Megaloblastic anaemia

Question 1

What is the main cause of megaloblastic anaemia?

Answer 1

Vitamin B12 or folate deficiency. Can occur in B12 and folate metabolism or via defects in DNA synthesis

Question 2

What is the main source of vitamin B12?

Answer 2

Also known as cobalamin, it is gained through eating food of animal origin, daily average intake is 1-3 micrograms

Question 3

How much vitamin B12 is stored in the body and where is it stored?

Answer 3

2-3mg, which is 3-4 years worth is stored in the liver. It can be excreted in bile but is usually reabsorbed in the intestines

Question 4

What is the process that occurs leading to B12 absorption? (long answer)

Answer 4

B12 is consumed with dietary proteins, which stimulates amylase in saliva when chewing to secrete R-protein. Inside the stomach, parietal cells secrete intrinsic factor (IF) and chief cells secrete pepsin. Pepsin catabolizes dietary proteins and separates B12. Once separated the R-protein will bind to the free B12 protecting it from the stomach acid. The B12, R-protein and IF will then move into the duodenum. At the same time, the pancreas will secrete digestive enzymes that will catabolise the R-protein and free up the B12 allowing it to bind to IF. The IF and B12 can then be absorbed into the enterocytes of the ileum as the enterocytes only recognise B12-IF complex.

Question 5

What does intrinsic factor do to help B12 besides being absorbed by enterocytes in the duodenum?

Answer 5

They protect B12 from being catabolized by intestinal bacteria

Question 6

What happens to B12 following absoption into the enterocytes?

Answer 6

From the enterocytes, it passes into the portal blood and attaches to transcobalamin to deliver B12 to the bone marrow and other tissues. Once B12 is in the bone marrow it can bind to a receptor of erythroid precursors and be endocytosed, it is then able to perform as a co-factor in crucial biochemical pathways

Question 7

You can have megaloblastic anaemia while having normal B12 levels, what could be the problem?

Answer 7

Deficiency in transcobalamin

Question 8

What is the main source of folate in our diet and what is the average adult daily requirement?

Answer 8

Vegetables, it is recommended getting roughly 100-200 mg/day

Question 9

How is folic acid absorbed and used in the DNA synthesis pathway? (long answer)

Answer 9

It is converted to 5-methyl tetrahydrofolate by methyl THF reductase. It can then be absorbed in the jejunum, where folate binding proteins are present. Once inside the cell, it is converted to THF by methionine synthetase and vitamin B12, which also results in the methylation of homocysteine to methionine. THF can then be converted to THF polyglutamate and combine with the methionine to form 5,10-methylene THF polyglutamate, which is used to convert dUMP to dTMP changing istelf in the process to DHF polyglutamate and being recycled back into THF polyglutamate

Question 10

Why are B12 and folate required in the body?

Answer 10

They are used for a rate limiting step of DNA synthesis

Question 11

What happens if there is a deficiency of B12 in terms of DNA synthesis?

Answer 11

It will cause a builup of homocysteine, which will move into the serum and can be measures for B12 deficiency, but ultimately results in the lack of DNA synthesis

Question 12

What happens if there is a lack of folate in terms of DNA synthesis?

Answer 12

Prevents the formation of THF polyglutamate, the conversion of homocysteine to methionine can still take place but ther is nothing to convert. Resulting in the lack of DNA synthesis

Question 13

Besides DNA synthesis what else is B12 involved with?

Answer 13

It has a role in the production of key intermediaries in the Krebs cycle. Specifically deoxyadenosyl B12 assits in the conversion of methylmalonyl coenzyme A to succinyl CoA, which is required for the Krebs cycle

Question 14

What are some lab findings suggesting B12 deficiency?

Answer 14

Macrocytosis (enlarged RBC), anisocytosis (unequal sizes), hypersegmented neutrophils, poikilocytosis (abnormal shapes), Howell jolly bodies (nucleus present in RBC), basophilic stippling (left over ribosomal RNA)

Question 15

What test could you use to determine whether an individual with B12 deficiency is due to malabsorption or poor diet? (long answer)

Answer 15

Schillings test - patient is given B12 via intramuscular injection, the B12 will move to the liver and saturate all the receptors, then the patient is given radiolabeled B12, it should be absorbed in the intestines and move to the liver, but because it is saturated it should be excreted in the urine over 24hrs indicating a dietary problem. If not the case then give the patient an oral dose of radio B12 and IF and monitor urine levels over 24 hrs. If B12 is then present in the urine indicates pernicious anaemia

Question 16

What is pernicious anaemia?

Answer 16

Presents as B12 deficiency but is actually the absent or almost absence of IF causing B12 not being able to be absorbed in the intestines

Question 17

If the B12 deficiency is not due to malabsorption or poor diet what could be the cause? (long answer)

Answer 17

It suggests less common causes, would go on to administer radio B12 and antibiotic. If B12 is present in the urine it indicates an overgrowth of bacteria in the ileum. Lastly, if that still doesn’t work (very rare) administer B12 and pancreatic enzymes, if it works indicates insufficiency in pancreas enzymes meaning R-protein remains bound to B12 indicating chronic pancreatitis