The molecular biological analysis of autosomal amelogenesis imperfecta, and the gene diagnosis.

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Amelogenesis imperfecta is a group of inherited disorders affecting enamel formation that are characterized by clinical and genetic heterogeneity. It is genetically classified into two forms, X-linked type caused by the mutated amelogenin gene and autosomal type. So far, mutations of the gene encoding enamelin, kalliklein 4, enamelysin and DLX3 were found to cause autosomal amelogenesis imperfecta, and they are likely to be much more prevalent than X-linked form. Furthermore, ameloblastin is one of the extracellular matrix proteins in tooth enamel and thought to be responsible for autosomal amelogenesis imperfecta since tests of ameloblastin-null mice developed severe enamel hypoplasia. Hence, the ameloblastin gene is also considered to be a candidate responsible for autosomal amelogenesis imperfecta. We investigated the ameloblastin, enamelin and enamelysin genes of 4 Japanese patients using single-strand conformational polymorphism (SSCP) analysis and DNA sequencing. One single nucleotide substitution and a trinucleotide deletion were identified in the translated region of the ameloblastin gene. However, the nucleotide substitution does not result in the change of the encoded amino acid residue, which means it is a synonymous substitution of amino acid residue. The trinucleotide deletion is a gene polymorphism that has no effect on the phenotype of human tooth at least in Japanese as we reported in the previous grant 'Scientific Research (B) no.15390633'. Subsequently, we focused our attention on the promoter regions of ameloblastin gene of the patients. As a consequence, the amplification product by PCR (polymerase chain reaction) derived from a patient was composed of two products and one of which showed the smaller size the expected. It indicates that one of the allelic genes might have a deletion in the promoter region of the amelobnlastin gene of the patient.