3-methylcrotonyl-CoA carboxylase deficiency (3-MCCD) is a condition that makes it hard for the body to break down the amino acid leucine, which is important for growth and development.
The symptoms of 3-MCCD can vary a lot from person to person. The first sign is often a metabolic crisis, where someone might have low blood sugar, vomiting, low energy, trouble feeding, irritability, and weak muscles. These crises can be triggered by infections, not eating for a long time (fasting), or eating too much protein. Some people may also have neurological problems like metabolic stroke, weakness on one side of the body, or reduced blood flow to the brain. Symptoms can start in infancy or childhood, but some people don’t show symptoms until adulthood or may never have symptoms at all.
There is no cure for 3-MCCD. Treatment focuses on managing symptoms. Many people without symptoms may never need treatment. Common treatments include a low-leucine diet, limited protein intake, and possibly L-carnitine supplements. It’s important to avoid fasting, especially in infants and young children. Some babies might need a special formula without leucine to prevent metabolic crises. In serious illnesses, an IV glucose regimen may be used as an emergency treatment.
3-MCCD is caused by pathogenic (disease-causing) variants in the MCCC2 gene and exhibits autosomal recessive inheritance. This means that both parents must be carriers to have a 25% chance of having a child with the condition. The risk of being a carrier is based on a person’s ancestry or ethnic background.
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Written August 2024
