Hypodontia and microdontia involving the maxillary lateral incisor occurs in clefts
involving the lip and palate.
X-linked hypohidrotic ectodermal dysplasia is characterized in males by thin sparse
hair, dry skin, absence of sweating and therefore heat intolerance, and multiple
missing teeth. These children are at risk due to their inability to cool themselves and
may die in infancy if undiagnosed. This condition, while rare, is of particular
importance as the dental professional be the first to come to a diagnosis, and thus to
introduce families to support mechanisms. In heterozygous females the changes are
milder and may be restricted to the teeth, although a distinctive facial profile (slight
retrusion of the maxilla) may be recognized. Most commonly, one or both maxillary
lateral incisors and/or the second premolars are missing. In some patients one
maxillary lateral incisor may be of peg form. The responsible gene is the ED1 gene on
the X chromosome which encodes the protein ectodysplasin-A.
Autosomal dominant inheritance of missing teeth is seen in families with mutations in
the MSX1 gene on chromosome 4. Missing third molars and second premolars are the
most common finding. These families may also have clefting segregating with the
missing teeth. Mutations in the MSX1 gene are also seen in the tooth-nail (Witkop)
syndrome.
A pattern of autosomally dominant inheritance of missing teeth, particularly molars, is
seen as a result of mutations in the PAX9 gene on chromosome 14.
Some patients having a solitary median maxillary central incisor have been found to
have mutations in the sonic hedgehog (SHH) gene on chromosome 7 (783HFig. 13.3).
784H
Fig. 13.2 Hypohidrotic ectodermal
dysplasia; erupted permanent teeth.785H
Fig. 13.3 Median maxillary central
incisor syndrome.