Pattern of organization of human mitochondrial pseudogenes in the nuclear genome

Mitochondrial tRNAGlu 14693A>G Mutation, an "Enhancer" to the Phenotypic Expression of Leber's Hereditary Optic Neuropathy

Leber's hereditary optic neuropathy (LHON), a maternally inherited ocular disease, is predominantly caused by mitochondrial DNA (mtDNA) mutations. Mitochondrial tRNA variants are hypothesized to amplify the pathogenic impact of three primary mutations. However, the exact mechanisms remained unclear. In the present study, the synergistic effect of the tRNAGlu 14693A>G and ND6 14484T>C mutations in three Chinese families affected by LHON is investigated. The m.14693A>G mutation nearly abolishes the pseudouridinylation at position 55 of tRNAGlu, leading to structural abnormalities, decreased stability, aberrant mitochondrial protein synthesis, and increased autophagy. In contrast, the ND6 14484T>C mutation predominantly impairs complex I function, resulting in heightened apoptosis and virtually no induction of mitochondrial autophagy compared to control cell lines. The presence of dual mutations in the same cell lines exhibited a coexistence of both upregulated cellular stress responses to mitochondrial damage, indicating a scenario of autophagy and mutation dysregulation within these dual-mutant cell lines. The data proposes a novel hypothesis that mitochondrial tRNA gene mutations generally lead to increased mitochondrial autophagy, while mutations in genes encoding mitochondrial proteins typically induce apoptosis, shedding light on the intricate interplay between different genetic factors in the manifestation of LHON.

Comparative Genome Microsynteny Illuminates the Fast Evolution of Nuclear Mitochondrial Segments (NUMTs) in Mammals

The escape of DNA from mitochondria into the nuclear genome (nuclear mitochondrial DNA, NUMT) is an ongoing process. Although pervasively observed in eukaryotic genomes, their evolutionary trajectories in a mammal-wide context are poorly understood. The main challenge lies in the orthology assignment of NUMTs across species due to their fast evolution and chromosomal rearrangements over the past 200 million years. To address this issue, we systematically investigated the characteristics of NUMT insertions in 45 mammalian genomes and established a novel, synteny-based method to accurately predict orthologous NUMTs and ascertain their evolution across mammals. With a series of comparative analyses across taxa, we revealed that NUMTs may originate from nonrandom regions in mtDNA, are likely found in transposon-rich and intergenic regions, and unlikely code for functional proteins. Using our synteny-based approach, we leveraged 630 pairwise comparisons of genome-wide microsynteny and predicted the NUMT orthology relationships across 36 mammals. With the phylogenetic patterns of NUMT presence-and-absence across taxa, we constructed the ancestral state of NUMTs given the mammal tree using a coalescent method. We found support on the ancestral node of Fereuungulata within Laurasiatheria, whose subordinal relationships are still controversial. This study broadens our knowledge on NUMT insertion and evolution in mammalian genomes and highlights the merit of NUMTs as alternative genetic markers in phylogenetic inference.

Numerous insertions of mitochondrial DNA in the genome of the northern mole vole, Ellobius talpinus

BACKGROUND

Ellobius talpinus is a subterranean rodent representing an attractive model in population ecology studies due to its highly special lifestyle and sociality. In such studies, mitochondrial DNA (mtDNA) is widely used. However, if nuclear copies of mtDNA, aka NUMTs, are present, they may co-amplify with the target mtDNA fragment, generating misleading results. The aim of this study was to determine whether NUMTs are present in E. talpinus.

METHODS AND RESULTS

PCR amplification of the putative mtDNA CytB-D-loop fragment using 'universal' primers from 56 E. talpinus samples produced multiple double peaks in 90% of the sequencing chromatograms. To reveal NUMTs, molecular cloning and sequencing of PCR products of three specimens was conducted, followed by phylogenetic analysis. The pseudogene nature of three out of the seven detected haplotypes was confirmed by their basal positions in relation to other Ellobius haplotypes in the phylogenetic tree. Additionally, 'haplotype B' was basal in relation to other E. talpinus haplotypes and found present in very distant sampling sites. BLASTN search revealed 195 NUMTs in the E. talpinus nuclear genome, including fragments of all four PCR amplified pseudogenes. Although the majority of the NUMTs studied were short, the entire mtDNA had copies in the nuclear genome. The most numerous NUMTs were found for rrnL, COXI, and D-loop.

CONCLUSIONS

Numerous NUMTs are present in E. talpinus and can be difficult to discriminate against mtDNA sequences. Thus, in future population or phylogenetic studies in E. talpinus, the possibility of cryptic NUMTs amplification should always be taken into account.

Comprehensive Identification of Mitochondrial Pseudogenes (NUMTs) in the Human Telomere-to-Telomere Reference Genome

Practices related to mitochondrial research have long been hindered by the presence of mitochondrial pseudogenes within the nuclear genome (NUMTs). Even though partially assembled human reference genomes like hg38 have included NUMTs compilation, the exhaustive NUMTs within the only complete reference genome (T2T-CHR13) remain unknown. Here, we comprehensively identified the fixed NUMTs within the reference genome using human pan-mitogenome (HPMT) from GeneBank. The inclusion of HPMT serves the purpose of establishing an authentic mitochondrial DNA (mtDNA) mutational spectrum for the identification of NUMTs, distinguishing it from the polymorphic variations found in NUMTs. Using HPMT, we identified approximately 10% of additional NUMTs in three human reference genomes under stricter thresholds. And we also observed an approximate 6% increase in NUMTs in T2T-CHR13 compared to hg38, including NUMTs on the short arms of chromosomes 13, 14, and 15 that were not assembled previously. Furthermore, alignments based on 20-mer from mtDNA suggested the presence of more mtDNA-like short segments within the nuclear genome, which should be avoided for short amplicon or cell free mtDNA detection. Finally, through the assay of transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) on cell lines before and after mtDNA elimination, we concluded that NUMTs have a minimal impact on bulk ATAC-seq, even though 16% of sequencing data originated from mtDNA.

The constraints of allotopic expression

Allotopic expression is the functional transfer of an organellar gene to the nucleus, followed by synthesis of the gene product in the cytosol and import into the appropriate organellar sub compartment. Here, we focus on mitochondrial genes encoding OXPHOS subunits that were naturally transferred to the nucleus, and critically review experimental evidence that claim their allotopic expression. We emphasize aspects that may have been overlooked before, i.e., when modifying a mitochondrial gene for allotopic expression━besides adapting the codon usage and including sequences encoding mitochondrial targeting signals━three additional constraints should be considered: (i) the average apparent free energy of membrane insertion (μΔGapp) of the transmembrane stretches (TMS) in proteins earmarked for the inner mitochondrial membrane, (ii) the final, functional topology attained by each membrane-bound OXPHOS subunit; and (iii) the defined mechanism by which the protein translocator TIM23 sorts cytosol-synthesized precursors. The mechanistic constraints imposed by TIM23 dictate the operation of two pathways through which alpha-helices in TMS are sorted, that eventually determine the final topology of membrane proteins. We used the biological hydrophobicity scale to assign an average apparent free energy of membrane insertion (μΔGapp) and a "traffic light" color code to all TMS of OXPHOS membrane proteins, thereby predicting which are more likely to be internalized into mitochondria if allotopically produced. We propose that the design of proteins for allotopic expression must make allowance for μΔGapp maximization of highly hydrophobic TMS in polypeptides whose corresponding genes have not been transferred to the nucleus in some organisms.

