Maternal transmission of mitochondrial diseases

Abstract Given the major role of the mitochondrion in cellular homeostasis, dysfunctions of this organelle may lead to several common diseases in humans. Among these, maternal diseases linked to mitochondrial DNA (mtDNA) mutations are of special interest due to the unclear pattern of mitochondrial inheritance. Multiple copies of mtDNA are present in a cell, each encoding for 37 genes essential for mitochondrial function. In cases of mtDNA mutations, mitochondrial malfunctioning relies on mutation load, as mutant and wild-type molecules may co-exist within the cell. Since the mutation load associated with disease manifestation varies for different mutations and tissues, it is hard to predict the progeny phenotype based on mutation load in the progenitor. In addition, poorly understood mechanisms act in the female germline to prevent the accumulation of deleterious mtDNA in the following generations. In this review, we outline basic aspects of mitochondrial inheritance in mammals and how they may lead to maternally-inherited diseases. Furthermore, we discuss potential therapeutic strategies for these diseases, which may be used in the future to prevent their transmission.


Introduction
The mitochondrion gained its deserved reputation in cell biology due to its role as the cellular powerhouse, with most of the adenosine triphosphate (ATP) in eukaryotic cells being supplied by this organelle (Wallace, 2013). However, mitochondria play several functions in the cell that far exceed the role in ATP generation. These are linked with buffering of Ca +2 levels, innate immunity, apoptosis and biogenesis of iron-sulfur clusters (Yasukawa et al., 2009;Naon and Scorrano 2014;Stehling et al., 2014). Moreover, mitochondria closely interact with other organelles such as the endoplasmic reticulum (ER) and regulate several pathways in the cell (de Brito and Scorrano, 2009;Betz et al., 2013;Chen et al., 2014;Carreras-Sureda et al., 2017;Xu et al., 2017). As a result, perturbations in mitochondrial function may dramatically disturb cellular homeostasis, resulting in several common diseases in humans (Bach et al., 2003;Chen et al., 2007Chen et al., , 2010Schaefer et al., 2008;Misko et al., 2012;Schon et al., 2012;Sebastian et al., 2012;Eschbach et al., 2013;Payne et al., 2013;Schneeberger et al., 2013;Pareyson et al., 2015;Ramírez et al., 2017).

Mitochondria in female germ cells
The earliest stages of embryogenesis are characterized by rapid cell division (i.e., cleavage) that gives rise to blastocysts. During these stages, the embryo relies on maternal factors inherited from the oocyte (i.e., mRNAs, proteins and mitochondria), as the embryonic genome is transcriptionally inactive. Also, in agreement with the "embryo silent" hypothesis, mitochondria show low activity during these stages to protect embryonic cells from oxidative damage (Leese, 2012). At the blastocyst stage, increased protein synthesis and blastocoel expansion is accompanied by upregulation of mitochondrial activity in cells that give rise to extraembryonic tissues (i.e., the trophectoderm) (Trimarchi et al., 2000;May-Panloup et al., 2005;Hashimoto et al., 2017;St John, 2019). Activation of mitochondrial function is postponed, however, in the inner cell mass that originates the embryo proper. Mitochondrial architecture and function seem to remain underdeveloped in cells committed with germline specification, and mtDNA replication is only resumed with primordial germ cell (PGC) differentiation (Wassarman and Josefowicz, 1978;Motta et al., 2000;Cree et al., 2008;Wai 2 Chiaratti et al. et al., 2008;St John et al., 2010;Floros et al., 2018;Chiaratti et al., 2018;St John, 2019).
Among the hundreds of cells in the developing fetus, PGCs originate from a few dozen located at the basis of allantois. Yet, after migration to the genital ridge, PGCs proliferate quickly to generate in females millions of oogonia (Leitch et al., 2013). After entering meiosis, these primary oocytes receive a cover layer of somatic pre-granulosa cells, giving rise to primordial follicles still during fetal life. These follicles constitute the ovarian reserve that females carry throughout their reproductive life (Oktem and Urman, 2010). After puberty, the ovary provides an adequate environment for follicle growth and maturation (Clarke, 2017). During this period, the oocyte stockpiles several molecules that are required later during embryogenesis. This includes ã 1,000-fold increase in mitochondria (Jansen and De Boer, 1998;Cree et al., 2008;Wai et al., 2008;St John, 2019), which accounts for the largest mitochondrial content amongst all cells in mammals. In spite of this, mitochondria display several characteristics that suggest they are immature and low functional in oocytes (Arhin et al., 2018). In fact, oocytes lacking the pyruvate dehydrogenase E1 alpha 1 (PDHA1), a key gene required for mitochondrial activity, successfully develop during most part of oogenesis and are ovulated (Johnson et al., 2007). Thus, although mitochondria do play an essential role during the final steps of oocyte development, the "embryo silent" hypothesis likely extends to oogenesis too (Arhin et al., 2018). Accordingly, somatic cells surrounding the oocyte (i.e., cumulus cells) provide the oocyte with several energetic molecules, including amino acids, cholesterol, pyruvate, AMP, and ATP (Su et al., , 2009Sugiura et al., 2007). Moreover, the adenosine salvage pathway seems to be a key source of ATP, giving it can be generated from abundant amounts of cyclic AMP (cAMP) present in oocytes (Scantland et al., 2014).
