High-throughput sequencing in mitochondrial DNA research

High-throughput fluorescence quantification method based on inner filter effect and fluorescence imaging analysis

The application of the inner filter effect (IFE) in fluorescent substance determination is gaining popularity. In this paper, a theory of the fluorescence distribution along with the excitation light path is derived from our previous research about the spatial micro-element method. According to the relationship between the summation of fluorescence intensities along the vertical direction at a certain position on the excitation light path and the position, a high-concentration and wide-range fluorescent substance quantification method based on the IFE and fluorescence imaging analysis is proposed. Correspondingly, a high-throughput fluorescent substance quantification detection system is constructed. In order to validate the method, solutions of rhodamine B in different concentrations are used for principle validation, concentration prediction, and experimental investigation on the influence of integration time and lens distortion. The high-throughput system enables the simultaneous measurement of six samples, realizing the high-concentration and wide-range quantification of rhodamine B (100-600 mg/L) with high precision (R2 = 0.9992, MRE = 2.34 %). By setting the filter wheel, the system can measure the concentration of fluorescent substances with different emission wavelengths. The improvement of experimental device is expected to reduce the single sample capacity to tens of microliters and increase the overall sample quantity to tens or even hundreds. The proposed method and system are beneficial to fluorescence measurement in fields such as biomedicine and dye research and to the improvement of high-throughput fluorescence quantitative PCR instruments.

An Exploratory Case-Control Study for Mitochondrial DNA G10398A in Bipolar I Disorder Patients with a Family History of Affective Disorders

BACKGROUND

The mitochondrial DNA (mtDNA) G10398A polymorphism has been associated with bipolar disorder (BD). It leads to an amino acid substitution within NADH dehydrogenase subunit, thereby altering the mitochondrial complex I function. This exploratory case-control study assesses the association of mtDNA G10398A with the risk of BD and its relationship to clinical variables in Indian patients.

METHODS

Cases met the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition diagnosis of BD-I in remission and had a family history of BD or recurrent unipolar disorder in biological relatives. The healthy controls (HC) had no known illness and were screened negative for Family Interview for Genetic Studies. Participants were assessed using Clinical Pro forma, NIMH-Life Chart Method and Alda lithium response scale. The mtDNA G10398A was assessed with real-time polymerase chain reaction using TaqMan assay.

RESULTS

A total of 82 participants were recruited across cases and controls, with 42 patients (50% with maternal history) and 40 healthy individuals with similar demographic profiles. The mean age of onset was 25.16 (standard deviation [SD] 7.6) years, with illness for 11.59 years (SD: 7.18). Allele A was found in 50% of cases compared to 32.5% HC (odds ratio = 2.08; 95% confidence interval [CI]: 0.85-5.09). Findings remain non-significant for patients with maternal mood disorders (allele A: 38.9%; 21/42). Cases with allele G had significantly higher body mass index (BMI) (P = 0.008) than those with allele A.

CONCLUSION

The study adds information on mtDNA 10398A amongst Indian patient samples and healthy individuals. No significant group difference was found with respect to mtDNA G10398A. The positive association of allele G with higher BMI has potential clinical relevance that can be further investigated in larger samples.

Application of Next-Generation Sequencing (NGS) Techniques for Selected Companion Animals

Next-Generation Sequencing (NGS) techniques have revolutionized veterinary medicine for cats and dogs, offering insights across various domains. In veterinary parasitology, NGS enables comprehensive profiling of parasite populations, aiding in understanding transmission dynamics and drug resistance mechanisms. In infectious diseases, NGS facilitates rapid pathogen identification, characterization of virulence factors, and tracking of outbreaks. Moreover, NGS sheds light on metabolic processes by elucidating gene expression patterns and metabolic pathways, essential for diagnosing metabolic disorders and designing tailored treatments. In autoimmune diseases, NGS helps identify genetic predispositions and molecular mechanisms underlying immune dysregulation. Veterinary oncology benefits from NGS through personalized tumor profiling, mutation analysis, and identification of therapeutic targets, fostering precision medicine approaches. Additionally, NGS plays a pivotal role in veterinary genetics, unraveling the genetic basis of inherited diseases and facilitating breeding programs for healthier animals. Physiological investigations leverage NGS to explore complex biological systems, unraveling gene-environment interactions and molecular pathways governing health and disease. Application of NGS in treatment planning enhances precision and efficacy by enabling personalized therapeutic strategies tailored to individual animals and their diseases, ultimately advancing veterinary care for companion animals.

Mitochondrial dysfunction and oxidative stress in Alzheimer's disease, and Parkinson's disease, Huntington's disease and Amyotrophic Lateral Sclerosis -An updated review

Misfolded proteins in the central nervous system can induce oxidative damage, which can contribute to neurodegenerative diseases in the mitochondria. Neurodegenerative patients face early mitochondrial dysfunction, impacting energy utilization. Amyloid-ß and tau problems both have an effect on mitochondria, which leads to mitochondrial malfunction and, ultimately, the onset of Alzheimer's disease. Cellular oxygen interaction yields reactive oxygen species within mitochondria, initiating oxidative damage to mitochondrial constituents. Parkinson's disease, linked to oxidative stress, α-synuclein aggregation, and inflammation, results from reduced brain mitochondria activity. Mitochondrial dynamics profoundly influence cellular apoptosis via distinct causative mechanisms. The condition known as Huntington's disease is characterized by an expansion of polyglutamine, primarily impactingthe cerebral cortex and striatum. Research has identified mitochondrial failure as an early pathogenic mechanism contributing to HD's selective neurodegeneration. The mitochondria are organelles that exhibit dynamism by undergoing fragmentation and fusion processes to attain optimal bioenergetic efficiency. They can also be transported along microtubules and regulateintracellular calcium homeostasis through their interaction with the endoplasmic reticulum. Additionally, the mitochondria produce free radicals. The functions of eukaryotic cells, particularly in neurons, have significantly deviated from the traditionally assigned role of cellular energy production. Most of them areimpaired in HD, which may lead to neuronal dysfunction before symptoms manifest. This article summarises the most important changes in mitochondrial dynamics that come from neurodegenerative diseases including Alzheimer's, Parkinson's, Huntington's and Amyotrophic Lateral Sclerosis. Finally, we discussed about novel techniques that can potentially treat mitochondrial malfunction and oxidative stress in four most dominating neuro disorders.

Two Complete Mitochondrial Genomes of Leptobrachium (Anura: Megophryidae: Leptobrachiinae): Characteristics, Population Divergences, and Phylogenetic Implications

The mustache toads Leptobrachium boringii and Leptobrachium liui are two attractive species in Megophryidae, in which adult males have mustache-like keratinized nuptial spines on their upper lip. However, both are under threat due to multiple factors, of which scientific studies are still very limited. In this study, two new complete mitochondrial genomes of L. boringii and L. liui were sequenced, assembled, and annotated based on next-generation sequencing. The mitogenome lengths of L. boringii and L. liui were found to be 17,100 and 17,501 bp, respectively, with both containing 13 protein coding genes, 23 tRNAs, 2 rRNAs, and 1 non-coding control region. Nucleotide diversity analyses indicate that atp8, atp6, and nad2 showed higher nucleotide diversity than cox1, cox3, and cytb. The intraspecific genetic distances among three different populations of L. boringii exceed 4%, and those between two populations of L. liui reach 7%. Phylogenetic relationships support their division into two subfamilies of Megophryidae (Leptobrachiinae and Megophryinae) as well as two species groups within Leptobrachium, corresponding to the number of keratinized nuptial spines (10-48 in the L. boringii species group vs. 2-6 in the L. liui species group). The two new mitogenomes reported in this study provide valuable data for future molecular evolutionary and conservation studies of the genus Leptobrachium and other Megophryidae toads.

