Megaloblastic Anaemia: Initiation of DNA Synthesis in Excess of DNA Chain Elongation as the Underlying Mechanism

DNA flow cytometry of control Euglena and cell cycle blockade of vitamin B12-starved cells

Vitamin B12 starvation in Euglena induces a cell cycle arrest that leads to unbalanced growth. Microfluorometry and flow cytometry analyses of cellular DNA fluorescence after Hoechst 33258 staining were performed on control and vitamin B12-deficient cells. Convergent results are obtained with both methods. Histograms that represent arrested cells are unimodal, with a mode channel value nearly twice that of the G1 control cell peak. Dispersion of fluorescence values is great, and values from 2C and over 4C are observed and discussed. It appears that vitamin B12 starvation in Euglena leads to defective DNA synthesis. Blocked cells have different DNA content, corresponding to blockade of DNA replication during the S phase. A second block prevents the onset of mitosis even for 4C cells.

DNA chain elongation rates in marrow cells from vitamin B12-deficient patients and methotrexate-treated mice

The DNA synthesized by marrow cells from patients with vitamin B12 deficiency and mice previously given methotrexate (MTX), has been investigated. Suspensions of bone marrow cells were pulse-labelled with [methyl-3H]thymidine or deoxy [5-3H]cytidine for 30 s and the radioactivity in the DNA was chased thereafter in the presence of 10 microM non-radioactive nucleoside for periods up to 60 min. The rates of elongation of new daughter strands were then assessed by hydroxyapatite chromatography of alkali-denatured DNA samples. No significant differences were found between the average rates of elongation of daughter strands from control marrow cells on the one hand and the vitamin B12-deficient or the methotrexate-affected cells on the other. This is to be contrasted with the results of previous studies which have shown a retardation in the rates of movement of replication forks in stimulated, cultured lymphocytes obtained from vitamin B12- or folate-deficient patients.

Vitamin B12 binders (transcobalamins) in serum

The measurement of plasma or serum vitamin B12 (B12) binders is important in the diagnosis of congenital megaloblastic anemias, the myeloproliferative disorders and perhaps in other malignancies. Two classes of binders circulate in plasma, Transcobalamin II (TC II) and R-binders. The latter have been divided by some authors into transcobalamin I (TC I) and transcobalamin III (TC III), and the validity of this division is discussed. R-binders and TC II differ in their apparent molecular weight on gel filtration chromatography, by which method they can be separated reliably. However, the technique is time-consuming and cumbersome and a variety of rapid separation methods have been described in the literature. These are discussed and compared to the standard gel filtration separation method. TC I and III are separable on the basis of their differing charges, and the methods which have been described for accomplishing this are compared and critically reviewed. There has been some controversy in the literature as to whether plasma or serum should be used to measure circulating B12 binders. Leukocytes, which contain R-binders, release these in vitro and this release is greatest when blood is allowed to clot and the serum separated. Consequently, the use of plasma, sometimes with agents added to prevent leukocytic release of binder, has been advocated. Yet, none of the agents used eliminates artifact totally, and this aspect, too, is reviewed. Lastly, several techniques for the purification of B12 binders have been described. Some techniques result in pure binder preparations, while others result in preparations which are free of other binders but contaminated with non-B12 binding proteins. Both approaches have advantages and disadvantages, and these are discussed.

