Unequal partitioning of bovine mitochondrial genotypes among siblings

The role of mtDNA in oocyte quality and embryo development

The mitochondrial genome resides in the mitochondria present in nearly all cell types. The porcine (Sus scrofa) mitochondrial genome is circa 16.7 kb in size and exists in the multimeric format in cells. Individual cell types have different numbers of mitochondrial DNA (mtDNA) copy number based on their requirements for ATP produced by oxidative phosphorylation. The oocyte has the largest number of mtDNA of any cell type. During oogenesis, the oocyte sets mtDNA copy number in order that sufficient copies are available to support subsequent developmental events. It also initiates a program of epigenetic patterning that regulates, for example, DNA methylation levels of the nuclear genome. Once fertilized, the nuclear and mitochondrial genomes establish synchrony to ensure that the embryo and fetus can complete each developmental milestone. However, altering the oocyte's mtDNA copy number by mitochondrial supplementation can affect the programming and gene expression profiles of the developing embryo and, in oocytes deficient of mtDNA, it appears to have a positive impact on the embryo development rates and gene expression profiles. Furthermore, mtDNA haplotypes, which define common maternal origins, appear to affect developmental outcomes and certain reproductive traits. Nevertheless, the manipulation of the mitochondrial content of an oocyte might have a developmental advantage.

Mitochondrial DNA variants segregate during human preimplantation development into genetically different cell lineages that are maintained postnatally

Humans present remarkable diversity in their mitochondrial DNA (mtDNA) in terms of variants across individuals as well as across tissues and even cells within one person. We have investigated the timing of the first appearance of this variant-driven mosaicism. For this, we deep-sequenced the mtDNA of 254 oocytes from 85 donors, 158 single blastomeres of 25 day-3 embryos, 17 inner cell mass and trophectoderm samples of 7 day-5 blastocysts, 142 bulk DNA and 68 single cells of different adult tissues. We found that day-3 embryos present blastomeres that carry variants only detected in that cell, showing that mtDNA mosaicism arises very early in human development. We classified the mtDNA variants based on their recurrence or uniqueness across different samples. Recurring variants had higher heteroplasmic loads and more frequently resulted in synonymous changes or were located in non-coding regions than variants unique to one oocyte or single embryonic cell. These differences were maintained through development, suggesting that the mtDNA mosaicism arising in the embryo is maintained into adulthood. We observed a decline in potentially pathogenic variants between day 3 and day 5 of development, suggesting early selection. We propose a model in which closely clustered mitochondria carrying specific mtDNA variants in the ooplasm are asymmetrically distributed throughout the cell divisions of the preimplantation embryo, resulting in the earliest form of mtDNA mosaicism in human development.

Mitogenomic analysis of a 50-generation chicken pedigree reveals a rapid rate of mitochondrial evolution and evidence for paternal mtDNA inheritance

Mitochondrial genomes represent a valuable source of data for evolutionary research, but studies of their short-term evolution have typically been limited to invertebrates, humans and laboratory organisms. Here we present a detailed study of 12 mitochondrial genomes that span a total of 385 transmissions in a well-documented 50-generation pedigree in which two lineages of chickens were selected for low and high juvenile body weight. These data allowed us to test the hypothesis of time-dependent evolutionary rates and the assumption of strict maternal mitochondrial transmission, and to investigate the role of mitochondrial mutations in determining phenotype. The identification of a non-synonymous mutation in ND4L and a synonymous mutation in CYTB, both novel mutations in Gallus, allowed us to estimate a molecular rate of 3.13 × 10(-7) mutations/site/year (95% confidence interval 3.75 × 10(-8)-1.12 × 10(-6)). This is substantially higher than avian rate estimates based upon fossil calibrations. Ascertaining which of the two novel mutations was present in an additional 49 individuals also revealed an instance of paternal inheritance of mtDNA. Lastly, an association analysis demonstrated that neither of the point mutations was strongly associated with the phenotypic differences between the two selection lines. Together, these observations reveal the highly dynamic nature of mitochondrial evolution over short time periods.

Segregation of Naturally Occurring Mitochondrial DNA Variants in a Mini-Pig Model

The maternally inherited mitochondrial genome (mtDNA) is present in multimeric form within cells and harbors sequence variants (heteroplasmy). While a single mtDNA variant at high load can cause disease, naturally occurring variants likely persist at low levels across generations of healthy populations. To determine how naturally occurring variants are segregated and transmitted, we generated a mini-pig model, which originates from the same maternal ancestor. Following next-generation sequencing, we identified a series of low-level mtDNA variants in blood samples from the female founder and her daughters. Four variants, ranging from 3% to 20%, were selected for validation by high-resolution melting analysis in 12 tissues from 31 animals across three generations. All four variants were maintained in the offspring, but variant load fluctuated significantly across the generations in several tissues, with sex-specific differences in heart and liver. Moreover, variant load was persistently reduced in high-respiratory organs (heart, brain, diaphragm, and muscle), which correlated significantly with higher mtDNA copy number. However, oocytes showed increased heterogeneity in variant load, which correlated with increased mtDNA copy number during in vitro maturation. Altogether, these outcomes show that naturally occurring mtDNA variants segregate and are maintained in a tissue-specific manner across generations. This segregation likely involves the maintenance of selective mtDNA variants during organogenesis, which can be differentially regulated in oocytes and preimplantation embryos during maturation.

Mitochondrial DNA in Day 3 embryo culture medium is a novel, non-invasive biomarker of blastocyst potential and implantation outcome

In assisted reproduction technology, embryo competence is routinely evaluated on morphological criteria. Over the last decade, efforts in improving non-invasive embryo assessment have looked into the secretome of embryos. Human embryos release genomic DNA (gDNA) and mitochondrial DNA (mtDNA) into the culture medium, and the mtDNA/gDNA ratio is significantly correlated with embryo fragmentation. Here, we investigate whether mtDNA/gDNA ratio in embryo spent medium is correlated with blastulation potential and implantation. The mtDNA/gDNA ratio was assessed in 699 Day 3 culture media by quantitative polymerase chain reaction (qPCR) to investigate its correlation with embryo morphology, blastocyst development and implantation. A logistic regression model evaluated whether mtDNA/gDNA ratio in the secretome may improve the prediction of blastulation. We found that embryos that successfully developed into blastocysts exhibited a significantly higher mtDNA/gDNA ratio in the culture medium compared with those that arrest (P = 0.0251), and mtDNA/gDNA, combined with morphological grading, has the potential to predict blastulation better than morphology alone (P = 0.02). Moreover, mtDNA/gDNA ratio was higher in the media from good-quality embryos that reached the full blastocyst stage on Day 5 compared with those that developed more slowly (P < 0.0001). With respect to blastocyst morphology, higher trophectoderm quality was associated with a higher mtDNA/gDNA ratio in the culture medium. Finally, a high mtDNA/gDNA ratio in spent medium was associated with successful implantation outcome (P = 0.0452) of good-quality embryos. In summary, the mtDNA/gDNA ratio in the Day 3 embryo secretome, in combination with morphological grading, may be a novel, non-invasive, early biomarker to improve identification of viable embryos with high developmental potential.