Comparative analysis between high-grade serous ovarian cancer and healthy ovarian tissues using single-cell RNA sequencing

Introduction

High-grade serous ovarian cancer (HGSOC) is the most common histological subtype of ovarian cancer, and is associated with high mortality rates.

Methods

In this study, we analyzed specific cell subpopulations and compared different gene functions between healthy ovarian and ovarian cancer cells using single-cell RNA sequencing (ScRNA-seq). We delved deeper into the differences between healthy ovarian and ovarian cancer cells at different levels, and performed specific analysis on endothelial cells.

Results

We obtained scRNA-seq data of 6867 and 17056 cells from healthy ovarian samples and ovarian cancer samples, respectively. The transcriptional profiles of the groups differed at various stages of ovarian cell development. A detailed comparison of the cell cycle, and cell communication of different groups, revealed significant differences between healthy ovarian and ovarian cancer cells. We also found that apoptosis-related genes, URI1, PAK2, PARP1, CLU and TIMP3, were highly expressed, while immune-related genes, UBB, RPL11, CAV1, NUPR1 and Hsp90ab1, were lowly expressed in ovarian cancer cells. The results of the ScRNA-seq were verified using qPCR.

Discussion

Our findings revealed differences in function, gene expression and cell interaction patterns between ovarian cancer and healthy ovarian cell populations. These findings provide key insights on further research into the treatment of ovarian cancer.

The complete mitochondrial genome of Talpa martinorum (Mammalia: Talpidae), a mole species endemic to Thrace: genome content and phylogenetic considerations

The complete mitogenome sequence of Talpa martinorum, a recently described Balkan endemic mole, was assembled from next generation sequence data. The mitogenome is similar to that of the three other Talpa species sequenced to date, being 16,835 bp in length, and containing 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNA genes, an origin of L-strand replication, and a control region or D-loop. Compared to other Talpa mitogenomes sequenced to date, that of T. martinorum differs in the length of D-loop and stop codon usage. TAG and T-- are the stop codons for the ND1 and ATP8 genes, respectively, in T. martinorum, whilst TAA acts as a stop codon for both ND1 and ATP8 in the other three Talpa species sequenced. Phylogeny reconstructions based on Maximum Likelihood and Bayesian inference analyses yielded phylogenies with similar topologies, demonstrating that T. martinorum nests within the western lineage of the genus, being closely related to T. aquitania and T. occidentalis.

Mitochondrial Genomes Provide New Phylogenetic and Evolutionary Insights into Psilidae (Diptera: Brachycera)

Psilidae (Diptera: Brachycera) is a moderate-sized family currently placed in the superfamily Diopsoidea and contains some destructive agricultural and forestry pests. The systematic position and intrafamilial classification of rust flies are in need of further study, and the available molecular data of Psilidae are still limited. In this study, we present the mitochondrial genomes of 6 Psilidae species (Chamaepsilatestudinaria Wang and Yang, Chyliza bambusae Wang and Yang, Chy. chikuni Wang, Loxocera lunata Wang and Yang, L. planivena Wang and Yang and L. sinica Wang and Yang). Comparative analyses show a conserved genome structure, in terms of gene composition and arrangement, and a highly Adenine plus Thymine biased nucleotide composition of the 6 psilid mitogenomes. Mitochondrial evolutionary rates vary among the 6 species, with species of Chylizinae exhibiting a slower average rate than species of Psilinae. The length, the nucleotide composition, and the copy number of repeat units of the control region are variable among the 6 species, which may offer useful information for phylogenetic and evolutionary studies of Psilidae. Phylogenetic analyses based on 4 mitogenomic datasets (AA, PCG, PCG12RNA, and PCGRNA) support the monophyly of Psilidae, and the sister relationship between Chylizinae and Psilinae, while Diopsoidea is suggested to be non-monophyletic. Our study enlightens the future application of mitogenomic data in the phylogenetic and evolutionary studies of Psilidae, based on denser taxon sampling.

Mitochondrial DNA homeostasis impairment and dopaminergic dysfunction: A trembling balance

Maintenance of mitochondrial DNA (mtDNA) homeostasis includes a variety of processes, such as mtDNA replication, repair, and nucleotides synthesis, aimed at preserving the structural and functional integrity of mtDNA molecules. Mutations in several nuclear genes (i.e., POLG, POLG2, TWNK, OPA1, DGUOK, MPV17, TYMP) impair mtDNA maintenance, leading to clinical syndromes characterized by mtDNA depletion and/or deletions in affected tissues. In the past decades, studies have demonstrated a progressive accumulation of multiple mtDNA deletions in dopaminergic neurons of the substantia nigra in elderly population and, to a greater extent, in Parkinson's disease patients. Moreover, parkinsonism has been frequently described as a prominent clinical feature in mtDNA instability syndromes. Among Parkinson's disease-related genes with a significant role in mitochondrial biology, PARK2 and LRRK2 specifically take part in mtDNA maintenance. Moreover, a variety of murine models (i.e., "Mutator", "MitoPark", "PD-mitoPstI", "Deletor", "Twinkle-dup" and "TwinkPark") provided in vivo evidence that mtDNA stability is required to preserve nigrostriatal integrity. Here, we review and discuss the clinical, genetic, and pathological background underlining the link between impaired mtDNA homeostasis and dopaminergic degeneration.

Complete Mitogenome of Cruznema Tripartitum Confirms Highly Conserved Gene Arrangement within Family Rhabditidae

Mitochondrial genomes have widely been used as molecular markers in understanding the patterns and processes of nematode evolution. The species in genus Cruznema are free-living bacterivores as well as parasites of crickets and mollusks. The complete mitochondrial genome of C. tripartitum was determined through high-throughput sequencing as the first sequenced representative of the genus Cruznema. The genome is comprised of 14,067 bp nucleotides, and includes 12 protein-coding, two rRNA, and 22 tRNA genes. Phylogenetic analyses based on amino acid data support C. tripartitum as a sister to the clade containing Caenorhabditis elegans and Oscheius chongmingensis. The analysis of gene arrangement suggested that C. tripartitum shares the same gene order with O. chongmingensis, Litoditis marina, Diplocapter coronatus, genus Caenorhabditis, and Pristionchus pacificus. Thus, the mitochondrial gene arrangement is highly conserved in the family Rhabditidae as well as some species in Diplogasteridae.

Human genetic risk of treatment with antiviral nucleoside analog drugs that induce lethal mutagenesis: The special case of molnupiravir

This review considers antiviral nucleoside analog drugs, including ribavirin, favipiravir, and molnupiravir, which induce genome error catastrophe in SARS-CoV or SARS-CoV-2 via lethal mutagenesis as a mode of action. In vitro data indicate that molnupiravir may be 100 times more potent as an antiviral agent than ribavirin or favipiravir. Molnupiravir has recently demonstrated efficacy in a phase 3 clinical trial. Because of its anticipated global use, its relative potency, and the reported in vitro "host" cell mutagenicity of its active principle, β-d-N4-hydroxycytidine, we have reviewed the development of molnupiravir and its genotoxicity safety evaluation, as well as the genotoxicity profiles of three congeners, that is, ribavirin, favipiravir, and 5-(2-chloroethyl)-2'-deoxyuridine. We consider the potential genetic risks of molnupiravir on the basis of all available information and focus on the need for additional human genotoxicity data and follow-up in patients treated with molnupiravir and similar drugs. Such human data are especially relevant for antiviral NAs that have the potential of permanently modifying the genomes of treated patients and/or causing human teratogenicity or embryotoxicity. We conclude that the results of preclinical genotoxicity studies and phase 1 human clinical safety, tolerability, and pharmacokinetics are critical components of drug safety assessments and sentinels of unanticipated adverse health effects. We provide our rationale for performing more thorough genotoxicity testing prior to and within phase 1 clinical trials, including human PIG-A and error corrected next generation sequencing (duplex sequencing) studies in DNA and mitochondrial DNA of patients treated with antiviral NAs that induce genome error catastrophe via lethal mutagenesis.

