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CRISPR in the clinic

CRISPR in the clinic

Kat Arney
Genetic engineering, Health

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Unsurprisingly, the applications of CRISPR have exploded over the past few years. Scientists are already busy using the tool to engineer crops that are more resilient to extreme weather conditions, livestock that is more resistant to viruses to increase agricultural efficiency and even pigs that make better organ donors for humans. CRISPR is also being used in new tests for viruses, including a rapid test for COVID-19 that was approved by the FDA in May 2020.

But the most revolutionary applications of CRISPR lie in the clinic. So far, most clinical trials using CRISPR involve removing cells, such as blood cells, from a patient and modifying them before returning them to the patient’s body. Many ongoing trials are using this approach to modify immune T-cells so they can fight off various types of cancer - an approach known as CAR-T cells. 

Another CRISPR-based treatment that’s showing promise in clinical trials is for the genetic blood disorder sickle cell anaemia - an inherited condition affecting millions of people around the globe, caused by a mutation in a gene called beta-globin. 

When a person inherits two copies of the faulty beta-globin gene, the oxygen-carrying haemoglobin molecules in their blood cells are defective and form abnormal strands, making their red blood cells squish into a crescent or sickle shape, hence the name. These sickled cells die early, causing anaemia, and stick together, blocking arteries, causing organ damage and extreme pain. Many people with sickle cell anaemia need frequent hospital visits and blood transfusions, and most have their lives cut short.

Currently, the only cure is a bone marrow or stem cell transplant, which are both risky procedures. So, what if we could just fix the faulty gene in their blood cells with CRISPR?

One current clinical trial uses CRISPR to boost the production of fetal haemoglobin, a type of haemoglobin we all make at birth but is eventually switched off in favour of adult haemoglobin. The ideas is to produce enough fetal haemoglobin to compensate for the defective haemoglobin produced in sickle cell anaemia. To do this, doctors take stem cells from the patient’s bone marrow and use CRISPR to switch off the gene that usually shuts down fetal haemoglobin production after birth, switching it back on. The edited stem cells are returned to the patient, where they set up home in their bone marrow and hopefully start churning out fetal haemoglobin.

The first sickle cell patient to receive this pioneering treatment in the US was a young Mississippi woman called Victoria Gray. Doctors hoped that after the treatment, 20% of the haemoglobin in her body would be fetal haemoglobin. One year after treatment, nearly half of the haemoglobin in her blood was fetal haemoglobin and over 80% of her bone marrow cells contained the genetic modification needed to continue making the protein. What’s more, in the year after her treatment Grey has had no pain attacks or hospitalisations because of her condition. 

Unfortunately, just as they do for stem cell transplants, patients undergoing the CRISPR treatment still have to have chemotherapy first to kill off their stem cells and make room for the modified cells, with all the gruelling side effects and potential risks that it entails. Next, researchers hope to inject the CRISPR components directly into a patient’s body to alter the cells in vivo, removing the need for chemotherapy and making the whole process quicker and easier.

This so-called in vivo CRISPR, where the CRISPR technology is used inside a patient’s body, brings additional challenges. For a start, you’ve got to get the CRISPR editing complex into the correct cells, avoid inducing any accidental off-target edits where they’re not wanted, and avoid triggering an immune response. But in vivo CRISPR is already being used in some easily accessible tissues such as the eye, which is a handily confined space with little immune activity. 

The first people to receive in vivo CRISPR treatment were people with sight loss due to a rare genetic condition called Leber congenital amaurosis 10, which is caused by a mutation in a gene called CEP290 that disables light-detecting cells in the retina. Currently, there is no treatment for the disease, and it is a leading cause of childhood blindness. 

Ongoing clinical trials involve injecting CRISPR complexes that delete the mutation in the CEP290 gene, hopefully restoring the function of the light-detecting cells and, therefore also restoring sight. The first patients received the treatment in March 2020, and initial results are expected by the end of 2021. 

While the sickle cell studies involve editing cells outside the body and returning them, and the eye disease trials are testing editing inside the body, both techniques only make changes to the somatic cells of the body, not the egg or sperm cells, known as the germline. This means that any edits won’t be passed on to the next generation. 

But it’s here where the CRISPR story gets a little more unsettling.

