Gene Deletion in Adult Mice Creates a Game-Changing Cancer Model

Lab mice have been a great model for finding out a whole host of important genetic information in cancer research, but generating genetically engineered mice containing cancer-causing genes, through breeding or stem cell manipulation, is a long and expensive process.  Yesterday, researchers from MIT published a new method for genetically engineering mice that is much simpler than the typical method.  The researchers expect that this method, which uses the CRISPR/Cas gene editing system, will be a game-changer in cancer research.

How the CRISPR/Cas system works.  Source.

CRISPRs (clustered regularly interspaced short palindromic repeats) are sequences of DNA with short base repeats, found in segments of "spacer DNA".  They are often associated with Cas proteins, which essentially cut out segments of DNA.  This system is part of the prokaryotic immune system, protecting against invading viral DNA, but has since been exploited by researchers for targeted gene editing.  Genes encoding Cas proteins can be inserted into an organism, and along with specifically designed CRISPRs, can cut an organism's genome at the desired location.

Injection of vector into mouse tail veins directs
DNA into the liver.

The researchers, led by Dr. Tyler Jacks, used the CRISPR/Cas system to generate somatic cancer mutations in adult mice, rather than making alterations to mouse stem cells.  They did this by targeting a gene called Pten, a cancer suppressor that, when lost or mutated, has been associated with human cancers.  They inserted a vector co-expressing a Pten-targeting sgRNA and Cas9, and after verifying that the sgRNA was inducing Pten mutations, then they introduced CRISPR into liver cells.  This is actually done by injecting the CRISPR genes into the tail veins of mice, which delivers ~20% of the DNA into the liver.  They found that the CRISPR/Cas9/Pten-sgRNA was able to remove snippets of DNA from Pten, thus generating liver cells lacking tumor suppressing function in the same way that embryonic stem cell manipulation does.  This was enough to cause liver cancer in the mice after a few months.

The authors also used this same method to cut out a normal version of beta-catenin, a gene that makes cells cancerous if additional mutations occur, with an overactive version of the same gene.  This created a new mouse model for cancer research, in with a specific gene can be targeted and replaced in an adult!  The success level was fairly low (0.5%), but this is a very promising start.

The authors hope that this method will help speed up cancer genomic research by bypassing the engineering of embryonic stem cells and then several generations of breeding to generate mutant lines.  Ultimately, this can facilitate the study of any single genes or gene combinations that are suspected of initiating cancer formation in the liver.  Given that cancer-causing genes are being found by the truckload thanks to next generation sequencing efforts, it's critical to find ways to quickly study them, and the results of this study are definitely moving cancer research in that direction.  While this method has only been tested in liver cells, other delivery methods may be able to effectively target other organs.