While a hit-and-run typically leads to an arrest, in the world of epigenetic editing it turns out to be the key to evading cell cycle arrest (senescence) and promoting the transformation of a law-abiding cell into a cancerous criminal! The hypermethylation of promoters at the wrong time and wrong place is a trademark of cancer, and the labs of Gabriella Ficz (Queen Mary University of London, UK) and Tomasz Jurkowski (University of Stuttgart, Germany) sought to interrogate this phenomenon. To aid their investigation, this crime-solving team employed the ultra-potent dCas9-Dnmt3a-Dnmt3L methyltransferase previously established the Jurkowski lab.
The group turned to primary human myoepithelial cells from the healthy breast tissue of multiple donors and analyzed DNA methylation via the EPIC array. They chose to target a panel of tumor suppressor genes CDKN2A, RASSF1, HIC1 and PTEN, given the link between promoter hypermethylation and breast cancer.
The team delivered their designer methyltransferase and 26 guide gRNAs targeting the abovementioned promoters by transient transfection, where the term hit-and-run represents the temporary presence of their designer system in the edited cells. Here’s what they found:
- A greater than 20% increase in the methylation of the target genes
- The only decrease in gene expression caused by their construct occurs in CDKN2A transcripts
- The edited cells are hyper-proliferative and evade senescence
- However, the edited cells are not immortal and eventually enter cell cycle arrest through a different telomere-dependent mechanism
- RNA-seq revealed altered gene expression profiles related to senescence
- This subset of genes more closely resembles unmodified early passage cells
- By breaking up their panel of targets to investigate different regions of single genes, the team discovered that the repression of p16, a CDKN2A transcript, drove the observed alterations
First author Emily Saunderson shares, “This has been an amazing project to work on as there isn’t really a rule book yet when it comes to epigenetic editing using CRISPR so we’ve been learning as we go. I think a key factor to the success of the project has been the combination of expertise from different groups.”
Senior author Gabriella Ficz concludes, “It’s surprising that cells from several healthy individuals are so permissive to gaining this epigenetic change and that one ‘hit’ from an epigenetic editing tool is sufficient to set off this chain reaction of epigenetic inheritance and establish a cancer cell-like gene expression signature. Epigenetic fluctuations happen all the time in our cells. We know that, during ageing, our epigenome is progressively distorted – so called ‘epigenetic drift’. It will therefore be exciting to find out if this drift is responsible for initiating or accelerating ageing-associated diseases. Age is the biggest risk in cancer so our work highlights the importance of understanding the mechanism behind epigenetic drift.”
Drift on over to Nature Communications, November 2017