Genomic stability is dependent upon availability of sufficient thymidylate to support DNA synthesis and limit misincorporation of uracil into newly synthesized DNA.  Folate is required for one-carbon metabolism, which underpins purine base synthesis in the nucleus and mitochondria.  Within the mitochondria, deoxyuridylate is converted to thymidylate by tetrahydrofolate-dependent enzymes, such as serine hydroxymethyltranferase (SHMT2).  Although the impact of low folate levels or loss of Shmt2 has been documented in some cell lines, the impact of Shmt2 loss and folate deficiency has not been studied using nontransformed cells or primary tissues.  Fiddler and colleagues conducted a study to fill this void in our understanding and publish their results in the October 2021 issue of The Journal of Nutrition.

Mice (Shmt2+/+ or Shmt2+/-) were provided diets with sufficient folate levels or lacking folate for 7 weeks.  Tissue folate levels and uracil incorporation into liver mtDNA were determined.  Embryonic fibroblasts (MEF) from these mice were cultured in defined medium containing 0 or 25 nM folate and proliferative capacity and mitochondrial function were determined. 

Shmt2+/- led to reductions in cellular SHMT2 protein (48-67%), and these mice exhibited a 25% reduction in liver mitochondrial folate even when consuming the folate replete diet.  Reduction in liver folate in the heterozygote mice led to a greater than 20-fold increase in mtDNA uracil content when the folate containing diet was consumed, and when the diet lacking folate was consumed there was increased uracil incorporation for both mouse genotypes.  Cell proliferation, mitochondrial membrane potential and oxygen consumption rate were all decreased in MEF cells with decreased Shmt2 expression. 

These observations led the authors to conclude that Shmt2 heterozygosity and folate deficiency impairs mitochondrial thymidylate synthesis, increases uracil incorporation into mtDNA, and impairs mitochondrial function due to elevated uracil in mtDNA.

References

Fiddler JL, Xiu Y, Blum JE, Lamarre SG, Phinney WN, Stabler SP, Brosnan ME, Brosnan JT, Thalacker-Mercer AE, Field MS.  Reduced Shmt2 expression impairs mitochondrial folate accumulation and respiration, and leads to uracil accumulation in mouse mitochondrial DNA.  The Journal of Nutrition, Volume 151, Issue 10, October 2021, Pages 2882–2893, https://doi.org/10.1093/jn/nxab211.

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Nancy Turner, PhD

Nancy Turner is a Professor of Nutrition at Michigan State University.  Her research focus is on the role of diet on colon cancer and inflammatory bowel disease.  She is the author of many papers on this topic, as well as the co-editor of a book, and co-author of a textbook.