Are there any ethical differences to consider between the two existing mitochondrial replacement techniques? A recent article in Bioethics (Bioethics 29; 631–638, 2015) defends the advisability of using pronuclear transfer (PNT) compared to maternal spindle transfer (MST) to prevent the transmission of certain hereditary mitochondrial diseases.
MST consists of extracting the nuclear DNA from the ovum (egg) of the mother (whose mitochondria are “sick”) and placing it in a healthy enucleated ovum from a donor. Thus, the resulting ovum contains nuclear DNA from the mother and healthy mitochondria from the donor. The ovum is then fertilised in vitro. In contrast, in PNT, the replacement does not occur between ova, but between zygotes. In the first step, two ova are fertilised: one from the mother, containing the “sick” mitochondria, and one from a donor with healthy mitochondria. The DNA replacement occurs between zygotes, resulting in a zygote with the genetic contribution of the parents and the mitochondria of the donor; the other zygote is discarded.
The authors argue that, since MST is applied to the female gamete before fertilisation, it can somehow be said that the sole function of the technique is to allow the mother to have children of her own without the disease. In contrast, since PNT is applied to the zygote, this technique can be considered to act directly on the health of an already existing child, to cure it rather than to satisfy the mother’s desire to have healthy children. From this, the authors draw two conclusions. First of all, that the parents have stronger moral reasons to accept PNT than to accept MST, since not using PNT would directly harm their children, which would not be the case of MST.