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Smith-Lemli-Opitz syndrome (also SLOS, or 7-dehydrocholesterol reductase deficiency) is a metabolic and developmental disorder that affects many parts of the body.
This condition is characterized by distinctive facial features, small head size (microcephaly), mental retardation or learning disabilities, and behavioral problems. Malformations of the heart, lungs, kidneys, gastrointestinal tract, and genitalia are also common. Infants with Smith-Lemli-Opitz syndrome have weak muscle tone (hypotonia), experience feeding difficulties, and tend to grow more slowly than other infants. Most affected individuals have fused second and third toes (syndactyly), and some have extra fingers or toes (polydactyly).
Smith-Lemli-Opitz syndrome affects an estimated 1 in 20,000 to 40,000 births. It is most common in Caucasians of European ancestry, but very rare among African and Asian populations.
This disorder is inherited in an autosomal recessive pattern, which means two copies of the gene must be inherited to have the disorder.
Mutations in the DHCR7 gene cause Smith-Lemli-Opitz syndrome. The DHCR7 gene makes an enzyme called 7-dehydrocholesterol reductase. This enzyme is responsible for the final step in the production of cholesterol. Cholesterol is an essential nutrient that is necessary for normal embryonic development. Cholesterol is also a structural component of cell membranes and the protective substance covering nerve cells (myelin). Additionally, cholesterol plays an important role in the production of certain hormones and digestive acids.
Mutations in the DHCR7 gene reduce or eliminate the activity of 7-dehydrocholesterol reductase, preventing cells from producing enough cholesterol. A lack of this enzyme also allows potentially toxic byproducts of cholesterol production to build up in the blood and other tissues. The combination of low cholesterol levels and an accumulation of other substances likely disrupts the growth and development of many body systems. It is not known, however, how this disturbance in cholesterol production leads to the specific features of Smith-Lemli-Opitz syndrome.
A 2006 study of 14 children with Smith-Lemli-Opitz syndrome reported that most children with SLOS have an autism spectrum disorder. SLOS appears to have the most consistent relationship with autism of any single-gene disorder.[1]
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