DNA transfer prevents mitochondrial disease in humans

AusSMC: In a world-first study to be published in Nature, scientists have shown in human embryos a new technique developed last year in monkeys to treat maternally-inherited mitochondrial diseases.

Credit: Australian Science Media Centre
Credit: Australian Science Media Centre

The study describes the first ever transfer of genetic material between fertilised human eggs which has the potential to ‘treat’ human mitochondrial DNA (mtDNA) disease at a genetic level. The procedure involves transferring the nucleus from an egg which has defective mitochondria and transporting it into a donor egg with healthy mitochondria, creating an embryo free of mitochondrial disease.

Treatment options for patients with mtDNA disease are extremely limited and so the results of this study have the potential to help couples where the woman carries a mutation in her mitochondrial DNA that could cause disease in her children. Mitochondrial disease is a debilitating and potentially fatal genetic disorder that affects both children and adults by robbing the body’s cells of energy. Recent research suggests mitochondrial disease may affect one in 200 Australians.

The AusSMC and the UK SMC have rounded up comment from experts. The graphic displayed to the left has been created by the AusSMC – high resolution versions of it can be downloaded from the AusSMC website.

Prof Peter Braude, Head of Department of Women’s Health, King’s College London, said:

“This is promising work and shows the benefits that can be accrued from properly targeted and regulated embryo research. The difficult task now will be the experiments to demonstrate pronuclear transfer to be safe for first-in-human trials of what is effectively germ line therapy.”

Mr Tony Rutherford, Chairman of the British Fertility Society, said:

“Mitochondrial diseases are a relatively rare but important group of conditions with an incidence varying between 1 in 4,000 to 1 in 10,000. They are caused by mutations in the DNA that code for mitochondria, the energy producing organelles in a cell. An example is a condition known as MELAS (Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) which presents in childhood, and is associated with muscle weakness and discomfort, headaches, vomiting and seizures. Most sufferers will have stroke-like episodes before middle age, which on a repeated basis can lead to loss of vision, issues with movement and dementia. Mitochondrial disorders are inherited through the maternal cell line, and they usually occur when the majority of the mitochondrial DNA is abnormal. Unfortunately at present, there are no cures for mitochondrial diseases, and the only treatment option is supportive therapy.

“This preliminary study outlines a possible method to prevent the transmission of mitochondrial disorders. The female pronucleus, which carries the genetic message from the mother to the next generation, is removed from an egg that is known to carry a high percentage of mutated mitochondrial disease and transferred to a healthy donor egg, after its own genetic material has been removed. This proof of concept study shows that transferring a pronucleus appears to be possible in human eggs, but more importantly it shows that this technique leaves the abnormal mitochondrial DNA behind, so that when the newly constructed fertilised egg cleaves to form an embryo, it uses the normal mitochondria from the donor egg. Of course this study was performed on abnormally fertilised eggs and needs to be repeated in controlled conditions using healthy eggs to establish whether the technique is indeed viable in conditions closer to those employed in a clinical setting, and to demonstrate that the procedure is safe. It is important to put this work into context, as it is only the first step on the long road to potentially developing a treatment to prevent the transmission of these dreadfully debilitating disorders. The British Fertility Society supports such research and we look forward to seeing the results of more extensive studies in due course.”

Prof Justin St. John, Professor and Director, Centre for Reproduction & Development, Monash Institute of Medical Research, Monash University, Australia, said:

“Finding effective assisted reproductive technologies for women who are carriers of mitochondrial disorders is a priority for these patients. Although this work and previous work have addressed some of the issues and provide some promising preliminary outcomes, we do not believe embryos carrying even very low levels of mutant mitochondrial DNA will not give rise to mitochondrial disorders. This is especially as mitochondrial DNA segregates randomly during fetal development. During fetal development mitochondrial DNA is also extensively replicated and selection of mutant and non-mutant mitochondrial DNA for replication is again random. We know from one clinical case that mutant mitochondrial DNA that contributed less than 0.01% of the total mitochondrial DNA population at fertilisation was found in a male patient suffering from a mitochondrial myopathy. Consequently, it is really necessary to eliminate all mutant mitochondrial DNA in any assisted reproductive technology aimed at helping patients with mitochondrial disorders who wish to have children that would not be affected or be subsequent carriers of such severe metabolic disorders.”

Science Media Centre Fact Sheet

Mitochondrial DNA

Mitochondrial DNA (mtDNA) is DNA contained in the mitochondria in our cells – these are the energy-generating structures commonly referred to as the ‘batteries’ or ‘powerhouses’ of the cells

Mitochondria have their own genome which is separate from that contained within the cell nucleus – this consists of a circular molecule of DNA, containing the genes necessary for mitochondrial formation – the mitochondria The mitochondrial genome contains around 16,500 base pairs (the nuclear genome contains over 3 billion base pairs) and 37 genes (compared to around 23,000 genes in the nuclear genome).

It is thought to have evolved separately from nuclear DNA, when bacteria containing circular DNA became part of the precursors to cells that exist today – this is shown by the observation of similarities between mitochondrial and bacterial genomes.

Mitochondrial DNA is inherited solely through the maternal side (i.e. from the mother).

Mitochondrial diseases

Mitochondrial diseases are a group of disorders caused by damage to the mitochondrial DNA, or to nuclear DNA that contributes to mitochondrial components. A number of these conditions are associated with mitochondrial myopathy, which feature neuromuscular disease symptoms resulting from failure of the mitochondria to function properly.

Examples of mitochondrial diseases include:

– Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP) – a condition consisting of various symptoms affecting the nervous system, including numbness or pain in the limbs (sensory neuropathy), muscle weakness, and problems with balance and coordination (ataxia) as well as deterioration of light-sensing cells I the retina (retinitis pigmentosa).

– Leigh syndrome – a progressive degenerative disorder affecting the brain and nervous syndrome which mainly appears in infants during their first year of life

– Myoclonic Epilepsy with Ragged Red Fibres (MERRF) – a rare type of progressive epilepsy characterised by ‘ragged red’ muscle fibres

– Leber’s hereditary optic atrophy (LHON) — the onset in midlife (average age 30) of painless central visual loss that progresses over a period averaging 4 months, resulting in blindness in both eyes

They can affect either single or multiple organs, and can appear at any age and with variable frequency. The defects that cause them are transmitted by maternal inheritance, in accordance with the maternal pattern of mtDNA inheritance.

It’s important to note that these are not always clearly distinguishable conditions – there may be various symptoms present than can be attributable to more than one disorder, and careful examination may be required to make a diagnosis.

In most cases, the possibilities of treatment are limited, although there are options for managing the conditions.

Emerging research on mtDNA:

Researchers are working on ways to replace damaged mitochondrial DNA in cells as a means of preventing mitochondrial disease being passed on to the next generation. Research in this area is at an early stage and is tightly controlled – although could be permitted by the most recent Human Fertilisation and Embryology Act (2008) and subject to licensing by the Human Fertilisation and Embryology Authority.

The idea is to be able to extract genetic material (nuclear DNA) from the fertilised egg in which the mitochondrial DNA is damaged, and to insert it into another fertilised egg obtained from a donor in which the mitochondria are undamaged. The DNA is in the form of two pronuclei – one from the egg and one from the sperm that fertilised it – which exist separately before they fuse to form the nucleus with the full set of chromosomes (one set each from mother and father).

The egg is then allowed to develop in vitro with normal mitochondria, as per the (simplified) diagram above.