116: Silent but Stalling: A Synonymous mtDNA Variant Shapes CD8+ T Cells

Lareau CA et al., PNAS - Single-cell multiomic profiling identifies a mosaic synonymous mtDNA variant (m.7076A>G) in MT-CO1 that is selectively depleted in CD8+ effector memory T cells. Mechanistic assays show the variant forces wobble decoding, stalls mitochondrial ribosomes, and impairs differentiation of high-demand effector T cells. Key terms: mitochondria, synonymous mutation, mtDNA, CD8+ T cells, translation.

Study Highlights:
The study identifies a mosaic synonymous mtDNA mutation m.7076A>G in MT-CO1 present at ~47% heteroplasmy in a healthy donor and shows selective depletion of the mutant allele specifically in CD8+ effector memory/short-lived effector T cells. MT-CO1 transcript abundance is unchanged by genotype, but mitochondrial ribosome profiling reveals enrichment of the mutant allele in ribosome-protected fragments, consistent with translational stalling. The effect is explained by the limited mitochondrial tRNA pool: the mutant GGG codon requires wobble/super-wobble decoding by the single mt-tRNAGly, reducing codon:anticodon affinity and slowing translation. Short-lived effector CD8+ T cells have elevated translational and metabolic demands that amplify dependence on efficient MT-CO1 translation, driving lineage-specific purifying selection.

Conclusion:
A synonymous mtDNA change can have cell type–specific functional consequences by altering codon syntax and translation efficiency; single-cell multiomics reveals purifying selection of such variants in metabolically demanding immune states. These results expand annotation of mitochondrial variation and motivate targeted studies of codon optimality in mtDNA.

Music:
Enjoy the music based on this article at the end of the episode.

Article title:
Cell type–specific purifying selection of synonymous mitochondrial DNA variation

First author:
Lareau CA

Journal:
PNAS

DOI:
10.1073/pnas.2505704122

Reference:
Lareau CA, Zielinski S, Stickels RR, et al. Cell type–specific purifying selection of synonymous mitochondrial DNA variation. PNAS. 2025;122(30):e2505704122. doi:10.1073/pnas.2505704122

License:
Creative Commons Attribution License 4.0 (CC BY)

Support:
Base by Base – Stripe donations: https://donate.stripe.com/7sY4gz71B2sN3RWac5gEg00

Official website https://basebybase.com

On PaperCast Base by Base you’ll discover the latest in genomics, functional genomics, structural genomics, and proteomics.

Episode link: https://basebybase.com/episodes/cell-typespecific-purifying-selection-of-synonymous-mtdna-variation

QC:
This episode was checked against the original article PDF and publication metadata for the episode release published on 2025-08-24.

QC Scope:
- article metadata and core scientific claims from the narration
- excludes analogies, intro/outro, and music
- transcript coverage: Audited the scientific content describing (1) identification of the mosaic mtDNA variant m.7076A>G in MT-CO1, (2) CD8+ TEM–specific depletion of the mutant allele, (3) stable MT-CO1 mRNA across genotypes, (4) ribosome profiling showing translational stalling of the mutant codon, (5) wobble decoding due to mt-tRNA limit
- transcript topics: Identification of mosaic mtDNA variant m.7076A>G in MT-CO1; CD8+ TEM depletion of m.7076G allele; MT-CO1 transcript levels remain stable; Mitochondrial ribosome profiling and translation pausing; Glycine tRNA limitation and super-wobble decoding; SLEC metabolic dependence on OXPHOS and puromycin/SCENITH data

QC Summary:
- factual score: 10/10
- metadata score: 10/10
- supported core claims: 7
- claims flagged for review: 0
- metadata checks passed: 4
- metadata issues found: 0

Metadata Audited:
- article_doi
- article_title
- article_journal
- license

Factual Items Audited:
- m.7076A>G mosaic synonymous mtDNA variant in MT-CO1 present at ~47% heteroplasmy in donor PBMCs
- Mutant m.7076G allele selectively depleted in CD8+ effector memory (TEM) cells, not in other T cell subsets
- MT-CO1 mRNA levels are unchanged by genotype
- Mutant allele enriched in ribosome-protected fragments, indicating translational stalling
- CD8+ T cells with high metabolic demands (SLECs) show increased translational/metabolic activity and selective pressure against the mutant allele
- Limited mitochondrial tRNA pool (22 tRNAs); glycine decoding relies on wobble; GGG codon necessitates super-wobble decoding

QC result: Pass.