Amelogenesis imperfecta

Amelogenesis imperfecta (AI) is a disorder of tooth development. This condition causes teeth to be unusually small, discolored, pitted or grooved, and prone to rapid wear and breakage. Other dental abnormalities are also possible, and the defects vary among affected individuals. These problems can affect both primary (baby) teeth and permanent teeth.

It is a genetic disorder because the instructions to form the proteins for the enamel are encoded in the genes and are passed from generation to generation. New mutations are also possible.

Researchers have described at least 14 forms of amelogenesis imperfecta. These types are distinguished by their specific dental abnormalities and by their pattern of inheritance.

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Features

Amelogenesis imperfecta presents with abnormal formation of the enamel or external layer of teeth. Enamel is composed mostly of mineral, that is formed and regulated by the proteins in it. AI is due to the malfunction of the proteins in the enamel: ameloblastin, enamelin, tuftelin and amelogenin.

People afflicted with amelogenesis imperfecta have teeth with abnormal color: yellow, brown or grey. The teeth have a higher risk for dental cavities and are hypersensitive to temperature changes. This disorder can afflict any number of teeth. A thesis written by Tom, Tyrone, Tri and Leighton et al 2007 confirms that odontoblasts differentiate into ameloblasts^{[citation needed]}. The following year it was discovered that ameloblasts in fact do not differentiate from odontoblasts but from ectodermal cells^{[citation needed]}.

Genetics

Up to date, mutations in the AMELX, ENAM, MMP20, and KLK-4 genes have been found to cause amelogenesis imperfecta (non-syndromic form). The AMELX, ENAM, KLK-4 and MMP20 genes provide instructions for making proteins that are essential for normal tooth development. These proteins are involved in the formation of enamel, which is a hard, calcium-rich material that forms the protective outer layer of each tooth. Mutations in any of these genes alter the structure of these proteins or prevent the genes from making any protein at all. As a result, tooth enamel is abnormally thin or soft and may have a yellow or brown color. Teeth with defective enamel are weak and easily damaged.

Researchers are looking for mutations in other genes that may also cause amelogenesis imperfecta.

Amelogenesis imperfecta can have different inheritance patterns depending on the gene that is altered. Most cases are caused by mutations in the ENAM gene and are inherited in an autosomal dominant pattern. This type of inheritance means one copy of the altered gene in each cell is sufficient to cause the disorder.

Amelogenesis imperfecta is also inherited in an autosomal recessive pattern; this form of the disorder can result from mutations in the ENAM or MMP20 gene. Autosomal recessive inheritance means two copies of the gene in each cell are altered.

About 5% of amelogenesis imperfecta cases are caused by mutations in the AMELX gene and are inherited in an X-linked pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. In most cases, males with an X-linked form of this condition experience more severe dental abnormalities than affected females.

Other cases of this condition result from new gene mutations and occur in people with no history of the disorder in their family.

Treatment

Crowns are sometimes being used to compensate for the soft enamel.Usually stainless steel crowns are used in children which may be replaced by porcelain once they reach adulthood. In the worst case scenario, the teeth may have to be extracted and implants or dentures are required.

Epidemiology

The exact incidence of amelogenesis imperfecta is uncertain. Estimates vary widely, from 1 in 700 people in northern Sweden to 1 in 14,000 people in the United States.

References

• Winter GB, Brook AH (1975). "Enamel hypoplasia and anomalies of the enamel". Dent Clin North Am 19 (1): 3-24. PMID 162891
• Simmer JP, Hu JC (2001). "Dental enamel formation and its impact on clinical dentistry". J Dent Educ 65 (9): 896-905. PMID 11569606
• Aldred MJ, Savarirayan R, Crawford PJ (2003). "Amelogenesis imperfecta: a classification and catalogue for the 21st century". Oral Dis 9 (1): 19-23. PMID 12617253
• Nusier M, Yassin O, Hart TC, Samimi A, Wright JT (2004). "Phenotypic diversity and revision of the nomenclature for autosomal recessive amelogenesis imperfecta". Oral Surg Oral Med Oral Pathol Oral Radiol Endod 97 (2): 220-30. PMID 14970781
• Stephanopoulos G, Garefalaki ME, Lyroudia K (2005). "Genes and related proteins involved in amelogenesis imperfecta". J Dent Res 84 (12): 1117-26. PMID 16304440