How to edit your mitochondria
'the powerhouse of the cell'
Anna Broughton | 8 November 2015

Researchers have recently devised a new technique to cut out parts of your mitochondrial DNA which may cause harmful diseases.

Mitochondria are tiny structures (more scientifically named organelles) within cells which are responsible for releasing the energy needed for essential functions and reactions which support life. Thus, they are also known as “the powerhouse of the cell”. As mitochondria are so small many compact into each cell; in fact, there are approximately 300-400 in every cell of your body! However, if they fail, this can lead to cell damage, and in extreme cases death.

Scientists believe mitochondria evolved from free standing bacteria (those are the microbes present in your yoghurt) about 2 million years ago, and indeed they do have a similar appearance. Moreover, they have their own DNA (frequently shortened to mtDNA)  which differs from the more commonly known nuclear DNA as we only inherit mtDNA from our mothers, instead of our fathers as well. In addition, unlike nuclear DNA, each mitochondrion contains 5 to 10 copies of its genes (of which there are only 2 in the nucleus). Whilst this DNA does not code for characteristics, such as hair colour, if it is flawed, it can cause diseases which can be inherited.

Even though these mitochondrial diseases are rare (only affecting approximately 1/4000 people), they can be very difficult to live with. This is due to the patients suffering a lack of cellular energy which in turn causes symptoms such as stunted growth, muscle weakness and blindness. What is more, in some cases it can cause more serious conditions including diabetes, heart disease and seizures! Luckily the symptoms of these diseases can be improved by various treatment methods; for example, vitamin tablets can be taken. In addition, a few changes to patient lifestyle, in order to conserve more energy (by avoiding vigorous exercise and exposure to infections), can lessen these symptoms. However, even using these treatment techniques symptoms can persist and women can pass the diseases on to offspring. Hence, a new way of removing the disease causing mtDNA was needed.

Scientists at Salk Institute have used specially engineered molecular scissors to remove harmful mutations (these are random changes in the DNA code which result in a damaging characteristic being coded for) in mouse embryos so that only the healthy DNA stays.

Fortunately, it was successful as it resulted in healthy offspring. Since, it has led to more successful experiments involving faulty human mtDNA inserted into mouse eggs. These mouse egg cells each contained two different types of mitochondria which had different DNA (this is known as heteroplasmy). The scientists then injected some genetic instructions into each egg cell which cut the mtDNA at a specific point in just one of the mitochondria types, causing just these mitochondria to be destroyed. This resulted in there being more of one of the types of mitochondria in the egg cell than the other. (A technique called TALENs.) As most women who possess faulty mitochondria carry a mixture of both healthy and harmful mitochondria, this TALENs technique increases the number of healthy mitochondria and hence decreases the number of harmful mutant mitochondria in their egg cells, reducing the risk of the damaging mtDNA being passed on to their offspring.

This is the first attempt at editing genes in mitochondria in mammals’ cells and, if this procedure is proven safe, many claim it will provide a better, simpler alternative for women with harmful mtDNA who wish to have children than the currently used process of mitochondrial transfer. However, many argue that this new process of editing mtDNA may cause damage to genes that we need in the future. Furthermore, some have claimed that it could potentially result in harmful long term side effects - of which we are as of yet unaware - because many mitochondria are lost in the process. Only time will tell.

The process of mitochondrial transfer used today is an IVF procedure involving three parents. They consist of the mother (with harmful mtDNA), the father and a female donor with healthy mitochondria. This was recently legalized by the UK parliament in February 2015 and involves transferring healthy mitochondria from the female donor into an egg cell of the mother so that the child does not inherit any mitochondrial diseases. This could potentially help 2,400 women in the UK, so that they can give birth to their own children without worrying about them inheriting any mitochondrial diseases. However, this has caused much controversy as it means the child inherits DNA from three parents. In addition, the child may be at risk of unknown genetic risks in the long term (Some scientists are speculating that the offspring may be more susceptible to Alzheimer’s due to a lack of a protein, humanin).


So, in conclusion, my thoughts are in favour of the TALENS technique as it will allow women who suffer from mitochondria-related diseases to give birth to children with healthy mitochondria who will consequently live a longer life of higher quality.

 

Image used under Creative Commons license

http://api.ning.com/files/WVOgQi8nSORDqStxxrtBkp*jju1xAahRDtRZs*pIcT5EXonfadjIVMTjKB*lBUuK6htJeRTvcNasQ3l5TgP9voOjgAp*rOx0/mitochondriaandsmoothendoplasmicreticulumtw2012.png

James Routledge 2016