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Prime editing successfully corrects sickle cell mutation

Researchers have used prime editing to correct the mutation that causes sickle cell disease in patient-derived stem cells. The study, published in Nature Biomedical Engineering, showed that transplanting the edited cells into mice led to the rescue of the sickle cell phenotype.

A prime technique

Sickle cell disease is a blood disorder characterised by misshapen blood cells that cannot efficiently carry oxygen around the body. The condition can be debilitating, yet the disease is relatively common, impacting millions of individuals worldwide, particularly those of African descent. Currently, the only cure for the disease is stem cell transplant from a suitable healthy donor, yet these can be hard to come by.

The disease is caused by a mutation in the haemoglobin-Beta (HBB) gene, and as such there is potential to correct the mutation in the patient’s own cells pre-transplant. A significant amount of research has been carried out in this field, and previous work has shown that base editing could remove the disease-causing mutation,  but ultimately could not restore the wild-type variant.

In a bid to correct the mutation and rescue the sickle cell phenotype, the team from the Broad Institute and St. Jude’s Children’s Research Hospital used prime editing to alter the HBB gene. Prime editing works on a ‘search-and-replace’ model – using a guide RNA and Cas protein similar to CRISPR technology – but instead of creating potentially lethal double strand breaks, it nicks only one strand of the DNA. This means that prime editing is less likely to have harmful off-target effects.

Figure 1: Diagram describing a prime editor and the process of prime editing. Adapted from Matsoukas, 2020.

Long-term impacts

The researchers used the prime editing technology to correct the adenine-to-thymine point mutation that causes the condition. This function could not be performed with base editing, as it cannot convert thymine back to adenine. The prime editing tool was able to locate the mutation and successfully correct in it around 40% of patient-derived stem cell samples. Clinical efficacy is usually assumed at 20%, meaning the results are promising.

To further test the success of the technology, the stem cells were transplanted into sickle cell mouse models. The mice displayed normal haemoglobin production in around 45% of their red blood cells up to 17 weeks post-transplant, implying that the treated cells can persist long-term.

More to come

Prime editing is the first gene editing technology that has shown true potential to accurately and efficiently correct the HBB gene mutation with limited off-target effects. However, the team were keen to stress that there is much more testing to be done before the treatment could be used in humans.

Discussing the results in a press release, author Jonathon Yen stated, “Because of its unique versatility, prime editing has the potential to cure many more genetic diseases. It will be a challenge to get to the clinic. It will require extensive manufacturing development, process optimization and safety assessment. But the proof of concept is there. Our work now opens the door to developing cures for many hematological diseases.”

More on these topics

Gene Editing / Prime Editing / Sickle Cell