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

The Molecular Pathology of Odontogenic Tumors: Expanding the Spectrum of MAPK Pathway Driven Tumors

Odontogenic tumors comprise a heterogeneous group of lesions that arise from the odontogenic apparatus and their remnants. Although the etiopathogenesis of most odontogenic tumors remains unclear, there have been some advances, recently, in the understanding of the genetic basis of specific odontogenic tumors. The mitogen-activated protein kinases/extracellular signal-regulated kinases (MAPK/ERK) pathway is intimately involved in the regulation of important cellular functions, and it is commonly deregulated in several human neoplasms. Molecular analysis performed by different techniques, including direct sequencing, next-generation sequencing, and allele-specific qPCR, have uncovered mutations in genes related to the oncogenic MAPK/ERK signaling pathway in odontogenic tumors. Genetic mutations in this pathway genes have been reported in epithelial and mixed odontogenic tumors, in addition to odontogenic carcinomas and sarcomas. Notably, B-Raf proto-oncogene serine/threonine kinase (BRAF) and KRAS proto-oncogene GTPase (KRAS) pathogenic mutations have been reported in a high proportion of ameloblastomas and adenomatoid odontogenic tumors, respectively. In line with the reports about other neoplasms that harbor a malignant counterpart, the frequency of BRAF p.V600E mutation is higher in ameloblastoma (64% in conventional, 81% in unicystic, and 63% in peripheral) than in ameloblastic carcinoma (35%). The objective of this study was to review MAPK/ERK genetic mutations in benign and malignant odontogenic tumors. Additionally, such genetic alterations were discussed in the context of tumorigenesis, clinical behavior, classification, and future perspectives regarding therapeutic approaches.

KRAS mutations drive adenomatoid odontogenic tumor and are independent of clinicopathological features

Adenomatoid odontogenic tumor is a benign encapsulated epithelial odontogenic tumor that shows an indolent clinical behavior. We have reported in a few adenomatoid odontogenic tumors mutations in KRAS, which is a proto-oncogene frequently mutated in cancer such as lung, pancreas, and colorectal adenocarcinomas. We aimed to assess KRAS mutations in the hotspot codons 12, 13, and 61 in a large cohort of adenomatoid odontogenic tumors and to test the association of these mutations with clinical (age, site, tumor size, follicular/extrafollicular subtypes) and histopathological parameters. Thirty eight central cases were studied. KRAS codon 12 mutations were assessed by TaqMan allele-specific qPCR (p.G12V/R) and/or Sanger sequencing, and codon 13 and 61 mutations were screened by Sanger. Histological tumor capsule thickness was evaluated by morphometric analysis. Additionally, the phosphorylated form of the MAPK downstream effector ERK1/2 was investigated. Statistical analysis was carried out to test the association of KRAS mutations with clinicopathological parameters. KRAS c.35 G >T mutation, leading to p.G12V, was detected in 15 cases. A novel mutation in adenomatoid odontogenic tumor, c.34 G >C, leading to p.G12R, was detected in 12 cases and the other 11 were wild-type. Codon 12 mutations were not associated with the clinicopathological parameters tested. RAS mutations are known to activate the MAPK pathway, and we show that adenomatoid odontogenic tumors express phosphorylated ERK1/2. In conclusion, a high proportion of adenomatoid odontogenic tumors (27/38, 71%) have KRAS codon 12 mutations, which occur independently of the clinicopathological features evaluated. Collectively, these findings indicate that KRAS mutations and MAPK pathway activation are the common features of this tumor and some cancer types. Although it is unclear why different codon 12 alleles occur in different disease contexts and the complex interactions between tumor genotype and phenotype need clarification, on the basis of our results the presence of KRAS p.G12V/R favors the adenomatoid odontogenic tumor diagnosis in challenging oral neoplasm cases.

Odontomas and supernumerary teeth: is there a common origin?

The aim of the present work is to analyze all scientific evidence to verify whether similarities supporting a unified explanation for odontomas and supernumerary teeth exist. A literature search was first conducted for epidemiologic studies indexed by PubMed, to verify their worldwide incidence. The analysis of the literature data shows some interesting similarities between odontomas and supernumerary teeth concerning their topographic distribution and pathologic manifestations. There is also some indication of common genetic and immuno-histochemical factors. Although from a nosological point of view, odontomas and supernumeraries are classified as distinct entities, they seem to be the expression of the same pathologic process, either malformative or hamartomatous.

