High Levels Of Heteroplasmy Research Articles

The MELAS mutation m.3243A>G promotes reactivation of fetal cardiac genes and an epithelial-mesenchymal transition-like program via dysregulation of miRNAs

The pathomechanisms underlying oxidative phosphorylation (OXPHOS) diseases are not well-understood, but they involve maladaptive changes in mitochondria-nucleus communication. Many studies on the mitochondria-nucleus cross-talk triggered by mitochondrial dysfunction have focused on the role played by regulatory proteins, while the participation of miRNAs remains poorly explored. MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) is mostly caused by mutation m.3243A>G in mitochondrial tRNALeu(UUR) gene. Adverse cardiac and neurological events are the commonest causes of early death in m.3243A>G patients. Notably, the incidence of major clinical features associated with this mutation has been correlated to the level of m.3243A>G mutant mitochondrial DNA (heteroplasmy) in skeletal muscle. In this work, we used a transmitochondrial cybrid model of MELAS (100% m.3243A>G mutant mitochondrial DNA) to investigate the participation of miRNAs in the mitochondria-nucleus cross-talk associated with OXPHOS dysfunction. High-throughput analysis of small-RNA-Seq data indicated that expression of 246 miRNAs was significantly altered in MELAS cybrids. Validation of selected miRNAs, including miR-4775 and miR-218-5p, in patient muscle samples revealed miRNAs whose expression declined with high levels of mutant heteroplasmy. We show that miR-218-5p and miR-4775 are direct regulators of fetal cardiac genes such as NODAL, RHOA, ISL1 and RXRB, which are up-regulated in MELAS cybrids and in patient muscle samples with heteroplasmy above 60%. Our data clearly indicate that TGF-β superfamily signaling and an epithelial-mesenchymal transition-like program are activated in MELAS cybrids, and suggest that down-regulation of miRNAs regulating fetal cardiac genes is a risk marker of heart failure in patients with OXPHOS diseases.

Genomic and transcriptomic characterization of the mitochondrial-rich oncocytic phenotype on a thyroid carcinoma background

We conducted the first systematic omics study of the oncocytic phenotype in 488 papillary thyroid carcinomas (PTC) from The Cancer Genome Atlas. Oncocytic phenotype is secondary to PTC, being unrelated to several pathologic scores. The nuclear genome had low impact on this phenotype (except in specific copy number variation), which was mostly driven by the significant accumulation of mitochondrial DNA non-synonymous and frameshift mutations at high heteroplasmy levels. Energy and mitochondrial-related pathways were significantly enriched in oncocytic tumors that also displayed increased levels of expression for genes involved in autophagy and fusion of mitochondria. Our in vitro tests confirmed that autophagy is increased and functional while mitophagy is decreased in these tumors.

High-level heteroplasmy for the m.7445A>G mitochondrial DNA mutation can cause progressive sensorineural hearing loss in infancy

ObjectiveHearing loss caused by mutation of mitochondrial DNA typically develops in late childhood or early adulthood, but rarely in infancy. We report the investigation of a patient to determine the cause of his early onset hearing loss. Materials and methodsThe proband was a boy aged 1 year and 2 months at presentation. Newborn hearing screening test by automated auditory brainstem response generated “pass” results for both ears. His reaction to sound deteriorated by 9 months. Average pure tone threshold at 0.5, 1, and 2 kHz was 55 dB by conditioned orientation response audiometry. His father had congenital hearing loss, and his mother had progressive hearing loss since childhood.Invader assays and Sanger sequencing were performed to investigate genetic causes of the hearing loss in the proband, and heteroplasmy was assessed by PCR-restriction fragment length polymorphism, Sanger sequencing, and pyrosequencing. Additionally, mitochondrial function was evaluated by measurement of the oxygen consumption rate of patient skin fibroblasts. ResultsAn m.7445A > G mitochondrial DNA mutation and a heterozygous c.235delC (p.L79Cfs*3) mutation of GJB2 were detected in the proband. His mother carried the m.7445A > G mitochondrial DNA mutation, and his father was a compound heterozygote for GJB2 mutations (c.[235delC]; [134G > A; 408C > A]). Tissue samples from both the proband and his mother exhibited a high degree of heteroplasmy. Fibroblasts from the proband exhibited markedly reduced oxygen consumption rates. These data indicate that the proband had impaired mitochondrial function, resulting in hearing loss. ConclusionThis research demonstrates that hearing loss in a proband who presented in infancy and that of his mother resulted from a high level of heteroplasmy for the m.7445A > G mitochondrial DNA mutation, indicating that this alteration can cause hearing loss in infancy.

