Professor Edith Heard
Kat - Now itís time to delve back into the world of sex - specifically, sex chromosomes. In mammals, females have two X chromosomes, while males have an X and a Y. But this double dosage of the X chromosome can cause major problems for the ladies, as Professor Edith Heard, from the Curie Institute in Paris, explained to me.
Edith - The X chromosome is actually fairly large. It actually carries about a thousand genes. So, itís one of the bigger chromosomes at least in humans and mice. The fact that females have two of these and males only have one means that actually, it could cause serious problems in terms of the amount of protein products produced from the female X versus the male X. So, this is an imbalance that is actually intolerable during development and we know that a female that doesnít manage to deal with this imbalance, to have this double dose of the X chromosomes, actually dies very quickly.
Kat - How do females cope with this? What do they do to even out the amount of stuff that they're getting from their X chromosomes?
Edith - In mammals, the way this happens is that one of the two Xs is actually shut down. So, the chromosome stays there, there are two Xs, the DNA is there, but one of the two chromosomes actually becomes transcriptionally silent. So, that means its genes are no longer expressed. The genes are still physically there, but they just are no longer read. They donít produce RNA and they donít produce protein. So basically, you go from a double dose in terms of DNA, but only a single dose in terms of the RNA and the protein thatís produced from the female X chromosome or from the female double X chromosome. So, female cells produce the same amount of X-linked RNA and protein as male cells do, and thatís how the balance is achieved.
Kat - So theyíve basically just switched one of them completely off.
Edith - Almost completely. It turns out that there is actually one small part of the X chromosome that is identical to the Y chromosome and itís called the pseudo-autosomal region. So, that little bit of the chromosome actually stays on. We donít know exactly how it manages to avoid this shutting down, but it stays on and itís present in a double dose, but thatís okay because males actually have a double dose as well because they have it on their Y and on their X. And there are also actually a few other genes on the X chromosome that aren't in this pseudo autosomal region that seem to escape from this process of shutting down, this X chromosome inactivation process. And again, weíre not quite sure why, but we think that for some of them, it could be important that they escape and it might be something that is female specific or maybe XX specific. You need to have a little bit more of some products from the X chromosome in females. But overall, of those 1,000 genes, the overwhelming majority are actually shutdown and this happens very early on in development. Overall, itís actually fairly stable and in somatic cells, once the inactive X is shutdown, its state is very, very stably propagated.
Kat - Explain to me a little bit about how this process actually happens. How do you go from having two X chromosomes in a developing embryo at this very early stage to an adult female, all her cells just have one active X chromosome? What happens? What's the journey?
Edith - So, the journey is complex and we really donít understand it very well, but one of the mammals that weíve looked at most or we in general have looked at is the mouse. We think that the way it works in the mouse at least and probably also in humans is that there's a trigger to this process that is this RNA thatís called the X-inactive specific transcript (XIST). This RNA is only expressed from the X chromosome that will become shut down. So, this RNA is switched on early on in development and somehow in a female cell, this RNA or the gene that produces it knows that only one copy needs to be switched on. So, it produces this RNA and itís an amazing RNA because it actually coats the whole chromosome. It sort of smothers it and this coating of the chromosome is actually what leads to gene silencing and we donít know how that works. At the molecular level, we actually have no clue how this XIST RNA does its job. But once itís done its job, then itís no longer important because there are other changes that happen at the level of what's called chromatin. So, the DNA of the chromosome thatís going to get shut down is wrapped up in proteins and chromatin, the way the rest of the genome is actually. But the chromatin of the X thatís going to get shut down starts to change.
It starts to take on new modifications, new flavours we call them, so new proteins associate with it, and we think that these changes actually are what lock in the silent state. So, XIST RNA triggers the process and then these chromatin changes help to keep the silent state silent all through cell division. So, as the cells divide, the X that was shutdown will always be the one that get shutdown. It won't suddenly wake up and remember it should have been active. It actually stays inactive, thanks to these different changes that are mostly at the chromatin level and this is what we call epigenetics. These epigenetic changes are changes that actually allow a stable silencing of a state. In the case of the X, itís almost the whole X. And once this stable silencing has set up, itís the epigenetic marks that allow its propagation through cell divisions.
Kat - This is quite unusual because most RNAs you think, well itís a gene, it becomes active and makes an RNA that goes off and tells the cells to make a protein. So, this is quite weird. Are there any other RNAs that are like this X-inactivation transcript?
Edith - It used to be quite weird. It was discovered over 20 years ago, this XIST RNA. And somehow, it only accumulates over the chromosome that itís produced from and thatís a big mystery as well. How does it stay associated in cis as we say, with the chromosome that produces it? So, it did seem to be a very weird kind of RNA, but actually, in the last few years, there's been a whole flurry of discoveries of RNAs that seem to be like XIST. The problem is, we donít actually understand how XIST works and we know even less I think about how these other non-coding RNAs work. And although we can try to make parallels, itís not clear actually whether any two non-coding RNAs or non-protein coding RNAs are doing the same thing or behaving in the same way. So, I think this is a very recent and emerging field of non-coding RNA biology that lots and lots of people are now working on.
But as I said, XIST was discovered over 20 years ago and we still have no clue exactly how it does what it does, and I think thatís going to be the situation for many of these non-coding RNAs. So, I think itís very exciting field and lots of people are talking about non-coding RNAs now, but actually, I'm still not sure what they're all doing and I especially donít know what XIST is doing even though I've worked on it for so long.
Kat - That was Professor Edith Heard from the Curie Institute in Paris.