Molecular and genetic aspects of ameloblastoma pathogenesis

Background: Ameloblastoma (AM) is a benign epithelial odontogenic tumour characterized by its progressive growth, morbidity and a high rate of recurrence if not adequately removed. Worldwide, AM represents approximately 9 to 11% of all odontogenic tumours (OTs), whereas in Nigeria, this tumour accounts for 63-73% of all OTs. Although benign, its biological aggression and typical late presentation poses a significant therapeutic challenge. Current treatment methods include both conservative treatments and resection. There is mounting evidence that mutations in BRAF and SMO and activation of intracellular pathways, have important roles in the development of AM. Nevertheless, it has been insufficient to explain its tumourigenesis.
Purpose: To characterise the transcriptome and coding mutations in AM in vivo and in vitro, to identify relevant genes and molecular pathways that may be relevant in its aetiopathogenesis.
Methods: Whole-exome and RNA sequencing were performed in 13 mandibular AM samples and in two AM cell lines. Validation of putative genes was done with Sanger Sequencing, qPCR, western blotting, and immunohistochemistry.
Results: In addition to BRAF p.V600E mutation, each AM carried an average of three mutations affecting cancer driver genes. With an unsupervised hierarchical analysis, besides the two distinct transcriptome profiles associate with the BRAF mutation status, two other molecular clusters were found. BRAF WT and cluster 1 showed enrichment of the epithelial-mesenchymal transition (EMT) gene set, activation of oncogenic pathways such as EGFR, KRAS, and TGF-β. CDH11 and TGM2, two putative genes associated with EMT were upregulated, and thus selected and validated in tissues and cell lines.
Conclusions: Our results showed that even in the absence of BRAF mutation, AM shows activation of other oncogenic pathways, including MAPK. In addition, the presence of distinct transcriptome profiles among the samples suggests underlying inter-tumour molecular heterogeneity that may be considered for the use of molecular targeted therapy.