Tissue- and cell-type-specific manifestations of heteroplasmic mtDNA 3243A>G mutation in human induced pluripotent stem cell-derived disease model

Mitochondrial DNA (mtDNA) mutations manifest with vast clinical heterogeneity. The molecular basis of this variability is mostly unknown because the lack of model systems has hampered mechanistic studies. We generated induced pluripotent stem cells from patients carrying the most common human disease mutation in mtDNA, m.3243A>G, underlying mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome. During reprogramming, heteroplasmic mtDNA showed bimodal segregation toward homoplasmy, with concomitant changes in mtDNA organization, mimicking mtDNA bottleneck during epiblast specification. Induced pluripotent stem cell-derived neurons and various tissues derived from teratomas manifested cell-type specific respiratory chain (RC) deficiency patterns. Similar to MELAS patient tissues, complex I defect predominated. Upon neuronal differentiation, complex I specifically was sequestered in perinuclear PTEN-induced putative kinase 1 (PINK1) and Parkin-positive autophagosomes, suggesting active degradation through mitophagy. Other RC enzymes showed normal mitochondrial network distribution. Our data show that cellular context actively modifies RC deficiency manifestation in MELAS and that autophagy is a significant component of neuronal MELAS pathogenesis.

Dynamic regulation of mitochondrial genome maintenance in germ cells

Mitochondria play a crucial role in the development and function of germ cells. Mitochondria contain a maternally inherited genome that should be transmitted to offspring without reactive oxygen species‐induced damage during germ line development. Germ cells are also involved in the mitochondrial DNA (mtDNA) bottleneck; thus, the appropriate regulation of mtDNA in these cells is very important for this characteristic transmission. In this review, we focused on unique regulation of the mitochondrial genome in animal germ cells; paternal elimination and the mtDNA bottleneck in females. We also summarized the mitochondrial nucleoid factors involved in various mtDNA regulation pathways. Among them, mitochondrial transcription factor A (TFAM), which has pleiotropic and essential roles in mtDNA maintenance, appears to have putative roles in germ cell regulation.

Lack of Structural Variation but Extensive Length Polymorphisms and Heteroplasmic Length Variations in the Mitochondrial DNA Control Region of Highly Inbred Crested Ibis, Nipponia nippon

The animal mitochondrial DNA (mtDNA) length polymorphism and heteroplasmy are accepted to be universal. Here we report the lack of structural variation but the presence of length polymorphism as well as heteroplasmy in mtDNA control region of an endangered avian species - the Crested Ibis (Nipponia nippon). The complete control region was directly sequenced while the distribution pattern and inheritance of the length variations were examined using both direct sequencing and genotyping of the PCR fragments from captive birds with pedigrees, wild birds and a historical specimen. Our results demonstrated that there was no structural variation in the control region, however, different numbers of short tandem repeats with an identical motif of CA3CA2CA3 at the 3'-end of the control region determined the length polymorphisms among and heteroplasmy within individual birds. There were one to three predominant fragments in every bird; nevertheless multiple minor fragments coexist in all birds. These extremely high polymorphisms were suggested to have derived from the 'replication slippage' of a perfect microsatellite evolution following the step-wise mutational model. The patterns of heteroplasmy were found to be shifted between generations and among siblings but rather stable between blood and feather samples. This study provides the first evidence of a very extensive mtDNA length polymorphism and heteroplasmy in the highly inbred Crested Ibis which carries an mtDNA genome lack of structural genetic diversity. The analysis of pedigreed samples also sheds light on the transmission of mtDNA length heteroplasmy in birds following the genetic bottleneck theory. Further research focusing on the generation and transmission of particular mtDNA heteroplasmy patterns in single germ line of Crested Ibis is encouraged by this study.

Rapid mitochondrial DNA segregation in primate preimplantation embryos precedes somatic and germline bottleneck

The timing and mechanisms of mitochondrial DNA (mtDNA) segregation and transmission in mammals are poorly understood. Genetic bottleneck in female germ cells has been proposed as the main phenomenon responsible for rapid intergenerational segregation of heteroplasmic mtDNA. We demonstrate here that mtDNA segregation occurs during primate preimplantation embryogenesis resulting in partitioning of mtDNA variants between daughter blastomeres. A substantial shift toward homoplasmy occurred in fetuses and embryonic stem cells (ESCs) derived from these heteroplasmic embryos. We also observed a wide range of heteroplasmic mtDNA variants distributed in individual oocytes recovered from these fetuses. Thus, we present here evidence for a previously unknown mtDNA segregation and bottleneck during preimplantation embryo development, suggesting that return to the homoplasmic condition can occur during development of an individual organism from the zygote to birth, without a passage through the germline.

Sex-linked mitochondrial behavior during early embryo development in Ruditapes philippinarum (Bivalvia Veneridae) a species with the Doubly Uniparental Inheritance (DUI) of mitochondria

In most metazoans mitochondria are inherited maternally. However, in some bivalve molluscs, two mitochondrial lineages are present: one transmitted through females (F-type), the other through males (M-type). This unique system is called Doubly Uniparental Inheritance (DUI) of mitochondria. In DUI species, M-type mitochondria have to invade the germ line of male embryos during development, otherwise sperm would transmit F-type mtDNA and DUI would fail. The mechanisms by which sperm mitochondria enter the germ line are still unknown. To address this question, we traced the movement of spermatozoon mitochondria (M-type) in embryos of the DUI species Ruditapes philippinarum by fertilizing eggs with sperm stained with the mitochondrial-specific vital dye MitoTracker Green. As in Mytilus DUI species, in R. philippinarum the distribution of sperm mitochondria follows two different patterns: an aggregated one in which these organelles locate near the first cleavage furrow, and a dispersed one in which sperm mitochondria are scattered. The presence of the two mitochondrial patterns in these taxa, together with their absence in species with Strictly Maternal Inheritance (SMI), confirms that their occurrence is related to DUI. Moreover, a Real-Time qPCR analysis showed that neither M-type nor F-type mitochondria undergo replication boosts in the earliest embryo development. This is the first study on sex-linked mtDNA copy number carried out by qPCR analysis on embryos of a DUI species and the first time the segregation patterns of sperm mitochondria are described in a DUI system other than Mytilus.

Induced pluripotent stem cells generated from diabetic patients with mitochondrial DNA A3243G mutation

AIMS/HYPOTHESIS

The aim of this study was to generate induced pluripotent stem (iPS) cells from patients with mitochondrial DNA (mtDNA) mutation.

