Acquired Abnormality of Cobalamin Metabolism: Nitrous Oxide Inhalation
Nitrous oxide irreversibly oxidizes methylcobalamin to an inactive precursor; this inactivates methionine synthase. Megaloblastic anemia has occurred in patients undergoing prolonged N2O anesthesia (e.g., in intensive care units). A neuropathy resembling cobalamin neuropathy has also been described in dentists and anesthetists who are repeatedly exposed to N 2O. Methylmalonic aciduria does not occur as adocobalamin is not inactivated by N2O.
Antifolate Drugs A large number of epileptics, who are receiving long-term therapy with phenytoin or primidone, with or without barbiturates, develop low serum and red cell folate levels. The exact mechanism is unclear. Alcohol may also be a folate antagonist, as patients who are drinking spirits may develop megaloblastic anemia that will respond to normal quantities of dietary folate or to physiologic doses of folic acid only if alcohol is withdrawn. Macrocytosis of red cells is associated with chronic alcohol intake even when folate levels are normal.
Harrison's Internal Medicine Chapter 100. Megaloblastic Anemias
Megaloblastic Anemias: Introduction
The megaloblastic anemias are a group of disorders characterized by the presence of distinctive morphologic appearances of the developing red cells in the bone marrow. The cause is usually deficiency of either cobalamin (vitamin B 12) or folate, but megaloblastic anemia may arise because of genetic or acquired abnormalities affecting the metabolism of these vitamins or because of defects in DNA synthesis not related to cobalamin or folate (Table 100-1).
Biochemical Basis of Megaloblastic Anemia The common feature of all megaloblastic anemias is a defect in DNA synthesis that affects rapidly dividing cells in the bone marrow. All conditions that give rise to megaloblastic changes share in common a disparity in the rate of synthesis or availability of the four immediate precursors of DNA: the deoxyribonucleoside triphosphates (dNTPs): dA(adenine)TP and dG(guanine)TP (purines), dT(thymine)TP and dC(cytosine)TP (pyrimidines).
Table 100-4 Malabsorption of Cobalamin May Occur in the Following Conditions But Is Not Usually Sufficiently Severe and Prolonged to Cause Megaloblastic Anemia
Simple atrophic gastritis (food cobalamin malabsorption)
Gastric bypass surgery
Use of proton pump inhibitors
Deficiencies of cobalamin, folate, protein, ?riboflavin, ?nicotinic acid
Therapy with colchicine, para-aminosalicylate, neomycin, slow-release potas...
Tropical Sprue Nearly all patients with acute and subacute tropical sprue show malabsorption of cobalamin; this may persist as the principal abnormality in the chronic form of the disease, when the patient may present with megaloblastic anemia or neuropathy due to cobalamin deficiency. Absorption of cobalamin usually improves after antibiotic therapy and, in the early stages, folic acid therapy.
Fish Tapeworm Infestation
The fish tapeworm (Diphyllobothrium latum) lives in the small intestine of humans and accumulates cobalamin from food, rendering this unavailable for absorption.
Ineffective Hemopoiesis There is an accumulation of unconjugated bilirubin in plasma due to the death of nucleated red cells in the marrow (ineffective erythropoiesis). Other evidence for this includes raised urine urobilinogen, reduced haptoglobins and positive urine hemosiderin, and a raised serum lactate dehydrogenase. A weakly positive direct antiglobulin test due to complement can lead to a false diagnosis of autoimmune hemolytic anemia.
Causes of Cobalamin Deficiency
Cobalamin deficiency is usually due to malabsorption. The only other cause is inadequate dietary intake.
Folinic Acid (5-Formyl-Thf) This is a stable form of fully reduced folate. It is given orally or parenterally to overcome the toxic effects of methotrexate or other DHF reductase inhibitors.
Prophylactic Folic Acid
In many countries, food is fortified with folic acid (in grain or flour) to prevent neural tube defects. It is also used in chronic dialysis patients and in parenteral feeds.
IF is produced in the gastric parietal cells of the fundus and body of the stomach, and its secretion parallels that of hydrochloric acid. The IF-cobalamin complex passes to the ileum, where IF attaches to a specific receptor (cubilin) on the microvillus membrane of the enterocytes. Cubilin is also present in yolk sac and renal proximal tubular epithelium. Cubulin appears to traffic by means of amnionless (AMN), an endocytic receptor protein that directs sublocalization and endocytosis of cubulin with its ligand IF-cobalamin complex.
