INBORN ERRORS OF PROTEIN METABOLISM

HOMOCYSTINURIA

CAUSE

The protein building block methionine, ingested with food, is converted into cysteine during metabolic processes in healthy people.

In the hereditary disorder homocystinuria, this metabolic process is disrupted and an undesirable intermediate product (homocysteine) accumulates in the blood.

SYMPTOMS WHEN THE DISEASE IS UNTREATED

The high concentration of homocysteine in the blood can lead to a range of symptoms in various organs, including eye problems, osteoporosis, delayed intellectual and physical development, as well as vascular damage (arteriosclerosis). The last-named condition leads to a cluster of cardiovascular problems.

TREATMENT

Metabolic blockage and the harmful accumulation of homocysteine are prevented by a targeted low-protein diet, which also prevents the intake of methionine. However, to meet the protein requirements, a methionine-free but cysteine-rich dietary supplement is needed. The medication betaine can be combined with the dietary treatment.

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ISOVALERIC ACIDAEMIA

CAUSE

In the genetic metabolic disorder isovaleric acidaemia, the amino acid leucine is not broken down properly. Instead, isovaleric acid is produced, which accumulates in the body.

SYMPTOMS WHEN THE DISEASE IS UNTREATED

High concentrations of isovaleric acid damage the brain and the nervous system. Complications often appear in affected infants in the first few days after birth, which can become very serious if not treated. However, symptoms may appear only after several years.

TREATMENT

A low-protein diet is used to restrict leucine intake and thus the accumulation of isovaleric acid as well. If required, the diet must be supplemented with a leucine-free amino acid mixture.

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MAPLE SYRUP URINE DISEASE (MSUD)

CAUSE

Maple syrup urine disease (MSUD) is a hereditary metabolic disorder affecting the protein building blocks leucine, isoleucine and valine. These amino acids are ingested with food, but are processed only incompletely or not at all by the body and accumulate in the blood.
The metabolic product sotolon gives the urine of MSUD patients a spicy-sweet odour that resembles that of maple syrup, hence the name of the disease.

SYMPTOMS WHEN THE DISEASE IS UNTREATED

If the disease goes undetected in the newborn child and if the infant is given a normal diet containing protein, permanent brain damage can occur within a few days.
If the disease remains untreated, it can be fatal. Depending on the type of MSUD, the course of the disease can vary in severity.

TREATMENT

Treatment consists of a strict low-protein diet.
To ensure that protein deficiency does not result, the diet must be supplemented with a special amino acid mixture that contains all of the amino acids except leucine, isoleucine and valine. The requirements for these three amino acids are met by the small amount of natural protein in the diet.
If the disease is detected early and the diet is followed strictly, the affected child will develop in a normal, healthy manner.

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METHYLMALONIC ACIDAEMIA AND PROPIONIC ACIDAEMIA

CAUSE

Methylmalonic acidaemia and propionic acidaemia are both genetic disorders of propionate metabolism, which in healthy people produces energy from proteins ingested with food. In affected persons, the amino acids methionine, threonine, valine and isoleucine are not properly metabolised. Instead, metabolic products accumulate in the blood, and these can damage various organs.

SYMPTOMS WHEN THE DISEASE IS UNTREATED

If this metabolic disorder is not detected and not treated, it can lead to a variety of clinical symptoms. The degree of severity varies from mild neurological impairment to severe brain damage and death.

TREATMENT

The goal of treatment is to prevent the accumulation of harmful metabolic products. This is achieved with a strict low-protein diet. To prevent protein deficiencies, the diet is supplemented with an amino acid mixture that contains all of the necessary amino acids except methionine, threonine, valine and isoleucine.

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TYROSINAEMIA

CAUSE

Tyrosinaemia is a hereditary metabolic disorder in which the breakdown of the protein building block tyrosine is disrupted.
Depending on the type of disruption, different types of tyrosinaemia are distinguished, which vary in severity and in treatment.
Either the breakdown is completely blocked or disrupted to such an extent that toxic, undesirable decomposition products are generated.

SYMPTOMS WHEN THE DISEASE IS UNTREATED

If the disease remains undetected, tyrosine and phenylalanine, as well as toxic decomposition products of these two amino acids, accumulate in the blood. They can damage the brain as well as other organs (depending on the type of tyrosinaemia) and in some cases even lead to death.

TREATMENT

The goal of treatment is to prevent the accumulation of tyrosine, phenylalanine and toxic decomposition products. This is achieved with a strict low-protein diet. To prevent protein deficiencies, the diet is supplemented with a tyrosine-free amino acid mixture.

The medication nitisinone (NTBC) blocks the breakdown of tyrosine and thus the formation of toxic decomposition products. However, it is important that this drug is combined with a low-tyrosine diet.

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TYPE 1 GLUTARIC ACIDURIA

CAUSE

Type 1 glutaric aciduria is a congenital metabolic disorder that affects the breakdown of the amino acids lysine and tryptophan.
Due to a genetic defect, these amino acids cannot be broken down properly and thus accumulate. Harmful decomposition products are then deposited in the brain.

SYMPTOMS WHEN THE DISEASE IS UNTREATED

Deposits of the harmful decomposition products can result in irreversible brain damage. This often affects a region of the brain that is responsible for the coordination of movements,
leading to motor disability. These can vary greatly in severity.

TREATMENT

To prevent motor disability, the disorder must be detected as early as possible. Treatment consists of restricting lysine intake via a special low-lysine, i.e. low-protein diet.
The diet can be combined with the medication carnitine, which makes it easier to eliminate the harmful decomposition products via the urine.

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UREA CYCLE DISORDERS

CAUSE

In our diets, we often consume more protein – and therefore more amino acids (the protein building blocks) – than our bodies actually need. Excess amino acids are broken down in the body to ammonia. But the body cannot tolerate high levels of ammonia because it damages the brain. So the liver converts the toxic ammonia into non-toxic urea, which can then be excreted in the urine. This conversion takes place in a process known as the urea cycle. Inherited genetic defects may disrupt one or more of the steps in this process. As a result, toxic ammonia accumulates in the blood (this is called hyperammonaemia).

SYMPTOMS WHEN THE DISEASE IS UNTREATED

There are various types of urea cycle disorders, some of which are more severe than others. If severe disorders remain untreated, a life-threatening condition may develop. Drowsiness, rapid breathing, and vomiting are the first signs. Elevated levels of ammonia in the blood may also cause irritability, alterations in speech, and confusion. These are signs that the brain is involved.
But early diagnosis and treatment of the disorder can often prevent permanent damage.

TREATMENT

The main aim of treatment is to prevent an increase in the ammonia concentration in the blood. With acute symptoms, emergency measures usually require admission to hospital. A low protein diet and medication help to prevent high levels of ammonia in the long term, by easing the burden on the urea cycle. The diet has to be balanced in such a way that it provides the body with sufficient protein (including the essential amino acids), trace elements, and vitamins for normal growth but, at the same time, contains only just as much protein as can be tolerated. In addition, there must always be an adequate supply of energy, otherwise the body starts to break down its own protein, which may again lead to hyperammonaemia.

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