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Inborn errors of metabolism

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Metabolism is a sequence of chemical reactions that take place in cells in the body. These reactions are responsible for the breakdown of nutrients and the generation of energy in our bodies. Inborn errors of metabolism (IEM) are a group of disorders that causes a block in a metabolic pathway leading to clinically significant consequences.

Frequently Asked Questions

What are the different forms of IEM?

The different IEM are usually named for the enzyme that is not working properly. For example, if the enzyme carbamoyl phosphate synthetase 1 (CPS1) is not working, the IEM is called "CPS1 deficiency". A list of broad categories of IEM and some examples are listed below.

IEM Examples
Urea cycle disorders Ornithine transcarbamylase deficiency, citrullinemia, argininosuccinic aciduria, argininemia
Organic acidemias Propionic acidemia, methylmalonic aciduria, isovaleric acidemia, glutaric acidemia, maple syrup urine disease
Fatty acid oxidation defects Medium chain acyl-CoA dehydrogenase deficiency, carnitine palmitoyl transferase 1 deficiency, long chain hydroxyacyl-CoA dehydrogenase deficiency
Amino acidopathies Tyrosinemia, phenylketonuria, homocysteinuria
Carbohydrate disorders Galactosemia, fructosemia
Mitochondrial disorders MELAS, MERFF, pyruvate dehydrogenase deficiency

What causes the IEM and how are the different forms inherited?

The IEM are caused by mutations (or alterations) in the genes that tell our cells how to make the enzymes and the co-factors for metabolism. A mutation causes a gene to not function at all or not to function as well as it should. Most often these altered genes are inherited from parent(s), but they may also occur spontaneously.

When discussing how genetic conditions are passed on in a family, it is important to understand that we have two copies of most genes, with one copy inherited from our mother and one copy inherited from our father. This is not the case for the genes that are on our sex chromosomes (the "X" and "Y" chromosomes). These are different in men and women: men have only one X chromosome and therefore only one copy of the genes on that chromosome, while women have two X chromosomes and therefore have two copies of the genes on that chromosome. A father passes on his X chromosome to all of his daughters and his Y chromosome on to all of his sons. A mother passes on an X chromosome to each child.

What is the chance of having an IEM if someone else in the family has it?

The chance that someone else in the family has the same IEM as their relative depends on the inheritance pattern of the IEM, whether the at-risk family member is male or female, and the rest of the family history (how many relatives have been diagnosed with the disorder already and whether genetic testing has been performed in other relatives). In some cases, the age of the at-risk family member and whether or not they have shown any signs or symptoms of the disorder is helpful in estimating the chances that they also have the disorder.

We recommend talking to your metabolic specialist and/or genetic counselor to determine those relatives who may be at risk for having an IEM and for coordination of genetic testing, when appropriate. We are happy to talk with you about this.

What are the symptoms of IEM and how are they diagnosed?

In general, the earlier someone develops symptoms of an IEM, the more severe their disorder. The severity of symptoms is generally based (1) the position of the defective enzyme within the metabolic pathway and (2) whether or not there is any functional enzyme or co-factor being produced. However, other environmental and genetic factors may play a role in determining the severity of symptoms for a given patient.

IEM are multisystemic diseases and thus patients may present with a variety of symptoms, many of which depend on the specific metabolic pathway(s) involved. Some findings in patients with IEM may include elevated acid levels in the blood, low blood sugar, high blood ammonia, abnormal liver function tests, and blood cell abnormalities. Certain patients may also have neurologic abnormalities such as seizures and developmental delays. Growth may also be affected.

How are IEM treated?

Treatment of IEM is tailored to the specific disorder once a diagnosis is made. In general, the goals of treatment are to minimize or eliminate the buildup of toxic metabolites that result from the block in metabolism while maintaining growth and development. This may be accomplished by special modified diets, supplements and medications.

A doctor who specializes in metabolic disorders should see IEM patients on a regular basis. Severely affected patients will likely be seen on a more frequent basis than mild or moderately affected patients.

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