Mouse models have been great to science, they’ve given insight into humans in ways simply not possible in humans. However, while there are striking similarities in the epigenetics of mice and men, there are also undoubtedly some fundamental differences. Now, a team from the Broad Institute of MIT and Harvard consisting of the labs of Alexander Meissner, J. Keith Joung, and John Rinn has shown that not only does CRISPR-Cas9 work beautifully in human systems, where it can inactivate the DNMTs with precise mutations, but that study into human embryonic stem cells can reveal some fundamental differences between us and mice.

Here’s what went down:

Using the CRISPR-Cas9 genome editing system the DNA methyltransferases (DNMT1, DNMT3A and DNMT3B) were precisely inactivated in human embryonic stem cells (ESCs).

Using some whole genome sodium bisulfite sequencing they found that knocking out DNMT3A and DNMT3B alone or together results in viable pluripotent cell lines that have their very own distinct DNA methylation landscapes.

Interestingly, the DNMT1 mutation results in rapid cell death in human ESCs, which is not the case of mouse ESCs.

The group then overcame the immediate lethality in human ESCs by creating a a doxycycline-responsive DNMT1 rescue line that ultimately results in homozygous DNMT1 mutants. But once the inducer (doxycycline) is added to the mix there is a rapid global loss of DNA methylation and extensive cell death, driving home the absolute need for DNMT1 in human ESCs. Ultimately, using CRISPR-Cas9 the group was able to show not only can human ESCs be edited precisely, but they generated comprehensive single base-pair maps of the targets of the DNMTs, which sheds light on the differences between DNMT1 in human and mouse ESCs.

Go and take in the whole breakthrough at Nature Genetics, April 2015.