A cost-efficient method for sequencing the human whole mitochondrial genome utilizing multiplex PCR-based next generation sequencing

The mitochondrial genome of human is organized as a circular loop of double-stranded DNA with 16569 bp in length. Mitochondrial DNA (mtDNA) is extensively used as a biomarker in human evolution and population genetics due to the characteristics of maternal inheritance, multiple copies in cells, high mutational rates, high heteroplasmy levels and so on. Next-generation sequencing (NGS) has become a rapid and efficient approach to sequence mtDNA recently, while library-building is complex and expensive. Multiplex PCR has the advantage of high sensitivity and saving time and effort. Therefore, this technique was used to amplify whole complete mitochondrial genomes in Chinese in the present study. The whole mitochondrial genome was covered by 73 overlapped short amplicons, which was subsequently sequenced on an Illumina HiSeq X Ten instrument. The mean per-mtDNA-base read depth was higher than 20000×. 100% mitochondrial genome coverage was obtained across all the samples with a depth of coverage threshold of 100×. The data of high throughout sequencing was qualified for mutation analysis. The present sequencing method based on multiplex PCR leaves out the process of library-building. This sequencing methodology that we have developed is efficiency, economic and has a broad application prospect in the research of complex genetic disorders.

Extensive Heteroplasmy and Evidence for Fragmentation in theCallinectes sapidusMitochondrial Genome

Heteroplasmy is the presence of multiple genotypes in an individual and is commonly associated with mitochondria where thousands of copies of the genome can be present in a single organelle. Even in low frequencies, it can cause severe illness or death and strong selective pressures usually result in a low occurrence. Despite this, cloning and sequencing of a portion of the open reading frames of the nad2, nad4, and coI mitochondrial loci in the blue crab Callinectes sapidus (Rathbun, 1896) revealed abnormally high levels of heteroplasmy at all loci with as many as 24 haplotypes in a single individual and the dominant haplotype accounting for as little as 43.9% of the total sequences. A minimum spanning network comparing these individuals at each locus demonstrated an overlap of haplotypes between the female and themegalopa with almost no overlap of themale sequences. Illumina sequencing ofmuscle and testes froma single individual usingmultiple differentDNA-based amplificationmethods as well as cDNA amplification confirmed heteroplasmic circular mitochondrial DNA templates with portions of the reads containing unmapped highly repetitive regions at specific sequence positions in both tissue types. Thus, the blue crab appears to possess a highly degenerate and possibly fragmented mitochondrial genome that is present in germline tissue and linearly inherited. A general lack of fragmentation present in the cDNA sequences as well as site-specific sequence changes argues for mitochondrial editing as a way of maintaining sequence integrity. This is the first evidence for mitochondrial editing in a crustacean and a potential explanation for observations of extremely high population diversity when using blue crab mitochondrial markers.

Creation of cybrid lines of monocyte origin with high heteroplasmy level for mitochondrial genome mutation m.1555g>a

Creation of cybrid lines of monocyte origin with high heteroplasmy level for mitochondrial genome mutation m.1555g>a

Mitochondrial DNA Heteroplasmy Associations With Neurosensory and Mobility Function in Elderly Adults.

Mitochondrial DNA (mtDNA) heteroplasmy is a mixture of normal and mutated mtDNA molecules in a cell. High levels of heteroplasmy at specific mtDNA sites lead to inherited mitochondrial diseases with neurological, sensory, and movement impairments. Here we test the hypothesis that heteroplasmy levels in elderly adults are associated with impaired function resembling mild forms of mitochondrial disease. We examined platelet mtDNA heteroplasmy at 20 disease-causing sites for associations with neurosensory and mobility function among 137 participants from the community-based Health, Aging, and Body Composition Study. Elevated mtDNA heteroplasmy at four mtDNA sites in complex I and tRNA genes was nominally associated with reduced cognition, vision, hearing, and mobility: m.10158T>C with Modified Mini-Mental State Examination score (p = .009); m.11778G>A with contrast sensitivity (p = .02); m.7445A>G with high-frequency hearing (p = .047); and m.5703G>A with 400 m walking speed (p = .007). These results indicate that increased mtDNA heteroplasmy at disease-causing sites is associated with neurosensory and mobility function in older persons. We propose the novel use of mtDNA heteroplasmy as a simple, noninvasive predictor of age-related neurologic, sensory, and movement impairments.