The Mighty NUMT: Mitochondrial DNA Flexing Its Code in the Nuclear Genome

Nuclear-mitochondrial DNA segments (NUMTs) are mitochondrial DNA (mtDNA) fragments that have been inserted into the nuclear genome. Some NUMTs are common within the human population but most NUMTs are rare and specific to individuals. NUMTs range in size from 24 base pairs to encompassing nearly the entire mtDNA and are found throughout the nuclear genome. Emerging evidence suggests that the formation of NUMTs is an ongoing process in humans. NUMTs contaminate sequencing results of the mtDNA by introducing false positive variants, particularly heteroplasmic variants present at a low variant allele frequency (VAF). In our review, we discuss the prevalence of NUMTs in the human population, the potential mechanisms of de novo NUMT insertion via DNA repair mechanisms, and provide an overview of the existing approaches for minimizing NUMT contamination. Apart from filtering known NUMTs, both wet lab-based and computational methods can be used to minimize the contamination of NUMTs in analyses of human mtDNA. Current approaches include: (1) isolating mitochondria to enrich for mtDNA; (2) applying basic local alignment to identify NUMTs for subsequent filtering; (3) bioinformatic pipelines for NUMT detection; (4) k-mer-based NUMT detection; and (5) filtering candidate false positive variants by mtDNA copy number, VAF, or sequence quality score. Multiple approaches must be applied in order to effectively identify NUMTs in samples. Although next-generation sequencing is revolutionizing our understanding of heteroplasmic mtDNA, it also raises new challenges with the high prevalence and individual-specific NUMTs that need to be handled with care in studies of mitochondrial genetics.

Assembly-free quantification of vagrant DNA inserts

Inserts of DNA from extranuclear sources, such as organelles and microbes, are common in eukaryote nuclear genomes. However, sequence similarity between the nuclear and extranuclear DNA, and a history of multiple insertions, make the assembly of these regions challenging. Consequently, the number, sequence and location of these vagrant DNAs cannot be reliably inferred from the genome assemblies of most organisms. We introduce two statistical methods to estimate the abundance of nuclear inserts even in the absence of a nuclear genome assembly. The first (intercept method) only requires low-coverage (<1×) sequencing data, as commonly generated for population studies of organellar and ribosomal DNAs. The second method additionally requires that a subset of the individuals carry extranuclear DNA with diverged genotypes. We validated our intercept method using simulations and by re-estimating the frequency of human NUMTs (nuclear mitochondrial inserts). We then applied it to the grasshopper Podisma pedestris, exceptional for both its large genome size and reports of numerous NUMT inserts, estimating that NUMTs make up 0.056% of the nuclear genome, equivalent to >500 times the mitochondrial genome size. We also re-analysed a museomics data set of the parrot Psephotellus varius, obtaining an estimate of only 0.0043%, in line with reports from other species of bird. Our study demonstrates the utility of low-coverage high-throughput sequencing data for the quantification of nuclear vagrant DNAs. Beyond quantifying organellar inserts, these methods could also be used on endosymbiont-derived sequences. We provide an R implementation of our methods called "vagrantDNA" and code to simulate test data sets.

Precise and simultaneous quantification of mitochondrial DNA heteroplasmy and copy number by digital PCR

Mitochondrial DNA (mtDNA) is present in multiple copies and phenotypic consequences of mtDNA mutations depend on the mutant load surpassing a specific threshold. Additionally, changes in mtDNA copy number can impact mitochondrial ATP production, resulting in disease. Therefore, the precise determination of mtDNA heteroplasmy and copy number is crucial to the study of mitochondrial diseases. However, current methods can be imprecise, and quantifying small changes in either heteroplasmy or copy number is challenging. We developed a new approach to measure mtDNA heteroplasmy using a single digital PCR (dPCR) probe. This method is based on the observation that fluorescent-labeled probes in dPCR exhibit different intensities depending on the presence of a single nucleotide change in the sequence bound by the probe. This finding allowed us to precisely and simultaneously determine mtDNA copy number and heteroplasmy levels using duplex dPCR. We tested this approach in two different models (human and mouse), which proved faster and more internally controlled when compared to other published methods routinely used in the mitochondrial genetics field. We believe this approach could be broadly applicable to the detection and quantification of other mixed genetic variations.

Fishing for DNA? Designing baits for population genetics in target enrichment experiments: Guidelines, considerations and the new tool supeRbaits

Targeted sequencing is an increasingly popular next-generation sequencing (NGS) approach for studying populations that involves focusing sequencing efforts on specific parts of the genome of a species of interest. Methodologies and tools for designing targeted baits are scarce but in high demand. Here, we present specific guidelines and considerations for designing capture sequencing experiments for population genetics for both neutral genomic regions and regions subject to selection. We describe the bait design process for three diverse fish species: Atlantic salmon, Atlantic cod and tiger shark, which was carried out in our research group, and provide an evaluation of the performance of our approach across both historical and modern samples. The workflow used for designing these three bait sets has been implemented in the R-package supeRbaits, which encompasses our considerations and guidelines for bait design for the benefit of researchers and practitioners. The supeRbaits R-package is user-friendly and versatile. It is written in C++ and implemented in R. supeRbaits and its manual are available from Github: https://github.com/BelenJM/supeRbaits.

Complete Mitochondrial Genomes of Metcalfa pruinosa and Salurnis marginella (Hemiptera: Flatidae): Genomic Comparison and Phylogenetic Inference in Fulgoroidea

The complete mitochondrial genomes (mitogenomes) of two DNA barcode-defined haplotypes of Metcalfa pruinosa and one of Salurnis marginella (Hemiptera: Flatidae) were sequenced and compared to those of other Fulgoroidea species. Furthermore, the mitogenome sequences were used to reconstruct phylogenetic relationships among fulgoroid families. The three mitogenomes, including that of the available species of Flatidae, commonly possessed distinctive structures in the 1702-1836 bp A+T-rich region, such as two repeat regions at each end and a large centered nonrepeat region. All members of the superfamily Fulgoroidea, including the Flatidae, consistently possessed a motiflike sequence (TAGTA) at the ND1 and trnS₂ junction. The phylogenetic analyses consistently recovered the familial relationships of (((((Ricaniidae + Issidae) + Flatidae) + Fulgoridae) + Achilidae) + Derbidae) in the amino acid-based analysis, with the placement of Cixiidae and Delphacidae as the earliest-derived lineages of fulgoroid families, whereas the monophyly of Delphacidae was not congruent between tree-constructing algorithms.

Mitochondrial DNA variation and cancer

Variation in the mitochondrial DNA (mtDNA) sequence is common in certain tumours. Two classes of cancer mtDNA variants can be identified: de novo mutations that act as 'inducers' of carcinogenesis and functional variants that act as 'adaptors', permitting cancer cells to thrive in different environments. These mtDNA variants have three origins: inherited variants, which run in families, somatic mutations arising within each cell or individual, and variants that are also associated with ancient mtDNA lineages (haplogroups) and are thought to permit adaptation to changing tissue or geographic environments. In addition to mtDNA sequence variation, mtDNA copy number and perhaps transfer of mtDNA sequences into the nucleus can contribute to certain cancers. Strong functional relevance of mtDNA variation has been demonstrated in oncocytoma and prostate cancer, while mtDNA variation has been reported in multiple other cancer types. Alterations in nuclear DNA-encoded mitochondrial genes have confirmed the importance of mitochondrial metabolism in cancer, affecting mitochondrial reactive oxygen species production, redox state and mitochondrial intermediates that act as substrates for chromatin-modifying enzymes. Hence, subtle changes in the mitochondrial genotype can have profound effects on the nucleus, as well as carcinogenesis and cancer progression.

