Activation status of the X chromosome in human micronucleated lymphocytes

Chromosome-specific induction of micronuclei and chromosomal aberrations by mitomycin C: Involvement of human chromosomes 9, 1 and 16

Cytogenetic studies have shown that human chromosomes 1, 9, and 16, with a large heterochromatic region of highly methylated classical satellite DNA, are prone to induction of chromatid breaks and interchanges by mitomycin C (MMC). A couple of studies have indicated that material from chromosome 9, and possibly also from chromosomes 1 and 16, are preferentially micronucleated by MMC. Here, we further examined the chromosome-specific induction of micronuclei (MN; with and without cytochalasin B) and chromosomal aberrations (CAs) by MMC. Cultures of isolated human lymphocytes from two male donors were treated (at 48 h of culture, for 24 h) with MMC (500 ng/ml), and the induced MN were examined by a pancentromeric DNA probe and paint probe for chromosome 9, and by paint probes for chromosomes 1 and 16. MMC increased the total frequency of MN by 6-8-fold but the frequency of chromosome 9 -positive (9+) MN by 29-30-fold and the frequency of chromosome 1 -positive (1+) MN and chromosome 16 -positive (16+) MN by 12-16-fold and 10-17-fold, respectively. After treatment with MMC, 34-47 % of all MN were 9+, 17-20 % 1+, and 3-4 % 16+. The majority (94-96 %) of the 9+ MN contained no centromere and thus harboured acentric fragments. When MMC-induced CAs aberrations were characterized by using the pancentromeric DNA probe and probes for the classical satellite region and long- and short- arm telomeres of chromosome 9, a high proportion of chromosomal breaks (31 %) and interchanges (41 %) concerned chromosome 9. In 83 % of cases, the breakpoint in chromosome 9 was just below the region (9cen-q12) labelled by the classical satellite probe. Our results indicate that MMC specifically induces MN harbouring fragments of chromosome 9, 1, and 16. CAs of chromosome 9 are highly overrepresented in metaphases of MMC-treated lymphocytes. The preferential breakpoint is below the region 9q12.

Aneuploidy, inflammation and diseases

This review discusses how numerical aneuploidy may trigger inflammation in somatic cells and its consequences. Therefore we: i) summarized current knowledge on the cellular and molecular pathological effects of aneuploidy; ii) considered which of these aspects are able to trigger inflammation; iii) determined the genetic and environmental factors which may modulate the link between aneuploidy and inflammation; iv) explored the rôle of diet in prevention of aneuploidy and inflammation; v) examined whether aneuploidy and inflammation are causes and/or consequences of diseases; vi) identified the knowledge gaps and research needed to translate these observations into improved health care and disease prevention. The relationships between aneuploidy, inflammation and diseases are complex, because they depend on which chromosomes are involved, the proportion of cells affected and which organs are aneuploid in the case of mosaic aneuploidy. Therefore, a systemic approach is recommended to understand the emergence of aneuploidy-driven diseases and to take preventive measures to protect individuals from exposure to aneugenic conditions.

Inflammatory cytokine storms severity may be fueled by interactions of micronuclei and RNA viruses such as COVID-19 virus SARS-CoV-2. A hypothesis

In this review we bring together evidence that (i) RNA viruses are a cause of chromosomal instability and micronuclei (MN), (ii) those individuals with high levels of lymphocyte MN have a weakened immune response and are more susceptible to RNA virus infection and (iii) both RNA virus infection and MN formation can induce inflammatory cytokine production. Based on these observations we propose a hypothesis that those who harbor elevated frequencies of MN within their cells are more prone to RNA virus infection and are more likely, through combined effects of leakage of self-DNA from MN and RNA from viruses, to escalate pro-inflammatory cytokine production via the cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING) and the Senescence Associated Secretory Phenotype (SASP) mechanisms to an extent that is unresolvable and therefore confers high risk of causing tissue damage by an excessive and overtly toxic immune response. The corollaries from this hypothesis are (i) those with abnormally high MN frequency are more prone to infection by RNA viruses; (ii) the extent of cytokine production and pro-inflammatory response to infection by RNA viruses is enhanced and possibly exceeds threshold levels that may be unresolvable in those with elevated MN levels in affected organs; (iii) reduction of MN frequency by improving nutrition and life-style factors increases resistance to RNA virus infection and moderates inflammatory cytokine production to a level that is immunologically efficacious and survivable.

Micronuclei, inflammation and auto-immune disease

Auto-immune diseases (AUD) are characterized by an immune response to antigenic components of the host itself. The etiology of AUD is not well understood. The available evidence points to an interaction between genetic, epigenetic, environmental, infectious and life-style factors. AUD are more prevalent in women than in men; sex hormones play a crucial role in this sex bias. Micronuclei (MN) emerged as a new player in the induction of AUD, based on the capacity of DNA-sensors to detect self-DNA that leaks into the cytoplasm from disrupted MN and induce the cGAS-STING pathway triggering an innate auto-immune response and chronic inflammation. It was found that inflammation can induce MN and MN can induce inflammation, leading to a vicious inflammation-oxidative-DNA damage-MN-formation-chromothripsis cycle. MN originating from sex chromosome-loss may induce inflammation and AUD. We performed a systematic review of studies reporting MN in patients with systemic or organ-specific AUD. A meta-analysis was performed on lymphocyte MN in diabetes mellitus (10 studies, 457 patients/290 controls) and Behcet's disease (3 studies, 100 patients/70 controls) and for buccal MN in diabetes mellitus (11 studies, 507 patients/427 controls). A statistically significant increase in patients compared to controls was found in the meta-analyses providing an indication of an association between MN and AUD. A 36%-higher mean-MRi in buccal cells (3.8+/-0.7) was found compared to lymphocytes (2.8+/-0.7)(P = 0.01). The meta-MRi in lymphocytes and buccal cells (1.7 and 3.0 respectively) suggest that buccal cells may be more sensitive. To assess their relative sensitivity, studies with measurements from the same subjects would be desirable. It is important that future studies (i) investigate, in well-designed powered studies, the prospective association of MN-formation with AUD and (ii) explore the molecular mechanisms by which chromosome shattering in MN and the release of chromatin fragments from MN lead to the formation of auto-antibodies.

