Human enamel thickness and ENAM polymorphism

The tooth enamel development gene, enamelin (ENAM), showed evidence of positive selection during a genome-wide scan of human and primate DNA for signs of adaptive evolution. The current study examined the hypothesis that a single-nucleotide polymorphism (SNP) C14625T (rs7671281) in the ENAM gene identified in the genome-wide scan is associated with a change in enamel phenotype. African Americans were selected as the target population, as they have been reported to have a target SNP frequency of approximately 50%, whereas non-Africans are predicted to have a 96% SNP frequency. Digital radiographs and DNA samples from 244 teeth in 133 subjects were analysed, and enamel thickness was assessed in relation to SNP status, controlling for age, sex, tooth number and crown length. Crown length was found to increase with molar number, and females were found to have thicker enamel. Teeth with larger crowns also had thicker enamel, and older subjects had thinner enamel. Linear regression and generalized estimating equations were used to investigate the relationship between enamel thickness of the mandibular molars and ENAM SNP status; enamel in subjects with the derived allele was significantly thinner (P=0.040) when the results were controlled for sex, age, tooth number and crown length. The derived allele demonstrated a recessive effect on the phenotype. The data indicate that thinner dental enamel is associated with the derived ENAM genotype. This is the first direct evidence of a dental gene implicated in human adaptive evolution as having a phenotypic effect on an oral structure. A gene implicated in human evolution seems to have helped make our tooth enamel thinner. Scientists previously showed, using statistical analyses, that a variant of the tooth enamel development gene ENAM was favored by natural selection. Frank Roberts from the University of Washington, Seattle, USA, and colleagues compared tooth characteristics with ENAM gene differences in African-Americans, in whom the favored variant and the original variant occur in roughly equal proportions. Controlling for age, sex, tooth number and crown length, the researchers found that changes in the ENAM gene affected the thickness of the tooth enamel. Specifically, the variant that conferred an advantage in our species’ evolutionary past produced thinner dental enamel. According to the authors, this is the first direct evidence linking a gene with a known effect on tooth structure to adaptive evolution.