Various gene-therapy approaches take intention at Rett syndrome | Spectrum
Multi-pronged Approach: New therapies for Rett Syndrome look at the same goal from different angles.
Photo by William Mebane
In theory, researchers know how to reverse many of the debilitating effects of Rett syndrome: increasing the expression of the protein MECP2.
People with this condition typically have a mutated copy of the MECP2 gene and, as a result, a deficiency in its protein. This loss leads to intellectual disability, autism, seizures, and difficulty walking, speaking, and breathing.
In practice, however, it has proven complicated to get human cells to express the correct level of MECP2 protein. Unlike protein deficiency in some other genetic syndromes, MECP2 levels must be carefully titrated: Too little causes Rett syndrome; too much also leads to autism, seizures, and developmental delays.
The fact that MECP2 falls on the X chromosome raises another curve ball for potential therapies: Most girls with Rett syndrome who have two X chromosomes have one mutated and one functional copy of the gene – half of their cells So it already has sufficient levels of protein. This creates a dosing problem: indiscriminate increases in MECP2 levels can overdose these cells to treat the others. (Boys with mutations in MECP2 and only one copy of the gene rarely survive infancy.)
Traditional gene replacement therapy – where the mutated copy of MECP2 is swapped for a functional one – has been successful in mice, but it’s not obvious how to safely transfer this approach to humans, says Matthew Lyst, a research fellow at the School of Biological Sciences at the University of Edinburgh in Scotland.
For these reasons, a cadre of researchers is looking for alternative gene therapy approaches with RNA or microRNA. These different treatments all take slightly different approaches in attempting to circumvent Rett’s dosage problem.
“It takes a clever solution,” says Lyst.
One possible treatment targets RNA – the individual strands of genetic material that translate the DNA code to build the MECP2 protein. In Rett syndrome, MECP2 mutations repeat in the RNA, resulting in incorrect translation and no protein. But editing the RNA sequence can ensure that MECP2 is made anyway.
One of the advantages of RNA editing is that, unlike gene editing, it is potentially reversible, says John Sinnamon, assistant professor at Oregon Health & Science University in Portland, who did the work at the International Society for Autism’s annual event Research (INSAR) 2021 presented meeting in May. Not every type of RNA mutation can be edited, he says. Still, he and his colleagues have combined several techniques to combat about 45 percent of the mutations in people with Rett syndrome.
A technique made it possible for Sinnamon and his colleagues to process half of the MECP2 RNA in mice that carry a particular Rett mutation. It also increased MECP2 protein across multiple cell types, the team found.
“That was a fundamental proof” that this type of processing works in the brain of an animal, says Sinnamon. “The next step can be seen [if we can] get a sufficient level of editing to see a behavioral effect, ”and whether there are any negative results, he says.
A related approach called “RNA trans-splicing” is to target the part of the RNA that carries the mutation and replace it with an unmutated version.
As with RNA editing, the RNA trans-splice only affects those cells that carry a MECP2 mutation. But trans-splicing also has the advantage of being able to target a wider variety of Rett variants than RNA editing, said Stuart Cobb, neuroscience reader and research fellow at the University of Edinburgh, who presented the work at INSAR.
Newborn wild-type mice injected with a virus carrying an RNA trans-splice treatment show trans-spliced RNA throughout the brain, Cobb and colleagues have found, suggesting that the approach is effective.
At the moment, the process of altering the MECP2 RNA is inefficient, Cobb said during his talk, but the team is working to find better targets for providing the corrected part of the gene sequence.
Taysha Gene Therapies uses another strategy to control dosage. They build on standard gene replacement therapy and add a mechanism that regulates MECP2 expression.
The company’s new treatment, named TSHA-102, uses microRNAs – short strands of genetic material – to slow down production of the MECP2 protein before it gets too high, said Suyash Prasad, chief medical officer and head of research and development of the company, during an INSAR presentation.
Treatment uses a virus to deliver a functional copy of MECP2 into the nucleus, as well as an untranslated sequence that binds the microRNAs and slows production of the protein if it gets too high. As a result, each cell’s MECP2 levels stay within the “goldilocks” range, Prasad said: Not too high, not too low.
Wild-type mice that received this gene therapy treatment had no increased risk of premature death or showed no adverse behavioral effects, while 4 out of 10 wild-type mice that received therapy that indiscriminately increased MECP2 died prematurely.
And mice that don’t have MECP2 in their brains usually die by 10 to 12 weeks of age, but those treated with Taysha’s gene therapy survive an additional 5 weeks, the team found.
Like any gene therapy, these approaches involve risks such as adverse immune responses and long-term DNA damage. Such results can prevent people from participating in later studies. And as science advances, researchers, clinicians, families, and regulators must assess whether it is worth taking these risks.
One way to understand how the families of people with Rett Syndrome weigh the risks and benefits is to ask how Taysha did it in focus groups. Most of all, caregivers worry about their children’s inability to speak, gastrointestinal problems, and difficulty breathing, which suggests they would particularly appreciate treatment that could improve these characteristics. The company plans to use this feedback to inform about the upcoming clinical trial, Prasad said.
Ideally, TSHA-102 or the next treatment that makes it to trial will elevate MECP2 to an ideal level and alleviate any Rett characteristics. But it may not be possible to get these levels in all cells – or necessary, said Sinnamon at INSAR.
In mice, “any small increase in the amount of MECP2 re-expression appears to be beneficial” to the animals, and small improvements could have real benefits for people with Rett syndrome, he said.
A promising fact for any Rett therapy option is that neurons lacking MECP2 don’t die – they just don’t function normally. This means that successful treatment has the potential not only to slow the progression of Rett traits, but to reverse them entirely. And it also reduces the pressure to step in as early as possible to make a difference – a pressure that exists for many neurodevelopmental disorders.
Until clinical studies are underway, it is unclear how much MECP2 needs to be expressed and in how many cells for the improvements to be worth the risk, the researchers say.
Of the alternative therapies discussed, TSHA-102 is the furthest in the pipeline; The company plans to begin clinical trials later this year, Prasad said. Researchers working on the other approaches say it’s difficult to predict when their therapies might be at this stage – they still need to refine the techniques in mice.
But, says Sinnamon: “Animal models can only get you so far.”