The potential application of human PIG-A assay on azathioprine-treated inflammatory bowel disease patients

The rodent Pig-a assay has been used extensively as a potential regulatory assay for evaluating the in vivo mutagenicity of test substances. Although the assay can be conducted in different mammalian species, there have been only a few reports describing its use in humans, and rarely in genotoxicant-exposed human populations. In this study, PIG-A mutation frequencies (MFs) were evaluated in 36 azathioprine (AZA; human carcinogen)-treated inflammatory bowel disease (IBD) patients and 36 healthy volunteers. IBD patients exhibited a slight but statistically higher MF (6.10 ± 4.44 × 10^-6 ) than healthy volunteers (4.97 ± 2.74 × 10^-6 ) (P = 0.0489). The estimated relative risk for the exposed patients was 1.22 which indicated that AZA is a risk factor for inducing PIG-A mutation. However, the PIG-A MF showed no associations with AZA treatment duration or total AZA exposure. In addition, we performed the cytokinesis-block micronucleus test on the same samples. The frequencies of micronuclei (MN) and nuclear buds (NBUD) in IBD patients (MN: 4.70 ± 2.86‰; NBUD: 1.89 ± 0.95‰) were significantly higher than in healthy volunteers (MN: 1.47 ± 0.77‰, P < 0.001; NBUD: 0.90 ± 0.58‰, P = 0.004). MN frequency also had significant correlations with AZA treatment duration (P = 0.011) and total AZA exposure (P = 0.018). Our findings indicate that AZA-treated IBD patients have only a marginally significant increase in PIG-A MF; in contrast, a much stronger AZA-associated increase in genotoxicity was detected with the lymphocyte MN assay. Environ. Mol. Mutagen. 2019. © 2019 Wiley Periodicals, Inc.

X chromosome aneuploidy and micronuclei in fertile mares

Abnormalities of chromosomes are an important and well documented cause of disorders of sexual development, fertility problems and congenital anomalies in mammals. Detection of low-level 63,X/64,XX mosaicism during routine cytogenetic evaluation is a challenge because its clinical significance is not yet fully clear. This study describes the prevalence and levels of 63,X mosaicism for a cohort of fertile mares and compares the results with eight problem mares for which no clinical cause of sub-fertility was found. The study design allowed for the analysis of micronuclei which are biomarkers of genomic instability and can disturb cell divisions, drive cancer development or cause congenital diseases. Although 27% of the fertile mares were identified to be 63,X mosaics, the results showed that the rates of abnormal cells were very low (1-3%). Levels of abnormal cells in problem mares with 63,X mosaicism were similar or higher. The average rate of micronuclei in the blood of the fertile mares was ∼1%, well below the baseline (5%) which was proposed for peripheral blood of normal healthy humans. We found weak to modest, but not significant, correlations between the age of fertile mares and 63,X cells (Kendall's tau b = 0.2905; p > 0.05) as well as the rate of micronuclei (Kendall's tau b = 0.1896; p > 0.05). Likewise, the correlation between presence of a 63,X cell line and micronuclei rate was not significant (Kendall's tau b = 0.3201; p > 0.05). The presence of 63,X cells in rates greater than 3% may indeed indicate a higher risk for sub-fertility and eventually for associated health problems in such mares. Detection and elimination of mares with high level of X aneuploidies from breeding may have a positive effect on the fertility within the general horse population. This data may support the evaluation of problem mares with mosaic karyotypes involving the X chromosome.

The repeated cytogenetic analysis of subjects occupationally exposed to nanoparticles: a pilot study

The application of nanomaterials has been rapidly increasing during recent years. Inhalation exposure to nanoparticles (NP) may result in negative toxic effects but there is a critical lack of human studies, especially those related to possible DNA alterations. We analyzed pre-shift and post-shift a group of nanocomposite researchers with a long-term working background (17.8 ± 10.0 years) and matched controls. The study group consisted of 73.2% males and 26.8% females. Aerosol exposure monitoring during a working shift (involving welding, smelting, machining) to assess the differences in exposure to particulate matter (PM) including nanosized fractions <25–100 nm, and their chemical analysis, was carried out. A micronucleus assay using Human Pan Centromeric probes, was applied to distinguish between the frequency of centromere positive (CEN+) and centromere negative (CEN−) micronuclei (MN) in the binucleated cells. This approach allowed recognition of the types of chromosomal damage: losses and breaks. The monitoring data revealed differences in the exposure to NP related to individual working processes, and in the chemical composition of nanofraction. The cytogenetic results of this pilot study demonstrated a lack of effect of long-term (years) exposure to NP (total frequency of MN, P = 0.743), although this exposure may be responsible for DNA damage pattern changes (12% increase of chromosomal breaks—clastogenic effect). Moreover, short-term (daily shift) exposure could be a reason for the increase of chromosomal breaks in a subgroup of researchers involved in welding and smelting processes (clastogenic effect, P = 0.037). The gender and/or gender ratio of the study participants was also an important factor for the interpretation of the results. As this type of human study is unique, further research is needed to understand the effects of long-term and short-term exposure to NP.

Mapping the factors affecting the frequency and types of micronuclei in an elderly population from Southern Bohemia

The micronucleus assay is one of the most common methods used to assess chromosomal damage (losses or breaks) in human peripheral blood lymphocytes (PBL) in genetic toxicology. Most studies have focused on analyzing total micronuclei (MN), but identifying the content of MN can provide more detailed information. The main aim of this study was to map the factors affecting the frequency and types of micronuclei in binucleated cells (BNC) in elderly population. Fluorescence in situ hybridization using Human Pan Centromeric Chromosome Paint was used to identify centromere positive (CEN+) or centromere negative (CEN-) MN. A group of 95 men from Southern Bohemia, Czech Republic (average age 68.0±6.8 years) was followed repeatedly, in spring and fall 2014. The study participants were former workers of the uranium plant "MAPE Mydlovary" (processing uranium ore from 1962 to 1991), and controls. The general profile of individual types of MN, and the effect of the season, former uranium exposure, age, smoking status, weight, and X-ray examination on the level and type of MN were analyzed. The results of this study showed: (i) a stable profile of BNC with MN based on the number of MN during two seasons; (ii) an increase of the number of CEN+ MN from spring to fall; (iii) a lower frequency of the total MN in the exposed group than in controls with a significant difference in the percentage of aberrant cells (%AB.C.) in the fall; (iv) no clear effect of age, smoking and BMI on DNA damage in this group; (v) lower DNA damage levels in former uranium workers who received X-ray examination later in life. In summary, the results indicate a trend of seasonal changes of individual types of MN and suggest that former exposure can have a protective effect on the level of DNA damage in case of future exposure.

