28 August 2009
The mitochondria are sub-cellular stuctures that release energy from food to power the cell; they contain small amounts of their own DNA encoding a total of thirteen genes, although around 1500 additional genes that contribute to mitochondrial structure and function are found within the rest of the main genomic DNA of the cell inside the nucleus. The rate of mutation in mtDNA is much higher than that in nuclear DNA, and over time cells may accumulate a mixture of normal and mutant mtDNA (heteroplasmy). Since mitochondrial DNA (mtDNA) is inherited via the cytoplasm of maternal egg cells, heteroplasmy in egg cells can allow the transmission of mtDNA mutations to the woman’s offspring. Sperm cells, whilst containing mitochondria, do not normally contribute any to the embryo as they undergo selective destruction within the fertilised egg.
Inherited defects in mitochondrial DNA are known to be responsible for some serious rare genetic diseases including mitochondrial myopathies (neuromuscular disorders), the neurodegenerative disease Leigh syndrome, and Leber's hereditary optic neuropathy. Mitochondrial diseases may be highly variable, since the distribution of defective mitochondria in different body organs can vary; defective mitochondria in the muscular or nervous system typically have the most severe affects. Infant-onset mitochondrial diseases can be lethal in childhood; other have milder symptoms, but all tend to become progressively more severe with age [Lane N (2006) Nature 440 (7084), 600-602].
A new paper in Nature reports on a nuclear transfer procedure in rhesus macaques that could represent a step towards an effective therapy for human mitochondrial disease. The US researchers extracted DNA from the nucleus of one egg cell and transplanted it to another from which the nucleus had previously been removed – crucially, without simultaneously transferring any mitochondrial DNA [Tachibana M et al. (2009) Nature doi:10.1038/nature08368]. In all, fifteen embryos created using the technique (dubbed MII spindle–chromosomal complex transfer) were subsequently fertilised with sperm to create embryos with nuclear DNA from the first egg donor monkey and mitochondrial DNA from the second.
The embryos were implanted into nine female monkeys, of which three became pregnant. At the time of publication, one had given birth to twins and another to a single baby, with pregnancy reportedly ongoing in the third. The researchers report that the three monkey offspring are all healthy and that virtually none of the donor mother’s mitochondrial DNA has been detected (although all the donor monkeys used had normal, healthy mtDNA). A similar procedure in humans using eggs from healthy female donors could allow women with mitochondrial disease to conceive their own healthy genetic offspring, free from the mitochondrial mutations associated with disease; only the mitochondrial DNA (a tiny proportion of the whole genome) would come from the egg donor.
Currently, pre-implantation genetic diagnosis (PGD) has been used to screen embryos in an attempt to select those with few or no maternal disease-associated mtDNA mutations, but it isn’t possible to accurately predict risks because of the variability in how much healthy and mutated mtDNA may be passed to different embryos. David Thorburn, a mitochondrial disease specialist from the Murdoch Childrens Research Institute in Melbourne, Australia, reportedly commented: "It should be able to mimic the human situation more closely than mice. If proven safe [in humans] this could provide a huge advance" [Cyranoski D (2009) Nature doi:10.1038/news.2009.860].
Comment: This paper represents a potential step towards therapeutic cloning to avoid serious mitochondrial disease in humans, but many barriers remain. These include concerns about the long-term safety of the procedure, since previous efforts to transplant healthy mitochondria have been associated with birth defects; it will be desirable to observe larger numbers of primate offspring created in this manner. There are also possible ethical objections to the approach from some quarters as a form of germ-line (ie. permanent, heritable) genetic modification. Lead researcher Shoukhrat Mitalipov is quoted as having said that human trials could be taken forward in as little as two to three years (see BBC news report); although the particular cloning technique itself could be relatively easily transferred into the clinic in practical terms, it is likely that regulators will adopt a cautious approach until there is considerably more scientific evidence to support the safety and efficacy of the technique.