High-throughput sequencing in mitochondrial DNA research

Radiogenomics: A systems biology approach to understanding genetic risk factors for radiotherapy toxicity?

Adverse reactions in normal tissue after radiotherapy (RT) limit the dose that can be given to tumour cells. Since 80% of individual variation in clinical response is estimated to be caused by patient-related factors, identifying these factors might allow prediction of patients with increased risk of developing severe reactions. While inactivation of cell renewal is considered a major cause of toxicity in early-reacting normal tissues, complex interactions involving multiple cell types, cytokines, and hypoxia seem important for late reactions. Here, we review 'omics' approaches such as screening of genetic polymorphisms or gene expression analysis, and assess the potential of epigenetic factors, posttranslational modification, signal transduction, and metabolism. Furthermore, functional assays have suggested possible associations with clinical risk of adverse reaction. Pathway analysis incorporating different 'omics' approaches may be more efficient in identifying critical pathways than pathway analysis based on single 'omics' data sets. Integrating these pathways with functional assays may be powerful in identifying multiple subgroups of RT patients characterised by different mechanisms. Thus 'omics' and functional approaches may synergise if they are integrated into radiogenomics 'systems biology' to facilitate the goal of individualised radiotherapy.

Practicability of mitochondrial heteroplasmy detection through an Illumina genotyping array

MOTIVATION

High-throughput genomic data often contain unexpected information that can be mined for alternative applications. Despite the rise of high-throughput sequencing, Illumina genotyping arrays remain a driving force in large scale genetic and epidemiology studies. By processing and analyzing genotyping data of over 100,000 samples genotyped on Illumina genotyping arrays, we discovered evidence that indicates that mitochondrial heteroplasmy can be estimated from the fluorescence intensity data of the array. To verify our hypothesis, we conducted a sequencing validation study.

RESULT

Mitochondrial DNA targeted sequencing was performed on three samples that had been genotyped using the Illumina exome genotyping array. In each sample chosen, one heteroplasmy target was identified from the genotyping array, and sequencing data verified all three putative heteroplasmic sites. The estimated heteroplasmy level difference between that estimated from the genotyping fluorescence intensity and that directly measured from sequencing was 3.2% on average. Our analysis showed that an Illumina genotyping array can accurately and reliably estimate high-level heteroplasmy (>40%); however, intensity data from a genotyping array is not suitable for estimating low level heteroplasmy (<25%).

Haplotype diversity in mitochondrial genome in a Chinese Han population

Investigations into the use of mitochondrial genome (mtGenome) typing by massively parallel sequencing technologies are well underway in many areas, including forensic genetics. Previous studies have demonstrated that mtGenome sequencing data generated from Ion torrent personal genome machine (PGM) system were highly viable and reliable in forensic research. In this study, 145 whole mtGenomes from unrelated Chinese Han population were sequenced using the Ion PGM system. Results showed that 145 distinct haplotypes were obtained at a relatively high coverage with limited strand bias. The distribution of variants across the entire mtGenomes was illustrated and 70.74% of the variants were observed outside of the control region. An overall increase in the number of unique haplotypes as well as haplotype diversity were observed by detection of mtGenome compared with hypervariable region I/II (HV I/II) and control region (CR). This study demonstrates the substantially higher degree of haplotype resolution with whole-mtGenome sequences in comparison to HV I/II or CR that historically targeted for forensic testing, which shows the potential value of mtGenome typing in forensic testing in the future.

