MeCP2 Optimized for Brain Delivery Holds Promise as Protein Replacement Therapy for RTT, Early Study Suggests

A recombinant version of MeCP2 — the faulty protein in Rett syndrome — has the potential to enter the brain, encouraging its development as a protein replacement therapy for the disease, researchers say.

The scientists have also developed an assay that can accurately and sensitively detect MeCP2, which may help research into possible Rett treatments.

Their study, “An electrochemiluminescence based assay for quantitative detection of endogenous and exogenously applied MeCP2 protein variants,” was published in the journal Scientific Reports.

Methyl-CpG-binding protein 2 (MeCP2) is a protein involved in regulating how the information carried by genes is used to produce proteins. MeCP2 works by modifying the structure of chromatin, the bundle of protein and DNA that makes up chromosomes, thereby controlling which genes are turned on or off in the genome.

Although broadly expressed in several tissues, MeCP2 plays a particularly important role in the brain where its levels need to be tightly regulated, as both a deficiency or an excess of the protein can lead to severe neuronal disorders, such as Rett syndrome.

Approximately 90%–95% of Rett syndrome cases are due to mutations in the MECP2 gene which are thought to result from a shortage of functional MECP2 protein in the brain. In the vast majority of cases, Rett is not inherited but instead arises from sporadic mutations in one of the parent’s reproductive (germ) cells.

Scientists are investigating ways to restore the production of a working MeCP2 in brain cells, including gene therapy (delivery of a correct version of the gene to cells) or MeCP2 protein delivery, also known as protein replacement therapy.

These approaches need to ensure a careful balance of the MeCP2 protein, so as to provide enough, but not too much, of it in the brain, as this can lead to severe disabilities as well, such as MECP2 duplication syndrome.

Protein replacement therapy is thus being investigated as an alternative to gene therapy since dosing can be done more accurately with proteins. This approach is still in the early stages of development and has its challenges, namely the fact that for successful delivery to the brain, proteins need to cross the blood-brain barrier (BBB), a protective semipermeable barrier that separates circulating blood from brain cells.

Therefore, researchers at the Medical University of Vienna set out to develop a recombinant (lab-made) version of human MeCP2 that could be transported across the BBB.

Additionally, because “there is a strong need for an assay that can measure MeCP2 levels in a time-efficient, low-cost and high-throughput manner,” the team also developed an assay that enables MeCP2 quantification in a high-throughput format.

They adapted a method — called electrochemiluminescence-based immunoassay (ECLIA) — combining antibodies that recognize and bind to MeCP2 with “detection antibodies.” When stimulated by electricity, these detection antibodies trigger a series of electrochemical and chemical reactions that culminate in the production of a light signal proportional to the amount of MeCP2 present in the sample.

As a proof-of-concept of ECLIA’s broad applicability, the researchers used the assay to quantify a commercial version of the protein, as well as to evaluate MeCP2 levels in mouse brains and human fibroblasts (cells important for the structure and wound healing of tissues).

The assay demonstrated highly quantitative, accurate, and reproducible measurements with low variability between experiments.

Importantly, the tool was used to show that the recombinant version of human MeCP2 created by the team had the potential to overcome the challenge of crossing the BBB and deliver MeCP2 to nerve cells in the brain.

“In this work, we report for the first time on successful expression and purification of recombinant [MeCP2] proteins. In addition, we developed a new ECLIA, representing a quantitative tool to investigate appropriate dosage of MeCP2 in neurons of the [mouse] brain and to monitor the protein behavior in these cells,” the researchers concluded.

These encouraging results may support the development of a potential new protein replacement therapy for Rett syndrome.