NORD gratefully acknowledges Jerry Vockley, MD, PhD, Professor of Pediatrics and Human Genetics, University of Pittsburgh and Chief of Medical Genetics, Children's Hospital of Pittsburgh of UPMC, for assistance in the preparation of this report.
Short chain acyl-CoA dehydrogenase deficiency (SCADD) is a rare autosomal recessive genetic defect in fatty acid catabolism belonging to a group of diseases known as fatty acid oxidation disorders (FOD). It occurs because of a deficiency of the short-chain acyl-CoA dehydrogenase (SCAD) enzyme.
Although SCADD was initially thought to produce severe problems including progressive muscle weakness, hypotonia, acidemia, developmental delay, and even early death, it is now believed that this deficiency has no clinical relevance. Since the advent of expanded newborn screening programs using tandem mass spectrometry technology, many more SCADD infants are being detected, all of whom are asymptomatic.
When symptoms are present, additional diagnostic testing for another condition should be performed as the association is likely coincidental rather than causative.
Essentially all individuals identified through newborn screening have been healthy. Therefore, the variety of symptoms that have been reported in other individuals with SCAD deficiency are all likely coincidental. This situation has been accentuated by the existence of two very common variants in the SCAD gene that lead to blood and urine findings suggestive of SCADD but are not sufficiently severe to cause complete SCADD.
SCADD is an autosomal recessive condition caused by mutations in the Short Chain Acyl-Coenzyme A dehydrogenase (ACADS) gene leading to deficiency of the SCAD enzyme.
The SCAD enzyme is involved in the breakdown of complex fatty acids into more simple substances. This takes place in the cell’s mitochondria, small, well-defined bodies found in all cells in which energy is generated from the breakdown of complex substances into simpler ones (mitochondrial oxidation). Because this enzyme occurs at the very end of the fatty acid oxidation pathway, the compounds that accumulate can be utilized by other enzymes, preventing clinical symptoms from occurring.
More than 100 different mutations in the ACADS gene cause SCADD. Two common variations (polymorphisms) have also been found in the ACADS gene. It has been suggested that SCAD deficiency may be a risk factor that can make other neuromuscular disorders worse, but this remains unproven.
Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother.
Recessive genetic disorders occur when an individual inherits a non-working gene from each parent. If an individual receives one working gene and one non-working gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the non-working gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier, like the parents, is 50% with each pregnancy. The chance for a child to receive working genes from both parents is 25%. The risk is the same for males and females.
All individuals carry a few abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.
SCAD deficiency is thought to affect 1 in 40,000 to 100,000 newborns. In the US, ~10% of individuals have two copies of one of the common polymorphisms leading to potential identification of related metabolites in urine or blood.
Diagnosis of SCADD should be suspected on the basis of elevated ethylmalonic acid (EMA) excretion in urine or butyrylcarnitine (C4 carnitine) in blood. Patients with this finding should have whole gene sequencing. If a mutation is not identified and EMA excretion is persistent, additional clinical evaluation is warranted as another diagnosis is likely. The presence of the common polymorphisms generally leads to reduction of muscle SCAD activity to 50-67% of normal; rarely, patients with no other identifiable mutations have had complete loss of activity. However, there is little or no clinical utility in measuring enzyme activity and muscle biopsy is not recommended to diagnosis SCAD deficiency.
As noted above, expanded newborn screening with tandem mass spectrometry is identifying more infants with SCADD than in the past. Adjustment of the screening results interpretation can usually differentiate between individuals having the common polymorphisms vs. complete SCADD.
There is no need to treat SCADD.
Genetic counseling is recommended for patients and their families.
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