P-730 Detection of mitochondrial reversal following meiotic spindle transfer: a finding of importance for mitochondrial replacement therapies used for the purpose of avoiding mtDNA disease transmission
Authors: Spath K , Costa-Borges N , Nikitos E , Kostaras K , Calderón G C , Psathas P , Wells D 
Published: June 2023

Abstract

Study question
Following transfer of the meiotic spindle from a patient’s oocyte into an enucleated donor oocyte, do relative levels of patient and donor mtDNA remain stable?

Summary answer
mtDNA transferred along with the patient’s spindle typically remain at a low level. However, disproportionate expansion of the patient’s mtDNA can occur in some cases.

What is known already
Mitochondrial DNA disorders are caused by mutations in the mitochondrial genome, disrupting ATP production. They have few treatment options and no cure. All mitochondria are derived from the oocyte, therefore mtDNA disorders are maternally inherited. It has been proposed that disease transmission could be avoided if female mtDNA mutation carriers underwent meiotic spindle transfer (MST), removing the chromosomes (on the spindle) from an affected oocyte and transferring them into the healthy cytoplasm of a donor oocyte. However, research using embryonic stem cells has suggested that the small population of mitochondria transferred along with the spindle can sometimes undergo dramatic expansion.

Study design, size, duration
25 infertile couples were enrolled in a prospective pilot study evaluating MST as a treatment for infertility. The female participants had a mean age of 37.2, and an average of 6.4 previous unsuccessful IVF cycles (range 3-11) characterised by extremely poor embryo development and without any previous pregnancy. Importantly, none of the patients were carriers of a mtDNA disorder. Oocyte donors with previous successful IVF outcomes were matched with patients according to standard practice.

Participants/materials, setting, methods
Metaphase-II-spindles from patient oocytes were transferred into enucleated donor oocytes. Reconstructed oocytes underwent ICSI and the resulting embryos were transferred at the blastocyst stage. Mitochondrial genomes were sequenced to identify polymorphisms differing between the patient and oocyte donor. These variations were quantified in the embryos (blastocyst biopsies), during pregnancy (amniocentesis), in newborns (cord blood, cord tissue, urine), at 3-6 months and at one year (saliva, urine, blood), revealing the relative amounts of each mitochondrial type.

Main results and the role of chance
This MST pilot study resulted in the birth of six children, indicating that the procedure is compatible with the production of viable embryos, capable of producing healthy live births. The patient’s mtDNA was shown to represent &lt;1% of the total in all blastocysts produced, confirming that MST is highly reproducible and that relatively few mitochondria are transferred along with the spindle. For five of the six children, the proportion of the total mtDNA attributable to the patient appeared to be stable, remaining at very low levels in all of the samples from later developmental stages. However, in one child the small quantity of mtDNA transferred along with the spindle increased disproportionately with respect to the mtDNA of the oocyte donor, ultimately representing 30-60% of the total at birth, depending on the tissue tested. The precise timing of the expansion of one type of mtDNA at the expense of the other is not known but occurred sometime between the blastocyst stage and birth. By the time of birth, the levels of donor and patient mtDNA appeared to have stabilised (no further increases were seen at 6 months and one year). All of the children born remain developmentally normal and healthy.

Limitations, reasons for caution
After using MST, several pregnancies were achieved for patients with a long history of unsuccessful IVF attempts, associated with poor oocyte quality and a failure to produce blastocysts. However, this small pilot study lacked the controls necessary for a definitive evaluation of MST as a tool for infertility treatment.

Wider implications of the findings
All children born following MST were healthy. However, our results clearly demonstrate that a substantial degree of mtDNA ‘reversal’ is possible. Consequently, mitochondrial replacement therapies used for avoidance of mtDNA disorders might not always be successful, even when initial levels of mutant mtDNA in reconstructed oocytes are very low.

Trial registration number
ISRCTN11455145