The Rett Syndrome gene MECP2 –Function Redefined 14 years after its Discovery
It was almost exactly 14 years ago since the discovery of the genetic link to Rett syndrome was announced. The laboratories of Dr. Huda Zoghbi and Dr. Uta Francke searched to find the X-linked gene associated with Rett syndrome, and published their findings in the journal Nature in October 1999. The title of that seminal paper “Rett syndrome is caused by mutations in X-linked MECP2, encoding methyl-CpG-binding protein 2” said it all. Finally, the gene was known and the researchers could narrow in on why a mutation in this gene caused Rett syndrome. Prior to their findings, MECP2 was already being studied in other basic research laboratories. MECP2 was discovered by Dr. Adrian Bird who wrote the first publication about it in the journal Cell in 1992. Since then, there have been over 1,700 publications on MECP2 listed in the PubMED database. The gene and protein structure had been extensively studied. For a long time, MECP2 was considered a transcription factor – a protein that binds DNA and effects whether other genes are turned on or off. It indeed was shown to bind DNA, with more specificity for DNA that was modified by a chemical methyl group. In the case of MECP2, it was thought to act as a transcription factor that primarily turned off or repressed other genes for many years.
This past week, a new paper from Dr. Rudolf Jaenisch’s laboratory of the Whitehead Institute was published in the journal Cell Stem Cell that will change how we think about MECP2. Rudolf Jaenisch said in a recent press release "It was thought that this [MeCP2] protein globally repressed the expression of methylated DNA. What this work shows is when you do the analysis in a way that takes cell size into account—cell size is very different in Rett neurons compared to wild type—then suddenly we can see that the protein acts like a global activator. We've defined the function of MECP2 in a totally different way."
How did they redefine the function of MECP2? Using cutting edge technology, the researchers created new tools to model Rett syndrome at the cellular level. They generated two sets of human embryonic stem cell lines that represented both males and females; and within each set there were two cell lines that harbored either a normal or a mutant MECP2 gene. These stem cells have the capability of turning into any cell type such as neurons with the proper instructions. Another important note to make is that each set of cells were genetically identical except in the MECP2 gene. So using these new research tools, they made these stem cells turn into neurons and went on to characterize the cells and found that at the cellular level, the mutant cells appeared to have Rett-like features (small cell body, fewer dendrites, and a lowered ability to talk to each other). Once the cell lines were confirmed to have these features, they went on to study what was happening at the RNA and protein levels within the neurons because RNA and protein are ultimately elevated if a gene is turned on by a transcription activator. They discovered that the mutant cells had overall less RNA once they changed the way the data was analyzed and accounted for cell size differences between normal and mutant cells (which are much smaller). This study indicates that MECP2 would normally act to turn on genes because loss of MECP2 function in a mutant cell results in less RNA. They also showed that the overall protein levels were lower in the mutant cells, which is expected if there is less RNA. Finally, they concluded that a certain biological pathway was impaired and that by treating mutant cells with growth factors such as IGF-1 and BDNF to boost these pathways, one can rescue the phenotypes found in mutant cells.
So what does this all mean?
We have to start looking at MECP2 and the genes it 'turns on' differently. This will open up our continued search for relevant pathways to target therapeutically.
Their research describes these new cellular tools that we could use for in vitro testing of compounds (such as they did for IGF-1 and BDNF) for Rett and other neurologic disorders.
The paper points out that IGF-1 therapy is one that has great potential and illustrates the wisdom behind our support of the current Phase 2 IGF-1 clinical trial at Boston Children’s Hospital and the Phase 2 NNZ-2566 clinical trial at Baylor College of Medicine.
Yun Li, PhD, IRSF Postdoctoral Fellow Whitehead Institute at MIT
This work was funded in part by a 2012 International Rett Syndrome Foundation Postdoctoral Fellowship to Yun Li, PhD who is the first author on this paper. This fellowship was made possible by a generous donation from the Rett Syndrome Association of Massachusetts (RSAM) and its Co-President Maria McTernan who established Team Rett that ran in the Boston Marathon to raise the funds for Rett research.
IRSF would also like to highlight the power of our Mentored Training Fellowship Program in recruiting outstanding young scientists to Rett syndrome and would like to congratulate Dr. Li on such an elegant eye-opening study. “Not only does this paper bring a new technology to the forefront of Rett research but also gives us additional insights into potential therapies.” says IRSF CSO Steve Kaminsky.
This work was co-funded in part by the International Rett Syndrome Foundation, Rett Syndrome Association of Massachusetts, Simons Center for the Social Brain, Brain and Behavior Research Foundation, Croucher Foundation, Swedish Research Council, National Institutes of Health, Koch Institute, Curt Marble Cancer Research Fund, Simons Foundation Autism Research Initiative, and Ethel Louise Armstrong Foundation.
Global Transcriptional and Translational Repression in Human-Embryonic-Stem-Cell-Derived Rett Syndrome Neurons.
Li Y, Wang H, Muffat J, Cheng AW, Orlando DA, Lovén J, Kwok SM, Feldman DA, Bateup HS, Gao Q, Hockemeyer D, Mitalipova M, Lewis CA, Vander Heiden MG, Sur M, Young RA, Jaenisch R.
Cell Stem Cell. 2013 Oct 3;13(4):446-458.
Click here to read the IRSF Investigator Spotlight on Dr. Yun Li.