Deoxyuridine suppression: biochemical basis and diagnostic applications

Nitrous oxide abuse direct measurement for diagnosis and follow-up: update on kinetics and impact on metabolic pathways

Recreational use of nitrous oxide (N2O) has become a major health issue worldwide, with a high number of clinical events, especially in neurology and cardiology. It is essential to be able to detect and monitor N2O abuse to provide effective care and follow-up to these patients. Current recommendations for detecting N2O in cases of recreational misuse and consumption markers are lacking. We aimed to update current knowledge through a review of the literature on N2O measurement and kinetics. We reviewed the outcomes of experiments, whether in preclinical models (in vitro or in vivo), or in humans, with the aim to identify biomarkers of intoxication as well as biomarkers of clinical severity, for laboratory use. Because N2O is eliminated 5 min after inhalation, measuring it in exhaled air is of no value. Many studies have found that urine and blood matrices concentrations are connected to ambient concentrations, but there is no similar data for direct exposure. There have been no studies on N2O measurement in direct consumers. Currently, patients actively abusing N2O are monitored using effect biomarkers (biomarkers related to the effects of N2O on metabolism), such as vitamin B12, homocysteine and methylmalonic acid.

Plasma Metabolites Alert Patients With Chest Pain to Occurrence of Myocardial Infarction

Myocardial infarction (MI) is one of the leading causes of death worldwide, and knowing the early warning signs of MI is lifesaving. To expand our knowledge of MI, we analyzed plasma metabolites in MI and non-MI chest pain cases to identify markers for alerting about MI occurrence based on metabolomics. A total of 230 volunteers were recruited, consisting of 146 chest pain patients admitted with suspected MI (85 MIs and 61 non-MI chest pain cases) and 84 control individuals. Non-MI cardiac chest pain cases include unstable angina (UA), myocarditis, valvular heart diseases, etc. The blood samples of all suspected MI cases were collected not longer than 6 h since the onset of chest pain. Gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry were applied to identify and quantify the plasma metabolites. Multivariate statistical analysis was utilized to analyze the data, and principal component analysis showed MI could be clearly distinguished from non-MI chest pain cases (including UA and other cases) in the scores plot of metabolomic data, better than that based on the data constructed with medical history and clinical biochemical parameters. Pathway analysis highlighted an upregulated methionine metabolism and downregulated arginine biosynthesis in MI cases. Receiver operating characteristic curve (ROC) and adjusted odds ratio (OR) were calculated to evaluate potential markers for the diagnosis and prediction ability of MI (MI vs. non-MI cases). Finally, gene expression profiles from the Gene Expression Omnibus (GEO) database were briefly discussed to study differential metabolites' connection with plasma transcriptomics. Deoxyuridine (dU), homoserine, and methionine scored highly in ROC analysis (AUC > 0.91), sensitivity (>80%), and specificity (>94%), and they were correlated to LDH and AST (p < 0.05). OR values suggested, after adjusting for gender, age, lipid levels, smoking, type II diabetes, and hypertension history, that high levels of dU of positive logOR = 3.01, methionine of logOR = 3.48, and homoserine of logOR = 1.61 and low levels of isopentenyl diphosphate (IDP) of negative logOR = -5.15, uracil of logOR = -2.38, and arginine of logOR = -0.82 were independent risk factors of MI. Our study highlighted that metabolites belonging to pyrimidine, methionine, and arginine metabolism are deeply influenced in MI plasma samples. dU, homoserine, and methionine are potential markers to recognize MI cases from other cardiac chest pain cases after the onset of chest pains. Individuals with high plasma abundance of dU, homoserine, or methionine have increased risk of MI, too.

Diagnosis of megaloblastic anaemias

There are a large number of causes of megaloblastic anaemia. The most frequent are disorders resulting in vitamin B(12) or folate deficiency. The diagnostic process often consists first of establishing the presence of B(12) or folate deficiency and then of determining the cause of deficiency. The blood count, blood film, serum B(12) assay, and red cell and serum folate assays are the primary investigations. Other useful investigations include serum/plasma methylmalonic acid (MMA), plasma total homocysteine (tHCYS) and serum holo-transcobalamin II assays. All currently used tests have limitations regarding specificity or sensitivity or both and the metabolite assays are not widely available. An understanding of these limitations is essential in formulating any diagnostic strategy. The wide use of serum B(12) and metabolite assays has resulted in the increasingly early diagnosis of B(12) deficiency, often in patients without B(12)-related symptoms (subclinical deficiency). Food cobalamin malabsorption is the most frequent cause of a low serum B(12). At least 25% of low serum B(12) levels are not associated with elevated metabolite levels and may not indicate B(12) deficiency. Some of these are caused by partial deficiency of transcobalamine I.

