The Association of Specific EPHX1 and GSTM1 Gene Polymorphisms with Phenytoin- or Smoking-Associated Birth Defects

Abstract

Eleven percent of children exposed in utero to phenytoin show a pattern of fetal hydantoin syndrome (FHS) which may include cleft lip/palate. Also, in utero exposure to the products of cigarette smoke leads to reduced birth weight, prematurity, and small but significant risk of oral clefts. Cleft lip/palate occur at a frequency of 1 per 750 to 1000 livebirths in the United States. Microsomal epoxide hydrolase (mEH) participates in xenobiotic metabolism, including the detoxification of the bioactive arene oxide metabolite of phenytoin and polycyclic hydrocarbons found in cigarette smoke. Another enzyme that accelerates phenytoin arene oxide detoxification is glutathione S-transferase μ (GSTM). Our objective was to study the occurrence of selected DNA polymorphisms in genes encoding the drug metabolism enzymes, mEH and GSTM in individuals affected with FHS, fetal hydantoin effects, oral clefts, congenital heart defect, and other major malformations associated with maternal phenytoin use during the first trimester. We also studied infants born with an orofacial cleft whose mothers smoked tobacco during their pregnancy. The methods included polymerase chain reaction and allele-specific restriction enzyme digest on DNA samples of affected and non-affected individuals. Results: The allele frequencies at EPHX1 codon 113 in the phenytoin exposure population appeared to differ from the reported allele frequencies by Hassett et al. 1994, however statistically there was no significant difference where p = 0.14 . There was no apparent correlation between the EPHX1 and GSTM1 genotypes examined and mEH enzyme activity or phenotype associated with FHS. Our observed allele frequencies at EPHX1 codon 113 for the cigarette smoke exposure population were not comparable to allele frequencies reported by Hassett et al. 1994, where p = 0.0048. Our prevalence of the GSTM1 null genotype for the cigarette smoke exposure population was significantly higher than the prevalence of approximately 50 percent reported by others, where p < 0.0001. Overall these data do not support an association between the EPHX1 or GSTM1 polymorphisms tested and cigarette smoking-induced oral clefting. In conclusion, individual variation in mEH and GSTM activity may depend on factors other than those gene polymorphisms studied to date. Possible undetermined factors include additional 5' polymorphisms, post-transcriptional regulation, gene induction by environmental agents and variation in the levels of transcriptional proteins required for mEH expression. The participation of these factors must be characterized more definitively in order to allow prediction of individual activities on the basis of these integrated phenomena.