If mitochondria are not highly active in oocytes, why are they present in massive amounts before fertilization? This can be, at least, partially explained by downregulation of mitochondrial biogenesis during early embryogenesis; mitochondria are segregated among hundreds of embryonic cells without any increase in number up to the time of embryo implantation (Pikó and Taylor, 1987;Thundathil et al., 2005;Cree et al., 2008;Wai et al., 2008;St John, 2019). Therefore, a threshold number of mitochondria is necessary in oocytes to assure that every embryonic cell will inherit a minimum complement of mitochondria Wai et al., 2010). In keeping with this idea, extensive fragmentation of the mitochondrial network in oocytes allows for efficient segregation of mitochondria during early embryogenesis (Ashley et al., 1989;Cree et al., 2008;Ferreira et al., 2010;Lee et al., 2012b). Upregulation of pro-fission proteins (i.e., DRP1) and downregulation of MFN2 likely supports mitochondrial fragmentation during oogenesis (Udagawa et al., 2014;Machado et al., 2018;Hou et al., 2019;Zhang et al., 2019b). However, oocytes do retain fusion competence, as loss of DRP1 leads to mitochondrial elongation (Udagawa et al., 2014). Moreover, MFN1 is required for oocyte growth and ovulation; MFN1 loss impairs oocyte-somatic cell communication, disrupting folliculogenesis Hou et al., 2019;Zhang et al., 2019a,b).

Mitochondrial diseases originated from mtDNA mutations
Diseases caused by mutations in mtDNA are mostly severe and affect~1 in 4,300 people all over the world (Schaefer et al., 2008). In addition, almost every person (including healthy people) carries very low levels of mutant mtDNA (Payne et al., 2013) that may be passed down to following generations and associate with late-onset diseases, such as Parkinson disease, Alzheimer disease, and common cancers (Poulton et al., 2010;Wallace, 2011;Schon et al., 2012;Gorman et al., 2015;Stewart and Chinnery, 2015). With rare exceptions , mitochondria are inherited exclusively from the mother (Wallace and Chalkia, 2013). This uniparental pattern of inheritance is explained by the presence of several thousand mitochondria in the ovulated oocyte, against only dozens in the sperm (Wai et al., 2010). Additionally, the early embryo actively eliminates paternal mitochondria introduced into the oocyte during fertilization (Sutovsky et al., 1999;Rojansky et al., 2016). Although it is not clear why sperm mitochondria are excluded from the developing embryo, their elimination is in agreement with the "embryo silent" hypothesis, as sperm mitochondria are elongated, contain well-developed cristae and are highly active (Sutovsky et al., 1999;Ford, 2004;Ruiz-Pesini et al., 2007;Wai et al., 2010;Rojansky et al., 2016).
Mutations in mtDNA are much more frequent than in the nuclear DNA (Johnson and Johnson, 2001), which was initially thought to be explained by mtDNA proximity to ROS generation sites; the mitochondrial genome is attached to the inner mitochondrial membrane, close to complexes involved with the electron transport chain (Wallace, 2005). However, there is now data supporting that most mtDNA mutations originate from replication errors of the mtDNA polymerase (Kauppila et al., 2017). In humans, mice, and flies, for instance, transition mutations, which are indicative of replication errors, are more common than transversions, which often result from oxidative damage (Tomas, 1993;Zheng et al., 2006;Kennedy et al., 2013;Itsara et al., 2014). In fact, the machinery of DNA repair in the mitochondrion does not seem to be as effective as in the nucleus (Vermulst et al., 2008;Maynard et al., 2009;Kazak et al., 2012;Muftuoglu et al., 2014). Thus, intense replication of mtDNA during oogenesis makes it prone to replication errors (Wai et al., 2008;Mahrous et al., 2012;Wei et al., 2019).
