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Inherited Metabolic Disorders

Inherited metabolic disorders are genetic conditions that result in metabolism problems. Most people with inherited metabolic disorders have a defective gene that results in an enzyme deficiency. There are hundreds of different genetic metabolic disorders, and their symptoms, treatments, and prognoses vary widely.

What Is Metabolism?

Metabolism refers to all the chemical reactions taking place in the body to convert or use energy. A few major examples of metabolism include:

  • Breaking down the carbohydrates, proteins, and fats in food to release energy.
  • Transforming excess nitrogen into waste products excreted in urine.
  • Breaking down or converting chemicals into other substances and transporting them inside cells.

Metabolism is an organized but chaotic chemical assembly line. Raw materials, half-finished products, and waste materials are constantly being used, produced, transported, and excreted. The "workers" on the assembly line are enzymes and other proteins that make chemical reactions happen.

Causes of Inherited Metabolic Disorders

In most inherited metabolic disorders, a single enzyme is either not produced by the body at all, or is produced in a form that doesn't work. The missing enzyme is like an absentee worker on the assembly line. Depending on that enzyme's job, its absence means toxic chemicals may build up, or an essential product may not be produced.

The code or blueprint to produce an enzyme is usually contained on a pair of genes. Most people with inherited metabolic disorders inherit two defective copies of the gene -- one from each parent. Both parents are "carriers" of the bad gene, meaning they carry one defective copy and one normal copy.

In the parents, the normal gene copy compensates for the bad copy. Their enzyme levels are usually adequate, so they may have no symptoms of a genetic metabolic disorder. However, the child who inherits two defective gene copies cannot produce enough effective enzyme and develops the genetic metabolic disorder. This form of genetic transmission is called autosomal recessive inheritance.

The original cause of most genetic metabolic disorders is a gene mutation that occurred many, many generations ago. The gene mutation is passed along through the generations, ensuring its preservation.

Each inherited metabolic disorder is quite rare in the general population. Considered all together, inherited metabolic disorders may affect about 1 in 1,000 to 2,500 newborns. In certain ethnic populations, such as Ashkenazi Jews (Jews of central and eastern European ancestry), the rate of inherited metabolic disorders is higher.

Types of Inherited Metabolic Disorders

Hundreds of inherited metabolic disorders have been identified, and new ones continue to be discovered. Some of the more common and important genetic metabolic disorders include:

Lysosomal storage disorders: Lysosomes are spaces inside cells that break down waste products of metabolism. Various enzyme deficiencies inside lysosomes can result in buildup of toxic substances, causing metabolic disorders including:

  • Hurler syndrome (abnormal bone structure and developmental delay)
  • Niemann-Pick disease (babies develop liver enlargement, difficulty feeding, and nerve damage)
  • Tay-Sachs disease (progressive weakness in a months-old child, progressing to severe nerve damage; the child usually lives only until age 4 or 5)
  • Gaucher disease (bone pain, enlarged liver, and low platelet counts, often mild, in children or adults)
  • Fabry disease (pain in the extremities in childhood, with kidney and heart disease and strokes in adulthood; only males are affected)
  • Krabbe disease (progressive nerve damage, developmental delay in young children; occasionally adults are affected)

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