Somatic cell mutations: can they provide a link between aging and cancer?

Pharmacological relevance of CDK inhibitors in Alzheimer's disease

Evidence suggests that cell cycle activation plays a role in the pathophysiology of neurodegenerative diseases. Alzheimer's disease is a progressive, terminal neurodegenerative disease that affects memory and other important mental functions. Intracellular deposition of Tau protein, a hyperphosphorylated form of a microtubule-associated protein, and extracellular aggregation of Amyloid β protein, which manifests as neurofibrillary tangles (NFT) and senile plaques, respectively, characterize this condition. In recent years, however, several studies have concluded that cell cycle re-entry is one of the key causes of neuronal death in the pathogenesis of Alzheimer's disease. The eukaryotic cell cycle is well-coordinated machinery that performs critical functions in cell replenishment, such as DNA replication, cell creation, repair, and the birth of new daughter cells from the mother cell. The complex interplay between the levels of various cyclins and cyclin-dependent kinases (CDKs) at different checkpoints is needed for cell cycle synchronization. CDKIs (cyclin-dependent kinase inhibitors) prevent cyclin degradation and CDK inactivation. Different external and internal factors regulate them differently, and they have different tissue expression and developmental functions. The checkpoints ensure that the previous step is completed correctly before starting the new cell cycle phase, and they protect against the transfer of defects to the daughter cells. Due to the development of more selective and potent ATP-competitive CDK inhibitors, CDK inhibitors appear to be on the verge of having a clinical impact. This avenue is likely to yield new and effective medicines for the treatment of cancer and other neurodegenerative diseases. These new methods for recognizing CDK inhibitors may be used to create non-ATP-competitive agents that target CDK4, CDK5, and other CDKs that have been recognized as important therapeutic targets in Alzheimer's disease treatment.

Somatic inactivating PTPRJ mutations and dysregulated pathways identified in canine malignant melanoma by integrated comparative genomic analysis

Canine malignant melanoma, a significant cause of mortality in domestic dogs, is a powerful comparative model for human melanoma, but little is known about its genetic etiology. We mapped the genomic landscape of canine melanoma through multi-platform analysis of 37 tumors (31 mucosal, 3 acral, 2 cutaneous, and 1 uveal) and 17 matching constitutional samples including long- and short-insert whole genome sequencing, RNA sequencing, array comparative genomic hybridization, single nucleotide polymorphism array, and targeted Sanger sequencing analyses. We identified novel predominantly truncating mutations in the putative tumor suppressor gene PTPRJ in 19% of cases. No BRAF mutations were detected, but activating RAS mutations (24% of cases) occurred in conserved hotspots in all cutaneous and acral and 13% of mucosal subtypes. MDM2 amplifications (24%) and TP53 mutations (19%) were mutually exclusive. Additional low-frequency recurrent alterations were observed amidst low point mutation rates, an absence of ultraviolet light mutational signatures, and an abundance of copy number and structural alterations. Mutations that modulate cell proliferation and cell cycle control were common and highlight therapeutic axes such as MEK and MDM2 inhibition. This mutational landscape resembles that seen in BRAF wild-type and sun-shielded human melanoma subtypes. Overall, these data inform biological comparisons between canine and human melanoma while suggesting actionable targets in both species.

Erythroid differentiation of human induced pluripotent stem cells is independent of donor cell type of origin

