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WORLD OF REPRODUCTIVE BIOLOGY

Techniques to eliminate mitochondrial diseases are honed in a new study that outlines a path to preventing the transmission of damaged mitochondria from a mother to her offspring [1]. These promising findings in Nature come on the heels of a similar study published in the same journal online in October 2012 [2, 3]. Mitochondria and, consequently, mitochondrial diseases are transmitted by the mother to offspring through the oocyte. Daniel Paull et al. circumvented this scheme of nature by transplanting the nuclear DNA from one human oocyte into the cytoplasm of an oocyte from which a nucleus had been removed. To test whether the procedure worked, the authors parthenogenically activated the newly derived oocytes. The resulting embryos developed efficiently and took on the mitochondria of the host oocytes. Of the 18 oocytes, 7 made it to the blastocyst stage. The researchers generated three pluripotent stem cell lines from these blastocysts and passaged the cells for more than a year. Mitochondrial DNA transferred with the nuclear genome was initially present at low levels—below 1 percent—but waned to undetectable levels in the stem cell lines and in differentiated cells created from these lines. The mitochondria also were functional, for instance yielding oxygen consumption rates indistinguishable from controls. Such extreme manipulation of an oocyte raises concerns about introducing epigenetic alterations. To address this concern, the researchers examined gene expression in cell lines derived from manipulated embryos and found it to be comparable to that seen in control cell lines derived from unmanipulated embryos. The findings jibe with a study published online last October by Masahito Tachibana et al. [2], which employed a similar technique. Both techniques result in the replacement of donor mitochondria in resulting stem cells; however, in the study by Tachibana and colleagues, about half of the manipulated zygotes had an extra pronucleus, which may indicate an abnormal chromosome number. In the earlier study, researchers transferred spindle-chromosome complexes to host oocytes; in the new study, researchers were able to successfully transfer the nuclear DNA with an incompletely assembled spindle by cooling the nucleus, either by cryopreservation or by keeping it at room temperature, which resulted in reversible, partial depolymerization of the spindle.