Can migratory mitochondrial DNA activate oncogenes?

Mitochondrial DNA and carcinogenesis (review)

The role of the mitochondria in the process of carcinogenesis has drawn researchers' attention since the discovery of respiratory deficit in cells, particularly those characterized by rapid proliferation. The deficit was assumed to stimulate further differentiation of the cells and initiate the process of neoplastic transformation. As many as 25-80% of somatic mutations in mitochondrial DNA (mtDNA) are found in various neoplasms. These mutations are considered to trigger the neoplastic transformation through shifts of cell energy resources, an increase in the mitochondrial oxidative stress and modulation of apoptosis. The question arises as to whether the mtDNA mutations precede a neoplasm or whether they are a result of changes and processes that take place during neoplastic proliferation.

Mitochondrial DNA alterations in cancer

A number of studies have demonstrated the presence of mitochondrial DNA (mtDNA) mutations in cancer cells. In this article, we review mitochondrial genomic aberrations reported in solid tumors of the breast, colon, stomach, liver, kidney, bladder, head/neck, and lung. The tantalizing association of tumors with mtDNA mutations implicates these mutations in the process of carcinogenesis. Alterations in expression of mtDNA transcripts in a variety of cancer types are also reviewed. In solid tumors, elevated expression of mtDNA-genes coding for subunits of the mitochondrial electron respiratory chain may reflect mitochondrial adaptation to perturbations in cellular energy requirements. The role of mtDNA mutations and altered expression of mitochondrial genes in carcinogenesis is discussed. Mitochondrial DNA mutations can initiate a cascade of events leading to a continuous increase in the production of reactive oxygen species (persistent oxidative stress), a condition that probably favors tumor development.

Mitochondrial genome instability in human cancers

Malfunction of mismatch repair (MMR) genes produces nuclear genome instability (NGI) and plays an important role in the origin of some hereditary and sporadic human cancers. The appearance of non-inherited microsatellite alleles in tumor cells (microsatellite instability, MSI) is one of the expressions of NGI. We present here data showing mitochondrial genome instability (mtGI) in most of the human cancers analyzed so far. The mtDNA markers used were point mutations, length-tract instability of mono- or dinucleotide repeats, mono- or dinucleotide insertions or deletions, and long deletions. Comparison of normal and tumoral tissues from the same individual reveals that mt-mutations may show as homoplasmic (all tumor cells have the same variant haplotype) or as heteroplasmic (tumor cells are a mosaic of inherited and acquired variant haplotypes). Breast, colorectal, gastric and kidney cancers exhibit mtGI with a pattern of mt-mutations specific for each tumor. No correlation between NGI and mtGI was found in breast, colorectal or kidney cancers, while a positive correlation was found in gastric cancer. Conversely, germ cell testicular cancers lack mtGI. Damage by reactive oxygen species (ROS), slipped-strand mispairing (SSM) and deficient repair are the causes explaining the appearance of mtGI. The replication and repair of mtDNA are controlled by nuclear genes. So far, there is no clear evidence linking MMR gene malfunction with mtGI. Polymerase gamma (POLgamma) carries out the mtDNA synthesis. Since this process is error-prone due to a deficiency in the proofreading activity of POLgamma, this enzyme has been assumed to be involved in the origin of mt-mutations. Somatic cells have hundreds to thousands of mtDNA molecules with a very high rate of spontaneous mutations. Accordingly, most somatic cells probably have a low frequency of randomly mutated mtDNA molecules. Most cancers are of monoclonal origin. Hence, to explain the appearance of mtGI in tumors we have to explain why a given variant mt-haplotype expands and replaces part of (heteroplasmy) or all (homoplasmy) wild mt-haplotypes in cancer cells. Selective and/or replicative advantage of some mutations combined with a severe bottleneck during the mitochondrial segregation accompanying mitosis are the mechanisms probably involved in the origin of mtGI.

How infections with non-retroviral RNA viruses may be involved in the development of neoplasia

There have been reports of associations of infections with non-retroviral RNA viruses and tumour development. A hypothesis is proposed as to how non-retroviral RNA viruses may play a role in the development of neoplasia. It is based on a recent report of the detection of complementary DNA (cDNA) of the RNA virus lymphocytic choriomeningitis virus in mouse and hamster cells. This lends credence to the claim made in 1975 of the detection of cDNA copies of genomic DNA of three non-retroviral RNA viruses integrated into the DNA of host cells. Briefly, the hypothesis proposes that at least one cDNA fragment of a non-retroviral RNA virus is synthesized and integrated into the genome of the host cell in a way that could lead or contribute to tumour development. General approaches for testing the hypothesis are outlined.

Nuclear pseudogenes of mitochondrial DNA as a variable part of the human genome

Novel pseudogenes homologous to the mitochondrial (mt) 16S rRNA gene were detected via different approaches. Eight pseudogenes were sequenced. Copy number polymorphism of the mtDNA pseudogenes was observed among randomly chosen individuals, and even among siblings. A mtDNA pseudogene in the Y-chromosome was observed in a YAC clone carrying only repetitive sequence tag site (STS). PCR screening of human yeast artificial chromosome (YAC) libraries showed that there were at least 5.7 x 10(5) bp of the mtDNA pseudogenes in each haploid nuclear genome. Possible involvement of the mtDNA pseudogenes in the variable part of the human nuclear genome is discussed.

