MOLECULAR GENETICS OF AMELOGENESIS IMPERFECTA

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Amelogenesis Imperfecta (AI) is a collection of several genetic disorders affecting the formation of tooth enamel. This heterogeneous group of diseases not only present different types of inheritance (e.g. X-linked dominant or recessive and autosomal dominant or recessive) but also different clinical manifestations (e.g. hypoplasia, hypocalcification and hypomaturation). The etiology of AI, which occurs in the general population in a ratio of about 1:14,000 in the USA, remains largely unknown except for two families with x_linked AI where the defect was mapped to the Xp22 chromosome; the chromosomal position for the amelogenin structural gene. A deletion in the amelogenin gene was demonstrated in one of these families (Lagerstrom et al, 1991), and a non-sense mutation in the amelogenin gene was demonstrated in another X-linked AI individual (Aldred et al, 1992). Clearly, due to the heterogeneity presented by this disease, it appears that different types of mutations in the amelogenin gene could be responsible for the X-linked cases. Furthermore, autosomal inherited types of AI, which account for approximately 85% of all AI cases, can not be attributed to the amelogenin gene. This study proposes to characterize the molecular genotype of AI, in particular the autosomal inherited types of AI. We will test the hypothesis that autosomal types of AI are the result of defects in the structural or regulatory genes for non-amelogenin proteins like the acidic enamel proteins (tuftelin) or the enamel proteases involved in the processing of structural proteins. We propose to test this hypothesis by characterizing the human genes for the acidic enamel proteins and enamel proteases and then screen for mutations in DNA obtained from patients with autosomal inherited AI. In addition, we will screen for mutations in the amelogenin gene (and perhaps other genes) in X-linked inherited AI. Furthermore, the possibility of Y-linked inheritance, although very rare, will be explored. The following Specific Aims will be pursued: To characterize the major(s) mouse tuftelin cDNAs; determine the chromosomal localization of human tuftelins; determine the human genomic structure and organization of the tuftelin gene; determine the linkage of tuftelin to autosomal inherited types of AI; determine the human genomic structure and organization of the enamel-proteases genes; to examine and define the molecular basis of autosomal AI; examine and define the molecular basis of X-linked AI and examine and define the molecular basis for Y-linked AI. The results obtained will correlate specific gene products to the different types of AI, and will define the genetic alterations responsible for abnormal gene function(s). The biological significance of our molecular genetic findings will be determined by a correlation between the structural characterization of these proteins and the functional information obtained in other parts of this Program Project. The integration between our three subprojects will define the role of enamel proteins in biomineralization, and provide a basis to understand tissue-mediated mineralization during normal and abnormal enamel formation.