Are there any ethical differences to consider between the two existing mitochondrial replacement techniques? An 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.

Mitochondrial replacement techniques

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. Secondly, they suggest that MST could be considered a form of selective reproduction, ensuring that certain persons are not brought into existence, while PNT would be an eminently therapeutic technique.

However, we do not consider this to be a coherent argument. One the one hand, while it is true that PNT is applied directly to the individual who will suffer the disease, we must not forget that it requires the creation of another healthy individual who will then be sacrificed to cure the first; in MST, meanwhile, the therapy is applied to the mother (her ova) so that her descendents will not be affected. Therefore, if the zygote “patient” is really considered to be an individual of the human species, it is illogical to defend PNT as “more moral”, since the other zygote — which in this case should also be considered as an individual of the human species — must be sacrificed. On the other hand, we believe that the parental intention in both cases is to have a child free from the mitochondrial disease, and that there are no nuances as regards therapeutic or eugenic purposes. Although PNT is applied once the already child exists (in the state of zygote), that child has been produced by in-vitro fertilisation with the clear objective of applying the technique afterwards. It is also absurd to claim therapeutic, and not selective, intentionality when the technique necessarily implies eliminating of one of the zygotes, the one with the “sick” mitochondria.

Therefore, we do not believe that there are any valid arguments to advocate PNT over MST. However, in our opinion, there is indeed a significant difference in the moral plane between the two techniques, since in MST, gametes are manipulated, while in PNT, zygotes — already human individuals — are manipulated and destroyed. Accordingly, we believe that if the other ethical difficulties can be resolved, T MST could become an ethically acceptable option for families affected by mitochondrial diseases, while TPNT — on account of the procedure involved — can never be. The main problem with MST is that its application is presently bound to the subsequent use of in-vitro fertilisation, but this does not necessarily have to be so. Other less controversial assisted reproduction techniques, such as gamete intrafallopian tube transfer (GIFT) could be used if resources are invested in their development.

See Mitochondrial replacement used for the first time in viable human embryos

Lucia Gómez

Lucía Gómez-Tatay

Bioethics Observatory

Catholic University of Valencia