Megaloblastic A Flashcards
Macrocytic anaemia is characterized by abnormally large red blood cells with an MCV of? The causes can be broadly categorized into____&____ based on the appearance of developing erythroblasts in the bone marrow.
mean corpuscular volume, MCV >98 fL).
megaloblastic and non-megaloblastic,
Give me an overview of megaloblastic A
Megaloblastic Anaemias
Megaloblastic anaemias are a group of anaemias where erythroblasts in the bone marrow show a characteristic abnormality: the maturation of the nucleus is delayed relative to that of the cytoplasm. This condition is primarily caused by defective DNA synthesis, usually due to deficiencies in vitamin B12 or folate. Other less common causes include abnormalities in the metabolism of these vitamins (often drug-induced) or inherited defects in DNA synthesis.
What’s the pathophysiology of Megaloblastic A
Pathophysiology
Defective DNA Synthesis: The core defect in megaloblastic anaemias is in DNA synthesis, leading to asynchronous maturation of the nucleus and cytoplasm.
Morphological Changes: In the bone marrow, erythropoietic cells show a persistently open, loosely organized chromatin in the nucleus, while the cytoplasm displays changes typical of a later stage of maturation.
Vitamin B12 (Cobalamin)
Vitamin B12, also known as cobalamin, is synthesized by microorganisms. Animals acquire it through consumption of food of animal origin, internal production from intestinal bacteria (not in humans), or by eating bacterially contaminated foods. The vitamin is comprised of cobalamins, which have a cobalt atom at the center of a corrin ring, with different groups attached to the reactive center
Sources of Vitamin B12
Animal Products: Liver, meat, fish, and dairy produce.
Non-Animal Sources: Vitamin B12 does not naturally occur in fruits, cereals, or vegetables, except in small amounts due to contamination by insect parts during harvesting or by microorganisms in a natural environment.
Explain the absorption & Transportation of Vit. B12
Absorption of Vitamin B12
A normal diet contains a large excess of vitamin B12 compared to daily requirements. The process of B12 absorption involves several steps and key proteins:
Release from Food: B12 is released from its protein-bound form in food by the action of pepsin in the stomach.
Binding to Intrinsic Factor (IF): After release, B12 combines with intrinsic factor, a glycoprotein synthesized by gastric parietal cells.
Ileal Binding and Absorption: The IF–B12 complex binds to the specific receptor cubam in the ileum. This receptor is a complex of cubilin and amnionless proteins. Amnionless directs the endocytosis of the cubilin IF–B12 complex into the ileal cell, where B12 is absorbed and IF is destroyed.
Maximum Absorption: The maximum amount of B12 that can be absorbed from a single oral dose via the IF–cubam mechanism is about 1–2 μg.
In addition, some dietary B12 binds first to haptocorrin, a glycoprotein present in saliva and gastric juice. Release of B12 from haptocorrin for binding to IF depends on pancreatic proteases.
Transport of Vitamin B12: The Transcobalamins
Once absorbed from the ileal cell into the portal blood, B12 attaches to the plasma-binding protein transcobalamin (TC or transcobalamin II). This protein delivers B12 to the bone marrow and other tissues. The amount of B12 bound to TC in plasma is normally very low (<50 ng/L).
Transcobalamin Deficiency
Congenital TC Deficiency: This is caused by germline mutations in the TCN2 gene and results in megaloblastic anaemia because B12 fails to enter marrow and other cells from plasma. Serum B12 levels in TC deficiency are normal since most B12 in plasma is bound to haptocorrin.
Haptocorrin: Synthesized in saliva, gastric juice, milk, and by granulocytes and macrophages, haptocorrin-bound B12 in the blood does not transfer to marrow and is considered functionally inactive. Increased granulocyte production, as seen in myeloproliferative neoplasms and some liver diseases, can raise haptocorrin and B12 levels in serum significantly
What happens in Transcobalamin Deficiency?
Congenital TC Deficiency: This is caused by germline mutations in the TCN2 gene and results in megaloblastic anaemia because B12 fails to enter marrow and other cells from plasma. Serum B12 levels in TC deficiency are normal since most B12 in plasma is bound to haptocorrin.
