Inborn errors of metabolism are inherited disorders that are caused by a defect in a single gene. There are many inborn errors of metabolism. Some produce relatively unimportant physical features or skeletal abnormalities. Others produce serious disease and even death.

What is going on in the body?

The gene defect can cause a particular enzyme to be defective or missing. And because the enzyme isn't doing what it is supposed to do for the body, poor health may result.

Simply put, enzymes help the body by stimulating biological reactions inside cells. Enzymes are proteins that help the body use food, produce energy, and do work. When a particular enzyme is defective or missing, then the body isn't able to do something that it should do to maintain health. The enzyme involved determines what the body can't do and what the resulting physical features are.

Inborn errors of metabolism affect about 1 in every 5,000 babies born. Some of the more familiar inborn errors of metabolism are cystic fibrosis, hypothyroidism, sickle cell anemia, phenylketonuria (PKU) and Tay-Sachs disease.

What are the causes and risks of the disease?

The enzyme defects that lead to the inborn errors of metabolism are caused by abnormal genes. In most cases, the abnormal gene is autosomal recessive in character. Autosomal means that the gene is on one of the twenty-two non-sex chromosomes. Recessive means that there must be two copies of the gene, one inherited from each parent, in order for the enzyme defect to exist.

It is not possible to generalize about the nature of the enzyme defects or their locations on the chromosomes. Defects have been observed in virtually all biological reactions, and the responsible genes are scattered among the chromosomes. In many cases, the structure of the gene is known. More often, the chemicals in the body that are affected by the inborn error can be identified.

It is impossible to say how many people are affected by the various inborn errors of metabolism. After all, there are hundreds of such errors and record keeping has not been precise for most of them.