A Phylogenetic Analysis of 34 Chloroplast Genomes Elucidates the Relationships between Wild and Domestic Species within the GenusCitrus

Citrus genus includes some of the most important cultivated fruit trees worldwide. Despite being extensively studied because of its commercial relevance, the origin of cultivated citrus species and the history of its domestication still remain an open question. Here, we present a phylogenetic analysis of the chloroplast genomes of 34 citrus genotypes which constitutes the most comprehensive and detailed study to date on the evolution and variability of the genus Citrus. A statistical model was used to estimate divergence times between the major citrus groups. Additionally, a complete map of the variability across the genome of different citrus species was produced, including single nucleotide variants, heteroplasmic positions, indels (insertions and deletions), and large structural variants. The distribution of all these variants provided further independent support to the phylogeny obtained. An unexpected finding was the high level of heteroplasmy found in several of the analyzed genomes. The use of the complete chloroplast DNA not only paves the way for a better understanding of the phylogenetic relationships within the Citrus genus but also provides original insights into other elusive evolutionary processes, such as chloroplast inheritance, heteroplasmy, and gene selection.

Impaired respiratory function in MELAS-induced pluripotent stem cells with high heteroplasmy levels

Mitochondrial diseases are heterogeneous disorders, caused by mitochondrial dysfunction. Mitochondria are not regulated solely by nuclear genomic DNA but by mitochondrial DNA. It is difficult to develop effective therapies for mitochondrial disease because of the lack of mitochondrial disease models. Mitochondrial myopathy, encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) is one of the major mitochondrial diseases. The aim of this study was to generate MELAS-specific induced pluripotent stem cells (iPSCs) and to demonstrate that MELAS-iPSCs can be models for mitochondrial disease. We successfully established iPSCs from the primary MELAS-fibroblasts carrying 77.7% of m.3243A>G heteroplasmy. MELAS-iPSC lines ranged from 3.6% to 99.4% of m.3243A>G heteroplasmy levels. The enzymatic activities of mitochondrial respiratory complexes indicated that MELAS-iPSC-derived fibroblasts with high heteroplasmy levels showed a deficiency of complex I activity but MELAS-iPSC-derived fibroblasts with low heteroplasmy levels showed normal complex I activity. Our data indicate that MELAS-iPSCs can be models for MELAS but we should carefully select MELAS-iPSCs with appropriate heteroplasmy levels and respiratory functions for mitochondrial disease modeling.

A novel mitochondrial DNA m.7507A>G mutation is only pathogenic at high levels of heteroplasmy

We present a Dutch family with a novel disease-causing mutation in the mitochondrial tRNASer(UCN) gene, m.7507A>G. The index patient died during the neonatal period due to cardio–respiratory failure and fatal lactic acidosis. A second patient, his cousin, has severe hearing loss necessitating cochlear implants and progressive exercise intolerance. Laboratory investigations of both patients revealed combined deficiencies of the enzyme complexes of the mitochondrial respiratory chain in several tissues. Reduced levels of fully assembled complexes I and IV in fibroblasts by BN-PAGE associated with (near) homoplasmic levels of the m.7507A>G mutation in several tissues and a severe reduction in the steady-state level of mt-tRNASer(UCN) in fibroblasts were observed. The novel mitochondrial DNA mutation was shown to segregate with disease; several healthy maternal family members showed high heteroplasmy levels (up to 76 ± 4% in blood and 68 ± 4% in fibroblasts) which did not lead to any alterations in the activities of the enzyme complexes of the respiratory chain in fibroblasts or clinical signs and symptoms. We hereby conclude that the m.7507A>G mutation causes a heterogeneous clinical phenotype and is only pathogenic at very high levels of mtDNA heteroplasmy.

