In vitro DNA synthesis by megaloblastic bone marrow: effect of folates and cobalamins on thymidine incorporation and de novo thymidylate synthesis

Early Life Stage Folic Acid Deficiency Delays the Neurobehavioral Development and Cognitive Function of Rat Offspring by Hindering De Novo Telomere Synthesis

Early life stage folate status may influence neurodevelopment in offspring. The developmental origin of health and disease highlights the importance of the period of the first 1000 days (from conception to 2 years) of life. This study aimed to evaluate the effect of early life stage folic acid deficiency on de novo telomere synthesis, neurobehavioral development, and the cognitive function of offspring rats. The rats were divided into three diet treatment groups: folate-deficient, folate-normal, and folate-supplemented. They were fed the corresponding diet from 5 weeks of age to the end of the lactation period. After weaning, the offspring rats were still fed with the corresponding diet for up to 100 days. Neurobehavioral tests, folic acid and homocysteine (Hcy) levels, relative telomere length in brain tissue, and uracil incorporation in telomere in offspring were measured at different time points. The results showed that folic acid deficiency decreased the level of folic acid, increased the level of Hcy of brain tissue in offspring, increased the wrong incorporation of uracil into telomeres, and hindered de novo telomere synthesis. However, folic acid supplementation increased the level of folic acid, reduced the level of Hcy of brain tissue in offspring, reduced the wrong incorporation of uracil into telomeres, and protected de novo telomere synthesis of offspring, which was beneficial to the development of early sensory-motor function, spatial learning, and memory in adolescence and adulthood. In conclusion, early life stage folic acid deficiency had long-term inhibiting effects on neurodevelopment and cognitive function in offspring.

De novo mutations in folate-related genes associated with common developmental disorders

Folate deficiency is an environmental risk factor for several developmental disorders. De novo mutations (DNMs) also play important etiological roles in various developmental disorders. However, it remains unclear whether DNMs in folate-related genes (FRGs) contribute to developmental disorders. We obtained a list of 1,821 FRGs from folate metabolism pathways and the Comparative Toxicogenomics Database, along with data concerning DNMs in 15,404 cases and 3,391 controls from the Gene4Denovo database. We used a TADA-Denovo model to prioritize candidate disease-associated FRGs, and characterized these genes in terms of genic intolerance, functional networks, and expression patterns. Compared with the controls, FRGs were significantly enriched in likely damaging DNMs (ldDNMs) in patients with developmental disorders (1.54 ≤ odds ratio ≤ 3.39, P_adj ≤ 0.0075). Furthermore, FRGs with ldDNMs rather than with likely non-damaging DNMs (lndDNMs) overlapped significantly among the five developmental disorders included in the datasets. The TADA-Denovo model prioritized 96 candidate disease-associated FRGs, which were intolerant to genetic variants. Their functional networks mainly involved pathways associated with chromatin modification, organ development, and signal transduction pathways. DNMT3A, KMT2B, KMT2C, and YY1 emerged as hub FRGs from the protein–protein interaction network. These candidate disease-associated FRGs are preferentially expressed in the excitatory neurones during embryonic development, and in the cortex, cerebellum, striatum, and amygdala during foetal development. Overall, these findings show that DNMs in FRGs are associated with the risk of developmental disorders. Further research on these DNMs may facilitate the discovery of developmental disorder biomarkers and therapeutic targets, enabling detailed, personalized, and precise folate treatment plan.

The mitochondrial inner membrane protein MPV17 prevents uracil accumulation in mitochondrial DNA

Mitochondrial inner membrane protein MPV17 is a protein of unknown function that is associated with mitochondrial DNA (mtDNA)-depletion syndrome (MDS). MPV17 loss-of-function has been reported to result in tissue-specific nucleotide pool imbalances, which can occur in states of perturbed folate-mediated one-carbon metabolism (FOCM), but MPV17 has not been directly linked to FOCM. FOCM is a metabolic network that provides one-carbon units for the de novo synthesis of purine and thymidylate nucleotides (e.g. dTMP) for both nuclear DNA (nuDNA) and mtDNA replication. In this study, we investigated the impact of reduced MPV17 expression on markers of impaired FOCM in HeLa cells. Depressed MPV17 expression reduced mitochondrial folate levels by 43% and increased uracil levels, a marker of impaired dTMP synthesis, in mtDNA by 3-fold. The capacity of mitochondrial de novo and salvage pathway dTMP biosynthesis was unchanged by the reduced MPV17 expression, but the elevated levels of uracil in mtDNA suggested that other sources of mitochondrial dTMP are compromised in MPV17-deficient cells. These results indicate that MPV17 provides a third dTMP source, potentially by serving as a transporter that transfers dTMP from the cytosol to mitochondria to sustain mtDNA synthesis. We propose that MPV17 loss-of-function and related hepatocerebral MDS are linked to impaired FOCM in mitochondria by providing insufficient access to cytosolic dTMP pools and by severely reducing mitochondrial folate pools.

