kaylhele oliviero adriane
Jenifer Griffard
The ENAM gene provides instructions for making a protein called enamelin, which is essential for normal tooth development. Enamelin is involved in the formation of enamel, which is the hard, white material that forms the protective outer layer of each tooth. Enamel is composed mainly of mineral crystals. These microscopic crystals are arranged in organized bundles that give enamel its strength and durability. Although the exact function of enamelin is not well understood, this protein plays a key role in the formation and growth of crystals in developing enamel.
At least 14 mutations in the ENAM gene have been identified in people with a disorder of tooth development called amelogenesis imperfecta. Mutations in this gene cause autosomal dominant and autosomal recessive forms of this condition.
In the autosomal dominant form, one copy of the ENAM gene in each cell is altered. These mutations have a variety of effects on enamel formation. Some of these mutations reduce the amount of enamelin produced from one copy of the gene. Other mutations lead to the production of an abnormally short version of enamelin that is missing critical regions. A reduced amount of enamelin or an altered version of the protein can lead to severe problems with developing enamel or cause milder defects such as shallow pits or horizontal grooves in the teeth.
In the autosomal recessive form of amelogenesis imperfecta, two copies of the ENAM gene in each cell are altered. These mutations result in the production of an abnormal version of enamelin that prevents enamel from developing properly. People who inherit two mutated copies of the ENAM gene have severe defects in their enamel; as a result, this protective covering may be very thin or completely absent.
Hu et al. (1997) cloned and characterized mRNAs for porcine enamelin. In porcine enamel, enamelin is secreted as a 186-kD precursor protein, which undergoes a series of proteolytic cleavages to generate several polypeptides that reside in various layers of the enamel matrix. The enamelin protein has a 39-amino acid signal peptide and the secretory protein has 1,103 amino acids.
Amelogenesis imperfecta (AI) is an inherited defect of dental enamel formation that shows both clinical and genetic heterogeneity. Karrman et al. (1997) mapped a locus for autosomal dominant hypoplastic amelogenesis imperfecta (AI1B; 104500) to a 4-Mb YAC contig on chromosome 4q11-q21. Rajpar et al. (2001) analyzed a 3-generation family with an autosomal dominant smooth hypoplastic form of AI. A mutation (606585.0001) in the splice donor site of intron 7 of enamelin was found in affected members of this family.
In 3 unrelated patients with severe autosomal recessive local hypoplastic amelogenesis imperfecta and class II anterior openbite malocclusion (AI1C; 204650), Hart et al. (2003) identified homozygosity for a truncating mutation (606585.0003) in the ENAM gene. Heterozygous family members showed localized pitting enamel defects.
Using N-ethyl-N-nitrosourea (ENU) mutagenesis, Masuya et al. (2005) generated mice with AI-like phenotypes in the incisors and molars of the affected animals. Linkage analysis mapped mutations to a region of chromosome 5 that contained the Enam gene, and sequence analysis revealed 3 separate point mutations.
Hu et al. (2008) generated and characterized an Enam knockout/NLS beta-gal knockin mouse. Enam +/- mice showed nearly normal enamel in the maxillary incisors, but the mandibular incisors were discolored and tended to wear rapidly where they contacted the maxillary incisors. Enam -/- mice showed no true enamel. Radiography, microcomputed tomography, and light and scanning microscopy were used to document changes in the enamel of Enam -/- mice but did not discern any perturbations of bone, dentin, or any other tissue besides the enamel layer. Although a thick layer of enamel proteins covered normal-appearing dentin of unerupted teeth, von Kossa staining revealed almost a complete absence of mineral formation in this protein layer. However, a thin, highly irregular, mineralized crust covered the dentin on erupted teeth, apparently arising from the formation and fusion of small mineralization foci in the deeper part of the accumulated enamel protein layer. The results demonstrated ameloblast-specific expression of enamelin and revealed that enamelin is essential for proper enamel matrix organization and mineralization.