Comparison of two short DNA barcoding loci (COI and COII) and two longer ribosomal DNA genes (SSU & LSU rRNA) for specimen identification among quarantine root-knot nematodes (Meloidogyne spp.) and their close relatives

Root-knot nematodes (Meloidogyne spp.) are important pests of numerous crops worldwide. Some members of this genus have a quarantine status, and accurate species identification is required to prevent further spreading. DNA barcoding is a method for organism identification in non-complex DNA backgrounds based on informative motifs in short DNA stretches (≈600 bp). As part of the EU 7th Framework project QBOL, 15 Meloidogyne species were chosen to compare the resolutions offered by two typical DNA barcoding loci, COI and COII, with the distinguishing signals produced by two ribosomal DNA genes (small and large subunit rDNA; SSU ≈ 1,700 and LSU ≈ 3,400 bp). None of the four markers distinguished between the tropical species Meloidogyne incognita, M. javanica and M. arenaria. Taking P ID (Liberal) values ≥0.93 as a measure for species delimitation, the four mtDNA and rDNA markers performed well for the tropical Meloidogyne species complex, M. enterolobii, M. hapla, and M. maritima. Within cluster III A (Holterman et al. Phytopathology, 99, 227–235, 2009), SSU rDNA did not offer resolution at species level. Both mtDNA loci COI and COII did, whereas for LSU rDNA a longer fragment (≥700 bp) is required. The high level of mitochondrial heteroplasmy recently reported for M. chitwoodi (Humphreys-Pereira and Elling Nematology, 15, 315–327, 2013) was not found in the populations under investigation, suggesting this could be a regional phenomenon. For identification of RKNs, we suggest the combined use of SSU rDNA with one of three other markers presented here.

Dissecting the mechanisms underlying the accumulation of mitochondrial DNA deletions in human skeletal muscle.

Large-scale mitochondrial DNA (mtDNA) deletions are an important cause of mitochondrial disease, while somatic mtDNA deletions cause focal respiratory chain deficiency associated with ageing and neurodegenerative disorders. As mtDNA deletions only cause cellular pathology at high levels of mtDNA heteroplasmy, an mtDNA deletion must accumulate to levels which can result in biochemical dysfunction—a process known as clonal expansion. A number of hypotheses have been proposed for clonal expansion of mtDNA deletions, including a replicative advantage for deleted mitochondrial genomes inferred by their smaller size—implying that the largest mtDNA deletions would also display a replicative advantage over smaller mtDNA deletions. We proposed that in muscle fibres from patients with mtDNA maintenance disorders, which lead to the accumulation of multiple mtDNA deletions, we would observe the largest mtDNA deletions spreading the furthest longitudinally through individual muscle fibres by means of a greater rate of clonal expansion. We characterized mtDNA deletions in patients with mtDNA maintenance disorders from a range of ‘large’ and ‘small’ cytochrome c oxidase (COX)-deficient regions in skeletal muscle fibres. We measured the size of clonally expanded deletions in 62 small and 60 large individual COX-deficient f regions. No significant difference was observed in individual patients or in the total dataset (small fibre regions mean 6.59 kb—large fibre regions mean 6.51 kb). Thus no difference existed in the rate of clonal expansion throughout muscle fibres between mtDNA deletions of different sizes; smaller mitochondrial genomes therefore do not appear to have an inherent replicative advantage in human muscle.

Regulation of mitochondrial genome inheritance by autophagy and ubiquitin-proteasome system: implications for health, fitness, and fertility.

Mitochondria, the energy-generating organelles, play a role in numerous cellular functions including adenosine triphosphate (ATP) production, cellular homeostasis, and apoptosis. Maternal inheritance of mitochondria and mitochondrial DNA (mtDNA) is universally observed in humans and most animals. In general, high levels of mitochondrial heteroplasmy might contribute to a detrimental effect on fitness and disease resistance. Therefore, a disposal of the sperm-derived mitochondria inside fertilized oocytes assures normal preimplantation embryo development. Here we summarize the current research and knowledge concerning the role of autophagic pathway and ubiquitin-proteasome-dependent proteolysis in sperm mitophagy in mammals, including humans. Current data indicate that sperm mitophagy inside the fertilized oocyte could occur along multiple degradation routes converging on autophagic clearance of paternal mitochondria. The influence of assisted reproductive therapies (ART) such as intracytoplasmic sperm injection (ICSI), mitochondrial replacement (MR), and assisted fertilization of oocytes from patients of advanced reproductive age on mitochondrial function, inheritance, and fitness and for the development and health of ART babies will be of particular interest to clinical audiences. Altogether, the study of sperm mitophagy after fertilization has implications in the timing of evolution and developmental and reproductive biology and in human health, fitness, and management of mitochondrial disease.

