MACROCYTIC ANEMIASAnemias where the cells are BIG

MACROCYTIC ANEMIAS

Macrocytic anemias are characterized by large RBCs with a normal hemoglobin content.

Macrocytic anemias are classified as either megaloblastic or non-megaloblasticMegaloblastic anemias are associated with

defective DNA synthesis and therefore, abnormal RBC maturation in the bone marrow (a nuclear maturation defect). In megaloblastic anemias, the granulocytic and

megakaryocytic maturation are also affected and this leads to pancytopenia

MEGALOBLASTIC ANEMIAS

The biochemical basis for this is as follows: Megaloblastic dyspoiesis (abnormal

synthesis) occurs when the DNA synthesis in the hematopoietic system is disrupted or slowed down.

Other rapidly proliferating cells in the body are also affected.

Administration of drugs that interfere with DNA metabolism can be the cause of a megaloblastic anemia.

On rare occasions there is an inherited disorder that affects DNA synthesis

MEGALOBLASTIC ANEMIASHowever, the primary defect in DNA

replication is usually due to depletion of thymidine triphosphate which leads to retarded mitosis, and therefore retarded nuclear maturation. The depletion of thymidine triphosphate is usually

due to a deficiency of vitamin B12 or folic acid. Vitamin B12 deficiency is usually due to a

deficiency of a factor necessary for B12 absorption while a folic acid deficiency is usually due to an inadequate dietary intake.

Absorption and metabolism of vitamin B12 and folic acid: Vitamin B12 (cyanocobalamine)

Is found in meats, eggs, dairy products, and liver. Is absorbed via a binding protein called intrinsic

factor (IF)

MEGALOBLASTIC ANEMIAS Vitamin B12 and IF bind to mucosal cells in the ileum and

B12 enters. When B12 is released from the mucosal cell, it binds to

transport proteins in the bloodstream (transcobalamine IIII). Type II is the primary transport protein. Therefore a congenital deficiency in type II can lead to a megaloblastic anemia.

B12 is transported to the bone marrow for use or to the liver for storage.

VITAMIN B12 ABSORPTION

Mucosal cell

HOW IS B12 INVOLVED IN DNA SYNTHESIS?

Vitamin B12 is involved in the following reaction:

MEGALOBLASTIC ANEMIAS Decreased vitamin B12 therefore results in no conversion

of N5-methyl THF to THF. Thus, the net result is that dTMP for DNA synthesis is not made.

Humans need 3-5 ug of vitamin B12 per day. Since 70% of dietary B12 is absorbed, the diet needs to contain 5-7 ug/day.

MEGALOBLASTIC ANEMIAS Folic acid (pteroyl glutamic acid)

Is found in most foods, including eggs, milk, yeast, and liver.

Is abundant in green, leafy vegetables and is synthesized by many microorganisms.

Is destroyed by heat Absorption occurs in the small intestine

HOW IS FOLIC ACID INVOLVED IN DNA SYNTHESIS?

Is involved in the following reaction:

MEGALOBLASTIC ANEMIAS Therefore, with decreased folic acid, the net result is the

same as that for decreased vitamin B12 – there is decreased conversion of dUMP to dTMP, and thus, dTTP which is required for DNA synthesis.

Defective DNA synthesis may occur when dUTP gets used in place of dTTP because there is a great increase in erroneous DNA copying where dUTP is put in place of dTTP.

Humans need to get about 50 ug/day of folic acid from the diet

Causes of megaloblastic anemia Vitamin B12 deficiency

MEGALOBLASTIC ANEMIAS 85% is due to lack of intrinsic factor (IF) secondary to

gastric atrophy – this is called pernicious anemia (PA) and it occurs most often in older adults (over 40), most commonly in individuals of Northern European descent, and more commonly in women. Antibodies against the parietal cells in the G.I. tract and against IF are commonly found in these individuals.

After gastrectomy Malabsorption syndrome Inadequate dietary intake (rare) Defective or absent transport proteins (transcobalamine)

CAUSES OF B12 DEFICIENCY

MEGALOBLASTIC ANEMIAS Folic acid deficiency

Inadequate diet is the major cause and this occurs most often in the poor, elderly, and alcoholics.

When there is increased requirement such as in pregnancy, hemolytic anemia, leukemia, etc.

