Study Uses Brain Organoids to Model Rett Syndrome Mutations, Reveals Paths to Treatment

A study by neuroscientists at MIT's Picower Institute for Learning and Memory has used three-dimensional human brain tissue cultures, known as organoids, to model two different mutations that cause Rett syndrome. The research, published in Nature Communications, found that while the mutations share some common effects, they also produce distinct structural, functional, and molecular abnormalities. These differences led to successful correction by two different drug treatments in the organoid model.

The findings demonstrate that "individual mutations matter" and that the approach represents a method for personalizing treatment, even for a single-gene disorder. — Senior author Mriganka Sur

Background and Methodology

Rett syndrome is a developmental disorder most often caused by mutations in the MECP2 gene. More than 800 different mutations in this gene can cause the condition, with eight specific mutations accounting for over 60% of diagnosed cases.

This study focused on two of these mutations:

R306C: A mutation involving a single DNA base pair change, representing approximately 7-8% of Rett syndrome cases.
V247X: A rarer and typically more severe mutation involving a single DNA base deletion, which results in the production of an incomplete protein.

Researchers created brain organoids by reprogramming skin or blood cells donated by Rett syndrome patients with each specific mutation. These organoids were cultured for three months. The team used three-photon microscopy to analyze the organoids' structure and to image live neural activity. They also performed single-cell RNA sequencing to profile gene expression. Collaborators at Boston Children's Hospital provided electroencephalogram (EEG) measurements from a group of children with Rett syndrome for comparison with the organoid data.

Experimental Findings

The analysis revealed a combination of shared and mutation-specific abnormalities in the organoids compared to control organoids with non-mutated MECP2.

Common Abnormalities:

Reduced development of axon projections from neurons.
Reduced neuronal spiking activity.
Reduced synchronicity, or coordinated firing, between neurons.
Aberrations in molecular pathways involved in synapse development.

Mutation-Specific Differences: Abnormality R306C Mutation Organoids V247X Mutation Organoids Structure Structurally similar to control organoids. Showed structural differences, including larger size and altered layer thickness. Network Efficiency Exhibited a decrease in "small-world propensity" (SWP), a metric of efficient network structure. Exhibited an increase in SWP compared to controls. Neural Connectivity Not specifically reported as different from controls in this aspect. Showed significant differences in connectivity between excitatory and inhibitory neuron types. EEG Correlation EEG readings from patients indicated alterations in SWP properties, consistent with the organoid findings for both mutations.

Molecular Analysis and Targeted Treatments

Gene expression analysis identified distinct molecular signatures for each mutation type:

In R306C organoids, the gene HDAC2, which represses the expression of other genes, was overexpressed.
In V247X organoids, there was reduced expression of genes for certain GABA neurotransmitter receptors. Defects in the function of astrocyte cells were also observed.

Based on these specific molecular findings, researchers tested targeted drug treatments:

Treatment with an HDAC2 inhibitor restored neuronal activity and SWP to control levels in R306C organoids.
Treatment with the GABA agonist baclofen restored SWP to control levels in V247X organoids.

The researchers noted that both drugs have been studied in other disease contexts and are well-understood, suggesting potential for repurposing.

Researcher Statements and Next Steps

Lead author Tatsuya Osaki noted that the organoid platform provided mutation-specific insights that were not apparent in prior studies where the MECP2 gene was simply deleted.

The research team plans to apply this organoid platform to study four additional MECP2 mutations.

Research Support and Authors

The study was supported by the National Institutes of Health, a MURI grant, The Freedom Together Foundation, and the Simons Foundation.

The paper's authors include Mriganka Sur, Tatsuya Osaki, Charles Nelson, Chloe Delepine, Yuma Osako, Devorah Kranz, April Levin, and Michela Fagiolini.