Hereditary Metabolic Disease – Causes and Treatment

What Are Hereditary Metabolic Diseases?

Hereditary metabolic diseases, also known as inherited metabolic disorders or inborn errors of metabolism, are a large group of genetically determined conditions. In these diseases, a specific step in the body´s normal metabolism is disrupted because a particular enzyme, transport protein, or other molecule involved in metabolism is absent or does not function correctly. This leads to the accumulation of harmful substances or a deficiency of essential metabolic products in the body.

Several hundred different hereditary metabolic diseases have been identified. Although each individual condition is rare, they collectively represent a significant proportion of all genetic disorders.

Causes

Hereditary metabolic diseases are caused by mutations in individual genes that encode enzymes or other proteins involved in metabolism. These mutations can be inherited in different ways:

• Autosomal recessive: Both parents carry one defective gene copy each. A child is affected when it inherits both defective copies (e.g., phenylketonuria, cystic fibrosis).
• Autosomal dominant: A single defective gene copy is sufficient to cause the disease (e.g., familial hypercholesterolemia).
• X-linked: The gene defect is located on the X chromosome and more commonly affects males (e.g., hemophilia A).
• Mitochondrial: The defect lies in the mitochondrial genome and is inherited exclusively through the mother.

Types and Examples

Hereditary metabolic diseases can be classified according to the metabolic pathways affected:

Amino Acid Metabolism Disorders

In these conditions, certain amino acids cannot be properly broken down or converted. Well-known examples include:

• Phenylketonuria (PKU): Deficiency of phenylalanine hydroxylase leads to accumulation of phenylalanine, causing intellectual disability if untreated.
• Maple Syrup Urine Disease: Impaired breakdown of branched-chain amino acids, which can cause neurological damage.

Carbohydrate Metabolism Disorders

• Galactosemia: Inability to metabolize galactose (milk sugar), leading to liver and brain damage.
• Glycogen Storage Diseases (e.g., Pompe Disease): Disorders affecting the synthesis or breakdown of glycogen.

Lipid Metabolism Disorders

• Familial Hypercholesterolemia: Elevated LDL cholesterol levels due to defective LDL receptors, increasing the risk of heart attack.
• Sphingolipidoses (e.g., Gaucher Disease, Niemann-Pick Disease): Accumulation of fatty substances in organs.

Purine and Pyrimidine Metabolism Disorders

• Primary Gout: Elevated uric acid levels due to increased purine production.
• Lesch-Nyhan Syndrome: Severe neurological disorder caused by an enzyme defect in purine metabolism.

Lysosomal Storage Diseases

This group includes conditions such as Gaucher disease, Fabry disease, and the mucopolysaccharidoses, where substances cannot be broken down within the lysosomes of cells.

Symptoms

The symptoms of hereditary metabolic diseases are highly variable and depend on the specific condition and the affected metabolic pathway. Common signs and symptoms may include:

• Developmental delays or intellectual disability
• Neurological disorders (seizures, movement disorders)
• Enlargement of the liver and/or spleen (hepatosplenomegaly)
• Muscle weakness
• Skeletal abnormalities
• Unusual odor of urine, sweat, or breath
• Vomiting and poor feeding in infants
• Growth retardation

Many conditions manifest in infancy or early childhood, although some may not become apparent until adulthood.

Diagnosis

Diagnosing hereditary metabolic diseases can be complex. The following methods are used:

• Newborn Screening: Shortly after birth, a blood test screens for a range of metabolic disorders (e.g., PKU, maple syrup urine disease, and others), as part of national newborn screening programs.
• Biochemical Tests: Measurement of enzyme activities or specific metabolic products in blood, urine, or other body fluids.
• Molecular Genetic Testing: Identification of the causative gene mutation through DNA analysis.
• Imaging: E.g., MRI of the brain for neurological symptoms.
• Biopsy: In some cases, tissue samples (e.g., liver biopsy) are taken for further analysis.

Treatment

Treatment depends on the specific disorder. In many cases, a complete cure is not possible, but symptoms can be managed and complications prevented:

• Dietary Therapy: E.g., a low-phenylalanine diet for PKU; lactose-free diet for galactosemia.
• Enzyme Replacement Therapy (ERT): Intravenous administration of the missing enzyme, e.g., for Gaucher disease or Pompe disease.
• Substrate Reduction Therapy: Reducing the production of the substance that cannot be broken down, using specific medications.
• Gene Therapy: An increasingly researched approach in which the defective gene segment is corrected or replaced.
• Stem Cell or Bone Marrow Transplantation: Used for certain conditions (e.g., some mucopolysaccharidoses).
• Drug Therapy: E.g., uric acid-lowering agents for primary gout.
• Supportive Therapy: Physiotherapy, occupational therapy, speech therapy, and psychosocial support.

Prognosis

The prognosis depends greatly on the type of disease and the timing of diagnosis and treatment. With early diagnosis and consistent therapy, many patients can lead a largely normal life. Without treatment, however, some conditions can lead to severe and potentially life-threatening complications.

References

1. Scriver, C. R. et al. - The Metabolic and Molecular Bases of Inherited Disease. 8th edition. McGraw-Hill, New York 2001.
2. Saudubray, J. M., Garcia-Cazorla, A. - Inborn Errors of Metabolism Overview: Pathophysiology, Manifestations, Evaluation, and Management. Pediatric Clinics of North America, 2018.
3. National Institutes of Health (NIH), National Human Genome Research Institute - Inborn Errors of Metabolism. www.genome.gov (accessed 2024).