Relationship of Phenotype and Genotype in X-Linked Amelogenesis Imperfecta

J. T. Wright, P. S. Hart, M. J. Aldred, K. Seow, P. J. M. Crawford, S. P. Hong, C. W. Gibson, T. C. Hart

X-linked amelogenesis imperfectas (AI) resulting from mutations in the amelogenin gene (AMELX) are phenotypically and genetically diverse. Amelogenin is the predominant matrix protein in developing enamel and is essential for normal enamel formation. To date, 12 allelic AMELX mutations have been described that purportedly result in markedly different expressed amelogenin protein products. We hypothesize that these AMELX gene mutations result in unique and functionally altered amelogenin proteins that are associated with distinct amelogenesis imperfecta phenotypes. The AMELX mutations and associated phenotypes fall generally into three categories. (1) Mutations (e.g., signal peptide mutations) causing a total of loss of amelogenin protein are associated with a primarily hypoplastic phenotype (though mineralization defects also can occur). (2) Missense mutations affecting the N-terminal region, especially those causing changes in the putative lectin-binding domain and TRAP (tyrosine rich amelogenin protein) region of the amelogenin molecule, result in a predominantly hypomineralization/hypomaturation AI phenotype with enamel that is discolored and has retained amelogenin. (3) Mutations causing loss of the amelogenin C terminus result in a phenotype characterized by hypoplasia. The consistent association of similar hypoplastic or hypomineralization/hypomaturation AI phenotypes with specific AMELX mutations may help identify distinct functional domains of the amelogenin molecule. The phenotype-genotype correlations in this study suggest there are important functional domains of the amelogenin molecule that are critical for the development of normal enamel structure, composition, and thickness.