Role of deoxyuridine incorporation and DNA repair in the expression of human chromosomal fragile sites

Association between serum folate concentrations and all-cause mortality in U.S. adults: a cohort study based on National Health and Nutrition Examination Survey III

The association between serum folate and all-cause mortality in general population remains unclear. The objective of this study was to investigate the potential association between serum folate concentrations and all-cause mortality in a large, prospective, long-term U.S. cohort. Our study included adults from the National Health and Nutrition Examination Survey (NHANES) III, and mortality data was obtained by linking with the National Death Index (NDI) until December 31, 2019. Cox proportional hazard models were used to calculate hazard ratios (HR) and 95% confidence intervals (CI) to assess the association between serum folate concentrations and all-cause mortality. A total of 12,862 participants were included in this cohort study. After a median follow-up of 26.4 years [interquartile range (IQR), 15.4-28.7 years], a total of 5,299 deaths were recorded. The risk of death was lower by 12% per 1.0 g/L increase in log-transformed serum folate concentrations (HR, 0.88; 95% CI, 0.83-0.94). Compared with the lowest quartiles of serum folate level, the risk of death was lower in the second (HR, 0.84; 95% CI, 0.72-0.97), third (HR, 0.78; 95% CI, 0.68-0.91) and the highest quartiles (HR, 0.78; 95% CI, 0.69-0.88) in multivariable-adjusted model. In subgroup analyses, the inverse association between serum folate and all-cause mortality remained statistically significant for women, men and non-Hispanic White people. Higher serum folate levels were found to be significantly associated with reduced risk of all-cause mortality. However, further studies are needed to verify these findings and explore the underlying mechanism.

Perspectives on folate with special reference to epigenetics and neural tube defects

Folate is a micronutrient essential for DNA synthesis, cell division, fetal growth and development. Folate deficiency leads to genomic instability. Inadequate intake of folate during conception may lead to neural tube defects (NTDs) in the offspring. Folate influences the DNA methylation, histone methylation and homocysteine mediated gene methylation. DNA methylation influences the expression of microRNAs (miRNAs). Folate deficiency may be associated with miRNAs misregulation leading to NTDs. Mitochondrial epigenetics and folate metabolism has proved to be involved in embryogenesis and neural tube development. Folate related genetic variants also cause the occurrence of NTDs. Unmetabolized excessive folate may affect health adversely. Hence estimation of folate levels in the blood plays an important role in high-risk cases.

Folate intake and risk of colorectal cancer: a systematic review and up-to-date meta-analysis of prospective studies

PURPOSE

Colorectal cancer is one of the most commonly diagnosed and deadly cancers worldwide. Epidemiological studies on the relationship between folate intake and the risk of colorectal cancer have reported inconsistent findings since folate fortification in the USA. For this situation, we conducted a large number of data analyses to study the relationship between folate intake and colorectal cancer risk.

METHODS

PubMed and EMBASE databases were used to search the literature systematically. Eligible studies were reviewed and meta-analyzed to assess the relationship.

RESULTS

A total of 24 cohort studies involving 37 280 patients and 6 165 894 individuals were included. The results showed that high folate intake was associated with a reduced risk of colorectal cancer. The combined relative risk (RR) for the highest intake compared with the lowest was 0.88 [95% confidence interval (CI), 0.83-0.92, P  = 10 -4 ). Further studies indicated that the increase of folate intake may decrease the risk of colorectal cancer in people with medium or high alcohol consumption (RR = 0.97, 95% CI: 0.96-0.99, P  = 0.008; RR = 0.95, 95% CI: 0.92-0.98, P   =  0.003), but not in non-drinkers (RR = 1.00, 95% CI: 0.98-1.02, P  = 0.827). Next, high folate intake may decrease the risk of colon cancer (RR = 0.86, 95% CI: 0.81-0.92, P  = 10 -4 ) but not rectal cancer (RR = 0.92, 95% CI: 0.84-1.02, P  = 0.112). Additionally, the result that high folate intake may decrease the risk of colorectal cancer was observed in the USA and Europe but not in other regions.

CONCLUSION

High folate intake may be protective against colon cancer, particularly in people with middle or high alcohol consumption, but it still needs to be further confirmed.

Vitamins as Possible Cancer Biomarkers: Significance and Limitations

The Western-style diet, which is common in developed countries and spreading into developing countries, is unbalanced in many respects. For instance, micronutrients (vitamins A, B complex, C, D, E, and K plus iron, zinc, selenium, and iodine) are generally depleted in Western food (causing what is known as ‘hidden hunger’), whereas some others (such as phosphorus) are added beyond the daily allowance. This imbalance in micronutrients can induce cellular damage that can increase the risk of cancer. Interestingly, there is a large body of evidence suggesting a strong correlation between vitamin intake as well as vitamin blood concentrations with the occurrence of certain types of cancer. The direction of association between the concentration of a given vitamin and cancer risk is tumor specific. The present review summarized the literature regarding vitamins and cancer risk to assess whether these could be used as diagnostic or prognostic markers, thus confirming their potential as biomarkers. Despite many studies that highlight the importance of monitoring vitamin blood or tissue concentrations in cancer patients and demonstrate the link between vitamin intake and cancer risk, there is still an urgent need for more data to assess the effectiveness of vitamins as biomarkers in the context of cancer. Therefore, this review aims to provide a solid basis to support further studies on this promising topic.