Genetic landscape of human mitochondrial genome using whole genome sequencing

Increasing evidences suggest that mitochondrial dysfunction is implicated in diseases and aging, and whole-genome sequencing (WGS) is the most unbiased method in analyzing the mitochondrial genome (mtDNA). However, the genetic landscape of mtDNA in the Chinese population has not been fully examined. Here, we described the genetic landscape of mtDNA using WGS data from Chinese individuals (n = 3241). We identified 3892 mtDNA variants, of which 3349 (86%) were rare variants. Interestingly, we observed a trend toward extreme heterogeneity of mtDNA variants. Our study observed a distinct purifying selection on mtDNA, which inhibits the accumulation of harmful heteroplasmies at the individual level: (1) mitochondrial dN/dS ratios were much less than 1; (2) the dN/dS ratio of heteroplasmies was higher than homoplasmies; (3) heteroplasmies had more indels and predicted deleterious variants than homoplasmies. Furthermore, we found that haplogroup M (20.27%) and D (20.15%) had the highest frequencies in the Chinese population, followed by B (18.51%) and F (16.45%). The number of variants per individual differed across haplogroup groups, with a higher number of homoplasmies for the M lineage. Meanwhile, mtDNA copy number was negatively correlated with age but positively correlated with the female sex. Finally, we developed an mtDNA variation database of Chinese populations called MTCards (http://genemed.tech/mtcards/) to facilitate the query of mtDNA variants in this study. In summary, these findings contribute to different aspects of understanding mtDNA, providing a better understanding of the genetic basis of mitochondrial-related diseases.

Emerging methods for and novel insights gained by absolute quantification of mitochondrial DNA copy number and its clinical applications

The past thirty years have seen a surge in interest in pathophysiological roles of mitochondria, and the accurate quantification of mitochondrial DNA copy number (mCN) in cells and tissue samples is a fundamental aspect of assessing changes in mitochondrial health and biogenesis. Quantification of mCN between studies is surprisingly variable due to a combination of physiological variability and diverse protocols being used to measure this endpoint. The advent of novel methods to quantify nucleic acids like digital polymerase chain reaction (dPCR) and high throughput sequencing offer the ability to measure absolute values of mCN. We conducted an in-depth survey of articles published between 1969 -- 2020 to create an overview of mCN values, to assess consensus values of tissue-specific mCN, and to evaluate consistency between methods of assessing mCN. We identify best practices for methods used to assess mCN, and we address the impact of using specific loci on the mitochondrial genome to determine mCN. Current data suggest that clinical measurement of mCN can provide diagnostic and prognostic value in a range of diseases and health conditions, with emphasis on cancer and cardiovascular disease, and the advent of means to measure absolute mCN should improve future clinical applications of mCN measurements.

Predicting pathogenic non-coding SVs disrupting the 3D genome in 1646 whole cancer genomes using multiple instance learning

Over the past years, large consortia have been established to fuel the sequencing of whole genomes of many cancer patients. Despite the increased abundance in tools to study the impact of SNVs, non-coding SVs have been largely ignored in these data. Here, we introduce svMIL2, an improved version of our Multiple Instance Learning-based method to study the effect of somatic non-coding SVs disrupting boundaries of TADs and CTCF loops in 1646 cancer genomes. We demonstrate that svMIL2 predicts pathogenic non-coding SVs with an average AUC of 0.86 across 12 cancer types, and identifies non-coding SVs affecting well-known driver genes. The disruption of active (super) enhancers in open chromatin regions appears to be a common mechanism by which non-coding SVs exert their pathogenicity. Finally, our results reveal that the contribution of pathogenic non-coding SVs as opposed to driver SNVs may highly vary between cancers, with notably high numbers of genes being disrupted by pathogenic non-coding SVs in ovarian and pancreatic cancer. Taken together, our machine learning method offers a potent way to prioritize putatively pathogenic non-coding SVs and leverage non-coding SVs to identify driver genes. Moreover, our analysis of 1646 cancer genomes demonstrates the importance of including non-coding SVs in cancer diagnostics.

Applying genomic and transcriptomic advances to mitochondrial medicine

Next-generation sequencing (NGS) has increased our understanding of the molecular basis of many primary mitochondrial diseases (PMDs). Despite this progress, many patients with suspected PMD remain without a genetic diagnosis, which restricts their access to in-depth genetic counselling, reproductive options and clinical trials, in addition to hampering efforts to understand the underlying disease mechanisms. Although they represent a considerable improvement over their predecessors, current methods for sequencing the mitochondrial and nuclear genomes have important limitations, and molecular diagnostic techniques are often manual and time consuming. However, recent advances in genomics and transcriptomics offer realistic solutions to these challenges. In this Review, we discuss the current genetic testing approach for PMDs and the opportunities that exist for increased use of whole-genome NGS of nuclear and mitochondrial DNA (mtDNA) in the clinical environment. We consider the possible role for long-read approaches in sequencing of mtDNA and in the identification of novel nuclear genomic causes of PMDs. We examine the expanding applications of RNA sequencing, including the detection of cryptic variants that affect splicing and gene expression and the interpretation of rare and novel mitochondrial transfer RNA variants.

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.

Variations in the Mitochondrial Genome of a Goldfish-Like Hybrid [Koi Carp (♀) × Blunt Snout Bream (♂)] Indicate Paternal Leakage

Previously, a homodiploid goldfish-like fish (2n = 100; GF-L) was spontaneously generated by self-crossing a homodiploid red crucian carp-like fish (2n = 100; RCC-L), which was in turn produced via the distant hybridization of female koi carp (Cyprinus carpio haematopterus, KOC, 2n = 100) and male blunt snout bream (Megalobrama amblycephala, BSB, 2n = 48). The phenotypes and genotypes of RCC-L and GF-L differed from those of the parental species but were similar to diploid red crucian carp (2n = 100; RCC) and goldfish (2n = 100; GF), respectively. We sequenced the complete mitochondrial DNAs (mtDNAs) of the KOC, BSB, RCC-L, GF-L, and subsequent generations produced by self-crossing [the self-mating offspring of RCC-L (RCC-L-F₂) to the self-mating offspring of RCC-L-F₂ (RCC-L-F₃) and the self-mating offspring of GF-L (GF-L-F₂)]. Paternal mtDNA fragments were stably embedded in the mtDNAs of both lineages, forming chimeric DNA fragments. In addition to these chimeras, several nucleotide positions in the RCC-L and GF-L lineages differed from the parental bases, and were instead identical with RCC and GF, respectively. Moreover, RCC-L and GF-L mtDNA organization and nucleotide composition were more similar to those of RCC and GF, respectively, compared to parental mtDNA. Finally, phylogenetic analyses indicated that RCC-L and GF-L clustered with RCC and GF, not with the parental species. The molecular dating time shows that the divergence time of KOC and GF was about 21.26 Mya [95% highest posterior density (HPD): 24.41-16.67 Mya], which fell within the period of recent. The heritable chimeric DNA fragments and mutant loci identified in the mtDNA of the RCC-L and GF-L lineages provided important evidence that hybridizations might lead to changes in the mtDNA and the subsequent generation of new lineages. Our findings also demonstrated for the first time that the paternal mtDNA was transmitted into the mtDNA of homodiploid lineages (RCC-L and GF-L), which provided evidence that paternal DNA plays a role in inherited mtDNA. These evolutionary analyses in mtDNA suggest that GF might have diverged from RCC after RCC diverged from koi carp.

