Mitochondrial dysfunction central to thymidine kinase 2 deficiency (TK2d) may be better understood through a new gene-editing approach that maps how cells control mitochondrial DNA (mtDNA), according to a study published recently in Nature Structural and Molecular Biology.
These results offer potential paths toward targeted therapies. In TK2d, where mtDNA depletion drives muscle weakness and systemic disease, this study highlights how nuclear genes directly influence mtDNA copy number and mutation burden within individual cells.
Mitochondria carry their own DNA, but it is tightly regulated by the nucleus. Each cell contains many copies of mtDNA, and mutations may affect only some copies, a state called heteroplasmy. In TK2d and related mitochondrial diseases, both the number of mtDNA copies and the proportion of mutated DNA determine how severely tissues are affected. This study used a method called MitoPerturb-Seq to examine these processes at single-cell resolution.
“MitoPerturb-Seq provides a powerful forward genetic screening approach to discover the biological mechanisms driving age-dependent mtDNA [copy number] reduction or clonal expansion of damaging mtDNA mutations, thereby uncovering novel druggable targets to treat both rare and common mtDNA-related and neurodegenerative disorders,” the study’s authors explained.
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Researchers analyzed 11,990 cells carrying a disease-relevant mtDNA mutation and identified 6,551 high-quality cells for detailed study. They targeted 13 nuclear genes known to influence mitochondrial function. Disrupting the genes Tfam, Opa1 and Polg consistently reduced mtDNA levels in cells. This depletion did not significantly change the average mutation level but increased variability between cells, suggesting a “genetic bottleneck” effect that may push some cells past disease thresholds seen in TK2d.
This study also showed that mtDNA depletion triggered a stress response in cells. Many genes involved in protein production and metabolism were activated, partly controlled by the stress-related factor ATF4. However, only about 38.8% of affected genes were directly linked to ATF4, indicating that other pathways also contribute. This complexity may help explain why TK2d symptoms vary widely between patients and tissues.
Importantly, cells with depleted mtDNA grew more slowly, with delays across all phases of the cell cycle rather than at a single checkpoint. This contrasts with other mitochondrial defects and suggests that severe mtDNA loss, as seen in TK2d, broadly limits cellular energy and function. Despite these changes, mutation levels remained stable, reinforcing that mtDNA quantity, not just mutation load, is critical in disease progression.
For patients, these findings suggest that future treatments for TK2d may focus on maintaining or restoring mtDNA copy number rather than only reducing mutations. By identifying specific nuclear genes and pathways that regulate mtDNA, this research opens the door to therapies tailored to the underlying biology of mitochondrial depletion disorders.
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