Mitochondria contain their own genomes, and unlike nuclear genomes, mitochondrial genomes are inherited maternally. With a high mutation rate and little recombination, special selection mechanisms exist in the female germline to prevent the accumulation of deleterious mutations 1 – 5 . The molecular mechanisms underpinning selection remain poorly understood 6 . Here, using an allele-specific fluorescent in situ -hybridization approach to distinguish wildtype from mutant mtDNA, we have visualized germline selection for the first time. Selection first manifests in the early stages of Drosophila oogenesis, triggered by reduction of the pro-fusion protein Mitofusin. This leads to the physical separation of mitochondrial genomes into different mitochondrial fragments, preventing the mixing of genomes and their products, and thereby reducing complementation. Once fragmentated, mitochondria harboring mutant genomes are less able to make ATP, which marks them for selection through a process requiring the mitophagy proteins Atg1 and BNIP3. Surprisingly, a reduction in Atg1 or BNIP3 decreases the amount of wildtype mtDNA, suggesting a link between mitochondrial turnover and mtDNA replication. Remarkably, fragmentation is not only necessary for selection in germline tissues, but also sufficient to induce selection in somatic tissues where selection is normally absent. Our studies posit a generalizable mechanism to select against deleterious mtDNA mutations that may allow the development of strategies for treatment of mtDNA disorders.

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Serine hydroxymethyltransferase 2 (SHMT2) converts serine plus tetrahydrofolate (THF) into glycine plus methylene-THF and is upregulated at the protein level in lung and other cancers. In order to better understand the role of SHMT2 in cancer a model system of HeLa cells engineered for inducible over-expression or knock-down of SHMT2 was characterized for cell proliferation and changes in metabolites and proteome as a function of SHMT2. Ectopic over-expression of SHMT2 increased cell proliferation in vitro and tumor growth in vivo. Knockdown of SHMT2 expression in vitro caused a state of glycine auxotrophy and accumulation of phosphoribosylaminoimidazolecarboxamide (AICAR), an intermediate of folate/1-carbon-pathway-dependent de novo purine nucleotide synthesis. Decreased glycine in the HeLa cell-based xenograft tumors with knocked down SHMT2 was potentiated by administration of the anti-hyperglycinemia agent benzoate. However, tumor growth was not affected by SHMT2 knockdown with or without benzoate treatment. Benzoate inhibited cell proliferation in vitro, but this was independent of SHMT2 modulation. The abundance of proteins of mitochondrial respiration complexes 1 and 3 was inversely correlated with SHMT2 levels. Proximity biotinylation in vivo (BioID) identified 48 mostly mitochondrial proteins associated with SHMT2 including the mitochondrial enzymes Acyl-CoA thioesterase (ACOT2) and glutamate dehydrogenase (GLUD1) along with more than 20 proteins from mitochondrial respiration complexes 1 and 3. These data provide insights into possible mechanisms through which elevated SHMT2 in cancers may be linked to changes in metabolism and mitochondrial function.

Mitochondria are unusual organelles in that they contain their own genomes, which are kept apart from the rest of the DNA in the cell. While mitochondrial DNA (mtDNA) is essential for respiration and most multicellular life, maintaining a genome outside the nucleus brings with it a number of challenges. Chief among these is preserving mtDNA genomic integrity from one generation to the next. In this review, we discuss what is known about negative (purifying) selection mechanisms that prevent deleterious mutations from accumulating in mtDNA in the germline. Throughout, we focus on the female germline, as it is the tissue through which mtDNA is inherited in most organisms and, therefore, the tissue that most profoundly shapes the genome. We discuss recent progress in uncovering the mechanisms of germline mtDNA selection, from humans to invertebrates. Expected final online publication date for the Annual Review of Genomics and Human Genetics, Volume 22 is August 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.