Phenotype-genotype correlations in mouse models of amelogenesis imperfecta caused by Amelx and Enam mutations

Abstract

Mutations in human and in mouse orthologous genes Amelx and Enam result in a diverse range of enamel defects. In this study we aimed to investigate the phenotype-genotype correlation between the mutants and the wild-type controls in mouse models of amelogenesis imperfecta using novel measurement approaches. Ten hemi-mandibles and incisors were dissected from each group of Amelx(WT), Amelx(X/Y64H), Amelx(Y/Y64H), Amelx(Y64H/Y64H), and Enam(WT), Enam(Rgsc395) heterozygous and Enam(Rgsc395) homozygous mice. Their macro-morphology, colour and micro-topography were assessed using bespoke 2D and 3D image analysis systems and customized colour and whiteness algorithms. The novel methods identified significant differences (p ≤ 0.05) between the Amelx groups for mandible and incisor size and enamel colour and between the Enam groups for incisor size and enamel colour. The Amelx(WT) mice had the largest mandibles and incisors, followed in descending order of size by the Amelx(X/Y64H), Amelx(Y/Y64H) and Amelx(Y64H/Y64H) mice. Within the Enam groups the Enam(WT) incisors were largest and the Enam(Rgsc395) heterozygous mice were smallest. The effect on tooth morphology was also reflected by the severity of the enamel defects in the colour and whiteness assessment. Amelogenin affected mandible morphology and incisor enamel formation, while enamelin only affected incisors, supporting the multifunctional role of amelogenin. The enamelin mutation was associated with earlier forming enamel defects. The study supported the critical involvement of amelogenin and enamelin in enamel mineralization.

Fig. 1

Hemi-mandible and mandibular incisor 2D morphological measurements. a Hemi-mandible 2D morphometric variables: (1) overall length (mm); (2) ascending height (mm); (3) basal length (mm); (4) mandible angle (degrees); (5) coronoid-coronoid (mm); (6) diagonal length (mm); (7) mandible area (mm²), and (8) mandible perimeter (mm). b Mandibular incisor 2D morphometric variables: (1) overall length (mm); (2) angle of curvature (degrees); (3) width at midpoint (mm); (4) labial length (mm); (5) incisor perimeter (mm), and (6) incisor area (mm²). Left hemi-mandible and mandibular incisor shown from the buccal view. Scale bar = 11.0 mm.

Fig. 2

A selection of mandibular incisor 3D morphological measurements: (1) the line of x's demonstrate the path of the actual labial-length, whilst the straight line shows the projected 2D length; (2) actual width-at-midpoint (mm); (3) actual perimeter (mm); (4) marked surface-area (mm²).

Fig. 3

Mandibular incisor calibrated colour and whiteness. a Whole enamel surface region outlined. b Automatically separated cervical, middle and incisal anatomical surface region developmental stages. An automated algorithm calculated CIE L, A and B, and WI colour space values for each of the four regions of interest. Polarized images removed interference from surface reflections. Left mandibular incisor shown from the labial view.

Fig. 4

Mandibular incisor surface roughness assessment. Labial surface enamel regions, cervical (a), middle (b) and incisal (c), represent the stages of enamel formation, respectively. Right mandibular incisor shown from the labial view. Left: proximal end, right: distal tip.