Mitochondrial transfer: ready for the clinic?

For all of us that are a bit oxidized on our cell biology days, I would remind you that the mitochondrion is an intracellular organelle found in almost all cells of an organism. Mitochondria are small, and their number within a cell is related with the cell function; as you all know, sperm cells contain a few mitochondria along the tail to ensure motility, while highly metabolically active cells, like liver cells, can contain up to 3000 mitochondria each. The mitochondria main physiological role is to supply adenosine triphosphate (ATP) through the respiratory chain.

Mitochondria contain a circular genome which codes for some of the proteins needed for the mitochondrion own function. The human mitochondrial genome (mtDNA) encodes for 37 genes. One mitochondrion can contain two to ten copies of its own DNA. However, the mtDNA does not code for all of the protein necessary for the mitochondrion function; most of them are coded by genes in the cell nucleus.

Mitochondrial disorders often present themselves as syndromes affecting multiple organs and systems, with a concentration of neurological disorder, myopathies, and multiple endocrinopathy. Regardless of the nature of the specific alteration, diseases involving the mtDNA are invariantly transmitted by a woman to her children, as the developing embryo derives its mitochondria, and hence its mtDNA, entirely from the oocyte. As a corollary, the mode of inheritance of these mitochondrial diseases is similar to a dominant inheritance, although the penetrance in the offspring can vary.

Generating a child affected by mitochondrial disease is a risk undergone by an estimated 150 reproductive age women in UK yearly (paper NEJM). In many cases, the mother mutational load in her own cells would fall below the threshold to generate clinical symptoms, or cause a mild and often misdiagnosed syndrome, while enrichment in the germline and uneven segregation in the embryonic tissues will cause a clinically relevant disease in the offspring. In most cases, therefore, the mtDNA alteration becomes apparent in a newborn or small child, often leading to a severely decreased quality of life and early death.

Currently, these women can achieve motherhood of a healthy child either by conception with donor oocytes or adoption. Neither option will generate a child who is genetically related to the mother.

Over the last few years, a preventive technique has been proposed, which would allow for a woman carrying mtDNA alterations to conceive a likely healthy child genetically related to herself. The technique is based on a microsurgical manipulation of the woman oocyte in order to replace her faulty mitochondria with mitochondria coming from a donor oocyte.

The technique, called mitochondrial transfer, has been proposed in two variants. In one of them, called spindle transfer, the meiotic spindle of the affected woman is removed from her oocyte, and placed into an oocyte from a donor with healthy mitochondria, previously emptied of the donor meiotic spindle. The reconstructed oocyte is then fertilized with the partner sperm, and the resulting embryo will have the genome of both parents, as in a standard fertilization process, but accompanied by the mitochondria of the donor. A variant of this method is called pronuclear transfer, where the oocyte of the affected woman is first fertilized with the partner sperm, and when the 2 DNA of the parents start to form small nuclei called pronuclei, they are both removed from the affected oocyte and both transferred to the oocyte of the donor, previously emptied of the donor meiotic spindle.

In the case of pronuclear transfer, there has been one report of feasibility in the human species so far, carried out in abnormal embryos (cita Newcastle, nature). The technique of spindle transfer has been assayed in non-human primates (cita mitalipov), giving rise to two healthy rhesus monkey. In human, spindle transfer has been tested and apparently normal embryo have been developed to the blastocyst stage; these embryos have been used to derive embryonic stem cells, and to assay the presence of altered mtDNA in the differentiated cell populations derived by the stem cells, giving very encouraging results (cita mitalipov la segunda con humanos).

Regardless of the successful initial reports, a significant amount of safety data needs to be put in place before clinical translation of these techniques to the patients; since the prospective parents already have a safe option to parenthood, although without a genetic link to the child, the risk-benefit ratio should be heavily tilted towards the benefit, i.e. the risk for the mother and the future child should be minimal for the technique to be applied clinically. Such safety studies should start by analyzing the effect of mitochondria transfer on the resulting construct, among other things focusing on the preservation of the delicate and critical meiotic spindle, whose iatrogenic alteration might affect the mechanisms by which the DNA segregates in the reconstructed oocyte, leading to a non-viable or affected embryo.