B12 and folate are involved in DNA synthesis, therefore, if you are B12 and/or folate def, you cannot make DNA, specifically b/c you have a prob with making DMP (deoxythymidine monophosphate). Therefore, if you cannot make that, you cannot mature the nucleus (immature nuclei do not have a lot of DNA in them, but as you make more DNA, the nuclei become more matured, and the nucleus becomes smaller and more condensed). B/c DNA cannot be made, then you have large nucleus, and all nucleated the cells in your body are big – why they are called MEGAloblastic anemias. A good pathologist can dx B12 and folate def in a cervical pap smear, when looking at the squamous cells (cells look big – any cell with a nucleus has DNA in it, so any cell with DNA will be big – not just the hematopoeitic cells that are huge, ALL nucleated cells in the body are big – ie GI, squamous cells)
29. BM from a 65 year old woman with a macrocytic anemia, pancytopenia, and absent vibratory sensation in the lower extremities. What is the most likely diagnosis?
30. PB in a 35 year old alcoholic with a macrocytic anemia. What is the most likely diagnosis?
B12 aka cobalamin; B12 has cobalt in it. Circulating form of folate is methyltetrahydrofolate (tetra = four). Purpose of cobalamin (B12) is to take the methyl group off of
methyltetrahydrofolate. Then it’s called tetrahydrofolate. If you don’t get the methyl group off of folate, you will not make DNA. So, if you are B12 def, you can’t get the methyl group off and cannot make DNA. If you are def in folate, you can’t make DNA.
Cobalamin adds a methyl to group homocysteine; when you add a methyl group to
homocysteine, it becomes methionine. Methionine = aa for 1 carbon transfer rxns. (Methyl = CH3). If you are B12 or folate def, what are the serum homocysteine levels? High. With a high serum homocysteine, it produces thromboses, including MI’s; it damages endothelial
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cells, leading to thromboses, and predisposing to MI. So, what is MCC of increased homocysteine? It is NOT homocystinuria (rare auto rec dz), but B12 def or folate def, and folate is MC than B12. Therefore, the MCC of increased homocysteine is folate def, and have an increased incidence of thrombosis and MI. This is why cardiologists order serum homocysteine levels. In folate def, no methyl group to add to homocysteine (so homocysteine increases); with B12 def, no methyl group to add to methionine to make homocysteine therefore methionine increases.
Tetrahydrofolate is the start of the cycle, and leads to production of thymidilate synthase – this is where DNA is made. DUMP is converted to DDT, making DNA. Therefore, this substrate is necessary to make DNA. So, it is used in the making of DNA by an enzyme called
dihydrofolate reductase which converts oxidized dihydrofolate to tetrahydrofolate. Many drugs block dihydrofolate reductase – methotrexate, TMP-SMX. The drugs block DNA synthesis (ie decreasing DNA synthesis) thereby leading to macrocytic anemia. So, the functional B12 takes the methyl group from tetrahydrofolate and gives it to homocysteine to make methionine. And tetrahydrofolate will start the cycle for making DNA.
A. B12
1. B12 Reactions: B12 is humiliated by having to transfer methyl groups. This is an odd request – so whoever he asked said that they can take care of even chained FA’s, but we have a problem with ODD chained FA’s b/c we can only break down till proprionyl CoA, which leads to dementia and proprioception loss. B12 helps in odd chain FA
metabolism. Therefore, it is involved in proprionate metabolism, which is metabolism of an odd chain FA. Proprionate forms methylmalonyl CoA, where B12 comes in and helps convert methylmalonyl CoA to succinyl CoA, which can go into the TCA cycle. In B12 def, certain things will build up, such as proprionate and methylmalonyl CoA. Methylmalonyl CoA becomes methylmalonlylic acid, which is a sensitive and specific test for B12 def. So, with B12 def, get a methylmalonlylic acid test (which will be increased). Reason for
neurological problems is b/c proprionate metabolism; without B12, cannot convert odd chain FA’s into succinyl CoA, and they build up, and it screws up myelin (cannot syn myelin) – and leads to demyelination of posterior columns, and of the lateral corticospinal tract, along with dementia. B/c it is a posterior column dz, you will have probs with
proprioception, vibration; b/c you knock off the lateral cortical spinal tract, you will get UMN lesions (spasticity, babinski), and then dementia.
Will always tell you that you can have B12 def, and correct the anemia with high doses of folate, but cannot correct the neurologic dz. Therefore, must make the specific dx. B/c if you think its folate def and give folate, you will correct the hematologic problem, but not the neurological problem, therefore have B12 def. So, in differential of dementia, include B12 def (along with Alzheimer’s). You don’t have to have anemia with B12, but can have neurological probs. So, with dementia, get a TSH level (to throw out hypothyroidism), and a B12 level to rule out B12 def b/c these are REVERSIBLE causes of dementia.
Pure vegan vs. ovo-lactovegan: In ovo-lactovegan taking dairy products (which are animal products), therefore, do not have to take B12 supplements. However, a pure vegan does have to take B12 supplements.
2. Normal sequence of B12 absorption: Have to eat meats or dairy products to get B12.
The first thing B12 does is binds to R factor in saliva. R factor protects B12 from
destruction by acid in the stomach. Intrinsic factor (IF) made by parietal cells in the body fundus; they also make acid. IF is not destroyed by acid, therefore does not need anything to protect it. So the B12/R factor complex goes into the duodenum, where there is IF waiting for it. R factor must be cleaved off, which is done with enzymes from the functioning pancreas. Then, IF and B12 bind to e/o and take a long trip. Do not go to duodenum (Fe country), do not go to ligamentum of trietz in the jejunum (folate country); so they go all the way to the terminal ileum, where there are receptors for IF, and it is
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reabsorbed. This is the same place bile salts are reabsorbed, and the same place the Crohn’s dz hits. Therefore, it is fair to say that with Crohn’s dz, you also have bile salt reabsorption problems and B12 def.
