L1 and HERV-W retrotransposons are hypomethylated in human ovarian carcinomas

Abstract

Wide-spread hypomethylation of CpG dinucleotides is characteristic of many cancers. Retrotransposons have been identified as potential targets of hypomethylation during cellular transformation. We report the results of an preliminary examination of the methylation status of CpG dinucleotides associated with the L1 and HERV-W retrotransposons in benign and malignant human ovarian tumors. We find a reduction in the methylation of CpG dinucleotides within the promoter regions of these retroelements in malignant relative to non-malignant ovarian tissues. Consistent with these results, we find that relative L1 and HERV-W expression levels are elevated in representative samples of malignant vs. non-malignant ovarian tissues.

Figure 1

Hypomethylation and expresion of L1 and HERV-W elements in ovarian cancer. Genomic DNA was digested either with MspI (left) or HpaII (right), and hybridized with probes specific for the promoter regions of L1 (A) or HERV-W (B) elements. The restriction enzymes MspI and HpaII recognize the sequence CCGG but HpaII only cuts when the recognition sequence is unmethylated at the inner cytosine (i.e., CCGG) while MspI is indifferent to the methylation status of the inner cytosine. Brackets indicate bands from restriction cut sites internal to the elements (B = benign cystic mass; LMP = low-malignancy potential or borderline tumor; N = normal ovary. (C) Real time RT-PCR was performed to determine expression levels of LINE-1 and HERV-W elements in representative malignant and non-malignant samples. Normalized values (retroelement expression value divided by expression value of the RPS27A control gene. Shown is the average of 3 replicate assays per sample ± SE. RPS27A expression has been previously determined to be unchanged between the malignant and non-malignant samples examined in this study (see text for details).