Malabsorption – may be due to diseases of the upper small intestine such as ileitis, tropical sprue, and gluten sensitive enteropathy.

When there is increased loss such as in patients undergoing hemodialysis

Drug inhibition – examples are oral contraceptives, anticoagulant drugs, alcohol, and isoniazid

MEGALOBLASTIC ANEMIAS Clinical manifestations occur in two categories –

those found in folic acid or vitamin B12 deficiency, and those mainly found in B12 deficiency In both types of deficiency the symptoms include

pallor, weakness, lightheadedness, a smooth, sore tongue, and diarrhea alternating with constipation

In vitamin B12 deficiency, and occasionally folic acid defefiency, there are neurological disturbances including numbness and tingling of extremities, gait abnormalities, and mental disturbances.Older theories suggest that the B12 deficiency

leads to a defect in the degradation of propionyl CoA to succinyl CoA leading to an accumulation of propionyl CoA:

MEGALOBLASTIC ANEMIAS When there is a build up of propionyl CoA (3 carbons), it

is used in place of acetyl CoA (2 carbons) as a primer for fatty acid synthesis.

Fatty acids with odd numbers of carbons get incorporated into neural membranes

This leads to disruption of membrane function with subsequent demyelination of nerve fibers.

Newer theories suggest that the defiency of methionine leads to decreased production of S-adenosyl-methionine (SAM), a key intermediate in methylation reactions of myelin and this leads to demyelination

Lab findings Macrocytic, normochromic anemia (MCV=100-140,

MCHC is normal)) MCH is increased (due to increased cell size)

MEGALOBLASTIC ANEMIAS Hemoglobin and RBC counts are decreased WBC and platlet counts are decreased On a peripheral smear, a triad of things is commonly

seen: oval macrocytes, Howell Jolly bodies (nuclear DNA fragments), and hypersegmented neutrophils (5 or more lobes). In addition: Anisocytosis is usually moderate Poikilocytosis is striking with nucleated RBCs,

polychromatophilia, and cabot rings (spindle remnants). RBC dimorphism is seen with concomitant IDA. The absolute reticulocyte count is decreased because of

ineffective erythropoiesis.

PERIPHERAL SMEAR OF MEGALOBLASTIC ANEMIA

Oval Macrocyte

Howell Jolly Body

Cabot ring

PERIPHERAL SMEAR OF MEGALOBLASTIC ANEMIA

Hypersegmented neutrophil

MEGALOBLASTIC ANEMIAS The bone marrow will show hypercellularity, yet

there are decreased numbers of all cell types in the peripheral blood because ineffective hematopoiesis is occurring and many cells are dying prematurely in the bone marrow.

In folic acid deficiency – there will be decreased serum and RBC folate

In B12 deficiency – there will be decreased serum vitamin B12

Specific tests for PAGastric analysis – if there is no free HCl after

histamine stimulation, this may indicate PA since the same cells that secrete HCl, also secrete intrinsic factor (IF)

Test for antibodies to IF

MEGALOBLASTIC ANEMIASSchilling test – is the definative test for the

diagnosis of PA. ( Know this test!) The test measures the amount of an oral dose of

radioactively labeled B12 that is absorbed in the gut and excreted in the urine.

This is followed by an injection of unlabeled vitamin B12 to saturate all vitamin B12 receptors in the tissue and plasma. Thus any amount absorbed in the gut will be in excess, and will be filtered in the kidneys to appear in the urine.

If there is no radioactivity in the urine, this means that there is either malabsorption or PA.

The test is repeated, but this time the radioactively labeled B12 is accompanied by a dose of IF.

If absorption is now normal, this means that the patient has PA

SCHILLING TEST

SCHILLING TEST RESULTS

NON-MEGALOBLASTIC MACROCYTIC ANEMIA

NON-MEGALOBLASTIC MACROCYTIC ANEMIA Note that the macrocytic RBCs are not oval,

but are round. There are no hypersegmented neutrophils or

Howell-Jolly bodies

DIFFERENTIAL DIAGNOSIS WITH INCREASED MCV

DIFFERENTIAL DIAGNOSIS WITH A HIGH MCV

LAB RESULTS OF MEGALOBLASTIC VS NON-MEGALOBLASTIC MACROCYTOSIS