Inborn Errors of Metabolism
Inborn errors of metabolism are inherited disorders in which a genetic defect impairs a specific enzyme, transporter, or cofactor in a metabolic pathway, so that a substrate accumulates, a product is deficient, or an alternative pathway is activated. Individually rare but collectively significant, they span carbohydrate, amino acid, fatty acid, mitochondrial energy, and many other pathways, and they form a bridge between classical biochemistry and clinical genetics.
Definition
An inborn error of metabolism is a heritable disorder, usually monogenic, in which a defect in an enzyme or transport protein disrupts a metabolic pathway, producing disease through accumulation of a toxic precursor, deficiency of an essential product, or impaired energy production.
Scope
This area orients the reader to the inherited metabolic diseases as a group: the unifying concept of a blocked pathway, the broad mechanistic categories (intoxication-type, energy-deficiency-type, and storage/complex-molecule disorders), and the role of population newborn screening. Detailed disease families - glycogen storage diseases, mitochondrial cytopathies, aminoacidopathies and organic acidemias, fatty acid oxidation disorders, and urea cycle disorders - are treated in the child topics. It is a reference overview, not a clinical management resource.
Sub-topics
Key concepts
- Enzyme or transporter deficiency in a defined pathway
- Substrate accumulation versus product deficiency
- Intoxication-type disorders
- Energy-deficiency (mitochondrial) disorders
- Storage and complex-molecule disorders
- Autosomal recessive and other inheritance patterns
- Newborn screening by tandem mass spectrometry
- Acute metabolic decompensation
Mechanisms
A pathogenic variant reduces the activity of a particular enzyme, cofactor, or transporter, so flux through its pathway is interrupted. The clinical consequences follow from where the block lies: upstream substrate or its by-products may accumulate to toxic levels (the intoxication pattern seen in many aminoacidopathies, organic acidemias, and urea cycle disorders), a downstream product may be lacking, or the cell may fail to generate or use energy (the pattern in mitochondrial and fatty acid oxidation disorders). A useful organising scheme groups these conditions into small-molecule intoxication disorders, energy-metabolism disorders, and disorders of complex molecules. Because many pathways converge on shared intermediates, biochemical markers such as plasma amino acids, urine organic acids, and acylcarnitine profiles allow whole categories to be screened and triaged.
Clinical relevance
Inborn errors of metabolism illustrate how a single gene defect can disrupt biochemistry at the whole-organism level, and how biochemical reasoning underlies diagnosis. Recognising the broad categories explains why population newborn screening targets several of these conditions before symptoms appear. This entry describes the conceptual landscape and how evidence and screening are organised; it is not a basis for individual diagnostic or treatment decisions.
Epidemiology
Each individual disorder is rare, but as a group inborn errors of metabolism affect a meaningful fraction of newborns; expanded screening programmes using tandem mass spectrometry now detect dozens of conditions from a single dried blood spot, and population studies such as Wilcken and colleagues' newborn cohort have quantified how screening changes detection and outcomes.
History
Archibald Garrod introduced the phrase inborn errors of metabolism in his early twentieth-century Croonian Lectures, proposing that conditions such as alkaptonuria reflected inherited blocks in chemical pathways - an idea that anticipated the one gene one enzyme concept. Through the twentieth century, advances in enzymology, chromatography, and mass spectrometry expanded the catalogue of recognised disorders, and the introduction of newborn screening, later broadened by tandem mass spectrometry, moved many of these conditions from late clinical diagnosis to presymptomatic detection.
Key figures
- Archibald Garrod
- Jean-Marie Saudubray
- Bridget Wilcken
- Nenad Blau
Related topics
Seminal works
- garrod-1908
- wilcken-2003
- saudubray-2016
Frequently asked questions
- What does inborn error of metabolism mean?
- It is an inherited disorder in which a genetic defect impairs an enzyme, cofactor, or transporter in a metabolic pathway, so that a substance builds up, an essential product is lacking, or the cell cannot produce energy normally.
- Why are so many of these disorders included in newborn screening?
- Many inborn errors of metabolism cause irreversible harm before symptoms are obvious, yet leave detectable biochemical signatures. Tandem mass spectrometry can measure many of these markers from a single dried blood spot, allowing several disorders to be identified shortly after birth.