The following paper was released by the online journal Science Express at 2pm on February 8, 2007:
Reversal of Neurological Defects in a Mouse Model of Rett Syndrome
Jacky Guy, Jian Gan, Jim Selfridge, Stuart Cobb, Adrian Bird
Acknowledgement: This research was funded by grants from Wellcome Trust, Rett Syndrome Research Foundation and Rett Syndrome UK/Jeans for Genes
The Experiment: The laboratory of Dr. Adrian Bird at the University of Edinburgh, Scotland, genetically modified mice so that production of MeCP2 could be stopped for a period of time and then allowed to be produced and serve its normal function. This created the equivalent of a “temporary” mutation. The question was if the “temporary mutation” was in place long enough for all the mouse symptoms of Rett Syndrome to appear and run their course, would it then be too late to reverse symptoms and improve the disorder when normal MeCP2 function was restored.
The Study: The technology used in this study is called Cre-lox. The method provides a way of locking and then unlocking a gene. Mice were genetically modified to keep MECP2 silenced by inserting a foreign piece of DNA called a lox Stop cassette, thereby creating a model of the deficits seen in Rett.Syndrome. The lox Stop cassette could be spliced out, at will, by using a protein called Cre. Cre was kept anchored to an estrogen receptor in the cytoplasm of the cell while the mice, deprived of functioning MECP2, developed the Rett Syndrome symptoms. Symptomatic mice were then treated with a drug to release the Cre, which migrates to the nucleus and splices out the lox Stop cassette, unlocking MECP2 and allowing it to function normally. In this genetic manipulation, the normal gene is already present in the mouse model but is under the control of tamoxifen, an estrogen analogue, and is only activated by dosing with tamoxifen.
The Results: Turning off MeCP2 resulted in mice that had the full mouse symptoms of Rett syndrome (most but not all match human symptoms). When MeCP2 was gradually turned on in these mice, Rett syndrome symptoms were reversed even in mature mice where symptoms has been allowed to progress and some mice were days from dying. Long term potentiation (LTP), a cellular basis of learning and memory, is defective in mouse models of Rett syndrome. In this study LTP was defective when MeCP2 was locked and LTP was then restored to its normal function by the reversal experiments.
The Conclusions: This paper shows that while Rett syndrome has a neurodevelopmental component, it should not be considered a strictly neurodevelopmental disorder with a point of no return. The key finding is a proof of concept. The neurological abnormalities in mouse models of Rett syndrome are reversible, even after they are already evident. The exciting implication this suggests is that in humans treatment does not need to begin at birth or before the onset of typical features of Rett syndrome. While the genetic manipulation used in this study is not applicable to human treatments, it opens the door to the development of new therapeutic approaches.
Cautionary Notes: The authors’ conclusion suggests no direct therapeutic intervention in humans. Two cautionary notes are mentioned. One is that aggressive activation of the gene in male mice lacking any MeCP2 expression resulted in death of 9 of 17 animals. The remaining animals showed a return to ‘normalcy’. Thus, the effect of aggressive gene activation is all or none. The authors also acknowledge that studying the males is not the most proper approach – or, as stated in the paper, female mice with some cells already expressing the normal gene ‘may be the most appropriate model’ to study RS in humans. It appears that activation of the normal gene in these females was accomplished more slowly and did not produce the all or none phenomenon noted in the males.
To read the full paper, visit www.rettsyndrome.org.