Mitochondria single nucleotide variation across six blood cell types

It has been shown that heteroplasmic mitochondrial DNA variants can be tissue specific. However, whether mitochondrial DNA variants are specific by blood cell types has not been investigated. Motivated by this question and using mitochondria sequences extracted from RNAseq data from six distinct blood cell types (neutrophil, monocyte, myeloid dendritic, natural killer, T and B), we thoroughly compared SNPs and heteroplasmies among these cell types. Each cell type from each subject was sequenced at four time points used as biological replicates. We found that mitochondria content is low in neutrophil compared to the other five blood cell types. Subsequent analysis on the other five blood cell types showed that at the SNP level, there was no discrepancy. At the heteroplasmy level, we observed good concordances among all blood cell types. However, the allele frequencies of the heteroplasmy differed between blood cell types for certain heteroplasmic sites. Furthermore, we identified five tri-allelic sites (1610, 2617, 8303, 12146, 13710) that are likely caused by RNA editing. Three out of these five sites are located at the ninth position of tRNA genes, and are likely resulting from post-transcriptional methylation.

Mitochondria sequence mapping strategies and practicability of mitochondria variant detection from exome and RNA sequencing data

The rapid progress in high-throughput sequencing has significantly enriched our capacity for studying the mitochondrial DNA (mtDNA). In addition to performing specific mitochondrial targeted sequencing, an increasingly popular alternative approach is using the off-target reads from exome sequencing to infer mtDNA variants, including single nucleotide polymorphisms (SNPs) and heteroplasmy. However, the effectiveness and practicality of this approach have not been tested. Recently, RNAseq data have also been suggested as a good source for alternative data mining, but whether mitochondrial variants can be detected from RNAseq data has not been validated. We designed a study to evaluate the practicability of mtDNA variant detection using exome and RNA sequencing data. Five breast cancer cell lines were sequenced through mitochondrial targeted, exome, and RNA sequencing. Mitochondrial targeted sequencing was used as the gold standard to compute the validation and false discovery rates of SNP and heteroplasmy detection in exome and RNAseq data. We found that exome and RNA sequencing can accurately detect mitochondrial SNPs. However, the lower false discovery rate makes exome sequencing a better choice for heteroplasmy detection than RNAseq. Furthermore, we examined three alignment strategies and found that aligning reads directly to the mitochondrial reference genome or aligning reads to the nuclear and mitochondrial references genomes simultaneously produced the best results, and that aligning to the nuclear genome first and afterwards to the mitochondrial genome performed poorly. In conclusion, our study provides important guidelines for future studies that intend to use either exome sequencing or RNAseq data to infer mitochondrial SNPs and heteroplasmy.

RNAseq by Total RNA Library Identifies Additional RNAs Compared to Poly(A) RNA Library

The most popular RNA library used for RNA sequencing is the poly(A) captured RNA library. This library captures RNA based on the presence of poly(A) tails at the 3' end. Another type of RNA library for RNA sequencing is the total RNA library which differs from the poly(A) library by capture method and price. The total RNA library costs more and its capture of RNA is not dependent on the presence of poly(A) tails. In practice, only ribosomal RNAs and small RNAs are washed out in the total RNA library preparation. To evaluate the ability of detecting RNA for both RNA libraries we designed a study using RNA sequencing data of the same two breast cancer cell lines from both RNA libraries. We found that the RNA expression values captured by both RNA libraries were highly correlated. However, the number of RNAs captured was significantly higher for the total RNA library. Furthermore, we identify several subsets of protein coding RNAs that were not captured efficiently by the poly(A) library. One of the most noticeable is the histone-encode genes, which lack the poly(A) tail.

MitoRCA-seq reveals unbalanced cytocine to thymine transition in Polg mutant mice

Mutations in mitochondrial DNA (mtDNA) can lead to a wide range of human diseases. We have developed a deep sequencing strategy, mitoRCA-seq, to detect low-frequency mtDNA point mutations starting with as little as 1 ng of total DNA. It employs rolling circle amplification, which enriches the full-length circular mtDNA by either custom mtDNA-specific primers or a commercial kit, and minimizes the contamination of nuclear encoded mitochondrial DNA (Numts). By analyzing the mutation profiles of wild-type and Polg (mitochondrial DNA polymerase γ) mutant mice, we found that mice with the proofreading deficient mtDNA polymerase have a significantly higher mutation load by expanding the number of mutation sites and to a lesser extent by elevating the mutation frequency at existing sites even before the premature aging phenotypes appear. Strikingly, cytocine (C) to thymine (T) transitions are found to be overrepresented in the mtDNA of Polg mutated mice. The C → T transition, compared to other types of mutations, tends to increase the hydrophobicity of the underlying amino acids, and may contribute to the impaired protein function of the Polg mutant mice. Taken together, our findings may provide clues to further investigate the molecular mechanism underlying premature aging phenotype in Polg mutant mice.

