How the gene editing system known as CRISPR-Cas9 is transforming how we create animal models of disease.

Last year, a Chinese group became the first to inject a person with cells that contain genes edited using the CRISPR–Cas9 technique; future trials using the same system are in the works in the US and China.

But while much of the excitement around this hot new biological tool relates to its potential for treating human disease or editing human embryos, it is also profoundly changing how we create animal models of disease. The CRISPR-Cas9 system—which consists of a Cas9 enzyme that snips through DNA like a pair of molecular scissors and a small RNA molecule that directs the scissors to a specific sequence of DNA—is fast becoming the preferred methodology for engineering mice because it is much faster than conventional methods. From cancer to Huntington’s, scientists are using CRISPR-Cas9 to generate mouse models of disease and some scientists have begun using the CRISPR-Cas9 system to generate other animal models.

“To make a transgenic mouse it used to take 12-18 months using traditional techniques, says Iva Morse, Chief Scientific Officer of Research Models and Services at Charles River. “With the latest techniques such as the CRISPR-Cas9 technology it takes anywhere between 3 months to 9 months.”

Given that there are literally thousands of genetically modified animals, and sponsored efforts to knock out every one of those genes, the efficiencies made possible with the CRISPR-Cas9 system is destined to have an enormous impact on laboratories like Charles River that produce transgenic mice on an industrial scale—and the companies that use research animals—primarily mice and rats—to  develop and test their drug compounds. 

Last fall Charles River launched a full, end-to-end service offering CRISPR-Cas9 genome engineering technology through a licensing agreement with the Broad Institute of MIT and Harvard, and developed partnerships with three key groups globally for in vivo model creation services. Other laboratories have also harnessed the power of the CRISPR-Cas9 system to create models on an industrial scale and there are now mouse models—such as the NCG triple-immunodeficient mouse model deficient in T, B and Natural Killer cells—being generated with the CRISPR-Cas9 system.  

“The ability to cut the genome and modify it as game-changer in terms of model creation,” says Prem Premsrirut, Co-founder and Chief Executive Officer of Mirimus, which is partnering with Charles River in using CRISPR and another technology, RNA interference (RNAi) to engineer mouse models for clients.

Mirimus, which was founded in 2010 by scientists from Cold Spring Harbor and Harvard Medical School, started out using RNAi—a natural process of post-transcriptional silencing of genes by small fragments of the nucleic acid—to drive innovative approaches for loss-of-function genetic studies and provide a new path to cancer target identification and validation in vivo. Not long after the first major papers on CRISPR appeared, Mirimus jumped on that technology as well.

But it’s still very early days. While it is relatively easy using CRISPR to create knock-out models—where you knock out or inactivate a gene by replacing it or disrupting it with an artificial piece of DNA—it is much harder to knock in genes, says Premsrirut.

“We still don’t understand how CRISPR cuts in detail to get it to work efficiently every single time,” she said. “That process probably occurs 100 fold or even 1000 fold less than a standard knockout repair.”

CRISPR can also induce mutations where you don’t want them to be, says Premsrirut. “That is probably one of the dangers of CRISPR. While we can be modifying and creating designer genes we can also be creating mutations at the same time.”

This understandably has a number of scientists and ethicists concerned that the field is moving too fast and opening the door to problems that can’t be undone. “Since CRISPR-Cas9 was discovered there have been 1,000s of papers but we still don’t know enough about how it works and the potential for harm that it may cause,” says Premsrirut. “We do need to be careful. It’s a great technology but I think it’s important to also understand what the limitations are.”

 “Like any powerful technology, CRISPR comes with high risks,” Morse says. “The side effects of editing a human genome may not be visible for many, many years, possibly even generations. Being careful is key but proceeding is certainly justified. I do believe in our lifetime we will see diseases cured.”