What if the female embryo inherits one good copy of a gene
and one bad copy that is non-functional and disease-causing? Some of her cells would express the good copy
of the gene and be fine and other cells would express the bad copy and be
messed up. The severity of the disease
for this female will depend on how many cells inactivated the good copy and
where these cells are located in the body.
Imagine that X inactivation occurred at the 4 cell stage, where two
cells inactivate the good chromosome and the other two cells inactivate the bad
chromosome. Once an X chromosome is
inactivated, it will stay that way in all the cells that are formed from that
original cell in the 4 cell stage (see the figure below). If each one of those 4 cells divides the same
amount to form the final adult form, then you would expect half of her cells to
be messed up and half of them to be fine.
But what if the two cells with the active bad chromosome happen to be
cells that will divide way more and make way more future tissues of the
body? Then in the adult form, she would
have tons of messed up cells and probably have a much more severe version of
the disease.
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| In females one X chromosome is inactivated early in development (image from www.scoop.it) |
As I mentioned earlier, X inactivation actually happens
later on in embryonic development when there are more cells and each one can
choose to inactivate one chromosome or the other. If we consider the disease scenario, this
random nature of X inactivation can lead to huge variability in X-linked
disease expression in females. It’s also
important to think about how certain types of cells and tissues develop. If an entire tissue develops from a single
cell after X inactivation, then all of the cells in that tissue will have the
same inactivated chromosome.
Researchers at John
Hopkins University
visualized X inactivation by marking expression from each X chromosome with a
different fluorescent protein. Wu et al
published their beautiful images in a recent article in the journal Neuron.
Marking X chromosomes
The authors created two types of mice, which each had an
extra inserted gene on the X chromosome.
One type had a gene that encodes a red fluorescent protein called
tdTomato. The other mice had a gene for
the green fluorescent protein, or GFP, which was originally discovered in
jellyfish. They then mated these two
mice together and used the female offspring that had one X with tdTomato (Xt)
and one X with GFP (XG). If
the “red” chromosome is inactivated, then only GFP will be expressed and this
cell will look green, as will all of its daughter cells. This way they can look at the heterogeneity
of X chromosome expression in different parts of the body.
The results
Overall, the mice came in all different amounts of red and
green. For instance one mouse might be
nearly all green while its sibling is all red, again indicating that X
inactivation is a random process. In the
mice that had both red and green, it was interesting to see the different
patterns in the body. For instance, in
the intestine, cells of the same color were found in columns. That’s because the cells in the column
originate from one single stem cell, so they should all contain the same active
X chromosome.
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| X inactivation appears in columns in intestinal tissue, because cells from a single stem cell migrate together |
Another interesting finding was that skeletal muscle cells
expressed both red and green fluorescent proteins. This would seem to indicate that there is no
X inactivation in muscle, but this is not the case. Skeletal muscles are actually formed by
muscle progenitor cells (myoblasts) that fuse together, creating cells with
multiple nuclei and copies of the genome.
If a cell with an active “green” X chromosome and a cell with an active “red”
chromosome fuse together, then the muscle will express both proteins. This only works for skeletal muscle; cardiac
muscle in the heart does not develop by cell fusion, so these muscle cells are
either red or green. This is a great demonstration
of the differences in muscle development.
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| Skeletal muscle cells express both X chromosomes, because they are formed via cell fusion |
They also noticed clear differences between the left and
right side of the body, like in the tongue, retinas and brain. This indicates that progenitor cells stay
segregated to either the left or right side during development. In other words, there is not a lot of
migration between the two sides of the body, where a cell on the right side
would make cells for the left side of the body, and vice versa.
The mosaic brain
The main focus of this paper is on the heterogeneity in the
nervous system. They looked at two
different cell types in the brain: excitatory pyramidal cells and inhibitory
interneurons. These two types of neurons
develop from different areas of the embryonic brain. They found that inhibitory interneurons were
highly mixed. When they quantified the
fraction of red inhibitory cells in two different parts of the brain, the
values were very similar. On the other
hand, when looking at excitatory neurons, there was a lot of variability of
which X chromosome was inactivated, across different parts of the brain and in
different animals. If there was an
X-linked gene that affected excitatory neuron function, then the effects on
neuronal circuits would be different for different regions of the brain in an
individual. The authors suggest that
this could actually be a good thing, because it would allow females with
different genetic variants to respond to a range of stimuli, increasing the
dynamic range.
