Hydroxyurea increases plasma concentrations of microparticles and reduces coagulation activation and fibrinolysis in patients with sickle cell anemia.

Microparticles (MPs) are present in healthy subjects and their concentration increases in patients at high risk of thrombosis. We evaluated 10 patients with sickle cell anemia (SCA) treated with hydroxyurea (HU) and 13 SCA patients without this treatment. MP concentrations were determined by flow cytometry. Coagulation was evaluated using the thrombin-antithrombin complex (TAT) and D-dimers. Total MP concentrations were increased in the HU-treated group (265 × 10(6)/ml vs. 67.45 × 10(6)/ml; p = 0.0026), as well as MPs derived from RBC (67.83 × 10(6)/ml vs. 26.31 × 10(6)/ml; p = 0.05), monocytes (51.31 × 10(6)/ml vs. 9.03 × 10(6)/ml; p = 0.0084), monocytes with tissue factor (TF) expression (2.27 × 10(6)/ml vs. 0.27 × 10(6)/ml; p = 0.0058), endothelium (49.42 × 10(6)/ml vs. 7.23 × 10(6)/ml; p = 0.007) and endothelium with TF (1.42 × 10(6)/ml vs. 0.26 × 10(6)/ml; p = 0.0043). Furthermore, the concentrations of TAT (7.56 vs. 10.98 µg/l; p = 0.014) and D-dimers (0.65 vs. 1.29 µg/ml; p = 0.007) were reduced with HU. The MP elevation may suggest a direct cytotoxic effect of HU. Another explanation is a cell surface increase secondary to a megaloblastic process, resulting in increased vesicle release. In our opinion, the known benefits of HU on SCA patients, along with the reduction in coagulation activation, surpass its potential detrimental effect on MPs. Future studies should elucidate the role of MPs and demonstrate their significance in different contexts.

On the pathogenesis of erythroleukaemia (H0493).

In erythroleukaemia megaloblastic changes can co-exist with leukaemic changes in the marrow. The cause of the disease must therefore be such as can cause megaloblastosis and at the same time be mutagenic. Failure of the thymidylate synthelase reaction, the commonest cause of megaloblastic anaemia, can be eliminated in erythroleukaemia because (a) the dU suppression test is normal in the disease and (b) failure of the thymidylate synthelase reaction is not mutagenic. The deamination of both cytosine and adenine is mutagenic but the deamination of cytosine alone is apparent and the nucleotide of cytosine is the prime mutagenic nucleotide in leukaemia and cancer. Megaloblastic changes can result from an inadequate supply of any one of the four nucleotides that enter into the composition of DNA and it is suggested that an inadequate supply of the mutagenic nucleotide of cytosine, possibly through impaired synthesis, could cause both the megaloblastic and leukaemic changes in erythroleukaemia.

Bone marrow cells from vitamin B12- and folate-deficient patients misincorporate uracil into DNA.

Bone marrow cells from 15 patients with normal deoxyuridine (dU) suppression test results, 3 healthy subjects, and 11 patients with megaloblastic anemia caused by vitamin B12 or folate deficiency were examined for misincorporation of uracil into DNA. Cells were incubated with [5-3H] uridine for 2 hours and their DNA extracted. The DNA was hydrolyzed to deoxyribonucleosides with DNase 1, phosphodiesterase and alkaline phosphatase, and any dU present was separated from other deoxyribonucleosides by Aminex A6 chromatography. The quantity of dU/mg DNA and the radioactivity in the dU peak/mg DNA were then calculated. The results clearly showed that there was markedly increased uracil misincorporation into the DNA of vitamin B12- or folate-deficient marrow cells. Misincorporation of uracil into DNA may be an important biochemical lesion underlying both the megaloblastic change and the ineffectiveness of hematopoiesis in vitamin B12 and folate deficiency.

[The characteristics of iron metabolism in B12-deficient anemias (an evaluation of the functional status of megaloblastic erythropoiesis)]. / Osobennosti obmena zheleza pri B12-defitsitnykh anemiiakh (k otsenke funktsional'nogo sostoianiia megaloblastnogo éritropoéza).