Mitochondrial noncoding RNAs: new wine in an old bottle

Mitochondrial noncoding RNAs (mt-ncRNAs) include noncoding RNAs inside the mitochondria that are transcribed from the mitochondrial genome or nuclear genome, and noncoding RNAs transcribed from the mitochondrial genome that are transported to the cytosol or nucleus. Recent findings have revealed that mt-ncRNAs play important roles in not only mitochondrial functions, but also other cellular activities. This review proposes a classification of mt-ncRNAs and outlines the emerging understanding of mitochondrial circular RNAs (mt-circRNAs), mitochondrial microRNAs (mitomiRs), and mitochondrial long noncoding RNAs (mt-lncRNAs), with an emphasis on their identification and functions.

Mitochondrial DNA insert into CD40 ligand gene-associated X-linked hyper-IgM syndrome

BACKGROUND

X-linked hyper-IgM (X-HIGM), which results from mutations in the CD40LG gene located on chromosome Xq26.3, is the most common form of HIGM. To date, more than 130 variants of the CD40L gene have been reported. We described a patient with novel de novo nuclear mitochondrial DNA sequences (NUMTs) in the CD40LG gene that have resulted in X-HIGM.

METHODS

Whole-exome sequencing (WES) analysis was used to screen for causal variants in the genome, and the candidate breakpoint was confirmed by Sanger sequencing.

RESULTS

A new mutation of CD40LG, which deletes A at position 17 followed by a 147-nucleotide from mitochondrial DNA copies insertion in exon 1, was detected in a 20-month-old boy harbouring an X-HIGM combined with immunodeficiency syndrome.

CONCLUSION

This is one of the few cases of a human genetic disease caused by nuclear mitochondrial DNA sequences (NUMTs). The presented data serve to demonstrate that de novo NUMT transfer of nucleic acid is a novel mechanism of X-HIGM.

Intronic tRNAs of mitochondrial origin regulate constitutive and alternative splicing

BACKGROUND

The presence of nuclear mitochondrial DNA (numtDNA) has been reported within several nuclear genomes. Next to mitochondrial protein-coding genes, numtDNA sequences also encode for mitochondrial tRNA genes. However, the biological roles of numtDNA remain elusive.

RESULTS

Employing in silico analysis, we identify 281 mitochondrial tRNA homologs in the human genome, which we term nimtRNAs (nuclear intronic mitochondrial-derived tRNAs), being contained within introns of 76 nuclear host genes. Despite base changes in nimtRNAs when compared to their mtRNA homologs, a canonical tRNA cloverleaf structure is maintained. To address potential functions of intronic nimtRNAs, we insert them into introns of constitutive and alternative splicing reporters and demonstrate that nimtRNAs promote pre-mRNA splicing, dependent on the number and positioning of nimtRNA genes and splice site recognition efficiency. A mutational analysis reveals that the nimtRNA cloverleaf structure is required for the observed splicing increase. Utilizing a CRISPR/Cas9 approach, we show that a partial deletion of a single endogenous nimtRNALys within intron 28 of the PPFIBP1 gene decreases inclusion of the downstream-located exon 29 of the PPFIBP1 mRNA. By employing a pull-down approach followed by mass spectrometry, a 3'-splice site-associated protein network is identified, including KHDRBS1, which we show directly interacts with nimtRNATyr by an electrophoretic mobility shift assay.

CONCLUSIONS

We propose that nimtRNAs, along with associated protein factors, can act as a novel class of intronic splicing regulatory elements in the human genome by participating in the regulation of splicing.

Mitochondrial Fostering: The Mitochondrial Genome May Play a Role in Plant Orphan Gene Evolution

Plant mitochondrial genomes exhibit unique evolutionary patterns. They have a high rearrangement but low mutation rate, and a large size. Based on massive mitochondrial DNA transfers to the nucleus as well as the mitochondrial unique evolutionary traits, we propose a "Mitochondrial Fostering" theory where the organelle genome plays an integral role in the arrival and development of orphan genes (genes with no homologs in other lineages). Two approaches were used to test this theory: (1) bioinformatic analysis of nuclear mitochondrial DNA (Numts: mitochondrial originating DNA that migrated to the nucleus) at the genome level, and (2) bioinformatic analysis of particular orphan sequences present in both the mitochondrial genome and the nuclear genome of Arabidopsis thaliana. One study example is given about one orphan sequence that codes for two unique orphan genes: one in the mitochondrial genome and another one in the nuclear genome. DNA alignments show regions of this A. thaliana orphan sequence exist scattered throughout other land plant mitochondrial genomes. This is consistent with the high recombination rates of mitochondrial genomes in land plants. This may also enable the creation of novel coding sequences within the orphan loci, which can then be transferred to the nuclear genome and become exposed to new evolutionary pressures. Our study also reveals a high correlation between the amount of mitochondrial DNA transferred to the nuclear genome and the number of orphan genes in land plants. All the data suggests the mitochondrial genome may play a role in nuclear orphan gene evolution in land plants.

Identification of Mitochondrial DNA (NUMTs) in the Nuclear Genome of Daphnia magna

This is the first study in which the Daphnia magna (D. magna) nuclear genome (nDNA) obtained from the GenBank database was analyzed for pseudogene sequences of mitochondrial origin. To date, there is no information about pseudogenes localized in D. magna genome. This study aimed to identify NUMTs, their length, homology, and location for potential use in evolutionary studies and to check whether their occurrence causes co-amplification during mitochondrial genome (mtDNA) analyses. Bioinformatic analysis showed 1909 fragments of the mtDNA of D. magna, of which 1630 were located in ten linkage groups (LG) of the nDNA. The best-matched NUMTs covering >90% of the gene sequence have been identified for two mt-tRNA genes, and they may be functional nuclear RNA molecules. Isolating the total DNA in mtDNA studies, co-amplification of nDNA fragments is unlikely in the case of amplification of the whole tRNA genes as well as fragments of other genes. It was observed that TRNA-MET fragments had the highest level of sequence homology, thus they could be evolutionarily the youngest. The lowest homology was found in the D-loop-derived pseudogene. It may probably be the oldest NUMT incorporated into the nDNA; however, further analysis is necessary.

Insertions of mitochondrial DNA into the nucleus—effects and role in cell evolution

We review the insertion of mitochondrial DNA (mtDNA) fragments into nuclear DNA (NUMTS) as a general and ongoing process that has occurred many times during genome evolution. Fragments of mtDNA are generated during the lifetime of organisms in both somatic and germinal cells, by the production of reactive oxygen species in the mitochondria. The fragments are inserted into the nucleus during the double-strand breaks repair via the non-homologous end-joining machinery, followed by genomic instability, giving rise to the high variability observed in NUMT patterns among species, populations, or genotypes. Some de novo produced mtDNA insertions show harmful effects, being involved in human diseases, carcinogenesis, and ageing. NUMT generation is a non-stop process overpassing the Mendelian transmission. This parasitic property ensures their survival even against their harmful effects. The accumulation of mtDNA fragments mainly at pericentromeric and subtelomeric regions is important to understand the transmission and integration of NUMTs into the genomes. The possible effect of female meiotic drive for mtDNA insertions at centromeres remains to be studied. In spite of the harmful feature of NUMTs, they are important in cell evolution, representing a major source of genomic variation.

