In this lecture, we’ll move on to look at structural abnormalities of the autosomes. To begin, let’s look at how
some of those abnormalities happen. We should be pretty familiar with the fact that there could be
duplications of regions of the chromosome as well as deletions of regions of the chromosome.
We have also seen that this could happen through translocations, exchange of pieces between chromosomes.
Now, let’s introduce some new terminology. How do we express? We’ve talked about the Goldilocks principle
with a large bowl, a small bowl, and just the right amount. Overexpression is when we have too much.
That’s clear. Then, there’s another term, haploinsufficiency. It means having half of the genetic information
that one should. Although this can happen in whole chromosomal deletions or additions, aneuploidies
and polyploidies, we can also see overexpression or haploinsufficiency for genes encoded
within the deleted or duplicated or translocated sections. Let’s look at first, a deletion situation.
Cri-du-chat is a condition that you’ll be required to know. Cri-du-chat gets its name because people that are
are suffering from it have a specific cat-like cry. Obviously, that’s how it got its name. It results from
a deletion in the short arm p region of chromosome 5. So, we call that 5p. You can see the megabase numbers
indicated there. You don’t need to know those specifically. The break of the chromosome though
can be highly variable. That means that the expression of this disorder is highly variable. The break could involve
an interstitial, so a region in the middle of that short arm or it could be a terminal deletion, a short
terminal deletion or a longer terminal deletion. You can see exhibited in these sections in red over here
that mental retardation is a large component of cri-du-chat. But if the break is more terminal,
then there will be less mental retardation. That is a feature that’s highly variable in the expression of cri-du-chat.
The symptoms, of course, we are missing a portion so we have half as much gene product
are due to haploinsufficiency. One of the interesting things about cri-du-chat is that it is due not necessarily to
a hereditary condition. Indeed, it’s a genetic disorder but it arises de novo 90% of the time.
So, it’s the first incidence that is seen of it. The incidence is fairly infrequent relative to the trisomies
that we’ve looked at previously. Here are some of the symptoms. We have hypertelorism,
hyper meaning over or larger space between the eyes or a large bridge of the nose. Another feature is epicanthus.
In this picture, you can see the epicanthal folds are marked, which is the normal epicanthal fold
that we see in some individuals, not all individuals. But here we’re talking about epicanthus,
which is the vertical fold that you see on either side of the bridge of the nose. You can see it in each of these images.
So epicanthus, the condition is having this vertical fold on either side of the bridge of the nose, so an extra fold.
Another significant feature is retrognathia which is a short, a smaller, and receding chin.
Cri-du-chat, here’s a sequence of images of development of cri-du-chat, pretty classical representation
from a four year old, nine year old, and twelve year old, the same individual traced through life.
So, you can see the general features of this syndrome.