The review of the literature data and the evidence obtained on 85 own patients with B12-deficiency anemia concerning iron metabolism underlie 3 variants proposed by the authors: pronounced manifestations of secondary sideroachresia before B12 therapy, utilization iron deficiency upon B12 introduction, dimorphic anemia (development of megaloblastic erythropoiesis in prior iron deficiency). The causes of megaloblastic erythropoiesis incompetence (slow rate, hemolytic component, secondary sideroachresia) are discussed.

Correction of the DNA synthesis defect in vitamin B12 deficiency by tetrahydrofolate: evidence in favour of the methyl-folate trap hypothesis as the cause of megaloblastic anaemia in vitamin B12 deficiency.

The critical disturbance of folate metabolism caused by vitamin B12 deficiency which results in megaloblastic anaemia remains controversial. Vitamin B12 is required in the methionine synthase reaction in which homocysteine is converted to methionine and methyl tetrahydrofolate (methyl THF) to THF. The 'methyl-folate trap' hypothesis suggested that failure of demethylation of methyl THF with consequent deficiency of folate co-enzymes derived from THF is the crucial lesion caused by vitamin B12 deficiency. A more recent theory suggested that reduced supply of methionine leads to reduced availability of 'activated formate' and hence of formyl THF and it is this defect that results in failure of folate co-enzyme synthesis. The present results, based on deoxyuridine suppression tests on 103 cases of megaloblastic anaemia, show that THF itself is equally capable of correcting the failure of thymidylate synthesis in vitamin B12 deficiency as in folate deficiency. Although not as effective as formyl THF in correcting the dU blocking test in vitamin B12 deficiency, this is equally so for the correction of the test by THF compared with formyl THF in folate deficiency. The results therefore favour the theory that it is in the supply of THF and not of 'active formate' or formyl THF that vitamin B12 plays a critical role in folate metabolism.

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

The de novo pathway of thymidylate synthesis (i.e., methylation of dUMP to dTMP) is directly folate dependent and indirectly vitamin B12 (cobalamins) dependent. In deficiency of these vitamins, this pathway is impaired, and exogenous deoxyuridine (dU) fails to suppress adequately in vitro incorporation of [3H]thymidine (3H-TdR) into DNA via the salvage pathway (i.e., abnormal dU suppression). This abnormality is corrected by the addition of folate compounds (analogues) and/or vitamin B12 depending on the nature of the underlying deficiency. We studied the effects of addition of PteGlu, 5-methyl THF (5-CH3-FH4), 5-formyl-THF (5-CHO-FH4), and hydroxy-cobalamin (OH-cbl) on 3H-TdR incorporation into DNA and thymidine kinase activity (salvage pathway), and on [3H]deoxyuridine (3H-dU) incorporation and dU suppression values (de novo pathway) in cultures of normal and megaloblastic bone marrows. The results showed that 3H-TdR incorporation into DNA and the salvage enzyme, thymidine kinase, activity were greater and 3H-dU incorporation into DNA less in megaloblastic cells as compared with normal cells. The addition of folates significantly reduced 3H-TdR incorporation and thymidine kinase activity and enhanced 3H-dU incorporation in folate and vitamin B12-deficient cells except that 5-CH3-FH4 had no effect on vitamin B12-deficient cells. None of these additives had any significant effect on normal cells. This study also showed that the addition of the deficient vitamin(s) to the "control tubes" in the dU suppression test is inappropriate, as these vitamins may at least partially correct the defect in cellular DNA synthesis caused by the deficiencies of these vitamins and may mask these deficiencies in the results of the in vitro correction of the dU suppression abnormalities in mild cases of megaloblastic anemia.

Thymidylate synthesis and utilization via the de novo pathway in normal and megaloblastic human bone marrow cells.

