Deranged DNA Synthesis by Bone Marrow from Vitamin B12‐Deficient Humans*

Abstract

S ummary. The synthesis of thymine DNA from deoxyuridine and uridine was studied by culturing human bone marrow for 1–3 hours in the presence of radioactive‐labelled nucleosides. In the current study, this in vitro system appeared useful to test the potential efficacy of agents (such as methionine) which might be used to improve or retard improvement of patients with megaloblastic anaemia. In the bone marrow from patients with B 12‐deficient or folate‐deficient megaloblastic anaemia, there was defective incorporation of deoxyuridine into thymine DNA. This defect was partially corrected by added B 12 in the B 12‐deficient but not the folate‐deficient marrows, and completely corrected by folic acid (pteroylglutainic acid) in both types of deficient marrow. The folate antagonist rnethotrexate blocked the corrective effect of B 12. Incorporation of labelled uridine into DNA of B 12‐deficient megaloblastic marrow was enhanced less by B 12 than by PGA. Vitamin B 12 antagonists failed to depress the incorporation of labelled uridine into DNA in bone marrows from both normal and B 12‐deficient patients. These results support the concept that inadequate DNA synthesis in B 12 deficiency is due in large measure to blockade in folate metabolism brought about by lack of B 12. 5‐Methyl‐tetrahydrofolate, which may accumulate in B 12 deficiency, failed to correct the defect in DNA synthesis, adding further evidence to the concept of metabolic trapping of this form of folate in B 12 deficiency. Thus, riboside reduction in man may be independent of B 12, as it is in E. coli, rather than dependent on B 12, as it is in L. leichmannii.