The Stitch RNA (StitchR) method could accelerate the development of new drugs that could significantly treat Duchenne Muscular Dystrophy.
Loss of the DNA code that makes certain big proteins that muscles need to work right can lead to conditions that make muscles weak and waste away. Gene treatments try to get this code back into the muscle, but this can be hard because the DNA code is so long. Researchers have found a new way to send the code. It will now be sent in pieces that will be put together inside muscle cells. This could lead to new gene treatments that can help people who are losing muscle or getting weaker.
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The DNA Length Problem in Gene Therapy
Gene therapy has been created to treat a number of diseases that cause muscles to waste away and get weaker, but more work is still being done to make it work better. It can be hard because some of the proteins that muscle cells need are very big. The DNA code of a gene tells the body how to make a protein. The length of this code is measured by chemical blocks called bases.
The DNA base code needs to be longer for bigger proteins. The coding sequence for the dystrophin protein, which is missing in people with Duchenne muscular dystrophy, is over 10,000 bases long. Most genes’ coding sequences are only about 1,000 bases long. For example, if you don’t have enough dysferlin protein, you can get limb girdle muscular dystrophy and some other types of muscular dystrophy. The chain of bases that make up its code is over 6,000.
The challenge of using empty shells of viruses
Researchers have discovered that using the empty shells of viruses is the most effective method for introducing coding sequences into the body’s cells. These shells, known as capsids, are used by viruses to encapsulate their genetic material and transport it to neighboring cells for infection. Instead, they can be utilized to deliver gene therapy as they can be produced artificially in labs without the dangerous viral coding. Currently, capsids from a virus known as adeno-associated virus, or AAV, are the most effective for gene therapy. AAV is adept at reaching a large number of our cells without inciting the immune system.
Nevertheless, the AAV capsid cannot physically accommodate the coding sequences of dystrophin and dysferlin. It can only contain a sequence as long as the virus’s own genetic code, which is less than 5,000 bases long, and is just 26 nanometers broad (a thousand of them would fit across a human hair).
Ribozymes could be the solution
This has been circumvented in a novel method by recent study. A class of molecule known as a ribozyme has been identified and investigated in the past several decades. Ribozymes are RNA sequences that have the ability to split themselves in two. Ribozymes are copies of DNA that are utilized to generate proteins. They accomplish this by leaving particular chemical markings on the freshly cut ends. Certain proteins in the cell may recognize such markers and sew the ends back together, according to research from New York and Massachusetts. They believed that this may be used to combine the ends of two distinct RNA molecules, like two components of a gene therapy. For Stitch RNA, they dubbed the method StitchR.
The scientists created two RNA molecules to test StitchR: one with the initial portion of the dysferlin coding sequence followed by a second ribozyme, and another with a ribozyme followed by the remaining portion of the dysferlin coding sequence. In order to induce limb girdle muscular degeneration in a mouse model, they loaded some AAV capsids with one molecule and other AAV capsids with the other molecule.
What is Stitch RNA and How Does It Work?
The ribozymes are activated once they enter the mouse muscle cells and cleave the corresponding molecules. By identifying the marks left by the cuts, the cell’s repair processes fuse the two pieces together, restoring the dysferlin protein to the mouse’s muscles.
This method was also tested using dystrophin by the researchers. Because the dystrophin coding sequence is still too large for the AAV capsid even after being trimmed in half, they provided the coding sequence for a slightly shorter but still functional protein. The method improved a number of muscle condition metrics in the mouse model of Duchenne muscular dystrophy.
Conclusion
Although they have been developed for many years as prospective treatments for Duchenne muscular dystrophy, the dysferlinopathies, and other disorders, methods that combine the coding sequences of big genes inside the target cell have not proven to be very successful. The outcomes in mouse models are promising, and this new approach might help with some of the difficulties. To find out if this research can result in new medications, more testing is necessary.
