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2017-04-01

The Scissor Ladies

I cannot claim that I ever was a fan of Edward Scissorhands. Not enough dark imagination, I guess. But, alas, it brought back the concept of genetic scissors that have bookmakers now give amazing odds on a Nobel Prize in Chemistry to Emmanuelle Charpentier and UC Berkeley’s Jennifer Doudna for their development of CRIPR/Cas9 technology that does not create Frankenscissors such as Edward’s, but is a precision tool for the manipulation of gene sequences by slicing DNA molecules at a chosen spot. Cas9 (or CRISPR associated protein 9) is an RNA-guided DNA endonuclease enzyme associated with the CRISPR adaptive immunity system consisting of segments of prokaryotic DNA that contain Clustered Regularly Interspaced Short Palindromic Repeats. Each repetition is followed by short segments of ‘spacer DNA’ from previous exposure to a bacteriophage virus or plasmid.

Now that I reliably lost 99.99% of my gentle readers, let me mention the rare consensus that the peculiar acronym describes “genome editing,” which, according to MIT Technology Review, is the most important discovery since the dawn of biotechnology in the 1970s. For Science, it was the Breakthrough of the Year 2015. Neither surprises if one takes a look at the perplexing list of awards and honors of the all-European scientist Charpentier. Her idea was a combination of Cas9, which was already known, with an RNA molecule that would dock onto CRISPR/Cas9 together with tracrRNA. It is how bacteria cut foreign viral genes out of their own DNA – the equivalent of a surgeon operating on herself.

Genome editing is considerably more precise than classical genetic engineering where to this day nobody could predict where exactly newly installed genes will be placed. This was a major point of contention on the part of critics of genetic engineering. CRISPR/Cas9, on the other hand, permits precise insertion of changes to the genome through directing the DNA “scissor” Cas9 by means of so-called “guide RNA” to the desired location. The “fracture” in DNA can subsequently be repaired in different ways. To produce guide RNA, one needs to know the sequence of the targeted gene or DNA segment – a set can be produced within one day for about $20, making the technology accessible to any lab. Most molecular biologists would never have dared to dream of a tool like CRISPR/Cas9: it is simple, fast, precise and cheap to cut and modify the genome of any organism – bacteria, plants, animals, humans. It was inspired by an antiviral defense mechanism bacteria use to eliminate virus segments of DNA from their own double helix. If one is to compare conventional gene technology with open-heart surgery, genome editing is the equivalent of a minimally invasive procedure.  It permits scanning of DNA and cutting out parts of it selectively. It has been used to render malaria mosquitoes harmless, to edit plant genomes, it may be used to eliminate and replace part of human DNA, leading to new therapeutic methods in dealing with genetic diseases.

Regulatory issues abound. Because CRISPR/Cas9 can be used in different ways, biologists distinguish three types: Type I performs a single-point editing of a base within the DNA sequence, exchanging one letter for another. In the case of Type II, just a few letters are “edited.” Type III, finally, introduces a larger piece of foreign DNA into the cut. Type I and Type II edits do not result in a genetically modified organism because their edits result in point mutations that also occur naturally by crossing and/or recombination. They occur naturally all the time and are the engine of evolution. Conventional breeding methods also change the genome. Mutagenesis, for example, exposes plants to chemicals or radiation, causing untold mutations. Nobody knows where they occur, and most are harmful. Such plants are considered “natural” and are marketed without additional safety checks anywhere. Why should plants with a more surgically and not randomly altered genome be at a legal or regulatory disadvantage?

Contrary to the U.S. and Canada, the EU and Switzerland place greater importance on the procedure generating a plant or livestock than on the final product. If elements are introduced into the genome of an organism that was prepared outside the organism, it is considered genetically modified under EU law. This would include all plants or animals edited through CRISPR/Cas9 or another genome editing technology. But there is a catch: the material introduced by this technology, namely the guide RNA and the Cas9 enzyme, do not exist in the final product. These organisms do not differ from their conspecifics – how should the use of technology be controlled when its application cannot be proven? Even the Swiss Commission of Experts on Biosafety expressed reservations about strict regulation of such organisms, finding that, since products cannot be distinguished from others generally, they should be treated as equivalent in terms of consumer safety. While in the U.S. first plants with edited genomes are brought to market, the EU has yet to decide how to characterize such organisms.  

CRISPR/Cas9 can help answer questions about the origin of congenital diseases and develop new drugs. It permits reproduction of genetic mutations that lead to disease, be it in an animal model, in plants, or in organoids. But it could also be used for eugenics by eliminating disfavored traits from the genome – arguably the most politically, morally, and ethically sensitive issue in genetics since the 1930s. Recent discoveries of certain genetic roots of crime make scientists nervous. Harvard’s George McDonald Church, who optimized CRISPR/Cas9 for human genome engineering, wants to influence the development of ova and sperm, and Chinese scientists, including Junjiu Huang at Sun Yat-sen University at Guangzhou, have manipulated embryos affected by genetic disorders. In 2015, beta-thalassemia, a blood disorder, was first edited in human zygotes. While Charpentier is adamantly opposed to editing the human embryo, there is no compelling reason other than existing EU regulation to abstain from exploring further use of this technology. Her colleague Doudna, also opposed to such experiments, on the other hand appears more resigned and realistic about the likelihood of such developments. If opposition to stem cell research is any guide, the futility of attempting to estop science and research becomes self-evident. Public debate is a useful necessity given the lightning speed of evolution in genome engineering. Since genetic interventions have become possible, one should remember the adage that if something can be done, it likely will be. Regulating rather than prohibiting in principle appears to hold more realistic promise.

1 comment:

  1. RNA and DNA was always a confusing subject for me. I never really understood how they worked. Until now. Thank you for the brilliant explanation.

    ReplyDelete