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.

RFC1 and non-syndromic cleft lip with or without cleft palate: an association based study in Italy

The molecular basis of orofacial development is largely unknown and needs to be unravelled. Non-syndromic cleft lip with or without cleft palate (NSCL/P) is the most common craniofacial malformation, with an incidence of about 1/700 live births, although variable according to ethnicity. Being a multifactorial disease, it arises as a result of an interplay between genetic and environmental factors. Several approaches have been developed to identify susceptibility genes. Genes belonging to the folate/homocysteine pathway are attracting increasing interest because folate supplementation before and during early pregnancy can reduce the risk of NSCL/P. We performed a family based association study in order to assess if a genetic variant of RFC1 could be involved in NSCL/P onset. We genotyped 404 unrelated probands and their relatives for three biallelic polymorphic variants (rs1051266, rs4818789 and rs3788205), that were selected because they produced conflicting results on previous investigations. Evidence of association was found between the investigated polymorphisms and NSCL/P in our sample of the Italian population, albeit with weak significance levels. Results from this investigation provided a support of previous studies suggesting a role of RFC1 in NSCL/P aetiology, reinforcing the concept that genetic predisposition to NSCL/P varies enormously within different ethnic groups.

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.

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.

Localized depletion: the key to colorectal cancer risk mediated by MTHFR genotype and folate?

Dietary folate has been consistently associated with reduced risk of colorectal cancer (CRC). One of the known biochemical roles of folate is donation of methyl moieties. DNA hypomethylation is an early and almost ubiquitous occurrence in tumor tissue. Therefore, it was originally suggested that adequate folate intake contributed to reduced risk of CRC by facilitating methyl-mediated silencing of oncogenes. Methylene tetrahydrofolate reductase (MTHFR) metabolizes 5,10-MTHF (important in DNA synthesis) to 5-MTHF (contributes to downstream methylation reactions by regeneration of methionine from homocysteine). A common polymorphism in the MTHFR gene (C677T) results in a thermolabile phenotype associated with increased homocysteine levels and DNA hypomethylation. Consistent with the folate/methylation hypothesis, it was originally proposed that C677T may increase risk of CRC due to hypomethylation of oncogenes. However, most subsequent studies have reported a reduced risk associated with this polymorphism. This is inconsistent with methylation as the mechanism by which folate and MTHFR genotype mediate CRC risk. The hypothesis presented here proposes that localized folate depletion combined with the effect of the C677T polymorphism on enzyme stability, impacts on the DNA synthesis pathway and accounts for the observed variation in risk associated with genotype and folate status.

Megaloblastosis: from morphos to molecules

OBJECTIVE

Megaloblastosis (i.e., megaloblastic transformation of erythroid precursor cells in the bone marrow) is the cytomorphological hallmark of megaloblastic anemia resulting from vitamin B12 and folate deficiency. It is characterized by a finely stippled lacy pattern of nuclear chromatin, which is believed to be an expression of deranged cellular DNA synthesis. However, the molecular basis of these cytomorphological aberrations still remains obscure. The current presentation describes the results of our studies on some molecular events associated with the development of megaloblastosis.

METHODS

Transmission electron microscopy was used to study megaloblasts as well as DNA fibers extracted from megaloblastic and normoblastic bone marrows with and without treatment with proteinase K during the extraction procedure; cellular DNA synthesis in bone marrow cultures was studied by incorporation of 3H-thymidine and deoxyuridine suppression test, while histone biosynthesis in bone marrow cells was studied by in vitro incorporation of 3H-tryptophan, 3H-lysine and 3H-arginine into histones.

RESULTS

Derangement of DNA synthesis occurred due to an impaired de novo pathway of thymidylate synthesis in both vitamin-B12- and folate-deficient human megaloblastic bone marrows as well as in the bone marrows of rhesus monkeys and rats with experimentally induced folate deficiency. Interestingly, folate-deficient monkeys developed frank megaloblastic bone marrows, but folate-deficient rats did not. On the other hand, megaloblastic changes in the bone marrow of human patients with myelodysplastic syndrome and erythroleukemia were not associated with this DNA synthetic abnormality. Biosynthesis of predominantly arginine-rich histones in megaloblastic bone marrows was markedly reduced as compared to normoblastic bone marrows, which was consistently associated with elongation and despiralization of chromosomes and finely stippled nuclear chromatin in megaloblasts.

