Concise reviews: Assisted reproductive technologies to prevent transmission of mitochondrial DNA disease

Abstract

While the fertilized egg inherits its nuclear DNA from both parents, the mitochondrial DNA is strictly maternally inherited. Cells contain multiple copies of mtDNA, each of which encodes 37 genes, which are essential for energy production by oxidative phosphorylation. Mutations can be present in all, or only in some copies of mtDNA. If present above a certain threshold, pathogenic mtDNA mutations can cause a range of debilitating and fatal diseases. Here, we provide an update of currently available options and new techniques under development to reduce the risk of transmitting mtDNA disease from mother to child. Preimplantation genetic diagnosis (PGD), a commonly used technique to detect mutations in nuclear DNA, is currently being offered to determine the mutation load of embryos produced by women who carry mtDNA mutations. The available evidence indicates that cells removed from an eight-cell embryo are predictive of the mutation load in the entire embryo, indicating that PGD provides an effective risk reduction strategy for women who produce embryos with low mutation loads. For those who do not, research is now focused on meiotic nuclear transplantation techniques to uncouple the inheritance of nuclear and mtDNA. These approaches include transplantation of any one of the products or female meiosis (meiosis II spindle, or either of the polar bodies) between oocytes, or the transplantation of pronuclei between fertilized eggs. In all cases, the transferred genetic material arises from a normal meiosis and should therefore, not be confused with cloning. The scientific progress and associated regulatory issues are discussed.

Keywords: Mitochondria; Mitochondrial DNA; Preimplantation genetic diagnosis; Pronuclear transfer; Spindle transfer; Zygote.

Figure 1

Schematic drawing showing progression from prophase of meiosis I (GV stage) to completion of meiosis II following fertilization. The diploid maternal genome contained in the large nucleus (GV) of the prophase I oocyte is packaged into bivalent chromosomes formed during meiotic recombination when pairs of replicated parental homologs become linked at the sites of reciprocal DNA exchange. Oocytes enter M phase of first meiotic division (MI) in response to hormonal stimulation and undergo anaphase of MI when bivalents are converted to dyad chromosomes, consisting of a pair of chromatids (at least one of which is a recombinant). Half of the dyads are ejected in the first polar body. The dyads remaining in the oocyte align on the second meiotic division (MII) spindle poised to undergo anaphase in response to sperm entry. During anaphase of MII dyads are resolved to single chromatids and half is lost in the second polar body. The chromatids remaining in the oocyte become surrounded by a nuclear membrane to form the female pronucleus. The products of the first and second meiotic divisions each contain a unique genome. Abbreviation: GV, germinal vesicle.

Figure 2

Schematic drawing showing approaches to meiotic genome transfer that has been tested in human oocytes/zygotes for potential clinical application to reduce the risk of transmitting mtDNA disease. (A): Pronuclear transfer: MII-arrested oocytes obtained from the affected woman and a healthy donor are fertilized and the pronuclei are transferred in a karyoplast from the affected woman's fertilized egg to the enucleated donor egg. (B): Spindle transfer: oocytes obtained from an affected woman and a healthy donor are enucleated by removal of the MII spindle and its chromosomes in a karyoplast. The karyoplast from the affected woman is fused with the enucleated oocyte from the healthy donor. Reconstituted oocytes are then fertilized and undergo the second meiotic division followed by formation of the male and female pronuclei. Abbreviations: MII, second meiotic division; PN, pronuclei.