Enzyme and membrane markers in leukaemia: recent developments

Terminal deoxynucleotidyl transferase (TdT) assay has proved a valuable test for distinguishing lymphoblastic from myeloblastic leukaemias, particularly in adults whose blast cells are often negative for the c-ALL antigen. The immunofluorescence assay, particularly when used in combination with antisera to surface membrane antigens, has proved a sensitive technique for detecting small numbers of lymphoblasts in extramedullary sites, for example, testis or cerebrospinal fluid, or of residual Thy-ALL blasts in the marrow, which might otherwise be difficult to recognise. Differences in concentration of several enzymes concerned in purine metabolism have been detected between the blast cells in the various acute leukaemias. Adenosine deaminase (ADA) concentrations tend to be higher in Thy-ALL than in other forms of leukaemia, but the wide overlap reduces the diagnostic value of this assay. Thy-ALL blasts, however, appear to be selectively and exquisitely susceptible to inhibition of ADA by the drug deoxycoformycin, which has now been used sucessfully in a number of other wise resistant patients with Thy-ALL to obtain a complete remission. The recently introduced technique for the production of monoclonal antibodies has substantially widened the reagents available for analysing the membrane characteristics of bone marrow stem cells and of cell lineages derived from them. These have revealed previously unsuspected heterogeneity among different cases of acute lymphoblastic leukaemia, for example, among Thy-ALL blasts from different patients, and they have also delineated minor populations of immature thymocytes from which these leukaemic cells are derived. The potential use of these antibodies to prevent graft-versus-host disease by selective removal of T-lymphocytes from donor bone marrow before allogeneic bone marrow transplantation, or to prevent recurrence of Thy-ALL and other lymphoblastic leukaemias or lymphomas by selective removal of leukaemic or lymphoma malignant cells before autologous transplantation, is reviewed.

Conversion of partially single-stranded replicating DNA to double-stranded DNA is delayed in megaloblastic anaemia

DNA from phytohaemagglutinin-stimulated lymphocytes which had been pulse-labelled for 1 min with [3H]deoxycytidine eluted as partially single-stranded DNA from columns of benzoylated napthoylated DEAE-cellulose. The label was transferred progressively into the double-stranded DNA fraction upon incubation in the presence of unlabelled deoxycytidine. The rate of transfer was slower in untreated lymphocytes from patients with megaloblastic anaemia than in corresponding control cells. A similar delay was also observed in normal lymphocytes treated with methotrexate or hydroxyurea. A close temporal correlation between the joining of Okazaki pieces (measured by alkaline sucrose gradients) and the transfer of the pulse label to double-stranded DNA suggested that the latter process represented the filling of gaps between Okazaki pieces. We suggest that this gap-filling step is retarded in megaloblastic anaemia and in cells treated with methotrexate or hydroxyurea.

Significance of phenotypic and chromosomal abnormalities in X-linked mental retardation (Martin-Bell or Renpenning syndrome)

With the exception of macro-orchidism, three families with X-linked mental retardation showed diagnostic concordance of clinical features among the affected males. Since macro-orchidism was a variable feature among the otherwise identically affected males in one family, we question the existence of a separate entity of X-linked mental retardation characterized only be testicular enlargement. The X chromosome marker of Lubs was expressed, under the culture conditions of Sutherland, in lymphocytes of the affected males of two families, one with and the other without megalotestes. Two affected members of the third family, with megalotestes, did not show the marker. Telomeric structural changes similar to the mar(X) (qter) formation were found on certain autosomes, notably, chromosome 6 in some of the affected males, potential and obligate carrier females, and in both related and unrelated normal males. These autosomal markers appear to represent a nonspecific response to either in vivo or in vitro folate deficiency. Caution against premature introduction of this test for prenatal diagnosis, in the face of current ignorance regarding diagnostic specificity, is urged.

Defective DNA synthesis in megaloblastic anaemia. Studies employing velocity sedimentation in alkaline sucrose gradients

1. Autoradiographic experiments revealed that the average size of the replicating unit (replicon) in human phytohaemagglutunin-stimulated lymphocytes is 45 (+/- 1.3) micron. 2. A 5 min pulse of [3H]thymidine labelled DNA chains of approximately 40 S (15 micron) in control lymphocytes as revealed by velocity sedimentation in alkaline sucrose density gradients. Upon chasing in the absence of [3H]-thymidine the labelled DNA increased in size. By 6 h the bulk of the label co-sedimented with full-sized chromosomal DNA. 3. In untreated lymphocytes from patients with megaloblastic anaemia due to vitamin B-12 or folate deficiency or lymphocytes treated with methotrexate (10(-5) M) or hydroxyurea (5 . 10(-4) M) the increase in size of pulse-labelled DNA was slower than in control cells. 4. The block in maturation of pulse-labelled DNA to bulk DNA was not permanent. At 24 h of chase 75-80% of the pulse-label in both control and megaloblastic lymphocytes co-sedimented with bulk DNA. 5. We conclude that the lesions seen in DNA synthesis in megaloblastic anaemia due to folate or vitamin B-12 deficiencies occur through impaired biosynthesis of nucleotide precursors of DNA. Possible explanations of why the defects in DNA synthesis cause altered morphology of proliferating cells in megaloblastic anaemia are suggested.

