Folate rescues vitamin B ₁₂ depletion-induced inhibition of nuclear thymidylate biosynthesis and genome instability

Abstract

Clinical vitamin B deficiency can result in megaloblastic anemia, which results from the inhibition of DNA synthesis by trapping folate cofactors in the form of 5-methyltetrahydrofolate (5-methylTHF) and subsequent inhibition of de novo thymidylate (dTMP) biosynthesis. In the cytosol, vitamin B functions in the remethylation of homocysteine to methionine, which regenerates THF from 5-methylTHF. In the nucleus, THF is required for de novo dTMP biosynthesis, but it is not understood how 5-methylTHF accumulation in the cytosol impairs nuclear dTMP biosynthesis. The impact of vitamin B depletion on nuclear de novo dTMP biosynthesis was investigated in methionine synthase-null human fibroblast and nitrous oxide-treated HeLa cell models. The nucleus was the most sensitive cellular compartment to 5-methylTHF accumulation, with levels increasing greater than fourfold. Vitamin B depletion decreased de novo dTMP biosynthesis capacity by 5-35%, whereas de novo purine synthesis, which occurs in the cytosol, was not affected. Phosphorylated histone H2AX (γH2AX), a marker of DNA double-strand breaks, was increased in vitamin B depletion, and this effect was exacerbated by folate depletion. These studies also revealed that 5-formylTHF, a slow, tight-binding inhibitor of serine hydroxymethyltransferase (SHMT), was enriched in nuclei, accounting for 35% of folate cofactors, explaining previous observations that nuclear SHMT is not a robust source of one-carbons for de novo dTMP biosynthesis. These findings indicate that a nuclear 5-methylTHF trap occurs in vitamin B depletion, which suppresses de novo dTMP biosynthesis and causes DNA damage, accounting for the pathophysiology of megaloblastic anemia observed in vitamin B and folate deficiency.

Methionine synthase is essential for cancer cell proliferation in physiological folate environments

“…In both A549 and T.T cells, MTR is essential for proliferation when 5methyl THF is the sole folate source ( Figure 2B-C). MTR knockout cells display high steadystate levels of 5-methyl THF regardless of folate source ( Figure 2D-E), consistent with previous results 27 . This trapping of folate as 5-methyl THF in the MTR knockout cells may explain the small decrease in proliferation observed in A549 MTR knockout cells compared to EV control cells when cultured in folic acid.…”

Joint effects of folate and vitamin B12 imbalance with maternal characteristics on gestational diabetes mellitus

“…Metabolically, both folate and vitamin B 12 are required for nucleic acid synthesis and methyl group generation, and are linked through the remethylation of Hcy to methionine. 13 Vitamin B 12 deficiency will inhibit DNA synthesis by trapping folate as 5-methyltetrahydrofolate, 24 and reduction of mitochondrial DNA in obese people is considered to be associated with insulin resistance. 25 Recent studies revealed that folic acid could play a role in the development of diabetes via natural killer cell dysfunction, 26 and low vitamin B 12 levels in pregnancy alter adipose-derived circulating microRNAs, which may mediate an adipogenic and insulin resistance.…”

Safety of folic acid

“…Given the close biochemical relationship between folate and vitamin B 12 , it is biologically plausible that, among individuals with low vitamin B 12 status, those with different underlying folate status could have different short‐term responses to injections of vitamin B 12 . It is well established in the literature that elevated intake of folate or folic acid can functionally compensate for vitamin B 12 deficiency, as first reported for the masking of vitamin B 12 deficiency–related megaloblastic anemia by folic acid . Hence, vitamin B 12 ‐deficient individuals with elevated folate status would be expected to exhibit less improvement in functional indicators as a result of vitamin B 12 therapy than those with lower folate status because of the functional compensation afforded by folate in vitamin B 12 deficiency.…”

“…However, elevated folic acid intake is not known to rescue neurological pathology resulting from vitamin B 12 deficiency. There are no proposed biological premises or suggested mechanisms whereby elevated folic acid intake exacerbates vitamin B 12 deficiency at the level of metabolism, cellular physiology, or human pathogenesis …”

“…There are no proposed biological premises or suggested mechanisms whereby elevated folic acid intake exacerbates vitamin B 12 deficiency at the level of metabolism, cellular physiology, or human pathogenesis. 58 Concerns for adverse effects of high folic acid intakes on cognition in vitamin B 12 deficiency arose initially from case reports. 59 An observational study of data collected in the NHANES found that "In seniors with low vitamin B 12 status, high serum folate was associated with anemia and cognitive impairment.…”