Nutritional folate deficiency augments the in vivo mutagenic and lymphocytotoxic activities of alkylating agents

The Timing and Duration of Folate Restriction Differentially Impacts Colon Carcinogenesis

Diet plays a crucial role in the development of colorectal cancer (CRC). Of particular importance, folate, present in foods and supplements, is a crucial modulator of CRC risk. The role of folate, and, specifically, the synthetic variant, folic acid, in the primary prevention of CRC has not been fully elucidated. Animal studies varied considerably in the timing, duration, and supplementation of folates, leading to equivocal results. Our work attempts to isolate these variables to ascertain the role of folic acid in CRC initiation, as we previously demonstrated that folate restriction conferred protection against CRC initiation in a β-pol haploinsufficient mouse model. Here we demonstrated that prior adaptation to folate restriction altered the response to carcinogen exposure in wild-type C57BL/6 mice. Mice adapted to folate restriction for 8 weeks were protected from CRC initiation compared to mice placed on folate restriction for 1 week, irrespective of antibiotic supplementation. Through analyses of mTOR signaling, DNA methyltransferase, and DNA repair, we have identified factors that may play a critical role in the differential responses to folate restriction. Furthermore, the timing and duration of folate restriction altered these pathways differently in the absence of carcinogenic insult. These results represent novel findings, as we were able to show that, in the same model and under controlled conditions, folate restriction produced contrasting results depending on the timing and duration of the intervention.

Genotoxicity and cytotoxicity of chromium, copper, manganese and lead, and their mixture in WIL2-NS human B lymphoblastoid cells is enhanced by folate depletion

Heavy metal exposure or dietary deficiency is associated with increased genetic damage, cancer and age-related diseases. Folate (vitamin B9) required for DNA repair and synthesis may increase cellular susceptibility to metal induced genotoxicity. This study investigated the interactive effects of folic acid deficiency and sufficiency on genome instability and cytotoxicity induced by chromium (VI), copper (II), manganese (II), lead (IV), and their mixture (CCMP) in WIL2-NS human B lymphoblastoid cells. WIL2-NS cells were cultured in folic acid deficient (20 nM) and replete (2000 nM) RPMI 1640 medium treated with different concentrations (0.00-1000 μM) of the metals and CCMP for 48 h. Chromosomal damage and cytotoxicity were measured using the Cytokinesis-block Micronucleus Cytome assay. CCMP, Cr, Pb, Cu and Mn induced concentration dependent, increases in cells with chromosome damage (micronuclei, nucleoplasmic bridges, nuclear buds) and necrotic cells and decreased nuclear division index. The metals exhibited different cytotoxic and genotoxic potentials (CCMP>Cr>Pb>Cu>Mn) in both folate deficient and sufficient cells, with the cytogenotoxic effects being greater in folate deficient cells. Significant interaction between the metals and folic acid suggests that folic acid deficiency exacerbated cell proliferation inhibition and genome instability induced by metals. Folate deficiency, increasing metal concentration, and their interactions explained 3-11%, 74-92% and 4-12% of the variance of DNA damage biomarkers. In conclusion, exposure to the tested metals (0.01-1000 μM) increased chromosomal DNA damage in WIL2-NS cells and this was exacerbated by folate deficiency.

Folate and colorectal cancer in rodents: a model of DNA repair deficiency

Fortification of grains has resulted in a positive public health outcome vis-a-vis reduced incidence of neural tube defects. Whether folate has a correspondingly beneficial effect on other disease outcomes is less clear. A role for dietary folate in the prevention of colorectal cancer has been established through epidemiological data. Experimental data aiming to further elucidate this relationship has been somewhat equivocal. Studies report that folate depletion increases DNA damage, mutagenesis, and chromosomal instability, all suggesting inhibited DNA repair. While these data connecting folate depletion and inhibition of DNA repair are convincing, we also present data demonstrating that genetic inhibition of DNA repair is protective in the development of preneoplastic colon lesions, both when folate is depleted and when it is not. The purpose of this paper is to (1) give an overview of the data demonstrating a DNA repair defect in response to folate depletion, and (2) critically compare and contrast the experimental designs utilized in folate/colorectal cancer research and the corresponding impact on tissue folate status and critical colorectal cancer endpoints. Our analysis suggests that there is still an important need for a comprehensive evaluation of the impact of differential dietary prescriptions on blood and tissue folate status.