Damage to mitochondrial DNA induced by the hepatocarcinogen diethylnitrosamine in ovo

Different doses of the hepatocarcinogen diethylnitrosamine (DEN) were injected into fertilized turkey eggs 8 days before hatching. The embryos were removed from the eggs after 4 days and liver samples were shock frozen. Mitochondrial DNA (mtDNA) was purified from the samples. Electrophoresis on agarose gels with native mitochondrial DNA and with ribonuclease-treated mitochondrial DNA revealed a DEN-induced effect on the molecular size of the mtDNA. The content of mtDNA of the regular size of 16 kb dose-dependently decreased, whereas the amount of mtDNA fragments of various size increased. Fluorescent staining of the electrophoresis gels allowed the densitometric quantification of the mitochondrial DNA of the regular band at 16 kb and the amount of fragments of irregular size (smear). The diethylnitrosamine-induced effect was dose-dependent over the whole dose range from 1.24 to 6.2 mmol/kg. Even the lowest dose (10 mg DEN per egg) showed clear-cut effects. Mitochondrial damage and malfunction may be mechanistically involved in the neoplastic transformation and in aging phenomena. The in ovo model is a simple and rapid approach for investigations on chemically induced alterations of mtDNA.

Evolutionary trail of the mitochondrial genome as based on human 16S rDNA pseudogenes

In the course of studies on mutations in human mitochondrial (mt) DNA, we have uncovered and sequenced four new nuclear pseudogenes corresponding to bp 2457-2657 of the mt 16S rDNA. The four genes and their homologies with human mtDNA are E2 (62.4%), K10 (74.4%), E1 (84.6%) and LE6 (93.2%). When these five pseudogene sequences and another previously reported pseudogene sequence are compared with each other, they display what appears to be an ordered series of steps from a hypothetical common ancestor. The sequence of the hypothetical ancestor closely resembles that found in a wide variety of present-day mammalian mt genomes. The pseudogene sequences suggest an evolutionary trail of mt mutation dominated by base pair transitions punctuated by integration into the nuclear genome. Once integrated into the nuclear genome, the pseudogenes appear to follow the distinctive nuclear mutational pathway in which GC to AT transitions predominate and CpG sequences are preferentially eliminated.

Mitochondrial DNA alterations as ageing-associated molecular events

Mitochondrial DNA (mtDNA) is a naked double-stranded circular extrachromosomal genetic element continuously exposed to the matrix that contains great amounts of reactive oxygen species and free radicals. The age-dependent decline in the capability and capacity of mitochondria to dispose these oxy-radicals will render mtDNA more vulnerable to mutations during the ageing process. During the past 3 years, more than 10 different types of deletions have been identified in the mtDNA of various tissues of old humans. Some of them were found only in a certain tissue but some others appeared in more than one organ or tissue. The 4977-bp deletion is the most prevalent and abundant one among these deletions. Skeletal muscle is the target tissue of most ageing-associated mtDNA deletions and has often been found to carry multiple deletions. The onset age of the various deletions in mtDNA varies greatly with individual and type of the deletion. The 4977-bp deletion has been independently demonstrated to occur in the mtDNA of various tissues of the human in the early third decade of life. However, the 7436-bp deletion was only detected in the heart mtDNA of human subjects in their late thirties. The others appeared only in older humans over 40 years old. No apparent sex difference was found in the onset age of these ageing-associated mtDNA deletions. The various ageing-associated deletions could be classified into two groups. Most of the deletions belong to the first group, in which the 5'- and 3'-end breakpoints of the deletion are flanked by 4-bp or longer direct repeats. The deletion in the second group occurs less frequently and shows no distinct repeat sequences flanking the deletion sites. These two groups of mtDNA deletions may occur by different mechanisms. The first group is most probably caused by internal recombination or slippage mispairing during replication of mtDNA by the D-loop mechanism. The deleted mtDNA and the deleted DNA fragment may be further degraded or escape from the mitochondria and get translocated into the nucleus. The latter route has been substantiated by many observations of inserted mtDNA sequences in the nuclear DNA. Thus, the fragments of migrating mtDNA may change the information content and expression level of certain nuclear genes and thereby promote the ageing process or cause cancer. Similar ageing-associated alterations of mtDNA have also been observed in aged animals and plants. I suggest that mtDNA deletions and other mutations to be discovered are molecular events generally associated with the ageing process.

New evidence for the insertion of mitochondrial DNA into the human genome: significance for cancer and aging

We have observed and characterized in detail two cases of mitochondrial DNA fragments which have inserted into the nucleus of HeLa cells. In one case three non-sequential but contiguous regions of mitochondrial DNA with 92% homology to human cytoplasmic mitochondrial DNA inserted into the nuclear genome. In the second case the mitochondrial DNA sequence encoding cytochrome c oxidase subunit III was contiguous with and 5' of exons 2 and 3 of the c-myc oncogene and the chimeric gene was transcribed. Models are presented that describe mechanisms for the transfer of mitochondrial DNA into the nucleus involving fragmentation of mitochondrial DNA through aging and/or oxidative damage, anomalous processing or escape of mitochondrial DNA and RNA fragments from autophagic vacuoles, and insertion of mitochondrial DNA sequences, in some instances after reverse transcription of mitochondrial RNA, into the nuclear genome.

Cancer: involvement of replicative origins?

The assumption is made that movable genetic elements related to cancer are replicons and/or replicative origins. Some evidence is provided that certain mobile Alu-origins may represent candidates for precursors of chromosomal insertions. They could incorporate and interact with "fixed" chromosomal origins converting them to "procaryotic" ones. In other words, cancer is considered the result of a short and compact "re-evolution" from eucaryotic to "procaryotic" replication units.