The existence of a DNA repair machinery inside mitochondria is well established, but not fully characterized (Scheibye-Knudsen et al., 2015). Most genes encoding for factors involved in this machinery are shared with the nucleus; alternative variants of these genes allow for the protein to be targeted either to the nucleus or the mitochondrion (Muftuoglu et al., 2014;Scheibye-Knudsen et al., 2015). The best-known pathway of DNA repair in mitochondria is base excision repair (BER). Yet, several other enzymes involved with mismatch repair (MMR), non-homologous end joining (NHEJ), and direct repair have been reported in mitochondria (Maynard et al., 2009(Maynard et al., , 2010Ruhanen et al., 2010;Halsne et al., 2012;Kazak et al., 2012;Sharma et al., 2014;Scheibye-Knudsen et al., 2015). Moreover, although homologous recombination (HR) has not been proved to contribute with mtDNA repair (Kazak et al., 2012;Hagström et al., 2014;Scheibye-Knudsen et al., 2015), mitochondria do import RAD51, one of the most prominent enzymes of HR (Sage et al., 2010;Chen, 2013). RAD51 has also been linked with mtDNA synthesis under replicative stress (Sage and Knight, 2013), and in oocytes RAD51 is required for mitochondrial function (Kim et al., 2016).
Given that most cells contain several mtDNA molecules, a de novo mutation creates a condition termed heteroplasmy, characterized by the co-existence of two or more mtDNA genotypes (i.e., wild-type and mutant mtDNAs) within the same cell or organelle. Heteroplasmy commonly protects the cell, as most mtDNA mutations are recessive. Unless the mutation level exceeds a critical threshold necessary to cause a biochemical defect (i.e., above 60-90%), the mutation effect will be masked by wild-type molecules (Schon et al., 2012;Aanen et al., 2014;Haig, 2016). In addition, a mechanism known as the mitochondrial genetic bot-tleneck (Hauswirth and Laipis, 1982;Olivo et al., 1983;Hauswirth et al., 1984;Jenuth et al., 1996;Burgstaller et al., 2018) acts in the germline to rapidly re-establish homoplasmy (i.e., the presence of a single mtDNA genotype). This mechanism is based on the absence of mtDNA replication during early embryogenesis, which forces wild-type and mutant mtDNAs to segregate. Also, few cells among the hundreds present in the embryo differentiate into PGCs, resulting in a sampling effect that efficiently selects one mtDNA genotype to populate the following generation (Stewart and Chinnery, 2015;Burgstaller et al., 2018). However, the selected genotype can be either wild-type or mutant, generating genetic variability to be put to test at the cellular, organismal, or population level (Figure 1).
Mutations in mtDNA may vary considering their effect on mitochondrial function from neutral to deleterious. Among deleterious mutations, those affecting tRNA are the most frequent in humans. This is counter-intuitive though, as tRNA genes account for only 10% of the total coding capacity of mtDNA (Schon et al., 2012). However, in comparison with protein-coding genes, tRNA mutations are considered to be less severe, as higher levels (above 90%) are required to cause a biochemical defect (Yoneda et al., 1995). This finding is in agreement with several works that have provided evidence in support of purifying selection acting in 4 Chiaratti et al. Figure 1 -Mitochondrial kinetics in the female germline. Throughout germline development, the number of mitochondrial DNA (mtDNA) molecules per cell varies from 10 5 -10 6 in mature oocytes (before fertilization), 10 2 -10 3 in primordial germ cells (PGCs) and 10 5 -10 6 back to mature oocytes. This variation in copy number accounts for the mitochondrial genetic bottleneck, which forces segregation of mtDNA molecules. In line with this, the mitochondrial network is fragmented in oocytes, allowing efficient partitioning of mitochondria among hundreds of cells until embryonic implantation. In addition, only few cells in the embryo differentiate into PGCs, supporting a sampling effect towards selection of a single mtDNA genotype to populate the future oocyte.