DNA damage and neurotoxicity of chronic alcohol abuse

Chronic alcohol abuse results in a variety of pathological effects including damage to the brain. The causes of alcohol-induced brain pathology are presently unclear. Several mechanisms of pathogenicity of chronic alcoholism have been proposed, including accumulation of DNA damage in the absence of repair, resulting in genomic instability and death of neurons. Genomic instability is a unified genetic mechanism leading to a variety of neurodegenerative disorders. Ethanol also likely interacts with various metabolic pathways, including one-carbon metabolism (OCM). OCM is critical for the synthesis of DNA precursors, essential for DNA repair, and as a methyl donor for various methylation events, including DNA methylation. Both DNA repair and DNA methylation are critical for maintaining genomic stability. In this review, we outline the role of DNA damage and DNA repair dysfunction in chronic alcohol-induced neurodegeneration.

Folate deficiency provides protection against colon carcinogenesis in DNA polymerase beta haploinsufficient mice

Aging and DNA polymerase beta deficiency (beta-pol(+/-)) interact to accelerate the development of malignant lymphomas and adenocarcinoma and increase tumor bearing load in mice. Folate deficiency (FD) has been shown to induce DNA damage repaired via the base excision repair (BER) pathway. We anticipated that FD and BER deficiency would interact to accelerate aberrant crypt foci (ACF) formation and tumor development in beta-pol haploinsufficient animals. FD resulted in a significant increase in ACF formation in wild type (WT) animals exposed to 1,2-dimethylhydrazine, a known colon and liver carcinogen; however, FD reduced development of ACF in beta-pol haploinsufficient mice. Prolonged feeding of the FD diet resulted in advanced ACF formation and liver tumors in wild type mice. However, FD attenuated onset and progression of ACF and prevented liver tumorigenesis in beta-pol haploinsufficient mice, i.e. FD provided protection against tumorigenesis in a BER-deficient environment in all tissues where 1,2-dimethylhydrazine exerts its damage. Here we show a distinct down-regulation in DNA repair pathways, e.g. BER, nucleotide excision repair, and mismatch repair, and decline in cell proliferation, as well as an up-regulation in poly(ADP-ribose) polymerase, proapoptotic genes, and apoptosis in colons of FD beta-pol haploinsufficient mice.

Influence of DNA damage and repair upon the risk of treatment related leukemia

Therapy-related myelodysplasia and acute myeloid leukemia (t-MDS/AML) are malignancies occurring after exposure to chemotherapy and/or radiotherapy. Several studies have addressed cumulative dose, dose intensity and exposure to specific agents of preceding cytotoxic therapy in relation to the risk of developing such leukemia. Since only a small percentage of patients exposed to cytotoxic therapy develop t-MDS/AML, it has been suggested that some genetic predisposition may be involved, specifically associated to polymorphisms in certain genes involved in chemotherapy/radiotherapy response - fundamentally genes intervening in drug detoxification and DNA synthesis and repair. A review is made of the genetic studies related to t-MDS/AML predisposition, focusing on the mechanistic findings of how specific chemotherapeutic drug exposure produces DNA damage and induces the chromosomal abnormalities characteristic of t-MDS/AML, the molecular pathways involved in repairing such drug induced damage, and the way in which they influence t-MDS/AML genesis. Specific issues are (a) the interaction of topoisomerase II inhibitors, alkylators and antimetabolite drugs with DNA repair mechanisms and their impact on t-MDS/AML leukemogenicity and (b) the influence of DNA polymorphisms in genes involved in DNA repair, drug metabolization and nucleotide synthesis, paying special attention to the relevance of folate metabolism. Finally, we discuss some aspects relating to study design that are most suitable for characterizing associations between drug exposure and genotypes related to t-MDS/AML risk - stressing the importance of the inclusion of chemotherapy-exposed control groups.

