Biologically female mammals have two X chromosomes, but what might surprise you is that one of those X's is turned off.
We first discovered there was something funny going on with X-chromosomes in 1948.
The first person to recognize something different about female animal cells was Murray Barr.
In 1948, he discovered in cats that female cells have a 'nucleolar satellite' which appeared as a black dot in the nucleus.
This was odd because a similar structure could not be found in males.
It was hypothesized that this 'satellite' material was related to sex chromosomes and it later became known as the 'Barr Body.'
Over the span of 11 years, the Barr Body was used to easily discriminate the biological gender of cells.
But in 1959, Susumu Ohno showed that the Barr body was actually a condensed X chromosome!
Two years later, Lyonization, or X-inactivation, was first proposed by Mary Lyon after she observed a correlation between which X ended up being condensed and the coat color of female mice.
Ok, that’s great, but why’s it important for one of the X chromosomes to be turned off?
Dosage.
This is a fundamental concept in genetics and gene duplications or deletions can increase or decrease how many copies of each gene are in the genome.
Issues with dosage are most apparent in diseases like Down Syndrome where individuals have an extra copy of chromosome 21.
This imbalance results in the production of more protein than is required for normal cellular function and also is responsible for the observed phenotypes.
So, X-inactivation ensures that just the right amount of protein is produced from the code written in the X chromosomes!
While genetics is pretty good at correlating genetic changes with broader phenotypes, it can’t prove a direct relationship.
And so the molecular sleuthing began in 1991 with Huntington Willard and his team who located the 'X inactivation center' and discovered a long non-coding RNA (lncRNA) that seemed to encapsulate and condense the inactived X chromosome.
They named this lncRNA the X-inactive specific transcript or, XIST, for short.
But there was still a nagging question about whether this actually turned off gene expression.
A good chunk of that answer came as a result of the work of Jeannie Lee, who, in 1996, was a postdoc in Rudolf Jaenisch’s lab.
Lee copied the XIST sequence into a yeast artificial chromosome (YAC) that expressed a common ‘reporter’ gene, LacZ.
When introduced into mice, this YAC (T) and the mouse X chromosome (X) both expressed XIST RNA (red cloud).
But the proof that XIST represses gene expression came when looking at LacZ.
The gel shows that LacZ is expressed in embryonic stem cells (Lanes 1 and 4) but not in differentiated fibroblasts (Lanes 2,3,5 and 6)!
This work represents the first direct evidence of a functional role for XIST and its broader involvement in repressing gene expression from the inactived X chromosome.
Brian Krueger