Therapy Using ‘Unstable’ Gene Seen to Help Mice in Rett Model

A form of gene therapy based on providing an unstable version of the MECP2 gene extended survival and improved motor function in a mouse model of Rett syndrome, a study reports.

The study, “Whole brain delivery of an instability-prone Mecp2 transgene improves behavioral and molecular pathological defects in mouse models of Rett syndrome,” was published in the journal eLife.

Rett syndrome is caused by mutations in the MECP2 gene, which carries instructions to make a protein called MeCP2. This protein is responsible for maintaining synapses (the sites where nerve cells communicate) and controlling the activity of several other genes.

Studies in mouse models of Rett have shown that reactivation of the MECP2 gene (the mouse equivalent to the human gene) in nearly 70% of the neurons found in the brain of adult animals eased motor and sensory difficulties associated with the disease.

Despite such promising results, the use of potential gene therapies is not straightforward, because MECP2 is active in different brain regions and its duplication in humans is associated with other severe neurological disorders.

“Thus, a successful gene therapy for RTT [Rett syndrome] has to deliver the correct MeCP2 dosage in a tight range that overlaps with endogenous [naturally produced] levels,” the researchers wrote.

To reach the brain, these therapies must also overcome challenges posed by the blood-brain barrier — a highly selective membrane that protects the brain from potential treats carried on circulating blood.

Researchers in Italy, in collaboration with colleagues in the U.S., may have found a way to overcome both challenges. Their strategy involved using a previously characterized, harmless adeno-associated virus (AAV) vector called AAV-PHP.eB to deliver an instability-prone version of MECP2 (called iMECP2) to neurons across the brains of adult mice engineered to mimic Rett symptoms.

Proteins making up the shell of the AAV vector had been modified to allow the virus to bypass the blood-brain barrier following intravenous administration (into the bloodstream), thereby increasing its efficacy at delivering its content to neurons in the brain.

To overcome concerns with dosage and gene duplication, researchers used a special version of the Mecp2 gene that generated a highly unstable messenger RNA — the template for the production of a protein, in this case Mecp2. The instability of the messenger RNA made MeCP2 production less efficient, but still able to generate normal protein levels.

To test the this gene therapy’s efficacy, investigators tested increasing doses of the AAV-PHP.eB-iMecp2 construct. Treatment was seen to extend the animals’ lifespan, improve their motor skills, and normalize the activity of several genes in the brain.

Brain levels of the MeCP2 protein also remained within a normal range following treatment.

Although this therapy was well-tolerated in female mice, a strong immune response was seen in males that required treatment with immunosuppressants — agents that reduce immune system activity. The adverse effects found in male animals was due to their lack of MECP2 from birth, the researchers suggested, with their systems recognizing the delivered gene as foreign.

“Altogether in this study we characterized an iMecp2 viral cassette [construct element] which sustained significant improvements … safety and efficacy leading to the long-term symptomatic amelioration of RTT mice,” the investigators wrote.

“These results might accelerate the introduction of new gene therapy strategies for RTT with clinical prospective,” they added.