Maintenance of X-inactivation: A Step Towards a Potential Treatment for Rett Syndrome
Not only do the brown and black splotches of fur on calico cats make them pretty cute, they also reflect a moment early in mammalian development called X-chromosome inactivation. While females have two X-chromosomes, males only have one, so to ensure that the expression of genes on the X-chromosome is equal between the sexes, female cells will inactivate one of their X-chromosomes. This process, known as gene dosage compensation, is easily visible in cats because the gene for coat color is found on the X-chromosome.
Problems can arise, however, when genes on the X-chromosome are mutated or lost, often resulting in sex-specific diseases. Rett Syndrome, for example, results from mutations in the gene MECP2 on the active X-chromosome . Males with the mutation die shortly after birth, but while females survive, they have neurodevelopmental defects . Prior work in mice has shown that specific loss of MECP2 expression in the brain results in symptoms of Rett Syndrome . One current avenue of research is to ask if reactivation of the inactive X-chromosome carrying a healthy copy of MECP2 in brain cells might be a therapeutic option for women born with Rett Syndrome.
An important molecular player in X-inactivation is a long non-coding RNA called Xist (X-inactive specific transcript). Xist spreads along the X-chromosome that will be inactivated and recruits other proteins to help silence it . While much is known about the initiation of X-inactivation, less is known about the factors involved in maintaining an inactivated X-chromosome.
In a new study published in Epigenetics & Chromatin, Adrianse et al asked if Xist is important for the continued maintenance of X-inactivation in mouse brain cells. To do this, they took advantage of the fact that when female mice inherit a mutation for Xist on the X-chromosome from their mother, they selectively silence the X-chromosome from their father. This quality allowed them to engineer an EGFP reporter driven by the MECP2 gene promoter on the paternal X-chromosome (Xp). Thus, if the inactive Xp became active, the cell would drive expression of EGFP and glow green.
The researchers then used the Cre-Lox system to generate female mice that went through normal X-inactivation, and then, by expressing Cre specifically in brain cells, lost Xist expression only in the brain. While these mice appeared perfectly healthy and survived as long as their litter mates with normal Xist expression, the researchers found that after loss of Xist, mouse brain cells, specifically neurons, lost expression of H3K27me3 and H2AK119ub1. Therefore, Xist is required for maintaining these repressive chromatin marks. They also saw that 2-5% of neurons and 0.1-0.2% of astrocytes expressed MECP2-EGFP, which indicated that they had reactivated the inactive X. Because only a small percentage of brain cells reactivated, Xist must not be the only factor controlling maintenance of X-inactivation.
Taking a look at the global effect of loss of Xist, the team found that genes on the X-chromosome increased expression as compared to genes on autosomes. In a similar trend, DNA methylation of CpG islands on the X-chromosome decreased compared to DNA methylation of those on autosomes. To get a better idea of which genes were changing expression, the researchers grouped genes based on their expression level and found that the most highly expressed genes were also more prone to reactivation compared to lowly expressed genes. This same effect was also reflected in the CpG methylation pattern of highly expressed genes, which showed reduction in methylation compared to CpG islands of genes with lower expression levels.
This new study shows that Xist is important for maintaining repressive chromatin marks and DNA methylation on the inactivated X-chromosome. Loss of Xist in the brain led to a low percentage of brain cells reactivating. The result that the mice remained healthy, indicates that this loss of dosage compensation was not detrimental to their survival. The tolerance of reactivation of a few brain cells in mice is a step forward in the potential use of X-reactivation to treat X-linked disorders like Rett Syndrome.
Main article: Adrianse RL, Smith K, Gatbonton-Schwager T, Sripathy SP, Lao U, Foss EJ, Boers RG, Boers JB, Gribnau J & Beldalov A (2018). Perturbed maintenance of transcriptional repression on the inactive X-chromosome in the mouse brain after Xist deletion. Epigenetics & Chromatin, 11 (50). https://doi.org/10.1186/s13072-018-0219-8
- Shah RR and Bird AP (2017). MeCP2 mutations: progress towards understanding and treating Rett syndrome. Genome Medicine, 9 (17). https://doi.org/10.1186/s13073-017-0411-7