METHODS

Skin biopsies were obtained from two diabetic patients with mtDNA A3243G mutation. The fibroblasts thus obtained were infected with retroviruses encoding OCT4 (also known as POU5F1), SOX2, c-MYC (also known as MYC) and KLF4. The stem cell characteristics were investigated and the mtDNA mutation frequencies evaluated by Invader assay.

RESULTS

From the two diabetic patients we isolated four and ten putative mitochondrial disease-specific iPS (Mt-iPS) clones, respectively. Mt-iPS cells were cytogenetically normal and positive for alkaline phosphatase activity, with the pluripotent stem cell markers being detectable by immunocytochemistry. The cytosine guanine dinucleotide islands in the promoter regions of OCT4 and NANOG were highly unmethylated, indicating epigenetic reprogramming to pluripotency. Mt-iPS clones were able to differentiate into derivatives of all three germ layers in vitro and in vivo. The Mt-iPS cells exhibited a bimodal degree of mutation heteroplasmy. The mutation frequencies decreased to an undetectable level in six of 14 clones, while the others showed several-fold increases in mutation frequencies (51-87%) compared with those in the original fibroblasts (18-24%). During serial cell culture passage and after differentiation, no recurrence of the mutation or no significant changes in the levels of heteroplasmy were seen.

CONCLUSIONS/INTERPRETATION

iPS cells were successfully generated from patients with the mtDNA A3243G mutation. Mutation-rich, stable Mt-iPS cells may be a suitable source of cells for human mitochondrial disease modelling in vitro. Mutation-free iPS cells could provide an unlimited, disease-free supply of cells for autologous transplantation therapy.

Polarity and asymmetry during mouse oogenesis and oocyte maturation

Cell polarity and asymmetry play a fundamental role in embryo development. The unequal segregation of determinants, cues, and activities is the major event in the differentiation of cell fate and function in all multicellular organisms. In oocytes, polarity and asymmetry in the distribution of different molecules are prerequisites for the progression and proper outcome of embryonic development. The mouse oocyte, like the oocytes of other mammals, seems to apply a less stringent strategy of polarization than other vertebrates. The mouse embryo undergoes a regulative type of development, which permits the full rectification of development even if the embryo loses up to half of its cells or its size is experimentally doubled during the early stages of embryogenesis. Such pliability is strongly related to the proper oocyte polarization before fertilization. Thus, the molecular mechanisms leading to the development and maintenance of oocyte polarity must be included in any fundamental understanding of the principles of embryo development. In this chapter, we provide an overview of current knowledge regarding the development and maintenance of polarity and asymmetry in the distribution of organelles and molecules in the mouse oocyte. Curiously, the mouse oocyte becomes polarized at least twice during ontogenesis; the question of how this phenomenon is achieved and what role it might play is addressed in this chapter.

Segregation of mtDNA throughout human embryofetal development: m.3243A>G as a model system

Mitochondrial DNA (mtDNA) mutations cause a wide range of serious diseases with high transmission risk and maternal inheritance. Tissue heterogeneity of the heteroplasmy rate (“mutant load”) accounts for the wide phenotypic spectrum observed in carriers. Owing to the absence of therapy, couples at risk to transmit such disorders commonly ask for prenatal (PND) or preimplantation diagnosis (PGD). The lack of data regarding heteroplasmy distribution throughout intrauterine development, however, hampers the implementation of such procedures. We tracked the segregation of the m.3243A > G mutation (MT-TL1 gene) responsible for the MELAS syndrome in the developing embryo/fetus, using tissues and cells from eight carrier females, their 38 embryos and 12 fetuses. Mutant mtDNA segregation was found to be governed by random genetic drift, during oogenesis and somatic tissue development. The size of the bottleneck operating for m.3243A > G during oogenesis was shown to be individual-dependent. Comparison with data we achieved for the m.8993T > G mutation (MT-ATP6 gene), responsible for the NARP/Leigh syndrome, indicates that these mutations differentially influence mtDNA segregation during oogenesis, while their impact is similar in developing somatic tissues. These data have major consequences for PND and PGD procedures in mtDNA inherited disorders. Hum Mutat 32:116–125, 2011. © 2010 Wiley-Liss, Inc.

The strength and timing of the mitochondrial bottleneck in salmon suggests a conserved mechanism in vertebrates

In most species mitochondrial DNA (mtDNA) is inherited maternally in an apparently clonal fashion, although how this is achieved remains uncertain. Population genetic studies show not only that individuals can harbor more than one type of mtDNA (heteroplasmy) but that heteroplasmy is common and widespread across a diversity of taxa. Females harboring a mixture of mtDNAs may transmit varying proportions of each mtDNA type (haplotype) to their offspring. However, mtDNA variants are also observed to segregate rapidly between generations despite the high mtDNA copy number in the oocyte, which suggests a genetic bottleneck acts during mtDNA transmission. Understanding the size and timing of this bottleneck is important for interpreting population genetic relationships and for predicting the inheritance of mtDNA based disease, but despite its importance the underlying mechanisms remain unclear. Empirical studies, restricted to mice, have shown that the mtDNA bottleneck could act either at embryogenesis, oogenesis or both. To investigate whether the size and timing of the mitochondrial bottleneck is conserved between distant vertebrates, we measured the genetic variance in mtDNA heteroplasmy at three developmental stages (female, ova and fry) in chinook salmon and applied a new mathematical model to estimate the number of segregating units (N(e)) of the mitochondrial bottleneck between each stage. Using these data we estimate values for mtDNA Ne of 88.3 for oogenesis, and 80.3 for embryogenesis. Our results confirm the presence of a mitochondrial bottleneck in fish, and show that segregation of mtDNA variation is effectively complete by the end of oogenesis. Considering the extensive differences in reproductive physiology between fish and mammals, our results suggest the mechanism underlying the mtDNA bottleneck is conserved in these distant vertebrates both in terms of it magnitude and timing. This finding may lead to improvements in our understanding of mitochondrial disorders and population interpretations using mtDNA data.

The implications of mitochondrial DNA copy number regulation during embryogenesis

Mutations of mitochondrial DNA (mtDNA) cause a wide array of multisystem disorders, particularly affecting organs with high energy demands. Typically only a proportion of the total mtDNA content is mutated (heteroplasmy), and high percentage levels of mutant mtDNA are associated with a more severe clinical phenotype. MtDNA is inherited maternally and the heteroplasmy level in each one of the offspring is often very different to that found in the mother. The mitochondrial genetic bottleneck hypothesis was first proposed as the explanation for these observations over 20 years ago. Although the precise bottleneck mechanism is still hotly debated, the regulation of cellular mtDNA content is a key issue. Here we review current understanding of the factors regulating the amount of mtDNA within cells and discuss the relevance of these findings to our understanding of the inheritance of mtDNA heteroplasmy.

