Human Diseases Associated with Abnormal Tooth Roots

Tooth development is a complicated process, characterized by reciprocal interactions between the dental epithelia and mesenchyme. During this process, inner and outer enamel epithelia form a doublelayered sheath, Hertwig's epithelial root sheath, after crown formation. A close association has been suggested between this root sheath (epithelia) and root dentin (generated by cells originating from the mesenchyme) ; however, there is limited information on the mechanism of root morphogenesis and elongation. Furthermore, most of the information has been obtained from mice and rats, and not from humans. Many human genetic diseases are associated with abnormal roots. As chromosomal abnormalities, Down syndrome, caused by trisomy 21, is frequently associated with short roots. In Turner syndrome, which is caused by monosomy or the partial absence of the X chromosome, characteristic extra multiroots are seen in the mandibular premolars. In monogenic disorders, oculo-facio-cardio -dental (OFCD) syndrome is associated with extremely long roots. BCOR, encoding the BCL6 interacting corepressor, has been identified as a responsible gene, but the precise mechanism of how this gene mutation results in abnormal roots is unknown. Clinical observations have led to the following two conclusions : 1) Abnormal tooth roots are accompanied by abnormal crowns (size, shape, or structure). This indicates that crown and root formation is a sequential process and the latter is closely related to the former. 2) Multiple root abnormality is seen syndromically but not solely as a dental condition. This implies that regulators of tooth development are likely to be involved in the development of other tissues, such as in the limbs, eyes, and heart.