CRISPR: A cure for genetic disease?
Is CRISPR the answer for curing Sickle Cell Disease and Transfusion-dependent β-thalassemia?
“We may be nearing the beginning of the end of genetic diseases.”
These are the words of the 2020 Nobel Prize in Chemistry co-recipient, Dr. Jennifer A. Doudna; the biochemist whose ground-breaking research led to the development of the CRISPR-Cas9 genome editing technology. Her discovery and development of clustered regularly interspaced short palindromic repeats (CRISPR) systems has ushered in a promising era of genetic possibility. Researchers are tirelessly developing treatments and even cures for genetic disorders and diseases using CRISPR.
Although the name may seem daunting, CRISPR is essentially a prokaryotic (bacteria and archaea) defense mechanism. It allows bacteria to recognize and protect themselves from foreign genetic material that is trying to hijack their cellular hardware.
Researchers use CRISPR and its CRISPR-associated enzymes (Cas) to genetically edit the DNA of different kinds of organisms. CRISPR is a programmable, easy-to use tool that allows researchers to perform precise, double-stranded cuts along a strand of DNA and either replace or delete specific protein-encoding segments (genes). Think of CRISPR as “molecular scissors.” If these cuts interrupt a gene, this could result in an organism who has gained or lost a specific characteristic through mutation.
CRISPR is a game-changer in molecular biology - it is the only method available that can perform precise and programmable double-stranded cuts in our DNA. This technology can be used with almost any type of organism, and it’s fast, cheap, and simple to use. One of the most important qualities of CRISPR is its potential in developing a cure for genetic conditions that can be inherited from generation-to-generation.
A recent paper published to the New England Journal of Medicine (NEJM) by Frangoul et al. outlined the group’s work with developing a treatment for both Sickle Cell Disease (SCD) and Transfusion-dependent β-thalassemia (TDT) using CRISPR-Cas9. Both diseases are monogenic, involving one single hemoglobin gene.
Hemoglobin is a critical protein in our red blood cells - it is responsible for carrying oxygen to our tissues and organs, and transferring carbon dioxide waste produced by our tissues and organs to the lungs for expulsion. There are two types of hemoglobin - fetal (γ) hemoglobin and adult (β) hemoglobin. Fetal hemoglobin differs from adult hemoglobin due to its stronger hold on oxygen, which allows oxygen to be transported from the mother to the fetus during pregnancy. Mutations in this hemoglobin gene result in the development of blood diseases such as SCD and TDT. SCD and TDT patients are unable to transport enough oxygen to their tissues and organs due to misshapen red blood cells. With an annual estimation of 60,000 diagnosed TDT cases and 300,000 diagnosed SCD cases, the drive to develop a genetic cure for these two deleterious diseases is strong.
The paper analyzed the group’s first clinical trial that used CRISPR-Cas9 genome editing technology to downregulate, or reduce the production of the protein, BCL11A. This reduction of BCL11A consequently increases the production of fetal (γ) hemoglobin. BCL11A limits the production of γ hemoglobin during an infant’s 1st year of life and induces the manufacturing of adult (β) hemoglobin.
Other studies have found that when natural mutations occur in the BCL11A gene, the BCL11A protein is broken - it is unable to limit the production of γ hemoglobin. As a result, people who possess this natural mutation are protected against the effects of SCD and TDT due to the upregulation (or increase) of γ hemoglobin. This increase in γ hemoglobin means that these patients now have red blood cells that are able to efficiently transport oxygen throughout their bodies.
Frangoul and team decided to make a controlled mutation in the BCL11A gene using CRISPR-Cas9. The study enrolled volunteers between the ages of 18-35; however, the paper focused on the data from two patients: 19-year old Patient A with SCD and 33-year old Patient B with TDT.
Frangoul used CRISPR-Cas9 to perform a pre-programmed, double-stranded cut in the BCL11A gene of hematopoietic (a cell that can develop into any kind of blood cell) and progenitor (a cell that can develop into a specific type of cell) stem cells. These genetically-edited stem cells were extensively screened for unwanted mutations using multiple methods. Finding no extraneous mutations, both patients underwent myeloablative conditioning, a method of full-body radiation therapy. It is essentially like a smart phone factory reset - radiation “deletes” the body’s bone marrow; the site of blood cell and stem cell production. Once the cells in the bone marrow are “deleted,” the bone marrow is “reset” with the new, genetically-edited stem cells. Both patients were monitored closely over a 12-month period.
So what did they find?
During 12-months post-treatment, both SCD and TDT patients exhibited long-lasting increases in γ hemoglobin. Bone marrow was retrieved from the patients and screened in order to see how many stem cells possessed the BCL11A CRISPR-induced mutation.
The average number of cells showed 80% (+/- 6%) of the stem cells had the desired mutation.
Both patients responded positively towards the treatment, which suggests a potential cure to SCD and TDT.
It is important to note, however, that both patients had multiple “adverse effects” to the treatment, some being classified as “severe.” These effects included pneumonia, veno-occlusive liver disease, and sepsis. The paper suggests that these effects were connected to the myeloablative conditioning that both patients underwent before being transfused with the genetically-edited cells. This conditioning is highly intensive, so it is not surprising that the patients did not respond well to it.
Limitations?
Besides the severe effects associated with the myeloablative conditioning pre-treatment, the paper brought up some unanswered questions; for example, whether clonal diversity is present and whether the treatment is long-term.
In order to answer these questions, additional research and post-clinical trial studies with the same participants are required.
Scientists have been researching and developing treatments for blood conditions for decades. Maybe now, with the development of the CRISPR technology, they may actually develop a hereditary cure. This research is definitely promising, however, additional study is required before any assumptions can be made in good faith. This research has made one thing clear - we are one step closer to developing a cure for common genetic diseases and those inflicted with them.