Brookes et al. (2017) characterized a mouse model of AI that was caused by a G-to-T transversion in the Enam gene that changed a polar and possibly phosphorylated ser55 residue to a hydrophobic ile residue. In heterozygous S55I mutant mice, secretory stage ameloblasts appeared to function normally, but late secretory/early maturation stage ameloblasts lost contact with the underlying enamel extracellular matrix and showed vacuoles containing amelogenin, ameloblastin, and enamelin, with delay in onset of mineralization, and finally evidence of endoplasmic reticulum (ER) stress and apoptosis. Initial normal function followed by abnormal function in S55I heterozygous ameloblasts resulted in teeth with a structurally sound inner layer and impaired outer layer that was quickly lost following eruption, presumably due to mastication. Homozygous S55I mice had a more severe phenotype with absence of differentiated ameloblasts, failure to develop secretory Tomes processes, and lack of enamel deposition. Brookes et al. (2017) concluded that the S55I mutation in mouse Enam causes defects in enamel protein secretion by the ER, followed by ER stress and the unfolded protein response, leading to ameloblast apoptosis.
There is a high frequency of amelogenesis imperfecta in dog breeds, which is likely a consequence of inbreeding for a different trait. Gandolfi et al. (2013) identified a homozygous 5-bp deletion in exon 10 of the ENAM gene (c.1991_1995del) in Italian greyhounds with AI. The condition was restricted to the deciduous and permanent teeth and was manifested by enamel roughening/thinning and brownish mottling. The affected teeth were often small and pointed with increased gaps. About 14% of Italian Greyhounds were affected and 30% were carriers.
In Parson Russell terriers with AI, Hytonen et al. (2019) identified a homozygous missense mutation in exon 8 of the ENAM gene (P239L). The variant had a carrier frequency of 9% in tested animals of that breed.
In a 3-generation family with autosomal dominant hypoplastic local amelogenesis imperfecta (AI1B; 104500), Rajpar et al. (2001) identified a G-to-A transition in the splice donor site following exon 7 of the ENAM gene (c.841+1G-A). The mutation disrupted a HphI site, which permitted confirmation of cosegregation with the disease phenotype within the family. The mutation was predicted to exclude exon 7 from the mature mRNA of enamelin, but since expression is limited to ameloblasts, confirmation of the effect on splicing using RT-PCR was not possible. The mutation was not detected in 200 control chromosomes.
Song et al. (2012) stated that they had identified the same heterozygous splicing mutation, which they designated c.534+1G-A, in the ENAM gene in affected members of a 3-generation Chinese family (family 6) with hypoplastic AI. The mutation segregated with the phenotype and was not present in 200 controls.
In affected members of 6 families from northern Sweden with type IB AI (AI1B; 104500), Mardh et al. (2002) identified a heterozygous c.438A-T transversion in exon 4 of the ENAM gene, resulting in a lys53-to-ter (K53X) substitution. This nonsense mutation was predicted to result in a truncated peptide of 52 amino acids, as compared with 1142 amino acids of the full-length protein. Mardh et al. (2002) screened 6 additional small families with local hypoplastic AI and identified the same mutation in affected members of 1 family.
In 3 unrelated Turkish patients with severe autosomal recessive local hypoplastic amelogenesis imperfecta and openbite malocclusion (AI1C; 204650), Hart et al. (2003) identified a homozygous 2-bp insertion (c.13185insAG; g.13185_13186insAG) in exon 10 of the ENAM gene, resulting in a premature stop codon (Pro422fsTer448). The predicted truncated protein is 695 amino acids shorter than the wildtype protein of 1,142 amino acids. All 6 parents and other family members who were heterozygous for the mutation had localized hypoplastic enamel pitting defects without malocclusion, which the authors did not consider to be a mild form of amelogenesis imperfecta. Hart et al. (2003) concluded that the 2-bp insertion mutation manifested a dose-dependent effect.
Pavlic et al. (2007) identified the same mutation in heterozygosity in a father and son (family 1) with hypoplastic amelogenesis imperfecta (AI1B; 104500). The son presented with chalky whitish enamel with localized hypoplastic alterations. The father had openbite malocclusion and yellowish enamel with horizontal grooves in the cervical half of the crowns of the teeth.
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