Mitochondrial DNA (mtDNA) mutations are among the most frequent genetic alterations in human cancers, yet their molecular mechanisms in shaping tumor progression and metastasis under physiological conditions remain poorly understood. Here, we show that mtDNA heteroplasmy acts as a tumor cell-intrinsic rheostat that orchestrates fate decisions between progression and suppression. Moderate heteroplasmy induces a ribosomal storm of mitochondrial translation and ribosome biogenesis, driven by leucyl-tRNA synthetase 2 (LARS2). This response enhances metabolic fitness and licenses the secretion of S100A8/A9, enabling tumor cells to actively remodel the immune microenvironment through recruitment of myeloid-derived suppressor cells and exclusion of CD8 T cells, thereby promoting immune evasion and metastatic progression. In contrast, high heteroplasmy impairs tumorigenesis due to oxidative phosphorylation collapse. Disruption of LARS2, mitochondrial translation, or S100A9 signalling abrogates tumor progression. Our findings establish mtDNA heteroplasmy as a key regulator of cancer evolution, linking mitochondrial genetic variation to immune modulation, and reveal the LARS2-mitochondrial translation-S100A8/A9 axis as a therapeutically targetable programme within the pro-tumorigenic window of mtDNA heteroplasmy. ### Competing Interest Statement The authors have declared no competing interest. National Natural Science Foundation of China, 82271897, 32570717 Zhejiang Provincial Natural Science Foundation of China, LZYQ25C070001 the Westlake University Research Center for industries of the Future, WU2022C013 Key R&D Program of Zhejiang, 2024SSYS0033 Westlake Education Foundation

Bystander editing remains a major limitation of current base editors, hindering their precision and therapeutic potential. Here, we present a de novo protein design strategy that creates a structurally rigid interface between a DNA-binding TALE domain and a cytosine deaminase, forming a unified editing module termed TALE-oriented deaminase (TOD). Cryo-EM analysis of TOD-DNA complexes confirms that this precise spatial architecture tightly restricts the deaminase activity window, thereby minimizing unwanted deamination. To further enhance editing specificity, we develop a split version, termed DdCBE-TOD, which virtually eliminates off-target editing. As a proof of concept, we apply DdCBE-TOD to generate a mitochondrial disease mouse model and to correct a pathogenic mutation associated with MERRF syndrome in patient-derived cells, achieving single-nucleotide precision. This work introduces a generalizable and computationally guided approach for ultra-precise base editing, offering a promising platform for both mechanistic studies and therapeutic correction of single-nucleotide mutations.

Oocytes play a crucial role in transmitting maternal mitochondrial DNA (mtDNA), essential for the continuation of species. However, the effects of mitochondrial reactive oxygen species (ROS) on mammalian oocyte maturation and mtDNA maintenance remain unclear. We investigated this by conditionally knocking out the Sod2 gene in primordial follicles, elevating mitochondrial matrix ROS levels from early oocyte stages. Our data indicates that reproductive aging in Sod2 conditional knockout females begins at 6 months, with oxidative stress impairing oocyte quality, particularly affecting OXPHOS complex II and mtDNA-encoded mRNA levels. Despite unchanged mtDNA mutation load, mtDNA copy numbers exhibited significant variations. Strikingly, reducing mtDNA copy numbers by reducing mtSSB protein, crucial for mtDNA replication, accelerated reproductive aging onset to three months, underscoring the critical role of mtDNA copy number maintenance under oxidative stress conditions. This research provides new insights into the relationship among mitochondrial ROS, mtDNA, and reproductive aging, offering potential strategies for delaying aging-related fertility decline. Elevating mitochondrial ROS induces female reproductive aging without increasing maternal mtDNA mutation load where deducing mtDNA copy numbers accelerates this phenomenon, highlighting the importance of maintaining mtDNA quantity under oxidative stress.

