Homocystinuria-megaloblastic anaemia, cblG complementation type (MTR gene)
OMIM number: 250940
Comments closing date: 08/05/2026
Homocystinuria megaloblastic anaemia, cblG type, is a rare inherited condition caused by changes in both copies of the MTR gene. This gene provides instructions for making methionine synthase, an enzyme that uses vitamin B12 to convert homocysteine into methionine, an important building block for growth and development. When the enzyme does not work properly, homocysteine builds up in the body and methionine levels become low. This imbalance can affect many organs, including the brain and bone marrow, leading to megaloblastic anaemia (a type of anaemia where blood cells are large and not fully developed). Symptoms can vary widely. Many children show signs in infancy, although some people are not diagnosed until later in childhood or adulthood. Common features include developmental delay, weak muscle tone, feeding problems, tiredness, seizures, small head size (microcephaly) and megaloblastic anaemia. Blood and urine tests usually show high homocysteine and low methionine.
