Smoking behavior and the C677T allele of the methylenetetrahydrofolate reductase (MTHFR) gene.

Although little is known about the speci®c genetic factors that in ̄uence cigarette smoking behavior, it is known that genetic factors make a substantial contribution to tobacco use in both humans and animals [Collins, 1990]. For example, in a study of female twin pairs, intrapair correlation for smoking was higher for MZ twins (.797) than DZ twins (.443) [Heath et al., 1999]. In the Virginia Twin Study of Adolescent Behavioral Development, heritabilities for liabilities to lifetime and current tobacco use were estimated to be 84% and 82% respectively [Maes et al., 1999]. The ®nding that polymorphic variation of the gene coding for the P450 enzyme CYP2A6 that metabolizes nicotine correlated with smoking behavior raised the possibility that individuals with impaired nicotine metabolism needed fewer cigarettes to maintain constant blood and brain nicotine concentrations and that the unpleasant effects experienced by people learning to smoke lasted longer in people with impaired nicotine metabolism [Pianezza et al., 1998]. Another genetic factor that may in ̄uence smoking behavior is a polymorphic variation of the MAOB gene in individuals with Parkinson disease (PD) compared with control individuals [Checkoway et al., 1998]. For unknown reasons, individuals with PD are less likely to smoke than control individuals. The study con®rmed the reduced risk of PD among individuals who had ever smoked (adjusted odds ratio [AOR] 0.46, 95% con®dence interval [CI] 0.26±0.81, P0.007). A reduced risk of PD related to pack-years of smoking was found for individuals with the G allele of MAOB (AOR 0.24, 95% CI 0.10±0.55) but an increased risk of PD was found for individuals carrying the A allele (AOR 1.44, 95% CI 0.45±4.68). These ®ndings suggested an unexplained interaction between cigarette smoking and this MAOB polymorphism. In the course of a study of the methylenetetrahydrofolate reductase (MTHFR) gene C677T allele as a risk factor for low birth weight and preterm delivery, we found that this allele was unequally distributed between smokers and non-smokers. Data were examined from 232 gravidas who were participants in the Camden Study of Adolescent Pregnancy. All of these individuals had identi®ed themselves as Hispanic and all had given their country of origin of Hispanic descent as Puerto Rico. Hispanics were selected because population strati®cation has been demonstrated for the C677T allele, which has a lower frequency among individuals of African±American background [Motulsky, 1996; Stevenson et al., 1997]. Individuals of Puerto Rican background are a syncretic blend of three populations (Caribe Native Americans, Africans, and Spanish). DNA was extracted from buffy coat leukocytes of each gravida. Data collected included smoking history (cigarettes per day), parity, ethnicity, circulating folate, and time of blood drawing in gestation. To determine genotypes for the C677T allele of MTHFR, genomic DNA was ampli®ed by the polymerase chain reaction (PCR) using speci®c primer pairs [Frosst et al., 1995]. Hinf Idigested PCR products were separated by acrylamide gel electrophoresis. Genotypes were carried out in duplicate and checked for consistency; ambiguous or inconsistent samples were repeated in duplicate. No other gene loci, e.g. those related to nicotine metabolism, were studied since the goal of the study was to determine whether the C677T allele was a major contributor to adverse pregnancy outcome. Logistic regression was used to examine the in ̄uence of homozygosity for the heat-labile allele (genotype T,T) on smoking status controlling for potential confounding variables (age, parity, clinic pay status, inadequate gestational gain). Adjusted odds ratios (AOR's) and 95% con®dence intervals (CIs) were computed from the logistic regression coef®cient and the corresponding covariance matrix. Genotype frequencies for the sample (n232) were: genotype C,C (44.8%, n104), genotype C,T (47.8%, n 111), and genotype T,T (7.3%, n 17). The observed distribution of genotypes was compared with that expected according to the Hardy-Weinberg equilibrium: no difference was found (w1.57, 2df, 2-sided P 0.46). Maternal genotype did not signi®cantly in ̄uence background and pregnancy characteristics Grant sponsor: NIEHS Center of Excellence at UMDNJRWJMS; Grant number: P30ES05022-10; Grant sponsor: National Institutes of Health; Grant numbers: HD-18269, ES07437; Grant sponsor: NJ Governor's Council Fellowship on Prevention of Mental Retardation and Developmental Disabilities.