Quasi-Mendelian paternal inheritance of mitochondrial DNA: A notorious artifact, or anticipated behavior?

Reanalysis of mtDNA mutations of human primordial germ cells (PGCs) reveals NUMT contamination and suggests that selection in PGCs may be positive

The resilience of the mitochondrial genome (mtDNA) to a high mutational pressure depends, in part, on negative purifying selection in the germline. A paradigm in the field has been that such selection, at least in part, takes place in primordial germ cells (PGCs). Specifically, Floros et al. (Nature Cell Biology 20: 144-51) reported an increase in the synonymity of mtDNA mutations (a sign of purifying selection) between early-stage and late-stage PGCs. We re-analyzed Floros' et al. data and determined that their mutational dataset was significantly contaminated with single nucleotide variants (SNVs) derived from a nuclear sequence of mtDNA origin (NUMT) located on chromosome 5. Contamination was caused by co-amplification of the NUMT sequence by cross-specific PCR primers. Importantly, when we removed NUMT-derived SNVs, the evidence of purifying selection was abolished. In addition to bulk PGCs, Floros et al. reported the analysis of single-cell late-stage PGCs, which were amplified with different sets of PCR primers that cannot amplify the NUMT sequence. Accordingly, there were no NUMT-derived SNVs among single PGC mutations. Interestingly, single PGC mutations show adecreaseof synonymity with increased intracellular mutant fraction. More specifically, nonsynonymous mutations show faster intracellular genetic drift towards higher mutant fraction than synonymous ones. This pattern is incompatible with predominantly negative selection. This suggests that germline selection of mtDNA mutations is a complex phenomenon and that the part of this process that takes place in PGCs may be predominantly positive. However counterintuitive, positive germline selection of detrimental mtDNA mutations has been reported previously andpotentially may be evolutionarily advantageous.

The contribution of mitochondrial DNA alterations to aging, cancer, and neurodegeneration

Mitochondrial DNA (mtDNA) is as a double-stranded molecule existing in hundreds to thousands copies in cells depending on cell metabolism and exposure to endogenous and/or environmental stressors. The coordination of mtDNA replication and transcription regulates the pace of mitochondrial biogenesis to guarantee the minimum number of organelles per cell. mtDNA inheritance follows a maternal lineage, although bi-parental inheritance has been reported in some species and in the case of mitochondrial diseases in humans. mtDNA mutations (e.g., point mutations, deletions, copy number variations) have been identified in the setting of several human diseases. For instance, sporadic and inherited rare disorders involving the nervous system as well higher risk of developing cancer and neurodegenerative conditions, including Parkinson's and Alzheimer's disease, have been associated with polymorphic mtDNA variants. An accrual of mtDNA mutations has also been identified in several tissues and organs, including heart and muscle, of old experimental animals and humans, which may contribute to the development of aging phenotypes. The role played by mtDNA homeostasis and mtDNA quality control pathways in human health is actively investigated for the possibility of developing targeted therapeutics for a wide range of conditions.

A Systematic Review of the Impact of Mitochondrial Variations on Male Infertility

According to current estimates, infertility affects one in four couples trying to conceive. Primary or secondary infertility can be due either to both partners or only to the man or the woman. Up to 15% of infertility cases in men can be attributed to genetic factors that can lead to irreversible partial or complete spermatogenic arrest. The increased use of assisted reproductive technology (ART) has provided not only insights into the causes of male infertility but also afforded a diagnostic tool to detect and manage this condition among couples. Genes control a variety of physiological attributes, such as the hypothalamic–pituitary–gonadal axis, development, and germ cell differentiation. In the era of ART, it is important to understand the genetic basis of infertility so as to provide the most tailored therapy and counseling to couples. Genetic factors involved in male infertility can be chromosome abnormalities or single-gene disorders, mitochondrial DNA (mtDNA) mutations, Y-chromosome deletions, multifactorial disorders, imprinting disorders, or endocrine disorders of genetic origin. In this review, we discuss the role of mitochondria and the mitochondrial genome as an indicator of sperm quality and fertility.

Mitochondrial DNA: Consensuses and Controversies

In the course of its short history, mitochondrial DNA (mtDNA) has made a long journey from obscurity to the forefront of research on major biological processes. mtDNA alterations have been found in all major disease groups, and their significance remains the subject of intense research. Despite remarkable progress, our understanding of the major aspects of mtDNA biology, such as its replication, damage, repair, transcription, maintenance, etc., is frustratingly limited. The path to better understanding mtDNA and its role in cells, however, remains torturous and not without errors, which sometimes leave a long trail of controversy behind them. This review aims to provide a brief summary of our current knowledge of mtDNA and highlight some of the controversies that require attention from the mitochondrial research community.

