Answers:

  • How was this experiment designed?
    Rett Syndrome is caused by mutations in a gene called MECP2. Mutations in this gene, in turn, lead to a faulty MECP2 protein which is unable to function properly. Dr. Adrian Bird of the University of Edinburgh designed an experiment to answer a key question: Can full blown symptoms in a mouse model of Rett Syndrome be reversed by introduction of normal MECP2 protein?

    In order to answer this, Dr. Bird and colleagues designed an experiment in which the mouse MECP2 gene was initially shut down so that no protein could be made. Once the mice developed Rett-like symptoms, the gene was turned back on, allowing it to make functioning MECP2 protein.

    To silence the mouse MECP2 gene, research associate Jacky Guy inserted foreign pieces of DNA, called lox-Stop cassettes, within the gene. As the genetically modified mouse develops, the lox-Stop cassette acts like a lock, keeping the MECP2 gene silent in every cell.

    To turn the gene back on after the animals developed symptoms, Jacky Guy activated Cre recombinase (Cre), which is able to unlock MECP2 by splicing out the lox-Stop cassettes. As Cre is not normally found in mice, it was introduced by breeding the MECP2/lox-Stop mice to mice that have Cre in every one of their cells. The offspring therefore had both an MECP2 gene that is silenced by the lox-Stop cassettes and also Cre in all their cells. Importantly, Cre cannot delete the lox-Stop cassette at this stage, as it is kept anchored in the cytoplasm of the cell, away from the cell nucleus where the silenced MECP2 gene is found. In order for the Cre to be able to do its job and allow the MECP2 gene to turn back on it must enter the nucleus of the cell. This was achieved by treating the mice with a drug called tamoxifen, which breaks the bonds that trap Cre in the cytoplasm. Once free, Cre moves into the nucleus where it splices out the lox-Stop cassette and activates the dormant MECP2 gene.

    Remarkably, when MECP2 was activated by tamoxifen treatment of animals with Rett-like symptoms, the condition was largely reversed over a period of 4 weeks. Tremors disappeared, normal mobility, gait, breathing and weight were restored. Analysis of brain function showed that long-term potentiation (LTP), the quantifiable measurement of the ability of neurons to respond to stimulation, was also normalized after the reversal.




  • Why is this experiment so crucial?
    This work establishes the principle of reversibility in a mature mouse model of Rett and therefore raises the possibility that the symptoms of Rett may also be reversible in children and adults. Furthermore, the reversibility of symptoms could be achieved in mice that were very far into the disease process and days away from dying. In simple terms, gradually restoring normal amounts of MECP2 protein in to the Rett mice makes their symptoms disappear.




  • How is this different from earlier research showing Rett symptoms can be prevented?
    Preventing symptoms from developing in Rett mouse models did not tell us whether there was any hope that already existing damage could be repaired. Dr Bird's lab has accomplished a progressive reversal of severe symptoms in mature Rett mouse models.




  • What does this experiment mean for my child with Rett?
    It suggests that full blown symptoms may be amenable to treatment, even in mature patients. It had previously been expected that there would be a relatively short time window for treating Rett Syndrome symptoms, probably during the early stages of the disorder or even before symptom onset.




  • Will giving tamoxifen injections to my child with Rett be of any help?
    No. The use of tamoxifen is relevant only to the mice created for this research.




  • Based on the results from Dr Bird's lab, what are the next steps?
    Reversibility suggests that, once therapies are identified, it is not too late to improve the lives of children and adults with Rett Syndrome. The search for Rett Syndrome treatments is therefore boosted by these findings. Now the technical challenge is to learn more about the role played by MECP2 in the brain and to use this knowledge to identify drugs and other therapeutic approaches.




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