Written By: Dr. Ananya Mukherjee
Department of Plant Science and innovation
University of Nebraska-Lincoln, USA
CRISPR is something you must have come across in your science news feed in the last couple of years or so. Even if you have not, you may have seen the big news that took social media by storm – the genome-edited babies in China, Lulu and Nana. One might wonder what is this fuss all about and why is this technique in the news all of a sudden? To make sense of it all its important to delve a little into the history of CRISPR and why it became bigger than any other “editing” strategies that have been around for a while.
The scientist’s quest of genome or gene editing is not anything new. For several decades we have been striving to find techniques which will efficiently target genes of interest and make the changes we want them to make but with minimal off-targets. The reason why CRISPR is preferred over other tools is its efficiency, ease of manipulation and most importantly precision. Nearly 33 years ago Francisco Mojica from the University of Alicante, Spain found what we now know as CRISPR or Clusters of Regularly Interspaced Short Palindromic Repeats in E. Coli. Soon after that such regions of repeat DNA were found in another domain Archea as well. However, since then no other organism domain has shown CRISPR like repeats. Although the very fact that these unusual repeat sequences showed up or are “conserved” in two domains, making them the most widely distributed repeat sequences in prokaryotes. It’s hypothesized that these repeats which are part of the bacterial immune system came about to fight the selective pressure created by viruses. In 2005 Alexander Bolotin, French National Institute for Agricultural Research found what we now know as Cas9, a helper enzyme of CRISPR that acts like molecular scissors to make specific edits in DNA. The beauty of CRISPR perhaps is in the fact that the gene we wish to edit doesn’t need to be paired with enzymes like in other genetic methods, but Cas9 can be led to the gene via something called a “guide” RNA.
Soon after in 2007, CRISPR was used for S.thermophilus, which is used in the dairy and yoghurt industry, to resist bacteriophage attacks. Ever since its discovery, there has been no looking back for CRISPR. From agricultural applications like creating genetically edited crops tolerant to drought, crop diseases and higher yield to in-vitro human cells and animal models showing its applications in curing or reversing genetic defects like cystic fibrosis.
In a recent article, National Geographic interviewed scientist Zachary Lipman from Cold Spring Harbor who is trying to put in fields high-yield tomatoes. This is not very surprising because since 2013 disease-resistant varieties of tomato are being developed by scientists, showing how far this technique has come. Even abiotic stress such as chill which tomato is very susceptible to, has been tackled with CRISPR by the introduction of genes that can resist cold weather. Essentially it comes down to knowing which gene or group of genes to target so that off-target effect is minimal and the right objective is achieved. Aside from tomato, strawberry, banana, grape, apple, watermelon, and kiwifruit are some more fruits which have been edited by CRISPR. Complex and large genomes such as date palm are now being targeted to be edited by CRISPR to tackle similar growth-related issues in a changing climate. Food crops are being manipulated with the help of CRISPR to produce everything from low gluten wheat to virus-resistant cacao in West Africa that can produce more chocolate. Mushrooms which are known for spoiling easily are being edited to have a longer shelf life. China has scientists working on improving yields in rice. It’s no secret that with the boom in population and a changing climate the world is soon going to be in a massive struggle to have enough food for every mouth. With such attempts on a staple food crop, we can benefit in the right direction. These are just some exciting examples. There are lists of agriculture benefits that we have gotten from CRISPR and most of these crops are under varying stages of success.
Gene drives is another new application of CRISPR where a certain favourable trait can be passed down from parent to the next generation and thereby propagate in the entire population such as disease resistance or in agriculture- herbicide or pesticide resistance. In October of this year, David Liu of the Broad Institute of MIT and Harvard and postdoctoral fellow Dr. Andrew Anzalone came up with “prime editing”, which is a new form of CRISPR Cas9. The beauty of this technique stems from its ability to change the 4 bases of DNA A.T, G and C into any other base as deemed fit. Previous techniques were not as flexible and could not theoretically cure diseases like sickle cell anaemia which requires a specific A to T mutation to correct it. As of now several human trials for CRISPR to treat the disease have been approved and are all on the way. These include sickle cell anaemia to replace the lethal mutation, T cell editing to treat various types of cancer, photoreceptor cell editing to treat Night Blindness and many others.
Hence almost every biological system has been affected by the development of CRISPR.
This now brings us to the big CRISPR story which had the world in the crossfire of several debates regarding the ethical concerns of such a fast developing and precise technique like CRISPR. In June 2017 a couple visited Southern University of Science and Technology in Shenzhen China to meet He Jiankui. He, a biophysicist, was attempting to edit an embryo to remove the surface protein CCR5 that HIV uses for establishing infection. In November 2018, twins Lulu and Nana were born with edited genomes and it was known that another woman was also pregnant with a third CRISPR baby. Despite public outrage, He has not admitted to any wrongdoing and has even published a video promotion of the project. This is mainly because sometime in 2017 the US National academy of sciences decided that human trials if strictly regulated are permissible and it’s up to the governments to decide what can and cannot be done, as reported in Science. Of course, there has been public criticism by Nobel laureates like David Baltimore but there are also scientists who have emailed words of praise to He. He since his controversial appearance in the Hong Kong summit of genome editing in 2018, has been believed to be under house arrest. He was let go from his university position and not made any public appearances since then.
On the face of it, such an edit seems to be a good step in the right direction. After all, any future parent would want his or her kid to be free from certain deadly disease like HIV. The main cause for concerns is the fact that the exact effect of the mutations induced are not known and CRISPR, despite being a sophisticated technique, does have off-target effects. There is no telling if some cells had started to divide in the embryo before the edit was introduced and a “mosaic” effect may happen in such a case. Taking into consideration all of these dangers, it’s hard to say if the attempt made was a success or not. However, this is a very slippery slope because designer babies may soon become the future. CRISPR is not without its off-targets and such off-targets in humans can have deadly consequences and may even be inherited as mutations by generations. Editing embryos may not be the best way to eradicate HIV which is more prevalent in the continent of Africa. Editing babies perhaps would be able to slow down HIV in 30 years but by then more techniques will come up to stabilize the spread anyway. There is also the fact that if humans start doing away with all diseases the population boom may be too much for the world to handle climate change and food shortage breathing down our necks. As of now, CRISPR has only grown stronger with a new report showing it can cut and splice whole chromosomes. In the University of Pennsylvania in Philadelphia clinical trials to treat cancer cells have been approved and are underway. The scientific community has no doubt made great strides to use this gene-editing technique positively but only time will tell if the progress of CRISPR continues an upward trajectory both scientifically and ethically.