Insight into the molecular pathogenesis of odontogenic lesions

BACKGROUND

Odontogenic tumors, derived from epithelial, ectomesenchymal, and/or mesenchymal elements of the tooth-forming apparatus, constitute a heterogeneous group of lesions, including hamartomas, benign and malignant neoplasms with metastatic capabilities.

HIGHLIGHT

This review provides a comprehensive overviewof the pathogenesis of odontogenic tumors and explains the associated molecular events in the context of hallmarks of cancer established by Hanahan D and Weinberg RA. Diagrammatic representations depicted in the article would facilitate easier understanding.

CONCLUSION

A better understanding of the pathogenesis of the lesions may assist in determining patient's prognosis and devising better targeted therapeutic treatment, thus, reducing the morbidity and mortalityof patients.

Molecular pathology of odontogenic tumors

Odontogenic tumors are lesions derived from the elements of the tooth-forming apparatus and are found exclusively within the jawbones. This review represents a contemporary outline of our current understanding of the molecular and genetic alterations associated with the development and progression of odontogenic tumors, including oncogenes, tumor-suppressor genes, oncoviruses, growth factors, telomerase, cell cycle regulators, apoptosis-related factors, regulators of tooth development, hard tissue-related proteins, cell adhesion molecules, matrix-degrading proteinases, angiogenic factors, and osteolytic cytokines. It is hoped that better understanding of related molecular mechanisms will help to predict the course of odontogenic tumors and lead to the development of new therapeutic concepts for their management.

K-Ras gene status and expression of Ras/mitogen-activated protein kinase (MAPK) signaling molecules in ameloblastomas

BACKGROUND

To clarify the roles of rat sarcoma (Ras)/mitogen-activated protein kinase (MAPK) signaling pathway in oncogenesis and cytodifferentiation of odontogenic tumors, K-Ras gene status and expression of Ras, Raf1, MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK)1, and ERK1/2 proteins were analyzed in ameloblastomas as well as in tooth germs.

METHODS

Paraffin sections of 10 tooth germs and 46 benign and 6 malignant ameloblastomas were examined immunohistochemically for the expression of K-Ras, Raf1, MEK1, and ERK1/2. Frozen tissue samples of 22 benign ameloblastomas and 1 malignant (metastasizing) ameloblastoma were analyzed by direct DNA sequencing to detect K-Ras gene alteration.

RESULTS

Immunohistochemical reactivity for K-Ras, Raf1, MEK1, and ERK1/2 was detected in both normal and neoplastic odontogenic epithelium, and these molecules were reactive chiefly with odontogenic epithelial cells neighboring the basement membrane. Plexiform ameloblastomas showed slightly stronger expression of these Ras/MAPK signaling molecules than follicular ameloblastomas. Keratinizing cells and granular cells showed decreased reactivity for the signaling molecules. Basal cell ameloblastomas showed slightly stronger reactivity for the signaling molecules than did the other subtypes. K-Ras immunoreactivity in malignant ameloblastomas was lower than that in dental lamina of tooth germs. Direct DNA sequencing showed a GGT to GCT point mutation at codon 12 of K-Ras gene in one ameloblastoma.

CONCLUSION

Expression of K-Ras, Raf1, MEK1, and ERK1/2 in tooth germs and ameloblastomas suggests that Ras/MAPK signaling pathway functions to regulate cell proliferation and differentiation in both normal and neoplastic odontogenic epithelium. K-Ras gene status implied that K-Ras mutations might play a minor role in oncogenesis of odontogenic epithelium.

Immunohistochemical localization of Galphaq, PLCbeta, Galphai1-2, PKA, and the endothelin B and extracellular Ca2+-sensing receptors during early amelogenesis

Antibodies specific to Galphaq, PLCbeta, Galphai 1-2, and PKA were immunohistochemically (IHC) localized in the pre-ameloblasts up to initial dentin matrix deposition and continued in the distal ends of the pre-secretory ameloblasts to the beginning of enamel matrix secretion. It was hypothesized that the endothelin B receptor (ETBR) and/or the extracellular Ca2+-sensing receptor (CaR) would localize in the same locations as their known downstream signal transduction pathway (STP) effectors during events related to early amelogenesis. Localization was similar for the 4 signal transduction pathway elements and the CaR. The ETBR was not localized in any of the cells of the enamel organ. These findings indicate that the CaR and its related STPs are expressed in the pre-ameloblasts and pre-secretory ameloblasts in positions where they may be able to detect increases in extracellular Ca2+ concentrations observed in the pre-dentin matrix in a previous study. These observations are consistent with the hypothesis that increased levels of free Ca2+ in the pre-dentin matrix serve as a primary signal for modification of gene expression important to amelogenesis.

