15 April 2010
A new paper in Nature reports on the use of an approach termed pronuclear transfer to prevent transmission of mitochondrial disease. Genetic forms of mitochondrial disease, whilst rare, can have devastating consequences, so there is considerable interest in finding ways to prevent them in affected families. Work last year used a technique called metaphase II spindle transfer to combine monkey egg cells with healthy mitochondria (from which the nucleus had been removed) with the nucleus from a monkey carrying mitochondrial mutations. The resultant egg cells, when fertilised, gave rise to apparently healthy baby monkeys that were the genetic offspring of the diseased mother – but with normal mitochondria (see previous news).
The latest research uses a different approach, pronuclear transfer – using human zygotes produced by in vitro fertilisation (IVF) rather than egg cells [Craven L. et al. (2010) doi:10.1038/nature08958]. Pronuclei are the bodies containing genetic material derived from the sperm and egg cells that are present in the zygote following fertilisation, but before the two fuse to form a single nucleus. The researchers created zygotes using egg cells from women with mitochondrial mutations and healthy sperm cells, then transferred either one or two pronuclei from the zygotes into another zygote from a healthy mother (ie. with mitochondria that did not contain mutations) from which the pronuclei had been removed. The resultant zygotes were grown to the blastocyst stage (70-100 cell embryos, day 5-6 after fertilisation). The proportion of zygotes reaching this stage was around 8% - half that of artificially created zygotes that had not been manipulated.
The researchers also looked at the proportion of donor (ie. mutated) mitochondrial DNA (mtDNA) in embryos grown following pronuclear transfer and found it quite variable; one contained no donor mtDNA at all, the ideal scenario, but others showed levels ranging from 2- 40%. Refinement of the pronuclear transfer technique to minimise the amount of celullar material transferred along with the pronuclei reduced this to below 2%.
The authors conclude that the technique, along with the previously published alternative approach, has the potential to be used to ‘treat’ (rather, prevent or ameliorate) human mtDNA disease, and weight the relative advantages and disadvantages of each. They are careful to note that this sort of genetic manipulation is only appropriate for use in families affected by serious forms of disease that ‘can affect multiple family members with catastrophic consequences’, as per the example of a woman with seven affected children, of which six died shortly after birth and the seventh has very serious medical problems (see BBC news report).
Comment: Calls for further investment in this line of research seem justified, given the severity of the conditions it is intended to prevent. However, there are two areas that will continue to require scrutiny. The first is the safety of the procedure; the researchers themselves point out that artificial manipulation of embryos can result in abnormal development. The second is bioethical concerns; generic issues relating to any form of cloning, and specifics relating to this particular technique – which involves the creation and manipulation of multiple zygotes, as opposed to unfertilised egg cells.