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Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>Mutations in the MT-TK gene are the most common cause of MERRF, occurring in more than 80 percent of all cases. Less frequently, mutations in the MT-TL1, MT-TH, and MT-TS1 genes have been reported to cause the signs and symptoms of MERRF. People with mutations in the MT-TL1, MT-TH, or MT-TS1 gene typically have signs and symptoms of other mitochondrial disorders as well as those of MERRF.</p><p>The MT-TK, MT-TL1, MT-TH, and MT-TS1 genes are contained in mitochondrial DNA (mtDNA). Mitochondria are structures within cells that use oxygen to convert the energy from food into a form cells can use through a process called oxidative phosphorylation. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. The genes associated with MERRF provide instructions for making molecules called transfer RNAs, which are chemical cousins of DNA. These molecules help assemble protein building blocks called amino acids into full-length, functioning proteins within mitochondria. These proteins perform the steps of oxidative phosphorylation.</p><p>Mutations that cause MERRF impair the ability of mitochondria to make proteins, use oxygen, and produce energy. These mutations particularly affect organs and tissues with high energy requirements, such as the brain and muscles. Researchers have not determined how changes in mtDNA lead to the specific signs and symptoms of MERRF.</p><p>A small percentage of MERRF cases are caused by mutations in other mitochondrial genes, and in some cases the cause of the condition is unknown.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/myoclonic-epilepsy-with-ragged-red-fibers/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Myoclonic epilepsy with ragged-red fibers?"},{"bodyHtml":"<p>MERRF is characterized by muscle twitches (myoclonus), weakness (myopathy), and progressive stiffness (spasticity). When the muscle cells of affected individuals are stained and viewed under a microscope, these cells usually appear abnormal. These abnormal muscle cells are called ragged-red fibers. Other features of MERRF include recurrent seizures (epilepsy), difficulty coordinating movements (ataxia), a loss of sensation in the extremities (peripheral neuropathy), and slow deterioration of intellectual function (dementia). People with this condition may also develop hearing loss or optic atrophy, which is the degeneration (atrophy) of nerve cells that carry visual information from the eyes to the brain. Affected individuals sometimes have short stature and a form of heart disease known as cardiomyopathy. Less commonly, people with MERRF develop fatty tumors, called lipomas, just under the surface of the skin.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/myoclonic-epilepsy-with-ragged-red-fibers\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"myoclonic-epilepsy-with-ragged-red-fibers","sourceDocuments":[{"name":"Myoclonic epilepsy with ragged-red fibers","reviewed":"2014-05","sections":{"causes":{"fetchedAt":"2026-10-20T20:23:37.891844+00:00","label":"Causes","pageSha256":"dbaa9ea10f2739adc1d31fa91a4672efe1838c85f19f8da559998066f19dfd69","sourceUrl":"https://medlineplus.gov/genetics/condition/myoclonic-epilepsy-with-ragged-red-fibers/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/myoclonic-epilepsy-with-ragged-red-fibers"}],"sourceNote":"MedlinePlus descriptions and Causes; original HPO annotations.","takeawayHtml":"Cause explanations are from MedlinePlus Genetics. Feature values below retain their original sources and patient groups.","title":"Myoclonic epilepsy with ragged-red fibers"},"schemaVersion":1}