Alterations in miRNA processing and expression in pleomorphic adenomas of the salivary gland

Genome-wide microRNA (miRNA) expression profiling of salivary gland pleomorphic adenomas revealed a distinct expression signature consisting largely of upregulated miRNAs compared with matched normal tissue. Microarray data were confirmed by quantitative real time RT-PCR (q-RTPCR). Five miRNA genes upregulated in the tumours were found in close proximity to fragile sites and/or cancer associated genomic regions. Interestingly, q-RTPCR revealed an increase in the expression of components of the miRNA processing machinery (Dicer, Drosha, DGCR8 and p68) in tumours suggesting that the deregulation of miRNA expression may result from increased miRNA biogenesis. Target gene prediction analysis of the altered miRNAs indicated that genes in a number of signalling pathways important in tumourigenesis including WNT, MAPK and JAK-STAT were overrepresented. Significantly, the oncogene PLAG1 was overexpressed in our cohort and may be potentially regulated by these miRNAs. This is the first study to examine changes in the miRNA milieu in pleomorphic adenoma, the most common salivary gland tumour. This study has demonstrated an upregulation of both miRNAs genes and an upregulation of the miRNA processing machinery. These changes may be potential underlying mechanisms for the development of these benign tumours.