Disruption of Cytosolic Folate Integrity Aggravates Resistance to Epidermal Growth Factor Receptor Tyrosine Kinase Inhibitors and Modulates Metastatic Properties in Non-Small-Cell Lung Cancer Cells

Patients with advanced-stage non-small-cell lung cancer (NSCLC) are susceptible to malnutrition and develop folate deficiency (FD). We previously found that folate deprivation induces drug resistance in hepatocellular carcinoma; here, we assessed whether disrupted cytoplasmic folate metabolism could mimic FD-induced metastasis and affect the sensitivity of NSCLC cells to epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs). We examined whether cytosolic folate metabolism in NSCLC cells was disrupted by FD or the folate metabolism blocker pemetrexed for 1–4 weeks. Our results revealed an increase in NF-κB overexpression–mediated epithelial-mesenchymal transition biomarkers: N-cadherin, vimentin, matrix metalloproteinases (MMPs), SOX9, and SLUG. This finding suggests that the disruption of folate metabolism can drastically enhance the metastatic properties of NSCLC cells. Cytosolic FD also affected EGFR-TKI cytotoxicity toward NSCLC cells. Because SLUG and N-cadherin are resistance effectors against gefitinib, the effects of SLUG knockdown in folate antagonist–treated CL1-0 cells were evaluated. SLUG knockdown prevented SLUG/NF-κB/SOX9-mediated invasiveness and erlotinib resistance acquisition and significantly reduced pemetrexed-induced gelatinase activity and MMP gene expression. To summarize, our data reveal two unprecedented adverse effects of folate metabolism disruption in NSCLC cells. Thus, the folic acid status of patients with NSCLC under treatment can considerably influence their prognosis.

Thermodynamic Stability of DNA Duplexes Comprising the Simplest T → dU Substitutions

Members of the uracil-DNA glycosylase (UDG) enzyme family recognize and bind uracil, sequestering it within the binding site pocket and catalyzing the cleavage of the base from the deoxyribose, leaving an abasic site. The recognition and binding are passive and rely on innate dynamic motions of DNA wherein base pairs undergo thermally induced breakage and conformational fluctuations. Once the uracil breaks from its base pair, it can be recognized and bound by the enzyme, which then alters its conformation for sequestration and catalysis. Our results suggest that the thymine to uracil substitution, which differs only by a single methyl group, causes a destabilization of the duplex thermodynamics, which would lead to an increase in the population of the extrahelical state and increase the probability of uracil being recognized and excised from DNA by UDG. This destabilization is dependent on the identity of the nearest-neighbor base-pair stacks; a G·C nearest neighbor leads to thermal and enthalpic destabilization that is weaker that that seen with two A·T neighbors. In addition, uracil substitution yields a nearest-neighbor increase in the counterion uptake of the duplexes but decreases the level of immobilization of structural water for all substituted duplexes regardless of the neighbor identity or number of substitutions.

Folate promotes S-adenosyl methionine reactions and the microbial methylation cycle and boosts ruminants production and reproduction

Folate has gained significant attention due to its vital role in biological methylation and epigenetic machinery. Folate, or vitamin (B₉), is only produced through a de novo mechanism by plants and micro-organisms in the rumen of mature animals. Although limited research has been conducted on folate in ruminants, it has been noted that ruminal synthesis could not maintain folate levels in high yielding dairy animals. Folate has an essential role in one-carbon metabolism and is a strong antiproliferative agent. Folate increases DNA stability, being crucial for DNA synthesis and repair, the methylation cycle, and preventing oxidation of DNA by free radicals. Folate is also critical for cell division, metabolism of proteins, synthesis of purine and pyrimidine, and increasing the de novo delivery of methyl groups and S-adenosylmethionine. However, in ruminants, metabolism of B₁₂ and B₉ vitamins are closely connected and utilization of folate by cells is significantly affected by B₁₂ vitamin concentration. Supplementation of folate through diet, particularly in early lactation, enhanced metabolic efficiency, lactational performance, and nutritional quality of milk. Impaired absorption, oxidative degradation, or deficient supply of folate in ruminants affects DNA stability, cell division, homocysteine remethylation to methionine, de novo synthesis of S-adenosylmethionine, and increases DNA hypomethylation, uracil misincorporation into DNA, chromosomal damage, abnormal cell growth, oxidative species, premature birth, low calf weight, placental tube defects, and decreases production and reproduction of ruminant animals. However, more studies are needed to overcome these problems and reduce enormous dietary supplement waste and impaired absorption of folate in ruminants. This review was aimed to highlight the vital role of folic acid in ruminants performance.

Folate Deficiency Facilitates Genomic Integration of Human Papillomavirus Type 16 DNA In Vivo in a Novel Mouse Model for Rapid Oncogenic Transformation of Human Keratinocytes

Background

Epidemiologic and in vitro studies suggest independent linkages between poor folate and/or vitamin B-12 nutrition, genomic human papillomavirus (HPV) type 16 viral integration, and cancer. However, there is no direct evidence in vivo to support the causative role of poor folate nutrition in HPV16 integration into the cellular genome.

Objective

We tested the hypothesis that folate deficiency enables the integration of HPV16 into the genome of HPV16-harboring keratinocytes, and could thereby influence earlier transformation of these cells to cancer in an animal model.

