have an alteration in the amelogenin gene on the short arm of the X chromosome.
Affected males cannot pass on the condition to their sons (by virtue of passing on
their Y chromosome to their sons) but their daughters (to whom they necessarily pass
on their X chromosome) will all inherit the mutant gene. Such daughters will always
show some dental features although these might be subtle in some cases. These
heterozygous females can pass on the mutant gene to children of either sex. (830HFig.
13.26).
The enamel in both sexes may be hypoplastic, hypomineralized, or show elements of
both features. The appearance seen will be the result of the exact nature of the change
in the amelogenin gene and the sex of the patient.
Males, by virtue of having a single X chromosome, will be more severely and
uniformly affected. The enamel may be thin (hypoplastic⎯reduced in quantity) or
discoloured (with affected mineralisation) or a combination of both (831HFig. 13.27).
Females within the same family who inherit the affected gene will show a vertical
pattern of markings of the enamel, either vertical ridges and grooves (the equivalent
of the male, uniform hypoplasia), with or without discolouration or loss of
translucency of the enamel (where the mineralization is affected) (832HFig. 13.28).
Aetiology
The amelogenin gene, which encodes the enamel protein amelogenin, is located on
the short arm of the X chromosome. Mutations in the gene are responsible for most
cases of X-linked amelogenesis imperfecta but there also appears to be another gene
on the long arm of the X chromosome which is responsible for similar clinical
appearances in another family.
Genetic enamel defects associated with generalized disorders
Widespread enamel defects can be seen in a number of conditions with extraoral
manifestations. These include conditions such as epidermolysis bullosa, tuberous
sclerosis, oculo-dento-osseus dysplasia, as well as the amelogenesis imperfecta
associated with tricho-dento-osseous syndrome. The exact genomic relationship
between these and other conditions and amelogenesis imperfecta remains to be
established in most cases.
Key Points
Amelogenesis imperfecta
- Inheritance,
- Autosomal dominant,
- Autosomal recessive,
- X-linked,
- Apparently sporadic.
Phenotype
Hypoplastic +/- hypomineralization (hypocalcification to hypomaturity)
Pure hypoplasia or hypomineralization are probably rare