What are the Biochemical Functions of Vitamin B12?
Vitamin B12 acts as a coenzyme in two critical biochemical reactions:
Methionine Synthase (as Methyl B12): B12 acts as a cofactor for methionine synthase, which methylates homocysteine to methionine using methyltetrahydrofolate (methylTHF) as the methyl donor.
Methylmalonyl CoA Mutase (as Deoxyadenosyl B12): B12 assists in the conversion of methylmalonyl coenzyme A (CoA) to succinyl CoA, an important intermediate in the citric acid cycle.
Folic (pteroylglutamic) acid is the parent compound of the folate family
How is folate & folic acid absorbed in the body?
Absorption:
Folates are absorbed and converted to methylTHF in the bloodstream.
Inside cells, methylTHF is demethylated to THF and then converted to folate polyglutamates by adding multiple glutamate residues.
Folic acid is a poor substrate for dihydrofolate reductase; higher doses (200-400 µg) enter the plasma unchanged, to be reduced in the liver or excreted in urine.
What’s the biochemical function of folate
Biochemical Reactions:
Folates are involved in single-carbon unit transfers in amino acid interconversions, such as converting homocysteine to methionine and serine to glycine, and in synthesizing purine precursors of DNA.
What’s the Biochemical Basis for Megaloblastic Anaemia?
DNA Synthesis
DNA Synthesis Process:
DNA is made up of building blocks called deoxyribonucleoside monophosphates (dNMPs).
These building blocks are derived from deoxyribonucleoside triphosphates (dNTPs).
Role of Folate (Vitamin B9):
Folate is essential for the synthesis of one of the DNA building blocks called thymidine monophosphate (dTMP).
dTMP is crucial for DNA replication because it is one of the four nucleotides that make up DNA.
5,10-Methylene THF Polyglutamate:
This is a form of folate that acts as a coenzyme (helper molecule) for the enzyme that produces dTMP.
Without sufficient 5,10-methylene THF polyglutamate, the body can’t produce enough dTMP.
Consequence of Folate Deficiency:
If there’s a shortage of 5,10-methylene THF polyglutamate, the production of dTMP is limited.
This limitation causes a deficiency of dTTP (the triphosphate form used in DNA synthesis).
As a result, the S phase of the cell cycle (where DNA is replicated) is prolonged because the cell cannot complete DNA synthesis efficiently.
This delay can cause cells to die through a process called apoptosis.
Role of Vitamin B12 (Cobalamin)
Conversion of MethylTHF to THF:
Vitamin B12 is needed to convert methylTHF (a form of folate found in the bloodstream) into THF.
This reaction also involves converting homocysteine to methionine, an essential amino acid.
Importance of THF:
THF (tetrahydrofolate) is the form of folate that can be converted into various polyglutamate forms inside the cell.
These polyglutamates are the active forms of folate used in DNA synthesis and other cellular processes.
Indirect Role of B12 in Folate Function:
Without enough B12, the conversion of methylTHF to THF is impaired.
This reduces the availability of THF and, consequently, the active folate polyglutamates, including 5,10-methylene THF polyglutamate.
As a result, dTMP synthesis is impaired, leading to the same problems seen with folate deficiency: delayed DNA replication and cell death.
Summary
Folate Deficiency: Directly limits the production of dTMP, which is necessary for DNA synthesis, causing problems in cell division and leading to cell death.
B12 Deficiency: Indirectly causes the same problem by preventing the formation of active folate forms (polyglutamates) needed for dTMP production.
Both deficiencies disrupt DNA synthesis, leading to the characteristic large, immature red blood cells seen in megaloblastic anaemia.
Other Causes:
Other congenital or acquired causes of megaloblastic anaemia, such as antimetabolite drug therapy, inhibit purine or pyrimidine synthesis at different steps, resulting in reduced supply of DNA precursors.
Folate Cycle During dTMP Synthesis
Folate as a Coenzyme:
Folate, in its active form (tetrahydrofolate or THF), is essential for the synthesis of thymidine monophosphate (dTMP), a DNA building block.
During this process, folate is converted from THF to dihydrofolate (DHF).
