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MA 19 - Mesothelioma: Bench to Bedside (ID 680)
- Event: WCLC 2017
- Type: Mini Oral
- Track: Mesothelioma
- Presentations: 1
MA 19.06 - Multiple Mechanisms Contribute to Downregulation of Tumour Suppressor microRNAs in Malignant Pleural Mesothelioma (ID 9745)
11:00 - 12:30 | Author(s): B. McCaughan
Malignant pleural mesothelioma (MPM) is a disease with an almost invariably fatal diagnosis with limited therapeutic options. Characteristic patterns of deregulated microRNA expression have been demonstrated in MPM, and many downregulated microRNAs have been shown to have tumour suppressor activity. However, apart from silencing of miR-34b/c by promoter hypermethylation and co-deletion of miR-31 with the CDKN2A locus, the mechanisms responsible for downregulation of other tumour suppressor miRNAs such as miR-16 are yet to be elucidated.
Tumour samples (n=60) were from MPM patients undergoing extrapleural pneumonectomy, and samples of pleura (n=23) collected from patients undergoing cardiac surgery were used as normal controls. MPM cells lines were obtained from the ATCC. Expression levels of mature microRNAs in MPM tumour samples and cell lines, and pri-miRs and miRNA host genes in cell lines, were determined by RT-qPCR. Copy number variation (CNV) was analysed by droplet digital PCR (ddPCR), and methylation was inferred by miRNA expression following decitabine treatment. MYC was analysed by Western blot, and expression modulated by siRNAs.
Analysis of microRNA expression in tumour samples revealed a consistent and significant downregulation of miR-15a (4-fold, P<0.01), 15b (10-fold, P<0.01), 16 (22-fold, P<0.05), 34a (1.6-fold, P<0.05), 34b (1.8-fold, P<0.01), 34c (2.3-fold, P<0.0001) and 193a (3.1-fold, P<0.001) compared with normal pleura. Copy number variation analysis showed evidence of heterozygous loss for miR-193a (4 of 5 cell lines) and miR-15a/16-1 (2 of 5), but no change in miR-15b/16-2. Treating cell lines with the demethylating agent decitabine resulted in dramatic upregulation only in the case of miR-34c. RNAi-mediated knockdown of c-MYC led to upregulation of miR-15b and 16, and to a lesser extent miR-15a, as well as a consistent increase in the miR-15b/16-2 host gene SMC4 and the miR-15a/16-1 host gene DLEU2. Analysing the expression of these microRNAs in the tumour samples revealed a strong correlation between miR-15b and 16 (R=0.793) and miR-34b and 34c (R=0.753), but not between others.
Our data suggest that a combination of deletion, hypermethylation and transcriptional regulation contribute to the downregulation of miR-15a/b, 16, 34a/b/c and 193a. In MPM, unlike other cancers, the downregulation of miR-15a/16-1, miR-15b/16-2 appears to be due to transcriptional changes rather than deletion or promoter hypermethylation. MYC appears to contribute to miR-16 downregulation primarily via control of SMC4 and the miR-15b/16-2 locus, suggesting that the transcriptional control of miR-16 expression by c-Myc contributes to the malignant phenotype of MPM.
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