NGS-based accurate and efficient detection of circulating cell-free mitochondrial DNA in cancer patients

Mitochondrial DNA (mtDNA) mutations are closely implicated in the pathogenesis of multiple cancers, making circulating cell-free mtDNA (ccf-mtDNA) as a potential non-invasive tumor biomarker. However, an effective approach to comprehensively profile ccf-mtDNA mutations is still lacking. In this study, we first characterized ccf-mtDNA by low-depth whole-genome sequencing (WGS) and found that plasma DNA samples exhibited a dramatic decrease in mtDNA copy number when compared with fresh tumor tissues. Further analysis revealed that plasma ccf-mtDNA had a biased distribution of fragment size with a peak around 90 bp. Based on these insights, we developed a robust captured-based mtDNA deep-sequencing approach that enables accurate and efficient detection of plasma ccf-mtDNA mutations by systematic optimization of probe quantity and length, hybridization temperature, and PCR amplification cycles. Moreover, we found that placement of isolated plasma for 6 h at both 4°C and room temperature (RT) led to a dramatic decrease of ccf-mtDNA stability, highlighting the importance of proper plasma sample processing. We further showed that the optimized approach can successfully detect a substantial fraction of tumor-specific mtDNA mutations in plasma ccf-mtDNA specifically from hepatocellular carcinoma (HCC) patients but not from colorectal cancer (CRC) patients, suggesting the presence of a potential cancer-specific difference in the abundance of tumor-derived mtDNA in plasma.

Neuro-ophthalmic manifestations of mitochondrial disorders and their management

The visual system has high metabolic requirements and is therefore particularly vulnerable to mitochondrial dysfunction. The most commonly affected tissues include the extraocular muscles, photoreceptors, retinal pigment epithelium, optic nerve and visual cortex. Hence, the most common manifestations of mitochondrial disorders are progressive external ophthalmoplegia, macular pattern dystrophy, pigmentary retinopathy, optic neuropathy and retrochiasmal visual field loss. With the exception of Leber hereditary optic neuropathy and stroke-like episodes seen in mitochondrial encephalopathy, lactic acidosis and stroke-like episodes, the majority of neuro-ophthalmic manifestations have an insidious onset. As such, some patients may not recognize subtle progressive visual symptoms. When mitochondrial disorders are highly suspected, meticulous examination performed by an ophthalmologist with targeted ancillary testing can help confirm the diagnosis. Similarly, neuro-ophthalmic symptoms and signs may be the first indication of mitochondrial disease and should prompt systemic investigations for potentially life-threatening associations, such as cardiac conduction defects. Finally, the ophthalmologist can offer symptomatic treatments for some of the most disabling manifestations of these disorders.

Complete Mitochondrial Genome of Three Species of the Genus Microtus (Arvicolinae, Rodentia)

The 65 species of the genus Microtus have unusual sex-related genetic features and a high rate of karyotype variation. However, only nine complete mitogenomes for these species are currently available. We describe the complete mitogenome sequences of three Microtus, which vary in length from 16,295 bp to 16,331 bp, contain 13 protein-coding genes (PCGs), two ribosomal RNA genes, 22 transfer RNA genes and a control region. The length of the 13 PCGs and the coded proteins is the same in all three species, and the start and stop codons are conserved. The non-coding regions include the L-strand origin of replication, with the same sequence of 35 bp, and the control region, which varies between 896 bp and 930 bp in length. The control region includes three domains (Domains I, II and III) with extended termination-associated sequences (ETAS-1 and ETAS-2) in Domain I. Domain II and Domain III include five (CSB-B, C, D, E and F) and three (CSB-1, CSB-2, and CSB-3) conserved sequence blocks, respectively. Phylogenetic reconstructions using the mitochondrial genomes of all the available Microtus species and one representative species from another genus of the Arvicolinae subfamily reproduced the established phylogenetic relationships for all the Arvicolinae genera that were analyzed.

An innovative data analysis strategy for accurate next-generation sequencing detection of tumor mitochondrial DNA mutations

Next-generation sequencing technology has been commonly applied to detect mitochondrial DNA (mtDNA) mutations, which are reported to be strongly associated with cancers. However, several key challenges still exist regarding bioinformatics analysis of mtDNA sequencing data that greatly affect the detection accuracy of mtDNA mutations. Here we comprehensively evaluated several key analysis procedures in three different sample types. We found that a trimming procedure was essential for improving mtDNA mapping performance in plasma but not tissue samples. Mapping with a revised Cambridge reference sequence and human genome 19 reference was strongly suggested for mtDNA mutation detection in plasma samples because of the extreme abundance of nuclear DNA of mitochondrial origin. Moreover, our results showed that a setting of 3 mismatches was most appropriate for mtDNA mutation calling. Importantly, we revealed the presence of a negative logarithmic relationship between mtDNA site sequencing depth and minimum detectable mutation frequency and built an innovative and efficient filtering strategy to increase the accuracy and sensitivity of mutation detection. Finally, we verified that higher sequencing depth was required for a PCR-based compared with a capture-based enrichment strategy. We established an innovative data analysis strategy that is of great significance for improving the accuracy of mtDNA mutation detection for different types of tumor samples.

Mitochondrial DNA enrichment reduced NUMT contamination in porcine NGS analyses

Genetic associations between mitochondrial DNA (mtDNA) and economic traits have been widely reported for pigs, which indicate the importance of mtDNA. However, studies on mtDNA heteroplasmy in pigs are rare. Next generation sequencing (NGS) methodologies have emerged as a promising genomic approach for detection of mitochondrial heteroplasmy. Due to the short reads, flexible bioinformatic analyses and the contamination of nuclear mitochondrial sequences (NUMTs), NGS was expected to increase false-positive detection of heteroplasmy. In this study, Sanger sequencing was performed as a gold standard to detect heteroplasmy with a detection sensitivity of 5% in pigs and then one whole-genome sequencing method (WGS) and two mtDNA enrichment sequencing methods (Capture and LongPCR) were carried out. The aim of this study was to determine whether mitochondrial heteroplasmy identification from NGS data was affected by NUMTs. We find that WGS generated more false intra-individual polymorphisms and less mapping specificity than the two enrichment sequencing methods, suggesting NUMTs indeed led to false-positive mitochondrial heteroplasmies from NGS data. In addition, to accurately detect mitochondrial diversity, three commonly used tools-SAMtools, VarScan and GATK-with different parameter values were compared. VarScan achieved the best specificity and sensitivity when considering the base alignment quality re-computation and the minimum variant frequency of 0.25. It also suggested bioinformatic workflow interfere in the identification of mtDNA SNPs. In conclusion, intra-individual polymorphism in pig mitochondria from NGS data was confused with NUMTs, and mtDNA-specific enrichment is essential before high-throughput sequencing in the detection of mitochondrial genome sequences.