The immunohistochemical localization of the interferon-gamma and granulocyte colony-stimulating factor receptors during early amelogenesis in rat molars

Previous studies, in which the known janus kinase and signal transducer and activator of transcription (STAT) isoforms were immunohistochemically mapped in developing rat molars, implicated a sizeable list of cytokine superfamily receptor (CSR)/signal-transduction pathway (STP) linkages in the cells of the enamel organ involved in the events leading directly to early amelogenesis. Various combinations of upregulated janus kinases and STATs are known to be linked to single or small groups of CSRs. On the basis of the previous observations it was hypothesized that the interferon-gamma receptor (IFNgamma r) and the granulocyte colony-stimulating factor receptor (G-CSF receptor) would be localized in specific sites in the cells of the enamel organ during early amelogenesis. To verify this, whole-head, freeze-dried sections were here obtained at the level of the mandibular first and second molar from newborn and 5-day-old rats. These sections were not demineralized or fixed, reducing the possibility of false-negative results. Antibodies to the IFNgamma r and the G-CSF receptor were localized using a modification of the avidin-biotin complex method. In the newborn rats, IFNgamma r was localized in the preameloblasts in the cervical loop, the proximal and distal ends of presecretory ameloblasts, the outer enamel epithelium, the dental lamina, and in bone. In 5-day-old rats, it was confined to the proximal ends of the presecretory and secretory ameloblasts. The G-CSF receptor was observed in the molars of newborn rats in the preameloblasts, the proximal and distal ends of the presecretory ameloblasts, outer enamel epithelium, and in bone. In 5-day-old rats, G-CSF receptor was localized in the preameloblasts, the proximal ends of presecretory and secretory ameloblasts, the stellate reticulum, the outer enamel epithelium, and in bone. These findings indicate that the IFNgamma r and the G-CSF receptor, and their downstream STP linkages, are upregulated in the cells of the enamel organ and may be involved in the events leading directly to early enamel formation.

The immunohistochemical localization of stat-2, -3, -4 and -5 during early enamel and dentine formation in rat molars

STATs (signal transduction and activators of transcription) are key components of the signal transduction pathways in the cytokine receptor superfamily-linked pathway. STATs are activated directly by members of the Jak (Janus kinase) family and, when activated, migrate to the nucleus to modify gene expression to produce a variety of cellular responses. Individual cytokines activate specific combinations of the Jak/STAT isoforms. A previous study localized the known Jak isoforms and STAT-1 in 5-day-old rat molars during the early stages of enamel and dentine formation. The present study was undertaken to localize immunohistochemically STAT isoforms STAT-2. -3, -4 and -5 in association with events involved in early dentine and enamel formation in 5-day-old rat molars. Each of the isoform localization patterns was different from the others. Combining the results of the previous study with the present findings, it appears that all of the known Jaks and STATs-1, -2, -3, -4 and -5 are located in the cells directly involved in early enamel or dentine formation. Using colocalization patterns of the individual Jaks and STATs, individual receptor locations may be predicted. In the proximal ends of differentiated ameloblasts, several cytokine receptors [interleukin (IL) -5, -6, -7, -9, -10, -12, growth hormone granulocyte colony-stimulating factor interferon-alpha/beta. -gamma] are predicted. In other areas of the early odontogenic cells, the proximal ends of differentiating ameloblasts are predicted to have IL-7 receptors, inner enamel epithelium IL-6 and IL-10 receptors, and stratum intermedium cells IL-6 receptors. In the early developing dentine, differentiating odontoblasts are predicted to have IL-6 and IL-10 receptors, and differentiated odontoblasts no cytokine receptors identified by known Jak/STAT combinations. Mapping of the Jak and STAT isoforms in the cells involved in early enamel and dentine formation indicates that a sizeable list of ligands and their respective cytokine receptor/pathway complexes are involved in the regulation of these processes.