Near-Identical Segregation of mtDNA Heteroplasmy in Blood, Muscle, Urinary Epithelium, and Hair Follicles in Twins With Optic Atrophy, Ptosis, and Intractable Epilepsy

Mitochondrial DNA (mtDNA) disorders have emerged as major causes of inherited neurologic disease. Despite being well recognized for more than 2 decades, the clinical presentation continues to broaden. The phenotypic heterogeneity is partly owing to different percentage levels of mutant mtDNA heteroplasmy in different tissues, but the factors influencing this are poorly understood. This case report describes monozygotic male twins with ptosis, optic atrophy, and recent-onset intractable myoclonic epilepsy. The assessment of respiratory chain enzyme activities in the muscle from 1 twin revealed a severe and isolated defect involving mitochondrial complex I. Mitochondrial DNA sequencing revealed a pathogenic m.14487T>C MTND6 mutation, which was present at very high levels of heteroplasmy in muscle (84%) and lower levels in blood (15%), urinary epithelium (75%), and buccal mucosa (58%). Of particular interest, his identical twin was found to harbor very similar levels of the m.14487T>C mutation in his blood, urine, buccal mucosa, and hair follicle DNA samples, while the presence of low levels in the mother's tissues confirmed maternal transmission. It was shown that m14487T>C can also cause the unusual combination of optic atrophy, ptosis, and encephalomyopathy leading to intractable seizures. Near-identical heteroplasmy levels in different tissues in both siblings support a nuclear genetic mechanism controlling the tissue segregation of mtDNA mutations.

Human Mitochondrial tRNA Mutations in Maternally Inherited Deafness

Mutations in mitochondrial tRNA genes have been shown to be associated with maternally inherited syndromic and non-syndromic deafness. Among those, mutations such as tRNALeu(UUR)3243A>G associated with syndromic deafness are often present in heteroplasmy, and the non-syndromic deafness-associated tRNA mutations including tRNASer(UCN)7445A>G are often in homoplasmy or in high levels of heteroplasmy. These tRNA mutations are the primary factors underlying the development of hearing loss. However, other tRNA mutations such as tRNAThr15927G>A and tRNASer(UCN)7444G>A are insufficient to produce a deafness phenotype, but always act in synergy with the primary mitochondrial DNA mutations, and can modulate their phenotypic manifestation. These tRNA mutations may alter the structure and function of the corresponding mitochondrial tRNAs and cause failures in tRNAs metabolism. Thereby, the impairment of mitochondrial protein synthesis and subsequent defects in respiration caused by these tRNA mutations, results in mitochondrial dysfunctions and eventually leads to the development of hearing loss. Here, we summarized the deafness-associated mitochondrial tRNA mutations and discussed the pathophysiology of these mitochondrial tRNA mutations, and we hope these data will provide a foundation for the early diagnosis, management, and treatment of maternally inherited deafness.

Intraspecific variability and genetic structure in Meloidogyne chitwoodi from the USA

Meloidogyne chitwoodi is a quarantine pathogen and a severe problem of potato. Intraspecific variation and genetic structure are not well characterised for M. chitwoodi, but are critical to avoid misidentification and to optimise management strategies. The objective of this study was to analyse the morphological and molecular variation of four M. chitwoodi isolates, representing all races and pathotypes currently known in the USA. Despite statistically significant morphological variation among adult females, morphometrics were not able reliably to distinguish M. chitwoodi isolates. In contrast to morphology, molecular traits that are determined by nuclear ribosomal genes were stable across all isolates. Malate dehydrogenase, esterase and superoxide dismutase isozyme phenotypes were conserved in all isolates, which is an important finding for diagnostics. To gain insight into the genetic structure of M. chitwoodi, we analysed a mitochondrial DNA segment including a partial region of COII, tRNA-His and 16S rRNA genes. Genetic structure was weak and marked by high haplotype and low nucleotide diversity. We found a high level of mitochondrial heteroplasmy in M. chitwoodi. Taken together, our results indicate that there is significant intraspecific morphological and molecular variation in M. chitwoodi. Consequences for resistance breeding in potato and directions for phylogeographic studies to trace the origin of M. chitwoodi are discussed.