Genetic factors in isolated and syndromic laryngeal cleft

A laryngotracheoesophageal cleft (LC) is a rare congenital anomaly of the upper aerodigestive tract resulting from the absence of fusion of the posterior cricoid lamina, which affects an abnormal communication between the larynx, trachea and esophagus. The genetic etiology of LC remains elusive. The involvement of genetic factors in the development of LC is suggested by reports of familial occurrence, and the increased prevalence of component features among first-degree relatives of affected individuals and murine knockout models. No consistent pattern of inheritance has been found in nonsyndromic patients, except for cases associated with described syndromes. Once the syndrome related to the laryngeal cleft is considered, an active search for the cleft must be initiated. The genetic evaluation of patients with LCs should be guided by the type and location of the malformation, specific medical history and a detailed physical examination. The application of genetic approaches, such as microarrays and exome sequencing might lead to elucidating the etiology of LCs.

Mitoepigenetics and Its Emerging Roles in Cancer

In human beings, there is a ∼16,569 bp circular mitochondrial DNA (mtDNA) encoding 22 tRNAs, 12S and 16S rRNAs, 13 polypeptides that constitute the central core of ETC/OxPhos complexes, and some non-coding RNAs. Recently, mtDNA has been shown to have some covalent modifications such as methylation or hydroxylmethylation, which play pivotal epigenetic roles in mtDNA replication and transcription. Post-translational modifications of proteins in mitochondrial nucleoids such as mitochondrial transcription factor A (TFAM) also emerge as essential epigenetic modulations in mtDNA replication and transcription. Post-transcriptional modifications of mitochondrial RNAs (mtRNAs) including mt-rRNAs, mt-tRNAs and mt-mRNAs are important epigenetic modulations. Besides, mtDNA or nuclear DNA (n-DNA)-derived non-coding RNAs also play important roles in the regulation of translation and function of mitochondrial genes. These evidences introduce a novel concept of mitoepigenetics that refers to the study of modulations in the mitochondria that alter heritable phenotype in mitochondria itself without changing the mtDNA sequence. Since mitochondrial dysfunction contributes to carcinogenesis and tumor development, mitoepigenetics is also essential for cancer. Understanding the mode of actions of mitoepigenetics in cancers may shade light on the clinical diagnosis and prevention of these diseases. In this review, we summarize the present study about modifications in mtDNA, mtRNA and nucleoids and modulations of mtDNA/nDNA-derived non-coding RNAs that affect mtDNA translation/function, and overview recent studies of mitoepigenetic alterations in cancer.

Nuclear Integrants of Organellar DNA Contribute to Genome Structure and Evolution in Plants

The transfer of genetic material from the mitochondria and plastid to the nucleus gives rise to nuclear integrants of mitochondrial DNA (NUMTs) and nuclear integrants of plastid DNA (NUPTs). This frequently occurring DNA transfer is ongoing and has important evolutionary implications. In this review, based on previous studies and the analysis of NUMT/NUPT insertions of more than 200 sequenced plant genomes, we analyzed and summarized the general features of NUMTs/NUPTs and highlighted the genetic consequence of organellar DNA insertions. The statistics of organellar DNA integrants among various plant genomes revealed that organellar DNA-derived sequence content is positively correlated with the nuclear genome size. After integration, the nuclear organellar DNA could undergo different fates, including elimination, mutation, rearrangement, fragmentation, and proliferation. The integrated organellar DNAs play important roles in increasing genetic diversity, promoting gene and genome evolution, and are involved in sex chromosome evolution in dioecious plants. The integrating mechanisms, involving non-homologous end joining at double-strand breaks were also discussed.

Maternally Inherited Diabetes Mellitus Associated with a Novel m.15897G>A Mutation in Mitochondrial tRNAThr Gene

We report here the clinical, genetic, and molecular characteristics of type 2 diabetes in a Chinese family. There are differences in the severity and age of onset in diabetes among these families. By molecular analysis of the complete mitochondrial genome in this family, we identified the homoplasmic m.15897G>A mutation underwent sequence analysis of whole mitochondrial DNA genome, which localized at conventional position ten of tRNA^Thr, and distinct sets of mtDNA polymorphisms belonging to haplogroup D4b1. This mutation has been implicated to be important for tRNA identity and stability. Using cybrid cell models, the decreased efficiency of mitochondrial tRNA^Thr levels caused by the m.15897G>A mutation results in respiratory deficiency, protein synthesis and assembly, mitochondrial ATP synthesis, and mitochondrial membrane potential. These mitochondrial dysfunctions caused an increase in the production of reactive oxygen species in the mutant cell lines. These data provide a direct evidence that a novel tRNA mutation was associated with T2DM. Thus, our findings provide a new insight into the understanding of pathophysiology of maternally inherited diabetes.

Mitochondrial DNA: Epigenetics and environment

Maintenance of the mitochondrial genome is essential for proper cellular function. For this purpose, mitochondrial DNA (mtDNA) needs to be faithfully replicated, transcribed, translated, and repaired in the face of constant onslaught from endogenous and environmental agents. Although only 13 polypeptides are encoded within mtDNA, the mitochondrial proteome comprises over 1500 proteins that are encoded by nuclear genes and translocated to the mitochondria for the purpose of maintaining mitochondrial function. Regulation of mtDNA and mitochondrial proteins by epigenetic changes and post-translational modifications facilitate crosstalk between the nucleus and the mitochondria and ultimately lead to the maintenance of cellular health and homeostasis. DNA methyl transferases have been identified in the mitochondria implicating that methylation occurs within this organelle; however, the extent to which mtDNA is methylated has been debated for many years. Mechanisms of demethylation within this organelle have also been postulated, but the exact mechanisms and their outcomes is still an active area of research. Mitochondrial dysfunction in the form of altered gene expression and ATP production, resulting from epigenetic changes, can lead to various conditions including aging-related neurodegenerative disorders, altered metabolism, changes in circadian rhythm, and cancer. Here, we provide an overview of the epigenetic regulation of mtDNA via methylation, long and short noncoding RNAs, and post-translational modifications of nucleoid proteins (as mitochondria lack histones). We also highlight the influence of xenobiotics such as airborne environmental pollutants, contamination from heavy metals, and therapeutic drugs on mtDNA methylation. Environ. Mol. Mutagen., 60:668-682, 2019. © 2019 Wiley Periodicals, Inc.

Alignment-free approaches for predicting novel Nuclear Mitochondrial Segments (NUMTs) in the human genome

The nuclear human genome harbors sequences of mitochondrial origin, indicating an ancestral transfer of DNA from the mitogenome. Several Nuclear Mitochondrial Segments (NUMTs) have been detected by alignment-based sequence similarity search, as implemented in the Basic Local Alignment Search Tool (BLAST). Identifying NUMTs is important for the comprehensive annotation and understanding of the human genome. Here we explore the possibility of detecting NUMTs in the human genome by alignment-free sequence similarity search, such as k-mers (k-tuples, k-grams, oligos of length k) distributions. We find that when k=6 or larger, the k-mer approach and BLAST search produce almost identical results, e.g., detect the same set of NUMTs longer than 3 kb. However, when k=5 or k=4, certain signals are only detected by the alignment-free approach, and these may indicate yet unrecognized, and potentially more ancestral NUMTs. We introduce a "Manhattan plot" style representation of NUMT predictions across the genome, which are calculated based on the reciprocal of the Jensen-Shannon divergence between the nuclear and mitochondrial k-mer frequencies. The further inspection of the k-mer-based NUMT predictions however shows that most of them contain long-terminal-repeat (LTR) annotations, whereas BLAST-based NUMT predictions do not. Thus, similarity of the mitogenome to LTR sequences is recognized, which we validate by finding the mitochondrial k-mer distribution closer to those for transposable sequences and specifically, close to some types of LTR.

