A dance of histones silences transposable elements in pluripotent stem cells – EurekAlert


IMAGE: The mechanism of a peculiar type of heterochromatin, used by embryonic stem cells to silence ‘parasitic’ DNA-elements within the context of their highly dynamic pluripotent chromatin. view more 

Credit: Simon Elsässer

So-called transposons are abundant DNA-elements found in every eukaryotic organism as a consequence of their ability to jump and multiply within the host genome. Their activity represents a threat to the integrity of the host genome and thus the host cell engages a number of protective mechanisms to silence the expression of transposons. It is known that some of these mechanisms fail in cancer cells and also ageing cells, leading to a mobilization of transposons with largely unknown consequences. Histones, the proteins that package the genome in the eukaryotic nucleus, are key to the most fundamental line of defense to transposons. By forming a highly compacted array, so-called heterochromatin, they render the associated DNA sequence inert to being read and expressed. Heterochromatin is defined by characteristic modifications to histone proteins and DNA, such as histone H3 K9 trimethylation and DNA CpG methylation.

Simon Elsässer’s team studied endogenous retroviral elements (ERVs), a particularly active and abundant family of transposable elements in the mouse genome, which are in fact remnants of once-active viruses. Curiously, while they