Methods

HPV16-harboring human keratinocytes [(HPV16)BC-1-Ep/SL] were differentiated into 3-dimensional HPV16-organotypic rafts under either folate-replete or folate-deficient conditions in vitro. These were then subcutaneously implanted in severely immunocompromised female Beige Nude XID (Hsd: NIHS-LystbgFoxn1nuBtkxid) mice (4-6 wk old, 16-18 g) fed either a folate-replete diet (1200 nmol folate/kg diet) or a progressively folate-deficient diet (600 or 400 nmol folate/kg diet) for 2 mo prior to raft-implantation surgery, and indefinitely thereafter. The tumors that subsequently developed were characterized for onset, pattern of growth, morphology, HPV16 oncogene expression, and HPV16-genomic integration.

Results

All HPV16-organotypic rafts developed in either folate-replete or physiologic low-folate media in vitro and subsequently implanted in folate-replete mice eventually transformed into aggressive malignancies within weeks. When compared to HPV16-high folate-organotypic raft-derived tumors from mice fed either a 1200 or 600 nmol folate/kg diet, those raft-derived cancers that developed in mice fed a 400 nmol folate/kg diet expressed significantly more HPV16 E6 (1.8-fold more) and E7 (2.8-fold more) oncogenic proteins (P = 0.001), and revealed significantly more HPV16-integration sites in genomic DNA (2-fold more), either directly into, or in the vicinity of, cellular genes (P < 0.05).

Conclusions

This unprecedented animal model for the consistent rapid transformation of differentiated (HPV16)BC-1-Ep/SL-derived organotypic raft-keratinocytes to cancer in Beige Nude XID mice confirms that dietary folate deficiency can profoundly influence and modulate events leading to HPV16-induced carcinogenesis, and facilitates genomic integration of HPV16 DNA in vivo.

Folic acid and diseases - supplement it or not?

INTRODUCTION

folic acid is a water soluble vitamin, which is synthetically-produced and found in fortified foods and supplements. Folate is found naturally in plants, such as the dark green leafy vegetables. Folate is not synthesized de novo by humans, therefore the daily requirements are met from the dietary intake of folic acid supplements or food rich in this vitamin. Folate deficiency could lead to numerous common health problems. Hyperhomocysteinemia and the possibility of malignancy developments are the long term consequences of this deficit albeit contradictory findings on these claims.

METHODS

the articles included in this review focused on recent updated evidence-based reports and meta-analyses on the associations of the serum folate/folic acid and the various diseases found globally.

RESULTS

the benefit of folic acid supplementation in the pre-conception period for the prevention of neural tube defects (NTDs) was well established and it was suggested that counseling sessions should be given to women with previous pregnancies affected by NTDs. However, supplementation of folic acid and its medicinal effects in the treatment of other diseases were contradictory and unclear.

CONCLUSION

more detailed investigations into the health benefits of folic acid are needed before it could be recommended for supplementation, treatment or prevention of some of the diseases discussed in this review.

Associations between intake of folate and related micronutrients with molecularly defined colorectal cancer risks in the Iowa Women's Health Study

Folate and related micronturients may affect colorectal cancer (CRC) risk, but the molecular mechanism(s) remain incompletely defined. We analyzed associations between dietary folate, vitamin B6, vitamin B12, and methionine with incident CRC, overall and by microsatellite instability (MSS/MSI-L or MSI-H), CpG island methylator phenotype (CIMP-negative or CIMP-positive), BRAF mutation (negative or positive), and KRAS mutation (negative or positive) status in the prospective, population-based Iowa Women's Health Study (IWHS; 55-69 years at baseline; n = 41,836). Intake estimates were obtained from baseline, self-reported food frequency questionnaires. Molecular marker data were obtained for 514 incident CRC cases. Folate intake was inversely associated with overall CRC risk in age-adjusted Cox regression models, whereas methionine intake was inversely associated with overall CRC risk in multivariable-adjusted models [relative risk (RR) = 0.81; 95% CI = 0.69-0.95; P trend = 0.001 and RR = 0.72; 95% CI = 0.54-0.96; P trend = 0.03 for highest vs. lowest quartiles, respectively]. None of the dietary exposures were associated with MSI, CIMP, BRAF, or KRAS defined CRC subtypes. These data provide minimal support for major effects from the examined micronutrients on overall or molecularly defined CRC risks in the IWHS cohort.

Folate (vitamin B9) and vitamin B12 and their function in the maintenance of nuclear and mitochondrial genome integrity

Folate plays a critical role in the prevention of uracil incorporation into DNA and hypomethylation of DNA. This activity is compromised when vitamin B12 concentration is low because methionine synthase activity is reduced, lowering the concentration of S-adenosyl methionine (SAM) which in turn may diminish DNA methylation and cause folate to become unavailable for the conversion of dUMP to dTMP. The most plausible explanation for the chromosome-breaking effect of low folate is excessive uracil misincorporation into DNA, a mutagenic lesion that leads to strand breaks in DNA during repair. Both in vitro and in vivo studies with human cells clearly show that folate deficiency causes expression of chromosomal fragile sites, chromosome breaks, excessive uracil in DNA, micronucleus formation, DNA hypomethylation and mitochondrial DNA deletions. In vivo studies show that folate and/or vitamin B12 deficiency and elevated plasma homocysteine (a metabolic indicator of folate deficiency) are significantly correlated with increased micronucleus formation and reduced telomere length respectively. In vitro experiments indicate that genomic instability in human cells is minimised when folic acid concentration in culture medium is greater than 100nmol/L. Intervention studies in humans show (a) that DNA hypomethylation, chromosome breaks, uracil incorporation and micronucleus formation are minimised when red cell folate concentration is greater than 700nmol/L and (b) micronucleus formation is minimised when plasma concentration of vitamin B12 is greater than 300pmol/L and plasma homocysteine is less than 7.5μmol/L. These concentrations are achievable at intake levels at or above current recommended dietary intakes of folate (i.e. >400μg/day) and vitamin B12 (i.e. >2μg/day) depending on an individual's capacity to absorb and metabolise these vitamins which may vary due to genetic and epigenetic differences.