A Novel Approach for the Identification of Pharmacogenetic Variants in MT-RNR1 through Next-Generation Sequencing Off-Target Data

Specific genetic variants in the mitochondrially encoded 12S ribosomal RNA gene (MT-RNR1) cause aminoglycoside-induced irreversible hearing loss. Mitochondrial DNA is usually not included in targeted sequencing experiments; however, off-target data may deliver this information. Here, we extract MT-RNR1 genetic variation, including the most relevant ototoxicity variant m.1555A>G, using the off-target reads of 473 research samples, sequenced through a capture-based, custom-targeted panel and whole exome sequencing (WES), and of 1245 diagnostic samples with clinical WES. Sanger sequencing and fluorescence-based genotyping were used for genotype validation. There was a correlation between off-target reads and mitochondrial coverage (r_customPanel = 0.39, p = 2 × 10⁻¹³ and rWES = 0.67, p = 7 × 10⁻²¹). The median read depth of MT-RNR1 m.1555 was similar to the average mitochondrial genome coverage, with saliva and blood samples giving comparable results. The genotypes from 415 samples, including three m.1555G carriers, were concordant with fluorescence-based genotyping data. In clinical WES, median MT-RNR1 coverage was 56×, with 90% of samples having ≥20 reads at m.1555 position, and one m.1494T and three m.1555G carriers were identified with no evidence for heteroplasmy. Altogether, this study shows that obtaining MT-RNR1 genotypes through off-target reads is an efficient strategy that can impulse preemptive pharmacogenetic screening of this mitochondrial gene.

Mitochondrial DNA intra-individual variation in a bumblebee species: A challenge for evolutionary studies and molecular identification

Mitochondrial DNA (mtDNA) regions have been widely used as molecular markers in evolutionary studies and species identification. However, the presence of heteroplasmy and NUMTs may represent obstacles. Heteroplasmy is a state where an organism has different mitochondrial haplotypes. NUMTs are nuclear pseudogenes originating from mtDNA sequences transferred to nuclear DNA. Evidences of heteroplasmy were already verified in the bumblebee Bombus morio in an earlier study. The present work investigated in more detail the presence of intra-individual haplotypes variation in this species. Heteroplasmy was detected in individuals from all the ten sampled locations, with an average of six heteroplasmic haplotypes per individual. In addition, some of these heteroplasmic haplotypes were shared among individuals from different locations, suggesting the existence of stable heteroplasmy in B. morio. These results demonstrated that heteroplasmy is likely to affect inferences based on mtDNA analysis, especially in phylogenetic, phylogeographic and population genetics studies. In addition, NUMTs were also detected. These sequences showed divergence of 2.7% to 12% in relation to the mitochondrial haplotypes. These levels of divergence could mislead conclusions in evolutionary studies and affect species identification through DNA barcoding.

Genetic and phenotypic landscape of the mitochondrial genome in the Japanese population

The genetic landscape of mitochondrial DNA (mtDNA) has been elusive. By analyzing mtDNA using the whole genome sequence (WGS) of Japanese individuals (n = 1928), we identified 2023 mtDNA variants and high-resolution haplogroups. Frequency spectra of the haplogroups were population-specific and were heterogeneous among geographic regions within Japan. Application of machine learning methods could finely classify the subjects corresponding to the high-digit mtDNA sub-haplogroups. mtDNA had distinct genetic structures from that of nuclear DNA (nDNA), characterized by no distance-dependent linkage disequilibrium decay, sparse tagging of common variants, and the existence of common haplotypes spanning the entire mtDNA. We did not detect any evidence of mtDNA–nDNA (or mtDNA copy number–nDNA) genotype associations. Together with WGS-based mtDNA variant imputation, we conducted a phenome-wide association study of 147,437 Japanese individuals with 99 clinical phenotypes. We observed pleiotropy of mtDNA genetic risk on the five late-onset human complex traits including creatine kinase (P = 1.7 × 10⁻¹²).

Kenichi Yamamoto et al. report a genetic analysis of mitochondrial DNA (mtDNA) and a phenome-wide association study in Japanese individuals from the BioBank Japan Project. They describe the genetic landscape of the mitochondria and identify pleiotropic mtDNA variants associated with 5 late-onset complex traits.

Characterization of background noise in MiSeq MPS data when sequencing human mitochondrial DNA from various sample sources and library preparation methods

Improved resolution of massively parallel sequencing (MPS) allows for the characterization of mitochondrial (mt) DNA heteroplasmy to levels previously unattainable with traditional sequencing approaches. An essential criterion for the reporting of heteroplasmy is the ability of the MPS method to distinguish minor sequence variants (MSVs) from system noise, or error. Therefore, an assessment of the background noise in the MPS method is desirable to identify the point at which reliable data can be reported. Substitution and sequence specific error (SSE) was evaluated for a variety of sample types and two library preparations. Substitution error rates ranged from 0.18 to 0.49 per 100 nucleotides with C positions generally having the highest rate of misincorporation. Comparison of error rates across sample types indicated a significant increase for samples with damaged DNA. The positions of error were varied across datasets (pairwise concordance 0-68%), but had greater consistency within the damaged samples (80-96%). The most commonly observed motif preceding error in forward reads was CCG, while GGT was most common in reverse reads, both consistent with previous findings. The findings illustrate that for datasets containing samples with damaged DNA, reporting thresholds for heteroplasmy may have to be modified and individual sites with error levels exceeding thresholds should be scrutinized. Collectively, the shifting error profiles observed across the various sample types and library preparation methods demonstrates the need for an assessment of error under these varying circumstances. Characterization of the applicable background noise will help to ensure that thresholds are reliably set for detection of true MSVs.

The mitochondrial genome of the ornate sheep tick, Dermacentor marginatus

The ornate sheep tick, Dermacentor marginatus, is widespread in Europe. Its vector role of various zoonotic pathogens received much attention in these regions. However, the genomic resources of the ticks are limited. In this study, the complete mitochondrial genome of a single female D. marginatus collected in Slovakia was sequenced through the Illumina HiSeq sequencing platform. The mitochondrial genome is 15,067 bp long and contains 13 protein-coding genes, two ribosomal RNA genes and 22 transfer RNA genes. The overall G+C content is 21.6%. The gene order is identical to that of Metastriata ticks. The codon usage pattern is similar with that of other tick species. As in other ticks, two truncated tRNA genes were observed. Two control regions were found between tRNA-Leu and tRNA-Cys, tRNA-Ile and rrnS, respectively. The mitochondrial genome contains three noncoding regions, which is similar to that in D. nitens. The noncoding region located between rrnS and tRNA-Val is shorter than that of other Dermacentor species. Phylogenetic analyses indicate that D. marginatus is clustered with other Dermacentor species. These findings are helpful for exploring the systematics and evolution of ticks in the future.

Heteroplasmy in the complete chicken mitochondrial genome

Chicken mitochondrial DNA is a circular molecule comprising ~16.8 kb. In this study, we used next-generation sequencing to investigate mitochondrial heteroplasmy in the whole chicken mitochondrial genome. Based on heteroplasmic detection thresholds at the 0.5% level, 178 cases of heteroplasmy were identified in the chicken mitochondrial genome, where 83% were due to nucleotide transitions. D-loop regionwas hot spot region for mtDNA heteroplasmy in the chicken since 130 cases of heteroplasmy were located in these regions. Heteroplasmy varied among intraindividual tissues with allele-specific, position-specific, and tissue-specific features. Skeletal muscle had the highest abundance of heteroplasmy. Cases of heteroplasmy at mt.G8682A and mt.G16121A were validated by PCR-restriction fragment length polymorphism analysis, which showed that both had low ratios of heteroplasmy occurrence in five natural breeds. Polymorphic sites were easy to distinguish. Based on NGS data for crureus tissues, mitochondrial mutation/heteroplasmy exhibited clear maternal inheritance features at the whole mitochondrial genomic level. Further investigations of the heterogeneity of the mt.A5694T and mt.T5718G transitions between generations using pyrosequencing based on pedigree information indicated that the degree of heteroplasmy and the occurrence ratio of heteroplasmy decreased greatly from the F0 to F1 generations in the mt.A5694T and mt.T5718G site. Thus, the intergenerational transmission of heteroplasmy in chicken mtDNA exhibited a rapid shift toward homoplasmy within a single generation. Our findings indicate that heteroplasmy is a widespread phenomenon in chicken mitochondrial genome, in which most sites exhibit low heteroplasmy and the allele frequency at heteroplasmic sites changes significantly during transmission events. It suggests that heteroplasmy may be under negative selection to some degree in the chicken.

