Despite the potential for tissue specific

Despite the potential for tissue specific accumulation of dysfunctional mitochondria, MRT understandably is of considerable appeal to couples coping with a devastating mitochondrial disease as such intervention provides the only hope for a healthy child. Depending on the tissues and proportion of diseased mitochondria, this drift toward homoplasmy might not be problematic in first generation males or females. However, because of the uniparental inheritance and mitochondrial mecamylamine manufacturer within the prospective female germline, selection bias in favor of the “contaminating mitochondria” as they pass through the bottleneck owing to their shared maternal origin with the nuclear genome would be a significant limitation to MRT and potentially detrimental, as it could conceivably lead to reversion to the diseased donor mitochondria population. Thus, MRT females even with an otherwise healthy life would face the same mitochondrial disease and infertility as their mothers. A recent study that examined the fate of healthy donor mitochondria in recipient human oocytes indicates that in the majority of cases the donor mitochondria are eliminated over time; however, their data also point to persistence and even complete reversion or drift toward the mutant donor mitochondria in a significant number of cases (Yamada et al., 2016). This study both sheds light on and raises important basic research questions about the still poorly understood mechanisms underlying maternal inheritance of mitochondria, as well as translational implications and caution regarding the transgenerational effectiveness of this promising therapeutic strategy.
Mechanisms for organelle level sorting of mitochondria in oocytes The existence of a mitochondrial bottleneck during early oogenesis is widely recognized, but remains incompletely understood, both in terms of timing and mechanism. So far, emphasis has been on the population of mitochondria in late stage oocytes and fertilized embryos. Like the potential advantages and reasons why uniparental inheritance of mitochondria might be important, how and when individual mitochondria are selected by the oocyte, and what features of oocyte cell biology might render it uniquely suited for transgenerational transmission of mitochondria remain open questions. It is possible that this property is intrinsic to differences in meiosis between males and females – although meiosis in both sexes generates four haploid cells, all of which become sperm in males, whereas only one oocyte is produced in females. Bottlenecks are characterized by selection of only a limited population by a strong purifying mechanism. As oogenesis progresses, oocytes undergo considerable growth as they actively produce stores, including RNAs, proteins, and mitochondria in preparation for the later stages of meiosis and the developmental events of early embryogenesis prior to activation of the zygotic genome. Early differentiated or primary oocytes are small and have fewer mitochondria, which will undergo amplification to produce the plentiful mitochondria of the significantly larger fertilization-competent late oocytes; therefore, the primary oocyte stage fulfills the bottleneck requirement for a limited population. That a bottleneck exists is clear, as quantitative assessment of the mitochondrial numbers in mouse provided evidence for reduction of mitochondrial numbers in oocytes (Cree et al., 2008), as well as for alternative mechanisms that would involve selection and amplification rather than strictly elimination of mitochondria in mouse (Cao et al., 2009; Wai et al., 2008). In addition to containing comparably fewer mitochondria, primary oocytes also form unique transient subcellular structures that are not present in later stage oocytes, specifically the Balbiani body. The Balbiani body, also known as the mitochondrial cloud because of the enrichment of these organelles in this nonmembrane bound subcellular compartment, is an ancient and conserved asymmetric feature of oocytes (reviewed in (Marlow, 2010; Kloc et al., 2004). Little is known about how this structure forms or is regulated because it is only transiently present in primary oocytes and thus has been difficult to access, but it has been long appreciated as a potential sorting or amplification depot for oocyte mitochondria.