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Smith-Lemli-Opitz syndrome (also called 7-dehydrocholesterol reductase deficiency) is a 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 is inherited in an autosomal recessive pattern.

Smith-Lemli-Opitz syndrome affects an estimated 1 in 20,000 to 40,000 births. This condition is most common in Caucasians of European ancestry. It is very rare among African and Asian populations. This condition is inherited in an autosomal recessive pattern, which means two copies of the gene in each cell are altered.

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.

This article incorporates public domain text from the Genetics Home Reference, a service of the U.S. National Library of Medicine

• Richard I. Kelley on SLOS
• Overview of condition, Genetics Home Reference at NLM