Epigenetic memory in induced pluripotent stem cells, which is related to the somatic cell type of origin of the stem cells, might lead to variations in the differentiation capacities of the pluripotent stem cells. In this context, induced pluripotent stem cells from human CD34(+) hematopoietic stem cells might be more suitable for hematopoietic differentiation than the commonly used fibroblast-derived induced pluripotent stem cells. To investigate the influence of an epigenetic memory on the ex vivo expansion of induced pluripotent stem cells into erythroid cells, we compared induced pluripotent stem cells from human neural stem cells and human cord blood-derived CD34(+) hematopoietic stem cells and evaluated their potential for differentiation into hematopoietic progenitor and mature red blood cells. Although genome-wide DNA methylation profiling at all promoter regions demonstrates that the epigenetic memory of induced pluripotent stem cells is influenced by the somatic cell type of origin of the stem cells, we found a similar hematopoietic induction potential and erythroid differentiation pattern of induced pluripotent stem cells of different somatic cell origin. All human induced pluripotent stem cell lines showed terminal maturation into normoblasts and enucleated reticulocytes, producing predominantly fetal hemoglobin. Differences were only observed in the growth rate of erythroid cells, which was slightly higher in the induced pluripotent stem cells derived from CD34(+) hematopoietic stem cells. More detailed methylation analysis of the hematopoietic and erythroid promoters identified similar CpG methylation levels in the induced pluripotent stem cell lines derived from CD34(+) cells and those derived from neural stem cells, which confirms their comparable erythroid differentiation potential.

Reprogramming and the mammalian germline: the Weismann barrier revisited

The germline represents a unique cell type that can transmit genetic material to the next generation. During early embryonic development, somatic cells give rise to a small population of cells known as germ cells, which eventually differentiate into mature gametes. Germ cells undergo a process of removing and resetting relevant epigenetic information, mainly by DNA demethylation. This extensive epigenetic reprogramming leads to the conversion of germ cells into immortal cells that can pass on the genome to the next generation. In the absence of germline-specific reprogramming, germ cells would preserve the old, parental epigenetic memory, which would prevent the transfer of heritable information to the offspring. On the contrary, somatic cells cannot reset epigenetic information by preserving the full methylation pattern on imprinting genes. In this review, we focus on the capacity of germ cells and somatic cells (soma) to transfer genetic information to the next generation, and thus revisit the Weismann theory of heredity.

Mouse embryonic stem cells, but not somatic cells, predominantly use homologous recombination to repair double-strand DNA breaks

Embryonic stem (ES) cells give rise to all cell types of an organism. Since mutations at this embryonic stage would affect all cells and be detrimental to the overall health of an organism, robust mechanisms must exist to ensure that genomic integrity is maintained. To test this proposition, we compared the capacity of murine ES cells to repair DNA double-strand breaks with that of differentiated cells. Of the 2 major pathways that repair double-strand breaks, error-prone nonhomologous end joining (NHEJ) predominated in mouse embryonic fibroblasts, whereas the high fidelity homologous recombinational repair (HRR) predominated in ES cells. Microhomology-mediated end joining, an emerging repair pathway, persisted at low levels in all cell types examined. The levels of proteins involved in HRR and microhomology-mediated end joining were highly elevated in ES cells compared with mouse embryonic fibroblasts, whereas those for NHEJ were quite variable, with DNA Ligase IV expression low in ES cells. The half-life of DNA Ligase IV protein was also low in ES cells. Attempts to increase the abundance of DNA Ligase IV protein by overexpression or inhibition of its degradation, and thereby elevate NHEJ in ES cells, were unsuccessful. When ES cells were induced to differentiate, however, the level of DNA Ligase IV protein increased, as did the capacity to repair by NHEJ. The data suggest that preferential use of HRR rather than NHEJ may lend ES cells an additional layer of genomic protection and that the limited levels of DNA Ligase IV may account for the low level of NHEJ activity.

