Targeting the RNA
Type I myotonic dystrophy is the most common type of muscular dystrophy in adults. People with this disease inherit repetitive DNA segments that lead to the toxic accumulation of repetitive messenger RNA, which transfers genetic information from DNA into the mechanism of cellular protein production. As a result, people born with myotonic dystrophy experience progressive muscle wasting and weakness, as well as many other severe symptoms.
CRISPR-Cas9 is a genome editing technique that is increasingly being used in attempts to correct DNA defects associated with a variety of diseases. A few years ago, researchers at the University of California, San Diego School of Medicine modified this method to edit RNA instead of DNA (RCas9 method).
In a new study, Jin Yeo's group has shown that a single injection of RCas9 gene therapy can destroy toxic RNA and almost completely change the symptoms in a mouse model of myotonic dystrophy.
Standard CRISPR/Cas9 directs the Cas9 enzyme to cut a specific target gene (DNA), thereby allowing researchers to inactivate or replace it with a healthy one. RCas9 works similarly, but the Cas9 enzyme is directed to the RNA molecule, not DNA.
In a 2016 study, Yeo's group demonstrated that RCas9 successfully tracks RNA in living cells. In a 2017 study on laboratory models and cells obtained from patients, the group used RCas9 to eliminate 95% of aberrant RNA associated with type 1 and type 2 myotonic dystrophy, a type of ALS and Huntington's disease.
A new study has confirmed the effectiveness of RCas9 by reversing the course of type 1 myotonic dystrophy in a living organism – a mouse model of the disease.
Muscle tissue of a mouse model of myotonic dystrophy: fibers with RCas9 (green color) do not have toxic RNA (red color), whereas fibers without RCas9 (black color) have toxic RNA.
RCas9 is a type of gene therapy. The researchers loaded RCas9 inside a weakened virus, which is needed to deliver the system inside cells. They injected part of the mice with a single dose of therapy, the other with a simulated treatment.
RCas9 reduced the number of aberrant RNA repeats by more than 50%, the result varied slightly depending on the tissue. The treated mice became virtually indistinguishable from healthy mice.
The researchers feared that RCas9 proteins derived from bacteria could trigger an immune reaction in mice and quickly disappear, so they suppressed the animals' immune system during treatment. As a result, the viral vector and its cargo were preserved unharmed and completed the work. Moreover, there were no signs of muscle damage, on the contrary, the group noted an increase in the activity of genes involved in the formation of new muscles.
At least 20 human genetic diseases are caused by the accumulation of repetitive RNAs, so the CRISPR-Cas9 system targeting RNA has the potential to become an effective method of therapy for these incurable diseases.
It is not yet known whether RCas9-based therapies will work in humans and whether they can cause harmful side effects, such as an unwanted immune response. Preclinical studies will continue, and they will help researchers determine the potential toxicity and assess the long-term effects of the method.
Article by R.Batra et al. The sustained expression of Cas9 targeting toxic RNAs reverses disease phenotypes in mouse models of myotonic dystrophy type 1 is published in the journal Nature Biomedical Engineering.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on UC San Diego News Center: Twist on CRISPR Gene Editing Treats Adult-Onset Muscular Dystrophy in Mice.