Enlightenment of yeast mitochondrial homoplasmy: diversified roles of gene conversion

Mitochondria have their own genomic DNA. Unlike the nuclear genome, each cell contains hundreds to thousands of copies of mitochondrial DNA (mtDNA). The copies of mtDNA tend to have heterogeneous sequences, due to the high frequency of mutagenesis, but are quickly homogenized within a cell ("homoplasmy") during vegetative cell growth or through a few sexual generations. Heteroplasmy is strongly associated with mitochondrial diseases, diabetes and aging. Recent studies revealed that the yeast cell has the machinery to homogenize mtDNA, using a common DNA processing pathway with gene conversion; i.e., both genetic events are initiated by a double-stranded break, which is processed into 3' single-stranded tails. One of the tails is base-paired with the complementary sequence of the recipient double-stranded DNA to form a D-loop (homologous pairing), in which repair DNA synthesis is initiated to restore the sequence lost by the breakage. Gene conversion generates sequence diversity, depending on the divergence between the donor and recipient sequences, especially when it occurs among a number of copies of a DNA sequence family with some sequence variations, such as in immunoglobulin diversification in chicken. MtDNA can be regarded as a sequence family, in which the members tend to be diversified by a high frequency of spontaneous mutagenesis. Thus, it would be interesting to determine why and how double-stranded breakage and D-loop formation induce sequence homogenization in mitochondria and sequence diversification in nuclear DNA. We will review the mechanisms and roles of mtDNA homoplasmy, in contrast to nuclear gene conversion, which diversifies gene and genome sequences, to provide clues toward understanding how the common DNA processing pathway results in such divergent outcomes.

Bovine mtDNA D-loop haplotypes exceed mutations in number despite reduced recombination: an effective alternative for identity control

Mitochondrial (mt) DNA D-loop heterogeneity, haplotype distribution and possible sub-population structures within the relevant populations are important for DNA-based traceability. To gain insight into this distribution, we compared 1515 Bos taurus mtDNA D-loop sequences available from GenBank to 219 sequences that we sequenced de novo. A pronounced ambiguous trace typical of C-track length heteroplasmy was encountered in 5% of the samples, which were excluded from the analysis. Previously undescribed mutations and haplotypes were observed in 6% and 63% of the sequences, respectively. B. taurus haplotypes divided into the taurus, indicus and grunniens types and 302 variable sites formed the 858 taurus haplotypes detected. Fifty-five sites displayed a complex level of variation. As each level represents an independent mutation event, a total of 399 mutations were traced, which could potentially explain independent formation of less than half (47%) of the haplotypes encountered: most haplotypes were derived from different combinations of these mutations. We suggest that a mutational hotspot may explain these results and discuss the usefulness of mtDNA for identity and maternity assurance.

Strict sex-specific mtDNA segregation in the germ line of the DUI species Venerupis philippinarum (Bivalvia: Veneridae)

Doubly Uniparental Inheritance (DUI) is one of the most striking exceptions to the common rule of standard maternal inheritance of metazoan mitochondria. In DUI, two mitochondrial genomes are present, showing different transmission routes, one through eggs (F-type) and the other through sperm (M-type). In this paper, we report results from a multiplex real-time quantitative polymerase chain reaction analysis on the Manila clam Venerupis philippinarum (formerly Tapes philippinarum). We quantified M- and F-types in somatic tissues, gonads, and gametes. Nuclear and external reference sequences were used, and the whole experimental process was designed to avoid any possible cross-contamination. In most male somatic tissues, the M-type is largely predominant: This suggests that the processes separating sex-linked mitochondrial DNAs (mtDNAs) in somatic tissues are less precise than in other DUI species. In the germ line, we evidenced a strict sex-specific mtDNA segregation because both sperm and eggs do carry exclusively M- and F-types, respectively, an observation that is in contrast with a previous analysis on Mytilus galloprovincialis. More precisely, whereas two mtDNAs are present in the whole gonad, only the sex-specific one is detected in gametes. Because of this, we propose that the mtDNA transmission is achieved through a three-checkpoint process in V. philippinarum. The cytological mechanisms of male mitochondria segregation in males and degradation in females during the embryo development (here named Checkpoint #1 and Checkpoint #2) are already well known for DUI species; a Checkpoint #3 would act when primordial germ cells (PGCs) are first formed and would work in both males and females. We believe that Checkpoint #3 is a mere variation of the "mitochondrial bottleneck" in species with standard maternal inheritance, established when their PGCs separate during embryo cleavage.

Transmission of mitochondrial DNA diseases and ways to prevent them

Recent reports of strong selection of mitochondrial DNA (mtDNA) during transmission in animal models of mtDNA disease, and of nuclear transfer in both animal models and humans, have important scientific implications. These are directly applicable to the genetic management of mtDNA disease. The risk that a mitochondrial disorder will be transmitted is difficult to estimate due to heteroplasmy—the existence of normal and mutant mtDNA in the same individual, tissue, or cell. In addition, the mtDNA bottleneck during oogenesis frequently results in dramatic and unpredictable inter-generational fluctuations in the proportions of mutant and wild-type mtDNA. Pre-implantation genetic diagnosis (PGD) for mtDNA disease enables embryos produced by in vitro fertilization (IVF) to be screened for mtDNA mutations. Embryos determined to be at low risk (i.e., those having low mutant mtDNA load) can be preferentially transferred to the uterus with the aim of initiating unaffected pregnancies. New evidence that some types of deleterious mtDNA mutations are eliminated within a few generations suggests that women undergoing PGD have a reasonable chance of generating embryos with a lower mutant load than their own. While nuclear transfer may become an alternative approach in future, there might be more difficulties, ethical as well as technical. This Review outlines the implications of recent advances for genetic management of these potentially devastating disorders.

Is the mitochondrial cloud the selection machinery for preferentially transmitting wild-type mtDNA between generations? Rewinding Müller's ratchet efficiently

In animal mitochondrial DNA inheritance, it remains largely unclear where the mitochondrial genetic bottleneck localizes and how it works in rewinding Müller's ratchet. In a variety of different animals germ plasm mRNAs typically aggregate along with numerous mitochondria to form the mitochondrial cloud (MC) during oogenesis. The MC has been found to serve as messenger transport organizer for germ plasm mRNAs. Germ plasm RNAs in MC will specifically distribute to the primordial germ cells of the future embryo. It has been proposed that the MC might be the site where selected mitochondria accumulate for specific transmission to grandchildren but this idea received relatively little attention and the criterion by which mitochondria are selected remains unknown. Our recent results in zebrafish provided further evidence for selective mitochondria accumulation in the MC by showing that mitochondria with high-inner membrane potential tend to be recruited preferentially into the MC, and these mitochondria are transported along with germ plasm to the cortex of the vegetal pole. By analyzing the composition, behavior and functions of the MC, and in reviewing related literature, we found strong support for the proposition that the MC corresponds to the position and function of the mitochondrial genetic bottleneck.

