RNA-based Treatment Restores Motor Health in CMT1A Mouse Models

An RNA-based treatment normalized the levels of the key PMP22 protein as well as motor function and muscle strength in two mouse models of Charcot-Marie-Tooth type 1 A disease (CMT1A), including one of severe disease, a study shows.

This forms the basis of a possible precision medicine approach for CMT1A and other disorders, and will be further developed with a goal of moving it into clinical trials, the researchers said in a press release.

Their study “Squalenoyl siRNA PMP22 nanoparticles are effective in treating mouse models of Charcot-Marie-Tooth disease type 1 A” was published in the journal Communications Biology.

CMT1A, the most common form of CMT, is caused by a duplication of the PMP22 gene, leading to excessive production of the PMP22 protein. This excess leads to the demyelination (loss of myelin) of peripheral nerves, resulting in muscle weakness and shrinkage. (Myelin is the fat-rich layer that helps nerve cells send electrical signals more efficiently.)

Researchers in France developed a potential treatment using a special RNA molecule, called small interfering RNA (siRNA), to lower PMP22 levels and restore its normal function in cells.

The information coded in DNA gets transcribed into RNA molecules before being translated into a protein. siRNAs are short RNA molecules that can bind to a specific RNA, leading to its degradation and preventing protein production.

After testing several candidate siRNAs targeting the PMP22 RNA molecule, the researchers found one that led to a long-lasting reduction of about 50% in the levels of both RNA and its protein.

However, when the investigators injected the siRNA intravenously (into the vein) in a mouse model of CMT1A, they saw no drop in PMP22 protein levels. In agreement, the injection did not ease deficits in the mice’s locomotion and muscular strength.

The scientists thought this lack of effect was likely due to the siRNA’s instability, a common feature of these molecules.

To circumvent this, they coupled the siRNA with a biocompatible and biodegradable molecule called squalene, typically used in cosmetics and pharmacology. Squalene forms nanoparticles in the presence of water, and is a way of protecting siRNAs from degradation without affecting their performance.

The siRNA of PMP22 joined with squalene was able to reduce PMP22 levels in lab studies. The team then tested this molecule in two different mouse models of CMT1A, carrying one or two extra copies of the PMP22 gene. Compared to control (wild-type, healthy) animals, the two mouse models showed significant impairments in motor activity and poor muscle strength.

Mice received five intravenous injections of the siRNA-squalene nanoparticles, at a dose of 0.5 mg/kg twice a week for 20 days. The siRNA normalized the animals’ motor activity and muscular strength. Electrophysiological tests, which assess electrical properties, showed the animals’ nerve cells to be working better with nerve conduction comparable to those of control mice.

The siRNA nanoparticles also normalized (lowered) the levels of PMP22 protein and led to a significant increase in the amount of the proteins SOX10 and KROX20, which play an important role in myelin production.

Levels of a neurofilament known as NF-H, a marker of large axons, also rose to levels similar to those of control mice. Axons, or nerve fibers, are the long projections of a nerve cell that conduct electrical impulses away from the cell body to other nerve cells, muscles, and glands.

Nerve cell shape was also improved after treatment in both mouse models.

The benefits of the new treatment were deemed long lasting, with the effects preserved for over three weeks following the last (fifth) injection in mice with both moderate and severe CMT1A. Full locomotor recovery and muscle strength benefits were again seen after a second round of treatment, given after an interruption.

Overall, “treatment with siRNA PMP22-SQ [squalene nanoparticles] represents a potent promising therapy for CMT1A patients,” the researchers wrote.

“Possible applications of this therapeutic approach could go beyond the treatment of genetic diseases and may be extended to the normalization of gene expression [activity] altered by environmental factors, lifestyles, and age-related disorders,” they added.