Female Rett syndrome mouse model mimics patient gene dysregulation
Researchers: Preclinical models to test therapies should reflect actual mutations
A female mouse model of Rett syndrome manifested gene activity changes that began before the symptoms started and that mimicked those found in people with the condition.
Biological pathways affected across the disease’s progression also overlapped with other neurodegenerative and addiction disorders.
“Since RTT [Rett] in humans almost exclusively affects females, our results have important implications for translational medicine. First, preclinical models for testing new therapies should be female … ideally modeling actual human [Rett] mutations,” wrote the researchers, who noted “the overlap with other neurologic disease pathways … suggests that some existing drugs for neurodegenerative disorders could potentially be repurposed to [Rett].”
The study, “Sex-specific single cell-level transcriptomic signatures of Rett syndrome disease progression,” was published in Communications Biology.
Rett syndrome is mainly caused by mutations in the MECP2 gene, which provides instructions for producing a protein that regulates the activity of other genes by switching them on or off, which is important for the development of the brain and its function.
Rett almost exclusively affects females, since the MECP2 gene is in the X chromosome and females have two copies. This means they can inherit one X chromosome with the mutated gene and a normal X chromosome, that is, cells that express normal MeCP2, while others have the defective protein. This process is called mosaicism.
When do molecular changes in Rett occur?
Males have only one X chromosome. So a MECP2 gene mutation can’t be compensated by a healthy gene copy. Those who survive infancy typically have an earlier onset of symptoms. “However, it is not known when and in which cell types the molecular changes responsible for disease progression occur in MECP2 mutant females versus males,” wrote researchers at the University of California, Davis, who analyzed RNA levels to study gene activity in different types of cells from the cortex of a mouse model of Rett. The cortex is the outermost part of the brain.
A total of 93,798 cells of 14 different types, including neurons (nerve fibers) and non-neuronal cells, were analyzed from both sexes, at points in time that corresponded to presymptomatic, disease onset, and late disease stages.
At the presymptomatic stage, females with mutated MECP2 had 1,215 differentially expressed genes compared with control females, while only nine genes were altered in male mice. At disease onset, males had 73 genes with differential expression, while females had 47 genes with altered activity.
This means that in females, gene activity was most dysregulated before the onset of Rett onset, while the number of affected genes increased as the disease progressed in males. Still, the total number of dysregulated genes in males was 199, corresponding to 16.3% of those altered in females.
When the identified genes were categorized into networks, the scientists identified 18 different biological pathways consistently dysregulated as the disease progresses in females, compared with two in males.
The disease-relevant gene networks were enriched in pathways that contribute to normal brain function, including metabolism. These pathways are shared with neurodegenerative and addiction disorders.
Mutated genes’ effects on nonmutated neurons
The researchers also assessed the effects that neurons that carry the mutated gene had on non-mutated neurons, called non-cell-autonomous effects, in females. Before disease onset, neurons with a normal MECP2 gene had several dysregulated genes, likely due to the effect of mutated neurons.
Particularly, in excitatory (glutamatergic) neurons, which make it more likely that neighboring neurons propagate electric signals, eight pathways were dysregulated at all disease stages. These are also implicated in Parkinson’s, Alzheimer’s, and Huntington’s diseases, along with processes such as protein processing and modulating nerve cell communication via cannabinoid receptors. In inhibitory (GABAergic) neurons, which inhibit the activation of other nerve cells, only one pathway was affected across all time points. Still, several other Rett-related pathways were affected at certain disease stages.
“We showed that non-cell-autonomous effects in mosaic female [Rett] mice are responsible for … gene pathway dysregulations observed dynamically over time,” the researchers wrote.
Comparing mice to samples from females with Rett, the results showed that of the top 20 genes with increased expression in patients, 14 were also upregulated in female mice. Conversely, the top 20 genes with reduced activity in patients were also downregulated in female mice. Few genes overlapped between human Rett and Rett male mice.
“This demonstrates that [Rett] female mice are a better model for the dynamic [gene expression] dysregulation due to cellular complexities in Rett syndrome disease progression,” the researchers wrote.
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