A nomenclature for X-linked amelogenesis imperfecta

Enamelin and autosomal-dominant amelogenesis imperfecta

Dental enamel forms as a progressively thickening extracellular layer by the action of proteins secreted by ameloblasts. The most abundant enamel protein is amelogenin, which is expressed primarily from a gene on the X-chromosome (AMELX). The two most abundant non-amelogenin enamel proteins are ameloblastin and enamelin, which are expressed from the AMBN and ENAM genes, respectively. The human AMBN and ENAM genes are located on chromosome 4q13.2. The major secretory products of the human AMELX, AMBN, and ENAM genes have 175, 421, and 1103 amino acids, respectively, and are all post-translationally modified, secreted, and processed by proteases. Mutations in AMELX have been shown to cause X-linked amelogenesis imperfecta (AI), which accounts for 5% of AI cases. Mutations in ENAM cause a severe form of autosomal-dominant smooth hypoplastic AI that represents 1.5%, and a mild form of autosomal-dominant local hypoplastic AI that accounts for 27% of AI cases in Sweden. The discovery of mutations in the ENAM gene in AI kindreds proved that enamelin is critical for proper dental enamel formation and that it plays a role in human disease. Here we review how enamelin was discovered, what is known about enamelin protein structure, post-translational modifications, processing by proteases, and its potentially important functional properties such as its affinity for hydroxyapatite and influence on crystal growth in vitro. The primary structures of human, porcine, mouse, and rat enamelin are compared, and the human enamelin gene, its structure, chromosomal localization, temporal and spatial patterns of expression, and its role in the etiology of amelogenesis imperfecta are discussed.

The small bovine amelogenin LRAP fails to rescue the amelogenin null phenotype

Amelogenins are the most abundant secreted proteins in developing dental enamel. These evolutionarily-conserved proteins have important roles in enamel mineral formation, as mutations within the amelogenin gene coding region lead to defects in enamel thickness or mineral structure. Because of extensive alternative splicing of the primary RNA transcript and proteolytic processing of the secreted proteins, it has been difficult to assign functions to individual amelogenins. To address the function of one of the amelogenins, we have created a transgenic mouse that expresses bovine leucine-rich amelogenin peptide (LRAP) in the enamel-secreting ameloblast cells of the dental organ. Our strategy was to breed this transgenic mouse with the recently generated amelogenin knockout mouse, which makes none of the amelogenin proteins and has a severe hypoplastic and disorganized enamel phenotype. It was found that LRAP does not rescue the enamel defect in amelogenin null mice, and enamel remains hypoplastic and disorganized in the presence of this small amelogenin. In addition, LRAP overexpression in the transgenic mouse (wildtype background) leads to pitting in the enamel surface, which may result from excess protein production or altered protein processing due to minor differences between the amino acid compositions of murine and bovine LRAP. Since introduction of bovine LRAP into the amelogenin null mouse does not restore normal enamel structure, it is concluded that other amelogenin proteins are essential for normal appearance and function.

Unusual manifestations in X-linked amelogenesis imperfecta

This paper describes a female with X-linked amelogenesis imperfecta (XAI). This case is unusual in having taurodontism, pulpal calcifications, coronal defects prior to tooth eruption and unerupted teeth. These findings have been reported in some cases of autosomal dominant and autosomal recessive AI but have not previously been documented in XAI.

Identification of the enamelin (g.8344delG) mutation in a new kindred and presentation of a standardized ENAM nomenclature

The amelogenesis imperfectas (AI) are a genetically heterogeneous group of diseases that result in defective development of tooth enamel. Although X-linked, autosomal dominant and autosomal recessive forms of AI have been clinically characterized, only two genes (AMELX and ENAM) have been associated with AI. To date, three enamelin (ENAM) mutations have been identified. These mutations cause phenotypically diverse forms of autosomal dominant AI. Detailed phenotype-genotype correlations have not been performed for autosomal dominant AI due to ENAM mutations. We identified a previously unreported kindred segregating for the ENAM mutation, g.8344delG. Light and electron microscopy analyses of unerupted permanent teeth show the enamel is markedly reduced in thickness, lacks a prismatic structure and has a laminated appearance. Taken together these histological features support the enamelin protein as being critical for the development of a normal enamel thickness and that it likely has a role in regulating c-axis crystallite growth. Because there is growing molecular and phenotypic diversity in the enamelin defects, it is critical to have a nomenclature and numbering system for characterizing these conditions. We present a standardized nomenclature for ENAM mutations that will allow consistent reporting and communication.

Functional domains for amelogenin revealed by compound genetic defects

We have previously used the yeast two-hybrid assay and multiple in vitro methodologies to show that amelogenin undergoes self-assembly involving two domains (A and B). Using transgenic animals, we show that unique enamel phenotypes result from disruptions to either the A- or B-domain, supporting the role of amelogenin in influencing enamel structural organization. By crossbreeding, animals bearing two defective amelogenin gene products have a more extreme enamel phenotype than the sum of the defects evident in the individual parental lines. At the nanoscale level, the forming matrix shows alteration in the size of the amelogenin nanospheres. At the mesoscale level of enamel structural hierarchy, 6-week-old enamel exhibits defects in enamel rod organization caused by perturbed organization of the precursor organic matrix. These studies reflect the critical dependency of amelogenin self-assembly to form a highly organized enamel organic matrix, and that amelogenins engineered to be defective in self-assembly produce compound defects in the structural organization of enamel.