Senescence-dependent MutS alpha dysfunction attenuates mismatch repair

DNA damage and mutations in the genome increase with age. To determine the potential mechanisms of senescence-dependent increases in genomic instability, we analyzed DNA mismatch repair (MMR) efficiency in young and senescent human colonic fibroblast and human embryonic lung fibroblast. It was found that MMR activity is significantly reduced in senescent cells. Western blot and immunohistochemistry analysis revealed that hMSH2 and MSH6 protein (MutS alpha complex), which is a known key component in the MMR pathway, is markedly down-regulated in senescent cells. Moreover, the addition of purified MutS alpha to extracts from senescent cells led to the restoration of MMR activity. Semiquantitative reverse transcription-PCR analysis exhibited that MSH2 mRNA level is reduced in senescent cells. In addition, a decrease in E2F transcriptional activity in senescent cells was found to be crucial for MSH2 suppression. E2F1 small interfering RNA expression reduced hMSH2 expression and MMR activity in young human primary fibroblast cells. Importantly, expression of E2F1 in quiescent cells restored the MSH2 expression as well as MMR activity, whereas E2F1-infected senescent cells exhibited no restoration of MSH2 expression and MMR activity. These results indicate that the suppression of E2F1 transcriptional activity in senescent cells lead to stable repression of MSH2, followed by a induction of MutS alpha dysfunction, which results in a reduced cellular MMR capacity in senescent cells.

Autophagy, ageing and apoptosis: the role of oxidative stress and lysosomal iron

As an outcome of normal autophagic degradation of ferruginous materials, such as ferritin and mitochondrial metalloproteins, the lysosomal compartment is rich in labile iron and, therefore, sensitive to the mild oxidative stress that cells naturally experience because of their constant production of hydrogen peroxide. Diffusion of hydrogen peroxide into the lysosomes results in Fenton-type reactions with the formation of hydroxyl radicals and ensuing peroxidation of lysosomal contents with formation of lipofuscin that amasses in long-lived postmitotic cells. Lipofuscin is a non-degradable polymeric substance that forms at a rate that is inversely related to the average lifespan across species and is built up of aldehyde-linked protein residues. The normal accumulation of lipofuscin in lysosomes seems to reduce autophagic capacity of senescent postmitotic cells--probably because lipofuscin-loaded lysosomes continue to receive newly formed lysosomal enzymes, which results in lack of such enzymes for autophagy. The result is an insufficient and declining rate of autophagic turnover of worn-out and damaged cellular components that consequently accumulate in a way that upsets normal metabolism. In the event of a more substantial oxidative stress, enhanced formation of hydroxyl radicals within lysosomes jeopardizes the membrane stability of particularly iron-rich lysosomes, specifically of autophagolysosomes that have recently participated in the degradation of iron-rich materials. For some time, the rupture of a limited number of lysosomes has been recognized as an early upstream event in many cases of apoptosis, particularly oxidative stress-induced apoptosis, while necrosis results from a major lysosomal break. Consequently, the regulation of the lysosomal content of redox-active iron seems to be essential for the survival of cells both in the short- and the long-term.

Age-related accumulation of autosomal mutations in solid tissues of the mouse is gender and cell type specific

Most cancers in solid tissues increase with age and invariably contain causal mutations eliminating expression of one or more autosomal tumor suppressor genes. However, very little is known about the effect of age on autosomal mutations, often large in size, in cells of solid tissues. In this study, the frequency and spectrum of autosomal mutations were examined as a function of age for kidney epithelial cells and ear mesenchymal cells in B6D2F1 mice heterozygous for the selectable Aprt locus. Aprt mutant frequencies were found to increase with age in the kidneys of both male and female mice, but at all ages the mutant frequencies were approximately twice as high in the females, which in this strain have shorter lifespans than the males. An age-related increase in Aprt mutant frequencies was also observed for ear cells from female mice, but no significant increases in mutant frequencies were observed for the ear cells of male mice. A molecular analysis showed that the kidney and ear mutational spectra were distinct and that the age-related increases in mutant frequencies did not involve significant shifts in the mutational spectra. In total, the results demonstrate both gender and cell-type-specific patterns of autosomal mutational accumulation as a function of age in two solid tissues of the mouse.

