Download Pathology – Lecture 18: Megaloblastic Anemia 2/28/13 Megaloblastic

Pathology – Lecture 18: Megaloblastic Anemia
2/28/13
Megaloblastic Anemnia
 Introduction
o Caused by a failure of bone marrow blood-forming cells to make DNA
 Often d/t vit B12 or folate deficiency or DNA replication poisons
o Occur when the coenzyme forms of folate and vit B12 for normal DNA synthesis are
deficient, which impairs all cells to synthesize enough DNA per unit time
   #s of cells in the DNA synthesis stage of the cell cycle
 RNA isn’t affected, which  “giantism” of proliferating cells
o May also lead to malabsorption (damaged intestinal cells)
 Characteristics of Vit B12 and Folate
o Vit B12
 Circulates in the peripheral blood bound to various binder proteins
 Stores last 2-5 yrs
o Folate
 Heat labile
 Derivatives circulate largely unbound in the blood; many stored intracellularly
 Stores last 3-5 months
 Thus, folate deficiency is much more common than B12 deficiency
 Epidemiology
o Disease of older adults; median age 60 yo
o Genetic predisposition
o Many ppl will form Abs against multiple self-antigens
Vitamin B12
 Normal Vit B12 Metabolism
o Known as cobalamin
o Not synthesized; all must be acquired via diet
 Deficiencies seen in strictly vegetarian and macrobiotic diets
 Larger amts in animal products  stored excess intrahepatically
o Absorption
 Requires intrinsic factor (IF), secreted by the parietal cells of the fundus
 The action of pepsin frees B12 from food
 Free B12 binds w/ cobalophilins, or R-binders
 Pancreatic proteases release B12 in the duodenum, where it binds intrinsic factor
 Secreted into the plasma bound to transcobalamin II w/in ileal cells
  liver and other cells of the body (including rapidly proliferating cells in the
bone marrow and GI tract)
o Alternative uptake mechanism
 Not dependent on IF or an intact ileum
 Up to 1% of oral B12 can be absorbed this way  feasible to tx pernicious anemia w/
high doses of oral vit B12
 Microbe competetion from bacterial overgrowth of a blind loop or tapeworm
infestation (Diphyllobothrium latum) may  B12 deficiency
 Vit B12 Functions
o Two reactions require B12
 Methylcobalamin serves as a cofactor in conversion of homocysteine  methionine
by methionine synthase


This yields a methyl group that is recovered from N5-methyl FH4, the
principle form of folic acid
o This is converted to FH4, which is an immediate precursor of DNA
 A deficiency of FH4 d/t vit B12 deficiency is a fundamental
cause of impaired DNA synthesis
 B12 is also needed for synthesis of methionine, and if this can’t happen 
“internal” folate deficiency
 Result
o A lack of folate (FH4) is the proximate cause of anemia in vit B12
deficiency, since the administration of folic acid improves anemia
 Isomerization of methylmalonyl coenzyme A to succinyl Co-A
 B12 deficiency  ed plasma and urine levels of methylmalonic acid
o   methylmalonate and propionate  incorporation of abnormal
FAs into neuronal lipids
o This may predispose to myelin breakdown  neuro complications
Vit B12 Deficiency Etiology
o Usu. caused when the digestive system doesn’t adequately absorb B12 from food
 Causes
 Pernicious anemia (lack of intrinsic factor)
o D/t atrophic gastritis by autoimmune destruction of gastric parietal
cells
 Surgery (removal of all or part of the stomach and/or small intestine)
 Conditions affecting the small intestine
o Crohn’s dz, Celiac dz, bacterial growth, parasite
 Excessive EtOH consumption
 Autoimmune disorders (Graves’ dz or SLE)
 Long-term use of acid-reducing drugs
 Babies born to vegetarian mothers
Folate
 Metabolism of Folate
o Intestinal conjugases split polyglutamates into monoglutamates that are readily absorbed in
the proximal jejunum
o During intestinal absorption they are modified to 5-methyltetrahydrofolate, the normal
transport form of folate
 Anemia of Folate Defiency
o Folic acid deficiency  megaloblastic anemia
 Same characteristics as that caused by vit B12 deficiency
 Suppressed synthesis of DNA is the immediate cause of megaloblastosis
o FH4 derivatives act as intermediates in the transfer of one-carbon units such as formyl and
methyl groups
o FH4 serves as an acceptor of one-carbon fragments from serine and formiminoglutamic
acid
o Most important metabolic process depending on these transfers are:
 Purine synthesis
 Conversion of homocysteine  methionine (also needs B12)
 Deoxythymidylate monophosphate (dTMP) synthesis
 Probably the most important biologically b/c it’s needed for DNA synthesis
 Folate Deficiency Etiology
o Three major causes
 ed intake

