There are more than 6 billion letters, or nucleotides, of DNA in the genome. These contain all the information needed to create an individual organism. Certain sequences of DNA, called genes, contain instructions for making proteins that determine everything about how we look and how we function. We expect that there are some differences in those sequences that lead to differences in individuals, but sometimes these instructions contain mutations or major changes that can lead to serious diseases such as cancer.
Imagine that you need to know which changes in which sequences in that long string of 6 billion characters are important for targeting disease therapies. And once you identify some of those important genes, how do you fix those mutations?
The game-changing discovery in 2012 of a system called CRISPR revolutionized biomedical breakthroughs over the past decade. Scientists can use it to target, modify, modify and regulate genes and put any enzyme or protein they want anywhere in the genome. This allows them to find new therapeutic targets and understand how different genes affect cells in a way that was previously impossible.
But how do we apply CRISPR to understand cancer? We spoke with Traver Hart, PhD, associate professor of bioinformatics and computational biology, to learn more about CRISPR technology and how it can be used to advance cancer treatment.
What is CRISPR technology?
CRISPR stands for regularly spaced short cluster repeats. This is the mouth, so scientists refer to it as just CRISPR. These are repetitive sequences in the genetic code that were first found in bacteria and later found to be part of a new adaptive bacterial immune system against phages, which are viruses that attack bacteria.
This system combines CRISPR DNA sequences and a set of Cas (“cRISP as suchsociated”) to identify and destroy invading viral DNA. It also fuses a sample of this viral DNA between these CRISPR sequences so that it can easily identify the virus itself and attack it in the future. Thanks to this unexpected discovery in coli bacteria Bacteria, scientists can now harness this method and use it in a similar way inside human cells.
How does CRISPR work?
The main part of the CRISPR system is the Cas nuclease, a Cas protein that cuts strands of DNA. These Cas proteins can be programmed to find sequences of 17 to 24 letters by attaching an RNA guide that uniquely matches a specific DNA target. It is similar to a key that matches a lock. Researchers have a large library of guide RNAs available that can match specific parts of different genes in the human genome.
Once CRISPR is added to a cell, it looks for that matching target sequence in the DNA and binds to it, and the attached Cas protein is activated to do what the scientists asked it to do. Some Cas proteins – such as Cas9 – can cut or break DNA. This is the original protein found in bacteria. Others are designed to turn the gene on or off without having to cut it. This allows researchers to learn more about what happens if cells produce too much (overregulation) or too little (downregulation) of a particular protein and how this might affect the cell’s outcome.
How do we use CRISPR to study cancer in human cells?
Over the past several decades, studies have been conducted on yeast cells and other model organisms where scientists can efficiently edit genomes. The discovery of CRISPR was instrumental in changing that.
We can modify the genome directly in human cells with unprecedented ease thanks to CRISPR technology.
Once CRISPR cuts the target DNA, it is either repaired or replaced with a different sequence. Scientists use this method to eliminate human genes in cancer cells and determine which of these genes are necessary for cancer cells to grow without harming normal cells. This allows us to filter candidate genes for drug targets that can be highly tumor specific. My lab is trying to find better ways to kill cancer cells by simultaneously disrupting multiple genes using a different Cas protein called Cas12a. This gives us more knowledge about how different genes and proteins work together in cancer cells to promote cancer development.
A recent study by Yohei Yoshihama, Ph.D., and Ronald DePinho, MD, used CRISPR to screen cancer cells and identify a protein called JMJD1C as a candidate target in castration-resistant prostate cancer. Another study by Zhao Wang, Ph.D. And the Junjie Chen, Ph.D.used CRISPR to examine human cancer cells growing in mouse models and discovered a protein called KIRREL, which has been shown to be important for tumor suppression.
Can CRISPR repair genes in humans?
While the idea of being able to fix “bad” genes to treat diseases is a worthwhile endeavor, the science is not at the point where it can do so safely and effectively – yet. Researchers are looking into how CRISPR can be used to correct genetic defects that cause beta thalassemia and sickle cell anemia, diseases that affect the amount of hemoglobin in the body and cause patients to need frequent blood transfusions. If approved, this type of treatment, called exa-cell, would become the first medical treatment based on CRISPR technology, which is very exciting.
What’s next for CRISPR?
The possibilities are endless for the information that can be gleaned from using CRISPR systems, and only 10 years later, scientists have only scratched the surface. New Cas proteins and other enzymes are being studied, and there are still questions about how to make the CRISPR technology more specific so that it does not accidentally contain unintended targets.
Here at MD Anderson, our use of CRISPR continues to lead to a better understanding of how cancer cells function and help reveal many ways to target individual therapies specific to specific tumors that we hope will one day achieve our goal of eliminating cancer.