Oxidative stress, accumulation of biological 'garbage', and aging

Normal metabolism is associated with unavoidable mild oxidative stress resulting in biomolecular damage that cannot be totally repaired or removed by cellular degradative systems, including lysosomes, proteasomes, and cytosolic and mitochondrial proteases. Consequently, irreversibly damaged and functionally defective structures (biological 'garbage') accumulate within long-lived postmitotic cells, such as cardiac myocytes and neurons, leading to progressive loss of adaptability and increased probability of death and characterizing a process called aging, or senescence. Intralysosomal 'garbage' is represented by lipofuscin (age pigment), an undegradable autophagocytosed material, while extralysosomal 'garbage' involves oxidatively modified cytosolic proteins, altered biomembranes, defective mitochondria and other organelles. In aged postmitotic cells, heavily lipofuscin-loaded lysosomes perform poorly, resulting in the enhanced accumulation of defective mitochondria, which in turn produce more reactive oxygen species causing additional damage (the mitochondrial-lysosomal axis theory). Potential anti-aging strategies may involve not only overall reduction of oxidative stress, but also the use of intralysosomal iron chelators hampering Fenton-type chemistry as well as the stimulation of cellular degradative systems.

Tissue-specific time courses of spontaneous mutation frequency and deviations in mutation pattern are observed in middle to late adulthood in Big Blue mice

To better define the time course of spontaneous mutation frequency in middle to late adulthood of the mouse, measurements were made at 10, 14, 17, 23, 25, and 30 months of age in samples of adipose tissue, liver, cerebellum (90% neurons), and the male germline (95% germ cells). A total of 46 million plaque-forming units (pfus) were screened at the six time points and 1,450 circular blue plaques were harvested and sequenced. These data improve resolution and confirm the previously observed occurrence of at least two tissue-specific profiles of spontaneous mutation frequency (elevation with age in adipose tissue and liver, and constancy with age in neurons and male germ cells), a low mutation frequency in the male germline, and a mutation pattern unchanged with age within a tissue. These findings appear to extend to very old age (30 months). Additional findings include interanimal variation in spontaneous mutation frequency is larger in adipose tissues and liver compared with neurons and male germ cells, and subtle but significant differences in the mutation pattern among tissues, consistent with a minor effect of tissue-specific metabolism. The presumptive unaltered balance of DNA damage and repair with age in the male germline has evolutionary consequences. It is of particular interest given the controversy over whether or not increasing germline mutation frequency with paternal age underlies the reports associating older males with a higher incidence of some types of genetic disease. These most detailed measurements available to date regarding the time course of spontaneous mutation frequency and pattern in individual tissues help to constrain hypotheses regarding the role of mutational mechanisms in DNA repair and aging.

Is aging the price for memory?

Aging (senescence) is apparent in animals that possess long-lived postmitotic cells but is negligible in primitive species, such as hydras and other Cnidarians, all of whose cells are constantly renewed by cell division. This repetitive mitotic activity precludes the progressive intracellular accumulation of damaged biomolecules and organelles, which are obvious concomitants of aging in neurons and other long-lived cells of higher animals. We assume that the development of long-lived postmitotic cells, now found in the overwhelming majority of species, represented a useful evolutionary change. Probably, of particular importance was the evolution of long-lived neurons, which are required for long-term memory. However, the appearance of long-lived postmitotic cells not only increased fitness, but also gave rise to the aging process.

Oxidative mutagenesis, mismatch repair, and aging

A PubMed search for the term "oxidative stress" yields over 29,000 articles published on the subject over the past 10 years; more than 2000 of these articles also include the term "aging" in their title or abstract. Many theories of aging predict causal roles for oxidative stress in the myriad of pathological changes that occur as a function of age, including an increasing propensity to develop cancer. A possible link between aging and cancer is the induction and accumulation of somatic mutations caused by oxidative stress. This Review focuses on small mutational events that are induced by oxidative stress and the role of mismatch repair (MMR) in preventing their formation. It also discusses a possible inhibitory effect of oxidative stress on MMR. We speculate that a synergistic interaction between oxidative damage to DNA and reduced MMR levels will, in part, account for an accumulation of small mutational events, and hence cancer, with aging.

