Different mutations and their effects

In my last post I looked at a common mutation that causes deafness: the c.35DelG mutation. This type of mutation, where one of the DNA letters just goes missing, is called a 'frameshift deletion' mutation. Another one, 235DelC, is quite common among Asians and has pretty much the same effect.

Let's pretend that, instead of being a majestic molecule that encodes the very essence of life itself, DNA is a drab children's book. It still is only read as 3 letter words (remember codons). And it has the following line:

Pat and Ann ate ham and ran off

A frameshift mutation might delete a letter and give the following:

Pat and Ann ate ama ndr ano ffa

The sentence is now meaningless, just like the protein that such a mutation creates. But instead of deleting a letter, you could swap it with a different one:

Pat and Ann ate Pam and ran off

You know have a sentence with very different, and sinister, meaning. What are we subjecting our poor kids to? It's function has changed, and this is what happens with DNA - these mutations create proteins that do something, just not what they are meant to do.

Swapping one letter of DNA for another is called a missense mutation. When this happens with Connexin 26, you can get other problems as well as deafness. This is called Syndromic hearing loss.

The picture below shows the Connexin 26 molecule. Each of the coloured circles with a letter represents an amino acid. Scientists have noticed mutations that change a lot of different parts of the molecule. The ones that change the blue circles just cause deafness. But at the yellow parts, the mutations cause other problems too - mostly skin disorders, because that's where Cx26 is most active. And these tend to be missense mutations.

The skin problems can be severe, and can sometimes lead to loss of blood flow to fingers and toes, causing them to drop off, or to blindness.

But if these Cx26 mutations have such a big effect on skin, why does not having Cx26 leave your skin perfectly fine?

Scientists aren't sure, but they suspect it is because of 'gain of function'. The missense mutations mean that the Connexin 26 does things it isn't supposed to do. In this case it is likely it is letting more chemicals pass between cells, which might cause to much skin to grow (hyperkeratosis).

If there are no working Cx26, the skin doesn't seem to have any problems. They reckon this is because there are other Connexin proteins (like Cx30 or Cx43) that do more or less the same thing - you have redundancy there that isn't in the ear for some reason. It is possible that the missense Cx26 molecules also interfere with these other guys and stop them working properly. In genetics this is called Trans-dominance, where a mutated protein prevents healthy proteins from doing their job.

I wrote before about how Connexin molecules bunch together in groups of 6 to form the channels that connect cells. These aren't always the same Connexin molecules - Cx26 might combine with Cx30 or Cx43. These would give different channels, that let different molecules pass through. It might be that a mutated Cx26 is too eager to join up with these other Cx molecules. All the Cx43 and Cx30 proteins join up with the broken Cx26, instead of making their own channels that the skin needs.

So in this case, having a slightly-changed version of Cx26 can be worse than having none at all.

Most of this comes from a paper by Jack Lee & Thomas White, you can read it here if you fancy.
And if you want to know more about different mutation types here is a good guide.

Gap Junctions

The first thing to figure out is what healthy Cx26 actually does. Connexin 26 is a type of long molecule called a protein. It has another name: Gap Junction Beta 2. The mutation that causes my son's deafness is on a part of the DNA that makes this protein, which is why it is called the GJB2 gene.

So what is a gap junction?

Cells are basically bags of stuff - DNA, RNA, mitochondria, all sorts of sciency sounding stuff inside that make life as we know it happen. They are kind of like tea bags - water can pass through the outside of the cell (the cell membrane), and also oxygen and CO2, but not much else.

Stuff needs to get in and out of cells for life to happen. They need chemicals to do what they need to do - otherwise why would we eat? They also need to communicate to each other, and this is typically done through electrical signals; by passing ions (electrically charged atoms or molecules) between each other.

This is where gap junctions come in. Gap junctions are like tunnels connecting cells, allowing materials to pass between them.

Here's a picture robbed straight from Wikipedia:

 The blue plates are the edge of two cells, and the gap junctions are the yellow structures linking them together. From the top down, they look like a flower, with 6 petals. And each one of those petals is a Connexin protein.

A bit more about how the gap junctions work. There are many types of Connexin proteins, 21 in humans, and the gap junction can be made up of all the same type or different ones.

Half of a gap junction is called a connexon - note the second 'o'. When two cells are in contact, a connexon forms at the edge of each cell, and they connect to form a junction. The two connexons linking up don't have to be the same.

So from different combinations of Connexin proteins and connexons you can get different junctions, and these will let different materials through. From what I've read, the gap junctions made with Connexin 26 transport Potassium ions as well as some (unspecified) small molecules. This would suggest to me that these gap junctions are mostly used to send electrical signals between cells.

This fits in with my basic understanding of hearing: hair cells being wiggled by sound waves and sending a signal via nerve cells to the brain. If Connexin 26 is critical in sending these electrical signals, than I can imagine how not having it might cause deafness. But GJB2 mutations can cause other problems, particularly relating to skin growth, so maybe I'm off the mark.

One thing I would like to find out; how come GJB2 mutations don't necessarily cause profound deafness? They must be fairly critical for hearing, so why is there any hearing at all if they are not around? I suppose I can't answer that until I know their role.

The Question Remains

My son was discovered to be deaf at the young age of 5 weeks. A few months back, genetic testing showed that this was due to a genetic mutation - specifically the gene GJB2, which creates the Connexin 26 protein.

At the time, I had never heard of GJB2, or Connexin. The test results contained a lot of terminology and codes which I still haven't decrypted. The Dr. dealing with us, while friendly and supportive, didn't seem too knowledgeable either. Any question we asked lead to her immediately consulting google.

So this is what I did for the next few days. There was a lot of information out there, but it fell into two categories. The first was mostly articles from scientific journals, unreadable in a similar manner to the genetics results letter I received. The second category was the kind of overly simplistic stuff seen in hospital leaflets. It scratched the surface in terms of an explanation, but left me with more questions than answers. I wanted Goldilocks information.

My big question, and one I couldn't easily find a clear answer to is the following: What role does Connexin play in the hearing process, and how does the GJB2 mutation cause hearing loss?

I could find partial answers to this, but each one lead to more and more questions. So I started digging further, and am now stuck in a rabbit hole of genetics, cell molecular biology, and structural anatomy, none of which I really understand. But I m slowly trying to get to grips with it.

My aim is to update this blog weekly until I fully understand every aspect of the relationship between GJB2, Connexin 26 and deafness. Which will presumably be for a very long time.

My homework for next week:  GJB2 = Gap junction beta 2. So what the feck is a gap junction?

[p.s. if you are in the middle of doing your own research into this I found this link to be a good starting point]