CRISPR-Cas9, a revolutionary genetic editing tool that allows scientists to change the DNA code of an organism, has quickly become one of the most important medical advancements of our time. Theoretically, the possibilities of what can be done with targeted genetic editing are immense, and each new innovation gives hope to millions of people with inherited disorders across the globe.
However, like all things in life, CRISPR-Cas9 is not perfect. It sometimes likes to change bits of DNA that it shouldn’t, and sometimes it is simply not viable for use against a disease.
In an attempt to combat CRISPR-Cas9's limitations, researchers from the Weissman Lab at the Whitehead Institute have developed a new CRISPR technology called CRISPRon and CRISPRoff, which can target specific genes and turn them on or off without changing the DNA code. Their findings are published in the journal Cell.
“The big story here is we now have a simple tool that can silence the vast majority of genes,” says Weissman in a statement.
“We can do this for multiple genes at the same time without any DNA damage, with great deal of homogeneity, and in a way that can be reversed. It's a great tool for controlling gene expression.”
CRISPRoff uses the same fundamental targeting systems as CRISPR-Cas9, hence their close names. CRISPR-Cas9 is a two-part system, involving a CRISPR sequence and a Cas9 protein associated with it. The CRISPR sequence acts as a homing beacon – you can target it at a specific point in the genetic code of an organism and it searches out that sequence. Upon arrival, it unleashes Cas9, which attacks the DNA and cuts it with enzymes. Broken apart, the CRISPR-Cas9 then leaves the DNA sequence to repair itself using its own internal machinery, changing the sequence in the desired way in the process.
But changing a DNA sequence is permanent, and using the body's own internal machinery makes the process difficult to accurately target. What if we could remove the activity of a gene without fundamentally changing it?
To do so, the researchers turned to gene expression. Throughout the genome, genes are regularly turned on and off using the addition of simple chemical groups – this is called epigenetics. One important epigenetic process, called DNA methylation, involves the addition of a methyl group that essentially blocks the gene from being read by the cell – if the gene is hidden, it will not be turned into a protein and the gene is "silenced".
When this goes wrong, diseases can occur. A number of diseases are linked with this activation or silencing, including Prader-Willi syndrome, Fragile X syndrome, and some cancers.
CRISPRon/off utilizes epigenetic modification to genetically edit DNA, allowing scientists to turn genes "on and off" as they choose. Using small pieces of RNA that guide CRISPRon/off to a target site, the technology can add or remove methyl groups from specific sites in the gene, modifying their expression.
This change is inherited through cell divisions, making it an invaluable tool for anything from understanding the genome to developing therapies against epigenetic disease. The researchers are now hopeful that their new genetic editing can be used across a range of applications, improving the arsenal of tools scientists now have to fight genetic disorders.
“I think our tool really allows us to begin to study the mechanism of heritability, especially epigenetic heritability, which is a huge question in the biomedical sciences,” said the first author James Nuñez.