What is Amelogenesis Imperfecta?

There are at least 14 types of Amelogenesis imperfecta which are categorised according to the bad teeth abnormalities that they cause and by their pattern of inheritance.

The term Amelogenesis imperfecta means imperfect enamel. These defects are highly variable and are not related to any other health condition of the body, and both baby teeth and permanent teeth can be affected. There are a number of symptoms which include: -

• Thin tooth enamel
• Soft tooth enamel
• Pitted tooth enamel
• Rough tooth enamel
• The teeth are easily damaged
• Teeth appear yellow

The incidence of Amelogenesis imperfecta is unknown as estimates vary from country to country, but in the United States an approximate figure is 1 in every 14,000 people.

The genetic disorder is caused by mutations in genes that code for proteins involved in the manufacture of enamel. Tooth enamel is usually a calcium-rich tough protective substance but the mutated genes means that the proteins that make it are either malformed and poorly functioning or missing. This leads to abnormally thin and soft enamel.

A number of genes have been pinpointed as the root causes and they are: -

AMELX - which makes a protein called amelogenin, which is essential for normal tooth development. One copy of each gene is found on the sex chromosomes - X and Y

ENAM - which makes the protein enamelin, which is also essential for tooth development. The gene is located on chromosome 4 and no fewer than seven mutations have been identified which can cause Amelogenesis imperfecta.

MMP20 - which makes a protein called enamelysin, another protein essential for normal tooth development. It is found on chromosome 11 and at least two genetic mutations have been discovered.

In most cases the condition follows an autosomal dominant pattern of inheritance. This means that a child only needs to inherit one copy of the abnormal gene. If one parent has one mutated dominant gene then there is a 50% chance that a child will inherit the condition.

Amelogenesis imperfecta also follows an autosomal recessive pattern of inheritance which means that two abnormal copies of a gene are required for a child to be afflicted. This can result from mutations of the ENAM or MMP20 genes. If both parents are unaffected carriers of the condition then there is a 25% chance that a child will inherit the condition.

About 5% of cases follow an X-linked pattern of inheritance - this is when there are mutations on the AMELX gene. In this scenario males tend to have more severe dental problems than females.

And some cases of Amelogenesis imperfecta are caused by spontaneous mutations in the three genes mentioned in this article. The individuals here have no prior family history of the bad teeth condition.