Abdullatif, MAA; (2012) A pilot study of the genotype and phenotype in Amelogenesis Imperfecta and Molar Incisor Hypomineralization. Doctoral thesis , UCL (University College London).

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Abstract

Background Enamel is an external layer of the crown, and its production can be affected by genetic, systemic or environmental causes Amelogenesis Imperfecta (AI) is an inherited defect of dental enamel, and can be autosomal dominant, recessive, x-linked or sporadic. It can present as hypoplasia, hypomineralization or both. Mutations in several genes can cause defective enamel formation and have been linked to AI, e.g: AMELX (amelogenin), ENAM (enamelin), MMP20 (enamelysin) and KLK4 (kallikrein 4), although the correlation between genotype and phenotype is poorly understood. Molar Incisal Hypomineralization (MIH) is defined as an environmentally caused enamel defect of one to four permanent first molars, frequently associated with affected incisors, although the aetiology is unknown. The presence of MIH in siblings, and lack of obvious systemic cause suggests there may be an underlying genetic defect involved. When a patient presents in the early mixed dentition, it can be difficult to distinguish between AI and MIH in the absence of a clear family or medical history. Better understanding of the relationship between phenotype and genotype is required to aid diagnoses and management of these conditions. A pilot study was set up to determine the best method to collect data from patients, and establish a database to record dental anomalies. In the second part of this study, different machines were used to determine the most appropriate method to measure the physical proprieties of AI and MIH teeth. In the third part of the study DNA was extracted from AI and MIH patients to; (i) find the most common genes related to the AI patients in UK, and (ii) to check if there is genetic association in MIH patients. This was in order to correlate phenotype and genotype in AI and MIH patients. Aims To develop a dental anomalies clinic to identify patients with AI and MIH and create a data base. Apply a method to characterize phenotype vs. genotype for AI & MIH. Method Ethical approval was obtained. A dental anomalies clinic was established to record information (using DDE index) using a database in liaison with University of Strasbourg (Phenodent database). Phenotype analysis of MIH and AI teeth was done using Scanning Electron Microscope (SEM), hardness was obtained using both a Wallace indenter and an Atomic Force Microscope (AFM). To investigate the genotype, DNA was extracted from saliva samples using TaqMan protocol, and analysed for gene markers known to occur in inherited enamel defect conditions (Enam 2 Allele C and Allele T, Enam 1 Allele A and Allele G and MMp20 for Allele A and Allele T) and was applied on MIH patients for possible genetic association. Results 57 AI patients and 58 MIH patients were identified through the anomalies clinic. 8 MIH, 4 with AI and 8 control teeth were collected and analysed using SEM. Under higher magnifications, normal enamel had well organized prism and crystal structure, while the hypomineralised enamel in (AI and MIH) had less distinct prism borders and increased interprismatic space. In the AI teeth a glass like appearance and loss of prism layer were obvious. Over all, the hypomineralised enamel appeared more porous than the adjacent normal unaffected enamel. The average hardness ranged between 2.3 to 8.0 GPa for control teeth, between 0.004 to 0.027 GPa for AI teeth and from 0.07 to 0.40 for MIH teeth. Yellow/ brown opacities had lower hardness (0.07 GPa) compared to white/cream opacity (0.40 GPa). Strong association of AI and the ENAM 1 gene in UK. Conclusion Teeth diagnosed with Amelogenesis Imperfecta and Molar Incisor Hypomineralisation has significantly lower hardness values in the hypomineralised enamel compared with normal enamel. Yellow/ brown opacities had lower hardness values than white/ cream opacities. No correlation found between the phenotypic presentation of AI and MIH and the genotype of ENAM 1 was polymorphism.

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