Stem Cells in Focus
A CRISPR Method For Gene Editing: A Biomedical Breakthrough From a “Germ”
April 17, 2017
Bacteria. What do you think of when you hear this word? “Germs,” “antibiotics,” or “bleach” may come to mind, depending on the context. What about “powerhouse of scientific discovery?” That’s a string of words, but one that accurately describes the impact this single-celled organism is having on stem cell biology and human health.
Earlier this year, two separate research groups tested a potentially curative approach to treating sickle cell disease by precisely editing genes in human blood stem cells to correct the disease, which is caused by a single mutation in the gene that encodes hemoglobin. In sickle cell disease, normally round red blood cells collapse into an inflexible crescent shape blocking blood flow and starving the tissue of oxygen. The result is painful blockages in blood vessels, anemia and organ failure.
This gene editing approach is possible due to discoveries in bacteria.
How Researchers Developed CRISPR Technology
Like most biomedical discoveries, this one was built on decades of research. In the 1980s, scientists recognized that many bacteria contained clusters of repeated sections of DNA interspersed with unique sequences referred to as clustered regularly inter spaced palindromic repeats (CRISPRs).
It wasn’t until 2005, that researchers discovered that these unique sequences arise from previous viral infections and serve as a “memory” of previous viral exposure. DNA from an invading virus is processed into short sequences and inserted into the bacterial DNA—the CRISPRs. This record of viral marauders allows the bacteria to recognize and fight off viruses it had previously seen. Think of it as part of their immune system.
Scientists Jennifer Doudna from the University of California, Berkeley and Emmanuelle Charpentier at Umea Centre for Microbial Research, Sweden unleashed a revolution in molecular biology when they established how this process worked. CRISPR sequences from a previous infection are turned into short RNA sequences that direct cellular machinery known as Cas proteins to sequences that they match. Functioning as molecular scissors, the Cas proteins then chop the invading viruses up into smaller pieces rendering it harmless.
Because the targeted, genome editing capacity of the CRISPR/Cas9 system is driven by RNA, which can be easily custom-made in a laboratory, the researchers found the system was readily programmable and predicted at the time that the system could "offer considerable potential for gene targeting and genome editing applications."
The speculation set off a race to direct CRISPR/Cas9 to not only cut precise sequences of DNA, but also to adapt this process to edit the DNA in living cells. In 2013, Doudna and Charpentier, along with George Church from Harvard University and Feng Zhang from MIT, all accomplished this feat within a month of one another.
Collectively, these scientists demonstrated that an ancient immune system in bacteria could be repurposed as a programmable DNA editing tool. A simple, reliable system to edit DNA was realized.
Within four years, the CRISPR/Cas9 system streamlined genome editing to such a degree that it has become the platform of choice for many important applications across biology. These include genetically-modified animal models and cell lines that are helping researchers develop new drugs and develop new approaches to treating human disease.
The combination of two revolutionary technologies -- induced pluripotent stem (IPS) cell technology and CRISPR -- has opened opportunities once considered decades away. Scientists have used CRISPR to alter multiple genes in embryonic stem cells and IPS cells to model complex diseases using human cells. The ability to introduce multiple genetic changes that are associated with disease allows researchers to explore the complex biology of many multigenic diseases such as cancer and Parkinson's disease. CRISPR/Cas9 editing of human stem cells have established models for Rett Syndrome, HIV resistance, deafness, and other conditions.
The progress has been dizzying.
The exciting technology used by CRISPR/Cas9 has advanced science. However, it is not without its faults. It can make mistakes, making changes at sites other than the targeted DNA, known as “off-target” effects, and has often proven ineffective when human cells circumvent the technology and repair themselves as they would without intervention. Even with these faults, CRISPR/Cas9 technology is having a profound impact on science and the practice of medicine, including new ways to potentially treat sickle cell disease.