Hypodontia and Related Syndromes

Objectives:Congenital lack of teeth, with an estimated incidence ranging from 2.3% to 10% worldwide, comprise the most common dental abnormalities in humans. Usually, the pattern of missing teeth is symmetrical between the left and right sides of the dental arches, and the most common absent permanent teeth are the third molars, followed by lower second premolars and upper lateral incisors. Oligodontia or agenesis of more than six teeth (excluding the third molars), and its less severe form, hypodontia, can be inherited in autosomal dominant, autosomal recessive, or X-linked inheritance mode. These tooth agenesis can also be divided into two forms: oligodontia is often, but not always, observed to occur with other syndroms, including Rieger, Witkope, Wolf-Hirschhorn, Williams, Seckel syndrome, Kabuki make-up syndromes, ectodermal dysplasias, holoprosencephaly, Weyers acrofacial dysostosis, van der Woude syndrome, etc.

Teeth develop via a complex morphogenetic process involving the "epithelial-mesenchymal interactions". Different causes are possible for tooth agenesis. Environmental factors include trauma, chemotherapeutic agents, radiotherapy. Studies of odontogenesis using mice as models have indicated that teeth development is under strict genetic control, which determines the position, number, size, and shape of teeth. So far, more than 200 genes have been identified which are expressed during tooth development, and mutations in several of these genes can cause arrested tooth development. Genes that code for transcription factors(MSX1, PAX9, SHH), growth factors (FGF), growth factor receptors (FGFR), cytokines (TNF) and extracellular matrix molecules (BMP) have been shown to be essential for tooth morphogenesis at various stages. Combining clinical and molecular genetic studies, mechanisms of the differing forms of teeth agenesis will become more exact.

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