Amelogenesis Imperfecta; Genes, Proteins, and Pathways

Abstract

Amelogenesis imperfecta (AI) is the name given to a heterogeneous group of conditions characterized by inherited developmental enamel defects. AI enamel is abnormally thin, soft, fragile, pitted and/or badly discolored, with poor function and aesthetics, causing patients problems such as early tooth loss, severe embarrassment, eating difficulties, and pain. It was first described separately from diseases of dentine nearly 80 years ago, but the underlying genetic and mechanistic basis of the condition is only now coming to light. Mutations in the gene AMELX, encoding an extracellular matrix protein secreted by ameloblasts during enamel formation, were first identified as a cause of AI in 1991. Since then, mutations in at least eighteen genes have been shown to cause AI presenting in isolation of other health problems, with many more implicated in syndromic AI. Some of the encoded proteins have well documented roles in amelogenesis, acting as enamel matrix proteins or the proteases that degrade them, cell adhesion molecules or regulators of calcium homeostasis. However, for others, function is less clear and further research is needed to understand the pathways and processes essential for the development of healthy enamel. Here, we review the genes and mutations underlying AI presenting in isolation of other health problems, the proteins they encode and knowledge of their roles in amelogenesis, combining evidence from human phenotypes, inheritance patterns, mouse models, and in vitro studies. An LOVD resource (http://dna2.leeds.ac.uk/LOVD/) containing all published gene mutations for AI presenting in isolation of other health problems is described. We use this resource to identify trends in the genes and mutations reported to cause AI in the 270 families for which molecular diagnoses have been reported by 23rd May 2017. Finally we discuss the potential value of the translation of AI genetics to clinical care with improved patient pathways and speculate on the possibility of novel treatments and prevention strategies for AI.

Keywords: LOVD; Leiden Open Variant Database; ameloblasts; amelogenesis; amelogenesis genetics; amelogenesis imperfecta; biomineralization; enamel.

Figure 1

Ameloblast morphology, crystal development, and the final structure of the enamel. (A) Schematic cross section of the murine incisor. (B–D) Histology of the murine incisor. (B) During secretion, ameloblasts exhibit an elongated morphology with a cellular extension (the Tomes' process); (C) during transition the Tomes' process degenerates and the ameloblasts begin to reduce in height; (D) during maturation, the ameloblasts remove nearly all protein from the developing enamel and supply mineral ions to support crystallite growth; (E) immature enamel crystalites form during secretion by growth in their long axis; (F) by the end of secretion, the developing enamel is around 30% mineral and 25% matrix protein, with the remainder tissue fluid. By the end of maturation, the enamel is nearly 100% mineral; (G) the enamel crystallites grow in width and thickness during enamel maturation; (H) and (I) murine enamel has a decussating arrangement of enamel prisms; (J) and (K) human enamel is also arranged in a prismatic structure. Elements from this figure have been adapted from previously published figures and we acknowledge the following publications and publishers for the elements specified: Panels (A–D) were previously published by Barron et al. (2010). Panel (E) was previously published by Robinson (2014).

