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Pronuclear transfer may give families affected by mitochondrial disease a choice

19 April 2010
By Dr Kristina Mills and Dr Marita Pohlschmidt
Research Communications Officer and Director of Research, Muscular Dystrophy Campaign, UK
Appeared in BioNews 554
Research at Newcastle University funded by the Muscular Dystrophy Campaign has shown that it might be possible to prevent mitochondrial diseases being passed from mother to child. These often devastating conditions, which affect 3,500 people in the UK, can affect almost every part of the body, with symptoms including muscle weakness and pain, loss of sight and hearing, learning difficulties, seizures, diabetes, heart, liver and kidney disease and breathing difficulties. In the most severe cases, they lead to death in early childhood.

Mitochondrial diseases are caused by damaged DNA in the mitochondria, which are the 'batteries' of the cell and supply it with energy. Every cell contains thousands of mitochondria and each carries a small piece of DNA which contains 13 protein-coding genes. This is compared to 23,000 genes in the chromosomal DNA located in the nucleus of the cell. Mitochondria are passed down the maternal line, from a mother's egg to her child.

The research led by Professor Doug Turnbull (in collaboration with Dr Mary Herbert and Professor Alison Murdoch) showed for the first time that it is possible to transfer the nuclei containing the chromosomal DNA of a human embryo, into a donor egg. The donor egg previously had its own nuclear DNA removed. The human embryos manipulated in this way had minimal amounts of mitochondria transferred to the donor egg and, encouragingly, a proportion of them successfully developed in the laboratory for six to eight days. The success rate of embryo development to the blastocyst stage was rather low at 8.3%, but the embryos used were those donated to research because they had fertilised abnormally and could not be used for IVF. The researchers expect higher success rates with normal embryos, but this requires further research.

Pronuclear transfer in human embryos to prevent transmission of mitochondrial DNA disease

There is currently no effective treatment available for mitochondrial diseases and, due to the way that these conditions are inherited and their unpredictable nature, genetic counselling and prenatal testing are extremely difficult. If this technique, known as 'pronuclear transfer', were to come into clinical use, it would give families the option of having healthy children. The abnormal mitochondria would be exchanged for the healthy mitochondria of a donated egg. Less than 0.1% of the resulting baby's DNA would come from the donated egg, and these genes would only be involved in the production of energy via the mitochondria, and not any other characteristics. This is rather like replacing the batteries in a laptop; the laptop itself is unchanged and all of the information on the hard drive remains the same, but the new batteries allow it to work properly.

Recent research in the US has also given strength to this approach - it was shown that a similar technique tested on monkeys gave rise to apparently healthy offspring. The current Human Fertility and Embryology (HFE) Act does not allow embryos modified in this way to be implanted into the mother. If this legislation were to be changed, leading scientists are confident that it could be offered to families with mitochondrial disease within the next three years. Before this can happen, further studies to demonstrate that this technique is safe and efficient are required and Professor Doug Turnbull and Professor Alison Murdoch are currently liaising with the Human Fertilisation and Embryology Authority (HFEA) to discuss the details of the next steps. Their case can then be made to the Secretary of State who, since the amendment to the HFE Act in 2009, has the power to make provisions for this technique to be introduced into clinical practice.

Pronuclear transfer has been developed specifically for mitochondrial diseases and is not applicable to any other types of genetic conditions. It is an option that the families affected by these potentially-devastating conditions have been so desperately hoping for, because it means that it would break the chain of inheritance within a family line. The technique cannot stray into the realm of designer babies, as some may fear, because it cannot be used to alter the genes that are located in the chromosomes. Instead, it gives some parents the choice to have healthy children and also improves the quality and length of life of future generations.

The Muscular Dystrophy Campaign urges the government to carefully consider this new technology and take a positive view, as it will address the needs and rights of families affected by mitochondrial disease to have healthy children.

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