A Patient-Friendly Guide To X-Chromosome Reactivation Therapy

For many females with X-linked genetic disorders, research on their condition and available treatment options are limited. But scientists are now exploring a new possibility: what if the key to treatment lies in genes already present in a person’s cells, rather than relying on external medication or adding new genetic material? A gene therapy called X chromosome reactivation, still in the early stages of development, is showing promise as a potential treatment for women with X-linked disorders.

Human females have two X chromosomes and males have one X and one Y chromosome. If females were left with two fully functional X chromosomes, they would receive a “double dose” of the proteins made by genes on the X chromosome compared to males. To compensate for this, early in embryonic development, females undergo a process called X chromosome inactivation (XCI).

One of the X chromosomes in every cell is tightly condensed, essentially “turning it off” and leaving it sitting dormant in the cell. As those cells divide, their copies keep the same active and inactive X chromosomes. XCI is a random process, meaning that which of the two chromosomes in each cell gets turned off is randomized, so it doesn’t generally affect how X-linked genes are expressed.

Recently, researchers have turned to XCI to investigate potential treatments for certain X-linked genetic disorders. They hope that the inactivated X chromosome may hold the key to creating a treatment that will improve the symptoms of women and girls with X-linked conditions. For a female with an X-linked disorder, one of her X chromosomes carries a faulty gene, and one carries a healthy gene. When XCI occurs in her early development, some of the cells retain the faulty chromosome as the active chromosome, and some the healthy one (the proportion of this may affect the severity of her condition). If scientists are able to reactivate the healthy gene on the inactive chromosome, they may be able to treat the condition by using genetic material already present in the cell.

Existing research on X chromosome reactivation (XCR), as this process is named, has been primarily focused on treating Rett syndrome. Rett Syndrome is an X-linked disorder (although it is almost always caused by a spontaneous gene mutation rather than inherited) that primarily affects females. Males born with the condition have the faulty gene on the X chromosome in every one of their cells and unfortunately often die within a few days of birth, whereas females have one chromosome with the healthy gene and one with the mutated gene in each cell. Due to XCI, some cells have an active X chromosome with the healthy gene, and some with the mutated gene.

Rett Syndrome is a neurodevelopmental disorder that causes loss of speech abilities, delayed growth, seizures, breathing difficulties, loss of controlled hand movement, and sleep issues, among other symptoms. The condition is caused by a mutation on the “MECP2” gene on the X chromosome, which is involved in controlling gene regulation in the central nervous system (brain and spine). While individuals with Rett syndrome can live well into adulthood, they require life long treatment and constant care, and there is currently no known cure and limited treatment options.

Researchers have explored several pathways for treating Rett Syndrome through X-chromosome reactivation therapy. They looked at the different players in the cell involved in XCI and whether blocking them might reverse the inactivation process. The main molecule involved in carrying out XCI is a long-stranded RNA called “Xist”. However, many micro-RNAs (mi-RNAs, short RNA molecules that help regulate gene expression) help facilitate the process. A recent study found that one critical micro-RNA was miR-106a. When miR-106a is absent, Xist is destabilized and dissociates, and the inactive X chromosome is reactivated. Researchers introduced a synthetic DNA molecule that acts as a “sponge”, attracting miR-106a molecules and blocking them from being available at the X chromosome. Xist then becomes unstable, temporarily reversing inactivation. This provides a window in which the healthy MECP2 gene can be transcribed, and MeCP2 protein can be produced. When researchers treated mice with Rett syndrome using this gene therapy, the mice showed reduced symptoms and had longer lifespans. While the overall implications of reactivating a second copy of the gene are still being investigated, the therapy did not seem to have significant adverse effects on the mice. Overall, the study indicated that XCR holds promising potential.

In addition to Rett syndrome, researchers hope that X-chromosome reactivation therapy could treat a variety of X-linked neurodevelopmental disorders, such as fragile X Syndrome and CDKL5 deficiency disorder. While these methods of gene therapy are still undergoing research and development, and need further testing before they can move into the clinical trials stage, X-chromosome reactivation therapy is a promising avenue for future treatment.