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What is a mitochondrial disease?

We had both heard the term ‘mitochondria’, at some point before - Beth had a memory of it being mentioned in a biology lesson, Ollie thought he had read about it in a Richard Dawkins book.

We now know more about it than we could imagine.


Mitochondria have many similarities with bacteria. They have their own set of genes, entirely separate from the genes that give us our appearance and nature. They can be seen as very useful passengers, which live inside us, performing vital tasks that keep us healthy.


What do mitochondria do?


Mitochondria are often described as ‘cellular power plants’ as (along with other tasks) they generate most of the energy required by their host cell. They are the batteries that make the cell work. They exist in every cell of every living organism. Some cells may only contain a single mitochondrium, others may contain many thousands. They reproduce through ‘binary fission’ where the cell splits into two identical clones, both of which are identical to the ‘parent’ cell. As a result, healthy mitochondria create more healthy mitochondria, mitochondria with mutations create more mitochondria with the same mutations.


When does a mutation start to have an effect?


Something that surprised us when we were learning about mitochondria is that the threshold at which mitochondrial mutations become detectable is quite high - Beth’s mutation level was at around 50%, but she competed for her county at swimming - the mutation had absolutely no effect on her fitness levels. Someone could have mutation levels at 50%, 60%, maybe even 70%, and be an olympic athlete. Or they could have 100% healthy mitochondria, and be the class wimp.

But there is a threshold after which it will have an impact. It varies from person to person, from one type of mutation to another. It is an inexact science, somewhere between 30% and maybe 80%.

Casper had mutation levels of 98%. The poor little boy never had a chance.


Click here to read a PDF which details Casper’s condition


Where do we get our mitochondria from?


The eggs from the mother contain thousands of mitochondria. The sperm from the father contains a far, far smaller amount. In addition, when the sperm enters the egg, its mitochondria are marked and destroyed, so only mitochondria from the mother is used in producing a child.

As the eggs of the mother are created when that mother is still a foetus in her own mother’s womb, incredibly this means that a grandmother is carrying the mitochondria of her future grandchildren when she is pregnant. This also means that the eggs within the baby female foetus are even more tiny, and have fewer mitochondria (which will later divide and multiply). So in one egg, a small number of mitochondria may carry a mutation which divide and double and eventually become thousands within that egg, and within the child being born from that egg. In another, more fortunate egg, there might be fewer mitochondria at the very beginning - and so no bad mitochondria in the child.


This is why it was so important for us to have ‘Preimplantation Genetic Diagnosis’ (PGD) - to find out at a very early stage what proportion of healthy Mitochondria had been passed on.




PRONUCLEUR TRANSFER - leaving a bad neighbourhood.


Most people, if they found themselves in a warzone in Somalia, would prefer to move to a beach on the Seychelles. Even more so, they would do anything they could to move their child from a place of danger to somewhere safe. At a microscopic level, that is what Pronuclear Transfer does for a family suffering from mitochondrial mutations.


With Pronuclear Transfer, the cell nucleus - the part that holds the parents’ DNA, the part which determines skin, hair colour, how tall you are likely to be or what shape your nose comes out as - is transferred from the egg where it is surrounded by mutated mitochondria, to a donor egg, with the donor egg nucleus removed, but with all the healthy mitochondria in place.



Advantages of Pronuclear Transfer


PGD gives us the opportunity to pick an embryo with the lowest proportion of mutated mitochondria - but what if all the embryos have a high proportion? What if the threshold is very unclear? Is it fair to the child, or to the parent, or to the society that would support them, to take that risk? In most medical practices, even a one in one hundred risk would be considered too high to take. But with the limitations to what we can know about mitochondrial mutations, the risk taken even with PGD is higher than that. With Pronuclear Transfer, the danger of inherited mitochondrial mutations is reduced to zero.


Even when a child is lucky, and the level of mitochondrial mutation is below the threshold where it takes effect, this might not be the same for the following generation. Beth was lucky, the mutation on her was below the threshold. Casper was unlucky.


As you can tell, we consider this to be a positive development and are actively supporting it.  Beth was on Radio 4, on the 11th March 2011, when this was discussed.  

Click here to download the interview (9Mb) or click here to read the follow up article.


Objections to Pronuclear Transfer


The ‘3 Parent Debate’


Some people feel this presents ethical issues. Won’t this mean that a child has 3 parents? (the father’s genes, the mother’s genes, and some from the donor egg). Does this make it a ‘designer baby?’ We of course, are highly biased in our perspective - but we also think the facts make a solid argument.


   * There are roughly 23,000 genes in the human genome (in the human body). 37 of these come mitochondria (or 0.16%). So any child would          be 49.92% from the father,  49.92% from the mother, and 0.16% from the donor.

   * These mitochondrial genes assist in the running of the body - as we said above, they have no impact on appearance, shape or anything        else that might deserve a ‘designer baby’. The design is simply between healthy, and not healthy.

   * Could this approach create a race of supermen? The simple answer is no. Healthy mitochondria perform the same in everybody - so        whether you are an Olympic athlete or a couch potato, neither will be better than the other. It would not be creating a ‘superhuman’. Just        someone who will live.


Could the cell nucleus be damaged during transfer?


A theoretical, possible danger to Pronuclear Transfer, is the danger that while the cell is transferred into the healthy egg, it could suffer damage - and this could harm the child born from this egg.


This is a theoretical objection, based on the ‘what if something bad could happen’ concern, to be placed alongside other than ‘you just don’t know’. As with all new scientific advances, it is sensible to be careful. Similar concerns were doubtlessly raised before the birth of Louise Brown, the first ‘test tube baby’. There are now over 3 million people in the world alive today, born through IVF who would not be here if these concerns were not overcome.


Against the concerns, there are some counter arguments:


   * Newcastle University are working on methods to test the procedure, and confirm that no damage is done. This will be provable before any        procedures are carried out.

   * This procedure has been successfully tested on animals. There are some very healthy chimpanzees out there who have been born through        this procedure.

   * If there is no procedure which enables both mother and father to pass on their genes, their appearance, their essence, to their children        safely, then they will do so unsafely - through conceiving naturally. And hoping for the best. It happens now. It will continue to happen, until        there is a way for the illness to be prevented. And today, children are being born, falling ill, requiring a lifetime of support and dying,        because this treatment is not available. Even with an (unproven) small risk of cell nucleus damage, this would be a lower risk than the risk        currently undertaken.


Why should society (the hard pressed NHS) pay through taxes for parents to have this treatment?  


Apart from the tragic human cost when this treatment is not available, the price of supporting a disabled child, (even the cost of supporting Casper in his short life) is far greater than the cost of this treatment.


We might not need this treatment ourselves. But as you can tell, we are pretty firmly in favour.