It has been likened to the “find and replace” routine in a Word document. The difference is that Crispr-Cas9 can find the smallest DNA sequences within a genome and can not only replace them, but delete them, add to them, invert them or do just about anything you want to do so long as it’s within the possibilities of genetic engineering.
Crispr (pronounced “Crisper”) is the guide molecule that can find the part of the genome the scientist is interested in with extreme accuracy, while Cas9 is the “molecular scissors” that can cut and paste the double stranded DNA to create a genetic modification of exquisite precision. The discovery and development of Crispr-Cas9 promises nothing short of a revolution in gene editing – and human immunology.
The initial origins of the Crispr discovery go back to the late 1980s. But it wasn’t until 2007 when a yoghurt company identified it as an unexpected bacterial mechanism for fighting off viruses. Crispr, which stands for clustered regularly interspaced short palindromic repeats, turned out to be a kind of acquired immune system for bacteria which enabled them to recognise bacteriophages (viruses) that they had been exposed to in the past. Bits of viral genomes were found sandwiched between long stretches of palindromic repeat sequences within the bacterial genome acting as a memory bank of past infections.
In 2012, scientists combined the Crispr system with Cas9, a bacterial enzyme that can cut through double stranded DNA, and demonstrated the power of this new genetic tool in cutting and splicing DNA. It led to a revolution in gene editing because it would be possible to go in and correct the smallest genetic defect of any genome of any organism, including the human genome.
The research on Crispr exploded after 2012, with some 600 research papers that cited the gene-editing tool published within the following two years. Scientists had demonstrated that it worked on a range of organisms, from baker’s yeast, zebrafish, fruit flies, nematode worms, plants, mice, monkeys and human cells.
In 2015, Chinese scientists were the first to publish a study revealing the use of Crispr-Cas9 on editing the genes of non-viable human embryos to correct the mutation that leads to beta thalassaemia, a lethal heritable disorder. In 2016, researchers in China used Crispr on non-viable human embryos to alter the CCR5 gene to make the embryo HIV resistant, while researchers in the UK were given formal permission to genetically modify human embryos for basic research into the causes of miscarriages, provide the embryos were destroyed before the 14-day legal limit.
On 17 December 2015, the journal Science voted Crispr-Cas9 'Breakthrough of the Year', saying that it had “matured into a molecular marvel”. It is already being used in cancer immunotherapy to edit a patient’s own T-cell genome in order to remove the gene that “tells” these immune cells not to target cancerous tissue. Scientists have used Crispr to remove 62 copies of retrovirus DNA lurking within the pig genome, and in the process revived the moribund concept of xenotransplantation – pig-human organ transplants. A host of other uses in human immunology are expected to follow from this powerful gene-editing tool.