DLD

The DLD gene provides instructions for making a protein called dihydrolipoamide dehydrogenase. This protein is a part (a subunit) of several enzyme complexes (groups of enzymes that work together). These complexes are essential for the breakdown of certain molecules to produce energy in cells. Dihydrolipoamide dehydrogenase forms a subunit called the E3 component that is shared by several enzyme complexes.

Branched-chain alpha-keto acid dehydrogenase, or BCKD, is one of the enzyme complexes that include the E3 component. The BCKD enzyme complex performs one step in the normal breakdown of three protein building blocks (amino acids). These amino acids—leucine, isoleucine, and valine—are obtained from the diet. They are present in many kinds of food, particularly protein-rich foods such as milk, meat, and eggs. The breakdown of these amino acids produces molecules that can be used for energy.

The E3 component is also part of the pyruvate dehydrogenase complex. This enzyme complex plays an important role in the production of energy for cells. It converts a molecule called pyruvate, which is formed from the breakdown of carbohydrates, into another molecule called acetyl-CoA. The E3 component performs one part of this chemical reaction. The conversion of pyruvate is essential to begin the series of chemical reactions that produces adenosine triphosphate (ATP), the cell's main energy source.

maple syrup urine disease - caused by mutations in the DLD gene

More than 10 mutations in the DLD gene have been identified in people with maple syrup urine disease. Mutations in this gene are responsible for a variant form of the disorder that includes lactic acidosis, a potentially life-threatening complication caused by the buildup of lactic acid in the body's tissues.

Most DLD mutations change single amino acids in the E3 component of the BCKD enzyme complex. These mutations alter the structure of the E3 component, which prevents the enzyme complex from effectively breaking down valine, isoleucine, and leucine. As a result, these amino acids and their byproducts build up in the body. This accumulation is toxic to cells and tissues, particularly in the nervous system. The buildup of these substances can lead to seizures, developmental delay, and the other health problems associated with maple syrup urine disease.

Most people with maple syrup urine disease have reduced function of the other enzyme complexes that contain the E3 enzyme, which may account for the overlapping features of this condition and pyruvate dehydrogenase deficiency (described below).

pyruvate dehydrogenase deficiency - caused by mutations in the DLD gene

Mutations in the DLD gene cause pyruvate dehydrogenase deficiency in a small number of people. This condition is characterized by lactic acidosis, delayed development, and neurological problems. Most DLD gene mutations associated with pyruvate dehydrogenase deficiency change single amino acids in the E3 component, which results in a reduction of its function. This decrease leads to a reduction of pyruvate dehydrogenase complex activity. With decreased function of this complex, pyruvate builds up and is converted, in another chemical reaction, to lactic acid, causing lactic acidosis. In addition, the production of cellular energy is diminished. The brain, which is especially dependent on this form of energy, is severely affected, resulting in the neurological problems associated with pyruvate dehydrogenase deficiency.

Most people with pyruvate dehydrogenase deficiency also have reduced function of the other enzyme complexes that contain the E3 enzyme, which may account for the overlapping features of this condition and maple syrup urine disease (described above).