germ cells against deleterious mtDNA mutations (Rand, 2008) (Figure 2). For instance, Stewart and colleagues have shown that mice with a burden of mtDNA mutations are less likely to transmit to offspring non-synonymous changes in protein-coding genes (Stewart et al., 2008). In contrast, synonymous substitutions in protein-coding genes and mutations in tRNAs and rRNAs were present at higher levels (Stewart et al., 2008). Similar observations have been reported for flies, mice, and humans (Sato et al., 2007;Fan et al., 2008;Freyer et al., 2012;Sharpley et al., 2012;Hill et al., 2014;Ma et al., 2014;Li et al., 2016;Floros et al., 2018;Wei et al., 2019), suggesting a conserved mechanism of purifying selection was established early during evolution. Accordingly, Lieber et al. (2019) recently reported that mitochondrial fragmentation is required to drive selective removal of deleterious mtDNA during early oogenesis in Drosophila. Fragmentation likely enhances association between mitochondrial genotype and phenotype, favoring one genotype over another (Aanen et al., 2014;Haig, 2016). Nonetheless, at least in Drosophila, this mechanism does not rely on autophagic elimination of mutant mtDNA. Instead, mitophagic proteins enable preferential replication of wild-type mtDNA to outcompete their mutant counterparts (Hill et al., 2014;Ma et al., 2014;Lieber et al., 2019). In spite of the mounting evidence in support of a filter against mutant mtDNA in the female germline, this is not a resolved issue. Actually, there are conflicting data arguing against this filter, which has been generating much debate over the topic (Burr et al., 2018). Other questions involving the issue are: i) why would purifying selection be restricted to germline? ii) can one manipulate selection to avoid the accumulation of mutant mtDNA in somatic tissues? Whilst these questions remain unresolved, it is very likely that the purifying selection behaves differently for different mtDNA mutations and different nuclear genetic backgrounds.

Transmission of metabolic diseases linked to mitochondria dysfunction
Obesity and type II diabetes are currently recognized as the most endemic diseases in the human population. The frequency of these syndromes is increasing over the years; currently, nearly half of worldwide population suffers from obesity (Barnett, 2019;Blüher, 2019). Obesity and type II diabetes share similar metabolic alterations and are believed Mitochondrial disease inheritance 5 Hence, when a mutation in mtDNA arises, it can rapidly spread throughout the mitochondrial network. In this case, mutant (red circles) and wild-type (green circles) mtDNAs may co-exist, which is known as heteroplasmy. In comparison, homoplasmic mitochondria contain a single mtDNA genotype, either mutant or wild-type. Unless the mutation level exceeds a critical threshold necessary to cause a biochemical defect (i.e., above 60-90%; red mitochondria), the mutation effect will be masked by wild-type molecules (green mitochondria with both mutant and wild-type mtDNA). In germ cells (B), downregulation of fusion likely minimizes heteroplasmy within mitochondria, enhancing selection at the organellar level (i.e, stronger association between mitochondrial genotype and phenotype). In addition, decreased fusion leads to mitochondrial fragmentation, enhancing mtDNA segregation among embryonic cells. Hence, decreased levels of mtDNA in primordial germ cells (PGCs) makes possible selection at the cellular level (i.e., stronger association between mitochondrial genotype and cellular phenotype). Thus, as a result of selection against deleterious mutations, mature oocyte from the next generation may contain lower levels of mutant mtDNA.
to be highly correlated (Volaco et al., 2018). Transmission of these diseases to the following generations can occur through both parents, yet the maternal contribution has been shown to be larger (Shankar et al., 2008;Jungheim et al., 2010;Rattanatray et al., 2010;Ruager-Martin et al., 2010;Luzzo et al., 2012). In humans, for instance, offspring body mass index (BMI) correlated through three generations with maternal but not paternal BMI (Murrin et al., 2012). Likewise, maternal overnutrition in mice leads to offspring that are glucose intolerant and present increased cholesterol and body fat (Jungheim et al., 2010). These alterations can last up to the third generation, even when pups are fed a regular diet (Saben et al., 2016). Although epigenetic alterations in the nucleus play a major role in the regulation of these effects (Agarwal et al., 2018;Wang et al., 2018), other maternal factors have also been taken into account Saben et al., 2016).
