It has been seven years since the discovery that the CRISPR/Cas9 defence system, used by microbes to destroy viruses, could be re-engineered to edit the human genome. Since then researchers have carried out an array of experiments to explore potential applications.
Biophysist Dr He Jiankui sparked global controversy concerning the ethics of genome editing when he used CRISPR to genetically modify embryos, resulting in the birth of the first genome-edited babies (see BioNews 977).
'There's been a lot of appropriate caution in applying this to treating people, but I think we're starting to see some of the results of that work,' said Dr Edward Stadtmauer, a haematologist at the University of Pennsylvania, Philadelphia.
Over a dozen new clinical trials testing CRISPR therapy on diseases such as cancer, HIV and sickle cell anaemia were listed on the clinicaltrials.gov database last year. One trial in its early stages used CRISPR to treat sickle cell anaemia and beta-thalassaemia, both genetic blood disorders that result in the production of an abnormal form of the oxygen-carrying protein, haemoglobin.
Two patients with these disorders were treated by CRISPR Therapeutics in Cambridge, Massachusetts, and Vertex Pharmaceuticals in Boston, Massachusetts, using CRISPR to inactivate a gene that switches off the production of an alternative form of haemoglobin. Preliminary results of the study suggest that this therapy improved some of the symptoms but the participants will need to be followed for a longer period to be sure.
Results from two other trials, one in which genome-edited blood cells were transplanted into a man to treat HIV infection, and the other in which they were transplanted into three people to treat some forms of cancer, were less successful. In both cases, the transplanted cells flourished in the bone marrow of recipients, without any serious safety concerns, but did not produce a clear medical benefit. The study has been placed on hold while researchers explore ways to boost that percentage, says Hongkui Deng, a stem-cell researcher at Peking University, Beijing, China and a lead author of the work.
Other researchers are trying to move beyond editing cells in vitro. In July 2019 a clinical trial was launched to treat Leber congenital amaurosis 10 (LCA10), a rare genetic disease that causes blindness. The trial, launched by two pharmaceutical companies, Editas Medicine in Cambridge, Massachusetts, and Allergan in Dublin, Ireland, will be the first trial that uses CRISPR to edit cells inside of the body. The researchers are testing AGN-151587 (EDIT-101), which is a novel CRISPR treatment delivered via adeno-associated virus (AAV) directly to the eye's light-sensing photoreceptor cells to remove the mutation that causes LCA10.
There are some encouraging results so far from these trials. However the sample sizes are so small that it is difficult to draw a firm conclusion about their safety and efficacy in humans. Moving forward, larger groups of patients need to be studied over a substantial period of time.