[Thymine nucleotide synthesis and proliferation of bone marrow cells in megaloblastic anemias under the effect of vitamin B 12]

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)

Dysgranulopoietic neutropenia and abnormal monocytes in childhood vitamin B12 deficiency

Neutropenia and/or leukopenia (associated with elevated serum lysozyme levels) in three children with vitamin B12 deficiency were evaluated using soft agar culture and ultrastructural and cytochemical techniques. In two patients a marked increase in peripheral myeloid colony forming cells (CFC) was observed; whereas a marginal increase in CFC was present in the third, less symptomatic, patient. Marrow CFC was normal or slightly increased. Serum colony stimulating activity (CSA) was normal but elaboration of CSA by white blood cells was low. Normal maturation of the progenitors was present in vitro and serum inhibitors of myelopoiesis were absent. Megaloblastic neutrophils and monocytes with nuclear-cytoplasmic asynchrony were observed ultrastructurally in directly sampled marrow specimens. These cells contained autophagic and/or heterophagic vacuoles and an increase in cytoplasmic granules. Both monocytes and neutrophils also contained enlarged-disrupted centrioles. Many marrow macrophages contained phagocytic vacuoles, which enclosed disrupted neutrophils and cellular debris.

Biochemical mechanisms in the Killmann experiment: critique of the deoxyuridine suppression test

The degree of inhibition of [3H]thymidine incorporation into DNA by exogenous deoxyuridine is assayed in a procedure known as the deoxyuridine suppression test. We report studies of the biochemical basis of this phenomenon in phytohemagglutinin-stimulated lymphocytes, which suggest that its mechanism has not been fully understood. Results show that inhibition by deoxyuridine is caused only in part by expansion of the intracellular pools of nonradioactive dTMP and dTTP, which dilutes the specific radioactivity of the [3H]dTMP and [3H]dTTP derived from [3H]thymidine. Increased dTTP levels also inhibit thymidine kinase. In addition, thymidine kinase is competitively inhibited by intracellular deoxyuridine. Inhibition of thymidine kinase activity by both mebolites further decreases the specific radioactivity of [3H]dTMP and [3H]dTTP. Deoxyuridine also inhibits the incorporation of [3H]deoxyadenosine and [3H]deoxyguanosine into DNA in these cells. Exogenous deoxyuridine still inhibits [3H]thymidine incorporation in cells whose de novo thymidylate synthesis has been strongly inhibited by 5-fluorodeoxyuridine or methotrexate. In such drug-treated cells, exposure to high concentrations of exogenous deoxyuridine can partially overcome the inhibition of thymidylate synthetase with resulting increase in the severely depleted dTTP pools. This increase is associated with enhanced DNA synthesis, as measured by incorporation into DNA of labeled deoxyribonucleosides other than [3H]thymidine. We conclude that exogenous deoxyuridine has multiple effects on [3H]thymidine incorporation, which must be considered in interpretations of deoxyurindine suppression test results.

In vitro DNA synthesis by bone marrow cells and PHA-stimulated lymphocytes. Suppression by nonradioactive thymidine of the incorporation of 3H-deoxyuridine into DNA: enhancement of incorporation when inadequate vitamin B12 or folate is corrected

Previous studies demonstrated that excess deoxyuridine (dU) added to short-term bone marrow and PHA-stimulated lymphocyte cultures, blocks the incorporation of radioactive thymidine into DNA via the salvage pathway. In the current study, we investigated the effects of added thymidine (TdR) in varying concentrations (10(-6) to 1 mumol) on the incorporation of 3H-dU into thymine-DNA, i.e. we executed 'thymidine suppression tests.' Increasing concentrations of exogenous TdR caused progressive inhibition of 3H-dU incorporation into DNA, and decreasing 3H-dU incorporation was parallelled by increasing incorporation of added 14C-TdR. These findings demonstrate reciprocity of the salvage and the de novo pathways of thymine-DNA synthesis, presumably mediated by thymidine-triphosphate (dTTP), the common end product of both pathways, via feedback inhibition. In patients with folate and/or vitamin B12 deficiency, the addition of appropriate vitamins to marrow and lymphocyte cultures enhanced the incorporation of 3H-dU into DNA. As predicted, this was not observed in normal subjects. The enhancing effect of these vitamins on in vitro incorporation of 3H-dU into DNA by deficient cell systems was similar to their correcting effect on abnormal dU suppression. These findings support the theoretical concept that the dU suppression test defines biochemical megaloblastosis due to deficiency of folate and vitamin B12.

