MEGALOBLASTIC ANEMIAS COBALAMIN (VITAMIN B12) AND FOLATE DEFICIENCIES Prof. Dr. Sami Kartı

Introduction (I) The megaloblastic anemias result from interference in DNA synthesis The process interfering DNA synthesis affects not only developing erythroblasts, but also the granulocyte precursors, megakaryocytes, the lining of the GI tract, and other replicating cells throughout the body The most common causes of megaloblastic anemia are deficiencies of cobalamin (vitamin B12) and folic acid A similar process also occurs in patients on chemotherapy drugs that interfere with DNA synthesis, some nucleoside analogues used to treat HIV infection, and sometimes with medications that interfere with folate metabolism (antifolate drugs such as methotrexate, sulfa antibiotics, trimethoprim) Cobalamin deficiency can also be associated with neurologic abnormalities It is important to differentiate between megaloblastic anemias and macrocytic (non megaloblastic) anemias. Both are associated with an increase in red cell size (increased MCV)

Introduction (II) Causes of macrocytic anemia include liver disease, reticulocytosis, myelodysplasia, hypothyroidism and alcoholism. The MCV is usually only mildly elevated in most macrocytic anemias (~100–110 fL) but can be dramatically increased in the megaloblastic anemias (≥130 fL) Megaloblastic anemia is caused by interference with DNA synthesis, usually due to a deficiency of cobalamin or folic acid Pernicious anemia is megaloblastic anemia due to autoimmune chronic gastritis with destruction of the gastric parietal cells. Pernicious anemia is not synonymous with megaloblastic anemia; it is a subset of megaloblastic anemias Vitamin B12, strictly speaking, is cyanocobalamin; cobalamin is the generic term for members of the cobalamin family. Cyanocobalamin is an artifact of manufacturing, not a natural form, but it is used therapeutically. The terms vitamin B12 and cobalamin are used interchangeably

Folic Acid (I) Folic acid (folate) is actually pteroylglutamic acid. Folic acid is required for the transfer of one-carbon fragments such as methyl groups in numerous chemical reactions. Folic acid is synthesized by higher plants and microorganisms It is abundant in vegetables, fruit, cereals, and dairy products Folic acid is heat labile, and much is destroyed by cooking. Therefore, the primary dietary source for folic acid is fresh uncooked fruits and vegetables It is primarily absorbed in the jejunum. The daily requirement is ~50 μg The body has stores of approximately 5 to 10 mg, primarily in the liver

Folic Acid (II) An enterohepatic circulation is required for redistribution of hepatic folate stores to the rest of the body Folate stores can be exhausted in a few weeks to a few months (much faster if the enterohepatic circulation is disrupted) Pregnancy is worth special mention here. It is believed that folate deficiency during pregnancy predisposes the fetus to neural tube defects and that folate supplementation will reduce the risk of such defects Every woman who is pregnant, or who is considering becoming pregnant, should take folic acid

Cobalamin (I) The primary dietary sources of cobalamin are meat, eggs, milk, and cheese The daily requirement is ~0.1 μg/day. The average American diet contains ~5 to15 μg/day Body stores (primarily in the liver) contain 2 to 4 mg (2,000–4,000 μg). Thus, body stores can supply daily needs for many years Only strict vegans who consume no eggs, fish, cheese, or other dairy products are at risk of insufficient dietary intake Absorption of cobalamin from the GI tract is a multistep process, with several places for possible problems. Absorption requires intrinsic factor (IF), which binds to B12. There are specific receptors for the IF-B12 complex in the terminal ileum. B12 bound to IF is efficiently absorbed, but very little unbound B12 can be absorbed.

Cobalamin (II) Intrinsic factor is produced by gastric parietal cells, which also produce gastric acid. Dietary B12 is bound to proteins and must first be released by gastric acid and proteases. The steps in cobalamin absorption are as follows: –B12 is digested off from food protein by pepsin and gastric acid –The released B12 is then bound to B12-binding proteins (R proteins) produced by the salivary glands, which blocks binding of IF –The B12 is released from the R proteins by pancreatic enzymes, allowing IF to bind to the B12 –The B12-IF complex is absorbed in the terminal ileum Cobalamin circulates in the blood bound to proteins called transcobalamins The physiologically important transcobalamin is designated transcobalamin II (TCII); there are receptors on cell surfaces for the TCII-B12 complex There is an enterohepatic circulation of cobalamin, similar to that of folate

Pernicious anemia (I) Pernicious anemia is an autoimmune chronic gastritis, resulting in destruction of the parietal cells and loss of IF production It occurs in all ethnic groups, although the highest incidence appears to be in persons of Scandinavian, English, Scottish, and Irish descent In Caucasians, the average age of onset is about 60 years, although it can be seen at all ages, including children There is a familial predisposition to pernicious anemia