A population study using the human erythrocyte PIG-A assay

Erythrocyte-based PIG-A assay is sensitive and reliable in detecting exposure to mutagenetic agents in animal studies, but there are few data from human populations. In this study, we employed a method for detecting CD59 phenotypic variants, resulting from mutation in the PIG-A gene, in human red blood cells (RBCs), and determined the CD59-deficient RBC (RBC^CD59- ) frequencies in 217 subjects from general population. The majority of subjects had a relatively low mutant frequencies (MFs) (average, 5.25 ± 3.6 × 10^-6 , median, 4.38 × 10^-6 , for all subjects), but with males having a significantly greater MFs (5.97 ± 4.0 × 10^-6 ) than females (4.19 ± 2.5 ×10^-6 ). There was no correlation between MFs and age. In addition, MFs showed no difference between smoker and nonsmoker, and also no association with smoke duration in male subjects. However, there was a significant correlation between cigarette-pack-years which indicated that the MF was only slightly elevated with the increase of cigarette-pack-years. Moreover, intraindividual variations were investigated in three volunteer subjects over 300 days, and the MFs were relatively stable and repeatable. Furthermore, a pilot study by using white blood cell (WBC) assay based on labeling with FLAER was performed in volunteer subjects. The MFs of FLAER-deficient WBC (WBC^FLAER- ) and RBC^CD59- were consistently elevated in two subjects. Our findings provide baseline data that will be helpful in designing further studies using the PIG-A assay to monitor the genotoxic effects of carcinogens in human populations. Environ. Mol. Mutagen. 57:589-604, 2016. © 2016 Wiley Periodicals, Inc.

The mitotic tensegrity guardian tau protects mammary epithelia from katanin-like1-induced aneuploidy

The microtubule associated-protein tau has been identified as an effective positive prognostic indicator in breast cancer. To explore the physiological function of tau in early carcinogenesis, endogenous tau was knocked down in primary cultured human mammary epithelial cells. This resulted in chromosome-bridging during anaphase followed by micronucleation, both of which were suppressed by a further katanin-like1 knockdown. We also detected that the exogenously expressed katanin-like1 induction of cellular transformation is prevented by exogenous tau in rat fibroblasts. The mutant katanin-like1 (L123V) identified in breast cancer showed an increase in this transformation capacity as well as microtubule severing activity resistant to tau. The tau knockdown resulted in a loss of the kinetochore fibers on which tau is normally localized. This physical fragility was also observed in isolated tau-knockdown mitotic spindles, supporting the relevance of microtubule damage to the onset of transformation. The karyotyping of tau-knockdown cells showed increased frequency of loss of one X chromosome, further suggesting the involvement of tau in breast tumorigenesis. We propose that tau may contribute to tumor progression by protecting spindle microtubules from excess severing by katanin-like1. We also present data indicating that the microtubule-binding octapeptide NAP is a candidate modifier against the tau deficiency in tumor cells.

Assessment of individual susceptibility to baseline DNA and cytogenetic damage in a healthy Turkish population: evaluation with lifestyle factors

BACKGROUND

Cytogenetic biomarkers are most frequently used well-established endpoints in human population studies with their sensitivity for measuring exposure to genotoxic agents. They have an important role as early predictors of cancer risk. Identification of individual genotypes of metabolic gene polymorphisms helps to understand the modulation of cancer susceptibility by environmental exposures, such as cigarette smoking and other lifestyle factors.

AIM

To evaluate individual susceptibility to chemicals, we determined individual DNA damage related to glutathione S-transferase (GST) genotypes (GSTM1, GSTT1, and GSTP1) in a Turkish population.

METHODS

Peripheral blood lymphocytes (PBL) and DNA samples of 127 subjects were analyzed for the presence of DNA damage, using single-cell gel electrophoresis (the Comet assay), and for cytogenetic parameters (chromosomal aberrations [CAs], bleomycin-induced CA, and a cytokinesis-blocked micronucleus assay), and the polymerase chain reaction/restriction fragment length polymorphism method, respectively.

RESULTS

Individuals carrying a GSTT1-null allele showed higher frequencies of CA and micronucleus (MN) (p=0.026, p=0.003, respectively), whereas the GSTM1-null and GSTP1 mutant genotypes did not show any differences in cytogenetic parameters. Our findings demonstrated that none of the lifestyle factors (smoking, alcohol drinking, dietary habits, vitamin intake, and physical activity), except for vitamin intake (p=0.002), were significantly associated with the studied cytogenetic parameters.

CONCLUSION

Our results suggest that the GSTT1 gene polymorphism may influence the baseline cytogenetic frequency in a healthy population.

Estimation of a safe level for occupational exposure to vinyl chloride using a benchmark dose method in central China

OBJECTIVES

The aim of this study was to estimate a benchmark dose (BMD) for chromosome damage induced by vinyl chloride monomer (VCM) in VCM-exposed workers in central China and validate the published results in Shanghai.

METHODS

VCM-exposed workers who had been exposed to VCM for at least one year (n=463) and matched subjects not exposed to VCM or other toxins (n=273) were asked to participate in this study. Micronucleus (MN) frequency based on the cytokinesis-block micronucleus assay (CBMN) was used as a biomarker for chromosome damage induced by VCM exposure.

RESULTS

The MN frequency in the VCM-exposed workers was significantly higher than that in the control group, and multivariate Poisson regression suggested that gender, smoking status and VCM exposure were the significant factors influencing the risk of increased MN frequency. When subjects were further stratified according to gender and smoking status, the results showed that female VCM-exposed workers were more susceptible than the males to the risk of increased MN frequency. The MN frequency of smokers was significantly higher than that of nonsmokers in the control group. Our study also suggested that there was a strong dose-response relationship between VCM CED and the increased risk of MN frequency in the total group, males and females. The BMDL(10) was found to be 630.6, 670.2 and 273.7 mg-year for all VCM-exposed workers, males and females, respectively.

CONCLUSIONS

These results invite further scrutiny of the current VCM occupational exposure limits and warrant further study of the risk of VCM genotoxicity and carcinogenicity.

Chromosomal composition of micronuclei in human leukocytes exposed to mitomycin C

Micronuclei (MN) can be induced by different mutagenic substances. Even though this has been known for decades, it is still not clear which genetic content, especially which chromosomes, these MN are constituted of and if there are any influences on this content by the MN-inducing substance. Also, the interphase position, size, and gene density of a chromosome could influence its involvement in MN formation. To study some of these questions, fluorescence in situ hybridization using centromeric and whole-chromosome painting probes for chromosomes 3, 4, 6, 7, 9, 16, 17, 18, and X was applied in mitomycin C (MMC)-induced MN in human leukocytes. The obtained results showed that material from all studied chromosomes was present in MN. Also, there was no correlation between interphase position, size, and gene density of the studied chromosomes and their migration in MN. Interestingly, material derived from chromosomes 9 and 16 was overrepresented in MMC-induced MN. Finally, further studies using substances other than MMC are necessary to clarify if the MN-inducing mutagen has an influence on the chromosomal content of the MN.