Newer aspects of pernicious anemia

Although readily treatable with vitamin B12, pernicious anemia continues to captivate investigative endeavors of those interested in the pathophysiology and pathogenesis of this disorder. Notable advances have been made in understanding properties of intrinsic factor, vitamin B12-binding proteins, structure and de novo synthesis of vitamin B12, mechanism of action of vitamin B12-dependent enzymes in man, and metabolic consequences of reduced activities of these enzymes in pernicious anemia. Similarly, newer morphological observations have given information regarding pathogenesis of some of the cytological abnormalities found in megaloblasts, and recent cytochemical studies have shed light on abnormalities of nuclear and cytoplasmic constituents in vitamin B12-deficient cells. Both cellular and humoral factors may contribute to immune-mediated processes in pernicious anemia, although as yet, it has not been established with certainty that pernicious anemia is an autoimmune disorder. As we look ahead, it will be important to define the process or processes responsible for atrophic gastritis, which is the pathophysiological basis of pernicious anemia. Likewise, advances in biophysics used in the study of cell membranes, cell surface phenomena, and metallic ion transport may find applicability in the study of pernicious anemia and perhaps provide further insights into metabolic abnormalities responsible for the development of megaloblastosis.

DNA synthesis in isolated lymphocyte nuclei. Effects of megaloblastic anaemia due to folate or vitamin B-12 deficiency or antimetabolite drugs

DNA synthesis has been studied in nuclei isolated from phytohaemagglutinin-stimulated lymphocytes from normal subjects and patients with megaloblastic anaemia. Lymphocytes were incubated for 72 h, nuclei isolated and incorporation of tritiated deoxythymidine triphosphate ([3H]TTP) into DNA measured, usually over a 10 min incubation period. Preincubation of normal phytohaemagglutinin-stimulated lymphocytes with methotrexate (1 - 10(-5) M, 48--72 h), 5-fluorouracil (1 - 10(-6) M, 70--72 h), and 1-beta-D-arabinofuranosyl cytosine (cytosine arabinoside) (4 - 10(-5) M, 71--72 h) caused a mean rise in [3H]TTP incorporation of 1.7 (P less than 0.01), 1.7 (P less than 0.05) and 2.4 (P less than 0.0025) fold, respectively. Hydroxyurea (3 - 10(-4) M, 48--72 h) in two experiments caused a mean increase of 1.6 fold. Untreated vitamin B-12- and folate-deficient cells showed a 2.0-fold (P less than 0.05) increase above the incorporation when the deficiencies were corrected by addition of vitamin B-12 and folic acid between 0 and 72 h in vitro. The mean percentages of the incorporation due to ATP-independent synthesis in nuclei from normal untreated cells, 5-fluorouracil-treated, cytosine arabinoside treated and vitamin B-12- or folate-deficient cells were 56 +/- 7% S.E., 41 +/- 7%, 84 +/- 3% and 28 +/- 6%, respectively. 5-Fluorouracil caused a two-fold increase in the cytoplasmic fraction of DNA polymerase when added to phytohaemagglutinin-stimulated lymphocytes between 48 and 72 h of culture but had no significant effect when added between 70 and 72 h.

Cobalamin dependent methionine synthesis and methyl-folate-trap in human vitamin B12 deficiency

The activity of methionine synthetase (MS) is important for the rapid growth of human haematopoietic cells and cultured lymphoblastoid cells. The MS reaction is the only known metabolic step in which both vitamin B12 and folate are essential in a single enzyme reaction. In vitamin B12 deficiency the MS activity in bone marrow cells is significantly lower than that in normal bone marrow. Free tetrahydrofolic acid (H4PteGlu) is normally liberated from its metabolically inactive storage form, 5-methyl-H4PteGlu (CH3H4PteGlu), in the cobalamin-dependent MS reaction. Thus, in vitamin B12 deficiency H4PteGlu is not available in sufficient concentration to maintain the de novo synthesis of thymidylate and purines, and accords with the methyl-folate-trap hypothesis. After treatment with amethopterin (Methotrexate), the incorporation of 3H-deoxyuridine into cellular DNA is reduced. In proliferating normal cells this effect of methotrexate can be prevented (and the cells rescued) with CH3-H4PteGlu or with CHO-H4PteGlu (5-formyl-H4PteGlu; Leucovorin). On the other hand, in vitamin B12 deficient bone marrow cells this so-called rescue-effect could only be achieved with CHO-H4PteGlu and not with CH3-H4PteGlu. These observations also support the hypothesis of the methyl-folate-trap in vitamin B12 deficiency. Decreased MS activity in vitamin B12 deficiency seems to be the essential metabolic fault, which is responsible for secondary alterations of folate metabolims. Thus, measurement of MS activity may allow direct functional assessment of vitamin B12 deficiency, at least with regard to DNA metabolism.

