Genetic disorders in which chemical processes in the body are disrupted or faulty
Inborn errors of metabolism is a general term used to describe a group of genetic disorders in which the body chemistry (metabolism) is affected by an abnormal gene. In each disorder, this abnormal gene affects the production of a particular enzyme that is essential for specific metabolic processes. The disruption of these processes results in damage to one or more organs of the body, and the severity of a particular disorder depends on the chemical processes affected. Over 200 inborn errors of metabolism have been identified, but each disorder is extremely rare. Most of the abnormal genes that cause these in born metabolic disorders are inherited in an autosomal recessive manner (see Gene disorders).
Many inborn errors of metabolism lead to a build-up of harmful chemicals that cause damage to one or more organs in the body. In phenylketonuria, the accumulation of phenylalanine causes brain damage. In galactosaemia, the sugar galactose builds up to a dangerously high level and causes damage to the liver and brain. In Tay–Sachs disease, chemical build-up in the brain results in fatal brain damage. In MCADD, lack of an enzyme needed to convert fats into energy can lead to low levels of glucose and a high level of toxins in the blood. In the milder condition albinism, a deficient enzyme affects the production of the pigment melanin, which gives colour to the skin, eyes, and hair, but there is no harmful chemical build-up.
The symptoms of inborn errors of metabolism are usually present at or soon after birth, although in some cases symptoms may not appear until later in childhood. The symptoms may include unexplained illness and failure to thrive in a newborn, and drowsiness, floppiness, seizures, and developmental delay in older babies and children.
Couples planning to have children may be offered screening for the presence of an abnormal gene before beginning a pregnancy if there is a family history of an inborn error of metabolism or if a particular metabolic disorder is common in their ethnic group. A pregnant woman can also be screened for the condition before the baby is born (see Antenatal genetic tests). In the UK, all newborn infants are routinely given blood spot screening tests, which screen for various metabolic disorders as well as certain other conditions, such as cystic fibrosis.
Some inborn errors of metabolism can be treated easily. For example, the diet of an affected child can be adapted to restrict the intake of substances, such as phenylalanine or galactose, that his or her body cannot process. If the missing enzyme is produced by white blood cells, the condition may sometimes be cured by a stem cell transplant.
The outlook for inborn errors of metabolism depends on the disorder and how early it is diagnosed. Some disorders, such as albinism, rarely cause serious problems; others, such as galactosaemia, can be treated successfully if diagnosed soon after birth. However, there is no treatment for Tay–Sachs disease, which is usually fatal in early childhood.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.