The unresolved role of mitochondrial DNA in Parkinson's disease: An overview of published studies, their limitations, and future prospects

Parkinson's disease (PD), a progressive neurodegenerative disorder, has long been associated with mitochondrial dysfunction in both sporadic and familial forms of the disease. Mitochondria are crucial for maintaining cellular homeostasis, and their dysfunction is detrimental to dopaminergic neurons. These neurons are highly dependent on mitochondrial adenosine triphosphate (ATP) and degenerate in PD. Mitochondria contain their own genomes (mtDNA). The role of mtDNA has been investigated in PD on the premise that it encodes vital components of the ATP-generating oxidative phosphorylation (OXPHOS) complexes and accumulates somatic variation with age. However, the association between mtDNA variation and PD remains controversial. Herein, we provide an overview of previously published studies on the role of inherited as well as somatic (acquired) mtDNA changes in PD including point mutations, deletions and depletion. We outline limitations of previous investigations and the difficulties associated with studying mtDNA, which have left its role unresolved in the context of PD. Lastly, we highlight the potential for further research in this field and provide suggestions for future studies. Overall, the mitochondrial genome is indispensable for proper cellular function and its contribution to PD requires further, more extensive investigation.

Deciphering exome sequencing data: Bringing mitochondrial DNA variants to light

The expanding use of exome sequencing (ES) in diagnosis generates a huge amount of data, including untargeted mitochondrial DNA (mtDNA) sequences. We developed a strategy to deeply study ES data, focusing on the mtDNA genome on a large unspecific cohort to increase diagnostic yield. A targeted bioinformatics pipeline assembled mitochondrial genome from ES data to detect pathogenic mtDNA variants in parallel with the "in-house" nuclear exome pipeline. mtDNA data coming from off-target sequences (indirect sequencing) were extracted from the BAM files in 928 individuals with developmental and/or neurological anomalies. The mtDNA variants were filtered out based on database information, cohort frequencies, haplogroups and protein consequences. Two homoplasmic pathogenic variants (m.9035T>C and m.11778G>A) were identified in 2 out of 928 unrelated individuals (0.2%): the m.9035T>C (MT-ATP6) variant in a female with ataxia and the m.11778G>A (MT-ND4) variant in a male with a complex mosaic disorder and a severe ophthalmological phenotype, uncovering undiagnosed Leber's hereditary optic neuropathy (LHON). Seven secondary findings were also found, predisposing to deafness or LHON, in 7 out of 928 individuals (0.75%). This study demonstrates the usefulness of including a targeted strategy in ES pipeline to detect mtDNA variants, improving results in diagnosis and research, without resampling patients and performing targeted mtDNA strategies.

Detection of Heteroplasmic Variants in the Mitochondrial Genome through Massive Parallel Sequencing

Detecting heteroplasmies in the mitochondrial DNA (mtDNA) has been a challenge for many years. In the past, Sanger sequencing was the main option to perform this analysis, however, this method could not detect low frequency heteroplasmies. Massive Parallel Sequencing (MPS) provides the opportunity to study the mtDNA in depth, but a controlled pipeline is necessary to reliably retrieve and quantify the low frequency variants. It has been shown that differences in methods can significantly affect the number and frequency of the retrieved variants. In this protocol, we present a method involving both wet lab and bioinformatics that allows identifying and quantifying single nucleotide variants in the full mtDNA sequence, down to a heteroplasmic load of 1.5%. For this, we set up a PCR-based amplification of the mtDNA, followed by MPS using Illumina chemistry, and variant calling with two different algorithms, mtDNA server and Mutect. The PCR amplification is used to enrich the mitochondrial fraction, while the bioinformatic processing with two algorithms is used to discriminate the true heteroplasmies from background noise. The protocol described here allows for deep sequencing of the mitochondrial DNA in bulk DNA samples as well as single cells (both large cells such as human oocytes, and small-sized single cells such as human embryonic stem cells) with minor modifications to the protocol.

Evaluating heteroplasmic variations of the mitochondrial genome from whole genome sequencing data

BACKGROUND

Detecting heteroplasmic variations in the mitochondrial genome can help identify potential pathogenic possibilities, which is significant for disease prevention. The development of next-generation sequencing changed the quantification of mitochondrial DNA (mtDNA) heteroplasmy from scanning limited recorded points to the entire mitochondrial genome. However, due to the presence of nuclear mtDNA homologous sequences (nuMTs), maximally retaining real variations while excluding falsest heteroplasmic variations from nuMTs and sequencing errors presents a dilemma.

RESULTS

Herein, we used an improved method for detecting low-frequency mtDNA heteroplasmic variations from whole genome sequencing data, including point variations and short-fragment length alterations, and evaluated the effect of this method. A two-step alignment was designed and performed to accelerate data processing, to obtain and retain the true mtDNA reads and to eliminate most nuMTs reads. After analyzing whole genome sequencing data of K562 and GM12878 cells, ~90% of heteroplasmic point variations were identified in MitoMap. The results were consistent with the results of an amplification refractory mutation system qPCR. Many linkages of the detected heteroplasmy variations were also discovered.

CONCLUSIONS

Our improved method is a simple, efficient and accurate way to mine mitochondrial low-frequency heteroplasmic variations from whole genome sequencing data. By evaluating the highest misalignment possibility caused by the remaining nuMTs-like reads and sequencing errors, our procedure can detect mtDNA heteroplasmic variations whose heteroplasmy frequencies are as low as 0.2%.

Bioinformatics Tools and Databases to Assess the Pathogenicity of Mitochondrial DNA Variants in the Field of Next Generation Sequencing

The development of next generation sequencing (NGS) has greatly enhanced the diagnosis of mitochondrial disorders, with a systematic analysis of the whole mitochondrial DNA (mtDNA) sequence and better detection sensitivity. However, the exponential growth of sequencing data renders complex the interpretation of the identified variants, thereby posing new challenges for the molecular diagnosis of mitochondrial diseases. Indeed, mtDNA sequencing by NGS requires specific bioinformatics tools and the adaptation of those developed for nuclear DNA, for the detection and quantification of mtDNA variants from sequence alignment to the calling steps, in order to manage the specific features of the mitochondrial genome including heteroplasmy, i.e., coexistence of mutant and wildtype mtDNA copies. The prioritization of mtDNA variants remains difficult, relying on a limited number of specific resources: population and clinical databases, and in silico tools providing a prediction of the variant pathogenicity. An evaluation of the most prominent bioinformatics tools showed that their ability to predict the pathogenicity was highly variable indicating that special efforts should be directed at developing new bioinformatics tools dedicated to the mitochondrial genome. In addition, massive parallel sequencing raised several issues related to the interpretation of very low mtDNA mutational loads, discovery of variants of unknown significance, and mutations unrelated to patient phenotype or the co-occurrence of mtDNA variants. This review provides an overview of the current strategies and bioinformatics tools for accurate annotation, prioritization and reporting of mtDNA variations from NGS data, in order to carry out accurate genetic counseling in individuals with primary mitochondrial diseases.