The effect of dietary folic acid deficiency on the cytotoxic and mutagenic responses to methyl methanesulfonate in wild-type and in 3-methyladenine DNA glycosylase-deficient Aag null mice

Folic acid deficiency (FA-) augments DNA damage caused by alkylating agents. The role of DNA repair in modulating this damage was investigated in mice. Weanling wild-type or 3-methyladenine glycosylase (Aag) null mice were maintained on a FA- diet or the same diet supplemented with folic acid (FA+) for 4 weeks. They were then treated with methyl methanesulfonate (MMS), 100mg/kg i.p. Six weeks later, spleen cells were collected for assays of non-selected and 6-thioguanine (TG) selected cloning efficiency to measure the mutant frequency at the Hprt locus. In wild-type mice, there was no significant effect of either MMS treatment or folate dietary content on splenocyte non-selected cloning efficiency. In contrast, non-selected cloning efficiency was significantly higher in MMS-treated Aag null mice than in saline treated controls (diet-gene interaction variable, p=0.04). The non-selected cloning efficiency was significantly higher in the FA+ diet than in the FA- diet group after MMS treatment of Aag null mice. Mutant frequency after MMS treatment was significantly higher in FA- wild-type and Aag null mice and in FA+ Aag null mice, but not in FA+ wild-type mice. For the Aag null mice, mutant frequency was higher in the FA+ mice than in the FA- mice after either saline or MMS treatment. These studies indicate that in wild-type mice treated with MMS, dietary folate content (FA+ or FA-) had no effect on cytotoxicity, but FA- diet increased DNA mutation frequency compared to FA+ diet. In Aag null mice, FA- diet increased the cytotoxic effects of alkylating agents but decreased the risk of DNA mutation.

Effects of folate deficiency on gene expression in the apoptosis and cancer pathways in colon cancer cells

Folate is a B vitamin, deficiency of which appears to increase the risk of developing several malignancies including colorectal cancer. In contrast to the cancer-promoting effect of folate deficiency in normal tissues, several lines of evidence indicate that folate depletion suppresses the progression of existing neoplasms and enhance the sensitivity of cancer cells to chemotherapy. Folate mediates the transfer of one-carbon necessary for the de novo biosynthesis of purines and thymidylate, and hence is an essential factor for DNA synthesis and repair, and the maintenance of DNA integrity and stability. Folate deficiency induces DNA strand breaks, increases uracil misincorporation into DNA, impairs DNA repair and appears to induce apoptosis. Although the effects of folate depletion on DNA integrity and apoptosis and on subsequent cancer development, progression and treatment in colonic epithelial cells have been well characterized, it is largely unknown at present how folate depletion modulates specific upstream genes in apoptosis and cancer pathways that regulate these processes. We therefore investigated the effects of folate depletion on expression of genes involved in apoptosis and cancer pathways in four human colon adenocarcinoma cell lines in an in vitro model of folate deficiency. Apoptosis and cancer pathway-specific mini-microarray were used to screen for differentially expressed genes in response to folate deficiency, and the expression of seven most notably and consistently affected genes was confirmed by real time RT-PCR. Our data suggest that folate deficiency affects the expression of key genes that are related to cell cycle control, DNA repair, apoptosis and angiogenesis in a cell-specific manner. Cell-specificity in gene expression changes in response to folate deficiency is likely due to significant differences in molecular and phenotypic characteristics, growth rates and intracellular folate concentrations among the four cell lines.

Evaluation of cytogenetic and DNA damage induced by the antibacterial drug, trimethoprim

Trimethoprim, a widely used antimicrobial drug was tested for its effect on the level of nuclear DNA damage in cultured peripheral blood lymphocytes in terms of chromosome and DNA alterations. The extent of cytogenetic damage, expressed as chromosome breakage and chromosome loss, was evaluated employing the cytokinesis block micronucleus method (CBMN) in cultured peripheral blood lymphocytes coupled with fluorescence in situ hybridization (FISH) using a digoxigenin-labelled alphoid DNA probe specific for the centromere of all human chromosomes. The DNA breakage level was evaluated by the Comet assay. Cultures were set up by using blood samples from two healthy donors. A range of concentrations of the test agent (from 1 to 100 microg/ml) was used for the micronuclei (MN) frequency and cytogenetic origin of MN. For the Comet assay the range of doses used was from 0.5 to 150 microg/ml. From the results obtained it appears that this antifolic agent has a significant clastogenic potential, as detected by a dose-dependent increase of the incidence of C-MN and significantly greater than control levels at the highest concentrations tested (25,100 microg/ml). In addition, the results obtained in the Comet assay also show that trimethoprim induces a dose-dependent increase in the level of DNA breakage, this increase attaining statistical significance at the highest concentrations tested (25, 100, 150 microg/ml), which would confirm its genotoxicity.

