Transcriptional suppression of the miR-15/16 family by c-Myc in malignant pleural mesothelioma

Our plain-English summary of Transcriptional suppression of the miR-15/16 family by c-Myc in malignant pleural mesothelioma, by M. Williams, Y.Y. Cheng, M.B. Kirschner, et al (Oncotarget, 2019).

The study

• Authors: M. Williams, Y.Y. Cheng, M.B. Kirschner, K.H. Sarun, K. Schelch, P. Winata, B. McCaughan, S. Kao, N. Van Zandwijk, G. Reid
• Journal: Oncotarget
• Published: 2019;10(41):4125-4138
• Cited by: 14 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

MicroRNA downregulation is frequent in malignant pleural mesothelioma (MPM), but the mechanisms responsible for loss of miR-15/16 and miR-193a are yet to be elucidated and were investigated in this study. Copy Number Variation (CNV) of microRNA-coding genes was analyzed in MPM cells by digital droplet PCR (ddPCR) and revealed heterozygous loss of miR-193a and miR-15a/16-1, but no change in miR-15b/16-2. Epigenetic control of microRNA expression was inferred following decitabine and Trichostatin A (TSA) treatment which did not substantially affect microRNA expression. Knockdown of c-Myc expression led to upregulation of SMC4, miR-15b and 16, and to a lesser extent DLEU2 and miR-15a, whereas c-Myc overexpression repressed microRNA expression. Chromatin immunoprecipitation (ChIP) assays confirmed the interaction of c-Myc with the DLEU2 and SMC4 promoters. Tumor microRNA expression was determined in samples from MPM patients, with samples of pleura from cardiac surgery patients used as controls. In tumor samples, a strong correlation was observed between the expression of miR-15b and 16 (R2=0.793), but not miR-15a and 16. Our data suggest that in MPM, the downregulation of miR-15/16 is due to transcriptional repression by c-Myc, primarily via control of the miR-15b/16-2 locus, while miR-193a-3p loss is due to genomic deletion.

In plain English

This laboratory study looked at why small regulatory molecules called microRNAs (miR-15/16 and miR-193a) are reduced in malignant pleural mesothelioma. It did not study asbestos exposure. The authors used five mesothelioma cell lines, one non-cancerous mesothelial cell line, and 60 stored tumor samples from patients who had extrapleural pneumonectomy. One copy of the MIR193A gene was missing in most cell lines tested, and one copy of MIR15A/MIR16-1 was missing in two lines. Drugs that reverse chemical gene silencing changed miR-15/16 levels little in most cells. Lowering the c-Myc protein raised miR-16 about 2-fold in MSTO cells and 1.7-fold in H28 cells, and raised miR-15b 4.4-fold and 2-fold. Adding extra c-Myc to H28 cells lowered these microRNAs. c-Myc was found attached to the promoter regions of the host genes DLEU2 and SMC4. In the tumor samples, miR-15b and miR-16 levels were strongly correlated (R-squared of 0.793). The authors concluded that genomic deletion and transcriptional repression, rather than hypermethylation, contribute to the loss of these microRNAs, with c-Myc acting mainly through the miR-15b/16-2 locus.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory study using mesothelioma cell lines (gene copy-number testing, drug treatments, gene knockdown and overexpression, chromatin immunoprecipitation), plus microRNA analysis of archived tumor samples
• Who was studied: Mesothelioma cell lines MSTO-211H, H28, H2052, H2452 and VMC23, and the immortalized mesothelial cell line MeT-5A. Also 60 formalin-fixed tumor samples from a previously reported series of malignant pleural mesothelioma patients who had extrapleural pneumonectomy (Sydney), and 23 normal pleural tissue samples from patients having cardiac or aortic surgery.
• Exposure: No asbestos exposure was studied. The experiments examined the cancer-related protein c-Myc and the drugs decitabine and Trichostatin A (TSA) in cell lines.
• Main results:
  • Gene copy testing in cell lines found loss of one copy of MIR15A/MIR16-1 in H2052 and H2452 cells, no deletion of MIR15B/MIR16-2, and loss of one copy of MIR193A in most cell lines tested.
  • Decitabine raised miR-34c about 250-fold, but produced negligible changes in miR-15a, miR-15b and miR-16 in most cell lines. TSA did not substantially alter miR-15a, miR-15b, miR-16 or miR-193a-3p.
  • Knocking down c-Myc raised miR-16 2-fold in MSTO cells and 1.7-fold in H28 cells, and raised miR-15b 4.4-fold (MSTO) and 2-fold (H28), with little effect on miR-15a. It also significantly reduced proliferation of MSTO cells.
  • After c-Myc knockdown, pri-miR-15b rose 10.7-fold in MSTO and 2.7-fold in H28 cells, and SMC4 rose 5.9-fold in MSTO and 8.1-fold in H28 cells.
  • Overexpressing c-Myc in H28 cells lowered transcription from both the miR-15b/16-2 and miR-15a/16-1 loci.
  • Chromatin immunoprecipitation showed c-Myc associated with the SMC4 promoter (30-fold enrichment in MSTO, 11-fold in H28). DLEU2 promoter amplicons S, 1A and 1B showed 50-, 35- and 72-fold enrichment.
  • In the 60 tumor samples, miR-15b and miR-16 levels were strongly correlated (R2 = 0.793), but miR-15a and miR-16 were not (R2 = 0.3744 per the figure legend).
• The authors' conclusion: The authors concluded that a combination of genomic deletion and transcriptional repression, rather than hypermethylation, contributes to the downregulation of miR-193a, miR-15a/16-1 and miR-15b/16-2 in malignant pleural mesothelioma. In their view, c-Myc appears to regulate miR-15/16 mainly through the miR-15b/16-2 locus.

Topics

Medical terms in this study, explained in our Lexicon: Pleural Mesothelioma, Pleura.

Related studies

• A link between the fibroblast growth factor axis and the miR-16 family reveals potential new treatment combinations in mesothelioma — K. Schelch, M.B. Kirschner, et al. (2018)
• Dysregulated Expression of the MicroRNA miR-137 and Its Target YBX1 Contribute to the Invasive Characteristics of Malignant Pleural Mesothelioma — T.G. Johnson, K. Schelch, et al. (2018)
• Tumor Suppressor microRNAs Contribute to the Regulation of PD-L1 Expression in Malignant Pleural Mesothelioma — S.C. Kao, Y.Y. Cheng, et al. (2017)
• Long non coding RNAs (lncRNAs) are dysregulated in Malignant Pleural Mesothelioma (MPM) — C.M. Wright, M.B. Kirschner, et al. (2013)
• Restoring expression of miR-16: a novel approach to therapy for malignant pleural mesothelioma — G. Reid, M.E. Pel, et al. (2013)
• miR-193a-3p is a potential tumor suppressor in malignant pleural mesothelioma — M. Williams, M.B. Kirschner, et al. (2015)

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