An investigation into the development and fate of neutrophil giant metamyelocytes using the techniques of electron microscopy and high resolution autoradiography

The neutrophil giant metamyelocytes present in vitamin B12-and folate-deficient patients were studied using the techniques of electron microscopy and electron microscope autoradiography. The ultrastructural features of the cytoplasm of a proportion of these cells resembled those of promyelocytes and myelocytes, particularly with respect to the types of neutrophil granules present. This finding suggests that the giant metamyelocytes result from an abnormal type of development in promyelocytes and myelocytes which have been arrested or retarded in their progress through the cell cycle. The hypothesis that giant metamyelocytes eventually die within the marrow was supported by the observations that a significant proportion of these cells contain intracytoplasmic autophagic vacuoles, that some giant metamyelocytes suffer from a marked depression of RNA and protein synthesis and that degenerating giant metamyelocytes can be recognized within the cytoplasm of some bone marrow macrophages.

Thymidine kinase in megaloblastic anaemia

Thymidine kinase has been measured in phytohaemagglutinin (PHA)-stimulated lymphocytes from 13 normal subjects and eight patients with megaloblastic anaemia. The levels in normal subjects ranged from 0.20 to 2.10 units/mg protein (mean 0.903 units/mg protein) and in megaloblastic anaemia from 2.99 to 9.97 units/mg protein). All the patients showed raised levels of the enzyme which were partly but not completely reduced to normal by addition of folic acid in vitro. Vitamin B12 in vitro had a lowering effect in the five vitamin-B12-deficient patients and two patients with combined deficiencies but not in one 'pure' folate-deficient patient. Thymidine kinase activity was highest in the cells of the least anaemic patients, suggesting that the degree of anaemia in megaloblastic anaemia may be determined in part by the ability of the cells to utilize thymidine by the 'salvage' pathway when the de novo pathway of thymidylate synthesis is failing. The rise in thymidine kinase activity in megaloblastic anaemia is presumably due to induction of the enzyme. Addition of methotrexate or 5-fluorouracil, drugs known to inhibit de novo thymidylate synthesis, caused an increase in thymidine kinase activity in normal PHA-stimulated lymphocytes after 24 h (but not after 1 h) which could be completely blocked by addition of puromycin. Thymidine mono- and di-phosphate kinases were also measured in normal PHA-stimulated lymphocytes. The activities were substantially higher than that of thymidine kinase and their activities were unaffected by methotrexate addition.

The bone marrow colony forming cell in megaloblastic anaemia and iron deficiency anaemia

Bone marrow samples from patients with megaloblastosis and iron deficiency have been assayed for their content of in vitro colony forming cells (CFC), and compared with a group of normal patients. The concentration of these cells was found to be significantly increased in the megaloblastic group, while their content in the iron deficient patients was slightly higher than the controls. An in vitro thymidine suicide procedures was utilised to assay the proportion of CFC in the S phase of the cycle. This was found to be increased in the megaloblastic group and only slightly increased in the iron deficient group. The findings in megaloblastosis seem to be consistent with the concept of impaired DNA synthesis. As the CFC monitors an early granulocytic progenitor these data suggest some impairment in DNA synthesis or an abnormal increase in amplification in this myeloid stem cell compartment. Such alterations in granulopoietic proliferation may contribute to the ineffective granulopoiesis of megaloblastosis and accordingly may be an important factor in the development of neutropenia sometimes associated with this condition. The slightly increased CFC concentration and altered cell cycle status found in iron deficiency suggest that iron is not a major requirement for granulopoiesis.

Thymidine kinase activity in human bone marrow cells

The thymidine kinase activity per 10(6) DNA-synthesising marrow cells and the rate of incorporation of tritiated thymidine into the DNA of 10(3) DNA-synthesising marrow cells were estimated in 9 haematologically normal patients and 49 patients suffering from a variety of haematological disorders. Slight increases in thymidine kinase activity were found in 6 of the 31 patients with haematological diseases associated with normoblastic erythropoiesis and greater increases were found in 3 of the 18 patients with megaloblastic haemopoiesis due to vitamin B12 or folate deficiency. In the latter group, there was a statistically significant inverse correlation between haemoglobin levels and thymidine kinase activity. No correlation was found between thymidine kinase activity and the rate of incorporation of tritiated thymidine in either the normoblastic or megaloblastic group, suggesting that the level of thymidine kinase activity does not limit the rate of incorporation of exogenously supplied thymidine into the DNA of human bone marrow cells.