Circular RNA CpG Island Hypermethylation-Associated Silencing in Human Cancer

Although less than 2% of the human genome code for proteins, most studies in cancer research has focused in this small portion of our DNA. However, it has been recognized in the last years that non-coding RNAs (ncRNAs) also participate in cellular transformation. In this context, microRNAs and long noncoding RNAs (lncRNAs) can act as oncogenes or tumor suppressor genes. Recent work have also demonstrated that ncRNAs with growth-inhibitory functions can undergo promoter CpG island hypermethylation-associated silencing in tumorigenesis. Herein, we wondered whether circular RNAs (circRNAs), a type of RNA transcripts lacking 5’-3´ends and forming closed loops that are gaining relevance in cancer biology, are also a target of epigenetic inactivation in tumors. To tackle this issue, we have used cancer cells genetically deficient for the DNA methyltransferase enzymes in conjuction with circRNA expression microarrays. We have found that the loss of DNA methylation provokes a release of circRNA silencing. We have particularly identified that promoter CpG island hypermethylation of the genes TUSC3 (tumor suppressor candidate 3), POMT1 (protein O-mannosyltransferase 1), ATRNL1 (attractin-like 1) and SAMD4A (sterile alpha motif domain containing 4A) is linked to the transcriptional downregulation of both linear mRNA and the hosted circRNA. Although a role in the control of the parental gene has been shown for some circRNAs, we did not observe changes in TUSC mRNA levels upon TUSC3 circ104557 overexpression. Most importantly, data mining for 5’-end CpG island methylation of TUSC3, ATRNL1, POMT1 and SAMD4A in a large collection of human cancer cell lines and primary tumors showed that the epigenetic defect was commonly observed among different tumor types, where it was also associated with the diminished expression of the corresponding transcript. Our findings support a role for circRNA DNA methylation-associated loss in human cancer. We compared the circRNA expression profile of 3 DKO and 3 HCT116 cell line replicates by microarray analysis. We also compared the profiles with 3 normal colon patient samples.