Among the factors that contribute with maternal transmission of metabolic diseases, mitochondria are a main candidate giving their maternal-exclusive inheritance. In fact, mitochondrial defects in somatic tissues have been associated with obesity, diabetes and cardiovascular disease (Silva et al., 2000;Sarparanta et al., 2017;Ferey et al., 2019). For instance, mtDNA mutations impacting mitochondrial function and ATP production link with abnormal insulin release and b-cell development, insulin resistance, and diabetes (Poulton et al., 1998;Silva et al., 2000;Kaufman et al., 2015). In this context, Tanaka et al. (2002) demonstrated that single nucleotide polymorphisms in mtDNA (mtSNPs) may result in decreased energy expenditure, leading to obesity. Moreover, several studies have associated mtSNPs with type II diabetes and obesity (Rivera et al., 1999;Fuku et al., 2002;Okura et al., 2003;Guo et al., 2005). These mtSNPs can be located in genes coding for rRNAs, tRNAs, mRNAs (i.e., MT-CYB or MT-ATP6), and even in the non-coding region of mtDNA, the D-loop. Similarly, it was recently described that several mtDNA mutations in tRNAs lead to polycystic ovarian syndrome and metabolic alterations (Ding et al., 2018), both closely related to type II diabetes and obesity. Altogether, these findings provide evidence that mtDNA mutations may underpin maternal transmission of metabolic diseases.
Apart from mtDNA mutations, mitochondrial damage in oocytes has also been linked with increased risk of metabolic diseases in offspring. Obesity leads to increased lipid content in the follicular fluid, cumulus cells, and oocytes, which in turn damage organelles such as mitochondria and the ER (Wang et al., 2009;Wu et al., 2010;Fullston et al., 2015;Ruebel et al., 2017). Impaired ER function can lead to activation of the unfolded protein response (UPR) and Ca +2 release, further disrupting mitochondrial function (i.e., decreased Dym and increased ROS) and oocyte homeostasis (Wu et al., 2010Luzzo et al., 2012;Hou et al., 2016). Besides impacting oocyte competence and fertility Pasquariello et al., 2019), these mitochondrial abnormalities can be passed down to the following generations, increasing their risk to develop metabolic diseases (Saben et al., 2016). Hence, mice born to pregnant females under a high-fat/high-sucrose diet have impaired peripheral insulin signaling which associates with abnormal mitochondrial function and dynamics in skeletal muscle up to the third generation (Saben et al., 2016). Similar mitochondrial abnormalities were present in oocytes from the first and second generations, even though these were fed a regular diet (Saben et al., 2016). Therefore, apart from epigenetic alterations in the nucleus, mitochondria also contribute with the metabolic programing resulting from maternal overnutrition. Given that epigenetic marks in mtDNA regulate expression of this genome (Kobayashi et al., 2012;Sun et al., 2013;Sirard, 2019), it remains to be investigated whether these can also explain maternal transmission of metabolic diseases.

Treatment options for preventing mitochondrial disease transmission
Due to the poor understanding of the mechanisms regulating transmission of mitochondria-related diseases, there are few treatment options available to prevent their inheritance to the following generations (Craven et al., 2017). With respect to non-genetic alterations in mitochondria, the oocyte might benefit from treatments performed before fertilization, during the in vitro maturation. The idea is to expose the oocyte for a period of~24 h to drugs such as L-carnitine, rosiglitazone, salubrinal, or BGP-15, which potentially enhance mitochondria activity, decrease lipid content, and mitigate ER stress. In fact, treatments involving one or more of these drugs have been shown to mitigate the defects in the oocyte and the next generation (Wu et al., 2010Dunning and Robker, 2012;Liang et al., 2017). However, a major challenge in making these treatments available is to overcome the side effects of in vitro maturation (Lonergan and Fair, 2016;Yang and Chian, 2017). Given this is a critical period of oocyte development, which encompasses meiotic resumption from prophase I (dictyate) to metaphase II, any perturbation in oocyte homeostasis may lead to missegregation of chromosomes and aneuploidy (Greaney et al., 2017;Danadova et al., 2017). In addition, in vitro maturation on its own leads to metabolic alterations that mimic those of oocytes from obese donors (i.e., mitochondrial dysfunction and increased lipid content), potentially impacting the next generation (Farin et al., 2006;Li et al., 2014;del Collado et al., 2017a;del Collado et al., 2017b;Wang et al., 2018). Thus, these alternatives are not currently available in humans.