Humans are entirely dependent on dietary sources of folate
o Green veggies – best sources; certain fruits and animals – less
 Causes
o Nutritionally inadequate diet
 Mainly in chronic alcoholics, the indigent, and very elderly
 Megaloblastic anemia is generally accompanied by general
malnutrition and manifestations of other avitaminoses,
including cheilosis, glossitis, and dermatitis
o Impairment of intestinal absorption
 Celiac sprue; lymphoma
 Drugs (i.e. phenytoin and OCPs)
 ed requirements
 Pregnancy, infancy, hematologic derangements assoc. w/ hyperactive
hematopoiesis (hemolytic anemias) and disseminated cancer
 Impaired utilization
 Folic acid antagonists, such as methotraxate, inhibit dihydrofolate reductase
and  FH4 deficiency
Megaloblastic Anemia Pathology
 Morphology
o Peripheral blood findings in all megaloblastic anemias
 Macrocytic and oval red cells (macro-ovalocytes) – highly characteristic
 Lack central pallor and may appear “hyperchromic” w/o  MCHC
 Anisocytosis (varied size)
 Poikilocysosis (varied shape)
  reticulocyte count
 Early changes
 Hypersegmented neutrophils (also macropolymorphonuclear)
o Often appear before the onset of anemia
o 6+ lobes (normal 3-4)
o Bone marrow
 Hypercellular d/t ed hematopoietic precursors
 Megaloblastic changes detected at all stages of erythroid development
 Nuclear-to-cytoplasmic asynchrony
 Granulocyte precursors also display dysmaturation
o Giant metamyelocytes and band forms
 Abnormally large megakaryocytes w/ bizarre, multilobate nuclei
 Marrow hyperplasia
 Response to ed growth factor levels, such as EPO
 The derangement in DNA synthesis causes most precursors to undergo apoptosis in the
marrow (an example of ineffective hematopoiesis) and leads to pancytopenia
 Clinical Findings
o Folate deficiency
 Develops more rapidly (months)
o Vit B12 deficiency
 Develops more slowly (years)
 Neurologic Sxs, 2 to demyelination of the posterior and lateral columns
 Hematologic Findings
o Peripheral blood smear morphology
 Characteristically contains numerous oval macrocytes as well as schistocytes
 RBC fragmentation d/t the ed fragility of these large erythrocytes

 Basophilic stippling and Howell-Jolly bodies
 Nucleated RBCs may be seen when the HCT < 20%
 Hypersegmented neutrophils – appear very early
o Bone Marrow
 Characteristically hypercellular w/ erythroid and granulocytic hyperplasia
  mitotic activity
 Significant intramedullar cell death
 Megaloblastic changes
 Erythroid changes
o Nuclear-cytoplasmic asynchrony
 Myeloid changes
o Giantism of bands and metamyelocytes
o Nuclear hypersegmentation of mature granulocytes
 Masking of erythroid megaloblastosis
 Seen in concomitant iron deficiency (pregnancy, malabsorption, chronic
alcoholism, pernicious anemia)
o May seen an intermediate picture b/t iron deficiency and
megaloblastic anemia
Diagnostic Considerations
o Folate Deficiency
  folate levels in serum or red cells
 Normal methylmalonate concentrations
o Vitamin B12 Deficiency
  serum homocysteine and methylmalonic acid
 Neurological changes
 Can actually be made worse w/ folic acid administration
 Schilling test
 Measures vit B12 absorption
 Radioactive vit B12 given orally, w/ or w/o IF
 Urinary excretion of radioactivity is measured
 Not commonly used