bims-mitran 2023-04-23 papers

Issue of 2023–04–23
two papers selected by
Andreas Kohler, University of Graz

Dis Model Mech. 2023 Apr 01. pii: dmm049783. [Epub ahead of print]16(4):

Probing the pathogenicity of patient-derived variants of MT-ATP6 in yeast.

Emilia Baranowska, Katarzyna Niedzwiecka, Chiranjit Panja, Camille Charles, Alain Dautant, Jarosław Poznanski, Jean-Paul di Rago, Déborah Tribouillard-Tanvier, Roza Kucharczyk.

The list of mitochondrial DNA (mtDNA) variants detected in individuals with neurodegenerative diseases is constantly growing. Evaluating their functional consequences and pathogenicity is not easy, especially when they are found in only a limited number of patients together with wild-type mtDNA (heteroplasmy). Owing to its amenability to mitochondrial genetic transformation and incapacity to stably maintain heteroplasmy, and the strong evolutionary conservation of the proteins encoded in mitochondria, Saccharomyces cerevisiae provides a convenient model to investigate the functional consequences of human mtDNA variants. We herein report the construction and energy-transducing properties of yeast models of eight MT-ATP6 gene variants identified in patients with various disorders: m.8843T>C, m.8950G>A, m.9016A>G, m.9025G>A, m.9029A>G, m.9058A>G, m.9139G>A and m.9160T>C. Significant defect in growth dependent on respiration and deficits in ATP production were observed in yeast models of m.8950G>A, m.9025G>A and m.9029A>G, providing evidence of pathogenicity for these variants. Yeast models of the five other variants showed very mild, if any, effect on mitochondrial function, suggesting that the variants do not have, at least alone, the potential to compromise human health.

Keywords: MT-ATP6 ; ATP synthase; LHON; MILS; Mitochondrial DNA mutation; Mitochondrial diseases; Yeast; mtDNA

DOI: https://doi.org/10.1242/dmm.049783

PNAS Nexus. 2023 Apr;2(4): pgad105

Reduced methionine synthase expression results in uracil accumulation in mitochondrial DNA and impaired oxidative capacity.

Katarina E Heyden, Joanna L Fiddler, Yuwen Xiu, Olga V Malysheva, Michal K Handzlik, Whitney N Phinney, Linsey Stiles, Sally P Stabler, Christian M Metallo, Marie A Caudill, Martha S Field.

Adequate thymidylate [deoxythymidine monophosphate (dTMP) or the "T" base in DNA] levels are essential for stability of mitochondrial DNA (mtDNA) and nuclear DNA (nDNA). Folate and vitamin B12 (B12) are essential cofactors in folate-mediated one-carbon metabolism (FOCM), a metabolic network which supports synthesis of nucleotides (including dTMP) and methionine. Perturbations in FOCM impair dTMP synthesis, causing misincorporation of uracil (or a "U" base) into DNA. During B12 deficiency, cellular folate accumulates as 5-methyltetrahdryfolate (5-methyl-THF), limiting nucleotide synthesis. The purpose of this study was to determine how reduced levels of the B12-dpendent enzyme methionine synthase (MTR) and dietary folate interact to affect mtDNA integrity and mitochondrial function in mouse liver. Folate accumulation, uracil levels, mtDNA content, and oxidative phosphorylation capacity were measured in male Mtr+/+ and Mtr+/- mice weaned onto either a folate-sufficient control (C) diet (2 mg/kg folic acid) or a folate-deficient (FD) diet (lacking folic acid) for 7 weeks. Mtr heterozygosity led to increased liver 5-methyl-THF levels. Mtr+/- mice consuming the C diet also exhibited a 40-fold increase in uracil in liver mtDNA. Mtr+/- mice consuming the FD diet exhibited less uracil accumulation in liver mtDNA as compared to Mtr+/+ mice consuming the FD diet. Furthermore, Mtr+/- mice exhibited 25% lower liver mtDNA content and a 20% lower maximal oxygen consumption rates. Impairments in mitochondrial FOCM are known to lead to increased uracil in mtDNA. This study demonstrates that impaired cytosolic dTMP synthesis, induced by decreased Mtr expression, also leads to increased uracil in mtDNA.

Keywords: DNA; folate; methionine synthase; uracil; vitamin B12

DOI: https://doi.org/10.1093/pnasnexus/pgad105