We have measured the thymidylate synthetase activity of intact bone marrow cells using a 3H2O release assay. The mean thymidylate synthetase activity of vitamin B12- or folate-deficient megaloblastic marrow cells was reduced only in severely anaemic patients. There was a correlation between thymidylate synthetase activity and RBC in patients with megaloblastic haemopoiesis. The mean rate of incorporation into DNA of 6-3H deoxyuridine was similar in megaloblastic and normoblastic marrows. The rate of thymidylate synthesis exceeded its incorporation into DNA in all marrows, and the mean ratio between synthesis and incorporation was similar in normoblastic and megaloblastic patients, being independent of both thymidylate synthetase activity and RBC. Thus de novo thymine nucleotides were not utilized more efficiently in megaloblastic marrow cells. These data suggest that impaired thymidylate synthesis may not be the central defect in megaloblastic haemopoiesis, and that there is only a single pool of thymidine triphosphate in human bone marrow cells.

Megaloblastic change is a feature of colonies derived from an early erythroid progenitor (BFU-E) stimulated by monocytes in culture.

The morphology of stained preparations of cells from human bone marrow and peripheral blood erythroid colonies cultured in methylcellulose, were examined by light microscopy. Although the morphology of 7 d erythroid colonies (CFU-E) was largely normoblastic, bone marrow and peripheral blood erythroid burst (BFU-E) showed a variable degree of megaloblastic and culture system and the deoxyuridine suppression test demonstrated active thymidine synthesis. Megaloblastic morphology was correlated with the growth induced by the addition of monocytes to erythroid progenitors. It was concluded that megaloblastosis was a feature of the erythroblasts derived from an early BFU-E which required monocytes for their development.

Metabolism of thymine nucleotides synthesized via the 'de novo' mechanism in normal, megaloblastic and methotrexate-treated human cells and in a lymphoblastoid cell line.

Human bone-marrow cells and lymphocytes were incubated with [3H]deoxyuridine (dU) to study the metabolism of thymine nucleotides labelled via the thymidylate synthase (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1.45) step of the 'de novo' biosynthetic pathway. (1) Continuous labelling with [3H]dU was used to compare incorporation of label into DNA with the specific radioactivities of thymine nucleotides separated by paper chromatography. (2) Cells were also labelled with [3H]dU at 13 degrees C, and 'chased' in unlabelled medium at 37 degrees C in order to quantify the proportion of thymine nucleotides incorporated into DNA and the proportion degraded. Only 40% of labelled thymine nucleotides were incorporated into lymphocyte DNA during a 'chase', whereas 100% were incorporated by MOLT 4 cells (a lymphoblastoid cell line of thymic origin, Thy-ALL line). Unincorporated nucleotides were rapidly degraded in lymphocytes, but degradative activity was very low in MOLT 4 cells. The results described here reinforce our previous conclusions [Taheri, Wickremasinghe & Hoffbrand (1981) Biochem. J. 194, 451-461] that there is a single thymine nucleotide compartment in Thy-ALL cells, but at least two pools in lymphocytes and bone-marrow cells. This compartmentation of nucleotides in human cells is consistent with a model which proposes that deoxyribonucleotides are localized near replication forks by the activity of multienzyme complexes [Mathews, North & Reddy (1978) Adv. Enz. Regul. 17, 133-156]. Our results also suggest that thymine nucleotides derived by the 'de novo' mechanism may be more highly localized than those derived by salvage. In cells from patients with megaloblastic anaemia owing to deficiency of vitamin B12 or folate or in normal cells treated with methotrexate, there was a massive accumulation of labelled dUMP and decreased incorporation of label into DNA. There was no measurable incorporation of labelled deoxyuridine residues into DNA of megaloblastic cells, but deoxyuridine residues were detected in DNA of cells treated with methotrexate.

Erythremia with special reference to sideroblastic anemia.

