inborn error of metabolism
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What is an inborn error of metabolism?
Inborn errors of metabolism are rare genetic disorders caused by inherited defects that disrupt the body's metabolism. These errors typically involve mutated enzymes or transport proteins, which hinder the metabolic transformation of substances or their movement across cell membranes.
These diseases affect different aspects of metabolism, including the processing of amino acids, lipids, carbohydrates, and nucleic acids. Diagnosis involves biochemical and genetic testing, with newborn screenings crucial for early detection and intervention. Treatment varies depending on the specific pathway affected but often includes dietary adjustments and, in some cases, transplants or gene therapy. Most inborn errors of metabolism follow an autosomal recessive inheritance pattern.
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How are inborn errors of metabolism related to genetics?
Inborn errors of metabolism are genetic disorders caused by inherited defects that disrupt the body's metabolism. These errors typically arise from mutations in genes that encode enzymes or transport proteins involved in metabolic pathways.
The "one gene-one enzyme" principle connects a single gene to the production of a specific enzyme. Mutations in these genes can lead to defective enzymes with reduced or absent function, resulting in metabolic diseases.
Inheritance patterns vary, but inborn errors of metabolism usually follow an autosomal recessive pattern, where two copies of the mutated gene are needed to manifest the disease. However, dominant or sex-linked inheritance is also possible.
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What causes a genetic mutation leading to an inborn error of metabolism?
Inborn errors of metabolism arise from inherited genetic defects that disrupt the body's ability to process nutrients. These errors typically occur when a mutated gene leads to a deficiency or absence of a specific enzyme or transport protein. Enzymes are proteins that catalyze biochemical reactions, and transport proteins assist in moving compounds across cell membranes.
Most inborn errors of metabolism follow an autosomal recessive inheritance pattern, meaning that two copies of the mutated gene (one from each parent) must be inherited for the disease to manifest. However, in some instances, inheritance can be dominant or sex-linked. The dysfunctional enzyme disrupts metabolic pathways, potentially leading to a buildup of toxic substances or a deficiency in essential products.
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How are inborn errors of metabolism inherited from parents?
Inborn errors of metabolism (IEM) stem from inherited genetic defects that disrupt the body's metabolism. These errors typically arise from inheriting a mutated enzyme or transport protein, vital for metabolic processes.
IEM inheritance usually follows an autosomal recessive pattern, needing two copies of the mutated gene (one from each parent) for the disease to manifest. In some instances, dominant inheritance occurs, where only one copy of the mutated gene is sufficient to cause the condition. Additionally, sex-linked inheritance can also occur, where the mutated gene is carried on a sex chromosome.
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What are some common symptoms of inborn errors of metabolism?
Inborn errors of metabolism can manifest with a variety of symptoms, though some might not be obvious until early childhood. Symptoms include poor appetite, unusual food preferences (such as aversion to protein), failure to thrive, lethargy, and developmental delays. In some instances, symptoms can be confused with those of other diseases, delaying diagnosis.
These errors can injure virtually any tissue, with damage to the developing brain being a significant consequence in untreated or severe cases. Neurological disease may appear as encephalopathy, reflecting the accumulation of a normal metabolite that becomes toxic in excess. The consequences of metabolic imbalance may be severe, including intellectual disability, seizures, decreased muscle tone, organ failure, blindness, and deafness.
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How can genetic testing help diagnose and treat inborn errors of metabolism?
Genetic testing plays a crucial role in diagnosing and treating inborn errors of metabolism by detecting fundamental alterations in the genetic code that cause metabolic aberrations.
Diagnosis is achieved through biochemical tests like mass spectrometry for newborn screening, which identifies metabolic abnormalities in blood. Genetic testing can confirm diagnoses based on screening results and assess high-risk fetuses.
Treatment approaches vary depending on the specific metabolic pathway affected. Common interventions include diet therapy to restrict harmful nutrients, and organ transplants (bone marrow, liver, or kidney) to restore normal metabolism. Gene therapy, which introduces normal copies of the deficient gene into the patient's cells, is a prospective therapeutic approach.
