Developmental Defects of the Root

There are numerous hereditary conditions that can affect the shape, size, and composition of the dental roots. Many syndromes and chromosomal anomalies have associated tooth and dental root phenotypes. Therefore, the following section is not meant to be an exhaustive review but rather will highlight some of the critical components for root formation discussed in the previous section.

Taurodontism

There are many diverse conditions that are associated with taurodontism [Online Mendelian Inheritance in Man (OMIM) No. 272700] that result from the delayed invagination of the epithelial root sheath creating an elongated pulp chamber. Normally invagination of the epithelial root sheath occurs at the height of the furcation where it fuses to form the template and control the mor

Fig. 3. The maxillary primary molars in this child with X-linked hypohidrotic ED both exhibit taurodontism (arrows) as viewed in this panoramic radiograph.

Fig. 3. The maxillary primary molars in this child with X-linked hypohidrotic ED both exhibit taurodontism (arrows) as viewed in this panoramic radiograph.

Fig. 4. This young adult affected with TDO displays severe generalized taurodontism, generalized thin enamel, and a marked increase in the mandibular bone as viewed in this panoramic radiograph.

Fig. 4. This young adult affected with TDO displays severe generalized taurodontism, generalized thin enamel, and a marked increase in the mandibular bone as viewed in this panoramic radiograph.

Taurodontism Syndromes

phology of multi-rooted teeth. This root/pulp anomaly occurs in the normal population with a prevalence that varies substantially from one race to anther ranging from 1.5 to 48% of the population [MacDonald-Jankowski and Li, 1993; Toure et al., 2000]. Taurodontism is known to occur with many syndromes and chromosomal anomalies such as in individuals with Down syndrome, sex chromosome aneuploidy and hypodontia [Jaspers and Witkop, 1980; Seow and Lai, 1989; Alpoz and Eronat, 1997]. Given the purported abnormal epithelial root sheath function in the etiology of taurodontism it is not surprising that many conditions affecting epithelial-derived tissues also are associated with taurodontism.

The ectodermal dysplasias (EDs) are a clinically and genetically diverse group of conditions characterized by defects in at least two ectodermally derived appendages (e.g. teeth, hair, nails, and sweat glands). The most common form is X-linked hypohidrotic ED (OMIM 305100) that is caused by mutations in ED1 (ectodysplasin gene). Phenotypically similar autosomal dominant and recessive hypohidrotic EDs result from mutations in the ED1

receptor (EDAR; OMIM 129490 and 224900). These genes are active in the tumor necrosis factor pathway and are critical in both early and late events in odontogenesis. The prevalence and severity of taurodontism associated with ED is variable (fig. 3) [Crawford et al., 1991].

The amelogenesis imperfectas (AIs) are a group of genetically heterogeneous hereditary conditions affecting primarily the enamel. These conditions were classified into distinct AI subtypes by Witkop [1989] based on mode of inheritance and the phenotype of enamel and the presence of taurodontism. Those AI types having tau-rodontism as a phenotypic feature were designated hypo-plastic/hypomaturation AI with taurodontism (AI type IV) [Witkop, 1989]. Many cases originally reported as AI with taurodontism (OMIM 104510) were later reported to be individuals affected with the tricho-dento-osseous syndrome (TDO, OMIM 19032), due to the difficult clinical delineation between these conditions [Seow, 1993; Collins et al., 1999; Price et al., 1999]. Several studies indicate that the prevalence of taurodontism in AI cases is similar to that seen in unaffected controls [Seow, 1993;

Collins et al., 1999]. The different AI conditions are known to be caused by a variety of allelic and non-allelic mutations in a variety of genes including AMELX, ENAM, KLK4 and MMP20 [Wright, 2006]. There appear to be numerous AI types with molecular etiologies that remain to be discovered [Hart et al., 2003]. While the AI-associ-ated gene products are critical for normal enamel formation, they do not appear essential for normal root formation given that none of the AI phenotypes resulting from these mutations is known to have abnormal root morphology or structure and the dentin and cementum appear grossly normal.