Regeneration of Active Folate (THF):
For continued DNA synthesis, DHF needs to be converted back to THF.
This conversion is carried out by the enzyme dihydrofolate reductase (DHF reductase).
Inhibition of DHF Reductase:
Certain drugs inhibit DHF reductase, blocking the conversion of DHF to THF.
This inhibition prevents the regeneration of THF, halting the production of dTMP and, consequently, DNA synthesis
What are the drugs that Inhibit DHF Reductase?
Methotrexate:
Use: Primarily used to treat cancers (e.g., acute lymphoblastic leukaemia) and inflammatory diseases (e.g., rheumatoid arthritis, psoriasis).
Mechanism: Inhibits DHF reductase, blocking folate regeneration and thus DNA synthesis, which is particularly effective in cells with high turnover.
Pyrimethamine:
Use: Used mainly as an antimalarial drug.
Mechanism: Inhibits DHF reductase, but is weaker than methotrexate.
Trimethoprim:
Use: Used as an antibiotic, often in combination with a sulphonamide (e.g., co-trimoxazole).
Mechanism: Inhibits bacterial DHF reductase effectively, but is much less effective against the human enzyme.
What are the Causes of Megaloblastic Anemia
Causes of Megaloblastic Anemia
Vitamin B12 Deficiency
Vitamin B12 deficiency can arise from a variety of causes, including:
- Dietary Deficiency: Inadequate intake of vitamin B12, commonly seen in strict vegetarians or vegans.
- Malabsorption: Conditions such as pernicious anemia, Crohn’s disease, celiac disease, or gastric bypass surgery can impair the absorption of vitamin B12.
- Intrinsic Factor Deficiency: Lack of intrinsic factor, a protein necessary for vitamin B12 absorption, often due to autoimmune destruction of gastric parietal cells.
- Ileal Disease or Resection: The terminal ileum is crucial for B12 absorption, so any disease or surgical resection can lead to deficiency.
- Pancreatic Insufficiency: Pancreatic enzymes are necessary for the release of B12 from binding proteins in food.
- Drug Interference: Certain medications, such as metformin or proton pump inhibitors, can interfere with B12 absorption.
Folate Deficiency
Folate deficiency can be due to several factors:
- Dietary Deficiency: Inadequate intake of folate-rich foods such as leafy green vegetables, legumes, and fortified cereals.
- Increased Requirement: Conditions that increase cell turnover, such as pregnancy, hemolytic anemia, or malignancy, can increase folate demand.
- Malabsorption: Disorders affecting the upper small intestine, such as celiac disease or tropical sprue, can impair folate absorption.
- Alcoholism: Chronic alcohol consumption can interfere with folate metabolism and absorption.
- Drug Interference: Medications such as methotrexate, phenytoin, and trimethoprim can inhibit folate metabolism or absorption.
Combined Folate and B12 Deficiency
A deficiency in both vitamins can occur due to:
- Dietary Deficiency: Poor overall nutritional intake, often seen in cases of severe malnutrition or strict dietary restrictions.
- Malabsorption Syndromes: Conditions like celiac disease or tropical sprue that affect the absorption of both vitamins.
- Drug Interference: Use of medications that affect the metabolism of both folate and B12.
Abnormalities of Vitamin B12 or Folate Metabolism
These can be caused by:
- Genetic Conditions: Inherited disorders affecting the metabolism of B12 or folate, such as transcobalamin deficiency.
- Environmental Factors: Exposure to nitrous oxide, which inactivates vitamin B12.
- Medications: Drugs like methotrexate, phenytoin, or trimethoprim that interfere with folate metabolism.
Genetic mutations can disrupt DNA synthesis or the methionine cycle, leading to megaloblastic anemia. Examples include:
- Methionine Synthase Deficiency: A rare inherited disorder affecting the enzyme needed for homocysteine conversion to methionine.
These are rare genetic conditions that impair DNA or nucleotide synthesis, such as:
- Orotic Aciduria: A disorder that affects pyrimidine synthesis, leading to megaloblastic anemia.
