WIKIMEDIA; CHRISTOPH BOCK, MAX PLANCK INSTITUTE FOR INFORMATICSOur desire to manipulate genomes is not new; selective breeding, genetic and, more recently, genome engineering have greatly advanced our understanding of how genes shape phenotypes. However, epigenetic processes also influence how cells use genetic information. Like pointing out sections of a book with colored tags or Post-Its, the cell physically sticks chemical tags onto the genome, labeling features such as genes or regulatory elements. To date, millions of these tags—chromatin marks—have been profiled across different tissues and cell types through international efforts. Yet, until recently, we were not able to assess the influence of individual marks on gene activity because it was only possible to alter chromatin marks globally through mutational approaches or pharmacological inhibition. Emerging technologies for epigenome engineering now make it possible to interrogate the function of individual chromatin marks by adding them to, or removing them from, single locations of interest in the genome.
Targeted modification is achieved by fusing an existing chromatin modifying enzyme (or a functional part of such an enzyme) to a programmable DNA binding domain. Although programmable DNA binding domains have been around for some time, the recent appropriation of the bacterial CRISPR/Cas system from Streptococcus pyogenes has made it considerably easier to generate a targetable protein in the laboratory: the chromatin-modifying enzyme of choice is simply fused to the catalytically-inactive Cas9-protein (dCas9). dCas9 is then targeted to a specific genomic locus via a separate, synthetic RNA molecule known as the guide RNA (gRNA). The base sequence of the gRNA thus determines the DNA binding specificity of the fusion protein. A range of chromatin modifiers have already been engineered in this way, allowing researchers to ...
