The furore that accompanied the news that Chinese scientist He Jiankui had carried out gene-editing on twin girls threatens to overshadow the strides forward being made by biopharma companies developing therapies based on the technology.
He’s claim that he successfully altered the DNA of human embryos in vitro that were later born as twin girls – disabling the CCR5 gene in order to protect them from HIV infection – has injected a massive dose of controversy to the field even as it takes its first steps into human clinical testing. The fear is that that it could damage the public’s perception of gene editing and undermine the very real benefit that it could deliver to people with diseases that, up until now, have resisted all drug development efforts.
The technology used by He to modify the embryos was CRISPR/Cas9, a technique that involves introducing a break in a specific place within DNA to trigger a self-repair mechanism. However, instead of restoring the original sequence, CRISPR serves as a new template that can be used to change the sequence, allowing the genome to be edited practically on demand.
Artwork by Etienne Raimondeau, graphic designer and scientist
A few years ago, the discovery of the technique by researchers Jennifer Doudna Feng Zhang, and Emmanuelle Marie Charpentier was swiftly followed by widespread use of the technique as a research tool, and the creation of a clutch of companies aspiring to make CRISP/Cas9-based drugs a reality.
Now, the first CRISPR therapies are making their way into clinical trials. In November, the US Food and Drug Administration (FDA) approved Editas Medicine’s Investigational New Drug (IND) application for a phase 1/2 trial of its Allergan-partnered CRISPR drug EDIT-101 for LCA10, a rare form of blindness with no effective treatments.
The green light came after a delay of around a year caused by manufacturing issues, which allowed another candidate from CRISPR Therapeutics and Vertex Pharma – CTX001 for the rare blood disease beta thalassemia – to become the first CRISPR drug to start testing in Europe. The FDA placed the CTX001 programmed in beta thalassemia and a planned trial in sickle cell disease on a clinical hold shortly afterwards, but relaxed that last October.
The two candidates may both be based on CRISPR, but have big differences between them. CTX001 is an ex vivo therapy, meaning
the CRISPR technology is used to modify haematopoietic stem cells outside the body to express foetal haemoglobin, and then reinfused into the patient. The aim is to produce red blood cells that product the foetal haemoglobin variant that will dilute the cells producing defective adult haemoglobin.
In contrast, Editas is an in vivo therapy, with the CRISPR delivered directly under the retina of the eyes using an adeno-associated viral (AAV) vector, the same delivery vehicle being tested in gene therapies (and the technology which caused the FDA to raise questions that delayed the start of the trial). The idea is to modify the photoreceptor cells in the eye to eliminate a mutation in the CEP290 gene which causes LCA10. The company claims it is the first in vivo trial of a CRISPR drug to take place anywhere in the world.
However confidence in Editas and the whole gene-editing sector has been knocked in recent weeks by two notable setbacks – the resignation of Katrine Bosely from her role as CEO of Editas, and the failure of Sangamo Therapeutics’ rival zinc-finger nucleases (ZFNs) platform.
The first therapeutic use of CRISPR did not come from a US biotech team, however. Back in 2016, Chinese scientists led by oncologist Lu You at Sichuan University in Chengdu, injected CRISPR-modified cells into a patient with a form of lung cancer at the West China Hospital. The cells were gene-edited to lack PD-1, the immune checkpoint targeted by drugs such as Merck & Co’s Keytruda (pembrolizumab) and Bristol-Myers Squibb’s Opdivo (nivolumab), and the hope was that – without the brake of PD-1 – the edited cells would attack and defeat the cancer. The work was reported in the journal Nature and, since then, another Chinese group has tested a similar approach in head and neck cancer.
Meanwhile, CRISPR isn’t the only gene-editing technology being used in human therapeutics, although its devotees claim it is the simplest, easiest and cheapest to use.
Before CRISPR, the introduction of zinc-finger nucleases (ZFNs) and TALE nucleases (TALENs) in the genome engineering toolbox has already allowed gene manipulation in additional higher organisms such as human cells. These enzymes introduce double-stranded DNA breaks and can be engineered to target specific desired DNA sequences. The cell tries to fix the break using another copy of the sequence as a backup, such as the other unbroken chromosome in the pair but, by supplying a new template, the system can be forced to insert a desired sequence instead.
The ZFN approach is being pioneered by Sangamo Therapeutics, but shares in the company slumped in early February after interim results failed to show the treatment was effective against its rare genetic disease target. The phase 1/2 CHAMPIONS study did show that a single injection of SB-913 for the rare genetic disease mucopolysaccharidosis type II (MPS II) or Hunter syndrome was able to induce expression of the IDS enzyme that is deficient in the disease. However, the therapy wasn’t able to show an impact on biomarkers used to gauge the efficacy of MPS II drugs, and the company is now suggesting that it will have to switch its attention to a second generation of the ZFN design that it hopes will be more potent.
Sangamo CEO Sandy Macrae (below) attempted to convince investors that the results were a step on the path to an in vivo gene-editing therapy that is “the most difficult, cutting-edge application of our technology”, but the stock still lost almost a third of its value, implying investors had expected an efficacy signal.
TALENs work in a similar way to ZFNs and, while they tend to be bigger molecules, are considered to be easier and cheaper to make.
So far, the application of TALENs has been almost exclusively in ex vivo therapies, starting in 2015 when researchers from Great Ormond Street Hospital in London deployed the technology for the first time as a therapeutic. An 11-month old baby suffering from CD19-positive acute lymphoblastic leukaemia (ALL) was treated with modified donor T cells that had been engineered to attack leukaemia cells.
TALEN is now being deployed most widely in chimeric antigen receptor T cell (CAR-T) therapies, which generally involve harvesting T cells from patients, modifying them to attack tumour cells by recognising antigens on their surface, and then expanding their numbers and re-infusing them into patients.
Companies like Cellectis and partner Allogene are deploying TALEN to take a slightly different approach, using the gene-editing tool to create ‘off-the-shelf’ CAR-Ts that don’t require the harvesting stage. At this year’s American Society of Haematology (ASH) meeting, Allogene and partner Servier presented phase 1 data for their UCART19 candidate in ALL which revealed impressive response rates.
The approach is however also set to be tested as an in vivo therapy for cervical cancer caused by human papillomavirus (HPV) by researchers in China, who are comparing a TALEN plasmid to a CRISPR therapy.
Safety concerns
Like any emerging technology, there is still plenty to consider regarding safety as the first human trials start to generate results – even before He’s reckless bid for the scientific limelight.
For example, there’s no doubt that CRISPR- Cas9 has made gene editing very easy, but the tool has recently been found to be less precise than previously assumed. A Dutch team recently showed it can stimulate a little-known repair mechanism that can block the intended edit and also lead to dormant genes being activated, which could have serious consequences – for example if it led to the expression of a disease gene.
Meanwhile, earlier this year another group at the Wellcome Sanger Institute found that the technique can cause deletions that are far longer than expected, suggesting that these ‘may have pathogenic consequences’ and that therapies based on the approach must be subjected to rigorous scrutiny for possible harmful effects.
Gene-editing drug developers have acknowledged the findings but say that side effects such as cancer haven’t been seen in preclinical studies in cells and animal model, regardless of the specific technology deployed. What is clear that all the trials of gene-editing conducted in the west will be subjected to rigorous scrutiny, both before they start and after the results become available.
All told, what is clear is that while He’s experiment has backfired spectacularly, with reports suggesting he is now under house arrest in China, it is up to companies like Editas, CRISPR, Sangamo, Cellectis/Allogene and their peers to take the slow road to proving that gene- editing can deliver safe and effective therapeutics for patients with intractable diseases.