The immunohistochemical localization of signal-transduction pathway components Jak1, Jak2, Jak3, Tyk2 and STAT-1 during early enamel and dentine formation in rat molars

This study sought to localize immunohistochemically Janus kinase (Jak) and Tyk isoforms and STAT-1 in association with events involved in early dentine and enamel formation in the rat molar. The Jaks and STATs (signal transducers and activators of transcription) are key signal-transduction pathway components in the cytokine receptor-linked pathway. The histological sections were not demineralized or fixed, providing optimum conditions for immunohistochemical localization. It appears that all of the Jak isoforms and STAT-1 are involved in enamel formation. Jak2 and STAT-1 colocalized in the proximal ends of presecretory and secretory-stage ameloblasts, supporting work by others that growth hormone receptor is located at that site. The colocalization of Jak1, Jak2 and STAT-1 along the proximal ends of presecretory and secretory ameloblasts suggests that the interferon receptor is up-regulated in these cells as well. Also, colocalization of Jak3 and STAT-1 in the proximal ends of the ameloblasts and the cells of the stratum intermedium predicts the location of the interleukin-7 receptor in those locations. Jak1, Tyk2 and STAT-1, but not Jak2 or Jak3, stain was seen in the odontoblasts.

Odontogenic tumours in the v-Ha-ras (TG.AC) transgenic mouse

A line of homozygous transgenic mice (TG.AC) carrying a v-Ha-ras gene fused to the promoter of the zeta globin gene produces a variety of mesenchymal and epithelial neoplasms including odontogenic tumours. The 1-year incidence of odontogenic tumour formation in these mice was approx. 35%. Tumours formed more often in the mandible than maxilla. The various types of tumours frequently presented with: (1) primarily mesenchymal cells in a dense fibrous-like matrix, or (2) loose stroma surrounded by anastomosing cords of epithelial cells that exhibited squamous differentiation, or (3) odontomas forming mineralized tooth structures by well-differentiated odontoblasts and ameloblasts. Some tumours had areas with all three of these characteristics. Mineralized dentine and enamel in the odontomas were morphologically similar to those of normal murine teeth. Odontogenic tumours expressed the v-Ha-ras transgene that was primarily localized to the mesenchymal cells. Proliferating-cell nuclear antigen immunohistochemistry showed that the mesenchymal cells adjacent to the epithelial cords not only expressed the ras transgene but were also actively proliferating. The TG.AC mouse provides an excellent model for the study of odontogenic tumours and tooth development.

Immunodetection of the p21-ras products in human normal and preneoplastic tissues and solid tumors: a review

Immunohistochemical detection of p21-ras to identify and characterize preneoplastic or neoplastic lesions in human tissues is reviewed. Information concerning the commercially available antibodies is presented. Antibodies DWP, Ras-10, Y13-259, YA6-172, NCC-001, and NCC-004 are fully documented with respect to their behavior in appropriate specificity tests and appear to be reliable reagents. After reviewing the data we have identified three groups of tissues or organs with respect to positive immunostaining for p21-ras as the significant criterion of malignancy. These three groups comprise (1) tissues for which no definite conclusion could be drawn (colon, lung, bladder, ovary, and neural and odontogenic tissues) despite occasional claims to the contrary, (2) tissues for which conclusions were negative (pancreas and stomach), and (3) tissues for which p21-ras staining positively discriminated malignant from normal tissues (liver, uterus, and salivary gland). Immunohistochemically detectable levels of products from a mutated ras gene could be demonstrated in a fraction of the samples from colon, lung, and bladder carcinomas, as well as in some histologically normal tissues adjacent to a colon carcinoma. The possibility that a higher relative intensity of the immunostaining reaction for p21-ras might discriminate malignant tissues from normal tissues or benign lesions in breast, pancreas, stomach, lung, uterus, or thyroid samples is suggested. Further studies now appear warranted and a strategy is proposed to validate the conclusions reached thus far.