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concentration","onset_id":null,"reference":"ORPHA:859","source_release":"2026-06-23"},"sourceGroups":["orphanet"]}],"referenceLabels":{},"references":{"causeHtml":"<ul><li><a href=\"https://medlineplus.gov/genetics/condition/transcobalamin-deficiency\">Transcobalamin deficiency</a></li></ul>","identityNote":"MONDO cross-references identify native condition entries. Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>Mutations in the TCN2 gene cause transcobalamin deficiency. The TCN2 gene provides instructions for making a protein called transcobalamin. This protein attaches (binds) to cobalamin and transports the vitamin to cells throughout the body. Within cells, cobalamin helps certain enzymes carry out chemical reactions. Cobalamin plays a role in the processes that produce the building blocks of DNA (nucleotides) and break down various compounds such as fatty acids; these processes are needed for cell proliferation and the production of cellular energy.</p><p>Most TCN2 gene mutations that cause transcobalamin deficiency lead to a complete or near-complete lack (deficiency) of transcobalamin. Other TCN2 gene mutations result in a transcobalamin protein that cannot transport cobalamin to cells. The resulting lack of cobalamin within cells interferes with the functioning of certain enzymes, which impacts many cell activities. As a result, a wide range of signs and symptoms can develop including impaired growth, blood cell shortages, and neurological problems.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/transcobalamin-deficiency/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Transcobalamin deficiency?"},{"bodyHtml":"<p>The first signs of transcobalamin deficiency are typically a failure to gain weight and grow at the expected rate (failure to thrive), vomiting, diarrhea, and open sores (ulcers) on the mucous membranes such as the lining inside the mouth. Neurological function is impaired in affected individuals, and they can experience progressive stiffness and weakness in their legs (paraparesis), muscle twitches (myoclonus), or intellectual disability.</p><p>People with transcobalamin deficiency often develop a blood disorder called megaloblastic anemia. Megaloblastic anemia results in a shortage of red blood cells, and the remaining red blood cells are abnormally large. Individuals with transcobalamin deficiency may also have a shortage of white blood cells (neutropenia), which can lead to reduced immune system function. Decreased cellular cobalamin can lead to a buildup of certain compounds in the body, resulting in metabolic conditions known as methylmalonic aciduria or homocystinuria.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/transcobalamin-deficiency\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"transcobalamin-deficiency","sourceDocuments":[{"name":"Transcobalamin deficiency","reviewed":"2014-10","sections":{"causes":{"fetchedAt":"2026-10-20T20:25:30.230162+00:00","label":"Causes","pageSha256":"60625fb8063c576069dceb853eb5b6d7ce562d8884de5a279b487180bbddffa3","sourceUrl":"https://medlineplus.gov/genetics/condition/transcobalamin-deficiency/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/transcobalamin-deficiency"}],"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":"Transcobalamin deficiency"},"schemaVersion":1}