Congenital transcobalamin II deficiency due to errors in RNA editing

Transcobalamin II (TCII) is a plasma protein essential for the transport and cellular uptake of vitamin B12 (B12; cobalamin, Cbl). Congenital deficiency of functional TCII is an autosomal recessive genetic disorder that results in clinical B12 deficiency usually within several months following birth. In this report, we describe the molecular basis for TCII deficiency in two patients who developed a megaloblastic anemia in early infancy. The serum of both patients contained immunoreactive TCII that did not bind [57Co]Cbl. The fibroblasts from each patient secreted a similarly nonfunctional TCII, yet full-length TCII transcripts were identified by Northern blot. Overlapping cDNA fragments were generated by reverse transcription-polymerase chain reaction and several mutations were identified in the coding region of the cDNA, one of which was common to both patients. However, amplification of the corresponding regions of the gene from genomic DNA failed to identify these mutations. These findings were confirmed by replicate analyses and support the proposal that a variance in RNA editing is the likely mechanism for the mutations that resulted in the expression of a nonfunctional TCII protein in these patients.

Current concepts in cobalamin deficiency

The application of sensitive metabolic tests, such as the deoxyuridine suppression test and measurement of homocysteine and methylmalonic acid, to cobalamin status has identified the entity of mild, preclinical cobalamin deficiency. This state, common in the elderly, responds to cobalamin therapy. Preclinical deficiency may exist within the nervous system as well, although this requires further study. Nevertheless, it is well to remember that not all low cobalamin levels and not all abnormal metabolite results reflect cobalamin deficiency. Interpretation of metabolic results still requires caution, as do proposals to raise the cut-off point for low cobalamin levels to capture some normal levels that are associated with metabolic abnormality. The recognition of mild, preclinical deficiency has opened up many important issues. These include identifying its causes, what should be done about it, and what the clinical impact of the hyperhomocysteinemia itself is. Although malabsorptive disorders, especially food-cobalamin malabsorption, underlie about half of all cases of preclinical deficiency, no cause can be found in the remainder of these cases; poor dietary intake appears to be uncommon. In addition, unusual states of neurologically symptomatic cobalamin deficiency are being recognized, such as nitrous oxide exposure in patients with unrecognized deficiency and severe deficiency in children of mildly deficient mothers. All of these have broadened and complicated the picture of cobalamin deficiency while providing greater opportunities for prevention.

DNA damage in folate deficiency

Folate deficiency significantly increases uracil content and chromosome breaks (as measured by micronucleated cells) in human leukocyte DNA. Folate supplementation reduces both the uracil content of DNA and the frequency of micronucleated cells, indicating that uracil misincorporation may play a causative role in folate deficiency-induced chromosome breaks. A calculation is presented to explain how the levels of uracil found in DNA could cause chromosome breaks. Based on this calculation, the frequency of uracil repair events that might result in double-strand DNA breaks increases by 1752-fold. These results are consistent with clinical and epidemiological evidence linking folate deficiency to DNA damage and cancer.

Morphology, biology and biochemistry of cobalamin- and folate-deficient bone marrow cells

B12- or folate-deficient haemopoietic cells display abnormalities in their morphology under both the light and electron microscope, their cell kinetics and their capacity to synthesize protein. These abnormalities are maximal in the last dividing cell class and in non-dividing cells, presumably because B12 and folate uptake is largely confined to the most immature erythroid and granulocyte precursors. In patients with moderate or severe anaemia due to B12 or folate deficiency, erythropoiesis is markedly ineffective; intramedullary cell death occurs mainly in the early and late polychromatic megaloblasts. The damaged erythroblasts appear to display neoantigens or normally-hidden antigens at their cell surface and these react with naturally occurring antibodies. The opsonised erythroblasts are then recognised by macrophages via their IgG-Fc receptors and phagocytosed. Marrow cells from B12- or folate-deficient patients show a subnormal suppression of 3H-thymidine incorporation after pre-incubation with nonradioactive deoxyuridine, suggesting that such cells suffer from an impairment of the 5,10-methylene-THF-dependent methylation of deoxyuridylate to thymidylate. However, the exact mechanism by which B12 deficiency causes a reduced supply of this folate coenzyme is uncertain. Methylcobalamin is required for the 5-methyl-THF-dependent methylation of homocysteine to methionine and an impairment of this reaction will result in both reduced conversion of 5-methyl-THF to THF and in reduced methionine synthesis. There is controversy as to whether the reduced supply of THF or methionine is responsible for the reduced availability of 5,10-methylene-THF. Currently, the balance of evidence favours the hypothesis that the reduced supply of methionine leads to reduced synthesis of formyl-THF and, eventually, of 5,10-methylene-THF. Despite the evidence for impaired thymidylate synthesis, the duration of the S phase of megaloblasts appears to be normal or only modestly increased. Data on rates of DNA strand elongation are inconsistent, with subnormal rates reported in PHA-stimulated B12- or folate-deficient lymphocytes and normal rates in B12- or folate-deficient bone marrow cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Cobalamin-dependent metabolism in chronic myelogenous leukemia determined by deoxyuridine suppression test and the formiminoglutamic acid and methylmalonate excretion in urine