An alternative option to treat oocytes harboring mitochondria abnormalities, particularly those caused by mtDNA mutations, is known as mitochondrial replacement therapy (MRT; Figure 3). This method involves replacement of abnormal mitochondria in the oocyte by functional ones provided by a donated oocyte (Wolf et al., 2017). More specifically, ovulated oocytes at the metaphase-II stage are collected from both the patient and a "healthy" donor not containing mitochondrial abnormalities. With the aid of a micromanipulation set, the spindle from the donated oocyte is replaced by the patient's spindle. The resulting oocyte containing the patient's spindle and donated mitochondria is then fertilized to allow development to term. Provided that the large majority of mitochondria is replaced by donated ones, MRT has virtually the potential to prevent transmission of mitochondrial diseases. Yet,~1% of mitochondria from the patient's oocyte are transferred along with the spindle. This level can be even higher (up to 4%) when pronuclear zygotes are used instead of metaphase-II oocytes, which can lead in~15% of cases to a reversal back to the patient's mtDNA (Hyslop et al., 2016;Kang et al., 2016). Although hard to explain, rapid mtDNA segregation and bottleneck during preimplantation development might account for these quick shifts in mtDNA genotype (Lee et al., 2012a;Freyer et al., 2012). Alternatively, it has been proposed that a specific population of mtDNA is tagged in oocytes (i.e., from spindle-surrounding mitochondria) for replication during early development (Wolf et al., 2017). No matter the mechanism underlying these unexpected results, they highlight the need for careful studies before the clinical practice of MRT (Wolf et al., 2017;Craven et al., 2018).
With the advances in genome editing technologies, another potential strategy to prevent transmission of mitochondrial abnormalities is the targeted elimination of mutant mtDNA in oocytes or early embryos ( Figure 3). As a proof of concept, Reddy et al. (2015) used mitochondrial-targeted restriction endonucleases (mito-TALENs) to selectively eliminate mutant mtDNA in mice and humans. Although this strategy proved efficient,~10% of targeted molecules (i.e., mutant mtDNA) were left in oocytes, embryos and offspring produced after the use of mito-TALENs. Moreover, given that the mtDNA is not replicated during early embryogenesis (Pikó and Taylor, 1987;Thundathil et al., 2005;Cree et al., 2008), the use of mito-TALENs resulted in mtDNA-depleted embryos (Reddy et al., 2015). Although in the newborns the content of mtDNA was normal (Reddy et al., 2015), the lower levels of mtDNA (and likely of mitochondria too) in oocytes and embryos could lead to poorer developmental rates (Wai et al., 2010). Based on these uncertainties, mito-TALENs are not currently taken as a viable alternative to prevent transmission of mtDNA-linked diseases (Wolf et al., 2017).

Mitochondrial disease inheritance 7
Figure 3 -New technologies for preventing inheritance of mitochondrial diseases. The mitochondrial replacement therapy (MRT; A) proposes the replacement of a patient's mitochondria in oocytes by donor mitochondria. Towards that, mature oocytes arrested at the metaphase-II stage are collected from the patient and a donor. While the patient's oocytes are supposed to contain mutant (red) mitochondrial DNA (mtDNA), donor oocytes should contain only wild-type (green) mtDNA. Next, the spindle is removed from the patient's oocyte (donor karyoplast) to be injected into the donor oocyte from which the spindle was previously removed (donor cytoplast). Fertilization of the reconstructed oocyte should lead to a blastocyst, which can be used for embryonic stem cell (ESC) derivation. Although MRT allows transplantation of karyoplast with minimal carryover (~1%) of mutant mtDNA, recent data have provided evidence of a reversal in ESCs back to 100% mutant mtDNA (Hyslop et al., 2016;Kang et al., 2016). An alternative strategy to MRT is the nuclease-mediated elimination of mutant mtDNA (B), which relies on the use of mitochondrial-targeted restriction endonucleases (mito-TALENs). These nucleases are designed to selectively cut mutant mtDNA, but not wild-type molecules. However,~10% of targeted molecules were shown to be left uncut in newborns after use of mito-TALENs (Reddy et al., 2015).

Final considerations
Mitochondrial abnormalities have been linked with maternal transmission of important diseases in humans. Among these, mtDNA mutations in oocytes can be transmitted to the following generations and cause severe diseases. In addition, maternal obesity damages mitochondria in oocytes, leading to poor fertility and increased risk of metabolic diseases in offspring. Understanding how mitochondrial abnormalities are established and transmitted are of fundamental importance to mitigate their incidence in the human population. Moreover, treatment options involving manipulation of oocytes and early embryos are currently under consideration and may become available in the future to prevent transmission of mitochondria-associated diseases.

Conflict of Interests
The authors declare that there is no conflict of interest that could be perceived as prejudicial to the impartiality of the reported research.

Author contributions
MRC and MDC conceived the study. MRC and MDC reviewed previous publications. MRC, CHM, JDAN, MPG, AKP, FP and MDC wrote the manuscript. All authors read and approved the final version.