Genetic and environmental influences on spontaneous micronuclei frequencies in children and adults: a twin study

The primary aim of this study was to quantify genetic and environmental influences on the frequency of spontaneously occurring micronuclei in children and adults. To meet this aim, a total of 63 male and female twin pairs and 19 singletons (145 individuals) were evaluated, ranging in age from 7 to 85 years. Micronuclei frequencies significantly increased with age for both genders (r = 0.49, P < 0.001), with the lowest and highest rates being seen in the 7- to 9 (mean = 0.56%, SD = .28) and 60- to 69-year-olds (mean = 2.12%, SD = 1.0), respectively. This age effect was significantly more pronounced in females than males (P = 0.017). In addition to the main effect of age, the completion of puberty in either gender (P = 0.036) and menopause in females (P = 0.024) was associated with a significant increase in micronuclei frequencies. Genetic model fitting indicated that influences from both additive genetic (65.2% of variance) and unique environmental (34.8% of variance) sources best explained the observed micronuclei frequencies in monozygotic and dizygotic twin pairs. Self-reported health conditions associated with an increased frequency of micronuclei included a history of allergies (P < 0.007) and migraines (P = 0.026). Multivitamin use was also associated with increased micronuclei frequencies (P = 0.004). In contrast, significantly lower micronuclei frequencies were associated with arthritis (P = 0.002), as well as consuming fruit (P = 0.014), green, leafy vegetables (P < 0.001) and/or folate-enriched bread (P = 0.035). A sex-specific effect, resulting in a significantly increased frequency of micronuclei with tobacco usage, was observed for females (but not males). Gender differences also moderated the impact of vitamin D and calcium consumption. In conclusion, the frequency of spontaneously arising micronuclei in humans is a complex trait, being influenced by both heritable genetic and environmental components. Recognition of factors contributing to baseline levels of micronuclei should provide guidance to researchers in designing studies to evaluate agents hypothesised to influence chromosomal instability.

The effect of age, gender, diet and lifestyle on DNA damage measured using micronucleus frequency in human peripheral blood lymphocytes

Micronucleus (MN) frequency in cytokinesis-blocked peripheral blood lymphocytes (PBL) has become one of the best-established biomarkers for studying DNA damage occurring in vivo in humans. The application of this method in population biomonitoring studies requires a deep understanding of how lifestyle and common host variables may influence MN frequency in PBL. In this mini-review, an update is provided on results from studies reporting on the impact of age, gender, diet and lifestyle factors (e.g. exercise, alcohol, smoking and recreational drugs) on this biomarker. Evidence from these studies shows that each of these factors, either in isolation or in combination, can significantly influence MN frequency. Proper control for these factors is required to enable better measurement of the impact of other conditions, such as environmental exposure to genotoxins or a susceptible genetic background, on MN frequency in PBL.

Molecular mechanisms of micronucleus, nucleoplasmic bridge and nuclear bud formation in mammalian and human cells

Micronuclei (MN) and other nuclear anomalies such as nucleoplasmic bridges (NPBs) and nuclear buds (NBUDs) are biomarkers of genotoxic events and chromosomal instability. These genome damage events can be measured simultaneously in the cytokinesis-block micronucleus cytome (CBMNcyt) assay. The molecular mechanisms leading to these events have been investigated over the past two decades using molecular probes and genetically engineered cells. In this brief review, we summarise the wealth of knowledge currently available that best explains the formation of these important nuclear anomalies that are commonly seen in cancer and are indicative of genome damage events that could increase the risk of developmental and degenerative diseases. MN can originate during anaphase from lagging acentric chromosome or chromatid fragments caused by misrepair of DNA breaks or unrepaired DNA breaks. Malsegregation of whole chromosomes at anaphase may also lead to MN formation as a result of hypomethylation of repeat sequences in centromeric and pericentromeric DNA, defects in kinetochore proteins or assembly, dysfunctional spindle and defective anaphase checkpoint genes. NPB originate from dicentric chromosomes, which may occur due to misrepair of DNA breaks, telomere end fusions, and could also be observed when defective separation of sister chromatids at anaphase occurs due to failure of decatenation. NBUD represent the process of elimination of amplified DNA, DNA repair complexes and possibly excess chromosomes from aneuploid cells.

Cytogenetics of premature ovarian failure: an investigation on 269 affected women

The importance of X chromosome in the aetiology of premature ovarian failure (POF) is well-known but in many cases POF still remains idiopathic. Chromosome aneuploidy increase is a physiological phenomenon related to aging, but the role of low-level sex chromosome mosaicism in ovarian function is still undiscovered. Standard cytogenetic analysis was carried out in a total of 269 patients affected by POF: 27 chromosomal abnormalities were identified, including X chromosome and autosomal structural and numerical abnormalities. In 47 patients with 46,XX karyotype we performed interphase FISH using X alpha-satellite probe in order to identify X chromosome mosaicism rate. Aneuploidy rate in the patient group was significantly higher than the general population group. These findings underline the importance of X chromosome in the aetiology of POF and highlight the potential role of low-level sex chromosome mosaicism in ovarian aging that may lead to a premature onset of menopause.

Micronucleus frequencies in lymphocytes and reticulocytes in a pesticide-exposed population in Portugal

A wide range of chemical products known to be acutely toxic is currently used in the agricultural sector, including numerous pesticides with different compositions. Nevertheless, the effects in human health as result of chronic exposure to low levels are not yet completely understood. The methodology for determination of micronuclei (MN) in lymphocytes (CBMN) is well established, and accumulating data demonstrated a correlation to enhanced risk of cancer development. However, analysis of MN in reticulocytes (MN-RET) in humans is a recent tool on human biomonitoring. The aim of this study was to examine the influence of pesticide exposure on MN-RET and CBMN frequencies. In total, 177 individuals were studied (93 controls and 84 exposed). All individuals included in the exposed group were exposed regularly to various chemicals. Both MN-RET and CBMN were significantly higher in the exposed subjects compared to controls. The CBMN frequencies were quantitatively higher in females than males, especially within the exposed group. Smoking habits exerted no marked influence on the frequency of the biomarkers studied. A significant and positive correlation was found between both indicators. Within the exposed group, data showed that there was a significant correlation between MN-RET and recent exposure (exposure in the previous 10 d) that is not found when considering CBMN. It is conceivable that due to the short life span of reticulocytes, MN-RET were found to be more reliable to characterize recent genetic damage as opposed to CBMN.

