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New Genome-Editing Method Could Make Gene Therapy More Precise and Effective

A Boston startup wants to create precise genome-editing treatments that can address more types of disease than previous gene therapy methods.
November 27, 2013

A new startup, backed with $43 million in venture investments, aims to develop treatments that could cure inherited diseases with a one-time fix based on a new method of genome editing. The method offers great precision in changing the DNA sequence of a genome and can potentially treat diseases that other forms of gene therapy cannot.

The new method, to be commercialized by Editas Medicine, is based on a genome-editing system that was largely unknown just two years ago. The company’s founders say the method will allow them to go after currently untreatable diseases.

In the last two years, scientists have come to better understand that many microbes use a system of protein and RNAs (molecular cousins of DNA) to defend themselves against invading viruses. Researchers have adapted this bacterial immune system, referred to as CRISPR/Cas, to edit single base pairs of the human genome as well as larger stretches of DNA. “Big ideas like this emerge in the science world infrequently,” says Douglas Cole of Flagship Ventures, one of the three life-science-focused venture-capital firms investing in Editas.

The CRISPR/Cas system uses a “guide” RNA to bring the DNA-cutting Cas protein to a specific DNA sequence that contains a disease-causing mutation. Once the Cas protein cuts the DNA at that target site, the system replaces the faulty DNA sequence with a healthy version.

Harvard geneticist George Church, who cofounded Editas, says the technology’s ability to change single base pairs enables fundamentally new ways of thinking about gene therapy. Many inherited diseases, including cystic fibrosis and sickle-cell anemia, are caused by single base pair changes to the DNA sequence of genes; the precise CRISPR/Cas technology could correct these mutations in patients.

Church and other founders say the momentum behind other forms of gene therapy bolstered their decision to start Editas. With European approval of a gene therapy in 2012 (“Gene Therapy on the Mend as Treatment Gets Western Approval”) and the launch of new gene therapy companies (“Improved Virus Technology Spurs New Gene Therapy Startups”), the idea that inherited diseases could be cured with genetic medicines seems to be dusting off its precarious start.

But the traditional methods of gene therapy are limited. Generally, the treatments involve adding a functional copy of a gene to a patient’s cells. The healthy copy can either incorporate itself into the patient’s genome or it can remain a distinct entity, depending on the particular design of the treatment. This means the original dysfunctional copy of the gene remains. In cases where the dysfunctional gene produces nothing or a harmless, broken protein, this added gene method works. However, if the dysfunctional gene produces a dangerous version of a protein, or if other DNA mutations cause a gene to be overproduced to the point of toxicity, adding a healthy copy will not treat the resulting disease.

CRISPR/Cas could address these sorts of diseases, says Church. Take Huntington’s disease, an inherited neurodegenerative disorder caused by a toxic protein made by a dysfunctional gene. Traditional methods of gene therapy cannot address this disease, but CRISPR/Cas has the potential to correct the faulty DNA sequence.

“One of the powerful things about [Editas’s technology] is that it allows us to do things that there is no other approach to doing at all,” says Editas director and Third Rock Ventures partner Alexis Borisy.

Another advantage of the gene-editing potential of CRISPR/Cas technology is that the corrected gene remains in its normal chromosome location, which preserves the way the cell normally turns a gene on or off. With CRISPR/Cas, “all the natural program controls, the natural responses are all in place,” says Borisy.

Editas will need to address a number of issues with the technology to turn it into a medicine. For one, the CRISPR/Cas system can make unwanted cuts in the genome if its guide RNA nearly matches other DNA regions outside the gene that researchers want to treat. Another area to explore is how to best deliver the molecular tools into a patient’s cells. But the founders are confident that the expertise in the company, which includes many of the pioneers of CRISPR/Cas, will lead to ways to address these issues.

The company’s founders won’t talk specifically about the diseases they will try to address, but say they will focus on grievous diseases that are currently not treatable.    

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