Mitochondrial DNA: Radically free of free-radical driven mutations

Mitochondrial DNA has long been posited as a likely target of oxidative damage induced mutation during the ageing process. Research over the past decades has uncovered the accumulation of mitochondrial DNA mutations in association with a mosaic pattern of cells displaying mitochondrial dysfunction in ageing individuals. Unfortunately, the underlying mechanisms are far less straightforward than originally anticipated. Recent research on mitochondria reveals that these genomes are far less helpless than originally envisioned. Additionally, new technologies have allowed us to analyze the mutational signatures of many more somatic mitochondrial DNA mutations, revealing surprising patterns that are inconsistent with a DNA-oxidative damage based hypothesis. In this review, we will discuss these recent observations and new insights into the eccentricities of mitochondrial genetics, and their impact on our understanding of mitochondrial mutations and their role in the ageing process. This article is part of a Special Issue entitled: Mitochondrial Dysfunction in Aging.

Mitochondrial DNA heteroplasmy in the emerging field of massively parallel sequencing

Long an important and useful tool in forensic genetic investigations, mitochondrial DNA (mtDNA) typing continues to mature. Research in the last few years has demonstrated both that data from the entire molecule will have practical benefits in forensic DNA casework, and that massively parallel sequencing (MPS) methods will make full mitochondrial genome (mtGenome) sequencing of forensic specimens feasible and cost-effective. A spate of recent studies has employed these new technologies to assess intraindividual mtDNA variation. However, in several instances, contamination and other sources of mixed mtDNA data have been erroneously identified as heteroplasmy. Well vetted mtGenome datasets based on both Sanger and MPS sequences have found authentic point heteroplasmy in approximately 25% of individuals when minor component detection thresholds are in the range of 10-20%, along with positional distribution patterns in the coding region that differ from patterns of point heteroplasmy in the well-studied control region. A few recent studies that examined very low-level heteroplasmy are concordant with these observations when the data are examined at a common level of resolution. In this review we provide an overview of considerations related to the use of MPS technologies to detect mtDNA heteroplasmy. In addition, we examine published reports on point heteroplasmy to characterize features of the data that will assist in the evaluation of future mtGenome data developed by any typing method.

Maternal ancestry and population history from whole mitochondrial genomes

MtDNA has been a widely used tool in human evolutionary and population genetic studies over the past three decades. Its maternal inheritance and lack of recombination have offered the opportunity to explore genealogical relationships among individuals and to study the frequency differences of matrilineal clades among human populations at continental and regional scales. The whole mtDNA genome sequencing delivers molecular resolution that is sufficient to distinguish patterns that have arisen over thousands of years. However, mutation rate is highly variable among the functional and non-coding domains of mtDNA which makes it challenging to obtain accurate split dates of the mitochondrial clades. Due to the shallow coalescent time of mitochondrial TMRCA at approximately 100 to 200 thousand years (ky), mtDNA data have only limited power to inform us about the more distant past and the early stages of human evolutionary history. The variation shared by mitochondrial genomes of individuals drawn from different continents outside Africa has been used to illuminate the details of the colonization process of the Old World, whereas regional patterns of variation have been at the focus of studies addressing questions of a more recent time scale. In the era of whole nuclear genome sequencing, mitochondrial genomes are continuing to be informative as a unique tool for the assessment of female-specific aspects of the demographic history of human populations.