N-methyl-N-nitrosourea-induced changes in epithelial rests of Malassez and the development of odontomas in rats

Morphological changes in the epithelial rests of Malassez (ERM) and the development of odontogenic tumors in the molars of female Lewis rats treated at 4 weeks of age with a single intraperitoneal injection of 50 mg/kg of N-methyl-N-nitrosourea (MNU) were examined at 12, 18 and 30 weeks of age. Following MNU exposure, the total number and average area of ERM in the cervical and furcational regions of the first, second and third molars of the mandible and maxilla were compared with age-matched control animals. The number of ERM at each time point was significantly greater in the MNU-treated group compared to the control group, but there was no time-dependent increase in the number of ERM in either group. The area of ERM was significantly larger in the MNU-treated group compared to the control group at each time point, and it increased in a time-dependent manner in the MNU-treated group. No increases in the number or area of ERM were observed in the control group. At 30 weeks of age, 23% of the MNU-treated rats had developed odontomas (complex type) in the molar region as well as in the incisor region. Immunohistochemically, the expression of tyrosine receptor kinase A (TrkA) and cytokeratin 14 (CK14) decreased, whereas p63 expression remained high during ERM enlargement. In tumors, ameloblast-like cells were positive for amelogenin, TrkA and CK14 but negative for p63, whereas odontoblast-like cells were negative for all antigens examined. In conclusion, a single intraperitoneal injection of MNU caused the development of odontomas in the molar region; these tumors were possibly derived from ERM.

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.

Biological, cellular, and molecular characteristics of an inducible transgenic skin tumor model: a review

The genetically initiated Tg.AC transgenic mouse carries a transgene consisting of an oncogenic v-Ha-ras coding region flanked 5' by a mouse zeta-globin promoter and 3' by an SV-40 polyadenylation sequence. Located on chromosome 11, the transgene is transcriptionally silent until activated by chemical carcinogens, UV light, or full-thickness wounding. Expression of the transgene is an early event that drives cellular proliferation resulting in clonal expansion and tumor formation, the unique characteristics now associated with the Tg.AC mouse. This ras-dependent phenotype has resulted in the widespread interest and use of the Tg.AC mouse in experimental skin carcinogenesis and as an alternative carcinogenesis assay. This review examines the general biology of the tumorigenic responses observed in Tg.AC mice, the genetic interactions of the ras transgene, and explores the cellular and molecular regulation of zeta-globin promoted transgene expression. As a prototype alternative model to the current long-term rodent bioassays, the Tg.AC has generated a healthy discussion on the future of transgenic bioassays, and opened the doors for subsequent models for toxicity testing. The further exploration and elucidation of the molecular controls of transgene expression will enhance the usefulness of this mouse and enable a better understanding of the Tg.AC's discriminate response to chemical carcinogens.

mRNA expression and phenotype of odontogenic tumours in the v-Ha-ras transgenic mouse

Ameloblastomas are the most common odontogenic neoplasia in humans, and although typically considered locally invasive and benign, frequently recur subsequent to surgical resection. The Tg.AC transgenic mouse carrying the v-Ha-ras oncogene has been found to spontaneously develop ameloblastoma-like tumours (35% by 1 year of age) that are rare in the wild type FVB background strain.

OBJECTIVE

The purpose of this study was to characterise the mRNA expression of genes in the mouse tumours that are either expressed in human ameloblastomas or essential for normal odontogenesis and to correlate the expression to the histological phenotype.

STUDY METHODS

Histological, immunohistochemical and RT-PCR studies were used to evaluate clinically demonstrable odontogenic tumours occurring spontaneously in seven Tg.AC v-Ha-ras transgenic mice (homozygous, at 7 months of age or heterozygous at 11 months of age).

RESULTS

Most genes profiled were expressed in all tumour samples, however three (amelogenin, matrix metalloproteinase-20 (MMP-20) and Dlx7) displayed differential expression. In addition, only the most highly differentiated tumour stained positively for collagen. In most cases, the variable expression could be explained by reference to the histological phenotype, although differences in gene expression were apparent within the Type 2 and the mixed phenotype tumours.

CONCLUSIONS

These data confirm that many of the genes thought to be important in odontogenesis and odontogenic tumour formation in humans are also expressed in these murine ameloblastoma-like tumours however genes associated with terminal differentiation of ameloblasts demonstrate differential expression between the tumour phenotypes.

Astrocyte differentiation states and glioma formation

Gliomas are the most common primary malignancy in human central nervous system. Many similarities in cell morphology and expression of markers exist between cancerous cells and normal undifferentiated progenitor cells. At the molecular level, many important gene products are causally implicated in both the glial differentiation process and glial neoplasm formation. These observations raise the question of to what degree cell differentiation state influences glioma formation. In this review, we discuss new insights into the parallels between glial differentiation and glioma formation as well as the potential application of differentiation-inducing therapy.

The Tg.AC (v-Ha-ras) transgenic mouse: nature of the model

The Tg.AC (v-Ha-ras) transgenic mouse model provides a reporter phenotype of skin papillomas in response to either genotoxic or nongenotoxic carcinogens. In common with the conventional bioassay, the Tg.AC model responds to known human carcinogens and does not respond to noncarcinogens. It also does not respond to most chemicals that are positive in conventional bioassays principally at sites of high spontaneous tumor incidence. The mechanism of response of the Tg.AC model is related to the structure and genomic position of the transgene and the induction of transgene expression through specific mediated interactions between the chemicals and target cells in the skin.