Influence of physiologic folate deficiency on human papillomavirus type 16 (HPV16)-harboring human keratinocytes in vitro and in vivo

Although HPV16 transforms infected epithelial tissues to cancer in the presence of several co-factors, there is insufficient molecular evidence that poor nutrition has any such role. Because physiological folate deficiency led to the intracellular homocysteinylation of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and activated a nutrition-sensitive (homocysteine-responsive) posttranscriptional RNA operon that included interaction with HPV16 L2 mRNA, we investigated the functional consequences of folate deficiency on HPV16 in immortalized HPV16-harboring human (BC-1-Ep/SL) keratinocytes and HPV16-organotypic rafts. Although homocysteinylated hnRNP-E1 interacted with HPV16 L2 mRNA cis-element, it also specifically bound another HPV16 57-nucleotide poly(U)-rich cis-element in the early polyadenylation element (upstream of L2L1 genes) with greater affinity. Together, these interactions led to a profound reduction of both L1 and L2 mRNA and proteins without effects on HPV16 E6 and E7 in vitro, and in cultured keratinocyte monolayers and HPV16-low folate-organotypic rafts developed in physiological low folate medium. In addition, HPV16-low folate-organotypic rafts contained fewer HPV16 viral particles, a similar HPV16 DNA viral load, and a much greater extent of integration of HPV16 DNA into genomic DNA when compared with HPV16-high folate-organotypic rafts. Subcutaneous implantation of 18-day old HPV16-low folate-organotypic rafts into folate-replete immunodeficient mice transformed this benign keratinocyte-derived raft tissue into an aggressive HPV16-induced cancer within 12 weeks. Collectively, these studies establish a likely molecular linkage between poor folate nutrition and HPV16 and predict that nutritional folate and/or vitamin-B(12) deficiency, which are both common worldwide, will alter the natural history of HPV16 infections and also warrant serious consideration as reversible co-factors in oncogenic transformation of HPV16-infected tissues to cancer.

Folate and cancer: how DNA damage, repair and methylation impact on colon carcinogenesis

Inappropriate diet may contribute to one third of cancer deaths. Folates, a group of water-soluble B vitamins present in high concentrations in green, leafy vegetables, maintain DNA stability through their ability to donate one-carbon units for cellular metabolism. Folate deficiency has been implicated in the development of several cancers, including cancer of the colorectum, breast, ovary, pancreas, brain, lung and cervix. Generally, data from the majority of human studies suggest that people who habitually consume the highest level of folate, or with the highest blood folate concentrations, have a significantly reduced risk of developing colon polyps or cancer. However, an entirely protective role for folate against carcinogenesis has been questioned, and recent data indicate that an excessive intake of synthetic folic acid (from high-dose supplements or fortified foods) may increase human cancers by accelerating growth of precancerous lesions. Nonetheless, on balance, evidence from the majority of human studies indicates that dietary folate is genoprotective against colon cancer. Suboptimal folate status in humans is widespread. Folate maintains genomic stability by regulating DNA biosynthesis, repair and methylation. Folate deficiency induces and accelerates carcinogenesis by perturbing each of these processes. This review presents recent evidence describing how these mechanisms act, and interact, to modify colon cancer risk.

Use of a novel genetic mouse model to investigate the role of folate in colitis-associated colon cancer

Inflammatory bowel disease (IBD) patients are at high risk for developing folate deficiency and colon cancer. Since it is difficult to study the subtle global and gene-specific epigenetic mechanisms involved in folate-mediated tumor initiation and promotion, we have generated genetically modified mouse models by targeting the reduced folate carrier (RFC1) and folate-binding protein (Folbp1) genes. The transgenic mice were fed semi-purified diets for 8 weeks containing either normal (2 mg) or deficient (0.1 mg folate/kg diet) levels of folate. Compound heterozygous mice (Folbp1(+/-); RFC1(+/-)) fed an adequate folate diet exhibited a reduction in plasma folate concentrations compared to heterozygous (Folbp1(+/-)) and littermate wild-type mice (P<.05). In contrast, no differences were observed in colonic mucosa. Consumption of a low folate diet significantly reduced (three- to fourfold) plasma and tissue folate levels in all animal models, although plasma homocysteine levels were not altered. In order to elucidate the relationship between folate status and inflammation-associated colon cancer, animals were injected with azoxymethane followed by dextran sodium sulphate treatment in the drinking water. Mice were fed a normal folate diet and were terminated 5 weeks after carcinogen injection. The number of high multiplicity aberrant crypt foci per centimeter of colon was significantly elevated (P<.05) in compound Folbp1(+/-); RFC1(+/-) (3.5+/-0.4) mice as compared to Folbp1(+/-) (1.9+/-0.3) and wild-type control mice (1.1+/-0.1). These data demonstrate that the ablation of two receptor/carrier-mediated pathways for folate transport increases the risk for developing inflammation-associated colon cancer.

