Neuropathy ataxia retinitis pigmentosa syndrome

NARP and mitochondrial DNA-associated Leigh syndrome are part of a continuum of progressive neurodegenerative disorders.^[1]

These conditions affect mitochondria, which are structures within cells that convert the energy from food into a form that cells can use. NARP and MILS can be caused by mutations in the MT-ATP6 gene, which is contained within mitochondrial DNA. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. The most common genetic change in the MT-ATP6 gene replaces the DNA building block thymine with guanosine at position 8993 (written as T8993G).^[2]

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 mutation. Most individuals with NARP have the T8993G mutation in 70 to 90 percent of their mitochondria. When this mutation is present in a higher percentage of a person's mitochondria—greater than 90 percent—it causes the more severe features of mitochondrial DNA-associated Leigh syndrome. Because these two conditions can result from the same genetic changes and can occur in different members of a single family, researchers believe that they represent a spectrum of overlapping features instead of two distinct syndromes.^[3]