Rett Syndrome: Neurodevelopmental No Longer

Huda Zoghbi’s watershed discovery of the genetic cause of Rett Syndrome in 1999 ushered in a new era of research. The first mouse models for the disease came on the scene in 2001. The male mice are missing the Mecp2 protein completely and are called knockouts; the females, due to X chromosome inactivation, have approximately half of their cells lacking the protein.

These are what most people think of when discussing "Rett mice." However, in the past few years more types of mouse models have been created, each of them developed to answer a specific question and to teach us something about the disorder. Differences between these various models help form the foundation for much of the current drug discovery efforts.

Data regarding the latest animal model was published today in the high-profile journal, Science. The model was developed in the Zoghbi lab by MD/PhD student, Christopher McGraw. Through genetic engineering techniques he created mice that were missing Mecp2 only as adults.

MONICA COENRAADS, RSRT EXECUTIVE DIRECTOR, INTERVIEWS HUDA ZOGHBI, M.D.

MC: Dr. Zoghbi, please tell us about your decision to undertake this experiment and the results.

HZ: There were two main reasons we wanted to perform this study. We know that Rett symptoms start after birth and we wanted to understand whether there are any developmental components to the disease. In other words, did the Mecp2 protein have some function that is important during early development or childhood? That was the first question we wanted to answer.

The 2007 experiments from Adrian Bird’s lab told us that if Mecp2 is restored in adult mice that had developed abnormally without the protein, their Rett-like symptoms are reversed. We were curious to see whether brain cells that had properly developed and matured with Mecp2 being present, and had gone through typical experiences of learning and memory and then had the protein removed as adults – would their phenotype be milder? And the answer was a resounding NO. The big surprise for us was how similar the knockout mice that had Mecp2 missing from conception were to the mice that had Mecp2 missing only as adults. This told us that bypassing the critical period of development did not affect the severity of the symptoms.

The experiment tells us that you need Mecp2 all the time. It also tells us that you need Mecp2 not for development but rather to maintain normal brain function.

MC: So the timing of the appearance of Rett symptoms has nothing to do with development and everything to do with what happens in the brain after you remove Mecp2. Can we safely say now that Rett is not a neurodevelopmental disease?

HZ: Our experiments were done on mice and not humans so we must always be cognizant of that caveat. But I think you are right. It’s how long the cells are without the protein that matters.

MC: Your experiment certainly strengthens the idea that Rett is not neurodevelopmental. The reversal experiments of 2007 provided the first clue. You and I have been at meetings together where the issue has been debated. Some pointed out that the debate was not worth having because it was a matter of semantics. But I disagree. This is not just semantics; there are clinical implications.

HZ: You are right. It’s not semantics. I now call Rett a post-natal neurological disorder. Mecp2 is a factor that is critical for the normal function of brain cells. It’s a factor that is constantly needed for normal neurological function and this has implications for therapies. Therapies will need to be maintained for the long term.

MC: Your findings have implications for other diseases that are post-natal, like autism. Can you elaborate?

HZ: There are many disorders that show up after birth and we have assumed that, just like for Rett, the absence of a protein was affecting normal development. I think this paper is telling us that maybe this is not the case. In the case of Rett the protein affects transcription; in other cases the proteins are doing something different but the end effect is the same - some molecule is not being made in the right amount when it is needed in the brain cell.

So for disorders like Fragile X, Angelman Syndrome, Tuberous Sclerosis, if we take away their particular protein the cells are sensitive to the deficiency, but if we bring the protein back the chances for recovery are high.

MC: The key is that Rett symptoms are not hard-wired since the same symptoms can be found also in the adult knockout. That is hopeful, encouraging news.

It’s quite fascinating to me that despite being a rare disorder and having relatively small number of investigators working on Rett, the field seems to be tearing away at some long- standing neuroscience dogma.

Your discovery in 1999 made Rett the first sporadic neurological disorder that had a gene associated with it. Rett was the first childhood neurological disorder to be shown to be reversible, thereby teaching us about the plasticity of the brain. We now know that Rett is not developmental and this fact calls into question the neurodevelopmental status for other disorders such as autism, Fragile X, Angelman Syndrome, Tuberous Sclerosis and others. It’s quite remarkable.

It’s been almost 12 years since you discovered the genetic cause of Rett. Are we as far along as you would have expected in our search for treatments?

HZ: In many ways things are going well, as we’ve learned so much about the disease. We know the anatomy of the brain is normal, we know the cells can recover if you bring back this protein. Our challenge is that the protein is so essential for so many cells. Finding a pharmacological intervention that can hit a great majority of the cells will be key. I don’t underestimate the difficulties; it will take some very good pharmacology to bring the symptoms under control.

MC: I know I speak for every Rett family around the world – we are tremendously grateful that you are working on behalf of our children. Thank you for your commitment, your determination and your hard work.