Normal and Cut-Off Values of the Cytokinesis-Block Micronucleus Assay on Peripheral Blood Lymphocytes in the Croatian General Population

Mikronukleus (MN) test na limfocitima periferne krvi jedna je od najvažnijih metoda koje se primjenjuju u citogenetičkom nadzoru. Osnovni preduvjet za primjenu nekog testa u svrhu nadzora profesionalno izloženih populacija jest poznavanje normalnih vrijednosti promatranoga biološkog pokazatelja (biomarkera) u kontrolnoj populaciji. Baze podataka na razini opće populacije moraju se redovito obnavljati novim podacima. Cilj ovog istraživanja bio je utvrditi normalne i granične vrijednosti MN-testa na limfocitima periferne krvi 200 zdravih ispitanika obaju spolova iz opće populacije Republike Hrvatske te ispitati koji čimbenici pridonose spontanom nastanku MN. Na razini istražene populacije utvrđeno je prosječno (6,90±3,32) MN (medijan 7 MN), dok je raspon pojedinačnih vrijednosti iznosio 0 do 18 MN u 1000 binuklearnih stanica. Gornja granična vrijednost dobivena izračunavanjem 95. percentila za cjelokupnu promatranu populaciju iznosi 12,5 MN na 1000 limfocita. Utvrđeno je da na spontani nastanak MN utječu spol, dob i navika pušenja. Žene u prosjeku imaju više vrijednosti svih parametara MN-testa od muškaraca, a u njih je bio i naglašeniji porast vrijednosti citogenetičkog nalaza zbog navike pušenja. Kako su literaturni podaci o utjecaju pušenja cigareta na nastanak MN kontradiktorni, planiran je nastavak istraživanja radi razjašnjavanja utjecaja dnevno utrošenog broja cigareta i ukupnog trajanja pušačkog staža na vrijednosti parametara MN-testa. Usporedba rezultata s literaturnim podacima potvrdila je da su dobivene vrijednosti u skladu s vrijednostima MN-testa zabilježenim na općoj populaciji u drugim svjetskim laboratorijima. Normalne i granične vrijednosti MN-testa utvrđene u ovome istraživanju poslužit će kao osnova za usporedbu i tumačenje nalaza MN-testa u ispitanika izloženih populacija te daljnju nadogradnju laboratorijske baze podataka.

Antineoplastic Drugs as a Potential Risk Factor in Occupational Settings: Mechanisms of Action at the Cell Level, Genotoxic Effects, and Their Detection Using Different Biomarkers

U članku je prikazana osnovna podjela antineoplastičnih lijekova prema mehanizmima djelovanja na razini stanice. Objašnjeni su mehanizmi genotoksičnosti najvažnijih vrsta lijekova koji se primjenjuju u okviru uobičajenih protokola za liječenje zloćudnih novotvorina. Navedena je važeća klasifikacija antineoplastika prema kancerogenom potencijalu, podaci o mutagenom potencijalu te je prikazana njihova podjela u skladu s anatomsko-terapijsko-kemijskim sustavom klasifikacije. Sustavno su prikazani najvažniji rezultati svjetskih i hrvatskih istraživanja na populacijama radnika izloženih antineoplasticima, provedenih u razdoblju 1980.-2009. s pomoću četiri najčešće primjenjivane metode: analize izmjena sestrinskih kromatida, analize kromosomskih aberacija, mikronukleus-testa i komet-testa. Objašnjena su osnovna načela navedenih metoda te raspravljene njihove prednosti i nedostaci. Biološki pokazatelji daju važne podatke o individualnoj osjetljivosti profesionalno izloženih ispitanika koji mogu poslužiti unaprjeđenju postojećih uvjeta rada i upravljanju rizicima pri izloženosti genotoksičnim agensima. Na osnovi prednosti i nedostataka citogenetičkih metoda zaključeno je da je mikronukleus-test, koji podjednako uspješno dokazuje klastogene i aneugene učinke, jedna od najboljih metoda dostupnih za otkrivanje štetnih djelovanja antineoplastičnih lijekova koji su u aktivnoj primjeni.

Assessment of the genotoxicity of atenolol in human peripheral blood lymphocytes: correlation between chromosomal fragility and content of micronuclei

The antihypertensive drug atenolol was found to induce chromosome loss, detected as micronuclei in the peripheral lymphocytes of treated patients. The fundamental question which chromosomes the micronuclei were derived from remains to be answered. Analysis of structural chromosomal aberrations (CAs) and expression of fragile sites (FS) were pursued in this study. They revealed a significantly higher incidence of chromosomal aberrations (chromatid and chromosome breaks) in patients compared with controls, where 10 FS emerged as specific. Also, the band 17q12-21, where known fragile sites have not been reported, was only expressed in atenolol-treated patients. Fluorescence in situ hybridization using chromosome-specific probes revealed the preferential involvement of chromosomes 7, 11, 17 and X in the micronuclei (MN) of patients. The results also suggest a correlation between chromosomal fragility and content of MN, and support the findings for a linkage between hypertension and a locus on chromosome 17.

Cytogenetic perspective of ageing and longevity in men and women

Analysis of relationships between the ageing cell phenotype and the age of cell donors is one of the ways towards understanding the link between cellular and organismal ageing. Cytogenetically, ageing is associated with a number of gross cellular changes, including altered size and morphology, genomic instability, and changes in expression and proliferation. Genomic instability can be easily assessed by analyzing the level of cytogenetic aberrations. In this review, we focus on the differences in the level and profile of cytogenetic aberrations observed in donors of different age and gender. Centenarians are a small fraction of the population at the extreme of human longevity. Their inclusion in such studies may shed light on one of the basic questions: whether genome stability is better maintained in successfully aged individuals compared to the rest of the population. At the same time, comparing the profile of age-related amount of chromosomal aberrations in men and women may help explaining the commonly observed gender differences in longevity.

Effects of age and gender on micronucleus and chromosome nondisjunction frequencies in centenarians and younger subjects

Studies have shown a significant increase in chromosome aneuploidy with age. The aim of this study was to elucidate whether the age-related changes in the level of hypoploidy correlate with the occurrence of micronuclei (MN) and chromosome nondisjunction (ND) in men and women. We analyzed cytokinesis-blocked (binucleated) lymphocytes treated with cytochalasin B, from 127 donors varying in gender and age including 53 centenarians. Fluorescent in situ hybridization with probes specific for several autosomes (1, 4, 6, 8, 20) and for the sex chromosomes was applied to analyze the chromosomal content of MN and to analyze the frequency of reciprocal loss and gain due to ND in binucleated interphase cells. The general level of MN in Giemsa-stained preparations was higher in women and in both genders increased with age until approximately 70 years and ranged, depending on age group, from 0.5 to 1.4% in men and from 0.9 to 1.8% in women. Gender-related differences were mostly observed in the younger age groups (< or =50 years), with an almost two-fold difference between men and women (P < 0.005). Frequencies of autosome-positive MN in both genders and of sex chromosome-positive MN in men were comparable and remained unchanged in older groups. The frequency of X-positive MN in women was higher than the average frequency of autosome-positive MN and continued to increase until the oldest age. The frequency of NDs involving the analyzed chromosomes was on average two-fold higher in women than in men. In both genders, the frequency of NDs increased with age and was, on average, an order of magnitude higher than that of cells with MN, consistent with the previous reports that the efficiency of elimination of micronucleated cells is higher than of the cells presenting chromosome ND.