Role of Granulosa Cells in the Aging Ovarian Landscape: A Focus on Mitochondrial and Metabolic Function

Mitochondria are at the intersection of aging and fertility, with research efforts centered largely on the role that these specialized organelles play in the relatively rapid decline in oocyte quality that occurs as females approach reproductive senescence. In addition to various roles in oocyte maturation, fertilization, and embryogenesis, mitochondria are critical to granulosa cell function. Herein, we provide a review of the literature pertaining to the role of mitochondria in granulosa cell function, with emphasis on how mitochondrial aging in granulosa cells may impact reproduction in female mammals.

Mitochondrial DNA in forensic use

Genetic analysis of mitochondrial DNA (mtDNA) has always been a useful tool for forensic geneticists, mainly because of its ubiquitous presence in biological material, even in the absence of nuclear DNA. Sequencing, however, is not a skill that is part of the routine forensic analysis because of the relative rarity of requests, and the need for retention of necessary skill sets and associated accreditation issues. While standard Sanger sequencing may be relatively simple, many requests are made in the face of compromised biological samples. Newer technologies, provided through massively parallel sequencing (MPS), will increase the opportunity for scientists to include this tool in their routine, particularly for missing person investigations. MPS has also enabled a different approach to sequencing that can increase sensitivity in a more targeted approach. In these circumstances it is likely that only a laboratory that specialises in undertaking forensic mtDNA analysis will be able to take these difficult cases forward, more so because reviews of the literature have revealed significantly high levels of typing errors in publications reporting mtDNA sequences. The forensic community has set out important guidelines, not only in the practical aspects of analysis, but also in the interpretation of that sequence to ensure that accurate comparisons can be made. Analysis of low-level, compromised and ancient DNA is not easy, however, as contamination is extremely difficult to eliminate and circumstances leading to sequencing errors are all too easily introduced. These problems, and solutions, are discussed in the article in relation to several historic cases.

Interpreting NUMTs in forensic genetics: Seeing the forest for the trees

Nuclear mitochondrial DNA (mtDNA) segments (NUMTs) were discovered shortly after sequencing the first human mitochondrial genome. They have earlier been considered to represent archaic elements of ancient insertion events, but modern sequencing technologies and growing databases of mtDNA and NUMT sequences confirm that they are abundant and some of them phylogenetically young. Here, we build upon mtDNA/NUMT review articles published in the mid 2010 s and focus on the distinction of NUMTs and other artefacts that can be observed in aligned sequence reads, such as mixtures (contamination), point heteroplasmy, sequencing error and cytosine deamination. We show practical examples of the effect of the mtDNA enrichment method on the representation of NUMTs in the mapped sequence data and discuss methods to bioinformatically filter NUMTs from mtDNA reads.

Evidence for multi-copy Mega-NUMTs in the human genome

The maternal mode of mitochondrial DNA (mtDNA) inheritance is central to human genetics. Recently, evidence for bi-parental inheritance of mtDNA was claimed for individuals of three pedigrees that suffered mitochondrial disorders. We sequenced mtDNA using both direct Sanger and Massively Parallel Sequencing in several tissues of eleven maternally related and other affiliated healthy individuals of a family pedigree and observed mixed mitotypes in eight individuals. Cells without nuclear DNA, i.e. thrombocytes and hair shafts, only showed the mitotype of haplogroup (hg) V. Skin biopsies were prepared to generate ρ° cells void of mtDNA, sequencing of which resulted in a hg U4c1 mitotype. The position of the Mega-NUMT sequence was determined by fluorescence in situ hybridization and two different quantitative PCR assays were used to determine the number of contributing mtDNA copies. Thus, evidence for the presence of repetitive, full mitogenome Mega-NUMTs matching haplogroup U4c1 in various tissues of eight maternally related individuals was provided. Multi-copy Mega-NUMTs mimic mixtures of mtDNA that cannot be experimentally avoided and thus may appear in diverse fields of mtDNA research and diagnostics. We demonstrate that hair shaft mtDNA sequencing provides a simple but reliable approach to exclude NUMTs as source of misleading results.

Impact of Mitochondrial Genetic Variants in ND1, ND2, ND5, and ND6 Genes on Sperm Motility and Intracytoplasmic Sperm Injection (ICSI) Outcomes