Patterns of chromosomal fragmentation due to uracil-DNA incorporation reveal a novel mechanism of replication-dependent double-stranded breaks

There is growing evidence that spontaneous chromosomal fragmentation, one of the main contributors to genetic instability, is intimately linked to DNA replication. In particular, we proposed before that uracil incorporation in DNA triggers chromosomal fragmentation due to replication fork collapse at uracil-excision intermediates. We tested predictions of this model at the chromosomal level in the dut mutants of Escherichia coli, by determining the relationship between DNA replication and patterns of fragmentation in defined chromosomal segments. Here we show that the uracil-DNA-triggered chromosomal fragmentation: (i) has a gradient that parallels the replication gradient, (ii) shows polarity within defined segments pointing towards replication origins and (iii) reorganizes to match induced replication gradients, confirming its dynamic pattern. Unexpectedly, these fragmentation patterns not only support the replication fork collapse model, but also reveal another mechanism of the replication-dependent chromosomal fragmentation triggered by uracil excision.

An assay for uracil in human DNA at baseline: effect of marginal vitamin B6 deficiency

Improvements are made to our gas-chromatography-mass-spectrometry-based assay for quantifying low levels of DNA-uracil. Folate deficiency leads to increased deoxyuridine monophosphate/thymidylate (dUMP/dTMP) ratios and uracil misincorporation into DNA, which may increase cancer risk. Vitamin B6 (B6) deficiency might also result in increased DNA-uracil because B6 is a cofactor for serine hydroxymethyltransferase, which catalyzes the methylation of tetrahydrofolate (THF) to methylene-THF, the folate form that is required to convert dUMP to dTMP. However, the low baseline levels of DNA-uracil in healthy human lymphocytes are difficult to measure accurately. This version of the assay (Uracil assay V3) has an approximately 10-fold increase in signal strength over the previous method and a 10-fold lower detection limit (0.2 pg uracil). Five micrograms of DNA, the amount in about 1 ml of human blood, is a suitable amount for this assay. Using this improved assay, DNA-uracil was measured in lymphocytes from 12 healthy smoking or nonsmoking young men and women who consumed a B6-restricted diet (0.7 mg B6/day, or approximately half the recommended dietary allowance) for 28 days. DNA-uracil concentration was not significantly related to B6 status or smoking. More severe and/or prolonged B6 deficiency may be necessary to detect significant changes in DNA-uracil in humans. The average concentration of DNA-uracil in these subjects was found to be approximately 3,000 uracils per diploid lymphocyte, which is comparable to steady state levels of one of the oxidative adducts of DNA, 8-oxoguanine.

Sensitivity of markers of DNA stability and DNA repair activity to folate supplementation in healthy volunteers

We have previously reported that supplementation with folic acid (1.2 mg day⁻¹ for 12 week) elicited a significant improvement in the folate status of 61 healthy volunteers. We have examined effects of this supplement on markers of genomic stability. Little is known about the effect of folate supplementation on DNA stability in a cohort, which is not folate deficient. Preintervention, there was a significant inverse association between uracil misincorporation in lymphocyte DNA and red cell folate (P<0.05). In contrast, there were no associations between folate status and DNA strand breakage, global DNA methylation or DNA base excision repair (measured as the capacity of the lymphocyte extract to repair 8-oxoGua ex vivo). Folate supplementation elicited a significant reduction in uracil misincorporation (P<0.05), while DNA strand breakage and global DNA methylation remained unchanged. Increasing folate status significantly decreased the base excision repair capacity in those volunteers with the lowest preintervention folate status (P<0.05). Uracil misincorporation was more sensitive to changes in folate status than other measures of DNA stability and therefore could be considered a specific and functional marker of folate status, which may also be relevant to cancer risk in healthy people.

Tribromomethane exposure and dietary folate deficiency in the formation of aberrant crypt foci in the colons of F344/N rats

Folate and folic acid are forms of the B vitamin that are involved in the synthesis, repair, and functioning of DNA and are required for the production and maintenance of cells. Low levels of folate have been associated with several forms of cancer, including colon cancer. Aberrant crypt foci (ACF), identified as putative precursor lesions in the development of colon cancer, have been induced by the drinking water disinfection by-product, tribromomethane (TBM). To investigate whether ACF induced by TBM could be promoted by a diet devoid of dietary folate, male F344/N rats were exposed to 500 mg/l of TBM in drinking water and fed either a normal or no folate diet (NFD) for 26 weeks. At the conclusion of the study, colons were excised and examined for ACF. Rats exposed to TBM and fed a NFD, evident by significantly reduced serum folate concentrations and elevated serum homocysteine levels, had significant increases of ACF when compared to rats exposed to TBM and fed a normal diet. This study highlights the important role that diet, especially folate intake, represents in protecting the colon against TBM-induced ACF.

