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

Abstract

Amelogenesis imperfecta (AI) 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 AI (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 AI-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 Enam. 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(Rgsc514) heterozygotes showed severe breakage of the enamel surface, a phenotype that resembled local hypoplastic AI. 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 AI 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 AI.

Enamel Defects and Ameloblast-specific Expression in Enam Knock-out/lacZ Knock-in Mice

“…ENAM mutations show a dose effect, so that a single mutant allele causes a mild form of amelogenesis imperfecta, whereas having defects in both alleles eliminates the enamel layer (22,23). Mutations in the mouse enamelin gene have been induced with the mutagen N-ethyl-N-nitrosourea, and four separate Enam point mutations have been identified: p.S55I, pE57G, the splice donor site in exon 4, and pQ176X (24,25). The resulting enamel phenotypes include a rough and pitted enamel surface in heterozygous mice and enamel agenesis in the null condition.…”

Amelogenin Supra-Molecular Assembly in vitro Compared with the Architecture of the Forming Enamel Matrix

“…A recent report on ameloblastin-null mice has documented that ameloblastin functions as a cell adhesion protein controlling the differentiation state of the ameloblasts [Fukumoto et al, 2004]. As documented by recent transgenic and null mice model studies as well as linkage analysis of human pedigrees in cases of amelogenesis imperfecta, amelogenin, enamelin, ameloblastin, and enamelysin -matrix metalloproteinase (MMP-20) -have been shown to be critical for normal enamel formation [Lagerstrom-Fermer and Landegren, 1995;Gibson et al, 2001;Caterina et al, 2002;Paine et al, 2003;Fukumoto et al, 2004;Masuya et al, 2005].…”

Premature Stop Codon inMMP20Causing Amelogenesis Imperfecta

“…This principle also applies to the stages of amelogenesis. Amelogenin, enamelin and ameloblastin null mice all exhibit thin or absent enamel layers, which correlate with their critical expression during the secretory stage (Fukumoto et al, 2004;Gibson et al, 2001;Masuya et al, 2005). Humans with AMELX mutations in males (Kida et al, 2007) or ENAM mutations affecting both alleles (Ozdemir et al, 2005) have severely hypoplastic (thin) enamel.…”