Transcriptional Activation of Transposable Element (TE)-Associated Genes Is Frequently Associated with Altered Promoter Methylation in Placenta and Melanoma

Transposable elements (TEs) play important roles during development and disease, including through transcriptional activation of TE-associated genes during early human development. Moreover, based on the functional and epigenetic similarities between early development and cancer, TE-associated genes contribute not only to early human development, but frequently contribute to cancer progression. In this study, we hypothesised that recruitment of TE-associated genes during cancer onset occurs through epigenetic regulatory processes, especially involving DNA hypomethylation accompanied by transcriptional upregulation of early developmental pathways, such that, when reactivated inappropriately in later life, they may drive malignancy. It is unknown, however, to what extent DNA methylation changes are critically involved in the transcriptional activation of TE-associated genes. Accordingly, to investigate this we used the RepExpress tool to identify developmentally regulated TE-associated genes in placenta and human embryonic stem cells (hESCs), which we then investigated by targeted deep bisulfite sequencing (TDBS) to determine the methylation status of the identified TE-associated genes in placenta, somatic tissues, and melanoma cell lines. Outcomes suggest that DNA methylation may be one of the regulatory factors underscoring transcriptional activation of TE-associated genes, but that methylation is not necessarily the sole factor involved in regulating the transcriptional activation of TE-associated genes during malignant transformation.