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Ectodermal dysplasia syndromes affect the development of several epidermal/mesenchymal derived organs such as hair, teeth, and exocrine glands. Two mouse genes involved in these syndromes have been identified in recent studies, namely, TNF family ligand, ectodysplasin, encoded by the Tabby gene, and its receptor, edar, encoded by the downless gene. Both genes were expressed in epithelial layer of tooth and hair follicles thus regulating the morphogenesis. Recently, the expression of ectodysplasin was found to be upregulated by Wnt6, whereas edar was induced by activin [Beta]A in developing tooth cultures (J. Laurikkala et al., 2001, Dev. Biol. 229, 443-455). However, the downstream targets of ectodysplasin signaling are largely unknown. Therefore, we have initiated a search for target genes by comparing the differential expression pattern between wild-type and Tabby mice in both skin and tooth tissues using suppression subtractive hybridization (SSH). We have started to analyze a library of 1200 candidate clones derived from upper back skin of E14-stage wild-type and Tabby mice generated by this method. These clones have been picked by Genetix Qpix robot, and arraying them into a filter, hybridization, and sequencing of the clones are in progress.

Mammalian dentition is characterized by complex and diverse molar cusp patterns and subtle changes in molars are usually the earliest morphological evidence of modern orders in the fossil record. However, the exact ways that embryonic patterning of tooth shapes could change to result in the documented evolution have been difficult to characterize quantitatively. We have developed topographic methods using Geographic Information Systems (GIS) to analyze changes in developing teeth. We performed GIS analyses on cusp formation and gene expression in the first lower molar of two species of muroid rodents, the house mouse and the sibling vole. The evolutionary lineages of muroids are well represented in the fossil record, thus providing a good basis for discerning the types of developmental processes in evolutionary transformations. We identify two hierarchical developmental processes, prepatterning and propagation, that can be implicated in mouse and vole molar evolution. Firstly, digital elevation models and cross-correlation analysis show that evolution in lateral cusp topography, affecting location of cusps and crests, results from coordinated changes in molecular prepatterns preceding the first cusps. Secondly, evolutionary increase in longitudinal cusp number results from accelerated longitudinal growth and faster propagation of the established lateral topography. The diversity of mammalian molar cusp patterns may largely result from these two developmental processes, limiting the independence of individual morphological features used in evolutionary studies. GIS analyses on organ topography and gene expression enable new ways to quantify development and evolution.

The development of several ectodermal organs, including teeth, hair and sweat glands is impaired in mouse tabby (ta) and downless (dl) mutants and corresponding human ectodermal dysplasias. The protein product of ta gene, ectodysplasin, is a novel member of tumor necrosis factor (TNF) family. Dl mutants have an identical phenotype to ta mice and the dl gene encodes a TNF receptor like protein, a putative ectodysplasin receptor. We compared the expression patterns of ta and dl with mRNA in situ hybridisation and analysed their regulation in vitro by applying signal molecules with beads or cell aggregates on isolated dental epithelia in organ cultures. Ta transcripts were seen in early oral epithelium. At the bud stage tooth, ta was expressed in the outer enamel epithelium and dental lamina. This pattern persisted throughout the cap and bell stages. Dl was expressed in epithelial compartments complementary to ta, in epithelial signaling centers at E12-E14. Expression was downregulated after the disappearance of the enamel knot. Cell aggregates expressing wnt6 induced ta expression in the dental epithelium in vitro. None of the other signaling molecules analysed, e.g. BMP4, FGF10 and Shh induced ta expression. We conclude that ta and dl regulate tooth morphogenesis by mediating interactions between epithelial cells, and are associated with signaling networks regulating the formation and function of the enamel knot, a signaling center regulating tooth shape. Tiffs study was supported by the Finnish Academy and the Viikki Graduate School in Biosciences.

The aim of the study was to characterize the molecular relationship between ameloblastoma and keratocystic odontogenic tumor (KCOT) by means of a genome-wide expression analysis. Total RNA from 27 fresh tumor samples of 15 solid/multicystic intraosseous ameloblastomas and 12 sporadic KCOTs was hybridized on Affymetrix whole genome arrays. Hierarchical clustering separated ameloblastomas and KCOTs into 2 distinct groups. The gene set enrichment analysis based on 303 dental genes showed a similar separation of ameloblastomas and KCOTs. Early dental epithelial markers PITX2, MSX2, DLX2, RUNX1, and ISL1 were differentially overexpressed in ameloblastoma, indicating its dental identity. Also, PTHLH, a hormone involved in tooth eruption and invasive growth, was one of the most differentially upregulated genes in ameloblastoma. The most differentially overexpressed genes in KCOT were squamous epithelial differentiation markers SPRR1A, KRTDAP, and KRT4, as well as DSG1, a component of desmosomal cell-cell junctions. Additonally, the epithelial stem cell marker SOX2 was significantly upregulated in KCOT when compared with ameloblastoma. Taken together, the gene expression profile of ameloblastoma reflects differentiation from dental lamina toward the cap/bell stage of tooth development, as indicated by dental epithelium-specific transcription factors. In contrast, gene expression of KCOT indicates differentiation toward keratinocytes. [ABSTRACT FROM PUBLISHER]

Gene knockouts often lead to an early and total disruption of tooth development that makes them useless for the analysis of tooth crown formation. Because most of the dental characters which are commonly used in evolutionary studies develop during the crown formation, we investigated a gene mutation with a weaker tooth phenotype in order to compare developmental lability of different aspects of tooth shape. Ectodysplasin (Eda) is a cell signaling protein expressed during development of several ectodermal organs. Lack of functional Eda causes an ectodermal dysplasia syndrome in which the development of teeth, hair and ectodermal glands is disturbed but most teeth still develop. We analyzed the tooth phenotype of two mouse lineages. One was a natural mutant, Tabby which lacks functional Eda, and the other was a transgenic mouse line, K14-Eda which overexpresses Eda in its ectoderm. We examined how dental characters change when Eda activity levels rise from zero (Tabby) to normal (wild type controls), and much beyond normal (K14-Eda). We found that with increasing Eda levels, several aspects of tooth shape change: The number of cusps increases, cusp positions change, additional crests form, and number of teeth increases. These results indicate that different expression levels of one gene are capable of changing most of the character states of teeth used in taxonomy. While these changes in morphology represent extreme differences compared to typical population level variation, our results show how most tooth shape characters have the developmental potential for correlated changes during evolution.