Does mtDNA nucleoid organization impact aging?

Somatic cells in tissue culture package several copies of mitochondrial DNA (mtDNA) in aggregates known as nucleoids that appear to be remarkably stable. The clustering of multiple mtDNA genomes in a single nucleoid complex may promote the progressive age-related accumulation of deletion and point mutations in mtDNA in many somatic tissues, particularly in post-mitotic cells. In contrast, oocytes appear to have the ability to select against deleterious mutations in mtDNA, at least in mice. This fundamental difference suggests that oocytes may be better able to detect and remove defective mtDNA genomes than somatic cells, possibly due in part to the simpler organization of the mtDNA in smaller nucleoids. These observations suggest the hypothesis that a complex nucleoid structure containing several mtDNA molecules may impair the ability of the cell to select against deleterious mtDNA mutations, thereby contributing to age-related mitochondrial dysfunction.

New evidence confirms that the mitochondrial bottleneck is generated without reduction of mitochondrial DNA content in early primordial germ cells of mice

In mammals, observations of rapid shifts in mitochondrial DNA (mtDNA) variants between generations have led to the creation of the bottleneck theory for the transmission of mtDNA. The bottleneck could be attributed to a marked decline of mtDNA content in germ cells giving rise to the next generation, to a small effective number of mtDNA segregation units resulting from homoplasmic nucleoids rather than the single mtDNA molecule serving as the units of segregation, or to the selective transmission of a subgroup of the mtDNA population to the progeny. We have previously determined mtDNA copy number in single germ cells and shown that the bottleneck occurs without the reduction in germline mtDNA content. Recently one study suggested that the bottleneck is driven by a remarkable decline of mtDNA copies in early primordial germ cells (PGCs), while another study reported that the mtDNA genetic bottleneck results from replication of a subpopulation of the mtDNA genome during postnatal oocyte maturation and not during embryonic oogenesis, despite a detected a reduction in mtDNA content in early PGCs. To clarify these contradictory results, we examined the mtDNA copy number in PGCs isolated from transgenic mice expressing fluorescent proteins specifically in PGCs as in the aforementioned two other studies. We provide clear evidence to confirm that no remarkable reduction in mtDNA content occurs in PGCs and reinforce that the bottleneck is generated without reduction of mtDNA content in germ cells.

The mitochondrial DNA genetic bottleneck results from replication of a subpopulation of genomes

In mammals, mitochondrial DNA (mtDNA) sequence variants are observed to segregate rapidly between generations despite the high mtDNA copy number in the oocyte. This has led to the concept of a genetic bottleneck for the transmission of mtDNA, but the mechanism remains contentious. Several studies have suggested that the bottleneck occurs during embryonic development, as a result of a marked reduction in germline mtDNA copy number. Mitotic segregation of mtDNAs during preimplantation, or during the expansion of primordial germ cells (PGCs) before they colonize the gonad, is thought to account for the increase in genotypic variance observed among mature oocytes from heteroplasmic mothers. This view has, however, been challenged by studies suggesting that the bottleneck occurs without a reduction in germline mtDNA content. To resolve this controversy, we measured mtDNA heteroplasmy and copy number in single germ cells isolated from heteroplasmic mice. By directly tracking the evolution of mtDNA genotypic variance during oogenesis, we show that the genetic bottleneck occurs during postnatal folliculogenesis and not during embryonic oogenesis.

Rapid concerted evolution in animal mitochondrial DNA

Recombinational genetic processes are thought to be rare in the uniparentally inherited mitochondrial (mt) DNA molecules of vertebrates and other animals. Here, however, we document extremely rapid concerted microevolution, probably mediated by frequent gene conversion events, of duplicated sequences in the mtDNA control region of mangrove killifishes (Kryptolebias marmoratus). In local populations, genetic distances between paralogous loci within an individual were typically smaller (and often zero) than those between orthologous loci in different specimens. These findings call for the recognition of concerted evolution as a microevolutionary process and gene conversion as a likely recombinational force in animal mtDNA. The previously unsuspected power of these molecular phenomena could greatly impact mtDNA dynamics within germ cell lineages and in local animal populations.

The layered structure of human mitochondrial DNA nucleoids

Mitochondrial DNA (mtDNA) occurs in cells in nucleoids containing several copies of the genome. Previous studies have identified proteins associated with these large DNA structures when they are biochemically purified by sedimentation and immunoaffinity chromatography. In this study, formaldehyde cross-linking was performed to determine which nucleoid proteins are in close contact with the mtDNA. A set of core nucleoid proteins is found in both native and cross-linked nucleoids, including 13 proteins with known roles in mtDNA transactions. Several other metabolic proteins and chaperones identified in native nucleoids, including ATAD3, were not observed to cross-link to mtDNA. Additional immunofluorescence and protease susceptibility studies showed that an N-terminal domain of ATAD3 previously proposed to bind to the mtDNA D-loop is directed away from the mitochondrial matrix, so it is unlikely to interact with mtDNA in vivo. These results are discussed in relation to a model for a layered structure of mtDNA nucleoids in which replication and transcription occur in the central core, whereas translation and complex assembly may occur in the peripheral region.

The development of novel quantification assay for mitochondrial DNA heteroplasmy aimed at preimplantation genetic diagnosis of Leigh encephalopathy

PURPOSE

To perform preimplantation genetic diagnosis (PGD) of Leigh encephalopathy, we developed a rapid and reliable quantification assay for the percentage of T8993G mtDNA mutation and analyzed various specimens.

METHODS

We prepared the standard curve by measuring serial proportion of 8993T/G cloned plasmid DNA using real-time PCR, and measured (1) mutant DNA (known proportions by PCR-RFLP), (2) single lymphocytes from 46% mutant carrier, (3) 123 blastomeres from 20 abnormal embryos.

RESULTS

(1) These were within -5 - +6% error range, (2) mean 44.3%(11-70%), (3) Five embryos harbored T8993G mutation (4-22%). Embryos from same person indicated different degrees of heteroplasmy, and blastomeres from same embryo demonstrated limited dispersion of heteroplasmy (2-11%).

CONCLUSIONS

(1) This method provides rapid and reliable PGD for Leigh encephalopathy. (2) The variable heteroplasmy with somatic mitosis was suggested. (3) T8993G mutation was existed in undeveloped embryo, and the bottleneck theory was supported. The limited heteroplasmy dispersion of blastomeres from same embryo also supported reliability of PGD for T8993G mutation.

