The approach described in work takes advantage of the fact that every patient carries a cure within their own cells. But that cure is locked up by a life-long process called X-chromosome inactivation. Our goal has been to unlock the inactive X and restore expression of the good gene copy . The study report was published in PNAS Early Edition
Females carry two copies of the X chromosome , but within each cell only one copy is active, the other being silenced by an RNA molecule called Xist. Which copy remains active in which cell is randomly determined during embryonic development. While some X-linked disorders only produces symptoms in males, who carry a single X chromosome, females can be affected if a mutation is in a dominant gene on the active X chromosome.
The mutation leading to Rett syndrome affects the X chromosome gene for a protein called MECP2 , which is essential for normal neuronal development. While carrying this mutation before or after birth, females appear normal for the first year of life.
However, their physical and cognitive development slows and then regresses, leading to a constellation of symptoms. Carrette describes "resembling a combination of autism, cerebral palsy, Parkinson's, epilepsy and anxiety disorder." While females with Rett syndrome can survive in their 50s, they require round-the-clock care and assistance. The condition is the second only to Down syndrome as the most common cause of severe intellectual disability in females.
The possibility of reactivating the healthy X chromosome requires surmounting two primary challenges, the multiple processes involved in keeping the chromosome inactive and the potential for toxic effects. The MGH team targeted both the Xist molecule itself and the gene-silencing process of methylation, combining a Xist-binding antisense oligonucleotide resulting in a "dual modality" approach.
Their experiments in two mouse cell lines, each with a different method of distinguishing whether gene expression originated from active or inactive X chromosomes – revealed that the combined treatment increased MECP2 expression from the inactive chromosome up to 30,000-fold, depending on the duration of treatment .
Lee, a professor of Genetics at Harvard Medical School, said, Our approach to reactivating genes on the inactive X chromosome can be applied to other X-linked disorders CDKL5 disorder and a number of other neurodevelopmental syndromes. For Rett Syndrome, we are actively pursuing development of the two-modality approach.
However, researchers need a better female mouse model to test the reactivation drugs. This is an active area of ??research, and our preliminary data indicates that full expression of MECP2 protein is not required for improvements of symptoms. Thus, they are optimistic that their approach will eventually provide meaningful treatment for patients, but much more work remains to be done.
Antisense oligonucleotide drugs are very selective since they must precisely match their target RNA molecule, reducing the chance for off-target effects, and since Xist has a very specific role in X inactivation, no other cellular processes should be disturbed. The methylation inhibitor Aza is less selective, but the synergy produced in combination with the Xist antisense drug allows us to use relatively low doses, making this combination a feasible option.