Folate and its preventive potential in colorectal carcinogenesis

Based on a 15-year old hypothesis, it is believed that an adequate ingestion of folate vitamins decreases, whereas a nutritional depletion of folate increases the risk of colorectal cancer. The present article reviews the efforts to provide biochemical and epidemiological evidence for folate as a chemopreventive agent against colorectal carcinogenesis. BIOLOGICAL EVIDENCE: Tetrahydrofolates govern the intracellular one-carbon metabolism and account for proper DNA biosynthesis and macromolecular modification. Numerous experimental studies traced different molecular pathways and tried to link folate depletion with DNA instability and/or mutagenesis. However, none of the proposed underlying molecular mechanisms appear clearly defined. EPIDEMIOLOGICAL EVIDENCE: Numerous case-control and prospective studies have been conducted on folate and colorectal cancer, which all together miss a clinical bottom line. The recommendation of folate intake to prevent colorectal cancer is therefore not evidence-based.

Associations between Two Common Variants C677T and A1298C in the Methylenetetrahydrofolate Reductase Gene and Measures of Folate Metabolism and DNA Stability (Strand Breaks, Misincorporated Uracil, and DNA Methylation Status) in Human Lymphocytes In vivo

Objective: Homozygosity for variants of the methylenetetrahydrofolate reductase (MTHFR) gene is associated with decreased risk for colorectal cancer. We have investigated the relationships between two variants of the MTHFR gene (C677T and A1298C) and blood folate, homocysteine, and genomic stability (strand breakage, misincorporated uracil, and global cytosine methylation in lymphocytes) in a study of 199 subjects. Results: The frequencies of homozygosity for the C677T and A1298C variants of the MTHFR gene were 12.6% and 14.6%, respectively. Plasma homocysteine, folate, vitamin B12, 5-methyltetrahydrofolate, and RBC folate were determined in the C677T genotypes. Plasma folate was significantly lower (P &lt; 0.001) in the homozygous variants (6.7 ± 0.6 ng/mL) compared with wild-types (8.8 ± 0.4 ng/mL) and heterozygotes (9.1 ± 0.5 ng/mL). Homocysteine was significantly higher (P &lt; 0.05) in homozygous variants (13.2 ± 1.1 μmol/L) compared with homozygous subjects (10.9 ± 0.4 μmol/L). Homozygous variants had significantly lower (P &lt; 0.05) RBC folate (84.7 ± 6.3 ng/mL) compared with wild-types (112.2 ± 5.2 ng/mL) and heterozygous individuals (125.1 ± 6.6 ng/mL). No significant difference in RBC folate was observed between wild-types and heterozygotes. The A1298C variant did not influence plasma homocysteine, folate, 5-methyltetrahydrofolate, vitamin B12, or RBC folate. Lymphocyte DNA stability biomarkers (strand breaks, misincorporated uracil, and global DNA methylation) were similar for all MTHFR C677T or A1298C variants. Conclusion: Data from this study do not support the hypothesis that polymorphisms in the MTHFR gene increase DNA stability by sequestering 5,10-methylenetetrahydrofolate for thymidine synthesis and reducing uracil misincorporation into DNA.

Uracil in DNA, determined by an improved assay, is increased when deoxynucleosides are added to folate-deficient cultured human lymphocytes

Folate deficiency leads to increased dUMP/dTMP ratios and uracil misincorporation into DNA, which may increase cancer risk. We improved a previously described gas chromatography-mass spectrometry (GC-MS) assay for uracil in DNA and validated the assay by analyzing the DNA-uracil content of normal, primary human lymphocytes that were cultured in 0-3000 nM folic acid. In addition, the effects of nucleoside mixtures T or TdCA (T, thymidine; A, adenosine; dC, deoxycytidine) were investigated. Over 4 consecutive days, the inter- and intraassay coefficients of variation (CVs) were 2.3-3.9 and 0.6-2.2%. Mean recovery was 99.4%. Oligonucleotides containing 100 pg of uracil yielded a mean uracil measurement of 110.1 pg (CV=2.7%). Cells grown in different concentrations of folate showed a bimodal response, with maximum DNA-uracil at 12 nM, and minima at 0 and 3000 nM folate. Extremely folate-deficient cells may incorporate less uracil because DNA synthesis is reduced. A wide response to folate deficiency was seen in cells from different donors, suggesting that genetic background plays a critical role in individual susceptibility to DNA damage and cancer risk. Unexpectedly, TdCA supplementation caused increased DNA-uracil (vs 3000 nM folate for 10 days, P > 0.05), probably due to the conversion of deoxycytidine to deoxyuridine by cytidine deaminase, leading to elevated dUMP/dTMP ratios. This improved uracil assay could serve as a useful tool in the study of the mechanism of uracil misincorporation into DNA. The assay requires 3 microg of DNA per folate-deficient sample, but more may be required for baseline DNA-uracil detection in healthy humans.

On the nature of visible chromosomal gaps and breaks

From the earliest days of chromosomal aberration studies, the distinction, nature and origin of light-microscope observed "gaps" and "breaks" have been topics for debate and controversy. In this paper we survey, briefly, the various ideas that have appeared in the very extensive literature, and attempt to evaluate them in the light of our current understanding of chromosome structure and aberration formation. Attention is drawn to the problems of interpretation caused by G2/S cell imprecision.

