Amelogenesis imperfecta (AI) refers to a group of rare, inherited disorders characterized by abnormal enamel formation. The term is restricted to those disorders of enamel development not associated with other defects of the body. In AI, the layer of enamel is thin so that the teeth appear to be discolored, showing the color of the materials under the enamel. The teeth usually appear brown or some variant of that color.
Clinical researchers usually classify AI into four main types of which 14 subtypes are recognized. The main types are based on enamel effects and the subtypes are based on clinical appearance and mode of inheritance. The main types are: hypoplastic (Type 1); hypomaturation (Type II); hypocalcified (Type III); and hypomaturation/hypoplasia/taurodontism (Type IV). Amelogenesis imperfecta may be inherited as an X-linked, autosomal dominant, or autosomal recessive genetic trait, depending on the type.
Amelogenesis imperfecta is characterized by defective or missing tooth enamel. Secondary effects of this disorder may be early tooth loss, heightened susceptibility to disease of the tissues surrounding the teeth (periodontal) such as gums, cement, ligaments, and the bone in which the tooth root rests (alveolar). Sensitivity of the teeth to hot and cold is usually increased. The dental pulp (pulpa) in the root canal is exposed in some cases, and a so-called “open bite” may occur because the upper and lower jaws do not align properly. Another complication of AI is that the unsightly teeth may cause psychological problems. With orthodontic and periodontal restoration, however, the teeth will look normal and can remain functional throughout life.
The main types of AI may be distinguished by clinical appearance, enamel thickness, appearance on x-ray negatives, and mode of inheritance.
Type I hypomaturation AI is characterized by small to normal tops (crowns) of the teeth, upper and lower teeth that do not meet showing a poor bite, and teeth that vary in color from off-white to yellow-brown. The enamel thickness varies from thin and smooth to normal, ridged or pitted.
Type II hypomaturation AI is commonly associated with an open bite and creamy to yellow-brown roughly surfaced teeth that may be tender and sore. The enamel is gernally normal in thickness but is unusually brittle. Similar to Type I hypomaturation, Type II hypomaturation AI may also be inherited as any of the autosomal dominant, autosomal recessive, or X-linked traits.
Type III hypocalcified AI is seen in patients with an open bite and creamy to yellow-brown rough enamel-surfaced teeth that may be tender and sore. These teeth usually carry substantial precipitates of stony material from the fluids of the mouth (calculi).
Type IV hypomaturation/hypoplasia/taurodontism AI usually is characterized by smaller than normal teeth, the color of which may range from white to yellow-brown, and teeth that appear to be mottled or spotted. The enamel is thinner than normal with areas that are clearly less dense (hypomineralized) and pitted. Genetically these characteristics are transmitted as autosomal dominant traits.
Just as the classification of amelogenesis imperfecta is complex, so too is the contribution of genetics to this disorder. The malfunctioning (mutations) of several genes have been identified and the sites of these genes on various chromosomes have been determined. Exactly how these genes work singly or in concert with one another remains a mystery.
Genes responsible for hypoplastic AI (Type I) have been tracked to gene map loci Xp22, 3-p22.1, 4q31, and Xq22-q28.
Genes responsible for hypomaturation AI (Type II) are tentatively assigned to the X chromosome and to locations on chromosome 19 and chromosome 11.
Genes responsible for hypocalcified AI (Type III) have been tracked to gene map locations 4q21, 4q21, and the X chromosome.
A gene responsible for hypomaturation/hypoplasia/taurodontism AI (Type IV) has been traced to a gene map locus 17q21,3-q22.
Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 though 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 11p13” refers to band 13 on the short arm of chromosome 11. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother.
Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes form both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.
All individuals carry a few abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.
Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary for the appearance of the disease. The abnormal gene can be inherited from either parent, or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene form affected parent to offspring is 50% for each pregnancy regardless of the sex of the resulting child.
X-linked recessive genetic disorders are conditions caused by an abnormal gene on the X chromosome. Females have two X chromosomes but one of the X chromosomes is “turned off” and all of the genes on that chromosome are inactivated. Females who have a disease gene present on one of their X chromosomes are carriers for that disorder. Carrier females the abnormal gene that is “turned off”. A male has one X-chromosome and if he inherits an X chromosome that contains a disease gene, he will develop the disease. Males with X-linked disorders pass the disease gene to all of their daughters, who will be carriers. A male cannot pass an X-linked gene to his sons because males always pass their Y chromosome instead of their X chromosome to male offspring. Female carriers of an X-linked disorder have a 25% chance with each pregnancy to have a carrier daughter like themselves, a 25% chance to have a non-carrier daughter, a 25% chance to have a son affected with the disease, and a 25% chance to have an unaffected son.
X-linked dominant disorders are also caused by an abnormal gene on the X chromosome, but in these rare conditions females with an abnormal gene are affected with the disease. Males with an abnormal gene are more severely affected than females, and many of these males do not survive.
Amelogenesis imperfecta affects 1 of 14,000 to 16,000 children in the United States. Of this number, about 40% have the hypocalcified dominant type. The autosomal dominant and recessive forms of the disorder affect males and females in equal numbers. The sex-linked dominant type of the disorder affects twice as many males as females. The sex-linked recessive type affects only males.
Diagnosis of amelogenesis imperfecta is usually made by X-ray examination at the time the teeth erupt. By one to two years of age, the diagnosis can be made by visual examination.
Full crown restorations and a type of denture that caps defective teeth and corrects open bite are excellent treatments for this disorder. Desensitizing toothpaste can prevent painful sensitivity to heat and cold. Good oral hygiene is important. Genetic counseling is recommending for families of children with amelogenesis imperfecta.
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McKusick VA, ed. Online Mendelian inheritance in man (OMIM). The John Hopkins University. Amelogenesis Imperfecta 1, Hypoplastic Type; AIH1. Entry Number; 301200: Last Edit Date; 01/21/2005.
(See also OMIM Nos. 301201; 104530; 204650; 608563)
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(See also OMIM No. 204700)
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