Cells are the building blocks of our body, and the proper functioning of each cell contributes to our overall health. Within each cell there are several smaller structures, called organelles, each of which performs a specific task that is necessary for normal cellular function. The nucleus, for example, is an organelle that contains most of the cell’s DNA, or its genetic blueprint, and is necessary for normal gene expression or read- out of this blueprint. Another organelle, the mitochondrion, can be likened to the power station of the cell, and is necessary to break down sugars, protein, and fats to produce the energy that maintains normal cellular function. An average cell has thousands of mitochondria, although cells that require more energy, such as muscle or brain cells, generally have more. If the mitochondria do not work properly, or if their number is decreased, the cell’s energy supply can be depleted, disrupting normal cellular function, thereby causing disease.
An average cell contains approximately 4000 copies of mitochondria, although cells that require more energy, such as neurons and/or muscle cells, may have more. Because of the large number of mitochondria in each cell, dysfunction in a few of them has little effect. Only when a significant percent of mitochondria malfunction, reaching a critical threshold, do cellular functions become compromised and clinical symptoms occur. Because the distribution of normal to abnormal mitochondria can vary between different cells and/or different body regions, the clinical features of mitochondrial diseases are difficult to predict. Symptoms can thus vary widely, both within various organ systems, and also between patients.
Mitochondrial diseases were once thought to be rare genetic conditions presenting in childhood. A recent study has shown, however, that mitochondrial DNA mutations may occur in as many as one in 200 individuals. As we learn more about mitochondria and how they function within a cell and organism, it has also become clear that mitochondrial dysfunction may be involved in many more common conditions such as Parkinson’s disease, cancer, stroke, and even general ageing.