These can result from certain medications that inhibit key enzymes in nucleotide synthesis, including:
- Hydroxyurea: Used in the treatment of certain cancers and sickle cell anemia.
- Purine Synthesis Antagonists: Such as 6-mercaptopurine, which is used in the treatment of leukemia.
- Pyrimidine Antagonists: Such as cytosine arabinoside (cytarabine), used in chemotherapy.
What are the common Causes of Severe Vitamin B12 Deficiency
Common Causes in Developed Countries:
Pernicious Anemia: Also known as Addisonian anemia, it is an autoantibody-mediated condition where the immune system attacks the stomach’s parietal cells, leading to intrinsic factor deficiency. This is the most common cause of severe B12 deficiency.
Dietary Deficiency: Severe B12 deficiency can occur in strict vegans who do not take B12 supplements, as B12 is predominantly found in animal products.
Gastrectomy: Surgical removal of part or all of the stomach can lead to a lack of intrinsic factor production, impairing B12 absorption.
Small Intestinal Lesions: Conditions affecting the ileum, such as Crohn’s disease, can hinder B12 absorption.
Nitrous Oxide Exposure: Nitrous oxide can rapidly inactivate B12 in the body, leading to severe deficiency.
What are the less causes of of severe Vit B12 def
Less Common Causes:
Poor-Quality Diets: While entero-hepatic circulation can protect individuals with low dietary intake from severe deficiency, they may still develop mild B12 deficiency.
Chronic Conditions: Conditions that impair the entero-hepatic circulation or small intestine function can contribute to severe B12 deficiency.
What are the causes of mild Vit B12 def
Causes of Mild Vitamin B12 Deficiency
Most Frequent Cause Worldwide:
Inadequate Diet: The primary cause of mild B12 deficiency globally is insufficient dietary intake.
Other Causes:
Malabsorption Conditions:
Atrophic Gastritis: Particularly common in the elderly, this condition involves chronic inflammation of the stomach lining, reducing B12 absorption.
Chronic Pancreatitis: Inflammation of the pancreas can impair the release of pancreatic enzymes needed for B12 absorption.
Gluten-Induced Enteropathy (Celiac Disease): Damage to the small intestine impairs nutrient absorption, including B12.
HIV Infection: The virus can damage the intestinal lining, leading to malabsorption.
Zollinger-Ellison Syndrome: Excessive gastric acid production lowers the pH in the duodenum, inactivating pancreatic enzymes that release B12 from haptocorrin.
What medications can cause b12 def
Medication Effects:
Proton Pump Inhibitors (PPIs): Long-term use can reduce stomach acid necessary for B12 absorption.
Metformin: Commonly used in diabetes management, it can interfere with B12 absorption over prolonged use.
Cholestyramine: This bile acid sequestrant can interfere with B12 absorption.
Other Conditions:
Pregnancy: Serum B12 levels can fall during pregnancy but typically return to normal post-delivery.
Infant Deficiency: Prolonged B12 deficiency can lead to irreversible motor function impairment in infants
What can you tell me about pernicious Anemia / Pathophysiology
Pernicious Anemia (PA)
Pathophysiology
Autoimmune Attack on Gastric Mucosa: Pernicious anemia results from the immune system attacking the stomach lining.
Gastric Mucosa Atrophy: This leads to the stomach wall becoming thin, infiltrated with plasma cells and lymphocytes.
Destruction of Parietal Cells: This destruction causes a lack of stomach acid (achlorhydria) and nearly absent secretion of intrinsic factor (IF), which is necessary for vitamin B12 absorption.
Intestinal Metaplasia: The gastric mucosa may undergo a transformation to resemble intestinal tissue.
Serum Gastrin Levels: These are typically elevated in PA.
Helicobacter Pylori Infection: This bacterial infection can initiate autoimmune gastritis, leading to PA in older individuals or presenting as iron deficiency in younger patients
What’s the epidemiology involved in pernicious anemia
Epidemiology
Gender and Age: More common in females than males.
Associated Autoimmune Diseases: Frequently occurs alongside other autoimmune diseases, especially thyroid disorders.
Increased Cancer Risk: There is a higher incidence (2-3%) of stomach carcinoma in PA patients