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Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>Variants (also called mutations) in the CSTB gene cause progressive myoclonic epilepsy type 1. The CSTB gene provides instructions for making a protein called cystatin B. This protein reduces the activity of enzymes called cathepsins. Cathepsins help break down certain proteins in the lysosomes, which are compartments in the cell that digest and recycle different types of molecules. While the specific function of cystatin B is unclear, it may help protect the cells&#x27; proteins from cathepsins that leak out of the lysosomes.</p><p>In almost all affected individuals, progressive myoclonic epilepsy type 1 is caused by variants that affect the amount of cystatin B that is produced. One region of DNA that controls the activity of the CSTB gene has a particular repeating sequence of 12 DNA building blocks (nucleotides). This sequence is known as the dodecamer repeat. Normally, this sequence is repeated two or three times. However, in most people with progressive myoclonic epilepsy type 1, this sequence is repeated more than 30 times (called a repeat expansion). Most people with progressive myoclonic epilepsy type 1 have two copies of this variant.</p><p>A small number of people with progressive myoclonic epilepsy type 1 have one copy of the dodecamer repeat expansion and one copy of the CSTB gene with another type of variant. These other variants can include the substitution of a single nucleotide that impairs the gene&#x27;s function.</p><p>In individuals with progressive myoclonic epilepsy type 1, levels of cystatin B are only 5 to 10 percent of normal. This change is believed to cause the signs and symptoms of progressive myoclonic epilepsy type 1, but it is unclear how a reduction in the amount of cystatin B leads to the features of this disorder.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/progressive-myoclonic-epilepsy-type-1/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Progressive myoclonic epilepsy type 1?"},{"bodyHtml":"<p>People with progressive myoclonic epilepsy type 1 experience episodes of involuntary muscle jerking or twitching (myoclonus) that increase in frequency and severity over time. Episodes of myoclonus may be brought on by physical exertion, stress, light, or other stimuli. Within 5 to 10 years, the myoclonic episodes may become severe enough to interfere with walking and other everyday activities.</p><p>Affected individuals also usually have seizures that involve loss of consciousness, muscle rigidity, and convulsions (tonic-clonic or grand mal seizures). Like the myoclonic episodes, these may increase in frequency over several years. However, the seizures may be controlled with treatment. After several years of progression, the frequency of seizures may stabilize or decrease.</p><p>Eventually, people with progressive myoclonic epilepsy type 1 may develop problems with balance and coordination (ataxia) and speaking (dysarthria). They may also experience depression. Another feature of this condition is involuntary rhythmic shaking. This shaking is called intentional tremor because it worsens during intentional movements.</p><p>People with progressive myoclonic epilepsy type 1 may live into adulthood. Life expectancy depends on the severity of the condition and a person&#x27;s response to treatment. The severity of the condition can vary, even among members of the same family.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/progressive-myoclonic-epilepsy-type-1\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"progressive-myoclonic-epilepsy-type-1","sourceDocuments":[{"name":"Progressive myoclonic epilepsy type 1","reviewed":"2024-03","sections":{"causes":{"fetchedAt":"2026-10-20T20:24:27.411584+00:00","label":"Causes","pageSha256":"1d1936b96b7554cf01424cb82a5da08595cc928cffb6c9da6959d6f5df7d5e19","sourceUrl":"https://medlineplus.gov/genetics/condition/progressive-myoclonic-epilepsy-type-1/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/progressive-myoclonic-epilepsy-type-1"}],"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":"Progressive myoclonic epilepsy type 1"},"schemaVersion":1}