Abnormalities of Teeth qxd

Amelogenesis Imperfecta (AI)

There is a large group of inherited developmental defects in enamel collectively

referred to as amelogenesis imperfecta. At least 14 phenotypes have been identified

and autosomal dominant, recessive and X linked inheritance have been reported. The

conditions is rare, only about 1 in 14,000 have it. Establishing a pattern of inheritance

requires constructing a pedigree (family tree) of several generations, identifying those

with and without the condition. This is no minor task. Futhermore there is subtle over-

laps in the phenotypes so that you may not be able to identify a specific type of AI

when only a single or few patients are available for study. Much of what is known

about AI comes from northern Sweden where the condition is more common. Three

general categories of AI are recognized:

1. Hypoplastic type:  Inadequate formation of enamel matrix, both pitting and smooth

types exist. Enamel may be reduced in quantity but is of normal hardness.

2. Hypomaturation type:   A defect in the crystal structure of enamel leads to a mottled

enamel with white to brown to yellow colors.

3. Hypocalcified type:  A defect not in the quantity but in the quality of enamel. It is poor-

ly mineralized, soft and chips and wears easily.

Slides # 63 and #64 are examples of the hypo-

calcified type of AI. Slide # 65 is the smooth

hypoplastic type. Slide # 66 shows the mottled

appearance of the hypomaturation type.

*** Acquired (not inherited) defects in enamel may cause color and pitting defects, see

your text for more information.

The matrix of enamel is comprised mainly of a protein called amelogenin. The gene for this

protein is on the short (petit) arm of the X chromosome (Xp22.1). A number of mutations

have been described including deletions, missense and nonsense mutations.

Autosomal dominant AI has been traced to a gene on chromosome #4 near the site as the gene

for dentinogenesis imperfecta and dental dysplasia. (It appears that a region on #4 chromo-

some is a hot spot for dental developmental abnormalities). The AI gene at this site codes for

another enamel protein, enamelin. Enamelin is thought to serve as a “nucleation site for

hydroxyapatite crystals”

See:  (1) "Detection of a novel mutation in X-linked amelogenesis imperfecta"

Journal of Dental Research

79 (12):1978-82  2000

(2.) "An amelogenin gene defect associated with human x-linked AI"

Archives of Oral Biology

42:235-242  1997

(3) "Localization of a gene for autosomal dominant AI to chromosome 4q."

Human Molecular Genetics

3:1621-25  1994