The mitochondrial bottleneck occurs without reduction of mtDNA content in female mouse germ cells

Observations of rapid shifts in mitochondrial DNA (mtDNA) variants between generations prompted the creation of the bottleneck theory. A prevalent hypothesis is that a massive reduction in mtDNA content during early oogenesis leads to the bottleneck. To test this, we estimated the mtDNA copy number in single germline cells and in single somatic cells of early embryos in mice. Primordial germ cells (PGCs) show consistent, moderate mtDNA copy numbers across developmental stages, whereas primary oocytes demonstrate substantial mtDNA expansion during early oocyte maturation. Some somatic cells possess a very low mtDNA copy number. We also demonstrated that PGCs have more than 100 mitochondria per cell. We conclude that the mitochondrial bottleneck is not due to a drastic decline in mtDNA copy number in early oogenesis but rather to a small effective number of segregation units for mtDNA in mouse germ cells. These results provide new information for mtDNA segregation models and for understanding the recurrence risks for mtDNA diseases.

Human therapeutic cloning (NTSC)

Human therapeutic cloning or nuclear transfer stem cells (NTSC) to produce patient-specific stem cells, holds considerable promise in the field of regenerative medicine. The recent withdrawal of the only scientific publications claiming the successful generation of NTSC lines afford an opportunity to review the available research in mammalian reproductive somatic cell nuclear transfer (SCNT) with the goal of progressing human NTSC. The process of SCNT is prone to epigenetic abnormalities that contribute to very low success rates. Although there are high mortality rates in some species of cloned animals, most surviving clones have been shown to have normal phenotypic and physiological characteristics and to produce healthy offspring. This technology has been applied to an increasing number of mammals for utility in research, agriculture, conservation, and biomedicine. In contrast, attempts at SCNT to produce human embryonic stem cells (hESCs) have been disappointing. Only one group has published reliable evidence of success in deriving a cloned human blastocyst, using an undifferentiated hESC donor cell, and it failed to develop into a hESC line. When optimal conditions are present, it appears that in vitro development of cloned and parthenogenetic embryos, both of which may be utilized to produce hESCs, may be similar to in vitro fertilized embryos. The derivation of ESC lines from cloned embryos is substantially more efficient than the production of viable offspring. This review summarizes developments in mammalian reproductive cloning, cell-to-cell fusion alternatives, and strategies for oocyte procurement that may provide important clues facilitating progress in human therapeutic cloning leading to the successful application of cell-based therapies utilizing autologous hESC lines.

Germline bottlenecks, biparental inheritance and selection on mitochondrial variants: a two-level selection model

Selection on mitochondrial mutations potentially occurs at different levels: at the mitochondria, cell, and organism levels. Several factors affect the strength of selection at these different levels; in particular, mitochondrial bottlenecks during germline development and reduced paternal transmission decrease the genetic variance within cells, while they increase the variance between cells and between organisms, thus decreasing the strength of selection within cells and increasing the strength of selection between cells and organisms. However, bottlenecks and paternal transmission also affect the effective mitochondrial population size, thus affecting genetic drift. In this article, we use a simple model of a unicellular life cycle to investigate the effects of bottlenecks and paternal transmission on the probability of fixation of mitochondrial mutants and their frequency at mutation-selection equilibrium. We find that bottlenecks and reduced paternal transmission decrease the mean frequency of alleles with sm>sc (approximately), where sm and sc are the strengths of selection for an allele within and between cells, respectively, and increase the frequency of alleles with sm<sc. Effects on fixation probabilities are different; for example, bottlenecks reduce the fixation probability of mutants with sm>0 (unless sm is very small relative to sc) and increase the fixation probability of mutants with sm<0.

Comparison of two approaches to nuclear transfer in the bovine: hand-made cloning with modifications and the conventional nuclear transfer technique

The aim of the present study was to compare the in vitro and in vivo developmental competence of hand-made cloning (HMC) embryos with the conventional nuclear transfer (NT) method using five somatic cell lines and in vitro-fertilised (IVF; control) embryos. Modifications to the HMC procedure included fusion efficiency optimisation, effect of cytoplasmic volume and cloned embryo aggregation. The developmental competence of blastocysts from each of the treatment groups and cell lines used was assessed following transfer to 345 recipients. Vitrification was also used to enable management of recipient resources and to assess the susceptibility of membranes to cryopreservation following zona removal. Increasing cytoplasmic volume to 150% or aggregating two embryos improved the blastocyst development rate and increased the total cell number. Although HMC embryo transfers established a significantly higher pregnancy rate on Day 30 than fresh IVF or NT embryo transfers, the overall outcome in terms of cloned live births derived from either fresh or vitrified/thawed HMC or NT embryo transfers across the five cell lines did not differ. The birth and continued survival of clones produced with HMC technology with equivalent efficiency to NT shows that it can be used as an alternative method for the generation of cloned offspring in the bovine.

Birth of a cloned calf derived from a vitrified hand-made cloned embryo

The hand-made cloning (HMC) technique describes a simplified nuclear transfer process without the need for micromanipulators. The technique involves manual bisection of zona-free oocytes, selection of cytoplasts by Hoechst staining and fusion of a single somatic cell and two cytoplasts. In this proof-of-principle experiment, the objective was to examine the developmental competence of HMC embryos following embryo transfer. Modifications to the original method include not selecting of matured oocytes and simultaneous fusion of cytoplasts and karyoplast. Blastocyst rates for embryos cultured in the glass oviduct system as singles (10.5%; 24/228) or in pairs (16.1%; 36/224) did not differ significantly. Fresh and vitrified–thawed blastocysts were transferred to 16 synchronised recipients (three to four embryos per recipient). Ultrasound examination on Days 35–45 showed an initial pregnancy rate of 43.8% (7/16) and a pregnancy rate >8 months of 12.5% (2/16). A male cloned calf (42 kg) derived from a vitrified HMC blastocyst was delivered by Caesarean section on Day 271. The birth and ongoing survival (15 months, 243 kg) of a healthy and apparently normal calf, combining both HMC and vitrification technologies, provides a ‘proof of principle’ of the technology and a promising alternative to traditional nuclear-transfer techniques.

Mitochondrial DNA polymorphisms and fertility in beef cattle

Four hundred and twenty-two beef cattle of two different breeds (purebred Hereford and composite multibreed) were characterized by polymerase chain reaction-restriction fragment length polymorphism, using the restriction enzymes ApaI, AvaII, HindIII, PstI, SpeI, SspI and TaqI in two regions (the D-loop and the ND-5 gene) of mitochondrial DNA. The association between molecular haplotypes and records on calving rate, defined as the mean number of live calves born per year over 4 years, were examined by analysis of variance. A significant association was found between calving rate and mitochondrial polymorphisms in both breeds. This may have implications for genetically improving cow fertility.

