The underlying molecular pathways associated with the pathogenesis of ameloblastomas are not well established yet. Previous investigations have assessed the molecular and genetic alterations related mainly to apoptosis, allelic loss of tumour suppressor genes, deregulation of the Sonic Hedgehog signalling pathway, and the clonality of these tumours
[3, 24, 25].
Matrix metalloproteinases are involved in the degradation of collagen, as well as bone matrix, and have been shown to play a key role in the local invasiveness of ameloblastoma cells
[15, 26]. Overexpression of MMP-2 and MMP-9 was associated with the infiltrative behaviour of ameloblastomas, as well as that of several malignant neoplasms
[17, 27]. The suppression of MMP-2 activity was able to inhibit the invasiveness of ameloblastoma cells in vitro[14, 15]. Furthermore, it was suggested that increased expression of MMP-9 may be involved in the proliferation and invasive behaviour of ameloblastomas
Some papers, including studies from our research group, have demonstrated epigenetic alterations in odontogenic tumours
[5, 6, 9, 10, 28]. In the present study, we hypothesised that methylation may regulate the expression of MMP-2 and MMP-9 in ameloblastomas. We also investigated the association between methylation and the transcription levels of these genes. As most of the ameloblastoma samples were of the solid follicular type, we were not able to analyse possible associations between each clinical or histological type and the molecular data.
MMPs play an important role in collagen matrix remodelling in physiologic and pathologic processes, such as those found in basal membranes, dental follicle tissue and tumour metastasis
[27, 29]. Although MMP-2 is related to ameloblastoma pathogenesis, it seems to be constitutively expressed in physiologic tissues and many cell types and to exhibit characteristics of a housekeeping gene
[30–32]. Perhaps this could explain the similar expression levels of MMP-2 mRNA in ameloblastomas and healthy gingiva. Furthermore, our data suggest that MMP-2 expression in ameloblastomas may not be modulated by methylation because the methylation profile for this gene did not correlate with MMP-2 transcript levels in this odontogenic tumour.
The ameloblastomas presented an unmethylated profile of MMP-2 and MMP-9 genes compared to gingiva. Furthermore, along with unmethylated MMP-9, this tumour showed increased transcription of MMP-9 when compared to the control group. The important role of methylation in epigenetic silencing is well established, particularly through regulatory mechanisms of transcription. Accordingly, our data suggest that an unmethylated profile of the MMP-9 gene in ameloblastomas may be a permissive event allowing the binding of transcription factors to DNA, thus favouring MMP-9 gene transcription.
All of the ameloblastomas showed MMP-9 protein expression and were mostly unmethylated for MMP-9, so it was not possible to assess if the transcription of the gene was correlated with its methylation status. However, our study suggests that the increased transcription of MMP-9 in ameloblastomas could possibly be influenced by unmethylation of the gene. The evident protein expression, identified by zymography, provides additional evidence supporting the possible gene regulation by unmethylated MMP-9. It is interesting to note that hypomethylation of the MMP-2 and MMP-9 genes increases gene expression and contributes to cancer cell invasiveness and tumourigenesis in malignant neoplasms, such as prostate cancer and lymphoma