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inborn error of metabolism, any of multiple rare disorders that are caused by an inherited genetic defect and that alter the body’s ability to derive energy from nutrients. The term inborn error of metabolism was introduced in 1908 by British physician Sir Archibald Garrod, who postulated that inherited disorders such as alkaptonuria and albinism result from reduced activity or complete absence of enzymes involved in certain biochemical pathways. Garrod’s identification and categorization of inborn errors of metabolism represented an important conceptual advance in 20th-century medical genetics.
The overall estimated incidence of inborn errors of metabolism is approximately 1 in every 4,000 live births. However, incidence can vary within populations, depending on factors such as ethnic background.
Underlying causes and patterns of inheritance
The metabolic diseases that result from inborn defects involve different aspects of human metabolism, including the handling of amino acids, lipids, carbohydrates, and nucleic acids. In most instances the underlying cause is the inheritance of a mutated enzyme, the normal function of which is the metabolic transformation of one metabolite into another, or of a mutated transport protein, the normal function of which is to assist in the movement of a compound across a cell membrane.
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Inheritance of inborn errors of metabolism usually conforms to an autosomal recessive pattern (two copies of the mutant gene, one from each parent, must be inherited to produce the signs and symptoms of disease). In some cases, however, inheritance may be dominant (only one copy of the mutated gene is needed) or sex-linked (the mutated gene is carried on a sex [X or Y] chromosome).
Symptoms and effects on the brain
Although certain inborn errors of metabolism are apparent at or shortly after birth, others may not become obvious until early childhood. Certain symptoms vary according to the specific disorder, but, in general, affected individuals have a poor appetite or unusual food preferences (e.g., aversion to protein), may fail to thrive, may be lethargic, and may experience developmental delays. In some instances, symptoms are confused with those of other diseases or disorders, resulting in delayed diagnosis.
Inborn errors of metabolism can result in injury to virtually any tissue, but the most dramatic and characteristic consequence in untreated or severe cases is damage to the developing brain. Neurological disease often appears clinically as encephalopathy (abnormal brain function and structure). Encephalopathy reflects the accumulation of an otherwise normal metabolite that becomes toxic when present in excess concentration. An example is the extreme elevation of the amino acid phenylalanine that accompanies a congenital defect of phenylalanine hydroxylase, the mutant enzyme in classical phenylketonuria (PKU). The biochemical sequence that leads from phenylalanine accumulation to intellectual disability remains obscure, although it is likely that the underlying pathophysiology evokes alterations of brain energy metabolism, neurotransmitter synthesis, and myelin formation (myelin is the insulating material found around the axons of neurons).
Diagnosis and treatment of metabolic disorders
Inherited metabolic diseases are diagnosed based primarily on biochemical tests, which may employ any of several different chromatographic, electrophoretic, and enzymatic techniques for the isolation and quantitation of relevant metabolites in blood and urine. The ability to detect metabolic abnormalities in blood facilitated the development of newborn screening for metabolic disorders, in which mass spectrometry is used to screen for multiple disorders in dried spots of blood. Newborn screening attempts to catch metabolic diseases before they cause severe developmental delays or impairments.
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Genetic testing may also be used to diagnose inborn errors of metabolism or to confirm diagnosis based on screening or other biochemical findings. Genetic testing can unambiguously characterize fundamental alterations of the genetic code that give rise to metabolic aberrations. It is sometimes used in the assessment of fetuses at high risk for metabolic disease.
Treatment for inborn errors of metabolism depends on the specific biochemical pathway that has been affected. In general, however, diet therapy, or the purposeful interdiction of a potentially injurious nutrient, often attenuates or even prevents brain injury and permits normal neurological development. For many disorders, a bone marrow, liver, or kidney transplant has palliated the underlying lesion and afforded near-normal metabolism. A therapeutic prospect is gene therapy, or the administration of an agent that safely and efficiently carries normal copies of the deficient gene to cells of the affected patient, thereby reconstituting normal or near-normal enzymatic competence.