Mitochondrial determinants of cancer health disparities

Mitochondria, which are multi-functional, have been implicated in cancer initiation, progression, and metastasis due to metabolic alterations in transformed cells. Mitochondria are involved in the generation of energy, cell growth and differentiation, cellular signaling, cell cycle control, and cell death. To date, the mitochondrial basis of cancer disparities is unknown. The goal of this review is to provide an understanding and a framework of mitochondrial determinants that may contribute to cancer disparities in racially different populations. Due to maternal inheritance and ethnic-based diversity, the mitochondrial genome (mtDNA) contributes to inherited racial disparities. In people of African ancestry, several germline, population-specific haplotype variants in mtDNA as well as depletion of mtDNA have been linked to cancer predisposition and cancer disparities. Indeed, depletion of mtDNA and mutations in mtDNA or nuclear genome (nDNA)-encoded mitochondrial proteins lead to mitochondrial dysfunction and promote resistance to apoptosis, the epithelial-to-mesenchymal transition, and metastatic disease, all of which can contribute to cancer disparity and tumor aggressiveness related to racial disparities. Ethnic differences at the level of expression or genetic variations in nDNA encoding the mitochondrial proteome, including mitochondria-localized mtDNA replication and repair proteins, miRNA, transcription factors, kinases and phosphatases, and tumor suppressors and oncogenes may underlie susceptibility to high-risk and aggressive cancers found in African population and other ethnicities. The mitochondrial retrograde signaling that alters the expression profile of nuclear genes in response to dysfunctional mitochondria is a mechanism for tumorigenesis. In ethnic populations, differences in mitochondrial function may alter the cross talk between mitochondria and the nucleus at epigenetic and genetic levels, which can also contribute to cancer health disparities. Targeting mitochondrial determinants and mitochondrial retrograde signaling could provide a promising strategy for the development of selective anticancer therapy for dealing with cancer disparities. Further, agents that restore mitochondrial function to optimal levels should permit sensitivity to anticancer agents for the treatment of aggressive tumors that occur in racially diverse populations and hence help in reducing racial disparities.

Defining the momiome: Promiscuous information transfer by mobile mitochondria and the mitochondrial genome

Mitochondria are complex intracellular organelles that have long been identified as the powerhouses of eukaryotic cells because of the central role they play in oxidative metabolism. A resurgence of interest in the study of mitochondria during the past decade has revealed that mitochondria also play key roles in cell signaling, proliferation, cell metabolism and cell death, and that genetic and/or metabolic alterations in mitochondria contribute to a number of diseases, including cancer. Mitochondria have been identified as signaling organelles, capable of mediating bidirectional intracellular information transfer: anterograde (from nucleus to mitochondria) and retrograde (from mitochondria to nucleus). More recently, evidence is now building that the role of mitochondria extends to intercellular communication as well, and that the mitochondrial genome (mtDNA) and even whole mitochondria are indeed mobile and can mediate information transfer between cells. We define this promiscuous information transfer function of mitochondria and mtDNA as "momiome" to include all mobile functions of mitochondria and the mitochondrial genome. Herein, we review the "momiome" and explore its role in cancer development, progression, and treatment.

NumtS colonization in mammalian genomes

The colonization of the nuclear genome by mitochondrial DNA is an ongoing process in eukaryotes and plays an important role in genomic variability. Notwithstanding the DNA sequence availability of about 100 complete eukaryotic genomes, up to now NumtS distribution has been fully reported for a small number of sequenced eukaryotic species. With the aim to clarify the time and way of NumtS evolution, we explored the genomic distribution of NumtS in 23 eukaryotic species using an intra/interspecies in silico approach based on a cross-species similarity search and deeply investigate the evolution of NumtS in mammals. The intra- and interspecies analysis underlined how some mitochondrial regions that populated nuclear genomes can be considered as hotspots. Considering the large amount of NumtS we found in platypus and opossum genomes, we hypothesized the occurrence of an earlier colonization that happened prior to the Prototherian/Therian mammal divergence, approximately 160-210 million years ago. These events are still detectable due to the species-specific dynamics that have affected these genomes. Phylogenetic analyses of NumtS derived from two different mitochondrial DNA loci allowed us to recognize the unusual NumtS evolution that acted differently on primate and non-primate species' genomes.

Quantifying the Number of Independent Organelle DNA Insertions in Genome Evolution and Human Health

Fragments of organelle genomes are often found as insertions in nuclear DNA. These fragments of mitochondrial DNA (numts) and plastid DNA (nupts) are ubiquitous components of eukaryotic genomes. They are, however, often edited out during the genome assembly process, leading to systematic underestimation of their frequency. Numts and nupts, once inserted, can become further fragmented through subsequent insertion of mobile elements or other recombinational events that disrupt the continuity of the inserted sequence relative to the genuine organelle DNA copy. Because numts and nupts are typically identified through sequence comparison tools such as BLAST, disruption of insertions into smaller fragments can lead to systematic overestimation of numt and nupt frequencies. Accurate identification of numts and nupts is important, however, both for better understanding of their role during evolution, and for monitoring their increasingly evident role in human disease. Human populations are polymorphic for 141 numt loci, five numts are causal to genetic disease, and cancer genomic studies are revealing an abundance of numts associated with tumor progression. Here, we report investigation of salient parameters involved in obtaining accurate estimates of numt and nupt numbers in genome sequence data. Numts and nupts from 44 sequenced eukaryotic genomes reveal lineage-specific differences in the number, relative age and frequency of insertional events as well as lineage-specific dynamics of their postinsertional fragmentation. Our findings outline the main technical parameters influencing accurate identification and frequency estimation of numts in genomic studies pertinent to both evolution and human health.

Cellular and population level processes influence the rate, accumulation and observed frequency of inherited and somatic mtDNA mutations

Mitochondria are found in all animals and have the unique feature of containing multiple copies of their own small, circular DNA genome (mtDNA). The rate and pattern of mutation accumulation in the mtDNA are influenced by molecular, cellular and population level processes. We distinguish between inherited and somatic mtDNA mutations and review evidence for the often-made assumption that mutations accumulate at a higher rate in mtDNA than in nuclear DNA (nDNA). We conclude that the whole genome mutation accumulation rate is higher for mtDNA than for nDNA but include the caveat that rates overlap considerably between the individual mtDNA- and nDNA-encoded genes. Next, we discuss the postulated causal mechanisms for the high rate of mtDNA mutation accumulation in both inheritance and in somatic cells. Perhaps unexpectedly, mtDNA is resilient to many mutagens of nDNA but is prone to errors of replication. We then consider the influence of maternal inheritance, recombination and selection on the observed accumulation pattern of inherited mtDNA mutations. Finally, we discuss environmental influences of temperature and diet on the observed frequency of inherited and somatic mtDNA mutations. We conclude that it is necessary to understand the cellular processes to fully interpret the pattern of mutations and how they influence our interpretations of evolution and disease.

Numtogenesis as a mechanism for development of cancer

Transfer of genetic material from cytoplasmic organelles to the nucleus, an ongoing process, has implications in evolution, aging, and human pathologies such as cancer. The transferred mitochondrial DNA (mtDNA) fragments in the nuclear genome are called nuclear mtDNA or NUMTs. We have named the process numtogenesis, defining the term as the transfer of mtDNA into the nuclear genome, or, less specifically, the transfer of mitochondria or mitochondrial components into the nucleus. There is increasing evidence of the involvement of NUMTs in human biology and pathology. Although information pertaining to NUMTs and numtogenesis is sparse, the role of this aspect of mitochondrial biology to human cancers is apparent. In this review, we present available knowledge about the origin and mechanisms of numtogenesis, with special emphasis on the role of NUMTs in human malignancies. We describe studies undertaken in our laboratory and in others and discuss the influence of NUMTs in tumor initiation and progression and in survival of cancer patients. We describe suppressors of numtogenesis and evolutionary conserved mechanisms underlying numtogenesis in cancer. An understanding the emerging field of numtogenesis should allow comprehension of this process in various malignancies and other diseases and, more generally, in human health.

