Enamelin (Enam) is essential for amelogenesis: ENU-induced mouse mutants as models for different clinical subtypes of human amelogenesis imperfecta (AI)

Amelogenesis imperfecta (Al) is a group of commonly inherited defects of dental enamel formation, which exhibits marked genetic and clinical heterogeneity. The genetic basis of this heterogeneity is still poorly understood. Enamelin, the affected gene product in one form of Al (AIH2), is an extracellular matrix protein that is one of the components of enamel. We isolated three ENU-induced dominant mouse mutations, M100395, M100514 and M100521, which caused Al-like phenotypes in the incisors and molars of the affected individuals. Linkage analyses mapped each of the three mutations to a region of chromosome 5 that contained the genes encoding enamelin (Enam) and ameloblastin (Ambn). Sequence analysis revealed that each mutation was a single-base substitution in E nam . M100395 (Enam Rgsc395 ) and M100514 (Enam Rgsc514 ) were putative missense mutations that caused S to I and E to G substitutions at positions 55 and 57 of the translated protein, respectively. Enam Rgsc395 and Enam Rgs514 heterozygotes showed severe breakage of the enamel surface, a phenotype that resembled local hypoplastic Al. The M100521 mutation (Enam Rgsc521 ) was a T to A substitution at the splicing donor site in intron 4. This mutation resulted in a frameshift that gave rise to a premature stop codon. The transcript of the Enam Rgsc521 mutant allele was degraded, indicating that Enam Rgsc521 is a loss-of-function mutation. Enam Rgsc521 heterozygotes showed a hypomaturation-type Al phenotype in the incisors, possibly due to haploinsufficiency of Enam. Enam Rgsc521 homozygotes showed complete loss of enamel on the incisors and the molars. Thus, we report here that the Enam gene is essential for amelogenesis, and that mice with different point mutations at Enam may provide good animal models to study the different clinical subtypes of Al.