What Are Mitochondria and Why Do They Matter?

Mitochondria are often described as the “powerhouses” of our cells, producing the energy that allows our bodies to function. Mitochondrial disorders affect an estimated 1 in 5,000 people worldwide, making them one of the more frequently diagnosed inherited metabolic conditions, though still less common than cystic fibrosis, which affects about 1 in 3,000 newborns in Northern Europe. Mitochondria evolved from an ancient bacterium that joined an archaeal host, transforming into an organelle by integrating metabolism, genes, and protein systems. Mitochondria contain their own small set of DNA, known as mitochondrial DNA (mtDNA).  mtDNA).   

What Are Mitochondria and Why Do They Matter?

Mitochondria are often described as the “powerhouses” of our cells, producing the energy that allows our bodies to function. Mitochondrial disorders affect an estimated 1 in 5,000 people worldwide, making them one of the more frequently diagnosed inherited metabolic conditions, though still less common than cystic fibrosis, which affects about 1 in 3,000 newborns in Northern Europe. Mitochondria evolved from an ancient bacterium that joined an archaeal host, transforming into an organelle by integrating metabolism, genes, and protein systems. Mitochondria contain their own small set of DNA, known as mitochondrial DNA (mtDNA).  mtDNA).   

How Mitochondrial DNA Differs from Nuclear DNA 

While nuclear DNA (nDNA) is organised into chromosomes, mtDNA is smaller and circular in shape.
While nDNA comes from both parents, mtDNA is inherited exclusively from the mother, as spermatoid mitochondria are eliminated after fertilisation.
This maternal inheritance pattern means that variants in mtDNA can be transmitted consistently through the maternal line, sometimes affecting multiple generations. 

The majority of genetic diseases are caused by variants in nDNA. However, mitochondrial diseases can also arise from variants in mtDNA, which has a mutation rate up to 100 times higher than nDNA, though not all mutations will cause disease. When faulty and healthy mtDNA coexist in the same cell, a state called heteroplasmy, the proportion of the faulty DNA variants influences how severe the mitochondrial disease is. Even within the same family, symptoms can vary greatly, and this unpredictability makes mitochondrial disorders particularly difficult to diagnose and manage. 

The Ethical Challenges for Families 

For families affected by mitochondrial disease, genetic inheritance is not just a scientific issue but a personal one. As the condition is inherited maternally, women who carry mtDNA variants may face difficult decisions about family planning. 

Genetic counselling plays a central role in helping families understand their risks and options. In cases where in vitro fertilisation is used, preimplantation genetic diagnosis can screen embryos for mitochondrial mutations, while prenatal testing can detect them during pregnancy. Novel technologies bring their own ethical considerations, including questions about reproductive choice, accessibility, and the emotional impact of decision-making. 

Mitochondrial disease is particularly challenging for healthcare providers due to the wide variety of symptoms, often affecting multiple organs, especially those tissues with high energy requirements. This broad effect means patients require support from different medical specialists. In addition, families often need long-term psychological and social support as they navigate the realities of living with a rare disease. 

Looking Ahead: Research and Hope for the Future 

A promising therapy area is mitochondrial replacement therapy (MRT), sometimes called “three-parent IVF.” In the UK, two approaches are approved for mitochondrial replacement therapy. In maternal spindle transfer (MST), the mother’s nDNA is placed into a donor egg with healthy mitochondria before fertilisation. In pronuclear transfer (PNT), both the mother’s and donor’s eggs are fertilised first, and then the parents’ nDNA is moved into the into the donor zygote with its own DNA removed, alongside healthy mitochondria. In both cases, the resulting embryo carries the parents’ nDNA alongside donor mitochondria, ensuring that all cells develop with healthy energy production, offering the possibility of preventing the disease. There are, however, concerns about the long-term safety, as there is uncertainty if the mitochondrial – nDNA interactions will be affected by such a treatment. MRT is not only permanent but inheritable, news which should be considered with cautious optimism. Ongoing, large-scale, population studies, are improving our understanding of how common mtDNA variants are and what influences whether they could cause disease. This research could open the door to earlier diagnoses, better management strategies, and new treatments.