If you’ve ever entered “Williams Syndrome” into an internet search engine you’ve probably also received an answer that describes it as a complex condition caused by a microdeletion of a small number of genes from one copy of Chromosome 7, but what does that mean?
Williams Syndrome Genetics
The many possible symptoms of Williams Syndrome are described well in other articles on this site. My goal here is to share some understanding about what happens at a genetic level when someone has the Williams Syndrome deletion.
Three questions often arise:
1) Is it my fault?
2) Why does the deletion happen?
2) How can deleting, losing, just a few genes cause all the symptoms of Williams Syndrome?
If you’ll allow me just a few more minutes of your time I’ll do my best to explain the complex ideas behind the technical stuff.
The first two questions are actually easier to answer so we’ll start there. The third question will be answered in a related article on this site “Williams Syndrome Deletion – what do these genes do?”
To begin with the cells in each and every human being has a nucleus containing the chromosomes which hold the instructions on how to build a human being. You can imagine the nucleus is like a library containing bookshelves. Each chromosome is a different shelf containing different books in the series of books called “How to make a human being”.
These chromosomes are made of the chemical DNA and every cell in our body has 23 different chromosomes, but they come in pairs. So we have 23 pairs of chromosomes and within each pair, one comes from our Mom and one from our Dad. These chromosomes hold our genes, like individual books in this story of life. This means that for most of our genes we have two copies, not identical but very similar versions of the story of life. These tiny differences, together, are what makes each individual, well, an individual.
In someone with Williams Syndrome, at some stage, probably during the formation of the egg or sperm cell, maybe just after fertilization has occurred, a small piece of chromosome 7 was lost. A person with Williams Syndrome has lost just 25 of the more than 1000 genes from only one of their two copies of chromosome 7. This tiny difference, too small to be seen even down a microscope, is enough to cause what we know as Williams Syndrome.
Why does this deletion happen?
On one level, it’s a random event, it just happens to a very small number of cells as they grow and divide on a fairly rare and random basis. So the answer to question 1) is it really is no one’s fault.
One of the “normal” things about the way cells divide is that gene sequences on the chromosomes get copied – and sometimes the copying is less than perfect. Particularly during the formation of egg or sperm cells, sometimes chunks of genes get shuffled, it’s like trying to reorganise the books on the shelf by picking up a large block of books at the same time – easy to drop a few!
We do know something about why this particular set of genes is more likely to be affected by such shuffling and I explain this in more detail in another article and video – the simplified summary of how errors can happen is shown below:
Gene shuffling might sound strange but it can be a positive thing from an evolutionary point of view. It’s one of the things that helps to make each individual a unique human person.
If we go back to the idea of genes being like books arranged on a bookshelf, you can imagine that re-organising the books along a bookshelf can also make it easier to find the information or put related stuff together in the same section.
However, if you are trying to shuffle large groups of more than 10 or 20 books at a time, it wouldn’t be unusual for one or two to drop every now and then, get lost from the bookshelf and the dividing cell doesn’t really have a way to pick up dropped books (or deleted genes) from the floor.
The effects of these deletions are very different depending on which genes are involved. Many changes are fatal and the affected cells can never grow or mature.
Other changes (like those in Williams Syndrome) are not fatal, the cells can grow and, if these changes occur in the chromosomes of an egg or a sperm cell, the deletion is then copied into every cell as the new human being grows and develops.
Although each individual is potentially different, around 20 to 30 genes are usually affected in Williams Syndrome and their story is still being unravelled. Again, you’ll find more information in other articles on this website, so please do come and pay us a visit to find out more.
The hope is that, with better understanding will come the potential for treatment of the symptoms.
Do let me know if you found this article useful – and of course please ask any other questions you may have – I’ll see if I can find answers too!