Gene Therapy for a Neurological Disease: Rett Syndrome

(Posted on Sunday, March 3, 2024)

Gene therapy may offer hope for people with a rare childhood disease called Rett syndrome. 

Around one in 10,000 girls are born each year with Rett syndrome, a rare genetic condition that impacts brain development. People with Rett syndrome grow without issue for a time before severe problems with speech, memory, and coordination arise. The wide-ranging symptoms must be managed for life, as there is no cure. 

Published in Neuroscience Bulletin, one nascent gene therapy is designed to treat the condition at the source via direct injections. This preclinical research demonstrates encouraging proof-of-concept and poises the imagination toward an effective, simple-to-administer treatment for this challenging disease. 

What is Rett Syndrome?

Rett syndrome is a neurodevelopmental disorder that almost exclusively affects girls. Babies with the condition appear to grow as expected for the first six to 18 months of life before signs progressively emerge. Although the symptoms and their severity vary from person to person, most experience impairments that influence every aspect of their life, such as the ability to speak, breathe, walk and use their hands. People with Rett syndrome can also struggle with scoliosis, seizures, heart problems, issues eating and sleeping, and more. 

A targeted therapy for Rett syndrome does not yet exist. Trofinetide, a drug approved in 2023, can reduce brain swelling and alleviate core symptoms in children aged two and older. Still, individuals usually rely on a robust wheelhouse of interventions to support their functioning. This may involve various therapies, such as occupational and physical therapy, specific medicines for certain symptoms and lifelong caregiver support. Despite the symptoms, people with Rett syndrome can live well into middle age and beyond. 

Advantages of Direct Injection 

Gene therapy has the potential to treat Rett syndrome comprehensively. This is because almost all cases of Rett syndrome are caused by mutations found on an X chromosome gene called methyl CpG binding protein, or MECP2. This gene typically encodes a protein (MeCP2) which is essential for brain development. The protein facilitates the expression of several genes and communication between nerve cells. Targeting the mutated gene in people with Rett syndrome could improve symptoms at its source. 

Researchers from Shanghai, China, are turning to direct injection to deliver their gene therapy—a notable departure from standard methods. Most gene therapies take a person’s cells, alter them in a lab, and then reintroduce them to the body through intravenous infusion; like this, the treatment is forced to circulate the body and often results in lethal liver toxicity. 

An injection could alleviate issues by delivering the treatment directly to the site, far from the liver. This method also relies on cells already inside the individual to produce the gene therapy, which removes the expensive need to manipulate the cells outside the body. 

Injection Improves Symptoms 

In their preclinical study, researchers Zilong Qiu and colleagues assess if injectable gene therapy can improve symptoms in mouse models of Rett syndrome. To accomplish this, the team relied on CRISPR/Cas9 gene editing to knock out the MECP2 gene in male mice. The knockout simulates the mutated gene expression seen in people with Rett syndrome. As the mutation is X-linked and males carry only one X chromosome, these male mice do not live long without treatment. 

The plan entails packaging normal MECP2 genes into adeno-associated virus serotype 9 viral vectors. The treatment is then bilaterally injected into the brains of these knockout mice two days after their birth. Ideally, the injection should rescue the mutated genes in the brain and extend their lifespan with minimal liver damage.  

The team compared two different promoters—human synapsin (hSyn) versus cytomegalovirus early enhancer/chicken β-actin (CAG)—to drive the gene expression. These promoters affect how the gene is introduced and expressed in the mice neurons. Additionally, the promoters can help lower liver toxicity traditionally associated with intravenous administration of viral vector-packaged MECP2. 

The treatments partially rescued MeCP2 protein levels throughout the brain, but its distribution was unequal. The hypothalamus experienced the highest transduction efficiency and the hippocampus the least. The CAG promoter appears to cause a more widespread and moderate expression of MECP2 than the human promoter. 

Notably, the cytomegalovirus/chicken promoter outperformed the human-derived promoter. Although both promoters significantly increased the production of MeCP2 protein in the brain compared to mice given empty viral vectors, the cytomegalovirus/chicken promoter extended the life of knockout mice over 250% more than the other promoter; this significant result exceeds other injectable MECP2 treatments. The CAG promoter may have also restored protein expression in non-neuronal cells, thus contributing to the prolonged lifespan. Knockout mice that received the CAG-MECP2 injection demonstrated a higher density of cells expressing both MeCP2 protein and NeuN protein, a marker for mature neurons. 

Beyond lifespan, the CAG-MECP2 injection also mildly increased the body weight of these knockout mice and slightly improved their ability to move and walk compared to controls. 

Future Implications 

As this study illustrates, a focus on gene therapy could revolutionize how we treat Rett syndrome. The CAG-MECP2 injection enhances protein expression in the brain and shows positive effects beyond lifespan, including increased body weight and improved movement and walking abilities. While human testing is still needed to determine its full potential, this innovative technique promises to treat disease in an efficient and cost-effective way. The positive effect of direct injection echoes in other investigations, including aggressive ovarian treatment and in vivo CAR T therapy for leukemias and lymphomas.

This story was originally published in Forbes, and can be read online here.