Maple Syrup Urine Disease (MSUD) is an inherited disorder so named because one of its first signs is urine that has an odor reminiscent of maple syrup. The underlying defect disrupts the metabolism of certain amino acids. These are amino acids that have a branched side chain. Because they cannot be fully broken down, they accumulate in the urine, along with their metabolites (alpha-ketoacids) to give the distinctive smell. Left untreated, there is progressive neurodegeneration leading to death within the first months of life.

Three amino acids have branched side chains: valine, leucine, and isoleucine. They are an essential element in the diet and are broken down by the body to yield energy. One step in this breakdown involves the branched-chain alpha-ketoacid dehydrogenase (BCKDH) complex, which consists of three catalytic components and two regulatory enzymes. In total, six gene loci encode for the BCKDH, and mutations in different loci are responsible for the genetic variety seen in MSUD.

The Mennonite community of Lancaster County, Pennsylvania is particularly afflicted by MSUD, with over 1 of 176 individuals affected. This is due to a high carrier rate of a mutation in the E1alpha-subunit of the BCKDH complex. By contrast, the disease is rare in the general population.

Currently treatment consists of restricting the dietary intake of branched-chain amino acids to the absolute minimum that is needed for growth. However, studies have already shown that it is possible to transfer subunits of the BCKDH enzyme into cells using a retrovirus. Similar advances in gene therapy may provide a future cure.