Evolution of the metazoan mitochondrial replicase

The large number of complete mitochondrial DNA (mtDNA) sequences available for metazoan species makes it a good system for studying genome diversity, although little is known about the mechanisms that promote and/or are correlated with the evolution of this organellar genome. By investigating the molecular evolutionary history of the catalytic and accessory subunits of the mtDNA polymerase, pol γ, we sought to develop mechanistic insight into its function that might impact genome structure by exploring the relationships between DNA replication and animal mitochondrial genome diversity. We identified three evolutionary patterns among metazoan pol γs. First, a trend toward stabilization of both sequence and structure occurred in vertebrates, with both subunits evolving distinctly from those of other animal groups, and acquiring at least four novel structural elements, the most important of which is the HLH-3β (helix-loop-helix, 3 β-sheets) domain that allows the accessory subunit to homodimerize. Second, both subunits of arthropods and tunicates have become shorter and evolved approximately twice as rapidly as their vertebrate homologs. And third, nematodes have lost the gene for the accessory subunit, which was accompanied by the loss of its interacting domain in the catalytic subunit of pol γ, and they show the highest rate of molecular evolution among all animal taxa. These findings correlate well with the mtDNA genomic features of each group described above, and with their modes of DNA replication, although a substantive amount of biochemical work is needed to draw conclusive links regarding the latter. Describing the parallels between evolution of pol γ and metazoan mtDNA architecture may also help in understanding the processes that lead to mitochondrial dysfunction and to human disease-related phenotypes.

Complete mitochondrial genome of the Chinese endemic grasshopper Fruhstorferiola kulinga (Orthoptera: Acrididae: Podismini)

The whole-genome Illumina sequence of the Chinese endemic grasshopper Fruhstorferiola kulinga mitogenome was constructed and reported in this study. In all, the circular genome was obtained with 15,655 bp in length and contains 75.4% A + T. It typically consists of 37 genes, including 13 protein-coding genes (PCGs), 22 transfer RNAs (tRNAs), 2 ribosomal RNAs (rRNAs) and 1 D-loop region. All PCGs are initiated with ATN codons. Most of the PCGs use TAA as their stop codons, while the others use TAG as stop codons (COX1 and ND1). The size of the large and small ribosomal RNA genes are 1314 bp and 851 bp. The A + T-rich region (777 bp) showed strong resemblance to the other known Orthoptera insects. Our data would contribute to confirm the close relationship and other evolutionary researches of the F. kulinga.

Mitochondrial DNA somatic mutation in cancer

Cancer cells are known to drastically alter cellular energy metabolism. The Warburg effect has been known for over 80 years as pertaining cancer-specific aerobic glycolysis. As underlying molecular mechanisms are elucidated so that cancer cells alter the cellular energy metabolism for their advantage, the significance of the modulation of metabolic profiles is gaining attention. Now, metabolic reprogramming is becoming an emerging hallmark of cancer. Therapeutic agents that target cancer energy metabolism are under intensive investigation, but these investigations are mostly focused on the cytosolic glycolytic processes. Although mitochondrial oxidative phosphorylation is an integral part of cellular energy metabolism, until recently, it has been regarded as an auxiliary to cytosolic glycolytic processes in cancer energy metabolism. In this review, we will discuss the importance of mitochondrial respiration in the metabolic reprogramming of cancer, in addition to discussing the justification for using mitochondrial DNA somatic mutation as metabolic determinants for cancer sensitivity in glucose limitation.

Next-generation sequencing of the mitochondrial genome and association with IgA nephropathy in a renal transplant population

Kidneys are highly aerobic organs that are critically dependent on the normal functioning of mitochondria. Genetic variations disrupting mitochondrial function are associated with multifactorial disorders including kidney disease. This study sequenced the entire mitochondrial genome in a renal transplant cohort of 64 individuals, using next-generation sequencing, to evaluate the association of genetic variants with IgA nephropathy and end-stage renal disease (ESRD, n = 100).