Evolutionarily recent, insertional fission of mitochondrial cox2 into complementary genes in bilaterian Metazoa

Background

Mitochondrial genomes (mtDNA) of multicellular animals (Metazoa) with bilateral symmetry (Bilateria) are compact and usually carry 13 protein-coding genes for subunits of three respiratory complexes and ATP synthase. However, occasionally reported exceptions to this typical mtDNA organization prompted speculation that, as in protists and plants, some bilaterian mitogenomes may continue to lose their canonical genes, or may even acquire new genes. To shed more light on this phenomenon, a PCR-based screen was conducted to assess fast-evolving mtDNAs of apocritan Hymenoptera (Arthropoda, Insecta) for genomic rearrangements that might be associated with the modification of mitochondrial gene content.

Results

Sequencing of segmental inversions, identified in the screen, revealed that the cytochrome oxidase subunit II gene (cox2) of Campsomeris (Dielis) (Scoliidae) was split into two genes coding for COXIIA and COXIIB. The COXII-derived complementary polypeptides apparently form a heterodimer, have reduced hydrophobicity compared with the majority of mitogenome-encoded COX subunits, and one of them, COXIIB, features increased content of Cys residues. Analogous cox2 fragmentation is known only in two clades of protists (chlorophycean algae and alveolates), where it has been associated with piecewise relocation of this gene into the nucleus. In Campsomeris mtDNA, cox2a and cox2b loci are separated by a 3-kb large cluster of several antiparallel overlapping ORFs, one of which, qnu, seems to encode a nuclease that may have played a role in cox2 fission.

Conclusions

Although discontinuous mitochondrial protein genes encoding fragmented, complementary polypeptides are known in protists and some plants, split cox2 of Campsomeris is the first case of such a gene arrangement found in animals. The reported data also indicate that bilaterian animal mitogenomes may be carrying lineage-specific genes more often than previously thought, and suggest a homing endonuclease-based mechanism for insertional mitochondrial gene fission.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3626-5) contains supplementary material, which is available to authorized users.

Migration of mitochondrial DNA in the nuclear genome of colorectal adenocarcinoma

Background

Colorectal adenocarcinomas are characterized by abnormal mitochondrial DNA (mtDNA) copy number and genomic instability, but a molecular interaction between mitochondrial and nuclear genome remains unknown. Here we report the discovery of increased copies of nuclear mtDNA (NUMT) in colorectal adenocarcinomas, which supports link between mtDNA and genomic instability in the nucleus. We name this phenomenon of nuclear occurrence of mitochondrial component as numtogenesis. We provide a description of NUMT abundance and distribution in tumor versus matched blood-derived normal genomes.

Methods

Whole-genome sequence data were obtained for colon adenocarcinoma and rectum adenocarcinoma patients participating in The Cancer Genome Atlas, via the Cancer Genomics Hub, using the GeneTorrent file acquisition tool. Data were analyzed to determine NUMT proportion and distribution on a genome-wide scale. A NUMT suppressor gene was identified by comparing numtogenesis in other organisms.

Results

Our study reveals that colorectal adenocarcinoma genomes, on average, contains up to 4.2-fold more somatic NUMTs than matched normal genomes. Women colorectal tumors contained more NUMT than men. NUMT abundance in tumor predicted parallel abundance in blood. NUMT abundance positively correlated with GC content and gene density. Increased numtogenesis was observed with higher mortality. We identified YME1L1, a human homolog of yeast YME1 (yeast mitochondrial DNA escape 1) to be frequently mutated in colorectal tumors. YME1L1 was also mutated in tumors derived from other tissues. We show that inactivation of YME1L1 results in increased transfer of mtDNA in the nuclear genome.

Conclusions

Our study demonstrates increased somatic transfer of mtDNA in colorectal tumors. Our study also reveals sex-based differences in frequency of NUMT occurrence and that NUMT in blood reflects NUMT in tumors, suggesting NUMT may be used as a biomarker for tumorigenesis. We identify YME1L1 as the first NUMT suppressor gene in human and demonstrate that inactivation of YME1L1 induces migration of mtDNA to the nuclear genome. Our study reveals that numtogenesis plays an important role in the development of cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-017-0420-6) contains supplementary material, which is available to authorized users.

Non-coding RNAs: the dark side of nuclear-mitochondrial communication

Mitochondria are critical hubs for the integration of several key metabolic processes implicated in cell growth and survival. They originated from bacterial ancestors through endosymbiosis, following the transfer of more than 90% of their endosymbiont genome to the host cell nucleus. Over time, a mutually beneficial symbiotic relationship has been established, which relies on continuous and elaborate signaling mechanisms between this life-essential organelle and its host. The ability of mitochondria to signal their functional state and trigger compensatory and adaptive cellular responses has long been recognized, but the underlying molecular mechanisms involved have remained poorly understood. Recent evidence indicates that non-coding RNAs (ncRNAs) may contribute to the synchronization of a series of essential cellular and mitochondrial biological processes, acting as "messengers" between the nucleus and the mitochondria. Here, we discuss the emerging putative roles of ncRNAs in various bidirectional signaling pathways established between the host cell and its mitochondria, and how the dysregulation of these pathways may lead to aging-related diseases, including cancer, and offer new promising therapeutic avenues.

Templated Sequence Insertion Polymorphisms in the Human Genome

Templated Sequence Insertion Polymorphism (TSIP) is a recently described form of polymorphism recognized in the human genome, in which a sequence that is templated from a distant genomic region is inserted into the genome, seemingly at random. TSIPs can be grouped into two classes based on nucleotide sequence features at the insertion junctions; Class 1 TSIPs show features of insertions that are mediated via the LINE-1 ORF2 protein, including (1) target-site duplication (TSD), (2) polyadenylation 10–30 nucleotides downstream of a “cryptic” polyadenylation signal, and (3) preference for insertion at a 5′-TTTT/A-3′ sequence. In contrast, class 2 TSIPs show features consistent with repair of a DNA double-strand break (DSB) via insertion of a DNA “patch” that is derived from a distant genomic region. Survey of a large number of normal human volunteers demonstrates that most individuals have 25–30 TSIPs, and that these TSIPs track with specific geographic regions. Similar to other forms of human polymorphism, we suspect that these TSIPs may be important for the generation of human diversity and genetic diseases.

Neofunctionalization of Chromoplast Specific Lycopene Beta Cyclase Gene (CYC-B) in Tomato Clade

The ancestor of tomato underwent whole genome triplication ca. 71 Myr ago followed by widespread gene loss. However, few of the triplicated genes are retained in modern day tomato including lycopene beta cyclase that mediates conversion of lycopene to β-carotene. The fruit specific β-carotene formation is mediated by a chromoplast-specific paralog of lycopene beta cyclase (CYC-B) gene. Presently limited information is available about how the variations in CYC-B gene contributed to its neofunctionalization. CYC-B gene in tomato clade contained several SNPs and In-Dels in the coding sequence (33 haplotypes) and promoter region (44 haplotypes). The CYC-B gene coding sequence in tomato appeared to undergo purifying selection. The transit peptide sequence of CYC-B protein was predicted to have a stronger plastid targeting signal than its chloroplast specific paralog indicating a possible neofunctionalization. In promoter of two Bog (Beta old gold) mutants, a NUPT (nuclear plastid) DNA fragment of 256 bp, likely derived from a S. chilense accession, was present. In transient expression assay, this promoter was more efficient than the "Beta type" promoter. CARGATCONSENSUS box sequences are required for the binding of the MADS-box regulatory protein RIPENING INHIBITOR (RIN). The loss of CARGATCONSENSUS box sequence from CYC-B promoter in tomato may be related to attenuation of its efficiency to promote higher accumulation of β-carotene than lycopene during fruit ripening.