An underlying maternal folate metabolic abnormality has also been postulated. One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) (Fig. 100-1) caused by a common 677CÆT polymorphism in the MTHFR gene. In one study, the prevalence of this polymorphism was found to be higher in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% compared with 5% in control subjects. The polymorphism codes for a thermolabile form of MTHFR.
Nutritional Dietary folate deficiency is common. Indeed, in most patients with folate deficiency a nutritional element is present. Certain individuals are particularly prone to have diets containing inadequate amounts of folate (Table 100-5). In the United States and other countries where fortification of the diet with folic acid has been adopted, the prevalence of folate deficiency has dropped dramatically and is now almost restricted to high-risk groups with increased folate needs.
Serum Folate This is also measured by an ELISA technique. In most laboratories, the normal range is from 11 nmol/L (2.0 µg/L) to ~82 nmol/L (15 µg/L). The serum folate level is low in all folate-deficient patients. It also reflects recent diet. Because of this, serum folate may be low before there is hematologic or biochemical evidence of deficiency.
Most foods contain some folate. The highest concentrations are found in liver, yeast, spinach, other greens, and nuts (100 µg/100 g). The total folate content of an average Western diet is ~250 µg daily, but the amount varies widely according to the type of food eaten and the method of cooking. Folate is easily destroyed by heating, particularly in large volumes of water. Total-body folate in the adult is ~10 mg, the liver containing the largest store. Daily adult requirements are ~100 µg, so stores are only sufficient for 3–4 months in normal adults and severe folate deficiency may...
Psychiatric disturbance is common in both folate and cobalamin deficiencies. This, like the neuropathy, has been attributed to a failure of the synthesis of SAM, which is needed in methylation of biogenic amines (e.g., dopamine) as well as of proteins, phospholipids, and neurotransmitters in the brain (Fig. 100-1). Associations between lower serum folate or cobalamin levels and higher homocysteine levels and the development of Alzheimer's disease have been reported.
Serum Antibodies Two types of IF immunoglobulin G antibody may be found in the sera of patients with PA. One, the "blocking," or type I, antibody, prevents the combination of IF and cobalamin, whereas the "binding," or type II, antibody prevents attachment of IF to ileal mucosa. Type I occurs in the sera of ~55% of patients and type II in 35%. IF antibodies cross the placenta and may cause temporary IF deficiency in the newborn infant. Patients with PA also show cellmediated immunity to IF. Type I antibody has been detected rarely in the sera of patients without...
The megaloblastic anemias are a group of disorders characterized by the presence of distinctive morphologic appearances of the developing red cells in the bone marrow. The cause is usually deficiency of either cobalamin (vitamin B12) or folate, but megaloblastic anemia may arise because of genetic or acquired abnormalities affecting the metabolism of these vitamins or because of defects in DNA synthesis not related to cobalamin or folate (Table 100-1).
Bone Marrow The bone marrow is usually normal or hypercellular, but in 20% of cases it is sufficiently hypocellular to be confused with aplasia. No single characteristic feature of marrow morphology distinguishes MDS, but the following are commonly observed: dyserythropoietic changes (especially nuclear abnormalities) and ringed sideroblasts in the erythroid lineage; hypogranulation and hyposegmentation in granulocytic precursors, with an increase in myeloblasts; and megakaryocytes showing reduced numbers or disorganized nuclei.
Anemia occurs in ~80% of myeloma patients. It is usually normocytic and normochromic and related both to the replacement of normal marrow by expanding tumor cells and to the inhibition of hematopoiesis by factors made by the tumor. In addition, mild hemolysis may contribute to the anemia. A larger than expected fraction of patients may have megaloblastic anemia due to either folate or vitamin B12 deficiency. Granulocytopenia and thrombocytopenia are very rare.
T he science of human nutrition and its applications to health promotion continue to gain momentum. In
the relatively short time since the release of the first edition of this Encyclopedia, a few landmark
discoveries have had a dramatic multiplying effect over nutrition science: the mapping of the human genome,
the links between molecular bioenergetics and lifespan, the influence of nutrients on viral mutation, to name
Rogers syndrome is an autosomal recessive disorder result-ing in megaloblastic anemia, diabetes mellitus, and sensori-neural deafness. The gene associated with this disease
encodes for thiamine transporter 1 (THTR1), a member of
the SLC19 solute carrier family including THTR2 and the
reduced folate carrier (RFC). Using transient transfections
into NIH3T3 cells of a D93H mutant THTR1derived
from a Rogers syndrome family, we determined the
expression, post-translational modification, plasma mem-brane targeting and thiamine transport activity.