Altered erythrocyte pyrimidine activity in vitamin B12 or folate deficiency

The net activity of orotidylic pyrophosphorylase and decarboxylase, sequential enzymes which catalyse the formation of uridine monophosphate from orotic acid in de novo pyrimidine biosynthesis, has been evaluated in erythrocytes of patients with folate or cobalamin deficiency. In patients with normoblastic haemopoiesis and normal cobalamin and folate status a direct relationship exists between the maturity of the peripheral blood erythrocyte population, as indicated by G6PD activity, and net orotidylic activity. In contrast in cobalamin or folate deficiency this co-ordinate relationship is not observed and net orotidylic activity is relatively reduced. Fractionation of erythrocytes by centrifugation demonstrates that this inordinately low orotidylic activity consistently occurs in the young erythrocyte population and is reversed by specific replacement therapy. In vitamin B12 or folate deficiency an impressive array of evidence now exists to implicate altered folate metabolism for the observed alterations in purine and pyrimidine metabolism (Das & Herbert, 1976). Of these changes the cornerstone defect resulting in megaloblastic maturation is impaired methylation of deoxyuridine monophosphate to thymidine monophosphate (Hoffbrand et al, 1976). In this context the reduced serum uridine plus uracil levels in patients with vitamin B12 deficiency (Parry & Blackmore, 1976) and the haematological response of these patients to orotic acid therapy (Rundles & Brewer, 1958) are not readily explicable. Since the conversion of orotic acid to uridine monophosphate depends on the integrity of the coupled activities of orotidylic pyrophosphorylase and decarboxylase, this study has quantitated this capacity in peripheral blood erythrocytes in patients with vitamin B12 or folate deficiency.

Diagnosis and management of folate deficiency in low birthweight infants

Significant folate deficiency in 14 out of 37 preterm infants of birthweights 2.0 kg or less was found to be reliably and most conveniently diagnosed by abnormal morphological changes in peripheral blood and confirmed by the response to folic acid. Deficiency appeared to be more common in light-for-dates infants including the smaller of twins. Neither the clinical status nor the levels of haemoglobin or erythrocyte folate was a reliable guide to the presence of megaloblastic erythropoiesis in the very young preterm infant. 100-200 microgram folic acid a day, orally or IM, may be required to ensure an optimal haematological response, and this would be appropriate for therapeutic trial if the diagnosis is in doubt. This amount would also replenish tissue folate stores; larger doses are likely to exceed the requirements of small infants.

Mutant mouse L-cells: a model for megaloblastic anaemia

When temperature-sensitive (ts) mutant lines of mouse L-cells, ts AIS9 and ts CI, are shifted from 34C to 38.5C, a rapid inhibition of DNA synthesis and mitosis occurs. During this phase, cell and nuclear growth continues and results in a substantial increase in cell and nuclear volume. Such cellular modifications are also associated with a marked dispersal of the condensed chromatin masses of interphase nuclei, so that after 48-72 h of incubation at 38.5C, nuclear profiles of both ts cell lines bear a striking resemblance to the nuclear features characteristic of megaloblastic anaemia. Despite these marked alterations in nuclear chromatin organization, morphometric analysis indicates that the volume of condensed chromatin does not decrease. Current biochemical, cytological and morphometric data on the two ts lines of mutant mouse L-cells during expression of the mutation, suggest that they might provide a useful model to further elucidate cytological features of megaloblastic anaemia.