Genetic Diversity and Phylogenetic Analysis of South-East Asian Duck Populations Based on the mtDNA D-loop Sequences

The maternally inherited mitochondrial DNA (mtDNA) D–loop region is widely used for exploring genetic relationships and for investigating the origin of various animal species. Currently, domestic ducks play an important role in animal protein supply. In this study, partial mtDNA D–loop sequences were obtained from 145 samples belonging to six South-East Asian duck populations and commercial duck population. All these populations were closely related to the mallard duck (Anas platyrhynchos), as indicated by their mean overall genetic distance. Sixteen nucleotide substitutions were identified in sequence analyses allowing the distinction of 28 haplotypes. Around 42.76% of the duck sequences were classified as Hap_02, which completely matched with Anas platyrhynchos duck species. The neighbor-joining phylogenetic tree also revealed that South-East Asian duck populations were closely related to Anas platyrhynchos. Network profiles were also traced using the 28 haplotypes. Overall, results showed that those duck populations D-loop haplotypes were shared between several duck breeds from Korea and Bangladesh sub continental regions. Therefore, these results confirmed that South-East Asian domestic duck populations have been domesticated from Anas platyrhynchos duck as the maternal origins.

Generation of Rho Zero Cells: Visualization and Quantification of the mtDNA Depletion Process

Human mitochondrial DNA (mtDNA) is located in discrete DNA-protein complexes, so called nucleoids. These structures can be easily visualized in living cells by utilizing the fluorescent stain PicoGreen^®. In contrary, cells devoid of endogenous mitochondrial genomes (ρ⁰ cells) display no mitochondrial staining in the cytoplasm. A modified restriction enzyme can be targeted to mitochondria to cleave the mtDNA molecules in more than two fragments, thereby activating endogenous nucleases. By applying this novel enzymatic approach to generate mtDNA-depleted cells the destruction of mitochondrial nucleoids in cultured cells could be detected in a time course. It is clear from these experiments that mtDNA-depleted cells can be seen as early as 48 h post-transfection using the depletion system. To prove that mtDNA is degraded during this process, mtDNA of transfected cells was quantified by real-time PCR. A significant decline could be observed 24 h post-transfection. Combination of both results showed that mtDNA of transfected cells is completely degraded and, therefore, ρ⁰ cells were generated within 48 h. Thus, the application of a mitochondrially-targeted restriction endonuclease proves to be a first and fast, but essential step towards a therapy for mtDNA disorders.

Numts: an impediment to DNA barcoding of Polyclinids, Tunicata

The mitochondrial cytochrome c oxidase subunit I (COI) gene, a widely accepted molecular marker for species identification and classification, has been questioned because of the presence of Numts. In this study we found the presence of Numts in the COI chromatogram of two tunicates, Polyclinum indicum and Polyclinum madrasensis belonging to the genus Polyclinum. Numts were also present in our sequence (Accession Number: KJ944391) and in other sequences belonging to genus Polyclinum in the GenBank record. The GeneBank database of genus Polyclinum contains COI-like sequences and COI pseudogenes, but no record of COI gene from Polyclinids. The prevalence of Numts in Polyclinids belonging to Tunicata, is an impediment to DNA barcoding studies of Polyclinum species.

Landscape of insertion polymorphisms in the human genome

Nucleotide substitutions, small (<50 bp) insertions or deletions (indels), and large (>50 bp) deletions are well-known causes of genetic variation within the human genome. We recently reported a previously unrecognized form of polymorphic insertions, termed templated sequence insertion polymorphism (TSIP), in which the inserted sequence was templated from a distant genomic region, and was inserted in the genome through reverse transcription of an RNA intermediate. TSIPs can be grouped into two classes based on nucleotide sequence features at the insertion junctions; class 1 TSIPs show target site duplication, polyadenylation, and preference for insertion at a 5′-TTTT/A-3′ sequence, suggesting a LINE-1 based insertion mechanism, whereas class 2 TSIPs show features consistent with repair of a DNA double strand break by nonhomologous end joining. To gain a more complete picture of TSIPs throughout the human population, we evaluated whole-genome sequence from 52 individuals, and identified 171 TSIPs. Most individuals had 25–30 TSIPs, and common (present in >20% of individuals) TSIPs were found in individuals throughout the world, whereas rare TSIPs tended to cluster in specific geographic regions. The number of rare TSIPs was greater than the number of common TSIPs, suggesting that TSIP generation is an ongoing process. Intriguingly, mitochondrial sequences were a frequent template for class 2 insertions, used more commonly than any nuclear chromosome. Similar to single nucleotide polymorphisms and indels, we suspect that these TSIPs may be important for the generation of human diversity and genetic diseases, and can be useful in tracking historical migration of populations.

Stable heteroplasmy at the single-cell level is facilitated by intercellular exchange of mtDNA

Eukaryotic cells carry two genomes, nuclear (nDNA) and mitochondrial (mtDNA), which are ostensibly decoupled in their replication, segregation and inheritance. It is increasingly appreciated that heteroplasmy, the occurrence of multiple mtDNA haplotypes in a cell, plays an important biological role, but its features are not well understood. Accurately determining the diversity of mtDNA has been difficult, due to the relatively small amount of mtDNA in each cell (<1% of the total DNA), the intercellular variability of mtDNA content and mtDNA pseudogenes (Numts) in nDNA. To understand the nature of heteroplasmy, we developed Mseek, a novel technique to purify and sequence mtDNA. Mseek yields high purity (>90%) mtDNA and its ability to detect rare variants is limited only by sequencing depth, providing unprecedented sensitivity and specificity. Using Mseek, we confirmed the ubiquity of heteroplasmy by analyzing mtDNA from a diverse set of cell lines and human samples. Applying Mseek to colonies derived from single cells, we find heteroplasmy is stably maintained in individual daughter cells over multiple cell divisions. We hypothesized that the stability of heteroplasmy could be facilitated by intercellular exchange of mtDNA. We explicitly demonstrate this exchange by co-culturing cell lines with distinct mtDNA haplotypes. Our results shed new light on the maintenance of heteroplasmy and provide a novel platform to investigate features of heteroplasmy in normal and diseased states.

Alzheimer's disease is associated with altered expression of genes involved in immune response and mitochondrial processes in astrocytes

Alzheimer's disease (AD) is characterized by deficits in cerebral metabolic rates of glucose in the posterior cingulate (PC) and precuneus in AD subjects, and in APOEε4 carriers, decades before the onset of measureable cognitive deficits. However, the cellular and molecular basis of this phenotype remains to be clarified. Given the roles of astrocytes in energy storage and brain immunity, we sought to characterize the transcriptome of AD PC astrocytes. Cells were laser capture microdissected from AD (n = 10) and healthy elderly control (n = 10) subjects for RNA sequencing. We generated >5.22 billion reads and compared sequencing data between controls and AD patients. We identified differentially expressed mitochondria-related genes including TRMT61B, FASTKD2, and NDUFA4L2, and using pathway and weighted gene coexpression analyses, we identified differentially expressed immune response genes. A number of these genes, including CLU, C3, and CD74, have been implicated in beta amyloid generation or clearance. These data provide key insights into astrocyte-specific contributions to AD, and we present this data set as a publicly available resource.