Mitochondrial diseases, caused mainly by pathogenic mitochondrial DNA (mtDNA) mutations, pose major challenges due to the lack of effective treatments. Investigating the patterns of maternal transmission of mitochondrial diseases could pave the way for preventive approaches. In this study, we used DddA-derived cytosine base editors (DdCBEs) to generate two mouse models, each haboring a single pathogenic mutation in complex I genes (ND1 and ND5), replicating those found in human patients. Our findings revealed that both mutations are under strong purifying selection during maternal transmission and occur predominantly during postnatal oocyte maturation, with increased protein synthesis playing a vital role. Interestingly, we discovered that maternal age intensifies the purifying selection, suggesting that older maternal age may offer a protective effect against the transmission of deleterious mtDNA mutations, contradicting the conventional notion that maternal age correlates with increased transmitted mtDNA mutations. As collecting comprehensive clinical data is needed to understand the relationship between maternal age and transmission patterns in humans, our findings may have profound implications for reproductive counseling of mitochondrial diseases, especially those involving complex I gene mutations. Mitochondrial DNA mutations are subject to purifying selection in the female germline, limiting the transmission of pathogenic variants. In this study, the authors used two mouse models that harbor pathogenic mutations in mitochondrial complex I and observed that maternal age intensifies purifying selection processes.

The in vivo role for RNase H1 in mammalian mitochondria has been much debated. Loss of RNase H1 is embryonic lethal and to further study its role in mtDNA expression we characterized a conditional knockout of Rnaseh1 in mouse heart. We report that RNase H1 is essential for processing of RNA primers to allow site-specific initiation of mtDNA replication. Without RNase H1, the RNA:DNA hybrids at the replication origins are not processed and mtDNA replication is initiated at non-canonical sites and becomes impaired. Importantly, RNase H1 is also needed for replication completion and in its absence linear deleted mtDNA molecules extending between the two origins of mtDNA replication are formed accompanied by mtDNA depletion. The steady-state levels of mitochondrial transcripts follow the levels of mtDNA, and RNA processing is not altered in the absence of RNase H1. Finally, we report the first patient with a homozygous pathogenic mutation in the hybrid-binding domain of RNase H1 causing impaired mtDNA replication. In contrast to catalytically inactive variants of RNase H1, this mutant version has enhanced enzyme activity but shows impaired primer formation. This finding shows that the RNase H1 activity must be strictly controlled to allow proper regulation of mtDNA replication.

We report a role for the mitochondrial single-stranded DNA binding protein (mtSSB) in regulating mitochondrial DNA (mtDNA) replication initiation in mammalian mitochondria. Transcription from the light-strand promoter (LSP) is required both for gene expression and for generating the RNA primers needed for initiation of mtDNA synthesis. In the absence of mtSSB, transcription from LSP is strongly up-regulated, but no replication primers are formed. Using deep sequencing in a mouse knockout model and biochemical reconstitution experiments with pure proteins, we find that mtSSB is necessary to restrict transcription initiation to optimize RNA primer formation at both origins of mtDNA replication. Last, we show that human pathological versions of mtSSB causing severe mitochondrial disease cannot efficiently support primer formation and initiation of mtDNA replication.

Mitochondrial transcription factor A (TFAM) is compacting mitochondrial DNA (dmtDNA) into nucleoids and directly controls mtDNA copy number. Here, we show that the TFAM-to-mtDNA ratio is critical for maintaining normal mtDNA expression in different mouse tissues. Moderately increased TFAM protein levels increase mtDNA copy number but a normal TFAM-to-mtDNA ratio is maintained resulting in unaltered mtDNA expression and normal whole animal metabolism. Mice ubiquitously expressing very high TFAM levels develop pathology leading to deficient oxidative phosphorylation (OXPHOS) and early postnatal lethality. The TFAM-to-mtDNA ratio varies widely between tissues in these mice and is very high in skeletal muscle leading to strong repression of mtDNA expression and OXPHOS deficiency. In the heart, increased mtDNA copy number results in a near normal TFAM-to-mtDNA ratio and maintained OXPHOS capacity. In liver, induction of LONP1 protease and mitochondrial RNA polymerase expression counteracts the silencing effect of high TFAM levels. TFAM thus acts as a general repressor of mtDNA expression and this effect can be counterbalanced by tissue-specific expression of regulatory factors.

Pathogenic variants inFBXL4cause a severe encephalopathic syndrome associated with mtDNAdepletion and deficient oxidative phosphorylation. To gain further insight into the enigmatic pathophysiology caused byFBXL4 deficiency, we generated homozygousFbxl4knockout mice and found that they display a predominant perinatal lethality. Surprisingly, the few surviving animals are apparently normal until the age of 8-12 months when they gradually develop signs of mitochondrial dysfunction and weight loss. One-year-oldFbxl4knockouts show a global reduction in a variety of mitochondrial proteins and mtDNAdepletion, whereas lysosomal proteins are upregulated. Fibroblasts from patients withFBXL4 deficiency and humanFBXL4knockout cells also have reduced steady-state levels of mitochondrial proteins that can be attributed to increased mitochondrial turnover. Inhibition of lysosomal function in these cells reverses the mitochondrial phenotype, whereas proteasomal inhibition has no effect. Taken together, the results we present here show thatFBXL4 prevents mitochondrial removal via autophagy and that loss ofFBXL4 leads to decreased mitochondrial content and mitochondrial disease.

