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syndrome","evidence":"PCS","frequency_raw":"2/11","hpo_id":"HP:0001548","is_excluded":false,"label":"Overgrowth","onset_id":null,"reference":"PMID:22729224","source_release":"2026-06-23"},"sourceGroups":["published"]}],"referenceLabels":{},"references":{"causeHtml":"<ul><li><a href=\"https://medlineplus.gov/genetics/condition/megalencephaly-polymicrogyria-polydactyly-hydrocephalus-syndrome\">Megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome</a></li></ul>","identityNote":"MONDO cross-references identify native condition entries. Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>MPPH syndrome can be caused by mutations in the AKT3, CCND2, or PIK3R2 gene. The proteins produced from all three genes are involved in a chemical signaling pathway called the PI3K-AKT-mTOR pathway. This signaling influences many critical cell functions, including the creation (synthesis) of new proteins, cell growth and division (proliferation), and the survival of cells. The PI3K-AKT-mTOR pathway is essential for the normal development of many parts of the body, including the brain.</p><p>Mutations in the AKT3, CCND2, or PIK3R2 gene increase the activity of their respective proteins or prevent the proteins from being broken down when they should. As a result, chemical signaling through the PI3K-AKT-mTOR pathway is enhanced, which increases cell growth and division. In the brain, the increased number of cells leads to rapid and abnormal brain growth starting before birth. The rapid growth disrupts the structure and function of the developing brain. It is less clear how increased PI3K-AKT-mTOR signaling contributes to polydactyly, although the extra digits are probably related to abnormal cell proliferation in the developing hands and feet. CCND2 and PIK3R2 gene mutations are more likely to cause polydactyly than are AKT3 gene mutations.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/megalencephaly-polymicrogyria-polydactyly-hydrocephalus-syndrome/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome?"},{"bodyHtml":"<p>The problems with brain development cause a variety of neurological signs and symptoms. People with MPPH syndrome have delayed development and intellectual disability that ranges from mild to severe. About half of affected individuals develop recurrent seizures (epilepsy) beginning early in childhood. People with MPPH syndrome also have difficulty coordinating movements of the mouth and tongue (known as oromotor dysfunction), which leads to drooling, difficulty swallowing (dysphagia), and a delay in the production of speech (expressive language).</p><p>About half of people with MPPH syndrome have an extra finger or toe on one or more of their hands or feet (polydactyly). The polydactyly is described as postaxial because it occurs on the same side of the hand or foot as the pinky finger or little toe.</p><p>The brain abnormalities characteristic of MPPH syndrome are also found in a closely related condition called megalencephaly-capillary malformation syndrome (MCAP). However, MCAP includes abnormalities of small blood vessels in the skin (capillary malformations) and several other features that are not usually part of MPPH syndrome.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/megalencephaly-polymicrogyria-polydactyly-hydrocephalus-syndrome\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"megalencephaly-polymicrogyria-polydactyly-hydrocephalus-syndrome","sourceDocuments":[{"name":"Megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome","reviewed":"2017-01","sections":{"causes":{"fetchedAt":"2026-10-20T20:23:13.490651+00:00","label":"Causes","pageSha256":"434296308bcd12171f47950de3c37f665c32a2ece3f8d7070e6c9361ae98443b","sourceUrl":"https://medlineplus.gov/genetics/condition/megalencephaly-polymicrogyria-polydactyly-hydrocephalus-syndrome/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/megalencephaly-polymicrogyria-polydactyly-hydrocephalus-syndrome"}],"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":"Megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome"},"schemaVersion":1}