The Epigenetics Revolution

That’s a very anthropomorphic way of describing the event, but it’s a pretty good description. The ‘kiss’ only lasts a couple of hours or so, and it’s startling to think this sets a pattern that can persist in cells for the next hundred years, if a woman lives that long. This chromosomal smooch was first shown in 1996 by Jeannie Lee, who started out as a post-doctoral researcher in Rudi Jaenisch’s lab, but who is now a professor in her own right at Harvard Medical School, where she was one of the youngest professors ever appointed. She showed that essentially the two copies of the X find each other and make physical contact. This physical contact is only over a really small fraction of the whole chromosome, but it’s essential for triggering inactivation[110]. If it doesn’t happen, then the X chromosome assumes it is all alone in the cell, Xist never gets switched on, and there is no X inactivation. This is a key stage in chromosome counting.

It was Jeannie Lee’s lab that also identified one of the critical genes that controls Xist expression[111]. DNA is double-stranded, with the bases in the middle holding the strands together. Although we often envisage it as looking like a railway track, it might be better to think of it as two cable cars, running in opposite directions. If we use this metaphor, then the X Inactivation Centre looks a bit like Figure 9.4.

^{Figure 9.4 The two strands of DNA at a specific location on the X chromosome can each be copied to create mRNA molecules. The two backbones are copied in opposite directions to each other, allowing the same region of the X chromosome to produce Xist RNA or Tsix RNA.}

There is another non-coding RNA, about 40kb in length, in the same stretch of DNA as Xist. It overlaps with Xist but is on the opposite strand of the DNA molecule. It is transcribed into RNA in the opposite direction to Xist and is referred to as an antisense transcript. Its name is Tsix. The eagle-eyed reader will notice that Tsix is Xist backwards, which has an unexpectedly elegant logic to it.

This overlap in location between Tsix and Xist is really significant in terms of how they interact, but it makes it exceedingly tricky to perform conclusive experiments. That’s because it’s very difficult to mutate one of the genes without mutating its partner on the opposite strand, a sort of collateral damage. Despite this, considerable strides have been made in understanding how Tsix influences Xist.

If an X chromosome expresses Tsix, this prevents Xist expression from the same chromosome. Oddly enough, it may be the simple action of transcribing Tsix that prevents the Xist expression, rather than the Tsix ncRNA itself. This is analogous to a mortice lock. If I lock a mortice from the inside of my house and leave the key in the lock, my partner can’t unlock the door from the outside of the house. I don’t need to keep locking the door, just having the key in there is enough to stop the action of someone on the other side. So, when Tsix is switched on, Xist is switched off and the X chromosome is active.

This is the situation in ES cells, where both X chromosomes are active. Once the ES cells begin to differentiate, one of the pair stops expressing Tsix. This allows expression of Xist from that X chromosome, which drives X inactivation.

Tsix alone is probably not enough to keep Xist repressed. In ES cells, the proteins Oct4, Sox2 and Nanog bind to the first intron of Xist and suppress its expression[112]. Oct4 and Sox2 were two of the four factors used by Shinya Yamanaka when he reprogrammed somatic cells to the pluripotent iPS cell type. Subsequent experiments showed that Nanog (named after the mythical Celtic land of everlasting youth) can also work as a reprogramming factor. Oct4, Sox2 and Nanog are highly expressed in undifferentiated cells like ES cells, but their levels fall as cells start to differentiate. When this happens in differentiating female ES cells, Oct4, Sox2 and Nanog stop binding to the Xist intron. This removes some of the barriers to Xist expression. Conversely, when female somatic cells are reprogrammed using the Yamanaka approach, the inactive X chromosome is reactivated[113]. The only other time the inactive X is reactivated is during the formation of primordial germ cells in development, which is why the zygote starts out with two active X chromosomes.