Oligoribonucleases are conserved enzymes that degrade short RNA molecules of up to 5 nt in length and are assumed to constitute the final stage of RNA turnover. Here we demonstrate that REXO2 is a specialized dinucleotide-degrading enzyme that shows no preference between RNA and DNA dinucleotide substrates. A heart- and skeletal-muscle-specific knockout mouse displays elevated dinucleotide levels and alterations in gene expression patterns indicative of aberrant dinucleotide-primed transcription initiation. We find that dinucleotides act as potent stimulators of mitochondrial transcription initiation in vitro. Our data demonstrate that increased levels of dinucleotides can be used to initiate transcription, leading to an increase in transcription levels from both mitochondrial promoters and other, nonspecific sequence elements in mitochondrial DNA. Efficient RNA turnover by REXO2 is thus required to maintain promoter specificity and proper regulation of transcription in mammalian mitochondria.

Regulation of replication and expression of mitochondrial DNA (mtDNA) is essential for cellular energy conversion via oxidative phosphorylation. The mitochondrial transcription elongation factor (TEFM) has been proposed to regulate the switch between transcription termination for replication primer formation and processive, near genome-length transcription for mtDNA gene expression. Here, we report that Tefm is essential for mouse embryogenesis and that levels of promoter-distal mitochondrial transcripts are drastically reduced in conditional Tefm-knockout hearts. In contrast, the promoter-proximal transcripts are much increased in Tefm knockout mice, but they mostly terminate before the region where the switch from transcription to replication occurs, and consequently, de novo mtDNA replication is profoundly reduced. Unexpectedly, deep sequencing of RNA from Tefm knockouts revealed accumulation of unprocessed transcripts in addition to defective transcription elongation. Furthermore, a proximity-labeling (BioID) assay showed that TEFM interacts with multiple RNA processing factors. Our data demonstrate that TEFM acts as a general transcription elongation factor, necessary for both gene transcription and replication primer formation, and loss of TEFM affects RNA processing in mammalian mitochondria.

Replication of mammalian mitochondrial DNA (mtDNA) is an essential process that requires high fidelity and control at multiple levels to ensure proper mitochondrial function. Mutations in the mitochondrial genome maintenance exonuclease 1 (MGME1) gene were recently reported in mitochondrial disease patients. Here, to study disease pathophysiology, we generated Mgme1 knockout mice and report that homozygous knockouts develop depletion and multiple deletions of mtDNA. The mtDNA replication stalling phenotypes vary dramatically in different tissues of Mgme1 knockout mice. Mice with MGME1 deficiency accumulate a long linear subgenomic mtDNA species, similar to the one found in mtDNA mutator mice, but do not develop progeria. This finding resolves a long-standing debate by showing that point mutations of mtDNA are the main cause of progeria in mtDNA mutator mice. We also propose a role for MGME1 in the regulation of replication and transcription termination at the end of the control region of mtDNA.

Mutations of mtDNA cause mitochondrial diseases and are implicated in age-associated diseases and aging. Pathogenic mtDNA mutations are often present in a fraction of all mtDNA copies, and it has been widely debated whether the proportion of mutant genomes or the absolute number of wild-type molecules determines if oxidative phosphorylation (OXPHOS) will be impaired. Here, we have studied the male infertility phenotype of mtDNA mutator mice and demonstrate that decreasing mtDNA copy number worsens mitochondrial aberrations of spermatocytes and spermatids in testes, whereas an increase in mtDNA copy number rescues the fertility phenotype and normalizes testes morphology as well as spermatocyte proteome changes. The restoration of testes function occurs in spite of unaltered total mtDNA mutation load. We thus demonstrate that increased copy number of mtDNA can efficiently ameliorate a severe disease phenotype caused by mtDNA mutations, which has important implications for developing future strategies for treatment of mitochondrial dysfunction.