Use of transgenic animals for carcinogenicity testing: considerations and implications for risk assessment

Advances in genetic engineering have created opportunities for improved understanding of the molecular basis of carcinogenesis. Through selective introduction, activation, and inactivation of specific genes, investigators can produce mice of unique genotypes and phenotypes that afford insights into the events and mechanisms responsible for tumor formation. It has been suggested that such animals might be used for routine testing of chemicals to determine their carcinogenic potential because the animals may be mechanistically relevant for understanding and predicting the human response to exposure to the chemical being tested. Before transgenic and knockout mice can be used as an adjunct or alternative to the conventional 2-year rodent bioassay, information related to the animal line to be used, study design, and data analysis and interpretation must be carefully considered. Here, we identify and review such information relative to Tg.AC and rasH2 transgenic mice and p53+/- and XPA-/- knockout mice, all of which have been proposed for use in chemical carcinogenicity testing. In addition, the implications of findings of tumors in transgenic and knockout animals when exposed to chemicals is discussed in the context of human health risk assessment.

Current status and use of short/medium term models for carcinogenicity testing of pharmaceuticals--scientific perspective

Short- and medium-term rodent bioassays have been proposed under ICH guidelines for use in testing for the carcinogenic potential of pharmaceuticals. Further evaluation of these models is needed urgently and coordinated efforts are in progress worldwide to expand the available database. Models currently being investigated include transgenic mice (Tg-rasH2, Tg.AC, p53(+/-), XPA(-/-)) and neonatal mice. As more data become available on the performance of these assays, regulatory and industry scientists will be faced with the difficult challenge of determining how the performance (accuracy) of each assay will be measured and deciding which assays have value in the risk assessment process.

Photocarcinogenesis and susceptibility to UV radiation in the v-Ha-ras transgenic Tg.AC mouse

The v-Ha-ras transgenic Tg.AC mouse line has proven to be a useful model for the study of chemical carcinogenic potential. We undertook experiments designed to study the effect of the physical carcinogen, UV radiation, on tumorigenesis in this mouse strain. Following a total of three exposures on alternating days to a radiation source covering a cumulative UVR exposure range of 2.6-42.6 kJ per m2, squamous papillomas developed by 4 wk after initial exposure in a dose-dependent manner. Malignancies developed within 18-30 wk following the initial UVR exposure and were all diagnosed as squamous cell carcinoma or spindle cell tumors. In contrast to other mouse stains used in photocarcinogenesis studies, few p53 mutations were found in Tg.AC malignancies upon polymerase chain reaction-single stranded conformational polymorphism analysis of exons 4-8 followed by sequencing of suspicious bands; however, all tumors analyzed by in situ hybridization expressed the v-Ha-ras transgene. Immunohistochemical analysis of UVR-exposed skin taken 24 h after the last of three exposures (13.1 kJ per m2 total UVR) showed expression of p53 in hair follicles and in interfollicular epidermis, which indicates that the gene was functional. Thus, although there are some differences between the Tg.AC and other mouse models, these results suggest that the Tg.AC mouse may be a useful model for the study of acute exposure photocarcinogenesis.

Spontaneous and chemically induced proliferative lesions in Tg.AC transgenic and p53-heterozygous mice

Recently, the use of selected genetically altered mouse models in the detection of carcinogens after short-term chemical exposures has been evaluated. Studies of several chemicals conducted by the National Toxicology Program in Tg.AC transgenic and heterozygous p53-deficient mice have been completed recently and represent a major contribution to this effort, as well as the largest accumulation to date of toxicologic pathology data in these 2 lines of mice. The purpose of this report is to describe the proliferative target organ effects observed in this set of studies, as well as to present the tumor profile in the control groups of this data set. These findings provide a comprehensive toxicologic assessment of these 2 genetically altered mouse strains, which are of emerging importance in toxicologic pathology.

The genetic control of early tooth development

Most vertebrate organs begin their initial formation by a common, developmentally conserved pattern of inductive tissue interactions between two tissues. The developing tooth germ is a prototype for such inductive tissue interactions and provides a powerful experimental system for elucidation of the genetic pathways involved in organogenesis. Members of the Msx homeobox gene family are expressed at sites of epithelial-mesenchymal interaction during embryogenesis, including the tooth. The important role that Msx genes play in tooth development is exemplified by mice lacking Msx gene function. Msxl-deficient mice exhibit an arrest in tooth development at the bud stage, while Msx2-deficient mice exhibit late defects in tooth development. The co-expression of Msx, Bmp, Lefl, and Activin beta A genes and the coincidence of tooth phenotypes in the various knockout mice suggest that these genes reside within a common genetic pathway. Results summarized here indicate that Msxl is required for the transmission of Bmp4 expression from dental epithelium to mesenchyme and also for Lefl expression. In addition, we consider the role of other signaling molecules in the epithelial-mesenchymal interactions leading to tooth formation, the role that transcription factors such as Msx play in the propagation of inductive signals, and the role of extracellular matrix. Last, as a unifying mechanism to explain the disparate tooth phenotypes in Msxl- and Msx2-deficient mice, we propose that later steps in tooth morphogenesis molecularly resemble those in early tooth development.