A group of researchers at the Texas Children Neurological Research Institute (NRI), Duncan, Baylor College of Medicine, found a possible new therapy to cure Rett syndrome has been reported in the Science Translational Medicine journal, with the early signs of success in a neurodevelopmental disorder with no cure yet.
According to the corresponding author, Dr. Huda Zoghbi, the director of the Duncan NRI, Distinguished Service Professor at Baylor, and an investigator at the Howard Hughes Medical Institute, Rett syndrome is an uncommon genetic neurodevelopmental disorder that results in a developmental regression, usually past 6 to 18 months of normal development, followed by severe motor skill impairment, speech and communication impairment and communication impairment. The disorder affects girls mostly and it occurs in 1 out of 10,000 live births.
Loss-of-function mutations in MECP2 gene cause Rett syndrome, which plays a central role in normal brain functions since it controls the amount of various genes controlling neurological functions. These mutations cause the protein to disappear or code a faulty protein which cannot perform its normal function. Certain mutated forms of MeCP2 protein are also less abundant and/or reduced in terms of DNA binding, which is a key role in the functioning of this protein.
In mouse models of Rett syndrome, it has been demonstrated that the disorder is reversible, once normal MeCP2 protein is added in the brains of those mice, the conditions are reversible. Notably, it has been demonstrated by researchers that raising the concentration of a mutant MeCP2 protein that retains a minimal amount of functioning also enhances symptoms, such as survival, motor coordination and respiratory defects in mice.
The importance of this in that approximately 65 percent of patients with Rett syndrome have partially functional MeCP2, which is either less abundant or lacking DNA binding capacity, as compared to the normal level, according to the authors. Our study, on working with mouse models and cells obtained by patients with Rett syndrome, suggests evidence of concept that an increase in the levels of mutant MeCP2 in patients with the condition would be therapeutically beneficial.
Mechanism of the MECP2 gene
The creation of therapeutics that regulate the level of MeCP2 is not that simple. A mutation to MeCP2 that is too small leads to Rett syndrome, but an excess of MeCP2 leads to another neurological disorder, MECP2 Duplication Syndrome. The balance of the question has been so delicate that it has been difficult to come up with safe and efficient treatments.
Zoghbi said that before this research, they had already learned that the brain normally forms two versions of the MeCP2 protein E1 and E2, which differ slightly. These versions are the result of the same gene, which gets processed in one direction to form E1 and in another to form E2.
Consider a gene as a blueprint of a protein. MeCP2 has four ingredients, which are e1, e2, e3 and e4. In order to produce the MeCP2-E1 protein, cells simply mix ingredients e1, e3 and e4. In order to create MeCP2-E2, the cells mix all four ingredients, which makes ingredient e2 peculiar to this form of the protein. Both versions are produced by the brain, but E1 is prevalent.
“We also learnt that no cases of Rett syndrome individuals have been reported to have mutations on E2 protein. The condition is only caused by mutations that interfere with E1 protein,” said Tirumala. This is supported in studies on mice.
“All in all, we have known that MeCP2-E2 is a single ingredient below MeCP2-E1 in the gene and it is not so abundant as E1 and not linked to Rett syndrome and is not required to support the MeCP2 activities in the brain,” Tirumala said. “This made us hypothesize that by instructing the brain cells to omit the e2 ingredient, the production of additional MeCP2-E1 protein in patients with Rett syndrome will be promoted and will lead to a better disease outcome. We have experimental evidence in the mice and cells of patients with Rett-syndrome to support our hypothesis.”
The researchers first genetically removed the ingredient e2 of normal Mecp2 gene in mice and evaluated the outcome of the deletion on the abundance of the protein and its neurological activity. Tirumala said that the results of this approach gave 50 to 60 percent MeCP2 protein increment in normal mice.
The researchers subsequently used the same strategy on the cells that were obtained using the patients with Rett syndrome that contained MECP2 mutations that decrease the abundance and activity of the protein. They removed the ingredient e2 in this mutant MECP2 gene and evaluated the impact of this mutation on the abundance of this protein and the attributes of these cells. Tirumala said that they were excited to find that MeCP2 production improved when ingredient e2 was deleted. Notably, with the degree of mutation, these cells reappeared with part or all of their usual structure, their usual electrical functions and their capability to control the level of other genes.
Lastly, the team evaluated the therapeutic potential of such an approach. Does a blocking drug of ingredient e2 elevate MeCP2 protein?
Tirumala said the value of morpholinos was tested to stimulate MeCP2 protein production in mice. “Morpholinos are artificial molecules to be used in this instance to inhibit the production of MeCP2-E2 protein by preventing the e2 ingredient to enter the cell,” explained Tirumala. “This was interesting because our morpholinos found to increase MeCP2 protein in mice tremendously.”
The work by Zoghbi and colleagues forms the basis and offers preclinical support to a treatment method of Rett syndrome involving the enhancement of MeCP2 and offering some functional recovery, Zoghbi said. Even though morpholinos are not an option due to their toxicity, analogous ones, such as antisense oligonucleotide therapies that are already being employed to treat other diseases, might be created in Rett syndrome.