Boosting production of mutant protein causing Rett may be new treatment
Increasing MeCP2 in lab led to improvements in nerve cell function
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Modulating nerve cells to boost production of the mutant form of the MeCP2 protein that causes most cases of Rett syndrome may be a promising new therapeutic strategy for easing symptoms of the genetic condition.
That’s according to new research in which U.S. scientists investigated this key protein using nerve cells in a laboratory.
Because many mutant versions of MeCP2 still retain some activity, increasing their production — either through genetic changes or treatment — helped correct abnormal gene activity and improve nerve cell function in mice, as well as in nerve cells derived from patients, the team discovered.
The researchers suggested that these findings may inform the development of so-called antisense oligonucleotide therapies, already used for other conditions, to help many Rett patients who retain some MeCP2 protein function.
“Our work lays the foundation and provides preclinical evidence for a therapeutic approach for Rett syndrome that increases MeCP2 and confers functional improvement,” Huda Zoghbi, MD, the study’s lead author and a professor at the Baylor College of Medicine in Texas, said in a university news story detailing the work. Zoghbi is also the director of the Texas Children’s Duncan Neurological Research Institute.
Titled “Modulating alternative splicing of MECP2 is a potential therapeutic strategy for Rett syndrome,” the study was published in the journal Science Translational Medicine.
In nearly all cases, Rett syndrome is caused by mutations in the MECP2 gene, which lead to a loss or reduced function of the MeCP2 protein. Normally, MeCP2 binds to DNA and helps control when other genes are turned on or off. Without enough functional protein, gene activity is disrupted, impairing communication between nerve cells and leading to Rett symptoms.
Researchers tested new hypothesis in mice and patient cells
Previous research in Rett mouse models has shown that increasing normal levels of MeCP2 in the brain can reverse disease symptoms. Even increasing the levels of a mutant MeCP2 protein that retains some function can improve motor coordination, lung function, and survival.
“This is important because about 65% of patients with Rett syndrome have partially functional MeCP2 that either has decreased DNA binding or is less abundant than normal,” said Harini Tirumala, a graduate student in the Zoghbi lab and the study’s first author.
The MECP2 gene consists of four exons, which are segments of DNA that carry instructions to make proteins. Different combinations of exons lead to two slightly different versions of the MeCP2 protein: E1 and E2.
In MECP2-E1, the most abundant version in the brain, exon 2 is skipped (left out), resulting in a protein made from exons 1, 3, and 4 — which carry all known Rett-causing mutations. The template used for MECP2-E2 contains all four exons, and the protein is produced from exons 2, 3, and 4.
“We knew that MeCP2-E2 differs from MeCP2-E1 by a single ingredient in the gene, is less abundant than E1, is not associated with Rett syndrome and is not needed for MeCP2 function in the brain,” Tirumala said. “This led us to hypothesize that guiding brain cells to skip the [exon 2] ingredient would promote the production of more MeCP2-E1 protein in patients with Rett syndrome and improve disease outcomes.”
Work ‘provides proof of concept’ for increasing mutant MeCP2 protein
When the team genetically deleted exon 2 of the gene in healthy mice, MeCP2 protein production increased by as much as 50%-60%.
The researchers then tested the approach in nerve cells derived from a Rett patient carrying a mutation called G118E; this is a milder mutation that partially reduces MeCP2’s ability to bind DNA. Removing exon 2 increased MeCP2-E1 levels, improved the electrical activity of nerve cells, and corrected many abnormal patterns of gene activity.
In another patient-derived model carrying a more severe Rett mutation called T158M — which reduces protein stability and DNA binding — exon 2 removal led to only a small increase in MeCP2 protein. Even so, the researchers observed partial improvement in gene activity patterns.
“We were excited to see that deleting [exon 2] enhanced MeCP2 production,” Tirumala said. “Importantly, depending on the severity of the mutation, these cells recovered part or all of their normal structure, their normal electrical activity and their ability to regulate the levels of other genes.”
Our study provides proof of concept that increasing the levels of mutant MeCP2 in patients with the condition could provide therapeutic benefit.
Finally, the researchers tested a potential treatment approach in mice using a morpholino, a molecule that causes cells to skip exon 2 during gene processing and protein production. The treatment increased the level of the MeCP2 protein in living animals, a result Tirumala called “exciting.”
The researchers noted that morpholinos themselves are too toxic to use in people. However, similar technologies could be developed for Rett syndrome: One example is an antisense oligonucleotide therapy, which would target the template molecules used to produce proteins and can change how those molecules are read.
In the study abstract, the researchers noted that “these data set the stage for a potential therapeutic strategy using antisense oligonucleotides to promote isoform switching in patients with [Rett] who carry partially functioning alleles of MECP2.”
According to Tirumala, this work lays the foundation for developing new treatments for Rett.
“Our study provides proof of concept that increasing the levels of mutant MeCP2 in patients with the condition could provide therapeutic benefit,” Tirumala said.
