How to Keep an X Silent
CNRS researchers might have discovered the molecular event that triggers the chain reaction responsible for X inactivation–the silencing of one of the two X chromosomes during early development in mammalian female embryos.
In mammals, the X and Y chromosomes are responsible for differences between the sexes; males are XY and females are XX. Males have only one X chromosome, whereas females have two. One might therefore expect that females would have a double dose of gene products from the two X chromosomes. However, this is not the case. During early development, all cells in female embryos silence one of their two X chromosomes, chosen randomly, a process known as X inactivation. “This X-chromosome silencing usually doesn’t occur in males, but if it isn’t achieved in females, the embryos die very early on,” says CNRS researcher Edith Heard, 1
whose team at the Curie Institute in Paris recently identified molecular events underlying the initiation of this process. 2
Starting in the early 60s, researchers working on mice and humans gradually narrowed down the site of the X chromosome responsible for triggering inactivation, the X-inactivation center (Xic). In the early 90s, a gene termed Xist (X-inactive-specific-transcript) was identified within the Xic. Scientists showed that the Xist gene on one of the two X chromosomes becomes highly expressed very early during cell differentiation, producing a non-coding RNA that eventually coats the entire X chromosome and silences it.
“That was as much as we knew, but we still did not understand the nature of events that happened upstream of this up-regulation,” says Heard. “How does the cell know how many X chromosomes it has? How does it know that if it has more than one X, it must trigger inactivation?” In the recently published study, her team reports identifying a new region within the Xic–the “X-pairing region” (Xpr)– which brings the Xics of the two chromosomes together prior to the silencing process. “We think that this might be the critical step that tells the cell it has two X chromosomes,” says Heard.
Using 3D microscopy analysis on mouse embryonic stem (ES) cells, the team was able to show that the two Xpr regions come into close proximity within the nucleus just prior to Xist up-regulation. When introducing single copy Xpr transgenes in ES cells in different parts of the genome, they paired up with the endogenous Xic. Strikingly, when the Xpr region was introduced into male ES cells, the team observed frequent cell death, suggesting that having this extra copy of the Xpr triggered inactivation of the sole X in males.
In mice, ES cells provide a good model for studying X inactivation. In humans, XX ES cells are less well characterized but the authors hypothesize that this Xpr-mediated sensing mechanism is likely to be conserved. “The Xpr region contains sequences that are highly conserved between mammals,” notes Heard.
The authors speculate that this trans-chromosomal interaction mechanism could also explain the monoallelic expression of some genes located on non-sexual chromosomes–i.e., the fact that only one of the two copies of a gene is expressed. A recent study revealed that such genes represent approximately 10-15% of the human genome. Progress in understanding details of X inactivation could thus provide insight into the general regulation of monoallelic gene expression, which could have important implications for disease. Potentially harmful mutations may have more chances of being expressed if 10-15% of the genome is monoallelically expressed.
1. Dynamique nucléaire et plasticité du génome (CNRS / Institut Curie).
2. S. Augui et al., “Sensing X chromosome pairs before X inactivation via a novel X-pairing region of the Xic,” Science, 2007. 318: 1632-6.