Activation of neuronal genes via LINE-1 elements upon global DNA demethylation in human neural progenitors

DNA methylation is an important epigenetic modification that is thought to contribute to the maintenance of genomic integrity in somatic cells, in part through the silencing of transposable elements (TEs). In this study we used CRISPR/Cas9 mediated gene editing to disrupt DNMT1, the key maintenance methyltransferase in somatic human cells. Surprisingly, and in contrast to findings in mouse, inactivation of DNMT1 in human neural progenitor cells (hNPCs) resulted in viable proliferating cells that maintained the expression of appropriate marker genes. Removal of DNA methylation in hNPCs resulted in a specific activation of hominid-specific LINE-1 elements (L1s), while other classes of TEs remained silent. We also found that the transcriptionally activated L1s acted as alternative promoters for many protein-coding genes involved in neuronal functions, uncovering an L1-based transcriptional network influencing neuronal protein-coding genes. Our results prove novel mechanistic insight into the role of DNA methylation in somatic human cells.