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.