The relationship between micronuclei in human lymphocytes and selected micronutrients in vegetarians and non-vegetarians

A vegetarian diet results in higher intake of vitamins and micronutrients, which - although providing antioxidant defence - may lead to deficiency in other micronutrients involved in DNA metabolism and stability (such as vitamins belonging to the B group). The principal difference among various vegetarian diets is the extent to which animal products are avoided. We have performed a pilot study to determine the relationship between the micronucleus frequency in lymphocytes and diet, and we compared the levels of Vitamins C and E, beta-carotene, B(12), folic acid, homocysteine and total antioxidant capacity in healthy vegetarians and non-vegetarians. The vegetarian group, consisting of 24 volunteers (13 women and 11 men), were matched for age and sex with 24 volunteers (12 women and 12 men) with a traditional dietary habit. Among the vegetarians were 13 lacto-ovo-vegetarians with average duration of vegetarian diet 10.8 years (ranging from 5 to 26 years) and 11 lacto-vegetarians with average duration of vegetarian diet 8.2 years (ranging from 3 to 15 years). Homocysteine, Vitamins C and E and beta-carotene levels in plasma were assayed by HPLC, and serum folate and Vitamin B(12) were determined with Elecsys Immunoassay tests. The total antioxidant capacity of plasma was estimated by measuring the ferric-reducing activity in a spectrophotometric assay. Micronuclei were measured in cytokinesis-blocked lymphocytes. Vegetarians had significantly higher levels of Vitamin C and beta-carotene (but not Vitamin E) in plasma compared with non-vegetarians (P<0.001). There were no significant differences in serum levels of folic acid and Vitamin B(12) between the monitored groups. Levels of folic acid in vegetarians correlated with length of vegetarianism (r=0.62, P=0.001, N=24). Vegetarians had elevated levels of homocysteine compared with non-vegetarians (P=0.007), as did vegetarian women compared with non-vegetarian women (P=0.031). We did not find any differences in total antioxidant capacity or in micronucleus frequency between the groups. Micronuclei correlated with age (r=0.62, P<0.001, N=48), women having higher frequencies than men. Multifactorial regression analysis showed significant effects of age, sex and total antioxidant capacity on micronucleus frequency (N=48, P<0.001).

Chromosomal changes: induction, detection methods and applicability in human biomonitoring

The objective of this state of the art paper is to review the mechanisms of induction, the fate, the methodology, the sensitivity/specificity and predictivity of two major cytogenetic endpoints applied for genotoxicity studies and biomonitoring purposes: chromosome aberrations and micronuclei. Chromosomal aberrations (CAs) are changes in normal chromosome structure or number that can occur spontaneously or as a result of chemical/radiation treatment. Structural CAs in peripheral blood lymphocytes (PBLs), as assessed by the chromosome aberration (CA) assay, have been used for over 30 years in occupational and environmental settings as a biomarker of early effects of genotoxic carcinogens. A high frequency of structural CAs in lymphocytes (reporter tissue) is predictive of increased cancer risk, irrespective of the cause of the initial CA increase. Micronuclei (MN) are small, extranuclear bodies that arise in dividing cells from acentric chromosome/chromatid fragments or whole chromosomes/chromatids that lag behind in anaphase and are not included in the daughter nuclei in telophase. The cytokinesis-block micronucleus (CBMN) assay is the most extensively used method for measuring MN in human lymphocytes, and can be considered as a "cytome" assay covering cell proliferation, cell death and chromosomal changes. The key advantages of the CBMN assay lie in its ability to detect both clastogenic and aneugenic events and to identify cells which divided once in culture. Evaluation of the mechanistic origin of individual MN by centromere and kinetochore identification contributes to the high sensitivity of the method. A number of findings support the hypothesis of a predictive association between the frequency of MN in cytokinesis-blocked lymphocytes and cancer development. Recent advances in fluorescence in situ hybridization (FISH) and microarray technologies are modifying the nature of cytogenetics, allowing chromosome and gene identification on metaphase as well as in interphase. Automated scoring by flow cytometry and/or image analysis will enhance their applicability.

Human chromosomes with shorter telomeres and large heterochromatin regions have a higher frequency of acquired somatic cell aneuploidy

Both telomere shortening and increases in aneuploidy frequencies have been associated with aging. To test if these chromosomal attributes are correlated, chromosome-specific telomere lengths and aneuploidy frequencies were estimated and compared. Aneuploidy frequencies were determined for 10 autosomes (1, 3, 5, 8, 9, 10, 13, 16, 17, 21) and the X chromosome in lymphocytes, and for chromosomes 17 and X in buccal mucosa cells. Overall, chromosomal loss was seen more often than gain in lymphocytes, with the highest loss rates being observed for chromosomes X (3.03%), 17 (2.00%), and the autosomes having large blocks of heterochromatin (1 [1.93%]; 16 [1.53%]; and 9 [1.05%]). The frequencies of loss were significantly lower in the buccal mucosa cells compared to lymphocytes for chromosomes 17 (P = 0.006) and X (P = 0.003). However, the chromosome 17 trisomy frequencies did not vary between tissues. Using a semi-quantitative FISH assay to estimate chromosome-specific telomere length, a significant negative correlation (r = -0.379; P = 0.007) was seen for chromosomal aneuploidy and telomere length, with chromosomes having higher loss rates being noted to have shorter telomeres. Collectively, these studies show that acquired, spontaneous chromosomal loss is associated with multiple factors including the amount of heterochromatin, the chromosome's telomere length, and tissue-specific factors.