Imbalanced Base Excision Repair in Response to Folate Deficiency Is Accelerated by Polymerase β Haploinsufficiency

The mechanism by which folate deficiency influences carcinogenesis is not well established, but a phenotype of DNA strand breaks, mutations, and chromosomal instability suggests an inability to repair DNA damage. To elucidate the mechanism by which folate deficiency influences carcinogenicity, we have analyzed the effect of folate deficiency on base excision repair (BER), the pathway responsible for repairing uracil in DNA. We observe an up-regulation in initiation of BER in liver of the folate-deficient mice, as evidenced by an increase in uracil DNA glycosylase protein (30%, p < 0.01) and activity (31%, p < 0.05). However, no up-regulation in either BER or its rate-determining enzyme, DNA polymerase beta (beta-pol) is observed in response to folate deficiency. Accordingly, an accumulation of repair intermediates in the form of DNA single strand breaks (37% increase, p < 0.03) is observed. These data indicate that folate deficiency alters the balance and coordination of BER by stimulating initiation without subsequently stimulating the completion of repair, resulting in a functional BER deficiency. In directly establishing that the inability to induce beta-pol and mount a BER response when folate is deficient is causative in the accumulation of toxic repair intermediates, beta-pol-haploinsufficient mice subjected to folate deficiency displayed additional increases in DNA single strand breaks (52% increase, p < 0.05) as well as accumulation in aldehydic DNA lesions (38% increase, p < 0.01). Since young beta-polhaploinsufficient mice do not spontaneously exhibit increased levels of these repair intermediates, these data demonstrate that folate deficiency and beta-pol haploinsufficiency interact to increase the accumulation of DNA damage. In addition to establishing a direct role for beta-pol in the phenotype expressed by folate deficiency, these data are also consistent with the concept that repair of uracil and abasic sites is more efficient than repair of oxidized bases.

Diet modulates the toxicity of cancer chemotherapy in rats

The effects of diet and folate status on cyclophosphamide or 5-fluorouracil toxicity were studied in Fischer 344 rats maintained on either a cereal-based diet or a purified diet (AIN-93G). The rats fed the purified diet were divided into 3 groups: folate deficient (no dietary folic acid), folate replete (2 mg folic acid/kg diet), and high folate (2 mg folic acid/kg diet plus 50 mg/kg body weight folic acid intraperitoneally daily). The LD50 for cyclophosphamide was significantly higher for the cereal diet than for the purified diets, but there was no difference among the purified diets. Deaths were predicted by dose, diet, white blood cell count, and BUN on Day 4 after treatment. In the saline-treated rats fed the purified diet, hepatic total glutathione levels increased in the following order: folate deficient < folate replete < high folate. There was no significant difference in aldehyde dehydogenase activities or of microsomal P450 levels in livers from rats on the different diets. In the rats treated with 5-fluorouracil, the high folate rats developed more severe anemia, azotemia, and leukopenia than the other groups. Weight, white blood cell count, hematocrit, and BUN were important predictors of death. The kidneys from rats fed the cereal-based diet were histologically normal, but rats ingesting the purified diet had increasing renal pathology that correlated with folate intake. These results indicate that diet has an important influence on the toxicity of cyclophosphamide and 5-fluorouracil and that folate status modulates hepatic glutathione levels, which is a major cellular defense against oxidant and alkylating agent damage.

Dietary folate deficiency enhances induction of micronuclei by arsenic in mice

Folate deficiency increases background levels of DNA damage and can enhance the genotoxicity of chemical agents. Arsenic, a known human carcinogen present in drinking water supplies around the world, induces chromosomal and DNA damage. The effect of dietary folate deficiency on arsenic genotoxicity was evaluated using a mouse peripheral blood micronucleus (MN) assay. In duplicate experiments, male C57Bl/6J mice were fed folate-deficient or folate-sufficient diets for 7 weeks. During week 7, mice on each diet were given four consecutive daily doses of sodium arsenite (0, 2.5, 5, or 10 mg/kg) via oral gavage. Over the course of the study the folate-deficient diet produced an approximate 60% depletion of red blood cell folate. Folate deficiency by itself was associated with small but significant increases in MN in normochromatic erythrocytes (NCEs) and polychromatic erythrocytes (PCEs). Arsenic exposure was associated with significant increases in MN-PCEs in both folate-deficient and folate-sufficient mice. MN-PCE frequencies at the 10 mg/kg dose of arsenic were increased 4.5-fold over vehicle control in folate-deficient mice and 2.1-fold over control in folate-sufficient mice. At the 5 and 10 mg/kg doses of arsenic, MN-PCE levels were significantly higher (1.3-fold and 2.4-fold, respectively) in folate-deficient mice compared to folate-sufficient mice. Very few MN from either control or treated animals in either experiment exhibited kinetochore immunostaining, suggesting that the MN were derived from chromosome breakage rather than from whole chromosome loss. These results indicate that folate deficiency enhances arsenic-induced clastogenesis at doses of 5 mg/kg and higher.

