 95c for 7  min followed by 35 cycles of denaturation at 95c for 1  min , annealing at 53c for 1  min , extension at 72c for 1  min , and final extension at 72c for 10  min . the amplification of exon 4 was an initial denaturation at 94c for 5  min , followed by 35 cycles of denaturation at 94c for 30  s , annealing at 63c for 30  s , extension at 72c for 45  s , and final extension at 72c for 5  min . amplified products ( exon 3 : 163  bp , exon 4 : 357  bp ) were resolved by using 1.5% wt / vol agarose gels with ethidium bromide . following overnight digestion of the 14  l pcr product ( 163  bp ) with 10  u  psyi , these products were resolved with 2.5% agarose gel containing ethidium bromide . following digestion , the homozygous wild - type ( try 113 ) was identified by two dna bands ( 140 and 23  bp ) , whereas the homozygous mutant allele ( his 113 ) produced one band ( 163  bp ) , and the heterozygotes displayed a combination of all three ( 163 , 140 , and 23  bp ) dna fragments . again , after overnight digestion of the 14  l pcr product ( 357  bp ) with 10  u  rsai , the digested products resolved with 2.5% agarose gel containing ethidium bromide . after digestion , homozygous wild - type ( his 139 ) was identified by two dna bands ( 295 and 62  bp ) , whereas the homozygous mutant allele ( arg 139 ) produced three bands ( 174 , 121 , and 62  bp ) ; the heterozygotes displayed a combination of all four bands ( 295 , 174 , 121 , and 62  bp ) dna fragments . the distribution of the mephx genotypes , tobacco use , and family history of cancer between cases and controls was evaluated using the  test or fisher 's exact test ( when the expected number in any cell was less than five ) . in addition , the association between histopathologic type of brain tumor and mephx genotypes was evaluated using the  test . the strength of the association was assessed for each polymorphism individually with or and 95% ci from logistic regression models . all analyses were conducted using the statistical package for social sciences program ( spss , version 14 ) . in this study , samples were collected from 150 controls ( 75 for males and 75 for females ) and 105 cases ( 55 for males and 50 for females ) of primary brain tumor patients . the mean age of male and female patients was 49.6  16.3 and 47.5  17.6 , respectively . the mean age of male and female controls was 50.0  7.2 and 45.4  15.8 , respectively . no significant difference in the proportion of smokers was observed between case ( 25% ) and control ( 36% ) groups . the family history of cancer was more prevalent in cases ( 6% ) than in controls ( 16% ) . there were also no statistically significant differences between cases and controls in smoking status (  = 3.5 , p = 0.1 ) ( table 2 ) . however , there was a statistically significant association found between case and control groups for family history of cancer (  = 7.0 , p = 0.01 ) ( table 3 ) . the mephx exon 3 genotype distributions in controls and cases can be seen in table 4 . there was no significant association between mephx exon 3 genotype and primer brain tumor (  = 0.05 , p = 0.8 , crude or = 1.0 , 95% ci = 0.61.7 ) ( table 4 ) . we evaluated only 102 cases for exon 3 because there were no pcr products of the three cases of exon 3 . the mephx exon 4 genotype distributions in controls and cases are given in table 4 . there was a significant association between mephx exon 4 genotype and primer brain tumor incidence (  = 4.3 , p = 0.03 , crude or = 1.8 , 95% ci = 1.03.4 ) ( table 4 ) . the polymorphisms in the meh exon 4 and 3 were in hwe ( table 5 ) . we subdivided patients according to histopathologic type of brain tumor , so that we could evaluate the association between the polymorphisms of mephx and specific types of primer brain tumors . histopathologic data were available for 102 cases : 29% were meningioma , 38% were astrocytoma , 9% were hypophysis adenoma , and 24% were other types ( craniopharyngioma , acoustic neuroma , medulloblastoma , and colloid cyst ) of exon 3 . histopathologic data were available for 105 cases : 30% were meningioma , 37% were astrocytoma , 9% were hypophysis adenoma , and 24% were other types in exon 4 . there was no statistically significant association between mephx gene variants and tumor types ( table 6 ) . brain tumor is a predominant problem worldwide , in terms of both incidence and prevalence , and is a leading cause of tumor death . brain tumors affected more than 18,300 american males in 2003 , and the mortality rate was 72% ( i.e. , 13,100 men died of brain tumor in the same year ) . the most common brain tumors are gliomas , which originate in the supportive glia tissue : astrocytomas arise from astrocytes . brain tumors can strike anyone , but risk increases with age , certain lifestyles , and quality of the environment . exposure to chemical , physical , and biologic agents is considered to at least be a factor in the development of a brain tumor . our study was based on the hypothesis that the risk of a primary brain tumor would be modified by genetic polymorphism in enzymes involved in the metabolism of common chemicals , such as pah . genetic variability may be responsible for individual susceptibility to carcinogenic factors in brain tumors , as well as other tumors such as lung , ovarian , and colorectal [ 1820 ] . the most consistent epidemiologic finding for brain tumors is that primary ones are more common among males , whereas meningeal tumors are more common among females . cultural , ethnic , and geographic differences in risk factors may also influence differences in tumor incidence ; the incidence of malignant brain tumors in japan is less than half that in northern europe . in the united states , gliomas affect