A UCLA STUDY HAS SHED NEW LIGHT ON THE PROCESS by which an RNA molecule known as Xist plays a role in X chromosome inactivation during embryonic development. In mammalian development, every cell in the early female embryo shuts down one of its two copies of the X chromosome while leaving the other functional. For years, the mechanics behind this inactivation have been murky, but scientists from the Eli and Edythe Broad Center of Regenerat ive Medicine and Stem Cel l Research at UCLA have taken a major step forward in understanding the process. Their findings, based on research on mouse stem cells, upend previous assumptions about how X inactivation is initiated in female embryos and could lead to new ways to treat some genetic disorders. “X inactivation is one of the most fundamentally impor tant processes in development, and I think this study is a slam dunk in finally understanding it,” says Kathrin Plath, PhD, professor of biological chemistry.
Scientists have known for nearly three decades that, early in embryonic development , an RNA molecule known as Xist is required for X chromosome inactivation in order to prevent female cells from receiving a double dose of X-related proteins. In the absence of clear evidence, most in the field have assumed that many copies of Xist coat the targeted X chromosome or constantly move around between locations on the X to induce the silencing of more than 1,000 genes.
In the new study, Dr. Plath and her colleagues tagged individual molecules with fluorescence and used superresolut ion microscopy to watch the movements of Xist and interacting proteins as X chromosomes were being inactivated in the embryonic stem cells of female mice. They discovered that pairs of Xist were located at just 50 spots along the chromosome, for a total of 100 molecules of Xist.
“It was kind of shocking to us that from just 50 sites, Xist manages to silence a thousand genes,” says associate project scientist Yolanda Markaki, PhD. Instead of interacting directly with every gene on the chromosome, the Xist pairs act as protein magnets, recruiting thousands of proteins to their spots on the chromosome. The chromosome is then pulled into a tightly condensed shape so that every section is in the vicinity of one of these 50 large clouds of proteins. From there, genesilencing proteins within these complexes bind to each gene, shutting it off.
“The key insight here is that Xist RNA is not acting directly on the X chromosome, but is more of an architectural molecule that sets up proteins to do their job,” Dr. Plath says. “Now we know that to silence an entire chromosome, you only need 100 Xist molecules, so it’s easy to see how a few molecules are sufficient to set up little compartments of gene regulation.”
The observations also could point to new ways of treating diseases, she says. For example, the reactivation of the silenced X may serve as a strategy to treat diseases a s soci ated with the X chromosome in females, such as Rett syndrome.
— Sarah C.P. Williams
“Xist Nucleates Local Protein Gradients to Propagate Silencing across the X Chromosome,” Cell, November 4, 2021