Loss of Imprinting and Allelic Switching at the DLK1-MEG3 Locus in Human Hepatocellular Carcinoma

Abstract

Deregulation of imprinted genes is an important molecular mechanism contributing to the development of cancer in humans. However, knowledge about imprinting defects in human hepatocellular carcinoma (HCC), the third leading cause of cancer mortality worldwide, is still limited. Therefore, a systematic meta-analysis of the expression of 223 imprinted loci in human HCC was initiated. This screen revealed that the DLK1-MEG3 locus is frequently deregulated in HCC. Deregulation of DLK1 and MEG3 expression accompanied by extensive aberrations in DNA methylation could be confirmed experimentally in an independent series of human HCC (n = 40) in more than 80% of cases. Loss of methylation at the DLK1-MEG3 locus correlates linearly with global loss of DNA methylation in HCC (r2 = 0.63, p<0.0001). Inhibition of DNMT1 in HCC cells using siRNA led to a reduction in MEG3-DMR methylation and concomitant increase in MEG3 RNA expression. Allele-specific expression analysis identified loss of imprinting in 10 out of 31 informative samples (32%), rendering it one of the most frequent molecular defects in human HCC. In 2 cases unequivocal gain of bi-allelic expression accompanied by substantial loss of methylation at the IG-DMR could be demonstrated. In 8 cases the tumour cells displayed allelic switching by mono-allelic expression of the normally imprinted allele. Allelic switching was accompanied by gains or losses of DNA methylation primarily at IG-DMR1. Analysis of 10 hepatocellular adenomas (HCA) and 5 cases of focal nodular hyperplasia (FNH) confirmed that this epigenetic instability is specifically associated with the process of malignant transformation and not linked to increased proliferation per se. This widespread imprint instability in human HCC has to be considered in order to minimize unwanted side-effects of therapeutic approaches targeting the DNA methylation machinery. It might also serve in the future as predictive biomarker and for monitoring response to epigenetic therapy.

MEG3 inhibits proliferation and invasion and promotes apoptosis of human osteosarcoma cells

“…MEG3 is expressed in many human normal tissues, but has failed to express in a variety of human tumour cells, such as non-small cell lung cancer, bladder cancer, prostate cancer and other cancers (9)(10)(11)(12)(13)(14). Additionally, many studies have articulated that MEG3 gene plays an inhibiting role in various tumour cells (15)(16)(17)(18)(19)(20).…”

Epigenetic repression of long non-coding RNA MEG3 mediated by DNMT1 represses the p53 pathway in gliomas

“…MEG3, identified as a member of lncRNAs, is an imprinted gene with maternal expression which encodes a non-coding RNA (10). Dysregulation of MEG3 has been found in various human tumors including hepatocellular carcinoma, renal cell carcinoma and neuroblastoma (11,12). Moreover, MEG3 expression was lost in the majority of clinically non-functioning human pituitary tumors, and it suppressed cancer cell growth, stimulated p53-mediated transcriptional activation, and selectively activated p53 target genes (13,14).…”

Non-coding RNAs: New therapeutic targets and opportunities for hepatocellular carcinoma

“…Additionally, the tumor-suppressive effect of MEG3 has been confirmed in vivo and in vitro, and analyzing the accumulation of p53 using Kaplan-Meier analyses and Cox proportional regression deduced that MEG3 promoted HCC cell proliferation and apoptosis. It has been suggested that MEG3 may be a potential biomarker for predicting the survival rate of HCC patients [74] .…”