>> So it would be really interesting to see in the future which regions
they are demethylating at those times.
So one can imagine that there are going to be regions of the genome that
are prone to demethylation, or that are being actively demethylated, and
regions that maybe are protected.
So maybe there are regions that are protected from the demethylases,
and they are retained in a stable fashion.
Perhaps like CPG islands on the inactive X chromosome.
And then there are regions where they are actively being demethylated
and the reasons as to why this occurs, we could postulate on lots of them.
I mean, it could be that you need to maintain demethylation
of most CPG islands.
We certainly find that most CPG islands aren't methylated,
so maybe there's some process that keeps them unmethylated by
removing any methyl marks that are added there.
>> Sure, well there certainly are processes that
control the DNA methylation state of particular regions of the genome.
We already spoke about the one cell embryo having the male pronucleus behaving
slightly different to the female pronucleus
and we know that this is because of an oocyte specific factor
called STELLA which comes from the maternal oocyte and
selectively binds the DNA of the maternal pronucleus
and then offers protection to the maternal genome from demethylation.
>> From that active demethylation.
>> From active DNA methylation, right?
>> So we also know the DMR's or the imprint control regions that are found,
they are protected from any demethylation, active or passive,
during that pre-implantation period,
so there are proteins, I believe that also protect those regions at that time.
>> Yeah, that's right, too.
So, STELLA is believed to do that actually.
To bind two DMR's in the paternal genome,
whereas it leaves the rest of the paternal genome unbound,
and to protect from DNA demethylation.
There's also a protein called zinc finger proteins 57
which will bind the methylated DMR of an imprinted region
and then it recruits other factors, including the denovo DNA methyltransferases
to make sure these imprinted regions stay methylated.
>> Stay methylated.
>> Through rounds of global DNA demethylation.
>> So perhaps, just like we know about these ones for these really critical
regions for the imprinted genes, maybe there are also proteins that bind to other
regions in the genome that you really need to keep faithfully methylated
and that's how we get the stability.
>> Sure, definitely,
and there are regions such as repeat regions that we know have to remain
silent through DNA methylation
and indeed they maintain their DNA methylation
so there's still a lot to learn there.
>> Yeah, I guess the interesting thing as well would be to think how does this
happen?
Why does it happen?
And so there's going to be a really active field in years to come.
It's already an active field and I imagine over the next few years lots of
the mechanisms that we're discussing will hopefully be revealed.
>> Sure that's what makes it still an exciting field I guess.
>> [LAUGH] Exactly, that's why we both work in it.
So I guess one of the interesting things that demethylation has also been studied
in, other than in the brain and in primordial germ cell development and
in early embryonic development, has also been in reprogramming.
So this is where we've discussed in the other lectures in week four that we have,
and week five I think, where we take a somatic cell and
it gets reprogrammed back to an induced pluripotent state.
And this is really important for therapy I guess.
People are really interested in doing this so
that you can create a pluripotent cell line from a particular patient
and then create differentiated cells, for example blood cells or new epithelial
cells of different kinds so that they can have them for transplantation.
So to be self transplantation with their very own cells, rather than having
the problems that you have with graft versus host disease, normally.
But, reprogramming hasn't just existed or
been known about since this discovery was made about ten years ago, in about 2006.
>> So, that's right.
The first reprogramming was first done in the late 1950s,
by a young scientist called John Gurdon
who had a fairly simple question by today's standards,
but really wanted to know whether a somatic cell contained all
the genetic information to produce a whole new organism.
>> So, by that you mean that he thought that there was a potential differentiation
as we know it might actually be a result of throwing genes out to feel that.
>> Exactly, regressing, loss of- >> So you're not having him any more.
>> Loss of genes, exactly.
And so his question was if the somatic nucleus contained all the genes required,
then it should form a new organism.
So what he did was to take a somatic nucleus from a frog and
put it into the oocyte of a frog, and see if it produced an embryo,
and sure enough it did, and that really nicely answered his question I guess.
That there really are all the genes required as we know now.
But yeah and I guess suggested for one of the first times that it was,
rather than a loss of the genes, it's a silencing of the genes.
So it was one of the first times we noticed epigenetic silencing in action.
But I guess what John Gurdon didn't know when he did these experiments was that
there had to be complete reprogramming of that somatic nucleus to completely
remodel the epigenome in the way that we understand reprogramming now.
>> So then a long time later,
50 years later,
then Yamanaka's group came along, and they used four transcription factors.
In fact, they screened for all sorts of different factors that might,
the combinations of different factors that might enable reprogramming of
a somatic cell back to something that looked like an embryonic stem cell,
which they called an induced pluripotent cell.
And they happened upon four particular transcription factors,
that are now called the Yamanaka factors.
And they got this reprogramming to work, but it's incredibly inefficient.
Maybe, less than 1% of cells will do this even if they all have
these four transcription factors.
So extremely inefficient.
>> That's true.
>> When you just add these four factors.
>> That's true, but we know it can be a lot better or made a lot more efficient
because the reprogramming by somatic cell nucleotransfer happens
with almost 100% efficiency, compared to 1% efficiency.
It also happens much faster, and the reprogramming that you get appears
to remodel the epigenome more faithfully than by the [INAUDIBLE] method.
So if you compare a somatic cell nuclear transfer reprogrammed cell,
it very closely resembles an ES cell and
in a more homogeneous way than what will a Yamanaka factor
reprogrammed cells.
>> Yeah. So that,
normal biology does it better than we do with.
[LAUGH]. >> Yeah.
>> This does not recapitulate no more situations.
>> Exactly.
Which doesn't surprise me at all.
>> Right. >> But
it means that there are more factors to find
and we can improve the, improve this process.
And there's a lot to learn from whatever is contained in that oocyte.
>> Absolutely. As an interesting note, because in
the early 2000s, there were groups and probably still are groups that wanted to,
for livestock purposes, they wanted to be able to reproduce their bull, if you like,
that had just the right genetics or
a particular deer that had just the right genetics for their livestock purposes.
And so before the Yamanaka factors and
actually still probably the best way to do it.
They used to use an oocyte and of course in livestock, it's okay to get more oocytes,
that's not really the case for humans, of course.
There are ethical issues and problems of getting enough of them.
>> That's right. >> But
that's not an issue with livestock so they, or not such an issue, I guess.
And what they would do to try and increase the efficiency because
while it can be close to 100% efficient I think this is organism specific so
I think some organisms are more efficient than others.