Mitochondria are structures within cells that convert the energy from food into a form that cells can use. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA (known as mitochondrial DNA or mtDNA). In some cases, inherited changes in mitochondrial DNA can cause problems with growth, development, and function of the body’s systems. These variants (also known as mutations) disrupt the mitochondria’s ability to generate energy efficiently for cells.

Conditions caused by variants in mitochondrial DNA often involve multiple organ systems. The effects of these conditions are most pronounced in organs and tissues that require a lot of energy (such as the heart, brain, and muscles). Although the health consequences of inherited mitochondrial DNA alterations vary widely, frequently observed features include muscle weakness and wasting, problems with movement, diabetes, kidney failure, heart disease, loss of intellectual functions (dementia), hearing loss, and problems involving the eyes and vision.

Genetic changes that are not inherited (which are known as somatic variants) may also occur in mitochondrial DNA. Somatic variants occur in the DNA of certain cells (not sperm or egg cells) during a person’s lifetime and are not passed to future generations. Because mitochondrial DNA has a limited ability to repair errors, these variants tend to build up over time. A buildup of somatic variants in mitochondrial DNA has been associated with some forms of cancer and an increased risk of certain age-related disorders such as heart disease, Alzheimer's disease, and Parkinson's disease. Additionally, research suggests that the progressive accumulation of these variants over a person’s lifetime may play a role in the normal process of aging.