Sperm mitochondrial dysfunction causes the generation of an insufficient amount of energy needed for sperm motility. This will affect sperm fertilization capacity, and thus, most asthenozoospermic men usually require assisted reproductive techniques. The etiology of asthenozoospermia remains largely unknown. The current study aimed to investigate the effect of mitochondrial genetic variants on sperm motility and intracytoplasmic sperm injection (ICSI) outcomes. A total of 150 couples from the ICSI cycle were enrolled in this study. One hundred five of the male partners were asthenozoospermic patients, and they were subdivided into three groups according to their percentage of sperm motility, while forty-five of the male partners were normozoospermic. Genetic variants were screened using direct Sanger’s sequencing in four mitochondrial genes (nicotinamide adenine dinucleotide hydrogen (NADH) dehydrogenase 1 (ND1), NADH dehydrogenase 2 (ND2), NADH dehydrogenase 5 (ND5), and NADH dehydrogenase 6 (ND6)). We identified three significant variants: 13708G>A (rs28359178) in ND5, 4216T>C (rs1599988) in ND1, and a novel 12506T>A in ND5 with P values 0.006, 0.036, and 0.013, respectively. The medians of sperm motility, fertilization rate, embryo cleavage score, and embryo quality score were significantly different between men showing 4216T>C, 12506T>A, 13708G>A and wild type, Mann-Whitney P values for the differences in the medians were < 0.05 in all of them. The results from this study suggest that 13708G>A, 12506T>A, and 4216 T>C variants in sperm mitochondrial DNA negatively affect sperm motility and ICSI outcomes.

Mechanisms for sperm mitochondrial removal in embryos

BACKGROUND

Different animal species have different characteristics regarding the transmission of mitochondrial DNA. While some species have biparental mitochondrial inheritance, others have developed pathways to remove paternal mtDNA. These pathways guarantee the uniparental mitochondrial inheritance, so far well known in mammals, avoiding heteroplasmy, which may have the potential to cause certain mitochondrial diseases in the offspring.

SCOPE OF REVIEW

This review aims to address the main mechanisms that involve mitochondrial degradation in different animal species, as well as to describe what is present in the literature on the mechanisms involved in mitochondrial inheritance.

MAJOR CONCLUSIONS

Two theories are proposed to explain the uniparental inheritance of mtDNA: (i) active degradation, where mechanisms for paternal mitochondrial DNA elimination involve mitochondrial degradation pathway by autophagy and, in some species, may also involve the endocytic degradation pathway; and (ii) passive dilution, where the paternal mitochondria are diluted in the cells of the embryo according to cell division, until becoming undetectable.

GENERAL SIGNIFICANCE

This work brings a wide review of the already published evidence on mitochondrial inheritance in the animal kingdom and the possible mechanisms to mtDNA transmission already described in literature.

Mitochondrial DNA and Neurodegeneration: Any Role for Dietary Antioxidants?

The maintenance of the mitochondrial function is essential in preventing and counteracting neurodegeneration. In particular, mitochondria of neuronal cells play a pivotal role in sustaining the high energetic metabolism of these cells and are especially prone to oxidative damage. Since overproduction of reactive oxygen species (ROS) is involved in the pathogenesis of neurodegeneration, dietary antioxidants have been suggested to counteract the detrimental effects of ROS and to preserve the mitochondrial function, thus slowing the progression and limiting the extent of neuronal cell loss in neurodegenerative disorders. In addition to their role in the redox-system homeostasis, mitochondria are unique organelles in that they contain their own genome (mtDNA), which acts at the interface between environmental exposures and the molecular triggers of neurodegeneration. Indeed, it has been demonstrated that mtDNA (including both genetics and, from recent evidence, epigenetics) might play relevant roles in modulating the risk for neurodegenerative disorders. This mini-review describes the link between the mitochondrial genome and cellular oxidative status, with a particular focus on neurodegeneration; moreover, it provides an overview on potential beneficial effects of antioxidants in preserving mitochondrial functions through the protection of mtDNA.

Inheritance of mitochondrial DNA in humans: implications for rare and common diseases

The first draft human mitochondrial DNA (mtDNA) sequence was published in 1981, paving the way for two decades of discovery linking mtDNA variation with human disease. Severe pathogenic mutations cause sporadic and inherited rare disorders that often involve the nervous system. However, some mutations cause mild organ-specific phenotypes that have a reduced clinical penetrance, and polymorphic variation of mtDNA is associated with an altered risk of developing several late-onset common human diseases including Parkinson's disease. mtDNA mutations also accumulate during human life and are enriched in affected organs in a number of age-related diseases. Thus, mtDNA contributes to a wide range of human pathologies. For many decades, it has generally been accepted that mtDNA is inherited exclusively down the maternal line in humans. Although recent evidence has challenged this dogma, whole-genome sequencing has identified nuclear-encoded mitochondrial sequences (NUMTs) that can give the false impression of paternally inherited mtDNA. This provides a more likely explanation for recent reports of 'bi-parental inheritance', where the paternal alleles are actually transmitted through the nuclear genome. The presence of both mutated and wild-type variant alleles within the same individual (heteroplasmy) and rapid shifts in allele frequency can lead to offspring with variable severity of disease. In addition, there is emerging evidence that selection can act for and against specific mtDNA variants within the developing germ line, and possibly within developing tissues. Thus, understanding how mtDNA is inherited has far-reaching implications across medicine. There is emerging evidence that this highly dynamic system is amenable to therapeutic manipulation, raising the possibility that we can harness new understanding to prevent and treat rare and common human diseases where mtDNA mutations play a key role.