MeCP2-boosting Approaches Led to Toxic Events in Mice

Providing a working version of MECP2 — the gene mutated in most cases of Rett syndrome — to cells reversed disease-specific symptoms in a mouse model of a common, milder form of Rett syndrome, a study shows.

However, female mice treated with this approach showed signs of excessive, toxic levels of MeCP2, the protein coded by the MECP2 gene and missing or defective in Rett patients.

These findings add to increasing evidence pointing out the fine balance of protein levels that MeCP2-boosting strategies, including gene therapy, must achieve to be both safe and effective in female patients, especially those with milder MECP2 mutations.

The study, “Profiling beneficial and potential adverse effects of MeCP2 overexpression in a hypomorphic Rett syndrome mouse model,” was published in the journal Genes, Brain, and Behavior.

Affecting females almost exclusively, Rett syndrome is caused mostly by loss-of-function mutations in one of the copies the MECP2 gene, located on the X chromosome (one of the sex chromosomes). Loss-of-function mutations are those that completely impair the resulting protein’s function.

As such, gene therapy, which delivers a healthy copy of the disease-causing gene to cells, has emerged as a potential Rett treatment. However, according to researchers, for such an approach to be both safe and effective it has to overcome three main obstacles.

The first is the narrow therapeutic window of MeCP2 levels in the brain, as both deficiency and overproduction lead to cellular toxicity and disease. Notably, excessive MeCP2 levels lead to a neurodevelopmental disorder called MECP2 duplication syndrome (MDS), which shares some similarities with Rett syndrome.

The second obstacle is the variety of Rett-causing MECP2 mutations, which can lead to the production of a partially functional MeCP2 (called hypomorphic mutations), associated with milder symptoms.

Last, but not least, female Rett patients typically show a mosaic pattern of cells producing a working MeCP2 protein together with cells with a partially or fully nonfunctional protein. This is due to X-chromosome inactivation, a process that occurs only in females due to the presence of two X chromosomes (one from each biological parent) and that randomly “turns off” one of the chromosomes in each cell.

While previous studies in Rett mouse models have shown the promising potential of gene therapy in reversing Rett-like symptoms, most have not examined adverse side effects related to excessive MeCP2 levels in females, or used models with hypomorphic MECP2 mutations.

Now, a team led by researchers at the Vanderbilt University in Nashville, Tennessee, evaluated the effects of providing a working MECP2 gene to cells in both female and male mice carrying R133C, a common hypomorphic MECP2 mutation in Rett patients.

The mutation — one of the eight most common in Rett patients and accounting for 7% of all cases — leads to the production of a partially working MeCP2 protein, resulting in milder disease symptoms.

Results showed that providing a healthy MECP2 gene to cells rescued all Rett-like behavioral, respiratory, and motor deficits in male mice, supporting “the potential use of MeCP2-targeted approaches for male RTT [Rett syndrome] patients with the R133C mutation,” the researchers wrote.

While most of these deficits were corrected in female mice, MDS-like anxiety, motor coordination, and associative learning and memory symptoms also were present. Further analysis confirmed this was associated with a partial MeCP2 function, as the same approach in mice fully lacking the MECP2 gene did not result in such symptoms.

These findings point to a potential “narrower therapeutic window for MeCP2-targeted therapeutics when used with mild hypomorphic mutations” in female patients, and suggest that “personalized approaches may warrant consideration for the clinical assessment of MeCP2-targeted therapies,” the team wrote.

Furthermore, “this study contributes valuable safety data to ongoing efforts to develop MeCP2-targeted therapeutics that are both safe and effective,” and its results “highlight the need for similar investigations in other [disease-causing] hypomorphic mutations of RTT, and their inclusion in preclinical development efforts,” the researchers added.