Transmitochondrial differences and varying levels of heteroplasmy in nuclear transfer cloned cattle

To assess the extent of cytoplasmic genetic variability in cloned cattle produced by nuclear transplantation procedures, we investigated 29 individuals of seven male cattle clones (sizes 2-6) from two different commercial sources. Restriction enzyme and direct sequence analysis of mitochondrial DNA (mtDNA) detected a total of 12 different haplotypes. Transmitochondrial individuals (i.e., animals which share identical nuclei but have different mitochondrial DNA) were detected in all but one of the clones, demonstrating that mtDNA variation among cloned cattle is a very common phenomenon which prevents true genetic identity. The analyses also showed that the cytoplasmic genetic status of some investigated individuals and clones is further complicated by heteroplasmy (more than one mtDNA type in an individual). The relative proportions of different mtDNA-types in two animals with mild heteroplasmy were estimated at 2:98% and 4:96% in DNA samples derived from blood. This is in agreement with values expected from karyoplast-cytoplast volume ratios. In contrast, the mtDNA haplotype proportions observed in six other heteroplasmic animals of two different clones ranged from 21:79% to 57:43%, reflecting a marked increase in donor blastomere mtDNA contributions. These results suggest that mtDNA type of donor embryos and recipient oocytes used in nuclear transfer cattle cloning should be controlled to obtain true clones with identical nuclear and cytoplasmic genomes.

Genetic analysis of temperature-dependent transmission of mitochondrial DNA in Drosophila

In artificially induced mitochondrial DNA (mtDNA) heteroplasmy in Drosophila, the effects of chromosome substitution on temperature-dependent selection in mtDNA transmission were investigated. Using two strains of D. melanogaster, bw;e11 and y;bw;st, which showed a different temperature dependency in mtDNA transmission, chromosomes were substituted reciprocally, and mtDNA of D. mauritiana was introduced into each newly constructed strain. For each heteroplasmy, the transmission of mtDNA was examined at 25 degrees C and 19 degrees C. When either the second or the third chromosome of the y;bw;st strain was substituted with that of the bw;e11 strain, the temperature-dependent selection in mtDNA transmission was altered. The selection was not changed when either the second or the third chromosome of the bw;e11 strain was substituted with that of the y;bw;st strain, or even when both the second and the third chromosomes of the bw;e11 strain were substituted with those of the y;bw;st strain. These results suggest that the temperature-dependent selection in mtDNA transmission is co-operatively regulated by gene products that are encoded by the X, second and third chromosomes.

Microinjection of mitochondria into zygotes creates a model for studying the inheritance of mitochondrial DNA during preimplantation development

OBJECTIVE

To determine the effect of mutant mitochondria on preimplantation embryo development and of preimplantation embryo development on the survival of mutant mitochondrial DNA.

DESIGN

Laboratory research.

SETTING

Academic research laboratory.

PATIENT(S)

None.

INTERVENTION(S)

Mutant and wild-type mitochondria, fractionated from tissue obtained from a patient with MELAS syndrome, a mitochondrial disease, were microinjected into mouse zygotes. Control zygotes received either no injection or sham injection.

MAIN OUTCOME MEASURE(S)

Preimplantation embryo development and survival of mutant mitochondrial DNA as determined by polymerase chain reaction analysis.

RESULT(S)

After microinjection into zygotes, the MELAS mutation could be identified by polymerase chain reaction until the hatched blastocyst stage of embryo development. The survival of MELAS-injected zygotes, observed for 4 days after injection, did not differ from the survival of zygotes injected with wild-type mitochondria or from the survival of uninjected or sham-injected controls.

CONCLUSION(S)

It appears that preimplantation embryo development does not screen out mitochondrial DNA mutations introduced into fertilized oocytes, and low levels of mutant mitochondrial DNA do not disrupt early embryo development.

Heteroplasmy and organelle gene dynamics

This study assesses factors that influence the rates of change of organelle gene diversity and the maintenance of heteroplasmy. Losses of organelle gene diversity within individuals via vegetative segregation during ontogeny are paramount to resultant spatial and temporal patterns. Steady-state losses of organelle variation from the zygote to the gametes are determined by the effective number of organelles, which will be approximately equal to the number of intracellular organelles if random segregation prevails. Both rapid increases in organelle number after zygote formation and reductions at germ lines will reduce variation within individuals. Terminal reductions in organelles must be to very low copy numbers (<5) for substantial losses in variation to occur rapidly. Nonrandom clonal expansion and vegetative segregation during gametogenesis may be effective in reducing genetic variation in gametes. If organelles are uniparentally inherited, the asymptotic expectations for effective numbers of gametes and spatial differentiation will be identical for homoplasmic and heteroplasmic conditions. The rate of attainment of asymptote for heteroplasmic organelles, however, is governed by the rate of loss of variation during ontogeny. With sex-biased dispersal, the effective number of gametes is maximized when the proportional contributions of the sex having the higher dispersal rate are low.

Evidence from human oocytes for a genetic bottleneck in an mtDNA disease

We have examined oocytes from a patient with Kearn-Sayre syndrome caused by mtDNA rearrangements. In mtDNA diseases, mutant and wild-type mtDNA frequently coexist in affected individuals (the condition of heteroplasmy). The proportion of mutant mtDNA transmitted from mother to offspring is variable because of a genetic bottleneck, and the "dose" of mutant mtDNA received influences the severity of the phenotype. The feasibility of prenatal diagnosis is critically dependent on the nature and timing of this bottleneck. Significant levels of rearranged mtDNA were detectable in the majority of the patient's oocytes, by use of multiplex PCR, with wide variation, in the levels of mutant and wild-type molecules, between individual oocytes. We also used length variation in a homopolymeric C tract, which is often heteroplasmic in normal controls, to identify founder subpopulations of mtDNAs in this patient's oocytes. We present direct evidence that the number of segregating units (n) is three to five orders of magnitude less than the number of mitochondria in the human female oocyte. In some cases, the best estimate of n may correspond to a single mitochondrion, if it is assumed that intergenerational transmission of mtDNA can be treated as a single sampling event. The bottleneck appears to contribute a major component of the variable transmission from mother to oocyte, in this patient and in a control. That this bottleneck had occurred by the time that oocytes were mature advances the prospects for prenatal diagnosis of mtDNA diseases.

Genotypic and phenotypic changes in exhaustively grown cell lines from mitochondrial cytopathy patients

Understanding the pathobiology of mitochondrial (mt) DNA diseases involves both characterization of the effects of individual mutations on respiratory function and elucidation of the changes in mutation load and distribution (energy mosaicism) over serial cell generations. Whether a given mutation is stably maintained, or increases or decreases with cell growth, is one of the determinants as to whether a particular tissue will be affected by oxidative phosphorylation failure. In this study, we correlated mt genotype with biochemical phenotype in myoblasts from patients with pathogenic mtDNA mutations. The dominant process detected was a progressive elimination of mutant mtDNA genomes concomitant with an improvement in respiratory chain activity, suggesting that energetically normal cells have a growth advantage over those with a high mutation load. We propose that this elimination is by biased distribution of wild-type mtDNA to daughter cells, and that a similar mechanism could operate in vivo and contribute to both the clinical expression of mt disease and the maintenance of a predominantly wild-type mt genome pool across generations.