Mitochondrial tRNA mutations associated with deafness

Mitochondrial tRNA mutations are one of the important causes of both syndromic and non–syndromic deafness. Of those, syndromic deafness–associated tRNA mutations such as tRNALeu(UUR) 3243A>G are often present in heteroplasmy, while non–syndromic deafness–associated tRNA mutations including tRNASer(UCN) 7445A>G are often in homplasmy or in high levels of heteroplasmy. These tRNA mutations are the primary mutations leading to hearing loss. However, other tRNA mutations such as tRNAThr 15927G>A and tRNASer(UCN) 7444G>A may act in synergy with the primary mitochondrial DNA mutations, modulating the phenotypic manifestation of the primary mitochondrial DNA mutations. Theses tRNA mutations cause structural and functional alteration. A failure in tRNA metabolism caused by these tRNA mutations impaired mitochondrial translation and respiration, thereby causing mitochondr ial dysfunctions responsible for deafness. These data offer valuable information for the early diagnosis, management and treatment of maternally inherited deafness.

POG08 Novel mitochondrial tRNA mutation associated with migraine, pigmentary retinopathy, leukoencephalopathy and sensorineural deafness

A 34-year-old lady was referred to the headache clinic. She had a long history of migraines starting as a teenager with bilateral throbbing headaches associated with photophobia, phonophobia and nausea...

Complete mtDNA of Meretrix lusoria (Bivalvia: Veneridae) reveals the presence of an atp8 gene, length variation and heteroplasmy in the control region

The complete nucleotide sequence of the mitochondrial genome of the clam Meretrix lusoria (Bivalvia: Veneridae) was determined. It comprises 20,268 base pairs (bp) and contains 13 protein-coding genes, including ATPase subunit 8 ( atp8), two ribosomal RNAs, 22 transfer RNAs, and a non-coding control region. The atp8 encodes a protein of 39 amino acids. All genes are encoded on the same strand. A putative control region (CR or D-loop) was identified in the major non-coding region (NCR) between the tRNA Gly and tRNA Gln . A 1087 bp tandem repeat fragment was identified that comprises nearly 11 copies of a 101 bp motif and accounts for approximately 41% of the NCR. The 101 bp tandem repeat motif of the NCR can be folded into a stem–loop secondary structure. Samples of eight individuals from Hainan and Fujian provinces were collected and their NCR regions were successfully amplified and sequenced. The data revealed a highly polymorphic VNTR (variable number of tandem repeats) associated with high levels of heteroplasmy in the D-loop region. The size of the CR ranged from 1942 to 3354 bp depending upon the copy number of the repeat sequence.

Systemic Mitochondrial Dysfunction and the Etiology of Alzheimer's Disease and Down Syndrome Dementia

Increasing evidence is implicating mitochondrial dysfunction as a central factor in the etiology of Alzheimer's disease (AD). The most significant risk factor in AD is advanced age and an important neuropathological correlate of AD is the deposition of amyloid-beta peptide (Abeta40 and Abeta42) in the brain. An AD-like dementia is also common in older individuals with Down syndrome (DS), though with a much earlier onset. We have shown that somatic mitochondrial DNA (mtDNA) control region (CR) mutations accumulate with age in post-mitotic tissues including the brain and that the level of mtDNA mutations is markedly elevated in the brains of AD patients. The elevated mtDNA CR mutations in AD brains are associated with a reduction in the mtDNA copy number and in the mtDNA L-strand transcript levels. We now show that mtDNA CR mutations increase with age in control brains; that they are markedly elevated in the brains of AD and DS and dementia (DSAD) patients; and that the increased mtDNA CR mutation rate in DSAD brains is associated with reduced mtDNA copy number and L-strand transcripts. The increased mtDNA CR mutation rate is also seen in peripheral blood DNA and in lymphoblastoid cell DNAs of AD and DSAD patients, and distinctive somatic mtDNA mutations, often at high heteroplasmy levels, are seen in AD and DSAD brain and blood cells DNA. In aging, DS, and DSAD, the mtDNA mutation level is positively correlated with beta-secretase activity and mtDNA copy number is inversely correlated with insoluble Abeta40 and Abeta42 levels. Therefore, mtDNA alterations may be responsible for both age-related dementia and the associated neuropathological changes observed in AD and DSAD.