L1PA2 transposons contribute abundant regulatory sequences in MCF7 breast cancer cell line

Transposons, a type of repetitive DNA elements, can contribute cis-regulatory sequences and regulate the expression of human genes. L1PA2 is a hominoid-specific subfamily of LINE1 transposons, with approximately 4,940 copies in the human genome. Individual transposons have been demonstrated to contribute specific biological functions, such as cancer-specific alternate promoter activity for the MET oncogene, which is correlated with enhanced malignancy and poor prognosis in cancer. Given the sequence similarity between L1PA2 elements, we hypothesise that transposons within the L1PA2 subfamily likely have a common regulatory potential and may provide a mechanism for global genome regulation. Here we show that in breast cancer, the regulatory potential of L1PA2 is not limited to single transposons, but is common within the subfamily. We demonstrate that the L1PA2 subfamily is an abundant reservoir of transcription factor binding sites, the majority of which cluster in the LINE1 5UTR. In MCF7 breast cancer cells, over 27% of L1PA2 transposons harbour binding sites of functionally interacting, cancer-associated transcription factors. The ubiquitous and replicative nature of L1PA2 makes them an exemplary vector to disperse co-localised transcription factor binding sites, facilitating the co-ordinated regulation of genes. In MCF7 cells, L1PA2 transposons also supply transcription start sites to up-regulated transcripts. These transcriptionally active L1PA2 transposons display a cancer-specific active epigenetic profile, and likely play an oncogenic role in breast cancer aetiology. Overall, we show that the L1PA2 subfamily contributes abundant regulatory sequences in breast cancer cells, and likely plays a global role in modulating the tumorigenic state in breast cancer.