Marked taurodontism of the molar teeth (fig. 4) is a highly penetrant feature of TDO which is caused by mutations in the Distal-less homeobox gene DLX3. TDO derives its name from the three predominantly affected tissues, tricho-hair, dento-teeth, and osseous bone, which show variable severity in expression of the trait [Wright et al., 2000]. One family having a DLX3 mutation with no apparent hair or bone manifestations has been reported as being AI with taurodontism [Dong et al., 2005], however our evaluation of a large family with this same DLX3 mutation (CTdel) shows a marked hair phenotype suggesting this represents a TDO variant. Whether there is a distinct condition characterized by AI and taurodontism phenotypes remains controversial. While the expression of taurodontism in people with DLX3 mutations is variably expressed, the high penetrance of this trait suggests that DLX3 is an important regulator providing some degree of temporal control over the invagination of the epithelial root sheath.

Variations in Root Size and Structure

There are numerous hereditary conditions associated with abnormal development of the root that affect the size and structure. For example, dentinogenesis imperfectas (OMIM 125490 and 166240) and dentin dysplasia type II (OMIM 125420) frequently exhibit abnormal root morphology and often have a diminished root size (fig. 5). Mutations in the type 1 collagen genes (COL1A1 and COL1A2) and DSPP are associated with the dentinogen-esis imperfecta phenotype with roots that are commonly described as being short and constricted [Pallos et al., 2001; Malmgren et al., 2004]. Additionally, the Ehlers-Danlos conditions that are associated with collagen mutations (Ehlers-Danlos type I, collagen V mutation, OMIM 13000: Ehlers-Danlos type IV, collagen III mutation, OMIM 13050, and Ehlers-Danlos type VI, lysyl hy-droxylase mutation, OMIM 225400) also can be associated with abnormalities in dentin, root formation, and in

Fig. 5. This radiograph shows the shortened and minimally separated root morphology commonly seen in dentinogenesis imperfecta type I and type II.

some cases periodontal disease [Pope et al., 1992; Karrer et al., 2000]. Collectively, these conditions illustrate the importance of collagens and other extracellular matrix proteins in determining root morphology.

Dentin dysplasia type I (OMIM 125400) is a rare apparently autosomal dominant condition characterized by crowns that appear clinically normal. However, after the initial layer of normal dentin is formed, a cycle of odon-toblast death followed by new odontoblast recruitment again followed by odontoblast death occurs during repeated attempts at dentin formation. This pattern leads to a unique histological appearance of the dentin (cascading waterfall; fig. 6a) and markedly diminished root development (fig. 6b). The molecular etiology for this condition remains unknown at this time.

Hypophosphatasia (OMIM 146300 and 24150) can be inherited as an autosomal dominant or recessive trait and is caused by mutations in ALPL (tissue nonspecific alkaline phosphatase gene). The dentin can be hypomineral-ized, have an increased Tomes granular layer, and the root surfaces lack a normal cellular and acellular cemen-tum covering [Hu et al., 2000; van den Bos et al., 2005]. Consequently the teeth are frequently exfoliated prematurely with little or no root resorption. Loss of function of the tissue-nonspecific alkaline phosphatase alters the normal hydrolysis of pyrophosphate, which is an inhibitor of apatite crystal growth that is thought to be critical for normal cementum development [Nociti et al., 2002; van den Bos et al., 2005].

Rudimentary Tooth Dentin

Fig. 6. a This mineralized thin section of a tooth viewed with transmitted light microscopy shows the lack of normal root structure and the classical histological cascading waterfall dentin phenotype characteristic of dentin dysplasia type I. b Dentin dysplasia type I teeth frequently show pulp obliteration, only rudimentary root formation, and periapical abscess formation (note second primary molars) as illustrated in this panoramic radiograph.

Fig. 6. a This mineralized thin section of a tooth viewed with transmitted light microscopy shows the lack of normal root structure and the classical histological cascading waterfall dentin phenotype characteristic of dentin dysplasia type I. b Dentin dysplasia type I teeth frequently show pulp obliteration, only rudimentary root formation, and periapical abscess formation (note second primary molars) as illustrated in this panoramic radiograph.

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