2025 AMA Research Challenge – Member Premier Access

October 22, 2025

Virtual only, United States

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Background Molar–incisor hypomineralization (MIH) is a common developmental enamel defect in children, leading to higher risk of caries, post-eruptive enamel breakdown, and dental pain. Proper enamel formation depends on the expression and function of matrix proteins like ameloblastin (Ambn) and the protease matrix metalloproteinase 20 (Mmp20), which regulates mineral deposition and organic matrix removal during amelogenesis. We hypothesized that enamel hypomineralization caused by Ambn overexpression in mice could be rescued by increasing Mmp20 expression to restore the correct substrate-to-enzyme ratio essential for healthy enamel.

Methods Overexpression of Ambn and Mmp20 was achieved in separate mouse lines from transgenes (Tg). Ambn-Tg mice exhibited MIH-like phenotypes, and Mmp20-Tg mice displayed hypoplastic amelogenesis imperfecta. Crossbreeding of Ambn-Tg with Mmp20-Tg generated double-Tg mice overexpressing Ambn and Mmp20. Mandibular incisors of 7-week old mice (n=5/grp) were analyzed from the apical region to the incisal edge in sequential 1.0 mm volumes of interest (5 µm pixels) using isotropic micro-CT. Low, medium, and high thresholds were applied to quantify enamel volume and percent mineral volumes. Enamel mineral density was calibrated to hydroxyapatite phantoms (.25 and .75 g/cm3). We compared the Ambn-Tg+Mmp20-Tg to the wildtype (WT) controls. We ran a single factor ANOVA test and a two-tailed T-test to determine if there is a significant difference between the groups.

Results At eruption, the volumes of middle and high-density enamel in the Ambn and Mmp20 Tg’s demonstrated significant differences from the WT. Ambn-Tg had more middle density enamel volume (p=0.007). The Mmp20-Tg had significantly greater middle density (p=0.00001) and less higher density enamel (p=0.0000001) volumes compared to the WT, respectively. Ambn-Tg+Mmp20-Tg middle density and high-density enamel volumes were similar to the WT. Calculated enamel mineral densities were 38% (p=0.00001) and 8% (p=0.0005) lower in the Mmp20-Tg and Ambn-Tg groups compared to the WT, respectively. The Ambn-Tg+Mmp20-Tg enamel mineral density was similar to the WT. Visual analysis indicated that the incisor enamel of Mmp20-Tg and Ambn-Tg groups was distributed in circular patches containing regions of lower and higher enamel density. A similar visual inspection indicated that the Ambn-Tg+Mmp20-Tg enamel was distributed like the WT.

Conclusion These results show that enamel mineralization is highly sensitive to the balance of Ambn and Mmp20 expression. Imbalances can cause hypomineralized or hypoplastic enamel, while restoring the substrate-to-enzyme ratio can rescue normal enamel formation. This underscores the importance of maintaining balanced extracellular matrix protein and enzyme levels for healthy, functional enamel development.

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