Age-related changes in radiation-induced micronuclei among healthy adults

The aim of the present study was to establish the extent of in vitro radioresponse of lymphocytes among 62 healthy adults of both genders and to estimate the distribution of baseline micronuclei and radiosensitivity among individuals of the study population using the cytochalasin block micronucleus test. A younger study group consisted of 10 males (mean age, 22.4 years; range, 21-27) and 12 females (mean age, 24.8 years; range, 20-29), whereas an older study group consisted of 18 males (mean age, 35.1 years; range, 30-44) and 22 females (mean age, 38.5 years; range, 30-48). For evaluation of radiosensitivity blood samples were irradiated in vitro using 60Co gamma-ray source. The radiation dose employed was 2 Gy, the dose rate 0.45 Gy/min. The study revealed a significant gender effect on baseline micronuclei favoring females (Z = 3.25, P < 0.001), while yields of radiation-induced micronuclei did not differ significantly (Z = 0.56, P < 0.56) between genders. The distribution of baseline micronuclei among the individuals tested followed Poisson distribution in both study groups and in both genders, whereas the distribution of radiosensitivity among individuals of the older study group did not fulfill Poisson expectations (Kolmogorov-Smirnof test, P < 0.01). In contrast to a nonsignificant difference in radiosensitivity between males and females of the same age group (Z = 1.97, P < 0.56), a statistically significant difference in radiosensitivity between younger and older group for both genders was found (Z = 3.03, P < 0.03). Since the individuals tested were healthy, the observed variability in radiation response is considered to be an early effect of ageing.

Functional interaction between PARP-1 and PARP-2 in chromosome stability and embryonic development in mouse

The DNA damage-dependent poly(ADP-ribose) polymerases, PARP-1 and PARP-2, homo- and heterodimerize and are both involved in the base excision repair (BER) pathway. Here, we report that mice carrying a targeted disruption of the PARP-2 gene are sensitive to ionizing radiation. Following alkylating agent treatment, parp-2(-/-)-derived mouse embryonic fibroblasts exhibit increased post-replicative genomic instability, G(2)/M accumulation and chromosome mis-segregation accompanying kinetochore defects. Moreover, parp-1(-/-)parp-2(-/-) double mutant mice are not viable and die at the onset of gastrulation, demonstrating that the expression of both PARP-1 and PARP-2 and/or DNA-dependent poly(ADP-ribosyl) ation is essential during early embryogenesis. Interestingly, specific female embryonic lethality is observed in parp-1(+/-)parp-2(-/-) mutants at E9.5. Meta phase analyses of E8.5 embryonic fibroblasts highlight a specific instability of the X chromosome in those females, but not in males. Together, these results support the notion that PARP-1 and PARP-2 possess both overlapping and non-redundant functions in the maintenance of genomic stability.

Sex chromosome loss, micronuclei, sister chromatid exchange and aging: a study including 16 centenarians

In the present study we analysed the possible effect of age, sex and smoking on the mean values of micronucleus (MN) and sister chromatid exchange (SCE) frequencies on peripheral blood obtained from 38 subjects ranging in age from 16 to 63 years and 16 centenarians. The mean number of binucleated cells with micronuclei varied in function of age and sex (as demonstrated by the analysis of covariance (F=13.13; P<0.001), particularly evident was the increment observed in women with increasing age (interaction age/sex: F=5.53; P<0.05). Smoking habits had no effects on MN frequency (F=0.36; P>0.05). Sex (F=4.18; P<0.05) and smoking habits (F=14.64; P<0.001) influenced significantly SCE per cell frequencies, but age had no effects on them (F=2.45; P>0.05). The age-associated increase of sex chromosome loss was studied using fluorescence in situ hybridisation (FISH) on interphase nuclei. The loss of Y signals was observed in approximately 10% of interphase cells from the centenarians males, that is six times more often than in the younger control men (approximately 1.6%). The frequency of X signal loss (approximately 1.7%) in young women was similar to that observed in male controls of the same age but the incidence of the X chromosome aneuploidy in centenarian females was appreciably higher (approximately 22%) than that found for the Y chromosome in males. These results were correlated with the data on MN formation and a positive correlation between the percentage of aneuploid cells (FISH) and MN values was observed (r=0.50; P<0.05).

The application of spectral karyotyping (SKY) and fluorescent in situ hybridization (FISH) technology to determine the chromosomal content(s) of micronuclei

DNA loss by the process of micronucleation is associated with aging, cancer and environmental exposure. The primary aim of this study was to identify the chromosomal origin of the DNA excluded into micronuclei (MN). This was achieved using a novel application of SKY and FISH technologies. Cytochalasin B (Cyt B)-treated lymphocyte cultures from three females (aged 28, 42 and 72) were analyzed. SKY revealed that the majority of MN (89.8, 82.9, and 97.6% in the 28-, 42- and 72-year-old (y.o.), respectively) had a uniform, single color, suggesting that they were comprised of DNA from a single chromosome. Using a pancentromeric probe, most of the MN (82% in 28 y.o., 69% in 42 y.o. and 80% in 72 y.o.) had one centromere signal present. Overall, the confirmation studies (using FISH with chromosome-specific WCP) were in agreement with the SKY chromosomal assignments for 71.1% of the MN. Although the SKY analysis showed that all of the 23 chromosomes (22 autosomes and the X chromosome) could be present in the MN, overall, the X chromosome was seen most frequently. DNA from the X chromosome was seen in 50.6% of MN in the 42 y.o. individual, whereas in the 28 and 72 y.o. it was seen in 12.2 and 7.1% of MN, respectively. This difference (P<0.0001) in the frequencies of X chromosome exclusion into MN among individuals was independently confirmed using a single whole chromosome painting probe (WCP) for the X chromosome. SKY also showed variation in the frequency of autosomal exclusion into MN between chromosomes and between females. Collectively, this study supports the hypothesis that the majority of MN contain DNA from a single, monocentric chromosome. The use of SKY technology for the identification of the chromosomal content(s) of MN provides an opportunity for expansion of our knowledge of the chromosomal changes that accompany MN formation.

Selective elimination of acentric double minutes from cancer cells through the extrusion of micronuclei

Several lines of evidences from us or other authors had shown that tumor cells revert their phenotypes and differentiate by the elimination of oncogenes amplified on the acentric double minutes (DMs). The selective incorporation of DMs into the cytoplasmic micronuclei was thought to be involved in this elimination, however, the mechanism by which the content of micronuclei was eliminated from the cells remains to be discovered. In this report, we show the finding and the characterization of the extruded micronuclei in the culture fluid of human COLO 320DM tumor line, and suggest that the extrusion of micronuclei mediates the selective elimination of DMs. The extracellular micronuclei enriched with DMs had an apparently normal cytoplasmic membrane, decondensed chromatin and nuclear lamin protein, and their DNA did not suffer any extensive degradation. These characteristics were closely related to their cytoplasmic counterpart and clearly differentiated from the apoptotic bodies. We also developed a method for purifying the extracellular micronuclei. In this paper, the implications of the micronuclear extrusion are discussed.

