IN 2020 Jennifer Doudna, a professor from the University of California Berkeley, and Emmanuelle Carpentier, the founder and managing director of the Max Planck Unit for the Science of Pathogens in Berlin, have been awarded the ultimate science prize for their breakthrough research on CRISPR (Clusters of Regularly Interspaced Short Palindromic Repeats) technology.
CRISPR-Cas9 technology is a simple yet immensely powerful tool for the editing of genomes. It enables researchers to alter DNA sequences and thus modify the function of genes. CRISPRs are specialised regions of DNA containing the protein Cas9, an enzyme that acts like a pair of molecular scissors and is able to cut strands of DNA.
The CRISPR technology has its origin in the natural defence mechanism by which bacteria and archaea (single-celled micro-organisms) defend themselves against invading viruses. When the bacterial immune system gets threatened, the CRISPR-Cas system produces a chemical signal that triggers a second enzyme, which assists in chopping up and destroying the foreign invader’s DNA.
Emmanuelle Carpentier in 2011 discovered that CRISPR-Cas in bacteria can disarm viruses by cleaving their DNA, and then collaborated with Jennifer Doudna to successfully reproduce the bacteria’s genetic scissors in a test tube. They also reprogrammed the genetic scissors by simplifying it’s molecular components and proved that they could control it to cut any DNA molecule at a predetermined place. This allows scientists to rewrite the code of life where the DNA has been cut. When CRISPR technology is used in more complex organisms, it allows for the manipulation of genes, or “gene editing” resulting in a changing of the encoded messages and instructions of the gene.
CRISPR-Cas9 has become popular in recent years and paved the way for numerous applications in basic science, medicine and agriculture. It has been applied in the food and agricultural industries to create probiotic cultures, to vaccinate industrial cultures (eg yoghurt) against viruses, and to enhance the yield, drought tolerance, and improving the nutritional properties of crops. CRISPR is also promising in the creation of gene drives to control the spread of malaria by enhancing sterility among the disease vector, the female Anopheles gambiae mosquito, and to eliminate invasive species and reverse pesticide and herbicide resistance.
But the most exciting and promising use of CRISPR is the editing of the human genome to correct genetic defects, and could thereby not only treat but prevent many diseases. In 2013, the first reports were published of in vitro (laboratory) and animal studies that demonstrated that the technology can be effectively used in correcting genetic defects causing human diseases such as cystic fibrosis, cataracts and Fanconi anaemia (a rare disease affecting the bone marrow). Other promising applications are hypertrophic cardiomyopathy (a thickening of the heart muscle), Huntington’s disease (a breakdown of nerve cells in the brain), and certain mutations linked to breast and ovarian cancers. Scientist have even shown that CRISPR can knock HIV infections out of T cells, a type of white blood cell in mice, curing them from the virus.
Treating the untreatable
In the New England Journal of Medicine in June 2021 scientists published the preliminary results of a clinical trial that suggest CRISPR-Cas9 gene-editing can be deployed safely and effectively via the bloodstream into the human body to treat genetic diseases. In this small trial, six people with a rare and fatal hereditary condition, called transthyretin amyloidosis (a build-up of abnormal deposits of the amyloid protein in organs and tissues), received a single treatment with the gene-editing therapy targeting a protein made mainly in the liver. All patients showed a significant decline of up to 96% in the level of the misshapen TTR protein associated with the disease. If the production of the harmful protein is shut down, the disease symptoms are stopped from progressing and can even be reversed in some cases. Until this breakthrough there was little that doctors could do to treat this painful and incurable disease.
However, although the CRISPR technology is revolutionary it is not easy and without risk. The technique requires that the DNA-cutting enzyme, Cas9, and a piece of guide RNA, be directed to the precise place in the human genome. Since they are delivered intravenously, the RNA molecules coding the guide RNA and Cas9 protein were encased in nanoparticles made of biomolecules called lipids, to be taken up by the liver and also to protect them from degradation. The guide RNA then sends the Cas9 to snip the TTR gene to halt the production of the TTR protein causing the transthyretin amyloidosis.
Many more incurable diseases
Although most other CRISPR work has been done on animals, the direct delivery method developed by scientists of Intellia Therapeutics and Regeneron Pharmaceuticals has been promising for the treatment of humans. The intravenous delivery of the treatment make it possible that sickle-cell anaemia in future may not require the difficult and risky bone marrow transplant used in ongoing gene-editing trials. Until now, most gene-editing therapies entailed the removal of cells from the body, manipulation of the genes, and then the replacement of the cells.
Editas Medicine in Cambridge, Massachusetts, has encoded CRISPR-Cas9 components into a disabled virus and tested it in people with a hereditary disorder (Leber congenital amaurosis) that causes blindness. In this case the virus must be injected directly into the eye, where the gene therapy then takes place.
Other research all over the world focuses on cancer, blood disorders (beta-thalassemia), muscular dystrophy, and even Covid-19. Scientists at Stanford University have developed a method to use CRISPR/Cas13a to cut and destroy the genetic material of the Covid-19 virus to stop it from infecting human lung cells. This approach reduced the viral load by 90%. Scientists at the Georgia Institute of Technology used a similar approach in animals to destroy the virus before it even enters the cells.
A promising future
The techniques for delivering CRISPR-Cas9 component to various parts of the body are advancing rapidly and we will see a much broader application of genome editing in future. It looks promising that CRISPR may indeed in the future treat many incurable genetic diseases through the editing of human DNA. This prevention of diseases will certainly transform medical treatment and introduce a new era of medicine.
Professor Louis CH Fourie technology strategist
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