Skewed segregation of the mtDNA nt 8993 (T-->G) mutation in human oocytes

Rapid changes in mtDNA variants between generations have led to the bottleneck theory, which proposes a dramatic reduction in mtDNA numbers during early oogenesis. We studied oocytes from a woman with heteroplasmic expression of the mtDNA nt 8993 (T-->G) mutation. Of seven oocytes analyzed, one showed no evidence of the mutation, and the remaining six had a mutant load > 95%. This skewed expression of the mutation in oocytes is not compatible with the conventional bottleneck theory. A possible explanation is that, during amplification of mtDNA in the developing oocyte, mtDNA from one mitochondrion is preferentially amplified. Thus, subsequent mature oocytes may contain predominantly wild-type or mutant mitochondrial genomes.

A high observed substitution rate in the human mitochondrial DNA control region

The rate and pattern of sequence substitutions in the mitochondrial DNA (mtDNA) control region (CR) is of central importance to studies of human evolution and to forensic identity testing. Here, we report a direct measurement of the intergenerational substitution rate in the human CR. We compared DNA sequences of two CR hypervariable segments from close maternal relatives, from 134 independent mtDNA lineages spanning 327 generational events. Ten substitutions were observed, resulting in an empirical rate of 1/33 generations, or 2.5/site/Myr. This is roughly twenty-fold higher than estimates derived from phylogenetic analyses. This disparity cannot be accounted for simply by substitutions at mutational hot spots, suggesting additional factors that produce the discrepancy between very near-term and long-term apparent rates of sequence divergence. The data also indicate that extremely rapid segregation of CR sequence variants between generations is common in humans, with a very small mtDNA bottleneck. These results have implications for forensic applications and studies of human evolution.

Mitochondrial genotype segregation in a mouse heteroplasmic lineage produced by embryonic karyoplast transplantation

Mitochondrial genotypes have been shown to segregate both rapidly and slowly when transmitted to consecutive generations in mammals. Our objective was to develop an animal model to analyze the patterns of mammalian mitochondrial DNA (mtDNA) segregation and transmission in an intraspecific heteroplasmic maternal lineage to investigate the mechanisms controlling these phenomena. Heteroplasmic progeny were obtained from reconstructed blastocysts derived by transplantation of pronuclearstage karyoplasts to enucleated zygotes with different mtDNA. Although the reconstructed zygotes contained on average 19% mtDNA of karyoplast origin, most progeny contained fewer mtDNA of karyoplast origin and produced exclusively homoplasmic first generation progeny. However, one founder heteroplasmic adult female had elevated tissue heteroplasmy levels, varying from 6% (lung) to 69% (heart), indicating that stringent replicative segregation had occurred during mitotic divisions. First generation progeny from the above female were all heteroplasmic, indicating that, despite a meiotic segregation, they were derived from heteroplasmic founder oocytes. Some second and third generation progeny contained exclusively New Zealand Black/BINJ mtDNA, suggesting but not confirming, an origin from an homoplasmic oocyte. Moreover, several third to fifth generation individuals maintained mtDNA from both mouse strains, indicating a slow or persistent segregation pattern characterized by diminished tissue and litter variability beyond second generation progeny. Therefore, although some initial lineages appear to segregate rapidly to homoplasmy, within two generations other lineages transmit stable amounts of both mtDNA molecules, supporting a mechanism where mitochondria of different origin may fuse, leading to persistent intraorganellar heteroplasmy.

Heteroplasmic point mutations in the human mtDNA control region

As part of an investigation of the fixation mechanisms of mtDNA mutations in humans, we sequenced the first hypervariable segment of the control region in 180 twin pairs and found evidence of site heteroplasmy in 4 pairs. Significant levels of two mitochondrial haplotypes differing by a single point mutation were found in two MZ pairs, and within each pair, both members had similar levels of heteroplasmy. Two DZ pairs were found in which the predominant mitochondrial haplotype differed within the pair. We measured proportions of mitochondrial haplotypes within two twin pairs and their maternal relatives, using primer extension. In both maternal lineages, most family members were heteroplasmic, and the proportions of each genotype varied widely in different individuals. We used the changes in haplotype proportions within mother-offspring pairs to calculate the size range of potential bottlenecks in mitochondrial numbers occurring during development of the offspring. In most individuals, the most likely effective bottleneck sizes ranged from 3 to 20 segregating units, though in two individuals a small bottleneck was very unlikely and there was no upper limit on its possible size. We also used the data from this study, together with unpublished data from other populations, to estimate the frequency of site heteroplasmy in normal human populations. From this, we calculated that the rate of mutation and fixation in the first hypervariable segment of the human mtDNA control region is between 1.2 x 10(-6) and 2.7 x 10(-5) per site per generation. This range is in good agreement with published estimates calculated by other methods.

Prospects for DNA-based prenatal diagnosis of mitochondrial disorders

Mitochondria have their own DNA which is maternally inherited. Mitochondrial DNA (mtDNA) diseases are extremely variable because of the genetics of mtDNA and the unique pathogenesis of these disorders. This makes predicting the prognosis and the transmission of mtDNA disorders difficult. While mtDNA polymorphisms at a single base position are common, the overwhelming majority of the mitochondrial genomes within a single human individual are usually identical. When there is a point mutation difference between a mother and her offspring, there may be a complete switching of mtDNA type within a single generation. It is generally assumed that there is a genetic bottleneck whereby a single or small number of founder mtDNA(s) populate the organism, but it is not known at which stages the restriction/amplification of mtDNA subtype(s) occur, and this uncertainty impedes antenatal diagnosis for mtDNA disorders. Autosomally inherited disorders of mitochondrial function may be caused by mutations in genes for the components of the respiratory chain and for the machinery of mitochondrial biogenesis, which are nuclear-encoded. Accurate diagnosis of these disorders is important as prenatal diagnosis is available in a minority of cases.

Random genetic drift in the female germline explains the rapid segregation of mammalian mitochondrial DNA

Mitochondrial DNA (mtDNA) is maternally inherited in mammals. Despite the high genome copy number in mature oocytes (10(5)) and the relatively small number of cell divisions in the female germline, mtDNA sequence variants segregate rapidly between generations. To investigate the molecular basis for this apparent paradox we created lines of heteroplasmic mice carrying two mtDNA genotypes. We show that the pattern of segregation can be explained by random genetic drift occurring in early oogenesis, and that the effective number of segregating units for mtDNA is approximately 200 in mice. These results provide the basis for estimating recurrence risks for mitochondrial disease due to pathogenic mtDNA mutations and for predicting the rate of fixation of neutral mtDNA mutations in maternal lineages.