Mitochondrial DNA 4,977-bp deletion in paired oral cancer and precancerous lesions revealed by laser microdissection and real-time quantitative PCR

Oral cancer is the fourth leading cause of cancer deaths among men in Taiwan and is closely associated with areca quid chewing habits. Recent studies showed that mitochondrial DNA (mtDNA) mutations occur in various tumors, including oral cancers, and that the accumulation of mtDNA deletions could be an important contributor to carcinogenesis. Using laser microdissection, we have analyzed mtDNA deletions by pairwise comparisons in oral cancer, precancerous cells, and their adjacent submucosal stoma tissues in 12 patients with areca quid chewing history. Real-time quantitative polymerase chain reaction (RTQPCR) was performed to detect and quantify mtDNA with the 4,977-bp deletion in the histologically defined specified cell groups. Quantitative analysis of 60 samples by RTQPCR revealed that the average proportions of 4,977-bp deleted mtDNA over total mtDNA were 0.137%, 0.367%, and 0.001% in cancer, precancer cells, and lymphocytes of lymph node biopsies, respectively. Pairwise analysis of the proportion of mtDNA deletion in cancer, precancer, and their stroma tissues revealed a consistent trend among these patients. All of the patients (12/12) presented a higher proportion of mtDNA with 4,977-bp deletion in the lesions than in the lymphocytes, with average increases of 198-fold in cancer and 546-fold in precancer cells. A decrease in the proportion of deleted mtDNA was observed in 8 of 12 patients when the disease progressed from precancer to cancer lesions. Interestingly, 7 of 12 cancer tissues and 8 of 12 precancer lesions exhibited an average of 6.3-fold and 17.4-fold increases in the proportion of 4,977-bp deleted mtDNA in the stromal cells than in the lesion cells, respectively. The observation that the proportion of 4,977-bp deleted mtDNA in all oral lesions was higher than normal and consistently decreased during cancer progression from precancer to primary cancer suggests that accumulation and subsequent cytoplasmic segregation of the mutant mtDNA during cell division may play an important role in oral carcinogenesis. This study also demonstrates that laser microdissection combined with RTQPCR is an efficient and sensitive tool to gain insight into the role that mtDNA mutation may play in carcinogenesis.

Accumulation of point mutations in mitochondrial DNA of aging mice

Mitochondrial DNA (mtDNA) exists in a highly genotoxic environment created by exposure to reactive oxygen species, somewhat deficient DNA repair, and the relatively low fidelity of polymerase gamma. Given the severity of the environment, it was anticipated that mutation accumulation in the mtDNA of aging animals should exceed that of nuclear genes by several orders of magnitude. We have analyzed fragments amplified from the D-loop region of mtDNA from 2 to 22-month-old mice. The amplified 432 bp fragments were cloned into plasmid vectors, and plasmid DNAs from individual clones were purified and sequenced. None of 110 fragments from young mice contained a mutation, while 9 of 87 clones originating from old animals contained base substitutions (chi square = 11.9, P<0.001). The estimated mutation frequency in mtDNA from old mice was 11.6+/-2.7 or 25.4+/-7.8 per 10(5) nucleotides (depending on assumptions of clonality), which exceeds existing estimates for mutation frequencies for nuclear genes by approximately 1000-fold. Our data suggest that at 22 months of age, which roughly corresponds to 3/4 of the mouse natural life span, most mtDNA molecules carry multiple point mutations.

