DNA metabolism, B12, and Folate deficiency Flashcards
Describe how folate and B12 work together in metabolism/DNA synthesis.
- folate–> THF –> dUMP –> thymidine for DNA synthesis. Some of the THF is methylated though, and requires B12 as cofactor to the enzyme that demethylates it. Without B12, folate is useless and trapped as CH3-THF and DNA synthesis cannot occur
- B12 deficiency is also like having a folate deficiency
Where do humans get B12 and folate?
B12: animal proteins like beef, poultry, eggs, fish, and dairy products
Folate: dark green leafy vegetables, fruits and fruit juices, nuts, beans, peas, dairy products, poultry and meat, eggs, seafood, grains, and some beers (yeast)
Where does B12/folate metabolism take place?
-liver
3 reactions B12 serves a co-factor function. Be sure to state what is decreased or builds up due to a b12 deficiency.
- Turning CH3-THF into THF; decreased THF
- Turning homocysteine (HCY) into methionine; Increase HCY (toxic)
- Methylmalonyl CoA–>Succinyl CoA; Increase in MMA (methylmalonic acid=toxic to CNS)
How can overloading someone with folate bad for diagnostics?
-overloading with folate can mask a B12 deficiency and thus, you won’t treat the underlying cause
4 categories and examples of issures due to B12/folate deficiency
- Hematologic: megaloblastic anemia, low platelts, leukopenia
- Neuropsychiatric (B12): neural tube defects, ataxia, parasthesias, megaloblastic madness, depression, blindness
- Dermatologic: rashes, hyperpigmentation
- Oral/GI: diarrhea, malabsorption, chelitis, glossitis
* *3 and 4 due to EPITHELIAL ATROPHY bc these are sites of high cell turnover, but you cannot make DNA with these deficiencies, so you just lose these cells and cannot replace them
T/F: If you are B12 deficient, you must be anemia/macrocytic.
False; folate can mask a b12 deficiency and allow for DNA synthesis and cell division (erythropoiesis) to continue on even with B12 deficiency
3 groups of cobalamin-binding proteins, their source, and function.
- Intrinsic factor made by gastric parietal cells; promotes absorption of B12 in ileum
- Transcobalamin II made by all cells; transport of B12 to cells in blood
- R Proteins (Haptocorrin and Cobalaphilin) made by exocrine glands and phagocytes; protects B12 from degradation in stomach
Steps of B12 digestion and absorption
- Dietary B12 normally bound to proteins when ingested. Pepsin and acidic pH degrade protein and release B12
- B12 binds haptocorrin. In duodenum, pH increases and allows pancreatic proteases to degrade haptocorrin. B12 is released again and binds tightly to Intrinsic Factor produced by parietal cells.
- Mucosal cells of distal ileum have receptors (cubilin) for IF-B12 complex.
- B12 then enters blood bound to transcobalamin II and is delivered to all cells of the body, after being transported via portal system to liver
- Extensive enterohepatic circulation occurs and it is transported via bile duct back to duodenum
What 3 lab results point to a B12 deficiency? What else should be considered/tested? Which is most/more reliable?
3 labs: decrease in B12, increase in HCY, increase in MMA; decrease in B12 is less reliable because you can be functionally, not quantitatively deficient in B12
- also see low retic, blood smear, TSH
- consider Abs to IF
What is the historic test used to test B12?
- Schilling Test: give normal B12 to block all binding sites, then give radioactive PO, collect urine and analyze to see if 100% was excreted (due to blocking step in first step)
- First give IM block and PO radioactive, keep trying until successful with IF, Abx, then pancreatic enzymes
Why is diagnosis of B12 deficiency often delayed?
- interpreting lab results takes time and can be confusing
- neuropsych symptoms can hit before anemia, macrocytosis and it will often be overlooked in this case as the cause
4 general categories of B12 deficiency causes and examples.
- nutritional: vegetarian/vegan; breast milk from deficient mother; Only causes not due to decreased absorption
- Exocrine: pancreatic insufficiency
- Gastric: pernicious anemia, gastrectomy/atrophy, Proton-Pump inhibitors
- Intestinal: ileum resection, ileitis, blind-loop syndrome, fish tapeworm
Pernicious Anemia
- autoimmune destruction of gastric parietal cells (lymphocytic/plasma cell invasion with Anti-IF and anti-parietal cell Abs)
- associated with other autoimmune diseases
- account for 25-50% of B12 deficiency in adults
4 categories of folate deficiency causation and examples.
- Nutritional: old age, poverty, premature infancy, Goat’s Milk anemia
- Decreased absorption: celiac sprue, tropical sprue, intestinal disease
- Increased turnover: pregnancy, chronic hemolysis, exfoliative dermatitis
- Drugs: DHF reductase inhibitors (methotrexate, trimethoprim), antimetabolities, dRibonuclease inhibitors, antiepileptics
Congenital disorders leading to B12 and folate deficiencies
- Deficiencies of B12-associated proteins: cubilin, IF, transcobalamin II
- Deficiencies of folate-associated proteins: DHF reductase, HCY methyl transferase
Megaloblastic Anemia
- morphologically distinctive macrocytic anemia: impaired DNA synthesis interrupts cell cycle from G2 to M; nuclear and cytoplasmic dyssynchrony (cytoplasm matures normally, nucleus is lagging)
- large cells (megaloblasts), shortened lifespan (increase LDH, bilirubin)
- hypersegmented neutrophils
Is megaloblastic anemia always due to B12/folate deficiency?
No, can be due to antimetabolites that directly poison DNA production, like chemotherapy or antimicrobial agents
Neuropsych Pathology: mechanism
- in B12 deficiency, can develop in isolation due to folate masking
- often not reversible and affects brain and spinal cord
- elevated MMA prevents normal fatty acid synthesis, causing myeline destabilization and demyelination
Subacute Combined Degeneration of Spinal Cord
- gradual onset and progessive due to myelin loss (weakness, parasthesias in extremities and trunk, unsteady gait, + Romberg test and + babinski sign)
- often irreversible if B12 deficient for >6 months
If a patient is both folate and B12 deficient, which should you treat first?
-B12 because it can cause the irreversible neuropsychiatric events
Aside from Subacute Combined Degeneration of spinal cord, what are other neuropsych effects of B12 deficiency?
- Depression/Megaloblastic Madness/Somnolence
- Catecholamine synthesis (maintains moods) requires methionine to be formed, which needs B12 as cofactors
- Blindness: optic nerve atrophy, rare
Rx of B12 deficiency
- parenteral B12 is severe
- For pernicious anemia, oral B12 at high dose following parenteral replacement; law of mass action drives its uptake even without IF
- usually don’t need transfusion, as for most chronic anemias
Rx: folate deficiency
- oral folate of 400-1000 micrograms/daily
- response seen even in cases of malabsorption
