Neuropathy, ataxia, and retinitis pigmentosa (NARP) is a condition that causes a variety of signs and symptoms that mainly affect the nervous system. The condition typically begins in childhood or early adulthood, and the signs and symptoms usually worsen over time. Most people with NARP experience numbness, tingling, or pain in the arms and legs (sensory neuropathy); muscle weakness; and problems with balance and coordination (ataxia). Many affected individuals also have vision loss caused by changes in the light-sensitive tissue that lines the back of the eye (the retina). In some cases, the vision loss results from a condition called retinitis pigmentosa. This eye disease causes the light-sensing cells of the retina gradually to deteriorate.

Learning disabilities and developmental delays are often seen in children with NARP, and older individuals with this condition may experience a loss of intellectual function (dementia). Other features of NARP include seizures, hearing loss, and abnormalities of the electrical signals that control the heartbeat (cardiac conduction defects). These signs and symptoms vary among affected individuals.

NARP results from mutations in the MT-ATP6 gene. This gene is contained in mitochondrial DNA, also known as mtDNA. 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.

The MT-ATP6 gene provides instructions for making a protein that is essential for normal mitochondrial function. Through a series of chemical reactions, mitochondria use oxygen and simple sugars to create adenosine triphosphate (ATP), the cell's main energy source. The MT-ATP6 protein forms one part (subunit) of an enzyme called ATP synthase, which is responsible for the last step in ATP production. Mutations in the MT-ATP6 gene alter the structure or function of ATP synthase, reducing the ability of mitochondria to make ATP. It remains unclear how this disruption in mitochondrial energy production leads to muscle weakness, vision loss, and the other specific features of NARP.

Normal Function

The MT-ATP6 gene provides information for making a protein that is essential for normal mitochondrial function. Mitochondria are structures within cells that convert the energy from food into a form that cells can use. These cellular structures produce energy through a process called oxidative phosphorylation, which uses oxygen and simple sugars to create adenosine triphosphate (ATP), the cell's main energy source.

The MT-ATP6 protein forms one part (subunit) of a large enzyme called ATP synthase. This enzyme, which is also known as complex V, is responsible for the final step of oxidative phosphorylation. Specifically, one segment of ATP synthase allows positively charged particles, called protons, to flow across a specialized membrane inside mitochondria. Another segment of the enzyme uses the energy created by this proton flow to convert a molecule called adenosine diphosphate (ADP) to ATP.

Health Conditions Related to Genetic Changes

Leigh syndrome

Mutations in the MT-ATP6 gene have been found in approximately 10 percent of people with Leigh syndrome. Leigh syndrome is a progressive brain disorder that usually appears in infancy or early childhood. Affected children may experience delayed development, muscle weakness, problems with movement, or difficulty breathing.

MT-ATP6 gene mutations associated with Leigh syndrome change one DNA building block (nucleotide) in the MT-ATP6 gene. The most common genetic change replaces the nucleotide thymine with the nucleotide guanine at position 8993 (written as T8993G). The mutations that cause Leigh syndrome impair the function or stability of the ATP synthase complex, inhibiting ATP production and impairing oxidative phosphorylation.

Although the exact mechanism is unclear, researchers believe that impaired oxidative phosphorylation can lead to cell death because of decreased energy available in the cell. Certain tissues that require large amounts of energy, such as the brain, muscles, and heart, seem especially sensitive to decreases in cellular energy. Cell death in the brain and in other sensitive tissues likely cause the characteristic signs and symptoms of Leigh syndrome.

Mitochondrial complex V deficiency

MedlinePlus Genetics provides information about Mitochondrial complex V deficiency

Neuropathy, ataxia, and retinitis pigmentosa

Some of the mutations that cause Leigh syndrome are also responsible for a similar, but less severe, condition called neuropathy, ataxia, and retinitis pigmentosa (NARP). A small number of mutations in the MT-ATP6 gene have been identified in people with NARP. Each of these mutations changes one nucleotide in the MT-ATP6 gene. As in Leigh syndrome, the most common genetic change associated with NARP replaces the nucleotide thymine with the nucleotide guanine at position 8993 (written as T8993G). The mutations that cause NARP alter the structure or function of ATP synthase, reducing the ability of mitochondria to produce ATP. Although the precise effects of these mutations are unclear, researchers continue to investigate how changes in the MT-ATP6 gene interfere with ATP production and lead to muscle weakness, vision loss, and the other features of NARP.

Most of the body's cells contain thousands of mitochondria, each with one or more copies of mitochondrial DNA. The severity of some mitochondrial disorders is associated with the percentage of mitochondria in each cell that has a particular genetic change. People with Leigh syndrome due to an MT-ATP6 gene mutation tend to have a very high percentage of mitochondria with the mutation (from more than 90 percent to 95 percent). The less-severe features of NARP result from a lower percentage of mitochondria with the mutation, typically 70 percent to 90 percent. Because these two conditions result from the same genetic changes and can occur in different members of a single family, researchers believe that they may represent a spectrum of overlapping features instead of two distinct syndromes.

Charcot-Marie-Tooth disease

MedlinePlus Genetics provides information about Charcot-Marie-Tooth disease

Other disorders

A condition called familial bilateral striatal necrosis, which is similar to Leigh syndrome (described above), can also result from changes in the MT-ATP6 gene. In the few reported cases with these mutations, affected children have delayed development, problems with movement and coordination, weak muscle tone (hypotonia), and an unusually small head size (microcephaly). Researchers have not determined why MT-ATP6 mutations result in this combination of signs and symptoms in children with bilateral striatal necrosis.

Other Names for This Gene

• ATP synthase 6
• ATP synthase F0 subunit 6
• ATP6
• ATP6_HUMAN
• ATPase protein 6
• ATPase-6
• ATPASE6
• mitochondrially encoded ATP synthase 6
• MTATP6
• Su6m

Genomic Location

This condition is inherited in a mitochondrial pattern, which is also known as maternal inheritance. This pattern of inheritance applies to genes contained in mtDNA. Because egg cells, but not sperm cells, contribute mitochondria to the developing embryo, children can inherit disorders resulting from mtDNA mutations only from their mother. These disorders can appear in every generation of a family and can affect both males and females, but fathers do not pass traits associated with changes in mtDNA to their children.

Most of the body's cells contain thousands of mitochondria, each with one or more copies of mtDNA. The severity of some mitochondrial disorders is associated with the percentage of mitochondria in each cell that has a particular genetic change. Most individuals with NARP have a specific MT-ATP6 mutation in 70 percent to 90 percent of their mitochondria. When this mutation is present in a higher percentage of a person's mitochondria—more than 90 percent to 95 percent—it usually causes a more severe condition known as maternally inherited Leigh syndrome. Because these two conditions result from the same genetic changes and can occur in different members of a single family, and because some individuals with MT-ATP6 gene mutations have related signs and symptoms that do not follow the specific patterns of these conditions, researchers believe that the conditions may be part of a spectrum of overlapping features rather than two distinct syndromes.

The prevalence of NARP is unknown. This disorder is probably less common than a similar but more severe condition, Leigh syndrome, which affects about 1 in 40,000 people.