Mitochondrial Mutations in Focus
Mitochondrial DNA (mtDNA), located outside the protection of the cell nucleus, mutates at a much higher rate than nuclear DNA—10 to 100 times faster. With its compact structure of only 16,569 base pairs and lack of non-coding introns, mtDNA is highly vulnerable, where even a single mutation can have a significant impact. Mutations in all mitochondria within a cell result in homoplasmy, whereas partial mutations lead to heteroplasmy, which can vary in severity depending on the cell type. These variations can cause serious health issues, including heart problems.
Mitochondria and Reprogramming
While harmful mtDNA mutations often outcompete beneficial ones within certain cells, scientists have yet to determine why this happens or to what extent these mutations contribute to age-related health issues. Research has shown that reprogramming cells into iPSCs can significantly alter mitochondrial heteroplasmy, either amplifying or entirely erasing mutations. This is particularly critical since mitochondrial quality directly affects stem cell functionality.
Experimenting with Mutations
Researchers examined three cell lines: one with the A3243G point mutation affecting 89% of mitochondria and two with the Δ4977 deletion, which impacted a smaller percentage of mitochondria. The A3243G mutation severely impaired respiration, while Δ4977 had a milder effect. 
Reprogramming these cells into iPSCs revealed drastic changes. A3243G cells either retained similar mutation levels or eliminated them entirely. For Δ4977 cells, the mutation initially increased during reprogramming but sharply declined after four divisions, except in cells with very high mutation levels. This suggests that reprogramming creates a “selection pressure” favoring either dominant mutations or their removal.
Functional and Epigenetic Outcomes
The impact of mitochondrial mutations became apparent during differentiation. While Δ4977-mutated iPSCs differentiated into fat and bone cells without visible issues, they failed to function properly as cardiac cells. Mutations disrupted critical metabolic processes and oxidative stress pathways, leading to impaired cardiac functionality.
Interestingly, Δ4977-mutated iPSCs also exhibited significant differences in epigenetic age, surpassing the variations typically seen between cells reprogrammed from 20-year-olds and 100-year-olds, according to the Horvath clock.
Implications for Therapy and Research
These findings hold promise for mitochondrial disease research by offering a reliable source of cells with specific mutations. Clinically, the ability of iPSC reprogramming to eliminate accumulated mtDNA mutations ensures a safer starting point for therapeutic applications, particularly in cardiac and functional tissue engineering.
By reshaping mitochondrial mutations, iPSC technology offers hope for both advancing research and improving the safety of regenerative therapies.