The effect of folate deficiency on the cytotoxic and mutagenic responses to ethyl methanesulfonate in human lymphoblastoid cell lines that differ in p53 status

Folic acid deficiency acts synergistically with alkylating agents to increase genetic damage at the HPRT locus in Chinese hamster ovary cells in vitro and in rat splenocytes in vivo. The present studies extend these observations to human cells and, in addition, investigate the role of p53 activity on mutation induction. The human lymphoblastoid cell lines TK6 and WTK1 are derived from the same parental cell line (WI-L2), but WTK1 expresses mutant p53. Treatment of folate-replete or deficient WTK1 and TK6 cells with increasing concentrations (0-50microg/ml) of ethyl methanesulfonate (EMS) resulted in significantly different HPRT mutation dose-response relationships (P<0.01), indicating that folate deficiency increased the EMS-induced mutant frequency in both cell lines, but with a greater effect in TK6 cells. Molecular analyses of 152 mutations showed that the predominant mutation (65%) in both cell types grown in the presence or absence of folic acid was a G>A transition on the non-transcribed strand. These transitions were mainly at non-CpG sites, particularly when these bases were flanked 3' by a purine or on both sides by G:C base pairs. A smaller number of G>A transitions occurred on the transcribed strand (C>T=14%), resulting in 79% total G:C>A:T transitions. There were more genomic deletions in folate-deficient (15%) as compared to replete cells (4%) of both cell types. Mutations that altered RNA splicing were common in both cell types and under both folate conditions, representing 33% of the total mutations. These studies indicate that cells expressing p53 activity exhibit a higher rate of mutation induction but are more sensitive to the toxic effects of alkylating agents than those lacking p53 activity. Folate deficiency tends to reduce toxicity but increase mutation induction after EMS treatment. The p53 gene product did not have a major influence on the molecular spectrum after treatment with EMS, while folate deficiency increased the frequency of deletions in both cell types.

The effect of folate deficiency on the hprt mutational spectrum in Chinese hamster ovary cells treated with monofunctional alkylating agents

Folic acid deficiency acts synergistically with alkylating agents to increase DNA strand breaks and mutant frequency at the hprt locus in Chinese hamster ovary (CHO) cells. To elucidate the mechanism of this synergy, molecular analyses of hprt mutants were performed. Recently, our laboratory showed that folate deficiency increased the percentage of clones with intragenic deletions after exposure to ethyl methanesulfonate (EMS) but not N-nitroso-N-ethylurea (ENU) compared to clones recovered from folate replete medium. This report describes molecular analyses of the 37 hprt mutant clones obtained that did not contain deletions. Folate deficient cells treated with EMS had a high frequency of G>A transitions at non-CpG sites on the non-transcribed strand, particularly when these bases were flanked on both sides by G:C base pairs. Thirty-three percent of these mutations were in the run of six G's in exon 3. EMS-treated folate replete cells had a slightly (but not significantly) lower percentage of G>A transitions, and the same sequence specificity. Treatment of folate deficient CHO cells with ENU resulted in predominantly T>A transversions and C>T transitions relative to the non-transcribed strand. These findings suggest a model to explain the synergy between folate deficiency and alkylating agents: (1) folate deficiency causes extensive uracil incorporation into DNA; (2) greatly increased utilization of base excision repair to remove uracil and to correct alkylator damage leads to error-prone DNA repair. In the case of EMS, this results in more intragenic deletions and G:C to A:T mutations due to impaired ligation of single-strand breaks generated during base excision repair and a decreased capacity to remove O6-ethylguanine. In the case of ENU additional T>A transversions and C>T transitions are seen, perhaps due to mis-pairing of O2-ethylpyrimidines. Correction of folate deficiency may reduce the frequency of these types of genetic damage during alkylator therapy.