Background: What are mitochondria?

Mitochondria are small structures that are present in all the cells of the human body in multiple copies. Mitochondria are found in the cytoplasm, a gel-like substance inside the cell that surrounds the nucleus. Mitochondria produce the energy that each cell in the human body needs in order to function. Serious health problems can arise if we have mutations in the genes contained in the mitochondria, as this affects the energy available to the cell.

According to our current understanding, the 13 mitochondrial genes are only thought to govern the function of mitochondria.  The 25,000 genes typically contained in the nucleus of a cell provide the basis for how human bodies are built and for many of our unique personal characteristics.

Many of us have low levels of mutated mitochondrial DNA (mtDNA) in our cells and experience no adverse symptoms. However, when the mutated mtDNA copies make up around 60 per cent or more of the total of mtDNA copies in a cell_¹,  this causes serious health problems. Patients who are affected by mtDNA disorders may have mutations in either some, or all, of the mitochondria in their cells. This often impacts hardest on the organs of the body which require relatively large amounts of cellular energy to function properly, such as the brain, heart, kidneys and major muscle groups.

In the absence of an effective cure, and given the very poor outcomes for some children and young people affected, laboratory research is currently being carried out on techniques to prevent the transmission of inherited mitochondrial disorders.

Researchers are investigating options that could be offered to prospective parents seeking to prevent a disease-causing level of mutated mitochondria from being passed on to their children, and who are also wishing to use their own sperm and eggs to have a baby. No licensed technique is currently available which could fulfil both of these criteria.

Experimental techniques such as pronuclear transfer (PNT_⁾²  and maternal spindle transfer (MST)_³  have been developed in response. It  is currently unlawful under UK laws governing IVF and embryo research to offer such techniques for treatment. However, the legislation does allow for them to be introduced, subject to Regulations that can be made by the Secretary of State, if approved by Parliament. If they were permitted to be used in treatment, the expectation is that the resulting children would be born free from inherited mitochondrial disorders.

Next: Ethical questions arising

¹_{Taylor RW and Turnbull DM (2005) Mitochondrial DNA mutations in human disease Nature Reviews Genetics 6(5): 389-402.}

²_{Craven L, Tuppen H, Greggains G et al, (2010), Pronuclear transfer in human embryos to prevent transmission of mitochondrial DNA disease, Nature vol 465, p82–85}

³_{Tachibana M, Sparman M, Sritanaudomchai H, et al Mitochondrial gene replacement in primate offspring and embryonic stem cells. Nature. 7;461(7262):367-72. Epub, 2009. PMID: 19710649}