Angelman syndrome though is a completely different expression because it results from a microdeletion
on the maternal chromosome. Let’s look at the schematic again. Both parents have both chromosomes.
However, on the maternal chromosome, we see that the Angelman syndrome region, the red region is expressed.
However, on the paternal chromosome, that region is imprinted, meaning that it is turned off
and hypermethylated, so imprinting turned off. Now, if the maternal chromosome has a microdeletion
that spans that region of chromosome 15, then she does not have the Angelman syndrome genes
expressed or it does not have the Angelman gene syndrome expressed. Thus, the individual has no genes
expressed in that Angelman syndrome region of chromosome 15 and they develop Angelman syndrome.
Angelman syndrome is an image on the right side of the screen here of what an individual
looks like and here are a few more. Let’s take a look at some of the phenotypic features of Angelman.
Again, you’ll see they’re quite distinct from those that we see in Prader-Willi syndrome even though
the deletion is in the same region of the gene. Angelman is one of the researchers that identified this gene.
But prior to his time, this syndrome was known as the happy puppet syndrome. It might help you
remember the symptoms of Angelman because these children tend to smile and giggle a lot.
They’re ever so happy although fairly severely intellectually disabled. They also have pretty jerky movements
as well as short stature and they often exhibit seizures. They don’t talk a whole lot. Often, there are
developmental delays. The intellectual disabilities are much more severe than you see in Prader-Willi.
Another feature is that they often have fairly widely-spaced teeth. But in general for me in remembering it,
we have the happy puppet as Angelman and then a serious need for food in Prader-Willi syndrome.
Those would be the overall big features to remember to keep these syndromes in mind.
Now, Prader-Willi and Angelman syndrome are mostly derived from these deletions in the Q arm
of chromosome 15. Specifically, you can see the megabase numbers but that’s not really something
so important for you to remember. I’m going to introduce a whole another concept here.
Some of the rest of these conditions come from a uniparental disomy. If you have, in this case,
two maternal chromosomes that both have the Prader-Willi region hypermethylated,
then they would have no Prader-Willi gene product and thus develop Prader-Willi syndrome.
Less of the time, we see Angelman syndrome derived from a paternal uniparental disomy.
We’ll look at how this happens in just a moment. Because both of those chromosomes
are hypermethylated in the Angelman syndrome region, they would develop Angelman syndrome.
So what is uniparental disomy and how do we even get there? It sounds like a pretty strange term.
Let’s take a look at meiosis again. Here, we have two chromosomes. We’re going to go through
their homologous pair. Let’s say we have nondisjunction during meiosis I. Then the next division,
meiosis II, we see that each resulting oocyte is disomic, meaning it’s got both copies. In this case,
we’ll go with the chromosome 15. The sperm comes along and fertilizes the egg. Now, it’s triploid.
But one thing that can happen that I mentioned in a previous lecture is you can have a trisomy rescue
where the resulting zygote recognizes that it has three copies of this chromosome and kicks one out.
So now, we have a zygote that has two chromosomes but both of them are from maternal origin
in this situation. This can happen in another way. Here, it’s called heterodisomy where we have them
both from one parent. Now, let’s look at isodisomy. Let’s say meiosis I happens fine but meiosis II
doesn’t happen quite the way it should. We have nondisjunction of the sister chromatids resulting
in a disomic ovum which then could be fertilized by a sperm that either doesn’t have a chromosome
because it had nondisjunction of its own or it could have a chromosome that is also rejected
in trisomy rescue. Either way, we have two chromosomes that are from the same egg in this situation,
right and so isodisomy. Heterodisomy, we have two different chromosomes. I think I misspoke.
Heterodisomy, we have two different chromosomes from the same parent. Isodisomy, we have two
of exactly the same chromosome, so sister chromatids, exact copies of each other, isodisomy.
So disomy, two chromosomes, one parent; iso, both of them are exactly the same.
So again, although this is uncommon, I bring this up so that you can understand the concept
of uniparental disomy and how it might manifest itself in certain genetic conditions.
As I bring this lecture to a close, I look forward to seeing you in the next lecture
where we have some really exciting conditions looking at these sex chromosomes.