Transcription factors bind negatively selected sites within human mtDNA genes

Transcription of mitochondrial DNA (mtDNA)-encoded genes is thought to be regulated by a handful of dedicated transcription factors (TFs), suggesting that mtDNA genes are separately regulated from the nucleus. However, several TFs, with known nuclear activities, were found to bind mtDNA and regulate mitochondrial transcription. Additionally, mtDNA transcriptional regulatory elements, which were proved important in vitro, were harbored by a deletion that normally segregated among healthy individuals. Hence, mtDNA transcriptional regulation is more complex than once thought. Here, by analyzing ENCODE chromatin immunoprecipitation sequencing (ChIP-seq) data, we identified strong binding sites of three bona fide nuclear TFs (c-Jun, Jun-D, and CEBPb) within human mtDNA protein-coding genes. We validated the binding of two TFs by ChIP-quantitative polymerase chain reaction (c-Jun and Jun-D) and showed their mitochondrial localization by electron microscopy and subcellular fractionation. As a step toward investigating the functionality of these TF-binding sites (TFBS), we assessed signatures of selection. By analyzing 9,868 human mtDNA sequences encompassing all major global populations, we recorded genetic variants in tips and nodes of mtDNA phylogeny within the TFBS. We next calculated the effects of variants on binding motif prediction scores. Finally, the mtDNA variation pattern in predicted TFBS, occurring within ChIP-seq negative-binding sites, was compared with ChIP-seq positive-TFBS (CPR). Motifs within CPRs of c-Jun, Jun-D, and CEBPb harbored either only tip variants or their nodal variants retained high motif prediction scores. This reflects negative selection within mtDNA CPRs, thus supporting their functionality. Hence, human mtDNA-coding sequences may have dual roles, namely coding for genes yet possibly also possessing regulatory potential.

Implications of human genome structural heterogeneity: functionally related genes tend to reside in organizationally similar genomic regions

Background

In an earlier study, we hypothesized that genomic segments with different sequence organization patterns (OPs) might display functional specificity despite their similar GC content. Here we tested this hypothesis by dividing the human genome into 100 kb segments, classifying these segments into five compositional groups according to GC content, and then characterizing each segment within the five groups by oligonucleotide counting (k-mer analysis; also referred to as compositional spectrum analysis, or CSA), to examine the distribution of sequence OPs in the segments. We performed the CSA on the entire DNA, i.e., its coding and non-coding parts the latter being much more abundant in the genome than the former.

Results

We identified 38 OP-type clusters of segments that differ in their compositional spectrum (CS) organization. Many of the segments that shared the same OP type were enriched with genes related to the same biological processes (developmental, signaling, etc.), components of biochemical complexes, or organelles. Thirteen OP-type clusters showed significant enrichment in genes connected to specific gene-ontology terms. Some of these clusters seemed to reflect certain events during periods of horizontal gene transfer and genome expansion, and subsequent evolution of genomic regions requiring coordinated regulation.

Conclusions

There may be a tendency for genes that are involved in the same biological process, complex or organelle to use the same OP, even at a distance of ~ 100 kb from the genes. Although the intergenic DNA is non-coding, the general pattern of sequence organization (e.g., reflected in over-represented oligonucleotide “words”) may be important and were protected, to some extent, in the course of evolution.

Interference of Co-amplified nuclear mitochondrial DNA sequences on the determination of human mtDNA heteroplasmy by Using the SURVEYOR nuclease and the WAVE HS system

High-sensitivity and high-throughput mutation detection techniques are useful for screening the homoplasmy or heteroplasmy status of mitochondrial DNA (mtDNA), but might be susceptible to interference from nuclear mitochondrial DNA sequences (NUMTs) co-amplified during polymerase chain reaction (PCR). In this study, we first evaluated the platform of SURVEYOR Nuclease digestion of heteroduplexed DNA followed by the detection of cleaved DNA by using the WAVE HS System (SN/WAVE-HS) for detecting human mtDNA variants and found that its performance was slightly better than that of denaturing high-performance liquid chromatography (DHPLC). The potential interference from co-amplified NUMTs on screening mtDNA heteroplasmy when using these 2 highly sensitive techniques was further examined by using 2 published primer sets containing a total of 65 primer pairs, which were originally designed to be used with one of the 2 techniques. We confirmed that 24 primer pairs could amplify NUMTs by conducting bioinformatic analysis and PCR with the DNA from 143B-ρ0 cells. Using mtDNA extracted from the mitochondria of human 143B cells and a cybrid line with the nuclear background of 143B-ρ0 cells, we demonstrated that NUMTs could affect the patterns of chromatograms for cell DNA during SN-WAVE/HS analysis of mtDNA, leading to incorrect judgment of mtDNA homoplasmy or heteroplasmy status. However, we observed such interference only in 2 of 24 primer pairs selected, and did not observe such effects during DHPLC analysis. These results indicate that NUMTs can affect the screening of low-level mtDNA variants, but it might not be predicted by bioinformatic analysis or the amplification of DNA from 143B-ρ0 cells. Therefore, using purified mtDNA from cultured cells with proven purity to evaluate the effects of NUMTs from a primer pair on mtDNA detection by using PCR-based high-sensitivity methods prior to the use of a primer pair in real studies would be a more practical strategy.

Somatic mtDNA mutation spectra in the aging human putamen

The accumulation of heteroplasmic mitochondrial DNA (mtDNA) deletions and single nucleotide variants (SNVs) is a well-accepted facet of the biology of aging, yet comprehensive mutation spectra have not been described. To address this, we have used next generation sequencing of mtDNA-enriched libraries (Mito-Seq) to investigate mtDNA mutation spectra of putamen from young and aged donors. Frequencies of the “common” deletion and other “major arc” deletions were significantly increased in the aged cohort with the fold increase in the frequency of the common deletion exceeding that of major arc deletions. SNVs also increased with age with the highest rate of accumulation in the non-coding control region which contains elements necessary for translation and replication. Examination of predicted amino acid changes revealed a skew towards pathogenic SNVs in the coding region driven by mutation bias. Levels of the pathogenic m.3243A>G tRNA mutation were also found to increase with age. Novel multimeric tandem duplications that resemble murine control region multimers and yeast ρ⁻ mtDNAs, were identified in both young and aged specimens. Clonal ∼50 bp deletions in the control region were found at high frequencies in aged specimens. Our results reveal the complex manner in which the mitochondrial genome alters with age and provides a foundation for studies of other tissues and disease states.

Mitochondria are unique among animal organelles in that they contain their own multi-copy genome (mtDNA). For the past 20 years it has been known that tissues like brain and muscle accumulate somatic mtDNA mutations with age. Because individual mtDNA mutations are present at very low levels, few details are known about the spectrum of mutations associated with aging. Advances in sequencing technology now permit the examination of mtDNA mutations at high resolution. We have examined the spectrum of mtDNA mutations present in putamen, a brain region prone to the accumulation of somatic mtDNA mutations. We were able to quantify the accumulation of clonal and non-clonal deletions in the mtDNA coding region which are known to have a strong association with aging. Partial deletions and novel duplications of the mtDNA control region were also identified, and appear to be more prevalent than previously recognized, but levels showed weaker associations with age than coding region deletions. Single nucleotide variants accumulate fastest in the control region, with a skew towards the accumulation of pathogenic mutations in the coding region. Understanding how the mitochondrial genome alters with age provides a benchmark for studies of somatic mtDNA mutations and dissection of the role they play in normal aging and degenerative diseases.