Differential expression of mRNA-miRNAs related to intramuscular fat content in the longissimus dorsi in Xinjiang brown cattle

In this study, we examined the role of mRNAs and miRNAs in variations in intramuscular fat content in the longissimus dorsi muscle in Xinjiang brown cattle. Two groups of Xinjiang brown cattle with extremely different intramuscular fat content in the longissimus dorsi were selected for combined of miRNA and mRNA analysis using an RNA-Seq. In total, 296 mRNAs and 362 miRNAs were significantly differentially expressed, including 155 newly predicted miRNAs, 275 significantly upregulated genes, 252 significantly upregulated miRNAs, 21 significantly downregulated genes and 110 significantly downregulated miRNAs. The combined miRNA and mRNA analysis identified 96 differentially expressed miRNAs and 27 differentially expressed mRNAs. In all, 47 upregulated miRNAs had a regulatory effect on 14 differentially downregulated target genes, and 49 downregulated miRNAs had a regulatory effect on 13 upregulated target genes. To verify the sequencing results, 10 differentially expressed genes (DEGs) and 10 differentially expressed miRNAs were selected for qRT-PCR. The qRT-PCR results confirmed the sequencing results. The results of this study shed light on the molecular regulation of bovine adipose tissue, which might help with the development of new strategies for improving meat quality and animal productivity in beef cattle to provide healthier meat products for consumers.

Complete mitochondrial genome of the Iberian Mole Talpa occidentalis (Talpidae, Insectivora) and comparison with Talpa europaea

The complete mitogenome of Talpa occidentalis, the Iberian mole, was sequenced using a combination of the Illumina and Sanger methods. The 16,962 bp genome obtained contains 13 protein-coding genes, 22 transfer RNAs, 2 ribosomal RNAs, and a control region. Thirty-seven identical repetitions of a 10-nucleotide (CACACGTACG) repeat element were identified in the non-coding control region (D-loop). The number, order, and orientation of the mitochondrial genes are the same as in T. europaea, the only mitogenome published so far for this genus. These two mitogenomes differ only at the repeat element included in the control region. The phylogeny obtained for the Talpidae species using the protein-coding genes of these mitogenomes agrees with the current classification of this family.

Deciphering the Spectrum of Mitochondrial DNA Mutations in Hepatocellular Carcinoma Using High-Throughput Sequencing

Accumulation of mitochondrial DNA (mtDNA) mutations has been proposed to contribute to the initiation and progression of tumors. By using high-throughput sequencing strategies, we measured 33 specimens including 11 hepatocellular carcinoma (HCC) tissues, 11 corresponding adjacent tissues, and 11 normal liver tissues. We identified 194 single nucleotide variants (SNVs; including insert and deletion) in 33 liver tissues, and 13 somatic novel mutations were detected, including 7 mutations in the coding region. One of the seven somatic mutations (T7609C, 91.09%) is synonymous, which does not change amino acid coding; the other four somatic mutations (T6115C, 65.74%; G8387A, 12.23%; G13121A, 93.08%; and T14180C, 28.22%) could result in amino acid substitutions, potentially leading to mitochondrial dysfunction. Furthermore, two mutations in tRNA might influence amino acid transportation. Consistent with a previous study, we also found that mtDNA copy number was significantly reduced in HCC tissues. Therefore, we established a mitochondrial genome depletion cell line ρ0 and revealed that mtDNA loss reduced proliferation and migration in HCC cells but promoted their resistance to 5-fluorouracil. Our results suggested that somatic mtDNA mutations may cause mitochondrial dysfunction and affect chemoresistance of HCC cells. These new identified somatic mutations may serve as a reference for future studies of cancer mitochondrial genomes.

The source of cell-free mitochondrial DNA in trauma and potential therapeutic strategies

Mitochondria play a key role in the pathophysiology of post-injury inflammation. Cell-free mitochondrial DNA (cf-mtDNA) is now understood to catalyse sterile inflammation after trauma. Observations in trauma cohorts have identified high cf-mtDNA in patients with systemic inflammatory response syndrome and multiple organ failure as well as following major surgery. The source of cf-mtDNA can be various cells affected by mechanical and hypoxic injury (passive mechanism) or induced by inflammatory mechanisms (active mechanism). Multiple forms of cf-mtDNA exist; mtDNA fragments, mtDNA in microparticles/vesicles and cell-free mitochondria. Trauma to cells that are rich in mitochondria are believed to release more cf-mtDNA. This review describes the current understanding of the mechanisms of cf-mtDNA release, its systemic effects and the potential therapeutic implications related to its modification. Although current understanding is insufficient to change trauma management, focussed research goals have been identified to pave the way for monitoring and manipulation of cf-mtDNA release and effects in trauma.

Site-specific selection reveals selective constraints and functionality of tumor somatic mtDNA mutations

BACKGROUND

Previous studies have indicated that tumor mitochondrial DNA (mtDNA) mutations are primarily shaped by relaxed negative selection, which is contradictory to the critical roles of mtDNA mutations in tumorigenesis. Therefore, we hypothesized that site-specific selection may influence tumor mtDNA mutations.

METHODS

To test our hypothesis, we developed the largest collection of tumor mtDNA mutations to date and evaluated how natural selection shaped mtDNA mutation patterns.

RESULTS

Our data demonstrated that both positive and negative selections acted on specific positions or functional units of tumor mtDNAs, although the landscape of these mutations was consistent with the relaxation of negative selection. In particular, mutation rate (mutation number in a region/region bp length) in complex V and tRNA coding regions, especially in ATP8 within complex V and in loop and variable regions within tRNA, were significantly lower than those in other regions. While the mutation rate of most codons and amino acids were consistent with the expectation under neutrality, several codons and amino acids had significantly different rates. Moreover, the mutations under selection were enriched for changes that are predicted to be deleterious, further supporting the evolutionary constraints on these regions.

CONCLUSION

These results indicate the existence of site-specific selection and imply the important role of the mtDNA mutations at some specific sites in tumor development.

Tri-allelic heteroplasmies, DNA-RNA differences and their polynucleotide tract associations in the mitochondrial genome

The human mitochondrial genome has been extensively studied for its function and disease associations. Utilizing five types of high-throughput sequencing data on ten breast cancer patients (total N=50), we examined several aspects of the mitochondrial genome that have not been thoroughly studied, including the occurrence of tri-allelic heteroplasmy, the difference between DNA and RNA, and the variants association with polynucleotide tracts. We validated four previously reported and identified 23 additional tri-allelic positions. Furthermore, we detected 18 single nucleotide and seven InDel differences between DNA and RNA. Previous studies have suggested that some of these differences are caused by post transcriptional methylation. The rest can be accredited to RNA editing, polyadenylation or sequencing errors. Most importantly, we found that the tri-allelic positions, and differences between DNA and RNA, are strongly associated with polynucleotide tracts in the mitochondrial genome, suggesting DNA instability or difficulty sequencing around the polynucleotide tract regions.

Estimating relative mitochondrial DNA copy number using high throughput sequencing data

We hypothesize that the relative mitochondria copy number (MTCN) can be estimated by comparing the abundance of mitochondrial DNA to nuclear DNA reads using high throughput sequencing data. To test this hypothesis, we examined relative MTCN across 13 breast cancer cell lines using the RT-PCR based NovaQUANT Human Mitochondrial to Nuclear DNA Ratio Kit as the gold standard. Six distinct computational approaches were used to estimate the relative MTCN in order to compare to the RT-PCR measurements. The results demonstrate that relative MTCN correlates well with the RT-PCR measurements using exome sequencing data, but not RNA-seq data. Through analysis of copy number variants (CNVs) in The Cancer Genome Atlas, we show that the two nuclear genes used in the NovaQUANT assay to represent the nuclear genome often experience CNVs in tumor cells, questioning the accuracy of this gold-standard method when it is applied to tumor cells.

