The role of Glycine N‐methyltransferase (GNMT) in cellular 1‐carbon metabolism

Abstract

GNMT is a folate binding protein commonly diminished in human hepatoma. We have demonstrated that GNMT assists cellular methyl group homeostasis in vitro. In the present study we postulated that GNMT assists cellular 1‐carbon metabolism and helps maintain genome integrity by promoting nucleotide biosynthesis. 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, and in Gnmt wildtype (Gnmt(+/+)), heterozygote (Gnmt(+/‐)) and knockout (Gnmt(‐/‐)) mice under folate deplete, replete, or supplementation conditions. We discovered that GNMT‐ cells utilized more exogenous formate in purine synthesis, suggesting that GNMT may promote endogenous formate generation. Furthermore, compared to GNMT‐, GNMT+ cells had increased percentage of the thymidine specie derived from the mitochondria as formate, supporting our hypothesis. In conclusion, GMMT assists cellular 1‐carbon metabolism in numerous ways. GNMT helps methyl group homeostasis, assists formate generation; supports methylene‐folate dependent pyrimidine synthesis and formylfolate dependent purine syntheses; and minimizes uracil incorporation into DNA in folate depletion. Loss of GNMT impairs nucleotide biosynthesis. Over‐expression of GNMT improves DNA integrity by reducing uracil misincorporation in DNA both in vitro and in vivo. The present study gives new insights into the role of GNMT on cellular 1‐carbon metablic kinetic.