In details

The innate errors of metabolism and genetics

Let us now briefly describe two important diseases related to the action of “defective” genes.


Phenylketonuria (PKU) is a genetic disease due to the action of a recessive gene that manifests in homozygosis, which affects approximately 1 in 12,000 newborns in Brazil. People with this anomaly are unable to produce an enzyme that acts on the conversion of a phenylalanine amino acid to the tyrosine amino acid. Without this conversion, phenylalanine accumulates in the blood and is converted into a toxic substance that causes damage to the nervous system, culminating in mental retardation of the carrier. One such substance is phenylpyruvic acid, excreted in the urine, which explains the name given to the disease. A newborn child, recessive homozygous for PKU, has a healthy start to life, since the enzymes produced by the mother were transferred through the placenta, freeing her from the problem. However, as the days go by, the enzyme dies down and phenylalanine builds up.

In the 1950s, biochemical tests were developed to prevent the symptoms of the disease. A simple blood test (foot test) may reveal excess phenylalanine. Once the disease is recognized, children are poorly fed on phenylalanine (remember that phenylalanine is important in building metabolism as it is part of the structure of many proteins). Children treated in this way reach adulthood normally, and even if they feed on phenylalanine-containing substances at this stage, there will no longer be any risk as the development of the nervous system is already over.


Galactosemia is a rare genetic metabolic disorder. The deficiency of an enzyme in galactose metabolism does not allow it to be transformed into glucose, the body's main energy source. Accumulation of galactose or its metabolites is the cause of kidney, liver, brain and eye damage or even death in more severe cases.

The disease can manifest in children and newborns with metabolic deficiency as galactose is introduced into the diet via breast milk, milk in general, cheese and dairy products. The signs of the disease are usually vomiting, enlargement of the liver, yellowish pigmentation of the child, among others.

The damage caused by galactosemia may be minor if the disease is diagnosed early. Therefore, in many medical centers research on galactosemia is already being included in the so-called "foot test" (or neonatal screening), a simple measure that can greatly improve the prognosis of the disease.

Treatment is the basis of a severe galactose-free and lactose-free diet from the neonatal period to, in most cases, the rest of life with constant monitoring by measuring the level of galactose and its red blood cell metabolites. With severe treatment since the neonatal period, damage that may have occurred pre-diagnosis in the liver will be remedied, but the incidence of ovarian, language and speech complications, motor coordination and learning are high in the long run.