Alternative Names metabolic disorders, enzymatic disorders, inborn errors of metabolism
Definition Genetic Diseases are inherited disorders that are usually 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 protein to be defective or missing, because the protein isn't doing what it is supposed to do for the body. Poor health may result.
What are the signs and symptoms of the disease? The symptoms of inborn errors of metabolism vary greatly. Each one must be reviewed separately to know what the physical features are. Often, an infant will show signs of failure to thrive by failing to gain weight, not eating well, and generally showing developmental delay. Vomiting and diarrhoea are among other symptoms that may signal the doctor to test for an inborn error of metabolism.
What are the causes and risks of the disease? In most cases, the abnormal gene is inherited in an autosomal, recessive manner. 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 disease to occur.
It is not possible to generalise about the nature of the protein 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 genetic diseases. After all, there are hundreds of such diseases and record keeping has not been precise for most of them.
What can be done to prevent the disease? The protein defects that lead to the genetic diseases are caused by genetic abnormalities present from the time of conception. Therefore, nothing can be done to prevent them. The diseases caused by the inborn errors, however, may be preventable in some cases:
The missing product may be supplied to the body, allowing it to function normally. For example, an inherited disorder of metabolism known as acrodermatitis enteropathica causes skin problems. A chemical called zinc sulphate cures this problem if taken regularly.
A product that might build up because of the inborn error might be eliminated from the diet, allowing the body to function normally. For example, the strategy in treating phenylketonuria is to withhold phenylalanine from the diet.
Another way to prevent some genetic diseases is family planning. A couple who know that they are both carriers of an autosomal recessive error may choose to modify their family planning. They might choose not to have children at all. They also might choose to make use of one of the recent advances in assisted pregnancy or antenatal diagnosis. Genetic counselling can be very helpful.
How is the disease diagnosed? Physical features such as malformations of the skeleton or abnormal hair may suggest the presence of a genetic disease. Whether or not the physical features are obvious, diagnosis is confirmed by checking the blood or urine for one or more of the compounds involved. Tests for urine amino acids can be helpful. There may be too much of a compound, not enough of another, or abnormalities among others. The compounds may also be studied in tissue from biopsies. DNA tests may also confirm the diagnosis.
What are the long-term effects of the disease? Each genetic disease has its own pattern of long-term effects.
What are the risks to others? Parents who have had one child with a genetic disease (autosomal, recessive inheritance) are at risk to have other affected children. One in 4 pregnancies of such couples is expected to result in an affected child.
Some genetic diseases are more often found in certain racial and ethnic groups. Sickle cell anaemia, for example, is found among those of African descent. Those of European heritage are more likely to pass on defective genes for cystic fibrosis.
The children of women with some genetic diseases are at risk because of an unfavourable environment in the womb.
What are the treatments for the disease? Many of the genetic diseases can be treated effectively. Treatment depends on the precise nature of the problem and how readily the compounds involved can be eliminated or replaced.
What are the side effects of the treatments? Side effects of treatment depend on the disease being treated.
What happens after treatment for the disease? Life long treatment will often be required.
How is the disease monitored? Most genetic diseases are monitored by routine blood or urine tests.
Author: Ronald J. Jorgenson, DDS, PhD, FACMG Reviewer: HealthAnswers Australia Medical Review Panel Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia Last Updated: 1/10/2001 Contributors Potential conflict of interest information for reviewers available on request
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