The cobalamin metabolism in chronic myelogenous leukemia (CML) was evaluated in 18 newly diagnosed and untreated patients by formiminoglutamic acid (FiGlu) and methyl malonic acid excretion (MMA) tests. A deoxyuridine (dU) suppression test of bone marrow cells was compared in patients with acute myelogenous leukemia (N = 5), myelodysplastic disease (N = 3), untreated pernicious anemia (N = 16), folate deficiency (N = 7), and a hospital reference group without signs of cobalamin or folate deficiency (N = 22). All had normal MMA excretion but 3 of 15 patients had increased FiGlu excretion. In vitro thymidine uptake in bone marrow cells of CML patients were lower (mean 40 fmol/106 cells) than pernicious anemia patients (115 fmol/106 cells). Methotrexate (MTX) increased the uptake in all cases. Addition of formyl-THF, methyltetrahydrofolate (methyl-THF), and pteroylglutamic acid (PGA) tended to normalize the effect of MTX. In pernicious anemia methyl-THF only decreased the uptake in combination with CN-Cbl. dU suppression values were significantly higher (6.3%) in CML than in the reference group (4.4%), but significantly lower than in pernicious anemia (41.6%) and folate deficiency (28.5%). The dU suppression values in bone marrow cells of CML patients correlated significantly with the transferrin saturation. In buffy coat cells dU suppression values were even higher (9.3%) than in bone marrow cells of the same CML patients. Addition of folate forms and CN-Cbl did not change the dU suppression values in CML, as it did in pernicious anemia. MTX increased dU suppression values significantly in all patients, but more in CML (64.5%) than in pernicious anemia (48.6%) and controls (49.8%). The MTX effect was to some extent neutralized by folate analogues with formyl-THF as the most effective followed by methyl-THF and lastly PGA. Methyl-THF also neutralized MTX in pernicious anemia, but its effect was certainly enhanced by addition of CN-Cbl. Thymidine uptake and dU suppression patterns were not significantly changed in CML after treatment with busulfan for 1 week or in accelerated phase. We concluded that signs of cobalamin or folate deficiency (apart from one patient) cannot be demonstrated in untreated CML. However, dU suppression was significantly increased and more so in circulating myeloid cells than in bone marrow. This indicates a deranged metabolism of deoxynucleotides which is independent of cobalamin and folates, and a difference between bone marrow cells and circulating cells. dU suppression is a valuable indicator of cobalamin deficiency.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Misincorporation of uracil into the DNA of folate- and B12-deficient HL60 cells

HL60 cells were cultured for 10 days under various experimental conditions. They were then incubated with 1 mumol/l [5-3H] uridine for 2 hours and their DNA extracted. The DNA was hydrolysed to deoxyribonucleosides with phosphodiesterase and alkaline phosphatase and the hydrolysate subjected to Aminex A6 chromatography. The elution profiles showed that, when compared with control cells, DNA from cells grown in medium deficient in folate, B12 or both folate and B12 contained increased amounts of deoxyuridine (dU) and increased radioactivity in the dU peak. The data demonstrate that misincorporation of uracil into DNA occurs in a myeloid cell line cultured in growth medium deficient in folate, B12 or both folate and B12.

The use of radioisotopes in haematology

Radioisotopes used in haematology may be divided into four groups: 1. those used for in vivo studies, involving the labelling of cells in the blood or bone marrow and the use of labelled plasma albumin; 2. investigations involving surface counting over organs such as the bone marrow, spleen, liver and heart; 3. in vitro use of radioisotopes in the haematology laboratory and 4. isotopes used as part of imaging procedures. The shorter the half life of the isotope, the more limited patient exposure to radioactivity will be, but the greater the problems of starting and completing the investigation before the isotope has decayed. Isotopes studies should not be carried out in children or pregnancy unless there are exceptional clinical indications.

