A novel role of the tumor suppressor GNMT in cellular defense against DNA damage

Abstract

Glycine N-methyltransferase (GNMT) is a folate binding protein commonly diminished in human hepatoma yet its role in tumor development remains to be established. GNMT binds to methylfolate but is also inhibited by it; how such interactions affect human carcinogenesis is unclear. We postulated that GNMT plays a role in folate-dependent methyl group homeostasis and helps maintain genome integrity by promoting nucleotide biosynthesis and DNA repair. To test the hypothesis, GNMT was over-expressed in GNMT-null cell lines cultured in conditions of folate abundance or restriction. The partitioning of folate dependent 1-carbon groups was investigated using stable isotopic tracers and GC/MS. DNA damage was assessed as uracil content in cell models, as well as in Gnmt wildtype (Gnmt 1/1 ), heterozygote (Gnmt) and knockout (Gnmt 2/2 ) mice under folate deplete, replete, or supplementation conditions. Our study demonstrated that GMMT 1) supports methylene-folate dependent pyrimidine synthesis; 2) supports formylfolate dependent purine syntheses; 3) minimizes uracil incorporation into DNA when cells and animals were exposed to folate depletion; 4) translocates into nuclei during prolonged folate depletion.In conclusion, loss of GNMT impairs nucleotide biosynthesis. Over-expression of GNMT enhances nucleotide biosynthesis and improves DNA integrity by reducing uracil misincorporation in DNA both in vitro and in vivo. To our best knowledge, the role of GNMT in folate dependent 1-carbon transfer in nucleotide biosynthesis has never been investigated. The present study gives new insights into the underlying mechanism by which GNMT can participate in tumor prevention/suppression in humans.

In Vitro Interactions between 17β-Estradiol and DNA Result in Formation of the Hormone-DNA Complexes

“…On the basis of cellular functions, DNA is the key macromolecule, which is highly protected against damages coming from various substances including those called carcinogens [30,31]. Metabolism of estrogens via catechol estrogen pathway was previously characterized as a balanced set of activating and protective enzymes.…”

Nuclear Enrichment of Folate Cofactors and Methylenetetrahydrofolate Dehydrogenase 1 (MTHFD1) Protect de Novo Thymidylate Biosynthesis during Folate Deficiency

“…The primary structure of MTHFD1 does not contain a nuclear localization signal (NLS), although non-canonical three-dimensional NLSs are known to exist on other proteins. Similarly, the primarily cytosolic enzyme glycine N-methyltransferase, which transfers the methyl group from S-adenosylmethionine to glycine, was also shown to translocate to the nucleus in response to folate deficiency, where it also served to increase de novo purine and thymidylate synthesis and limit uracil in DNA (26). Glycine N-methyltransferase also lacks a canonical nuclear localization signal, and the mechanism by which it enters the nucleus remains unknown (27).…”

Targeted and untargeted metabolomics provide insight into the consequences of glycine‐N‐methyltransferase deficiency including the novel finding of defective immune function

“…Instead, we provide evidence that GNMT -/mice have impaired nicotinamide metabolism with increased PARP-1 activation suggesting that altered nucleotide metabolism is contributing to liver damage. GNMT -/mice have been reported to have increased DNA damage due to augmented folate metabolism (Wang et al, 2014) that thereby increases the need for PARP activity and demand for NAD + as a substrate. We propose that NAD + catabolism is greatly exacerbated by increased and aberrant methylation of nicotinamide and that this net increase in demand for NAD + biosynthesis cannot be met due to reduced catabolism of tryptophan through the kynurenine-quinolinate pathway.…”