CRISPR kids

Although many countries worldwide have bans on germline gene editing, there have already been attempts to alter the genetic code of human embryos, creating genetically modified humans with changes that would also be passed on to any children they might have.

In 2017, He Jiankui, a Chinese researcher and ex-professor at the Southern University of Science and Technology in Shenzhen, recruited families affected by HIV to a gene-editing study. The study involved editing embryos during IVF to disable a gene encoding a protein that allows HIV to enter immune cells, with the aim of giving the resulting babies lifelong immunity to HIV infection.

In 2018, He made headline news around the world when he announced the birth of twin girls, named Lulu and Nana, whose genomes had been altered in this way.  The international research community was outraged, calling the experiment irresponsible, premature and unjustified, and He was sentenced to three years in prison and a £350,000 fine for violating government bans on gene editing of embryos.

Aside from the ethical outrage, there’s another big question about He’s work that has tended to be overlooked: we don’t actually know for sure that it even worked.

He’s reports showed that edits were not made uniformly in all cells of early human embryos. Some cells had the intended edits, others had off-target edits, and some had no edits at all. What’s more, He only tested a few of the embryonic cells, and the edits to the other cells are unknown. 

As a result, it’s not clear what proportion of the twins’ immune cells have actually been edited, and scientists remain sceptical that the girls will have any additional immunity to HIV. Unfortunately, the only way to find out would be to expose them to the virus, and I’m sure you’ll agree that they have been subjected to enough dodgy, unethical science already. 

We will probably never know the effects of the off-target edits made to LuLu and Nana’s genomes and any changes they will pass on to their children, should they have them. So, does the failure of He’s editing and the international outrage that followed the girls’ birth mean we will never have any more gene-edited babies? 

There’s no doubt that CRISPR offers enormous potential for preventing genetic diseases. In theory, it’s simple, just edit the genes in embryos at the early stages, so every cell in the resulting baby’s body is free from the genetic disease. But right now, we don’t fully understand how our genes interact with each other, so editing genes in human embryos could have unintended and unexpected knock-on effects on different parts of the genome or the resulting baby’s health.

In 2019, ethicists and researchers called for a five year hold on human germline editing until the technique’s safety has been better investigated, moral and ethical issues explored, and acceptable uses agreed upon.  Currently, in the UK, editing somatic and embryonic cells for research is legal, but implanting edited embryos into the womb or using edited germ cells for reproduction remains against the law.  

So, for now, gene edited babies are off the table, but many scientists agree that it’s no longer a question of if it will happen, but when. 

The future of genome editing 

For now, CRISPR is undoubtedly the fastest and cheapest way to precisely make changes in an organism’s genome.  But it’s not the end of the story. Although CRISPR gets the job done, scientists worry about the potential impact of off-target edits. As a result, the next generations of gene editing technologies are focusing on increasing precision.

One exciting new technique, known as base editing, comes from Professor David Liu, a biochemist from Harvard University.

Rather than breaking the double-stranded DNA completely and relying on the cell to repair the breaks, as in CRISPR, his technique uses a clever molecular machine that makes specific tweaks to individual letters in the DNA code. One CRISPR-like component searches for and binds to a particular DNA sequence, while another performs a chemical reaction on the DNA letter you want to change at that site, making it look like another letter. 

Although it’s not a complete transformation, the change is close enough to fool the cell’s internal biological machinery, which fixes the mismatch in favour of the new version. So, where you once had a C opposite a G, you now have a T opposite an A, for example. And when that gene is read or the cell copies its DNA to divide, the change will be propagated.

Liu and his collaborators are hoping to use this technique to treat progeria, a genetic condition that leads to dramatically accelerated ageing, which is caused by a single DNA letter change. Initial tests in mice have been very promising, and the researchers are now working with pharmaceutical partners to bring base editing to the clinic.

The field of gene editing is moving at break-neck speed, with ideas that would be considered science fiction less than a decade ago, like correcting single DNA letters to cure disease or making GM babies, fast becoming a reality. But while these powerful tools offer genuine hope for people suffering from incurable genetic conditions, we should also make sure that they are used responsibly, so the consequences of their use remain positive for everyone. 

References:

Hello CRISPR

Hello CRISPR

Learning to edit: the early days of genome engineering

Learning to edit: the early days of genome engineering
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