Novel step is for the cell that reproduces a virus, whether useless or harmful, to develop some way to resist it. And, in fact, it was in this manner – as a bacteria’s defense against invading viruses – that the CRISPR-Cas process first emerged.
That process allows acquired characteristics to become inherited. In the course of a first infection, a small fragment of the viral genome – a kind of signature – is copied into the CRISPR genomic island (an extra piece of genome, outside of the parent genome text). As a result, the memory of the infection is retained across generations. When a descendant of the cell is infected with a virus, the sequence will be compared to the viral genome. If a similar virus has infected a cell’s parent, the descendant will recognize it, and ad hoc machinery will destroy it.
This complex process took many decades for scientists to decipher, not least because it controverted standard theories of evolution. But now scientists have figured out how to replicate the process, enabling humans to edit, with the utmost precision, specific genomes – the Holy Grail of genetic engineering for nearly 50 years.
This means that scientists can apply the CRISPR-Cas mechanism to correct problems in the genome – the equivalent of typos in a written text. For example, in the case of cancer, we would want to destroy those genes that allow the multiplication of tumor cells. We are also interested in introducing genes in cells that never gained them by natural genetic transfer.
There is nothing new about these objectives. But, with CRISPR-Cas, we are far better equipped to achieve them. Previous techniques left traces in the modified genomes, contributing, for example, to antibiotic resistance. A mutation obtained by CRISPR-Cas, by contrast, is not distinguishable from a mutation that emerged spontaneously. That is why the US Food and Drug Administration has ruled that such constructs do not need to be labeled as genetically modified organisms.
Previous techniques were especially arduous if one needed to modify several genes, because the process would need to occur sequentially. With CRISPR-Cas, the ability to perform genome modifications simultaneously has already enabled the creation of fungi and apples that do not oxidize, or turn brown, when they come into contact with air – a result that required several genes to be deactivated simultaneously. Such apples are already on the market, and are not considered genetically modified organisms.