Natural autoantibodies may play a role in ineffective erythropoiesis during megaloblastic haemopoiesis

Marrow aspirates from 11 patients with megaloblastic haemopoiesis and from 14 healthy individuals with normoblastic haemopoiesis were studied for antibodies associated with polychromatic/orthochromatic erythroblasts, using an 125I-labelled anti-human immunoglobulin reagent and autoradiography. In addition, the expression on these cells of receptors for FcIgG (FcR) and of the type I receptor for fragments of the third complement component (CR1) were investigated with receptor-specific monoclonal antibodies, 125I-labelled anti-mouse immunoglobulin and autoradiography. The percentages of immunoglobulin-positive erythroblasts were significantly greater in the megaloblastic than in the normoblastic marrows. Abnormally high percentages of labelled erythroblasts were present in patients without any manifestations of an autoimmune disorder. The percentage of labelled erythroblasts in the marrows of the patients correlated well with the degree of anaemia. FcR were absent on the majority of megaloblasts or normoblasts while the expression of CR1 was similar in both types of cell. The difference between the percentage labelling of megaloblasts and normoblasts was therefore unlikely to be due to greater binding of immune complexes with or without associated complement to megaloblasts than normoblasts. The megaloblast-bound immunoglobulin is, therefore, likely to have recognized abnormally expressed epitopes on the surface of megaloblasts. The results suggest that natural autoantibodies play a role in the destruction of erythroblasts during megaloblastic haemopoiesis.

The effects of folate deficiency on thymidylate synthetase activity, deoxyuridine suppression, cell size and doubling time in a cultured human myeloid cell line

To help understand the pathogenesis of megaloblastic anaemia, we have studied folate-deprived HL60 cells. Cells grown with no added folic acid developed macrocytosis, a prolonged doubling time, a grossly increased deoxyuridine-suppressed value, and markedly reduced thymidylate synthetase activity. Cells in medium containing 50 nmol/l added folic acid became macrocytic with similar biochemical changes, but their doubling time was only marginally prolonged. In 100 nmol/l added folic acid, there was slight macrocytosis and a normal doubling time, but marked biochemical alterations were still present. The findings demonstrate that folate deficiency causes diminished thymidylate synthetase activity and macrocytosis in human myeloid cells, but that such cells may nevertheless demonstrate no prolongation of their doubling time, indicating either that their supply of thymidine triphosphate (TTP) is sufficient, or that misincorporation of deoxyuridine triphosphate (dUTP) is occurring. This suggests that the ineffectiveness of haemopoiesis in folate deficiency may result from damage to bone marrow cells in some way other than arrest in S-phase by a reduced delivery of TTP to the DNA replication fork.

Deoxyuridine suppression: biochemical basis and diagnostic applications

The deoxyuridine (dU) suppression test evolved out of investigations into the biochemical basis of the megaloblastic changes seen in vitamin B12 and folate deficiency. Although the abnormality in dU suppression which occurs in vitamin B12- or folate-deficient states is assumed to reflect impaired methylation of deoxyuridylate, there is still no direct demonstration that this is so. Furthermore, there is evidence that reactions other than the methylation of deoxyuridylate are involved in the phenomenon of dU suppression. Nevertheless, in clinical practice abnormal dU suppression serves as a sensitive index of the presence of megaloblastosis due to vitamin B12 or folate deficiency. dU suppression is also abnormal in a number of conditions other than vitamin B12 or folate deficiency, but its overall specificity in detecting tissue dysfunction due to these two deficiency states is considerably higher than that of the serum vitamin B12 or red cell folate levels. Consequently, the test enables us simply and rapidly to define those patients in whom macrocytosis is unrelated to a deficiency of vitamin B12 or folate. For these reasons, the dU suppression test has been adopted by several laboratories across the world for investigating patients with (a) possible vitamin B12 or folate deficiency, (b) macrocytosis, and (c) megaloblastic erythropoiesis. Since the dU suppression test is abnormal in transcobalamin II deficiency and in some congenital disorders of vitamin B12 and folate metabolism, it is very useful in the investigation of obscure anaemias in infancy and childhood. In addition, it has contributed to our understanding of the mechanisms underlying the myelotoxicity of certain drugs, and particularly of nitrous oxide.