As reported in a recent commentary in Nature the UK has decided to allow human trials of mitochondrial replacement. This would involve a small group of women who have genetic defects in their mitochondria that could be transmitted to their offspring. The procedure would involve transfer of only the nucleus of the egg of the affected woman to a healthy enucleated egg from a donor with subsequent in vitro fertilization by the male partner. Simply put, it would result in a ‘three parent baby’ with male and female nuclei provided by the parents and mitochondrial DNA provided by the donor. The positive aspect is that it would allow women with mitochondrial defects to have normal children that share most of their genome. The mitochondrial genes account for only a tiny fraction of the total genome and thus the children born in this manner would be genetically very similar to those conceived normally. The aspect of concern is that this really is re-engineering the human germ line since the mitochondrial DNA could be passed on indefinitely through female offspring. On an ethical basis there has been great reluctance to engage in modification of the human germ line, but this would be a first step.
While the proposed trials are directed toward preventing offspring with genetic defects one could easily visualize a ‘slippery slope’ effect here. There is increasing momentum for using various pharmacological and genetic techniques to enhance human capabilities (I will be posting much more about this soon). Since mitochondria can contribute to endurance and athletic performance (1), it will be tempting to use this relatively simple technology to ‘improve’ one’s offspring with, for example, mitochondria from an outstanding female athlete. Clearly stringent monitoring will be needed.