Expression of aphidicolin-induced fragile sites and their relationship between genetic susceptibility in breast cancer, ovarian cancer, and non-small-cell lung cancer patients

Fragile sites are nonrandomly located gaps and/or breaks and their expres-sion can be induced by specific culture conditions. There are many reports in the literature that indicate that these sites can act as factors that predispose to specific chromosome aberrations and other complex rearrangement in the chromosome and their association with cancers. In the present study, the expression of the fragile sites induced by aphidicolin was evaluated on prometaphase chromosomes from peripheral blood lymphocytes of 55 patients with breast cancer patients belonging to different stages of the cancer, 25 patients with epithelial ovarian cancer, and 13 with non-small-cell lung cancer, 100 of their first-degree clinically healthy female relatives, and 100 normal age-matched healthy persons without a familial history of cancer. The frequency of expression of the fragile sites in cancer patients and their first-degree relatives was found to be statistically significant (P<0.05) than those of the controls. In different stages of breast cancer patients, 6q26 is the best-defined fragile site whereas 13q13 is confined to stage II and stage III patients only. The chromosomal aberration rate/cell in breast cancer patients was found to be 0.29+/-0.13, in epithelial ovarian cancer patients 0.38+/-0.14, and in non-small-cell lung cancer 0.29+/-0.11 as compared to 0.07+/-0.03 in controls, and was found to be statistically significant. Therefore, our results indicate that these fragile sites may be the unstable sites in the genome and, hence, can be used as suitable and reliable markers for genetic predisposition to breast cancer, epithelial ovarian cancer, and in non-small-cell lung cancer.

Biochemical and molecular aberrations in the rat colon due to folate depletion are age-specific

Elder adulthood and diminished folate status are each associated with an enhanced risk of colorectal carcinogenesis. We therefore examined whether these two factors are mechanistically related. Weanling male Sprague-Dawley rats (n = 44) and 1-y-old rats (n = 44) were each divided into three groups and fed diets containing 0, 4.5 or 18 micro mol folic acid/kg (deplete, replete and supplemented groups, respectively). Rats were killed at 0, 8 and 20 wk. The folate concentrations, the distribution of the different coenzymatic forms of folate, uracil incorporation into DNA and genomic DNA methylation were measured in the colonic mucosa. Folate-deplete and folate-replete elder rats had 30-45% lower colonic folate concentrations than young rats. Furthermore, 5-methyltetrahydrofolate was uniformly depleted in colons of the elder, folate-deplete rats, whereas this depletion occurred in only a minority of the younger rats. By the end of the experiment, the folate-deplete and folate-replete elder rats had approximately 50% more uracil incorporated into their colonic DNA than the corresponding young groups (P < 0.05). In elder rats, this uracil misincorporation was incremental across the three diet groups (P-test for trend < 0.05), whereas no excess uracil incorporation was observed in young rats. Neither age nor dietary folate affected genomic DNA methylation in the colon. In conclusion, the colon of elder rats is more susceptible to biochemical and molecular consequences of folate depletion than that of young rats. However, folate supplementation is as effective at sustaining adequate colonic folate status in elder rats as it is in the young.

Impact of folate deficiency on DNA stability

Convincing evidence links folate deficiency with colorectal cancer incidence. Currently, it is believed that folate deficiency affects DNA stability principally through two potential pathways. 5,10-Methylenetetrahydrofolate donates a methyl group to uracil, converting it to thymine, which is used for DNA synthesis and repair. If folate is limited, imbalances in the DNA precursor pool occur, and uracil may be misincorporated into DNA. Subsequent misincorporation and repair may lead to double strand breaks, chromosomal damage and cancer. Moreover, folate affects gene expression by regulating cellular S-adenosylmethionine (SAM) levels. 5-Methyltetrahydrofolate serves as methyl donor in the remethylation of homocysteine to methionine, which in turn is converted to SAM. SAM methylates specific cytosines in DNA, and this regulates gene transcription. As a consequence of folate deficiency, cellular SAM is depleted, which in turn induces DNA hypomethylation and potentially induces proto-oncogene expression leading to cancer. Data from several model systems supporting these mechanisms are reviewed here. There is convincing evidence that folate modulates both DNA synthesis and repair and DNA hypomethylation with altered gene expression in vitro. The data from in vivo experiments in rodents is more difficult to interpret because of variations in the animal and experimental systems used and the influence of tissue specificity and folate metabolism. Most importantly, the confounding effects of nutrient-gene interactions, together with the identification of polymorphisms in key enzyme systems and the influence that these have on folate metabolism and DNA stability, must be considered when interpreting evidence from human studies.

The role of folic acid and Vitamin B12 in genomic stability of human cells

Folic acid plays a critical role in the prevention of chromosome breakage and hypomethylation of DNA. This activity is compromised when Vitamin B12 (B12) concentration is low because methionine synthase activity is reduced, lowering the concentration of S-adenosyl methionine (SAM) which in turn may diminish DNA methylation and cause folate to become unavailable for the conversion of dUMP to dTMP. The most plausible explanation for the chromosome-breaking effect of low folate is excessive uracil misincorporation into DNA, a mutagenic lesion that leads to strand breaks in DNA during repair. Both in vitro and in vivo studies with human cells clearly show that folate deficiency causes expression of chromosomal fragile sites, chromosome breaks, excessive uracil in DNA, micronucleus formation and DNA hypomethylation. In vivo studies show that Vitamin B12 deficiency and elevated plasma homocysteine are significantly correlated with increased micronucleus formation. In vitro experiments indicate that genomic instability in human cells is minimised when folic acid concentration in culture medium is >227nmol/l. Intervention studies in humans show: (a) that DNA hypomethylation, chromosome breaks, uracil misincorporation and micronucleus formation are minimised when red cell folate concentration is >700nmol/l folate; and (b) micronucleus formation is minimised when plasma concentration of Vitamin B12 is >300pmol/l and plasma homocysteine is <7.5micromol/l. These concentrations are achievable at intake levels in excess of current RDIs i.e. more than 200-400microgram folic acid per day and more than 2microgram Vitamin B12 per day. A placebo-controlled study with a dose-response suggests that based on the micronucleus index in lymphocytes, an RDI level of 700microgram/day for folic acid and 7microgram/day for Vitamin B12 would be appropriate for genomic stability in young adults. Dietary intakes above the current RDI may be particularly important in those with extreme defects in the absorption and metabolism of these Vitamins, for which ageing is a contributing factor.