a greater proportion of caucasians than african - americans , but the incidence of meningioma is nearly equal . there have not been any studies to date in the literature that indicate an association between brain tumors and mephx gene polymorphism . in this study , 60% of meningeal tumors were more common among female patients , such that these results were consistent with the literature . we found no association between primary brain tumor incidence and mephx exon 3 polymorphism (  = 0.05 , p = 0.8 , or = 1.0 , 95% ci = 0.61.7 ) ( table 4 ) . some studies found a slight association between primary brain tumor and mephx exon 3 gene polymorphism . for example , de roos et al . reported a poorly increased risk associated with ephx1 113 : his / his for glioma was predominant with elevated risks among older subjects ( > 50 ) , females , and lifelong - smokers . according to several studies , an increased risk of cancer is associated with lower activity of the his / his protein product ; nevertheless , ephx1 high - activity alleles have been associated with an increased risk for various types of cancer [ 2427 ] . however , there may be other mechanisms that affect brain carcinogenesis , such as tumor suppressor gene with defects ( pten , rb1 , cdkn2a , cdkn2b , parf , p53 ) and proto - oncogenes ( cdk4 , mdm2 , egfr ) activation [ 28 , 29 ] . we found a poor association between primary brain tumor incidence and mephx exon 4 polymorphism (  = 4.3 , p = 0.03 , or = 1.8% , 95 ci = 1.03.4 ) ( table 4 ) . thus , exon 4 polymorphism is probably protective against the risk of primary brain tumor in terms of exposure to pah . there have been no reports about the relationship between brain tumors and mephx exon 4 polymorphism in the literature . the results of other studies , though , suggest a relationship between mephx exon 4 polymorphism and related cancer risks . wu et al . reported an increased risk of lung cancer among individuals with exon 4 ( rapid allele ) , while there is a decreased risk of lung cancer among individuals with exon 3 ( slow allele ) . no association between adenoma and meh genotypes ( exon 3 and 4 ) or meh activity was observed by cortessis et al . . in this study , there was no statistically significant difference between cases and controls in smoking status (  = 3.5 , p = 0.1 ) ( table 2 ) . our results suggest a minor role of tobacco smoking with brain tumor risk ; nevertheless , an increased risk of adult glioma with smoking unfiltered cigarettes was observed by lee et al . and silvera et al . some carcinogenic compounds found in tobacco smoke can not cross the blood - brain barrier , but some carcinogenic chemicals such as n - nitroso can cross it and may cause brain tumors [ 35 , 36 ] . the benzo ( a ) pyrene-7,8-dihydrodiol ( bap ) is oxidized to benzo ( a ) pyrene-7,8-dihydrodiol 9,10-epoxide ( bpde ) , which binds to the exocyclic amino group of guanine dna [ 37 , 38 ] . we found a statistically significant association between brain tumor incidence and family history of cancer (  = 7.0 , p = 0.01 ) ( table 3 ) . this result is consistent with some other studies , suggesting that individuals with a family history of cancer may have an increased brain tumor risk [ 39 , 40 ] . in one study , there were similar mechanisms for brain carcinogenesis , such as that of other organs . in our case - control study , an increase in risk was not observed for any histological type of brain tumor associated with mephx variant genotypes . this finding is inconsistent with other studies and may be due to the small number of brain tumor cases in our study . these results add to the literature about the contribution of variation in metabolic genes to the incidence of adult brain tumors . overall , these data do not provide strong evidence for the importance of genes involved in biotransformation of pahs and their metabolites for brain carcinogenesis ; however , specific genes may play a role in the context of both pahs and other exposures . we believe that the association between mephx enzyme polymorphisms and brain tumor incidence must be investigated by studies with a larger number of cases and controls . in conclusion , our results somewhat support the hypothesis that genetic susceptibility to brain tumors may be associated with mephx gene polymorphisms . additional research is still necessary to determine an individual 's susceptibility to brain tumor development .
meh is a critical biotransformation enzyme that catalyzes the conversion of xenobiotic epoxide substrates into more polar diol metabolites : it is also capable of inactivating a large number of structurally different molecules . 
 two polymorphisms affecting enzyme activity have been described in the exon 3 and 4 of the meh gene . 
 the hypothesis of this study is that inherent genetic susceptibility to a primary brain tumor is associated with meh gene polymorphisms . 
 the polymorphisms of the meh gene were determined with pcr - rflp techniques and 255 turkish individuals . 
 our results indicate that the frequency of the meh exon 4 polymorphism ( in controls ) is significantly higher than that of primary brain tumor patients ( or = 1.8 , 95% ci = 1.03.4 ) . 
 this report , however , failed to demonstrate a significant association between meh exon 3 polymorphism and primary brain tumor susceptibility in this population . 
 analysis of patients by both histological types of primary brain tumor and gene variants showed no association , although analysis of family history of cancer between cases and controls showed a statistically significant association ( 2 = 7.0 , p = 0.01 ) . 
 our results marginally support the hypothesis that genetic susceptibility to brain tumors may be associated with mephx gene polymorphisms .