Figure 2

Schematic diagram depicting the main events during amelogenesis. (A) Pre-secretory stage: Ameloblasts (blue) differentiate from the cells of the inner enamel epithelium (IEE), in response to reciprocal signaling between the IEE and the dental papilla. The basal lamina between the IEE and dental papilla breaks down so that the cells are in contact with the pre-dentine. The ameloblasts elongate and their nuclei shift to the proximal side of the cell, nearest the stratum intermedium (SI), resulting in reversal of the ameloblasts' polarity. At the distal end, closest to the pre-dentine, the Golgi apparatus and rough endoplasmic reticulum increase in size to increase the capacity for protein production, post translational modification and secretion. The non-dividing cell becomes further polarized as it forms a distal extension that will go on to form the Tomes' process (TP). Each ameloblast develops and maintains anchoring junctions to hold the ameloblast layer in alignment and to control what passes between them. (B–D) Secretory stage: During the secretory stage, a proteinaceous extracellular matrix is secreted from the ameloblast TP, as the ameloblast layer retreats from the dentine layer. To achieve this, ameloblasts produce large amounts of membrane bound, secretory granules containing enamel matrix proteins (EMPs). EMPs are constitutively secreted via exocytosis into the extracellular space at the distal end of the cell, on to the newly formed dentine. (B) Mineral immediately forms in this initial enamel matrix and forms a close association with the dentine mineral. This will form the aprismatic enamel. (C) The ameloblasts begin to move away from the dentine and further develop their TP at the distal end. EMPs are secreted from two aspects of the ameloblasts to produce enamel matrix that will go on to form the prismatic and interprismatic enamel. (D) As secretion progresses the TP lengthens and thins. The portion secreting the prismatic enamel is reduced before secretion ceases, therefore the final enamel formed will be aprismatic. (E) Transition stage: The transition stage is characterized by reduced EMP secretion and internal reorganization of the ameloblasts. Ameloblasts shorten to around half their original height and reduce in volume. Their nuclei become more central and the ER is reduced in size. The TP is completely lost and an atypical basal lamina is formed against the enamel matrix. Ameloblasts adhere to the enamel matrix via hemidesmosomes. The cells of the SI, stellate reticulum and the outer enamel epithelium form the papillary layer (PL). Capillaries invaginate into this layer and overlay the ameloblasts. The cells of the PL may assist ameloblasts in the maturation stage by participating in ion transport and removal of enamel protein products and water from the developing enamel. The ameloblast population reduces by around 25% at this stage through apoptosis. (F) and (G) Maturation stage: During the maturation stage the partially mineralized enamel matrix becomes fully mineralized by the breakdown and removal of residual EMPs, and the growth in width and thickness of enamel crystallites. These processes are achieved through repeated cyclical processes. The ameloblasts act as a gated barrier for the movement of ions and degraded proteins between the SI and the developing enamel and vice versa. To achieve this, the ameloblast membrane facing the enamel matrix modulates between ruffle ended (F) and smooth ended (G) morphologies. This is achieved in coordinated groups of ameloblasts across the developing enamel. Ruffle ended ameloblasts (RA) form membrane invaginations and tight junctions at the apical end, near the enamel surface, whereas smooth ended ameloblasts (SA) are more leaky. Enamel crystal growth generates large amounts of protons but it has also been shown that protons are pumped into the enamel by RA. Both RA and SA release bicarbonate ions into the enamel that act as a buffer to increase pH. A mildy acidic pH is found in enamel at RA regions and a more neutral pH in SA regions. During maturation around 25% of ameloblasts apoptose. (H) Post-maturation stage: The ameloblasts and other cells of the enamel organ, form the reduced enamel epithelium, which eventually contributes to the junctional epithelium of mature teeth. However, many of the ameloblasts apoptose before the formation of the junction epithelium is completed.

Figure 3

Clinical images that illustrate the variability of AI. (A) Hypoplastic AI is characterized by teeth without the curves associated with a normal enamel volume. (B) In hypomaturation AI enamel volume can be near-normal, but opaque with structural weaknesses that result in rapid post-eruptive enamel loss with enamel fracturing away to exposure the underlying dentine. (C) Brown discolouration and early post-eruptive enamel loss is typical of hypomineralised forms of AI. (D) Mixed AI phenotypes are frequently encountered. In this example a near-normal enamel volume is characterized by multiple focal pits that are most evident on the inset image, with variable colouration that includes focal opacities, but without premature fracturing of the enamel to reveal dentine.

Figure 4

The three “compartments” involved in amelogenesis: the enamel space (extracellular matrix; red), the enamel organ (ameloblast cell layer and supporting cells; blue) and the interface between the two (purple). Known AI proteins are grouped according to functions within compartments. Those with no causative mutations for AI identified are designated by square brackets.