The X chromosome frequently lags behind in female lymphocyte anaphase

Pancentromeric FISH and X-chromosome painting were used to characterize anaphase aberrations in 2,048 cultured lymphocytes from a healthy 62-year-old woman. Of 163 aberrant anaphases, 66.9% contained either chromosomes or their fragments that lagged behind. Characterization of 200 laggards showed that 49% were autosomes, 33. 5% were autosomal fragments, and 17.5% were X chromosomes. The X chromosome represented one-fourth of all lagging chromosomes and was involved much more often than would be expected by chance (1/23). Labeling of the late-replicating inactive X chromosome with 5-bromo-2'-deoxyuridine revealed that both X homologues contributed equally to the laggards. Among 200 micronuclei examined from interphase cells, the proportion of the X chromosome (31%) and autosomal fragments (50%) was higher than among anaphase laggards, whereas autosomes were involved less often (19%). These findings may reflect either selection or the fact that lagging autosomes, which were more proximal to the poles than were lagging X chromosomes, were more frequently included within the main nucleus. Our results suggest that the well-known high micronucleation and loss of the X chromosome in women's lymphocytes is the result of frequent distal lagging behind in anaphase and effective micronucleation of this chromosome. This lagging appears to affect the inactive and active X chromosomes equally.

The HUman MicroNucleus Project--An international collaborative study on the use of the micronucleus technique for measuring DNA damage in humans

The International Collaborative Project on Micronucleus Frequency in Human Populations (HUMN) was organized to collect data on micronucleus (MN) frequencies in different human populations and different cell types. The test procedures considered by this project are assays using human lymphocytes (cytokinesis-block method), exfoliated epithelial cells, and other cell types. Data (including descriptions of the populations monitored, detailed test protocols, and test results) are being obtained from a large number of laboratories throughout the world and are being entered into a unified database. The information will be used to: (1) determine the extent of variation of 'normal' values for different laboratories and the influence of other factors potentially affecting baseline MN frequency, e.g., age, gender and life-style; (2) provide information on the effect of experimental protocol variations on MN frequency measurements; (3) design and test optimal protocols for the different cell types; and (4) determine the extent to which MN frequency is a valid biomarker of ageing and risk for diseases such as cancer.

Age-dependent inclusion of sex chromosomes in lymphocyte micronuclei of man

Two-color centromeric FISH was used to study the inclusion of the X and Y chromosomes in micronuclei of cultured lymphocytes from 10 men representing two age groups (21-29 years and 51-55 years). In addition, pancentromeric FISH was separately performed to identify any human chromosomes in micronuclei. One hundred micronuclei per probe were examined from each donor. A higher mean frequency of Y-positive micronuclei was observed in the older men than in the younger men. In both age groups, the X chromosome was micronucleated clearly more often than expected by chance, and the Y chromosome was overrepresented in micronuclei among the older men but not among the younger men. In lymphocytes of four women, X-positive micronuclei were more frequent than they were in men, even after the fact that women have two X chromosomes was taken into account. Similar results were obtained in first-division lymphocytes identified by cytochalasin-B-induced cytokinesis block. In comparison with normal cells, these binucleate cells showed a higher frequency (per 1,000 nuclei) of X-positive micronuclei (in the older men) but a lower frequency of micronuclei harboring autosomes or acentric fragments. In conclusion, the results show that both the X chromosome and the Y chromosome are preferentially micronucleated in male lymphocytes, the Y chromosome only in older subjects. Although the X chromosome has a general tendency to be included in micronuclei, it is micronucleated much more often in women than in men, which is probably the main reason for the high micronucleus frequency in women that has been documented in many previous studies.

Influence of serum micronutrients on the incidence of kinetochore-positive or -negative micronuclei in human peripheral blood lymphocytes

The possible contribution of some selected serum micronutrients (beta-carotene, vitamins B12 and C, folic acid and alpha-tocopherol) to spontaneous chromosomal damage was investigated in human peripheral blood lymphocytes from 33 non-smoking healthy donors by the cytokinesis-block micronucleus assay. Labelling of micronuclei with antikinetochore serum was used to discriminate between kinetochore-positive and -negative micronuclei and thus between micronuclei which arise from whole chromosome loss and those which arise from chromosome breaks. Simple correlation analysis showed that age was significantly associated with the increased frequency of micronucleated cells, and this age-related increase in these cells was due to the increase in cells with both kinetochore-positive and -negative micronuclei. Serum micronutrient levels had no apparent significant effects on incidence of micronucleated cells except for the weak positive correlation between vitamin B12 levels and frequency of kinetochore-positive micronucleated cells. Multiple regression analysis with age and serum micronutrient levels as independent variables showed that (a) age was the most influential variable for the frequency of micronucleated cells, (b) the serum vitamin C level was associated with increased frequency of spontaneous micronucleated cells, and this increase was mainly due to the increase in cells with kinetochore-positive micronuclei, and (c) the serum folic acid level was significantly and negatively related to the frequencies of cells with both kinetochore-positive and -negative micronuclei. To avoid the predominant age-effect, we also performed separate multiple regression analysis with age-adjusted frequency of micronucleated cells as dependent variable. The results from this analysis again showed a significant and positive effect of serum vitamin C level on age-adjusted frequency of kinetochore-positive micronucleated cells, while marginal negative effect of folic acid on age-adjusted frequency of total micronucleated cells (P < 0.06) and kinetochore-positive micronucleated cells (P < 0.051) was detected. These results suggest that age and serum vitamin C are definitely variables for frequencies of spontaneous chromosome loss, and that serum folic acid is perhaps another important micronutrient which influence the frequency of spontaneous chromosomal damage.

Selective entrapment of extrachromosomally amplified DNA by nuclear budding and micronucleation during S phase

Acentric, autonomously replicating extrachromosomal structures called double-minute chromosomes (DMs) frequently mediate oncogene amplification in human tumors. We show that DMs can be removed from the nucleus by a novel micronucleation mechanism that is initiated by budding of the nuclear membrane during S phase. DMs containing c-myc oncogenes in a colon cancer cell line localized to and replicated at the nuclear periphery. Replication inhibitors increased micronucleation; cell synchronization and bromodeoxyuridine-pulse labeling demonstrated de novo formation of buds and micronuclei during S phase. The frequencies of S-phase nuclear budding and micronucleation were increased dramatically in normal human cells by inactivating p53, suggesting that an S-phase function of p53 minimizes the probability of producing the broken chromosome fragments that induce budding and micronucleation. These data have implications for understanding the behavior of acentric DNA in interphase nuclei and for developing chemotherapeutic strategies based on this new mechanism for DM elimination.