Our data suggest that variants in alcohol-metabolizing genes can modify the teratogenic effect of maternal alcohol consumption. The association of heavy alcohol drinking with the risk of oral clefts was present only if either the mother or the baby carried the ADH1C variant that reduced ethanol-to-acetaldehyde oxidation. There was no evidence of risk from alcohol drinking if the mother and infant both had only high-activity variants, and there was no evidence of risk from ADH1C genotype if the mother abstained.
This association is credible on several counts. First, the study was large and population-based, with detailed information on exposures that was collected soon after delivery and DNA samples from a high proportion of infants and their parents. Second, the teratogenicity of alcohol has been demonstrated in animal models (23
). A possible mechanism for alcohol-induced embryonic malformations is ethanol inhibition of retinoic acid synthesis during embryogenesis (24
). When consumed at high levels, ethanol competitively inhibits the production of retinoic acid (a metabolite of vitamin A), which is necessary for normal cranial neural crest development. Furthermore, alcohol is established as a teratogen in humans, most clearly in the etiology of fetal alcohol syndrome (1
). Third, genetic variations in alcohol metabolism plausibly modify the effective dose of a given amount of maternal drinking, as suggested by the absence of any effect of binge drinking in the high-activity group in our study.
Both the gene and the exposure have been associated with oral clefts. In our data, heavy alcohol consumption was associated with oral clefts without regard to genotype. This has been seen in some other epidemiologic studies (26
), though not all (29
). Such inconsistency could be due in part to the large amounts of alcohol necessary to produce this defect; heavy drinking during pregnancy is uncommon, and the small numbers of exposed women in many studies have made it difficult to assess this association. Infant ADH1C
genotype was associated with the risk of cleft lip and palate in our case-triad data and confirmed in a Danish data set (7
). Furthermore, alcohol-metabolizing genes are expressed in placental tissue during the first trimester of pregnancy (12
), when the critical stages of facial development occur.
genes contribute to alcohol-metabolizing potential by functioning as homo- or heterodimers (3
). Strong linkage disequilibrium across the ADH
gene region makes it difficult to separate the effects of individual ADH
genes. The highly active ADH1B
variant was too uncommon (minor allele frequency in controls = 0.032) to examine its possible effect, along with maternal alcohol consumption, on oral cleft risk. However, excluding mothers and infants with this ADH1B
variant moderately increased the risk estimate for clefts among persons in the intermediate-activity ADH1C
group. This suggests a possible interplay among ADH
genes, such that an increased metabolic rate in one may counteract the effect of slower metabolic rates in others.
While these findings are plausible, this study had some limitations. First, we cannot exclude the role of chance. The multiplicative interaction of alcohol effects across the high-activity and reduced-activity groups () did not reach statistical significance. Ideally we would like to replicate our findings in another population, but such a study would require DNA from case and control mothers and babies and information on maternal alcohol consumption in early pregnancy. French investigators who considered this question with a smaller study sample had participants with lower levels of drinking and little statistical power to detect genetic susceptibility to alcohol teratogenicity (31
Recall bias is a potential concern. Mothers who gave birth to healthy infants may have been more or less likely to admit to drinking alcohol during pregnancy than mothers of infants with oral clefts; this would have biased the association. However, it is unlikely that such bias would vary by the genotype of the mother or the infant.
Another important source of uncertainty regarding these results is the stronger association between binge drinking and oral clefts in the intermediate-activity group than in the low-activity group—not the dose-response pattern that would be predicted (17
). Given the imprecision of these estimates, the observed pattern may have been due to chance. It is also theoretically possible that a woman who drinks heavily and has slow alcohol metabolism (or whose fetus has slow alcohol metabolism) may be more likely to experience other alcohol-related problems such as infertility or fetal loss. Such women would not have entered our study, and their absence would have led to an underestimate of risk in this group. The role of ADH1C
in modifying the association between maternal drinking and oral clefts was unclear when mothers’ and children's genotypes were examined separately. Unless the mother's and child's genotypes are the same, classification by either mothers’ or children's genotypes misclassifies some proportion of the other group, thus potentially clouding a genetic effect. The finding of genetic susceptibility in the combined analysis suggests the importance of both maternal and fetal genotypes when exploring genetic susceptibility in pregnancy-related outcomes.
Taking all these factors into consideration, the data provide coherent—but not conclusive—evidence that possession of the slow-metabolizing ADH1C
variant by either the mother or the fetus increases the vulnerability of the fetus to alcohol-related oral clefts. This finding adds support to a causal interpretation of alcohol as a cause of oral clefts. Given that the majority of women and children of European descent carry at least 1 haplotype of the slow-metabolizing variant, these findings provide yet another reason for such women to be cautious in their alcohol consumption when considering pregnancy. Furthermore, data on maternal and fetal genetic susceptibility may help to identify other effects of drinking during pregnancy on a wider range of fetal problems that have been suspected but not proven, including other birth defects and impairment of childhood neurodevelopment (32