A Method for Next-Generation Sequencing of Paired Diagnostic and Remission Samples to Detect Mitochondrial DNA Mutations Associated with Leukemia

Somatic mitochondrial DNA (mtDNA) mutations have been identified in many human cancers, including leukemia. To identify somatic mutations, it is necessary to have a control tissue from the same individual for comparison. When patients with leukemia achieve remission, the remission peripheral blood may be a suitable and easily accessible control tissue, but this approach has not previously been applied to the study of mtDNA mutations. We have developed and validated a next-generation sequencing approach for the identification of leukemia-associated mtDNA mutations in 26 chronic myeloid leukemia patients at diagnosis using either nonhematopoietic or remission blood samples as the control. The entire mt genome was amplified by long-range PCR and sequenced using Illumina technology. Variant caller software was used to detect mtDNA somatic mutations, and an empirically determined threshold of 2% was applied to minimize false-positive results because of sequencing errors. Mutations were called against both nonhematopoietic and remission controls: the overall concordance between the two approaches was 81% (73/90 mutations). Some discordant results were because of the presence of somatic mutations in remission samples, because of either minimal residual disease or nonleukemic hematopoietic clones. This method could be applied to study somatic mtDNA mutations in leukemia patients who achieve minimal residual disease, and in patients with nonhematopoietic cancers who have a matched uninvolved tissue available.

DNA isolation protocol effects on nuclear DNA analysis by microarrays, droplet digital PCR, and whole genome sequencing, and on mitochondrial DNA copy number estimation

Potential bias introduced during DNA isolation is inadequately explored, although it could have significant impact on downstream analysis. To investigate this in human brain, we isolated DNA from cerebellum and frontal cortex using spin columns under different conditions, and salting-out. We first analysed DNA using array CGH, which revealed a striking wave pattern suggesting primarily GC-rich cerebellar losses, even against matched frontal cortex DNA, with a similar pattern on a SNP array. The aCGH changes varied with the isolation protocol. Droplet digital PCR of two genes also showed protocol-dependent losses. Whole genome sequencing showed GC-dependent variation in coverage with spin column isolation from cerebellum. We also extracted and sequenced DNA from substantia nigra using salting-out and phenol / chloroform. The mtDNA copy number, assessed by reads mapping to the mitochondrial genome, was higher in substantia nigra when using phenol / chloroform. We thus provide evidence for significant method-dependent bias in DNA isolation from human brain, as reported in rat tissues. This may contribute to array "waves", and could affect copy number determination, particularly if mosaicism is being sought, and sequencing coverage. Variations in isolation protocol may also affect apparent mtDNA abundance.

MitoRS, a method for high throughput, sensitive, and accurate detection of mitochondrial DNA heteroplasmy

Background

Mitochondrial dysfunction is linked to numerous pathological states, in particular related to metabolism, brain health and ageing. Nuclear encoded gene polymorphisms implicated in mitochondrial functions can be analyzed in the context of classical genome wide association studies. By contrast, mitochondrial DNA (mtDNA) variants are more challenging to identify and analyze for several reasons. First, contrary to the diploid nuclear genome, each cell carries several hundred copies of the circular mitochondrial genome. Mutations can therefore be present in only a subset of the mtDNA molecules, resulting in a heterogeneous pool of mtDNA, a situation referred to as heteroplasmy. Consequently, detection and quantification of variants requires extremely accurate tools, especially when this proportion is small. Additionally, the mitochondrial genome has pseudogenized into numerous copies within the nuclear genome over the course of evolution. These nuclear pseudogenes, named NUMTs, must be distinguished from genuine mtDNA sequences and excluded from the analysis.

Results

Here we describe a novel method, named MitoRS, in which the entire mitochondrial genome is amplified in a single reaction using rolling circle amplification. This approach is easier to setup and of higher throughput when compared to classical PCR amplification. Sequencing libraries are generated at high throughput exploiting a tagmentation-based method. Fine-tuned parameters are finally applied in the analysis to allow detection of variants even of low frequency heteroplasmy. The method was thoroughly benchmarked in a set of experiments designed to demonstrate its robustness, accuracy and sensitivity. The MitoRS method requires 5 ng total DNA as starting material. More than 96 samples can be processed in less than a day of laboratory work and sequenced in a single lane of an Illumina HiSeq flow cell. The lower limit for accurate quantification of single nucleotide variants has been measured at 1% frequency.

Conclusions

The MitoRS method enables the robust, accurate, and sensitive analysis of a large number of samples. Because it is cost effective and simple to setup, we anticipate this method will promote the analysis of mtDNA variants in large cohorts, and may help assessing the impact of mtDNA heteroplasmy on metabolic health, brain function, cancer progression, or ageing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3695-5) contains supplementary material, which is available to authorized users.

Improvements and impacts of GRCh38 human reference on high throughput sequencing data analysis

Analyses of high throughput sequencing data starts with alignment against a reference genome, which is the foundation for all re-sequencing data analyses. Each new release of the human reference genome has been augmented with improved accuracy and completeness. It is presumed that the latest release of human reference genome, GRCh38 will contribute more to high throughput sequencing data analysis by providing more accuracy. But the amount of improvement has not yet been quantified. We conducted a study to compare the genomic analysis results between the GRCh38 reference and its predecessor GRCh37. Through analyses of alignment, single nucleotide polymorphisms, small insertion/deletions, copy number and structural variants, we show that GRCh38 offers overall more accurate analysis of human sequencing data. More importantly, GRCh38 produced fewer false positive structural variants. In conclusion, GRCh38 is an improvement over GRCh37 not only from the genome assembly aspect, but also yields more reliable genomic analysis results.

The ability of human nuclear DNA to cause false positive low-abundance heteroplasmy calls varies across the mitochondrial genome

Background

Low-abundance mutations in mitochondrial populations (mutations with minor allele frequency ≤ 1%), are associated with cancer, aging, and neurodegenerative disorders. While recent progress in high-throughput sequencing technology has significantly improved the heteroplasmy identification process, the ability of this technology to detect low-abundance mutations can be affected by the presence of similar sequences originating from nuclear DNA (nDNA). To determine to what extent nDNA can cause false positive low-abundance heteroplasmy calls, we have identified mitochondrial locations of all subsequences that are common or similar (one mismatch allowed) between nDNA and mitochondrial DNA (mtDNA).

Results

Performed analysis revealed up to a 25-fold variation in the lengths of longest common and longest similar (one mismatch allowed) subsequences across the mitochondrial genome. The size of the longest subsequences shared between nDNA and mtDNA in several regions of the mitochondrial genome were found to be as low as 11 bases, which not only allows using these regions to design new, very specific PCR primers, but also supports the hypothesis of the non-random introduction of mtDNA into the human nuclear DNA.

Conclusion

Analysis of the mitochondrial locations of the subsequences shared between nDNA and mtDNA suggested that even very short (36 bases) single-end sequencing reads can be used to identify low-abundance variation in 20.4% of the mitochondrial genome. For longer (76 and 150 bases) reads, the proportion of the mitochondrial genome where nDNA presence will not interfere found to be 44.5 and 67.9%, when low-abundance mutations at 100% of locations can be identified using 417 bases long single reads. This observation suggests that the analysis of low-abundance variations in mitochondria population can be extended to a variety of large data collections such as NCBI Sequence Read Archive, European Nucleotide Archive, The Cancer Genome Atlas, and International Cancer Genome Consortium.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3375-x) contains supplementary material, which is available to authorized users.