A genetic method that changes how cells “read” the MECP2 gene could help raise levels of the protein disrupted in people with Rett syndrome, according to a new study.
Rett syndrome is caused by genetic changes in MECP2, an X-chromosome gene that regulates thousands of genes important for brain function. Some variants completely block the production of the MECP2 protein, whereas others encode a defective version or just less of it. The condition mainly affects girls, who are often also diagnosed with autism. Boys with the syndrome typically die by early childhood.
Trofinetide, the only drug approved by the U.S. Food and Drug Administration for Rett syndrome, can ease some traits related to the condition but does not address the underlying cause. By contrast, increasing MECP2 protein levels in mice can reverse many of the condition’s traits and extend survival, studies show. For developing treatments for people, though, titrating the right amount of MECP2 is tricky: Too little causes Rett syndrome, and too much can lead to a different neurological condition called MECP2 duplication syndrome.
Current gene therapy approaches use viruses to deliver a self-regulating copy of MECP2 to limit overproduction. But these vectors reach only a small fraction of brain cells near the injection site. Because MECP2 is active throughout the brain, researchers are exploring other strategies that can safely boost protein levels more broadly, says study investigator Huda Zoghbi, professor of molecular and human genetics at Baylor College of Medicine.
Zoghbi and her team prompted the cell’s splicing machinery, which typically cuts away chunks of nucleotide sequences within genes, to skip over a small segment of the MECP2 gene. That adjustment in turn boosted MECP2 protein production in mice. In neurons derived from people with Rett syndrome, deleting the same small segment of the MECP2 gene partially corrected some of the condition’s hallmarks.
“It’s a very clever study,” says Walter Kaufmann, adjunct professor of human genetics at Emory University School of Medicine, who was not involved in the research.
T
he work builds on previous studies that identified two versions of MECP2 messenger RNA, called e1 and e2, which differ in one key segment: The e2 form includes a short section called exon 2, whereas e1 does not.
To encourage cells to favor the e1 version, which is more efficiently translated into protein, Zoghbi and her team deleted exon 2 in MECP2 in wildtype mice. As a result, cells produced more of the e1 form, raising MECP2 protein levels by up to 60 percent in the brains of the modified mice.
Male mice, which carry only one X chromosome, tolerated the increase well, displaying only minor side effects such as a slight rise in anxiety-like behavior. Female mice, which naturally carry a mix of cells expressing either normal or altered MECP2—similar to girls with Rett syndrome—showed no obvious behavioral difficulties.
It told us something about the protein itself—that even small levels of change can make a significant difference.
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Harini Tirumala
The researchers then tested the approach in neurons grown from stem cells of people carrying different Rett-associated variants. In neurons with the G118E variant, which reduces both the stability and DNA-binding ability of MECP2, deleting exon 2 restored protein levels to near normal, improving neuronal functional and structural characteristics. Up to 65 percent of disrupted genes in these neurons showed partial recovery after MECP2 levels increased. In neurons carrying a more disruptive variant, MECP2 rose only slightly, but gene activity across hundreds of affected genes still shifted closer to normal.
Seeing such a strong effect from a moderate increase in the altered protein was “very exciting,” says study investigator Harini Tirumala, a former graduate student in Zoghbi’s lab. “It told us something about the protein itself—that even small levels of change can make a significant difference,” she says.
The team also developed a molecule similar to an antisense oligonucleotide that binds near exon 2 and prevents it from being included in the final mRNA. This exon-skipping approach increased levels of the e1 form of MECP2 in wildtype mice. The team reported the findings earlier this month in Science Translational Medicine.
K
aufmann compares the strategy to exon-skipping antisense oligonucleotides used for Duchenne muscular dystrophy, which don’t cure the condition but convert a severe form into a milder one by producing a shorter, yet still functional, protein. The Rett approach could similarly make many MECP2 variants more workable, he says.
The strategy, however, would apply only to people whose variants involve some residual MECP2 protein function. It would not work for those whose variants completely eliminate the protein or its activity. “There, we can’t help,” Zoghbi says. Given that about 65 percent of people with Rett syndrome still make a protein with some residual function, she estimates that many of those people would benefit from the approach.
The idea may also extend beyond Rett syndrome, Zoghbi says. In principle, similar strategies could be used for other genetic conditions in which different versions of a gene produce proteins with different efficiencies.
The proof of concept in human neurons and mice is strong enough to justify further work, says Mriganka Sur, professor of neuroscience at the Massachusetts Institute of Technology, who was not involved in the study. “It’s a very creative approach,” he says.
But the results so far come from experiments in healthy animals rather than established Rett models, so it remains unclear how much the exon-skipping approach could help people with the condition, Sur says.
Other strategies—such as one reported last year that uses small RNA switches to keep MECP2 levels from rising too high—are already in clinical trials and show promise, whereas the new approach “has a longer way to go,” he says.