An autopsy case of erythremia with sideroblastic tumor cell proliferation is described. A 60-year-old man was admitted to the hospital due to general fatigue and anorexia. Bone marrow aspiration revealed abnormalities in erythropoiesis (megaloblasts, 4%; sideroblasts, 84%; ring-formed, 39%, and PAS-positive, 5%). Therapy was directed to pulmonary tuberculosis. Anemia was not improved despite repeated whole blood and platelet transfusions. Serum iron and percentage saturation of the total iron-binding capacity rose during the course. Administration of vitamin B12, B6 or folic acid was inefffective. INAH was replaced by its derivative, IHMS, during the course, but the population of sideroblasts especially of ring-sideroblasts was invariably large (78%-100% and 39%-65% for total sideroblasts and ring-sideroblasts, respectively). He died with increasing abdominal pain and jaundice after three months' hospitalization. Main autopsy findings were: diffuse proliferation of atypical erythroblasts in the bone marrow, systemic lymph nodes, liver, spleen and kidneys. Most of the cells positively stained with iron. Tuberculosis of lungs with cavity formation. Discussion is focussed on the relationship between erythremia and sideroblastic anemia.

The effect of homocysteine, methionine, serine and glycine on DNA synthesis by human normoblastic and megaloblastic bone marrow cells.

Both glycine and methionine, when added to a suspension of human bone marrow cells, impaired the utilization of deoxyuridine for DNA synthesis, using either the uptake of 3H-deoxyuridine or the subsequent uptake of 3H-thymidine as an index. Homocysteine reduced the uptake of both 3H-deoxyuridine and 3H-thymidine, indicating interference with DNA synthesis after the stage of thymidylate synthesis. Another explanation that the decreased uptake of both substances by homocysteine was due to cell damage caused in vitro was suggested by the trypan blue viability test. Serine generally did not produce significant effects. No difference could be detected between the results in normoblastic and megaloblastic marrow.

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.

Periodic acid-schiff-positive megaloblasts in pernicious anemia.

Periodic acid-Schiff-positive material representing glycogen was found in bone marrow megaloblasts at all stages of maturation from 11 patients with severe untreated pernicious anemia. This accumulation of glycogen in megaloblasts, which are known to have disordered biosynthesis of DNA, RNA, and histones, suggests that carbohydrate metabolism may also be abnormal in vitamin B12 deficiency, perhaps as a result of an abnormality in the enzyme amylophosphorylase.

Detection of methionine in pernicious anemia megaloblasts and other types of erythroid precursors.

Utilizing a bacterial-agar overlay technic incorporating the methionine-requiring bacterium Leukonostoc mesenteroides, little or no bacterial growth was seen surrounding the megaloblasts and proerythroblasts of eight patients who had severe untreated pernicious anemia. Similarly, scant bacterial growth was observed in five cases of chronic erythremic myelosis (DiGuglielmo syndrome). Heavy bacterial growth, indicating ample amounts of methionine, was seen in two cases of autoimmune hemolytic anemia, and in two cases of severe untreated folate-deficiency anemia. The results are consistent with the "methyltetrahydrofolate trap" hypothesis in pernicious anemia, in which a defect in the methylcobalamin-dependent methyltransferase leads to reduced amounts of methionine. These studies also suggest that a similar methyltransferase defect does not occur in folate deficiency or autoimmune hemolytic anemia. The generation of methionine, as estimated by the present technic, may also be defective in chronic erythremic myelosis.

Proliferation of ineffective erythropoiesis with nuclear abnormalities and megaloblastoid appearance in preleukaemia.

Erythropoiesis of two patients in the early stage of acute leukaemia and two patients with refractory anaemia and hypercellular bone marrow (preleukaemia?) was studied with the cytophotometric-autoradiographic method. Megaloblastoid erythroblasts show a decreased proliferative activity in comparison to the morphologically normal cells and are in the maturation stage of E4 (early polychromatic normoblasts) mainly in the G1-phase; they are therefore largely comparable to the megaloblastoid erythroblasts in erythroleukaemia. Erythroblasts in preleukaemia with nuclear abnormalities are found in a high percentage in the G2-phase or are unlabelled with a DNA content of between diploid and tetraploid value. They show a similar proliferative behaviour to the megaloblasts in pernicious anaemia. Early polychromatic erythroblasts arrested in G2-phase can differentiate without mitosis into tetraploid mature erythroblasts (E5). They can divide elsewhere endomitotically, produce binucleated E5 or take up the DNA synthesis and become polyploid. E4 with nuclear abnormalities do not proliferate and are mainly found in the premitotic phase.