CRISPR Shows Promise for Enhancing Gene Therapy

Scientists at Washington University School of Medicine in St. Louis have developed a new method that could help increase the long-term effectiveness of gene therapy. Gene therapy, a procedure that inserts genes into a patient's cells, has shown promise in treating inherited genetic diseases, but has proven to be a challenging task. Typically, replacing a defective gene with a healthy gene is a short-lived treatment that only lasts for a few weeks.

In a study supported by the National Institutes of Health and published in Gene Therapy on Feb. 27, 2018, researchers combined the gene-editing tool Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) with a deactivated virus to deliver a healthy gene to a precise location in the bodies of living mice.

The study showed that the inserted gene remained properly activated in mice for a minimum of six months. According to the scientists, the typical duration for this type of gene expression is four to six weeks. The experiment was stopped at the six-month mark to study the mice, but scientists say that the length of sustained gene expression was equivalent to a fix that would last the lifespan of the mice.

“Over the years, one limit of gene therapy has been the difficulty in achieving long-term gene expression to treat disease,” said senior author David T. Curiel, MD, PhD, a professor of radiation oncology and cancer biology, in a university press release. “We have shown long-term expression of a gene that treats alpha-1-antitrypsin deficiency, which is the most common form of inherited emphysema. And now we’re applying this technique to hemophilia, a genetic condition in which blood does not clot properly.”

According to the university, viruses have been studied for their potential to deliver gene directly into cells for decades. To make gene therapy work, scientists have used this function to enable viruses to deliver healthy genes to counteract disease while rendering the viruses harmless. However, scientists have no control over where the virus inserts the gene into the cell’s DNA code, raising the risk of undesirable mutations, as stated by the university.

CRISPR gene-editing technology has changed this level of control. Scientists tell CRISPR what combination of DNA “letters” to look for in a cell and what sequence of DNA to replace them with or to insert into that location. The challenge noted by researchers with CRISPR is delivering it to the right place inside the body.

Curiel and his team have combined viral delivery methods with CRISPR to overcome the limits of both techniques using research gathered by their efforts and those of other groups. The researchers use adenovirus as the delivery vehicle and CRISPR as the navigator and the editor once it gets there, inserting the desirable gene into a part of the genome that is unlikely to cause problems.

“We targeted these viruses with CRISPR to a part of the genome that’s called a safe harbor,” said Curiel, director of the School of Medicine’s Biologic Therapeutics Center, in the release. “These are parts of the DNA sequence that are quiet, removed from more active areas, and where it’s unlikely for this type of editing to do anything harmful.”

According to the university, Curiel and his collaborators used adenovirus, a type of virus that can cause the common cold and has been shown to be more efficient at gene transfer than other viruses, as the delivery vehicle to maximize efficiency and longevity of the treatment.

Curiel states that adeno-associated virus, the virus used in the muscular dystrophy experiments, is less efficient in delivering its cargo than the adenovirus used in the current study. 

Challenging to produce and only able to carry a small amount of DNA, adeno-associated virus is appropriate for correcting this type of muscular dystrophy. However, correcting for blood serum deficiencies requires much higher levels of gene expression, according to the university. This is because corrected muscle proteins stick around for a while, but serum proteins in the blood must be renewed constantly. Adenovirus was chosen to sustain this constant renewal because it can carry more cargo, but safety is a concern for researchers.

The university reports that the mice in this study appeared to be healthy for the duration of the experiment, but questions remain about the possibility of a triggered immune response by adenovirus or the CRISPR protein machinery, triggering an immune response. 

Currently, Curiel and his team are investigating the safety and efficacy of the combined adenovirus and CRISPR techniques with a focus on improving gene therapy for hemophilia.

Sources: Washington University in St. Louis, Scientific Reports