3. Causes of B12 deficiency:
a) MCC B12 def = pernicious anemia; this is an autoimmune dz with destruction of the parietal cells; autoAb’s attack the parietal cells and there are autoAb’s against IF and destroys the parietal cells which are located in the body and fundus. Everything gets destroyed leading to an atrophic gastritis of the body and fundus. No parietal cells
= no acid = achyloridria, and no IF. Achyloridria is a major predisposing factor for gastric adenocarcinoma.
b) Causes of B12 def: pure vegan; chronic pancreatitis seen in alcoholics (this leads to B12 def b/c can’t cleave off the R factor); D. latum (fish tapeworm that eats B12 (rarest) – from fish in lake trout in lakes of Chicago); terminal ileum dz (Crohn’s). And bacterial overgrowth due to peristalsis prob and/or diverticular pouches and/or stasis. Whenever there is stasis you’ll get bacterial infection (also bladder infection); bacteria love B12 and bile salts with bacterial overgrowth. All of these will lead to B12 deficiency.
31. PB in a 67 year old woman with a macrocytic anemia arrow points to a tear drop cell, which may be seen in patients with B12 deficiency
B. Folate
Folate is seen in animal and plant products, therefore not seen in vegans. Folate has many pharm ties (ie dihydrofolate reductase). When you eat folate, it’s in a polyglutamate form, meaning you cannot reabsorb it in the jejunum; therefore it has to be converted to a monoglutamate form. Intestinal conjugase (in the small intestine) is responsible for this.
What drug blocks intestinal conjugase? Phenytoin. So, if they ask about pt on Phenytoin, with macrocytic anemia, hypersegmented neutrophils, neurological effects are NOT present – therefore folate def (b/c there are no neurological problems, this r/o b12 def.) Now you have monoglutamate, which is absorbed in the jejunum. There are 2 things that inhibit its absorption: (1) birth control and (2) alcohol (MCC folate def = alcoholism).
With B12, have 6-9 year supply in liver, therefore its uncommon to get. Folate only has 3-4 month supply – so, even if you have an excellent diet, you can have folate def if you are taking one of these two things.
34. PB findings in a woman on birth control pills who has a mild macrocytic anemia
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33. PB in an alcoholic with alcoholic hepatitis and a macrocytic anemia
Summary: circulating form of folate is methyltetrahydrofolate, and B12 takes the folate off, and gives it to homocysteine which becomes methionine; the methyltetrahydrofolate becomes tetrahydrofolate, and with the help of dihydrofolate reductase, DNA is made.
Example: pic with hypersegmented neutrophil (definition: 5 or more lobes!). Hypersegmented neutrophil indicates B12 or folate def, even if you don’t have anemia. It is the first thing that comes up before anemia. And if the neurological test is normal, it’s a folate def. Test for
proprioception: Rhomberg test – if you have post column dz, prob with proprioception b/c do not know where your joints are; does not show cerebellar ataxia (will have these with eyes opened AND closed). Use vibrating tuning fork to see if pt has proprioception on the malleous.
32. PB from a patient with Crohn’s disease and a macrocytic anemia
What is the most likely diagnosis?
The WBC is hypersegmented indicating either B12 or folate deficiency
Hematopoetic cells are made outside the sinusoids in the BM. It’s analogous to the cords of bilroth in the spleen (where there are fixed macrophages and then, the RBC’s and WBC’s have to get back into the sinusoids and circulate through holes. They get through, and are in
sinusoids). The same thing occurs in the BM – they have a place equivalent to the cords of bilroth and that is where they are made. To get into the circulations, they have to fit through lil, narrow holes to get into the sinusoids in the BM and into the blood stream. Something very big will not be able to get through the lil holes and into the sinusoids. Therefore, macrophages will want to feast on the macrocytic cells (WBC’s, RBC’s, platelets) that cannot get into the
sinusoids. So, the macrophages kill them all. So in the peripheral blood, will see NOTHING – pancytopenia; severe macrocytic anemia, neutropenia, thrombocytopenia – which is
characteristic of B12 /folate def. (everything in the marrow is too big and cannot get out into the circulation).
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Schilling’s test – good test for localizing B12 def. We know now that it’s a B12 deficiency, and we want to know what caused it. Steps for schilling test: Give radioactive B12 by mouth; they then collect the 24 hr urine to see if any comes out in the urine and nothing comes out, therefore prove that they have a problem absorbing B12.
1st step: give radioactive B12 and IF, collect urine for 24 hrs, and piles in the urine = Pernicious anemia; b/c added what was missing (IF); if it didn’t work, you can EXCLUDE pernicious anemia.
Say this didn’t work, then you:
2nd step: give 10 days worth of broad spectrum antibiotic; pt comes back and again you give them radioactive B12; see piles of radioactive B12 in the urine, what is dx? Bacterial
overgrowth b/c knocked off the bugs eating B12 Say this didn’t work, then you:
3rd step: pancreatic extract, swallow pills, then give radioactive B12; 24 hrs later, see what happens; if there is radioactivity in urine, pt has chronic pancreatitis.
If that didn’t work, could be Crohn’s, worm, etc.
Summary:
If B12 malabsorption was corrected by adding IF, pt has pernicious anemia If B12 corrected by adding an antibiotic, pt has bacterial overgrowth
If B12 is corrected by adding pancreatic extract, pt has chronic pancreatitis.