Genetic and environmental factors in cancer and neurodegenerative diseases

The aim of this review is to summarise the recent findings in the fields of carcinogenesis and neurodegenerative diseases, the both disorders are characterised by the contribution of different factors including the inheritance of mutated genes, and the exposure to endogenous or exogenous agents during the life. We first analysed the causative genes until now discovered in both processes, then we focused our attention on the role of environmental exposure, susceptibility factors, oxidative stress, apoptosis and aging to the development of such disorders. The genotype at a particular locus may account for an inter-individual susceptibility that can both increase or decrease the risk to develop the pathology especially after the exposure to environmental agents. The mechanism of apoptosis, that is an excellent strategy in order to eliminate damaged cells, seems to be lost during carcinogenesis, while it seems to be involved in the neuronal death in a lot of neurodegenerative disorders. Oxidative stress can both lead to DNA mutations or to the formation of damaged proteins, so being an important risk factor for the initiation and the progression of a disease: in fact it may be one of the causes or can arise as a consequence of a damage caused by other factors increasing then the first damage. It is well established that carcinogenesis is a multi-step process caused by series of successive mutations occurring into a cell and conferring to this cell a growth advantage, so that age is the largest risk factor for cancer in humans. Pathophysiology of neurodegenerative diseases is complex and likely involves multiple overlapping and perhaps redundant pathways of neuronal damage, characterised by the generation of anomalous proteins, often due to mutations in the corresponding gene, and by their subsequent accumulation into or outside specific areas of the brain.

Prevention of cancer in the older person

Both primary and secondary cancer prevention may improve cancer control among older persons. Although chemoprevention of cancer is feasible, the agents currently used for chemoprevention have several complications. As a result, the use of these substances should be individualized based on risk-benefit ratio. It is reasonable to implement screening for cancer of the breast and of the large bowel in persons with a life expectancy of 5 years and longer. No definite recommendation may be issued at present related to screening for prostate, lung, and cervical cancer. Ongoing clinical trials may answer some of these questions.

Detection of mutation in transgenic CHO cells using green fluorescent protein as a reporter

A novel approach was developed for rapidly estimating the frequency of specific mutations in genetically engineered Chinese hamster ovary (CHO) cells. We designed double-transgenic CHO cell lines that contain a transgene consisting of the sequence coding for green fluorescent protein under the control of a tetracycline (Tet) responsive promoter and a second transgene coding for the constitutively expressed Tet repressor. Cultures of these CHO cells were treated with gamma-radiation, N-methyl-N-nitrosourea or methyl methanesulfonate, and the fluorescence of individual cells from both control and treated cultures was measured by flow cytometry. The treatments increased the number of highly fluorescent cells, those with presumed mutations in the Tet-repressor gene. Mutant cells from gamma-radiation-exposed cultures were isolated by fluorescence-activated cell sorting, cultured, and individual clones expanded. A PCR-based analysis indicated that the highly fluorescent expanded cells had lost the transgene coding for the Tet repressor, suggesting that the system mainly detects large genetic alterations. A similar approach may be useful for making high-throughput in vivo models for mutation detection.

Aging is a deprivation syndrome driven by a germ-soma conflict

Evolution through natural selection can be described as driven by a perpetual conflict of individuals competing for limited resources. Recently, I postulated that the shortage of resources godfathered the evolutionary achievements of the differentiation-apoptosis programming [Rev. Neurosci. 12 (2001) 217]. Unicellular deprivation-induced differentiation into germ cell-like spores can be regarded as the archaic reproduction events which were fueled by the remains of the fratricided cells of the apoptotic fruiting body. Evidence has been accumulated suggesting that conserved through the ages as the evolutionary legacy of the germ-soma conflict, the somatic loss of immortality during the ontogenetic segregation of primordial germ cells recapitulates the archaic fate of the fruiting body. In this heritage, somatic death is a germ cell-triggered event and has been established as evolutionary-fixed default state following asymmetric reproduction in a world of finite resources. Aging, on the other hand, is the stress resistance-dependent phenotype of the somatic resilience that counteracts the germ cell-inflicted death pathway. Thus, aging is a survival response and, in contrast to current beliefs, is antagonistically linked to death that is not imposed by group selection but enforced upon the soma by the selfish genes of the "enemy within". Environmental conditions shape the trade-off solutions as compromise between the conflicting germ-soma interests. Mechanistically, the neuroendocrine system, particularly those components that control energy balance, reproduction and stress responses, orchestrate these events. The reproductive phase is a self-limited process that moulds onset and progress of senescence with germ cell-dependent factors, e.g. gonadal hormones. These degenerate the regulatory pacemakers of the pineal-hypothalamic-pituitary network and its peripheral, e.g. thymic, gonadal and adrenal targets thereby eroding the trophic milieu. The ensuing cellular metabolic stress engenders adaptive adjustments of the glucose-fatty acid cycle, responses that are adequate and thus fitness-boosting under fuel shortage (e.g. during caloric restriction) but become detrimental under fuel abundance. In a Janus-faced capacity, the cellular stress response apparatus expresses both tolerogenic and mutagenic features of the social and asocial deprivation responses [Rev. Neurosci. 12 (2001) 217]. Mediated by the derangement of the energy-Ca(2+)-redox homeostatic triangle, a mosaic of dedifferentiation/apoptosis and mutagenic responses actuates the gradual exhaustion of functional reserves and eventually results in a multitude of aging-related diseases. This scenario reconciles programmed and stochastic features of aging and resolves the major inconsistencies of current theories by linking ultimate and proximate causes of aging. Reproduction, differentiation, apoptosis, stress response and metabolism are merged into a coherent regulatory network that stages aging as a naturally selected, germ cell-triggered and reproductive phase-modulated deprivation response.