CONCLUSION

The impaired biosynthesis of predominantly arginine-rich nuclear histones appeared to be a common molecular event (a denominator) underlying the development of megaloblastosis with or without abnormal DNA synthesis.

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.

Folate deficiency, mismatch repair-dependent apoptosis, and human disease

The vitamin that is most commonly deficient in the American diet is folate. Severe folate deficiency in humans is known to cause megaloblastic anemia and developmental defects, and is associated with an increased incidence of several forms of human cancer. Although the exact mechanisms by which this vitamin deficiency may cause these diseases are not known at the present time, recent work has shown that folate deficiency also causes genomic instability and programmed cell death (or apoptosis). Additionally, it is known that the DNA mismatch repair pathway mediates folate deficiency-induced apoptosis. This review will first describe work suggesting that folate deficiency causes genomic instability and apoptosis, then discuss possible mechanisms by which the mismatch repair pathway could trigger folate deficiency-induced apoptosis, which has either protective or destructive effects on tissue.

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.

Cobalamin

Cobalamin (vitamin B12) is an essential nutrient derived exclusively from bacterial sources. It is an essential cofactor for three known enzymatic reactions. Untreated deficiency, caused by either the autoimmune disease pernicious anemia or nutritional lack, results in a macrocytic anemia and/or subacute combined degeneration of the spinal cord and is eventually fatal. Cobalamin in serum is bound to two proteins, transcobalamin and haptocorrin. The former is responsible for the essential delivery of cobalamin to most tissues. Inadequate tissue availability of cobalamin results in increased concentration of methylmalonic acid and homocyst(e)ine due to inhibition of methylmalonyl-CoA mutase and methionine synthase, respectively. Strict vegetarians have long been known to be at risk of cobalamin deficiency, which develops insidiously over many years. It is now clear that a significant number of the elderly and HIV-positive individuals are also at increased risk of deficiency. Any individual with reduced ability to split cobalamin from food-protein may also become deficient even though intrinsic factor is present. Diagnosis of cobalamin deficiency has frequently relied on total serum cobalamin and the Schilling test. Newer approaches such as analysis of methylmalonic acid, homocyst(e)ine, holotranscobalamin, anti-intrinsic factor antibodies, and serum gastrin may provide more cost-effective testing, as well as identify those with a covert deficiency.

Deoxyuridine suppression test on isolated rat bone marrow cells and the in vitro effect of bidisomide

The deoxyuridine suppression test was performed on isolated rat bone marrow cells in order to study the effect of bidisomide, a new Class I antiarrhythmic agent, on folate-dependent DNA synthesis. Methotrexate and 5-fluorouracil, two known inhibitors of DNA synthesis, were included in the study to validate the test system. Methotrexate and 5-fluorouracil, at a concentration of 5.5 mum, decreased thymidine incorporation into DNA by way of the de novo pathway (thymidylate synthase activity). The salvage pathway of DNA synthesis (thymidine kinase activity), however, was not affected by these anticancer drugs. Bidisomide up to 1 mm did not affect the folate-dependent thymidylate synthase activity, nor the thymidine kinase activity of isolated rat bone marrow cells.

Low holotranscobalamin II is the earliest serum marker for subnormal vitamin B12 (cobalamin) absorption in patients with AIDS

In AIDS, as previously found in pernicious anemia (PA), the earliest serum marker of subnormal vitamin B12 (cobalamin) absorption, and therefore of negative B12 balance, is low serum holotranscobalamin II (holo-TC II; B12-TC II) despite normal total serum B12 level, normal serum homocysteine, and normal classic (oral free radio-B12) Schilling test. This may be accompanied by subtle and insidious damage to hematopoietic, immunologic, neuropsychiatric, nutritional and alimentary systems, confirmed by correction on therapeutic trial with B12 therapy. Our studies suggest such selective B12 deficiency occurs in about half of the HIV-1 infected, in part due to frequent depression of B12 absorption by HIV-1 attack on the gastric mucosa and/or opportunistic infection attack on the small bowel, and in part due to a telescoping of the continuum of the stages of negative B12 balance in relation to damage to B12 delivery by the infective and/or systemic disease process. In AIDS, when total serum B12 is normal despite tissue depletion of B12, if the classic Schilling test does not reveal subnormal food B12 absorption, the food Schilling test does. We hypothesize that DNA-synthesizing cells of the hematopoietic, immunologic, neurologic and other systems which have surface receptors solely for holo-TC II, and which have low B12 stores, rapidly become dysfunctional due to B12 deficiency when holo-TC II is low, while cells (such as liver cells) which also have surface receptors for holohaptocorrin (B12-haptocorrin) remain B12-replete. We believe this to be another example of the concept of selective nutrient deficiency in one cell line but not another.