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Genome Editing Shown to Repair Sickle Cell Gene

Blood cells with sickle cell disease

Red blood cells with sickle cell disease (NIH.gov)

2 March 2015. Geneticists at the biotechnology company Editas Medicine show how genome editing techniques could repair mutations in the gene that causes sickle cell disease. Cecilia Cotta-Ramusino, a researcher with Editas, presented her findings today at the Keystone Symposium for Genomic Instability and DNA Repair in Whistler, British Columbia, Canada.

Sickle cell disease is a genetic blood disorder affecting hemoglobin that delivers oxygen to cells in the body. People with sickle cell disease have hemoglobin molecules that cause blood cells to form into an atypical crescent or sickle shape. That abnormal shape causes the blood cells to break down, lose flexibility, and accumulate in tiny capillaries, leading to anemia and periodic painful episodes. The disease is prevalent worldwide, and affects 70,000 to 80,000 people in the U.S., including about 1 in 500 people of African descent.

Editas Medicine develops therapies with the ability to turn off and on and repair genes causing disease. The company’s technology harnesses discoveries including clustered, regularly interspaced short palindromic repeats (CRISPR) and related CRISPR-associated protein 9, known as CRISPR/Cas9. With CRISPR/Cas9, the Cas9 protein binds to targeted RNA molecules generated by the human genome. The RNA molecules then guide Cas9 proteins to specific genes needing repair, making it possible to address root causes of many diseases.

The gene targeted in this case is the hemoglobin beta or HBB gene, where a mutation changes a protein building block that causes the components of hemoglobin to stick together in long rigid molecules. These abnormal molecules in turn bend red blood cells into the sickle or crescent shape characteristic of sickle cell disease.

Cotta-Ramusino and colleagues at Editas, applied their technology in a technique called gene conversion, where the mutated gene is repaired with a different, but closely related gene. The repair — performed in lab cultures, not with humans or lab animals — used material from the hemoglobin delta or HBD gene, a cousin of hemoglobin beta.

The team tested several Cas9 enzymes to repair the HBB gene, and found one known as D10A that in about 30 percent of the cases could repair HBB genes with HBD material. The tests show as well D10A enzymes could repair HBB genes without material from an external donor. The tests suggest that a potential gene conversion therapy could be designed with HBD genetic material from a person afflicted with sickle cell disease, eliminating the need to find a compatible donor.

“While the results are early and further work is needed to see if this approach could be used therapeutically, says Katrine Bosley, Editas Medicine’s CEO in a company statement, “these data suggest gene conversion as a possible new approach to genomic repair for certain kinds of genetic mutations.”

Editas Medicine, in Cambridge, Massachusetts, began in 2013 and licensed technologies developed by the company’s scientific founders and others from Harvard University, MIT, Massachusetts General Hospital, and Duke University.

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