The breakthrough in editing the human genome? – The emergence ofa new, even more powerful CRISPR tool
Unlike two years ago, today not many people are still associating the term „CRISPR“ with something like a muesli bar. Even though few people know that the term „clustered regularly interspaced short palindromic repeats“ is a technical term that at first described nothing more than certain sections of repeating DNA pieces in the genome of bacteria, it is no longer necessary to explain that it is also a powerful tool for editing genes. What a few years ago was a marginal area of bacteriophage research that attracted little attention beyond a small circle of highly specialized biologists has long since developed into one of the most important scientific and technological breakthroughs of this century so far with enormous revolutionary potential for applications in medicine and human genetics. And most recently the new genetic engineering tool has yet shown again that is still good for a few more surprises.
The discovery of CRISPR dates back to phage research in the 1980s. Phages are viruses that attack bacteria. Once infected with them, bacteria can integrate parts of the viral foreign DNA into their own DNA in the form of recurring short palindromes interrupted by other sequences (a palindrome is a sequence of characters that reads the same from the front as from the back, such as the name „Anna“). The name „CRISPR“ was suggested by phage researchers Francisco Mojica and Ruud Jansen in 2001, when they searched for some more interrupted palindromic repeats in gene sequences, as those that had already been discovered in numerous phages. The integrated DNA serves the bacteria for the purpose of recognition: As soon as a virus with this DNA attacksthem again, the bacterial cells recognize its DNA and can thus develop strategies for protection. For this purpose, the CRISPR-DNA is joined by a further enzyme, a so-called „Cas“ („CRISPR-associated“) protein, which has the ability to cut open the recognized gene sequence and thus render the virus harmless. Mother Nature has developed an entire series of Cas proteins, with very different degrees of efficiency when it comes to cutting up the target gene string. A version known as „Cas9“ has proven to be particularly useful.
Around the year 2012, gene engineers recognized that beyond the bacterial world the composite CRISPR/Cas9 complex can also be used for themanipulation of genes (also known by the more harmless sounding name „editing“ genes),for which it functions like a Lego brick finder and scissors at the same time: You simply equip it with a sequence that is exactly complementary to the desired DNA target sequence, whereupon the enzyme complex finds the desired target sequence in the DNA and cuts it open exactly at this location. This makes it possible to insert any desired gene sequence or remove one without replacing it. This method can thus be used for the fast and precise cutting and splicing of DNA in almost all living organisms, in plants, animals, bacteria, as well as in humans. Within a very short time CRISPR has transformed biology and opened up entirely new ways of treating diseases.
In the last few months, a lot has been written about CRISPR in the popular press, often linked to the concern that this technology serves as on big leverage for gene technology with all its promises and problems, as it makes it much easier to modify genes of living beings, to introduce modified DNA into the germ line of living beings, and thus to influence their properties permanently. The international headline the new method ultimately filled in November 2018, when the birth of the first babies genetically manipulated with CRISPR was reported from China.
What is not yet on the radar screens of most people, however, is that the possibilities of this new genetic technology still seem to be far from exhausted. Thus, in February 2019, an article appeared in the renowned scientific journal Naturedescribing a new enzyme complex that functions very similarly to Cas9 but is about 40% smaller (E. Nogales, J. Doudna et al., CasX enzymes comprise a distinct family of RNA-guided genome editors, Nature, 4 February). Small size is a huge advantage when trying to bring a corresponding gene editor into a cell. And CasX, as the new complex was baptized, could prove to be particularly more powerful for the use in humans, asit is likely tobe more easily accepted by the human immune system. Doctors fear that Cas9 could trigger an immune reaction in patients treated with CRISPR therapies. Such a problem should not occur with CasX, because the bacteria in which it was discovered do not befall the human body.
Genetic engineers have long since jumped on the bandwagon. „We aren’t just looking to uncover the next pair of molecular scissors. We want to build the next Swiss Army knife,“ says Jennifer Doudna, one of the discoverers of Cas9 and pioneer of the CRISPR technology, who was also instrumental in the discovery of CasX (and is a co-author of theNature study). CasX could be the decisive step towards safely editing the human genome. A small, barely noticed step in research could prove to be a huge step for mankind along a scary path.