Increased uracil misincorporation in lymphocytes from folate-deficient rats

The development of certain human cancers has been linked with inadequate intake of folates. The effects of folate deficiency in vivo on DNA stability (strand breakage, misincorporated uracil and oxidative base damage) in lymphocytes isolated from rats fed a diet deficient in folic acid was determined. Because the metabolic pathways of folate and other methyl donors are closely coupled, the effects of methionine and choline deficiency alone or in combination with folate deficiency were determined. Feeding male Hooded Lister rats a folate-free diet for 10 weeks created a moderate folate deficiency (25-50% (approx.) decrease in plasma, red blood cell and hepatic folate concentrations (P < 0.05) and a 20% rise in plasma homocysteine (P < 0.05)). Lymphocyte DNA strand breakage was increased successively in all groups after 4 weeks and 8 weeks on the diet (50-100% (approx.) after 8 weeks). Only low folate specifically and progressively induced uracil misincorporation throughout the study (100% (approx.) after 8 weeks). Neither folate deficiency nor choline/methionine deficiency altered oxidative DNA base damage. In summary, moderate folate deficiency in vivo is associated with a decrease in DNA stability, measured as increased DNA strand breakage and misincorporated uracil.

Folate deficiency causes uracil misincorporation into human DNA and chromosome breakage: implications for cancer and neuronal damage

Folate deficiency causes massive incorporation of uracil into human DNA (4 million per cell) and chromosome breaks. The likely mechanism is the deficient methylation of dUMP to dTMP and subsequent incorporation of uracil into DNA by DNA polymerase. During repair of uracil in DNA, transient nicks are formed; two opposing nicks could lead to chromosome breaks. Both high DNA uracil levels and elevated micronucleus frequency (a measure of chromosome breaks) are reversed by folate administration. A significant proportion of the U.S. population has low folate levels, in the range associated with elevated uracil misincorporation and chromosome breaks. Such breaks could contribute to the increased risk of cancer and cognitive defects associated with folate deficiency in humans.

Inhibitors of thymine nucleotide biosynthesis: antimetabolites that provoke genetic change via primary non-DNA targets

Folate antagonists and direct-acting inhibitors of thymidylate synthase are potent genotoxic antimetabolites. These agents induce genetic change not by attacking DNA, but by interfering with the control of DNA precursor metabolism. This review surveys the genetic effects attributable to selected representatives of this class of antimetabolites.

Type I bipolar disorder associated with a fragile site on chromosome 1

The objective of this paper is to study the association between chromosomal fragile sites and type I bipolar disorder. This case-control study compares bipolar patients with normal controls. Ten cases of type I bipolar disorder diagnosed according to DSM-III-R criteria and the Composite International Diagnostic Interview (CIDI) were selected from the Escola Paulista affective disorders outpatient clinic and 10 healthy controls (CIDI negative for psychiatric diagnoses) matched for sex and age were drawn from the otorhinolaryngologic outpatient clinic of the same hospital. The cytogenetic analysis was carried out with blood lymphocytes, which were cultured in a folic acid-free medium. A total of 100 mitoses per subject were blindly analyzed to the psychiatric diagnostic assignment, and fragile sites were identified according to a minimum expected frequency of events per band in conformity with a Poisson distribution. A higher frequency of chromosomal lesions for cases than controls was found for the following bands: 1q32, 5q31, and 11q23, the 1q32 being considered a fragile site. Although no evident neuropsychiatric etiological component has been mapped to the 1q32 region so far, this finding may lead to further investigation of a possible linkage between genetic markers of this region and bipolar disorder.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Deoxyribonucleoside triphosphate levels: a critical factor in the maintenance of genetic stability

DNA precursor pool imbalances can elicit a variety of genetic effects and modulate the genotoxicity of certain DNA-damaging agents. These and other observations indicate that the control of DNA precursor concentrations is essential for the maintenance of genetic stability, and suggest that factors which offset this control may contribute to environmental mutagenesis and carcinogenesis. In this article, we review the biochemical and genetic mechanisms responsible for regulating the production and relative amounts of intracellular DNA precursors, describe the many outcomes of perturbations in DNA precursor levels, and discuss implications of such imbalances for sensitivity to DNA-damaging agents, population monitoring, and human diseases.

Uridine enhances expression of the fragile X chromosome in human lymphocytes

We have studied the effect of uridine on the expression of fragile X (fra[X]) in lymphocyte cultures established in the folate and thymidine deficient medium TC199. The results indicate that uridine enhances the expression of fra(X) and gives a higher mitotic rate. The excess of uridine during DNA synthesis might further promote the previously suggested cycle of misincorporation and removal of deoxyuridine monophosphate when the pool of deoxythymidine triphosphate is continuously depleted.