Pernicious anemia (II) There is also a strong association between pernicious anemia and other autoimmune disorders, including thyroid disease (Graves’ disease, Hashimoto’s thyroiditis), Addison’s disease, vitiligo, and hypoparathyroidism Patients with pernicious anemia may have serum antibodies against gastric parietal cells (anti–parietal cell antibodies) or antibodies against intrinsic factor (anti-IF antibodies) Patients with pernicious anemia have an increased risk of gastric carcinoma compared to the general population; the increase in risk is significant, but the overall risk for the individual patient is low

Pathophysiology of Megaloblastic Anemia

Neurologic Disease in Cobalamin Deficiency (I) Cobalamin deficiency may be complicated by neurologic disease, a feature that separates cobalamin from folate deficiencies The degree of neurologic disorder does not correlate with the degree of anemia, and patients may have severe neurologic disease without significant hematologic abnormalities The primary neuropathologic change in cobalamin deficiency is demyelination of the dorsal and lateral columns of the spinal cord and the cerebral cortex Both sensory and motor systems are affected, leading to the terms subacute combined degeneration and combined system disease The earliest and most common symptom is paresthesias in the distal extremities

Neurologic Disease in Cobalamin Deficiency (II) The earliest changes seen on physical examination are decreased vibration and position sensation in the extremities. These symptoms progress to weakness, clumsiness, and an unsteady gait In severe disease, the patient may have severe weakness and spasticity In more advanced cases, the patients may have hyperreflexia, clonus, and positive Romberg and Babinski signs Early cerebral signs include depression and impairment of memory More severe cortical changes include delusions, hallucinations, and paranoid and schizophrenic states (megaloblastic madness); however, these are uncommon

Schilling Test (I) The standard method to diagnose pernicious anemia, once cobalamin deficiency is confirmed, is the Schilling test At first a large dose of unlabeled B12 is given intramuscularly, (The purpose of the intramuscular “cold” B12 is to saturate the B12-binding sites in the serum, and thereby flush all of the orally absorbed B12 into the urine, where it can be measured) At the same time as IM Vit. B12, radiolabeled cobalamin is given orally, and urine is collected for 24 hours The amount of radioactivity in the urine indicates how much B12 was absorbed orally

Schilling Test (II) Typically, recovery of <6% in the urine indicates malabsorption of B12 If the initial value is abnormal, a second stage is performed in which intrinsic factor is given together with the labeled B12 An increase in the amount of B12 absorbed during the second stage of the Schilling test indicates pernicious anemia Some experts believe that there is no reason to do a Schilling test in cobalamin deficiency; they presume that pernicious anemia is the most likely diagnosis, and treatment of cobalamin deficiency of any cause is the same.

Treatment of Megaloblastic Anemia (I) It is critical to determine whether the deficiency is due to folic acid or cobalamin; giving the wrong treatment is ineffective and potentially dangerous Oral folate supplementation is the treatment of choice for most cases of folate deficiency. One 0.4 or 1.0 mg (400 or 1,000 μg) tablet daily should be adequate. Parenteral therapy may be required for rare cases of folate malabsorption Treatment of pernicious anemia and other causes of cobalamin malabsorption requires parenteral therapy Available preparations include cyanocobalamin and hydroxocobalamin

Treatment of Megaloblastic Anemia (II) A typical treatment regimen would be 1,000 μg (1 mg) of cyanocobalamin weekly for 5 weeks, followed by 1,000 μg monthly for life It is critical that once cobalamin deficiency is documented, therapy must be continued for life. Discontinuing therapy may result in the devastating neurologic complications of cobalamin deficiency The response to therapy is usually dramatic, with rapid symptomatic improvement. Reticulocytosis should appear after about 2 to 3 days, with a maximum response at 5 to 8 days. The hemoglobin should begin to rise after about 1 week, with normalization of the hemoglobin by 4 to 8 weeks The granulocyte count usually reaches a normal level within 1 week; hypersegmentation of neutrophils usually disappears within 2 weeks

Treatment of Megaloblastic Anemia (III) The platelet count usually returns to normal within 1 week The bone marrow shows dramatic improvement, with disappearance of megaloblasts within 1 to 2 days The serum LDH and bilirubin drop rapidly It is important to monitor the response to therapy; failure to respond appropriately indicates either an incorrect diagnosis or some complicating factor such as coexistent iron deficiency Treatment with high doses of folic acid can partially reverse the hematologic manifestations of cobalamin deficiency but will not help the neurologic disease In fact, treatment of cobalamin deficiency with folic acid may accelerate the progression of neurologic disease