Molecular analysis of mitochondrial DNA mutations from bleomycin-treated rats

In our previous studies, we have shown the mutagenicity of bleomycin (BLM) at the nuclear hprt locus. In the present study we have analyzed mutagenic effects of BLM in mitochondrial DNA (mtDNA) using short extension-PCR (SE-PCR) method for detection of low-copy deletions. Fisher 344 rats were treated with a single dose of BLM and total DNA preparations from splenic lymphocytes were processed in SE-PCR assay. Spontaneous deletions were typically flanked by direct repeats (78.5%), while the in BLM-treated group, direct repeats were found in only 46.6% of breakpoints. The ratio between deletions based on direct repeats and random sequence deletions changed from 3.67 in control group to 0.87 in BLM-treated animals, which corresponds to an approximate 1.7-fold increase in the deletion mutation frequency. Furthermore, 62.5% of deletions not flanked by direct repeats in the treated group contained cleavage sites for BLM. The localization of breakpoints was not entirely random. We have found four clusters containing deletions from both groups indicative of deletion hot spots. The results indicate that BLM exposure may be associated with the induction of mtDNA mutations, and suggest the utility of SE-PCR method for evaluating drug-induced genotoxicity.

Accumulation of mitochondrial DNA deletions in human oral tissues -- effects of betel quid chewing and oral cancer

Accumulation of mitochondrial DNA (mtDNA) mutations in human tissues has been associated with intrinsic aging and environmental insult. Recently, mtDNA mutations have been detected in various tumors, including head and neck tumors. However, the factors affecting the occurrence and accumulation of mtDNA deletions in tumor tissues are poorly understood. In Taiwan, betel quid chewing is a major risk factor for oral cancer. Using polymerase chain reaction (PCR) techniques, we examined large-scale deletions of mtDNA in 53 pairs of tumor and non-tumor oral tissues from the patients with or without betel quid chewing history. The results revealed that irrespective of the history of betel quid chewing, the incidences of the 4977bp deletion and other deletions of mtDNA were lower in the tumor portion as compared with the non-tumor portion. The average proportions of the 4977bp deleted mtDNA in the tumor tissues of the betel quid chewers and non-betel quid chewers were 13- and 5-fold, respectively, lower than those in the corresponding non-tumor tissues. Moreover, the average proportion of 4977bp deleted mtDNA was significantly higher (P<0.05) in the non-tumor oral tissues of the patients with betel quid chewing history than that of the patients without the history of betel quid chewing. These results suggest that betel quid chewing may increase mtDNA mutation in human oral tissues and that accumulation of mtDNA deletions and subsequent cytoplasmic segregation of these mutations during cell division could be an important contributor to the early phase of oral carcinogenesis.