Amelogenesis imperfecta

Description

Amelogenesis imperfecta is a group of disorders of tooth development. In people with this condition, tooth enamel does not form properly. Tooth enamel is the hard, white material that is mainly made of mineral crystals and forms the protective outer layer of each tooth. People with amelogenesis imperfecta may have teeth that appear (erupt) later than normal, and their teeth may be unusually small, discolored, pitted, or grooved. Affected individuals can also have misalignment of the top and bottom teeth (malocclusion). Because of these teeth problems, people with amelogenesis imperfecta often have difficulty eating, sensitivity to certain foods and drinks, and pain when brushing their teeth. Their teeth are also prone to rapid wear, frequent cavities, and breakage. Other dental abnormalities are also possible. These problems can affect both primary (baby) teeth and permanent (adult) teeth.

Because amelogenesis imperfecta causes changes in a person's appearance, the condition can profoundly affect quality of life and self-esteem if not treated. In some people, the condition can lead to avoidance of social activities, anxiety, and other emotional or psychological issues.

Researchers have described at least four forms of amelogenesis imperfecta: hypoplastic (also referred to as type I), hypomaturation (type II), hypocalcified (type III), and hypomaturation and hypocalcified with taurodontism (type IV). These types are generally distinguished by their specific enamel abnormalities. The hypoplastic type is defined by thin enamel, the hypomaturation type is defined by weak or brittle enamel, and the hypocalcified type is defined by soft enamel. The hypomaturation and hypocalcified with taurodontism type involves weak and soft enamel and teeth that are too large for the size of their roots, which are small (this combination is known as taurodontism). These four types of amelogenesis imperfecta are further divided into subtypes according to their pattern of inheritance and their genetic cause.

Some doctors simplify the classification of amelogenesis imperfecta by grouping structural problems with enamel into one group (quantitative enamel defect) and strength or hardness issues with enamel into a second group (qualitative enamel defect).

Amelogenesis imperfecta can occur without any other signs and symptoms (isolated) or it can occur as part of a syndrome that affects multiple parts of the body (syndromic). A subtype of type I, known as amelogenesis imperfecta type IG, can affect the kidneys and is called enamel-renal syndrome. People with this condition may develop calcium deposits in the kidneys (nephrocalcinosis) later in life.

Frequency

The incidence of amelogenesis imperfecta varies widely depending on the population, from 1 in 700 people in northern Sweden to 1 in 14,000 people in the United States. Type I is the most common form, accounting for about half of all cases.

Causes

Variants (also called mutations) in more than 20 different genes can cause amelogenesis imperfecta. Variants in the SACK1H, FAM20A, AMELX, and ENAM genes account for over half of all cases. These genes, as well as many other genes that are associated with amelogenesis imperfecta, provide instructions for making proteins that are essential for normal enamel and tooth development.

Variants in the genes that are associated with amelogenesis imperfecta often lead to an altered protein structure or prevent the production of any functional proteins. As a result, tooth enamel does not form normally and may instead be thin, weak, or soft. The abnormal enamel results in teeth that are easily damaged and can become discolored.

Some people with amelogenesis imperfecta do not have a variant in any of the genes that are associated with this condition. In these people, the genetic cause of the condition is unclear. However, enamel defects can occur in combination with dozens of other conditions.

Inheritance

Amelogenesis imperfecta can have different inheritance patterns depending on the specific gene that is altered.

When amelogenesis imperfecta is caused by variants in the SACK1H gene, it is 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.

Autosomal dominant inheritance. Credit: U.S. National Library of Medicine.

When this condition is caused by variants in the FAM20A gene, it is inherited in an autosomal recessive pattern. Autosomal recessive inheritance means both copies of the gene in each cell must have a variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Autosomal recessive inheritance. Credit: U.S. National Library of Medicine.

When this condition is caused by variants in the ENAM gene, it can be inherited in either an autosomal dominant or an autosomal recessive pattern.

When amelogenesis imperfecta is caused by variants in the AMELX gene, it is inherited in an X-linked pattern. A condition is considered X-linked if the altered gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes in each cell. In males (who have only one X chromosome), a variant in the only copy of the gene in each cell is sufficient to cause the condition. In females (who have two copies of the X chromosome), one altered copy of the gene may or may not cause the features of the condition. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons. In most cases, males with amelogenesis imperfecta caused by AMELX gene variants experience more severe dental abnormalities than females with similar variants.

In some cases, amelogenesis imperfecta can result from new (de novo) variants in the gene that occur during the formation of reproductive cells (eggs or sperm) in an affected individual's parent or during early embryonic development. These affected individuals have no history of the disorder in their family.

Other Names for This Condition

• AI
• Congenital enamel hypoplasia

Where this page came from

This page was imported from MedlinePlus Genetics, National Library of Medicine. Courtesy of MedlinePlus from the National Library of Medicine; its genetics summaries and the illustrations credited to the library are in the public domain. Pictures credited to others are not copied.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.