CRISPR is possibly the hottest topic in science at the moment. In 2018, it was mentioned in more than 23,000 published papers, and that doesn't include the lay press. In addition to the prototypical Cas-9 enzyme, research has now uncovered several other Cas enzymes which can all be harnessed in different ways, while the original Cas-9 has been modified enabling it to bind to a wider range of sequences, further broadening its reach.

Part of the reason for the high level of interest is that CRISPR has the potential to transform such a variety of scientific areas, from treating medical conditions such as Duchenne muscular dystrophy, eradicating some genetically inherited conditions such as Huntington's disease, to agricultural applications, such as improving crop yield and resistance to pests, and even eradicating certain pests such as malaria-carrying mosquitoes. It is also being developed as a diagnostic tool so that ultimately patients could self-diagnose many conditions in the comfort of their own home. Clinical trials involving CRISPR are underway in China, the EU and the US for a range of conditions including beta-thalassemia and cancer but as yet, none have reported any results.

CRISPR is also controversial – it has led to an epic battle in the US Patent courts between the Broad Institute (led by Fang Zhang) and UC Berkeley (led by Jennifer Doudna) and there is ongoing litigation in the European Patent courts. There have long been concerns about off-target effects – where altering the DNA sequence in one part of the genome may have unforeseen consequences on another part of the genome. Some off-target effects can be predicted and investigated; however, without sequencing the entire edited genome in every cell, some may remain undiscovered. This concern is not unfounded. In summer 2018, three papers were published which raised concerns about off-target effects. Two of these papers reported that cells that had successfully been edited using CRISPR-Cas9 turned out to have a defective p53 gene suggesting that CRISPR'd cells might have the potential to cause cancer.

However, the most controversial issue surrounds the potential use of CRISPR to create genetically engineered humans. In 2015, following the first international summit on gene editing, the consensus was that there should be a global moratorium on heritable genome editing in humans until further research had been carried out into the potential risks.

In November 2018, prominent experts were gathering in Hong Kong for the second international summit on human genome editing. Over three days, they were due to discuss scientific advances in the field, ethical concerns over gene editing and debate how governments and regulators should respond. Whilst most of them were still en route, the news broke that a Chinese researcher, He Jiankui, had apparently successfully used CRISPR to create gene-edited babies (twins, whom he called Lulu and Nana) resistant to HIV infection. He was due to present his research at the summit and it quickly turned into a media storm.

The scientific community reacted with deep unease to the news, with many prominent researchers releasing statements denouncing the work. He's research has yet to be published or peer-reviewed, and there is limited information about how successful the technique has been. His presentation and the discussion following raised more questions than it answered and did little to allay the concerns of the scientists at the summit. One of the main points of discussion was over his choice of gene target. In 2017, the US National Academies produced a detailed report on human genome editing which included a list of criteria which should apply to clinical trials using heritable genome editing. The first criterion was the absence of reasonable alternative treatments.

He chose to edit the CCR5 gene – mutations in this gene have been associated with resistance to many (but crucially, not all) strains of HIV infection. The couples enrolled in the trial included a man who was HIV positive and who wanted children who were free of HIV. However, there are many ways to avoid transmission of HIV to children and there are already drugs available which can block CCR5. One consequence of inactivating the CCR5 gene is an increased susceptibility to West Nile virus and to more severe cases of flu. In one of the twins, only one copy of the gene was inactivated meaning that they were still likely to be susceptible to HIV infection. Commentators who have analysed the results presented at the summit have also suggested that not all cells in the other twin had inactivated CCR5. Mosaicism, where some cells carry the gene edit but others do not, is a known issue. It is also not clear what the new mutations introduced into the CCR5 gene will do. Normal practice would be to introduce the mutations into mice (or other animals) first to ensure that they worked as predicted. In his haste to be "first", it appears that He has carried out very limited studies in animals.

Following the summit, the committee released a strongly worded statement about He's research, calling for an independent assessment to verify his claim:

"Even if the modifications are verified, the procedure was irresponsible and failed to conform with international norms. Its flaws include an inadequate medical indication, a poorly designed study protocol, a failure to meet ethical standards for protecting the welfare of research subjects, and a lack of transparency in the development, review and conduct of the clinical procedures."

With the genie now out of the bottle, what does this mean for those developing therapies based on CRISPR? Clearly they will need to show robust evidence not only that the expected genetic change has occurred but that as far as possible there are no off-target effects. It is worth pointing out that for individuals with life-limiting or terminal conditions, off-target effects may well be an acceptable risk. The choice of gene target is also crucial. Would there have been the same reaction to He's work if he had chosen to eliminate a disease such as Huntingdon's?

The scientific community will be watching with great interest for the results of the first clinical trials due in 2019, as will regulators. It may be that the controversy around He's research leads regulators to step in and restrict germline editing before they otherwise would.

Dr Rachel Gribben, Senior Scientist, Hogan Lovells